Measurement method and associated measurement device

Abstract

A dark field metrology method is disclosed. A first partial electric field is determined from a measured parameter of first diffracted radiation data relating to a first portion of a diffraction order, and a second partial electric field is determined from a measured parameter of second diffracted radiation data relating to a second portion of the diffraction order. The first portion of the diffraction order and the second portion of the diffraction order are associated with respective portions of a detection pupil plane or a conjugate plane thereof. And determining the electric field of the diffraction order according to the first part electric field and the second part electric field.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to European application No. 22195453.0 filed on September 13, 2022 and European application No. 23160181.6 filed on March 06, 2023, and the entire contents of these European applications are incorporated herein by reference. Technical Field

[0003] The invention relates to a metrology method and an apparatus which can be used, for example, to determine properties of structures on a substrate. Background Art

[0004] A lithographic apparatus is a machine configured to apply a desired pattern onto a substrate. For example, a lithographic apparatus may be used in the manufacture of integrated circuits (ICs). For example, a lithographic apparatus may project a pattern (also often referred to as a "design layout" or "design") at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) disposed on a substrate (e.g., a wafer).

[0005] To project a pattern on a substrate, a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of a feature that can be formed on a substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. Smaller features may be formed on a substrate using a lithographic apparatus that uses, for example, extreme ultraviolet (EUV) radiation (having a wavelength in the range of 4 nm to 20 nm, such as 6.7 nm or 13.5 nm).

[0006] Low k1 lithography can be used to process features with dimensions smaller than the classical resolution limit of the lithographic apparatus. In such a process, the resolution formula can be expressed as CD = k1×λ / NA, where λ is the wavelength of radiation employed, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the "critical dimension" (usually the smallest feature size printed, but in this case it is the half pitch), and k1 is an empirical resolution factor. In general, the smaller the k1, the more difficult it becomes to reproduce a pattern on a substrate that is similar in shape and size to what the circuit designer planned in order to achieve specific electrical functionality and performance. To overcome these difficulties, complex fine-tuning steps can be applied to the lithographic projection apparatus and / or the design layout. For example, these fine-tuning steps include, but are not limited to, NA optimization, customized illumination schemes, use of phase-shifted patterning devices, various optimizations of the design layout, such as optical proximity correction (OPC, sometimes also referred to as "optical and process correction") in the design layout, or other methods generally defined as "resolution enhancement techniques" (RET). Alternatively, a tight control loop for controlling the stability of the lithographic apparatus can be used to improve pattern reproduction at low k1.

[0007] During photolithography, it is frequently desirable to measure the resulting structure, for example, for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes or various forms of metrology equipment (such as scatterometers). A general term referring to such tools may be metrology equipment or inspection equipment.

[0008] In many metrology applications, it is desirable to access the complete electric field (i.e., intensity and phase), rather than just the intensity. Holographic tools enable this, however they require a reference beam and very tight operation to ensure interference between the reference beam and the measurement radiation beam. Other phase acquisition methods may involve iterative acquisition of the phase, however these methods are computationally demanding.

[0009] It would be desirable to improve existing methods for measuring full electric fields in metrology applications. Summary of the invention

[0010] Embodiments of the invention are disclosed in the claims and in the detailed description.

[0011] In a first aspect of the present invention, a dark field measurement method is provided, comprising: obtaining at least first diffraction radiation data and second diffraction radiation data, the first diffraction radiation data being associated with a first part of a diffraction order, and the second diffraction radiation data being associated with a second part of the diffraction order, wherein the first part of the diffraction order and the second part of the diffraction order are associated with corresponding parts of a detection pupil plane or a conjugate plane thereof; determining a first part of an electric field according to measurement parameters of the first diffraction radiation data; determining a second part of an electric field according to measurement parameters of the second diffraction radiation data; and determining the electric field of the diffraction order according to the first part of the electric field and the second part of the electric field.

[0012] In a second aspect of the present invention, a dark field measurement device is provided, comprising: an illumination arrangement, the illumination arrangement being used to illuminate a structure on a substrate so as to produce at least one diffraction order; a detection arrangement, the detection arrangement being used to detect the at least one diffraction order or a part thereof; a spatial filter arrangement, the spatial filter arrangement being located in a detection pupil plane or a conjugate plane of the dark field measurement device, the spatial filter arrangement being operable to separately transmit at least a first part of the diffraction order and a second part of the diffraction order so that the first part and the second part are separately detected on the detection arrangement; and a processor, the processor being operable to: determine a first partial electric field based on measurement parameters of first diffraction radiation data associated with the first part of the diffraction order; determine a second partial electric field based on measurement parameters of second diffraction radiation data associated with the second part of the diffraction order; and determine the electric field of the diffraction order based on at least the first partial electric field and the second partial electric field.

[0013] These and other aspects and advantages of the apparatus and methods disclosed herein will be apparent from a consideration of the following description of exemplary embodiments and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0015] Figure 1 A schematic overview diagram depicting a lithographic apparatus;

[0016] Figure 2 A schematic overview diagram depicting a lithography unit;

[0017] Figure 3 Schematic diagram depicting overall lithography, showing the collaboration between three key technologies to optimize semiconductor manufacturing;

[0018] Figure 4 is a schematic diagram of the scatterometry equipment;

[0019] Figure 5 comprising (a) a schematic diagram of a dark field scatterometer for measuring a target according to an embodiment of the present invention using a first pair of illumination apertures, (b) details of a diffraction spectrum of a target grating for a given illumination direction, (c) a second pair of illumination apertures providing an alternative illumination mode when using the scatterometer for diffraction-based overlay measurements, (d) a third pair of illumination apertures combining the first pair of illumination apertures with the second pair of illumination apertures;

[0020] Figure 6 is a schematic diagram of a one-dimensional coordinate system associated with the concepts disclosed herein;

[0021] Figure 7 (a) is a schematic diagram of a measurement arrangement according to an embodiment, Figure 7 (b) is a first mask for performing the metrology method according to the embodiment, and Figure 7 (c) a complementary second mask for performing the metrology method;

[0022] Figure 8 is a flow chart describing a measurement method according to an embodiment;

[0023] Fig. 9 is a schematic diagram of a single acquisition measurement arrangement according to an embodiment; and

[0024] Fig.10 Depicted is a block diagram of a computer system for controlling a metrology method and / or apparatus such as disclosed herein. DETAILED DESCRIPTION

[0025] In this document, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g., having a wavelength of 365 nm, 248 nm, 193 nm, 157 nm, or 126 nm) and EUV (extreme ultraviolet radiation, e.g., having a wavelength in the range of about 5 nm to 100 nm).

[0026] The terms "reticle", "mask" or "patterning device" as used herein may be broadly interpreted to refer to a general purpose patterning device that can be used to impart a patterned profile to an incoming radiation beam that corresponds to a pattern to be formed in a target portion of a substrate. The term "light valve" may also be used in this context. In addition to classical masks (transmissive or reflective, binary, phase-shifting, hybrid, etc.), other examples of such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0027] Figure 1A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA comprises an illumination system (also referred to as an illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation), a mask support (e.g., a mask stage) MT configured to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to certain parameters, a substrate support (e.g., a wafer stage) WT configured to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support according to certain parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0028] In operation, the illumination system IL receives a radiation beam from a radiation source SO, for example via a beam delivery system BD. The illumination system IL may include various types of optical components for directing, shaping and / or controlling the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical components, or any combination thereof. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its profile at the plane of the patterning device MA.

[0029] The term "projection system" PS as used herein should be broadly interpreted as encompassing various types of projection systems, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as applicable to the exposure radiation used, and / or other factors, such as the use of an immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0030] The lithographic apparatus LA may 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 (e.g. water) so as to fill the space between the projection system PS and the substrate W, which is also known as immersion lithography. More information on immersion technology is given in US6952253 (which is incorporated herein by reference).

[0031] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also referred to as a "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or a step of preparing a substrate W for subsequent exposure may be performed on a substrate W located on one of the substrate supports WT while another substrate W on another substrate support WT is being used to expose a pattern on another substrate W.

[0032] In addition to the substrate support WT, the lithographic apparatus LA may also comprise a measurement platform. The measurement platform is arranged to hold sensors and / or cleaning means. The sensors may be arranged to measure properties of the projection system PS or properties of the radiation beam B. The measurement platform may hold a plurality of sensors. The cleaning means may be arranged to clean a part of the lithographic apparatus, for example a part of the projection system PS or a part of the system for providing immersion liquid. The measurement platform may be moved under the projection system PS when the substrate support WT is away from the projection system PS.

[0033] In operation, a radiation beam B is incident on a patterning device (e.g. a mask) MA held on a mask support MT and is patterned by a pattern (design layout) present on the patterning device MA. Having traversed the 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 measurement system IF, the substrate support WT can be accurately moved, for example, in order to position different target portions C in the path of the radiation beam B at focused and aligned positions. Similarly, a first positioner PM and possibly a further position sensor (which may be located in the position sensor) may be used to position the radiation beam B. Figure 1 The patterning device MA may be accurately positioned relative to the path of the radiation beam B using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the illustrated substrate alignment marks P1, P2 occupy dedicated target portions, they may be located in spaces between target portions. When the substrate alignment marks P1, P2 are located between target portions C, they are referred to as scribe line alignment marks.

[0034] As in Figure 2 As shown in , the lithography apparatus LA may form part of a lithography cell LC, sometimes also referred to as a lithocell or (lithography) cluster, which typically also includes equipment for performing pre-exposure and post-exposure processes on a substrate W. Conventionally, these equipment include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH and a bake plate BK, for example for regulating the temperature of the substrate W, for example for regulating the solvent in the resist layer. A substrate transport device or robot RO picks up the substrate W from the input / output ports I / O1, I / O2, moves the substrate between the different processing equipment, and transfers the substrate W to a feed station LB of the lithography apparatus LA. The devices in the lithography cell (which are also often collectively referred to as a track or coating and developing system) are typically controlled by a track or coating and developing system control unit TCU, which itself may be controlled by a management control system SCS, which may also control the lithography apparatus LA, for example via the lithography control unit LACU.

[0035] In order for substrates W exposed by lithographic apparatus LA to be correctly and consistently exposed, it is desirable to inspect the substrates to measure properties of the patterned structures, such as overlay errors between subsequent layers, line thickness, critical dimensions (CD), etc. For this purpose, an inspection tool (not shown) may be included in lithography cell LC. If errors are detected, then, for example, exposure of subsequent substrates or other processing steps to be performed on substrate W may be adjusted, particularly if other substrates W of the same batch or lot are yet to be inspected before being exposed or processed.

[0036] The inspection apparatus, which may also be referred to as a metrology apparatus, is used to determine properties of a substrate W, and in particular to determine how properties of different substrates W vary or how properties relating to different layers of the same substrate W vary from layer to layer. The inspection apparatus is alternatively configured to identify defects on the substrate W, and may for example be part of a lithography cell LC, or may be integrated into a lithography apparatus LA, or may even be a separate device. The inspection apparatus may measure properties on a latent image (the image in the resist layer after exposure), or on a semi-latent image (the image in the resist layer after a post-exposure bake step PEB), or on a developed resist image (wherein exposed or unexposed parts of the resist have been removed), or even on an etched image (after a pattern transfer step such as etching).

[0037] Typically, the patterning process in the lithography apparatus LA is one of the most important steps in the process, which requires a high accuracy in the size and placement of the structures on the substrate W. To ensure this high accuracy, the three systems can be combined in a so-called "holistic" control environment, such as in Figure 3 As schematically depicted in FIG, one of these systems is a lithography apparatus LA, which is (in practice) connected to a metrology tool MET (a second system) and to a computer system CL (a third system). The key to this "holistic" environment is to optimize the cooperation between these three systems to enhance the overall process window and provide a tight control loop to ensure that the patterning performed by the lithography apparatus LA remains within the process window. The process window defines the range of process parameters (e.g., dose, focus, overlay) within which a particular manufacturing process produces a defined result (e.g., a functional semiconductor device), and typically within which process parameter variations in the lithography process or patterning process are allowed.

[0038] The computer system CL can use (part of) the design layout to be patterned to predict which resolution enhancement techniques to use and perform computational lithography simulations and calculations to determine which mask layout and lithography equipment settings achieve the maximum overall process window for the patterning process (in Figure 3Typically, the resolution enhancement technique is arranged to match the patterning possibilities of the lithographic apparatus LA. The computer system CL may also be used to detect where within the process window the lithographic apparatus LA is currently operating (e.g. using input from a metrology tool MET) to predict whether defects may exist due to, for example, suboptimal processing (e.g. at Figure 3 0 (as depicted by the arrow pointing to “0” in the second scale SC2).

[0039] The metrology tool MET may provide input to the computer system CL to enable accurate simulations and predictions, and may provide feedback to the lithographic apparatus LA to identify possible drifts, e.g. in the calibration state of the lithographic apparatus LA (in Figure 3 SC3).

[0040] During lithography, it is desirable to frequently measure the structures being formed, for example for process control and verification. Various tools are known for making such measurements, including scanning electron microscopes or various forms of metrology equipment such as scatterometers. Examples of known scatterometers often rely on the provision of dedicated metrology targets, such as an underfilled target (a target in the form of a simple grating or overlapping gratings in different layers, which is large enough that the measurement beam produces a spot smaller than the grating) or an overfilled target (so that the illumination spot partially or completely includes the target). In addition, the use of a metrology tool (e.g. an angle-resolved scatterometer that illuminates an underfilled target such as a grating) allows the use of so-called reconstruction methods, in which the properties of the grating can be calculated by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulated results with the measured results. The parameters of the model are adjusted until the simulated interaction produces a diffraction pattern similar to the diffraction pattern observed from a real target.

[0041] Scatterometers are versatile instruments that allow measuring parameters of a lithographic process by having a sensor in the pupil or a plane conjugated to the pupil of the objective of the scatterometer (measurements are usually referred to as pupil-based measurements), or by having a sensor in the image plane or a plane conjugated to the image plane, in which case measurements are usually referred to as image-based or field-based measurements. Such scatterometers and related measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP1,628,164A, which are incorporated herein by reference in their entirety. The above scatterometers can measure multiple targets from multiple gratings in one image using light from soft x-rays and visible to the near IR band.

[0042] Figure 4 A measurement device, such as a scatterometer, is depicted in . It comprises a broadband (white light) radiation projector 2 which projects radiation 5 onto a substrate W. The reflected or scattered radiation 10 is passed to a spectrometer detector 4 which measures the spectrum 6 of the one-way reflected radiation 10 (i.e. a measurement of the intensity I as a function of the wavelength λ). From such data, a processing unit PU can reconstruct the structure or profile 8 which produced the detected spectrum, for example by rigorous coupled wave analysis and nonlinear regression or by comparison with a library of simulated spectra. Typically, for the reconstruction, the general form of the structure is known and some parameters are assumed from knowledge of the process by which the structure was manufactured, leaving only some parameters of the structure to be determined from scatterometry data. Such a scatterometer can be configured as a normal incidence scatterometer or an oblique incidence scatterometer.

[0043] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, reconstruction methods can be applied to the measured signals to reconstruct or calculate the properties of the grating. Such a reconstruction can, for example, result from simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measured results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to the diffraction pattern observed from a real target.

[0044] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, radiation emitted by a radiation source is directed onto a target and reflected or scattered radiation from the target is directed to a spectrometer detector which measures the spectrum (i.e. a measure of intensity as a function of wavelength) of the spectroscopically reflected radiation. From such data, the structure or profile of the target which gave rise to the detected spectrum can be reconstructed, for example by rigorous coupled wave analysis and nonlinear regression or by comparison with a library of simulated spectra.

[0045] In a third embodiment, the scatterometer MT is an ellipsometry scatterometer. An ellipsometry scatterometer allows the parameters of a lithography process to be determined by measuring the scattered radiation for each polarization state. Such a metrology device emits a polarization (such as linear, annular or elliptical) by using, for example, an appropriate polarization filter in the illumination section of the metrology device. A source suitable for the metrology device may also provide polarized radiation. Various embodiments of existing ellipsometry scatterometers are described in U.S. patent applications 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,449, 12 / 920,968, 12 / 922,587, 13 / 000,229, 13 / 033,135, 13 / 533,110 and 13 / 891,410, which are incorporated herein by reference in their entirety.

[0046] In one embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the overlap of two misaligned gratings or periodic structures by measuring the reflection spectrum and / or an asymmetry in the detection configuration (the asymmetry being related to the extent of the overlap). The two (usually overlapping) grating structures may be applied in two different layers (not necessarily consecutive layers) and may be formed to be at approximately the same position on the wafer. The scatterometer may have a symmetrical detection configuration as described, for example, in co-owned patent application EP1,628,164A, so that any asymmetry is clearly discernible. This provides a straightforward way to measure alignment errors in the gratings. Other examples for measuring the overlap error between two layers comprising a periodic structure when measuring the target through the asymmetry of the periodic structure may be found in PCT patent application publication number WO 2011 / 012624 or US patent application US 20160161863, which are incorporated herein by reference in their entirety. Other parameters of interest may be focus and dose. Focus and dose may be determined simultaneously by scatterometry as described in US Patent Application US2011-0249244, which is incorporated herein by reference in its entirety (or alternatively by scanning electron microscopy).

[0047] The target can be measured in an underfill mode or in an overfill mode. In the underfill mode, the measurement beam produces a spot that is smaller than the overall target. In the overfill mode, the measurement beam produces a spot that is larger than the overall target. In such an overfill mode, it is also possible to measure different targets simultaneously and thus determine different process parameters simultaneously.

[0048] The overall measurement quality of a lithography parameter performed using a specific target is at least partially determined by the measurement profile used to measure such lithography parameters. The term "substrate measurement profile" may include one or more parameters of the measurement itself, one or more parameters of one or more patterns measured, or both. For example, if the measurement used in the substrate measurement profile is based on optical measurement of diffraction, one or more of the measured parameters may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the direction of the radiation relative to the pattern on the substrate, etc. One of the criteria for selecting a measurement profile may, for example, be the sensitivity of one of the measurement parameters to process variations. More examples are described in U.S. Patent Application US2016-0161863 and published U.S. Patent Application US 2016 / 0370717A1, which are incorporated herein by reference in their entirety.

[0049] Figure 5 (a) Presents an embodiment of a measurement apparatus and more particularly a dark field scatterometer. Figure 5(b) illustrates in more detail the target T and the diffracted rays of the measuring radiation used to illuminate the target. 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 into a lithography apparatus LA (e.g. at a measuring station) or incorporated into a lithography cell LC. An optical axis having several branches running through the apparatus is represented by a 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, 11 and an objective lens 16. These lenses are arranged in a double sequence in 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 at the same time allows access to an intermediate pupil plane for spatial frequency filtering. Thus, the angular range over which the radiation is 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). Specifically, this selection can be made by inserting an aperture plate 13 of suitable form between lenses 12 and 11 in a plane that is a back-projected image of the objective lens pupil plane. In the illustrated example, the aperture plate 13 has different forms marked as 13N and 13S, thereby 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 the sake of description only. In a second illumination mode, the aperture plate 13S is used to provide similar illumination, but from the opposite direction marked as "south". By using different holes, other illumination modes are possible. The rest of the pupil plane is desirably dark, because any unnecessary light outside the desired illumination mode will interfere with the desired measurement signal.

[0050] like Figure 5 As shown in (b), the target T is placed with the substrate W 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 incident on the target T at an angle offset from the axis O causes a zero-order ray (solid line 0) and two first-order rays (dash-dotted line +1 and dash-dotted line -1). It should be remembered that in the case of a small overfilled target, these rays are only one of many parallel rays covering the area of ​​the substrate including the measurement target T and other features. Since 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. Depending on the point spread function of the small target, each order +1 and -1 will be further spread out over the angular range, rather than the single ideal ray shown. It should be noted that the grating pitch and the illumination angle of the target can be designed or adjusted so that the first-order rays entering the objective are closely aligned with the central optical axis. Figure 5 (a) and Figure 5The rays illustrated in (b) are shown slightly off-axis simply to enable them to be more easily distinguished in the illustration.

[0051] At least the 0th and +1st orders diffracted by the target T on the substrate W are collected by the objective lens 16 and directed back through the beam splitter 15. Figure 5 (a) illustrates both the first illumination mode and the second illumination mode by specifying diametrically opposed apertures labeled North (N) and South (S). When the incident 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 aperture plate 13N, the +1 diffracted ray labeled +1 (N) enters objective lens 16. In contrast, when the second illumination mode is applied using aperture plate 13S, the -1 diffracted ray (labeled 1 (S)) is the diffracted ray that enters lens 16.

[0052] The second beam splitter 17 divides the diffraction beam into two measurement branches. In the first measurement branch, the optical system 18 forms a diffraction spectrum (pupil plane image) of the target on a first sensor 19 (e.g., a CCD or CMOS sensor) using the zero-order diffraction beam and the first-order diffraction beam. 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 metrology device and / or normalize the intensity measurement of the first-order beam. The pupil plane image can also be used for many measurement purposes such as reconstruction.

[0053] In the second measurement branch, the optical systems 20, 22 form an image of the target T on a sensor 23 (e.g., a CCD or CMOS sensor). In the second measurement branch, an aperture stop 21 is provided in a plane conjugate to the pupil plane. The aperture stop 21 serves to block the zero-order diffraction beam so that the image of the target formed on the sensor 23 is formed only by the -1 or +1 order beams. The image captured by the sensors 19 and 23 is output to a processor PU that 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 this broad sense. If only one of the -1 order and the +1 order is present, an image of such a grating line will not be formed.

[0054] Figure 5 The specific forms of aperture plate 13 and field stop 21 shown in FIG. 1 are merely examples. In another embodiment of the invention, coaxial illumination of the target is used, and an aperture stop with an off-axis hole is used to pass substantially only one first-order diffracted light to the sensor. In yet other embodiments, instead of or in addition to the first-order beam, 2nd, 3rd and higher order beams ( Figure 5 ) can also be used in the measurement.

[0055] In order that the measuring radiation can be adapted to these different types of measurements, the aperture plate 13 may comprise a certain number of aperture patterns formed around a disk which is rotated to bring the desired pattern into position. Note that the aperture plates 13N or 13S may only be used to measure gratings oriented in one direction (X or Y, depending on the setup). For measuring orthogonal gratings, rotations of the target of up to 90° and 270° may be implemented. Figure 5 (c) and Figure 5 Different orifice plates are shown in (d). The use of these orifice plates and many other variations and applications of the apparatus are described in the previously published applications mentioned above.

[0056] As an alternative to a scatterometer, the measurement device may include a holographic microscope, such as a digital holographic microscope (DHM) or a digital dark field holographic microscope. Such devices are disclosed, for example, in US2019 / 0107781 and WO2021121733A1, which are incorporated herein by reference.

[0057] In a scatterometry metrology tool, light can be used to create an image of a structure on a substrate (e.g., at one or both of the image plane or pupil plane). Such a tool can measure the intensity or amplitude (or related measurement parameters, such as diffraction efficiency) of the detected light, e.g., after having been scattered by the structure, and use this to determine one or more parameters of interest of the structure. In many cases, it can be beneficial to know the composite field (i.e., amplitude and phase) of the light scattered by the structure.

[0058] In addition to the measured intensity / amplitude, there are also many existing methods that enable the phase to be determined. One such technique is the aforementioned DHM, however holographic methods are usually subject to strict stability requirements and / or very low acquisition times to suppress mechanical vibrations and / or drifts, and thus to achieve the necessary interference of the scattered radiation with the reference radiation at the detector.

[0059] Another known method involves performing multiple measurements under various conditions (e.g., different focus levels) and reconstructing the composite field of the scattered light using an iterative algorithm (e.g., as described in WO2021 / 121733, incorporated herein by reference). However, such an approach is computationally very demanding / very expensive and can therefore be slow, particularly because the accuracy of the phase acquisition required in the metrology is higher than for more conventional phase acquisition applications (such as microscopy and which only require the formation of an image). Other techniques are also known, such as using a mask with pinholes or similar holes in the detection branch of the tool, which allows interferometric measurements (e.g., as described in WO2020 / 254041, incorporated herein by reference).

[0060] A non-iterative method for determining the composite field of scattered radiation will now be described based on a causal relationship such as applied in the context of (one-dimensional) time versus angular frequency. This temporal causality stems from the fact that the electric field cannot travel faster than the speed of light, resulting in an electric field E=0 for the time t<0 constraint. This temporal / causal constraint results in a symmetric relationship for the electric field in the angular frequency domain. This symmetric relationship is often referred to as the Kramers-Kronig relationship (and its inverse, the second Kramers-Kronig relationship).

[0061] The proposed method is based on an optical analogy of this concept. The temporal constraint is replaced / implemented by positioning a mask in the pupil plane, so that the time coordinate t becomes a pupil coordinate, and the angular frequency coordinate ω becomes a sensor / camera (image plane) coordinate. Because the Cramer-Kronym relation involves a linear integral transform (essentially a convolution operation), the (one-dimensional) Cramer-Kronym expression expressed in terms of the Fourier transform can be extended to a two-dimensional analysis; i.e., the two dimensions of the pupil plane / image plane.

[0062] As an extension, it is also possible to further extend the analysis into three dimensions. In such a three-dimensional extension, in addition to the two pupil / spatial coordinates, another dimension may also be the wavelength, so that the method can determine a composite field from images measured for multiple individual illumination wavelengths. Such a three-dimensional Cramer-Kronym embodiment provides a solid model that couples spatial and through-wavelength behavior, which can be advantageous, for example, for one or more of the following: suppressing measurement noise, interpolating data, providing key performance indicators (KPIs) for protecting data consistency.

[0063] In the following discussion, the Fourier transform of the function g(t) is The following definitions will be used:

[0064]

[0065]

[0066] And function The inverse Fourier transform of will be:

[0067]

[0068]

[0069] where i represents the number of imaginary units, ω represents the sensor / image plane position (analogous to radial frequency in conventional Cramer-Kronym analysis, hence the symbol ω), and t represents the pupil plane position (analogous to time in conventional Cramer-Kronym analysis, hence the symbol t).

[0070] Figure 6 The diagram is used to illustrate the coordinate system for the acquisition of a one-dimensional composite field, on which the following analysis will be based. , pupil plane t, lens or free space L, Fourier transform symmetry axis z, masked area MK in pupil plane t has mask edge at t=s. Position t=r represents function The origin of g(t) lies at the pupil plane. Although the choice of the symmetric shifts r and s with respect to t can be made arbitrarily, more typically r=s will be chosen, ie the mask edge lies at the origin of the function g(t).

[0071] Initial selection:

[0072]

[0073] in is the electric field at the sensor and is the electric field at the pupil. We can also define To prevent Pick value. You can also further restrict To avoid Expand Phase.

[0074] against Symmetry restrictions on t Taking the Fourier transform and plugging in the above logarithmic expansion yields:

[0075]

[0076] This t restriction forces the logarithmic expansion to satisfy t symmetry, which enables the logarithmic expansion to be inserted into ω symmetry. Based on the above situation, it can be shown that the electric field can be constructed simply from its modulus (i.e., the measured image magnitude, or the square root of the measured image intensity), since the global phase shift of no interest is neglected, the electric field can be described as:

[0077]

[0078]

[0079] Assume that , t limit , and the sufficient convergence condition is satisfied. This t-limit is an alternative to the above, which has the advantage of being able to be implemented via a hardware mask or equivalent arrangement.

[0080] A sufficient convergence condition can be: , where E0 is a freely definable non-zero complex constant. The convergence condition is derived from the expansion of the real logarithm function (of the electric field) via a power series, which is used to derive the electric field described in Eq (1) This definition of . This power series expansion has a finite region of convergence. It is possible that this convergence condition can be relaxed in some cases.

[0081] Of course, the measured image will typically comprise a two-dimensional image. Given the linearity and separability of the two-dimensional Fourier transform, the image can be obtained by replacing the 2D Fourier transform with the 2D Fourier transform and by appropriately rotating the image by the angle Aligning the t-symmetry (ie aligning with respect to the applied pupil mask) the one-dimensional field acquisition relationship can be adapted to two dimensions. This results in a two-dimensional field acquisition relationship.

[0082]

[0083] Thus, the field acquisition method may include performing a partial or masked field acquisition from each of the corresponding measured field moduli (amplitudes), each measured field modulus being obtained from a corresponding partial (e.g., masked) measurement of two (or more than two) partial measurements, each partial measurement being associated with a corresponding portion of the field of interest (e.g., a corresponding portion of the diffraction orders from the structure of interest). Such a step may use, for example, equation Eq (2). Each partial field may then be combined together; for example, given the linearity of the electric field, the partial fields may be added together.

[0084] This can be most simply achieved by performing a first measurement of a first part (e.g., first half) of the diffraction order / field of interest (e.g., by masking a second part / half of the diffraction order at the (detection) pupil plane or a conjugate thereof and detecting the resulting image). The above can then be repeated for the second part / half of the diffraction order, with the first half being masked. From these measurements, two partial electric fields can be determined by calculation using the measured electric field modulus from the image; these masked electric fields can then be summed to determine the complete unmasked electric field. The t-limit can be met by ensuring that the mask edge of each mask is located at the electric field origin (e.g., it passes through the center of the diffraction order in the relevant direction).

[0085] Thus, a dark field measurement method is disclosed herein, comprising: obtaining at least first diffraction radiation data and second diffraction radiation data, the first diffraction radiation data being associated with a first portion of a diffraction order, and the second diffraction radiation data being associated with a second portion of the diffraction order, wherein the first portion of the diffraction order and the second portion of the diffraction order are associated with corresponding portions of a detection pupil plane or a conjugate plane thereof; determining a first portion of an electric field according to measurement parameters (e.g., amplitude) of the first diffraction radiation data; determining a second portion of an electric field according to measurement parameters (e.g., amplitude) of the second diffraction radiation data; and determining the electric field of the diffraction order according to the first portion of the electric field and the second portion of the electric field.

[0086] A dark field measurement device is also disclosed, comprising: an illumination arrangement, the illumination arrangement being used to illuminate a structure on a substrate so as to generate at least one diffraction order; a detection arrangement, the detection arrangement being used to detect the at least one diffraction order or a part thereof; a spatial filter arrangement, the spatial filter arrangement being located in a detection pupil plane of the dark field measurement device or a conjugate plane thereof, the spatial filter arrangement being operable to at least individually transmit a first part of the diffraction order and a second part of the diffraction order so that the parts are individually detected on the detection arrangement; and a processor being operable to: determine a first partial electric field based on measurement parameters (e.g., amplitude) of first diffracted radiation data associated with the first part of the diffraction order; determine a second partial electric field based on measurement parameters (e.g., amplitude) of second diffracted radiation data associated with the second part of the diffraction order; and determine the electric field of the diffraction order based on at least the first partial electric field and the second partial electric field.

[0087] Figure 7 (a) Schematically illustrates a measurement arrangement for performing, for example, masked measurements. It shows an illumination beam IL incident on a target T on a substrate W, resulting in a zero-order (specular) beam ZO and a first-order diffracted beam DO. The first-order diffracted beam DO is collected by an objective lens OL. In the pupil plane, a mask MK blocks (usually) half of the diffracted order. The rest of the diffracted order DO' is imaged via a lens IL onto a detector DET. Figure 7 (b) shows the detection optics pupil mask MK covering half of the detection NA. Such a mask can be used for θ = 0°, for example, so that the origin O of the diffraction order DO DO Properly aligned with the mask edge MKE. Figure 7 (c) shows the corresponding detection mask MK' for complementary measurement of the diffraction orders.The measurements using each detection mask MK, MK' can then be combined as described.

[0088] Although a spatial filter mask is described above for dividing the pupil, other spatial filter arrangements may be used instead. Figure 7 The pupil is shown divided into a masked half and an unmasked half, with the diffraction order origin centered on the mask edge, but the arrangement is shown in this way for ease of illustration. In practice, the position of the diffraction orders in pupil space will depend on the illumination wavelength and the pitch (i.e., periodicity) of the target. As such, a single pair of complementary (fixed) masks will only allow one wavelength / pitch ratio to be measured. This can be addressed by providing multiple fixed masks (e.g., on a wheel or other arrangement) that can be switched into the detection path dependent on wavelength and / or pitch. Improved configurability and flexibility can be achieved using configurable filter configurations, such as spatial light modulators or knife-edge apertures that can be arranged according to wavelength / pitch.

[0089] Spatial filter masks, or more particularly the apertures provided by these masks, can be complementary, in that the pupil spaces defined by the mask apertures do not overlap. However, this is not necessary, and overlap can be handled mathematically, for example, via subtraction or multiplication of fields / intensities.

[0090] Other alternative spatial filtering arrangements may include beam steering optics such as mirrors or wedges (e.g., one mirror or wedge for each half or portion of the diffraction order). The above enables simultaneous imaging of two portions of the diffraction order on different portions of a detector or on different detectors. A configurable mirror array (e.g., a digital micromirror device or similar device) may be used to achieve such simultaneous imaging, with configurability for different wavelengths / pitch. The above may also be achieved by providing programmable shifting of the mirrors or wedges to track any wavelength and / or pitch changes.

[0091] In a special case, the mask may be located on an edge of the detection pupil such that the origin of the electric field passes via the pupil edge (e.g., is located on such an edge). In such a case, one of the two images from the corresponding complementary masked measurements that have been described will have zero intensity. In such an embodiment, there is actually no need to perform a measurement corresponding to the zero intensity image, since its intensity can be assumed to be zero (just as its associated partial electric field may also be zero). In such an embodiment, only first diffracted radiation data is required, e.g., relating to a first portion of diffraction orders (another portion of such diffraction orders will not be captured by the detection NA, since it has an origin on the NA boundary). The step of obtaining second diffracted radiation data may comprise estimating said second diffracted radiation data to be zero, and the step of determining the second partial electric field may comprise determining this to be zero based on said estimation.

[0092] In a three-dimensional embodiment, as described above, multiple masks or appropriately configured SLMs or DMDs can be used to individually mask the diffraction orders of each wavelength. In this way, the two-dimensional measurements described above can be repeated for more than one illumination wavelength. By exploiting the linearity of the transformation, the Cramer-Krony teachings are extended from one dimension to two dimensions. When a third dimension is added (e.g., measurement wavelength), the same operations can be performed, but now the dimensions are measurement wavelength and time, where the speed of light limit (i.e., it cannot be exceeded) is in the time domain. Therefore, the three-dimensional embodiment is actually constructed the same as the two-dimensional embodiment, except that a three-dimensional Fourier transform is now applied and the value is obtained. The rotation of does not rotate in this third added (wavelength) dimension.

[0093] In summary, the electric field in equation (2) becomes three-dimensional. The Fourier transform and its inverse transform also become three-dimensional, and the three-dimensional field acquisition relationship can be expressed as:

[0094]

[0095] in The subscripts are relative to this third dimension.

[0096] Although the examples show two halves of the field of interest being measured separately, this is not a requirement. The method may include dividing the field of interest (e.g., the measured diffraction orders) into three or more parts and processing the parts in the manner described to determine corresponding partial fields, which can then be combined into a full field. The parts may also have different (e.g., irregular) shapes and / or sizes, as long as the t limit is observed. The parts need not include or cover all diffraction orders; in this case, this would be equivalent to an effective reduction in the detection numerical aperture (NA). The parts may include non-overlapping parts, although as previously described, this is not required and overlapping parts may be processed computationally.

[0097] Such an approach enables full-field measurements without the need for a reference beam, thereby supporting long acquisition times due to lower optomechanical stability requirements. This is further achieved without the need for iterative phase acquisition algorithms, which are accompanied by (typically) slow convergence rates and / or high computational costs.

[0098] For (digital) holography, it is required that the self-interference and cross-interference terms are separable in the pupil domain, such that one of the two cross-interference terms can be clipped off and an (inverse) Fourier transform can be applied to recover the entire composite electric field. Such separation is (usually) achieved by adjusting the reference wave incidence angle. Phase determination as described in this article relaxes this restriction, since it only requires that the diffraction orders (ideally) do not overlap in order to recover the composite field.

[0099] The proposed method requires only a small number of acquisitions. Typically, only two acquisitions are needed in a sequential approach (one acquisition for each of the two adjacent masks), and only a single acquisition is needed if the two images are measured in parallel (e.g. by implementing an adjacent masking functionality via a mirror or prism).

[0100] The proposed method can be performed relative to a reference target. In fact, due to the linearity of the resulting non-iterative phase acquisition relationship, a similar non-iterative phase acquisition relationship as disclosed herein can be applied to a combination (e.g., ratio) of two electric fields, for example, the ratio of the diffraction order electric field of interest to a reference electric field (produced by a reference target). If the reference electric field is close to the measured electric field, it is (more) likely that their ratio will satisfy the relevant convergence condition. In a one-dimensional version, such a field ratio can be determined as:

[0101]

[0102] in is the reference electric field. The two-dimensional equivalent would be:

[0103]

[0104] Thus, determining the first partial electric field may include determining a combination (e.g., a ratio) of the first partial electric field and the reference electric field based on the amplitude of the first portion of the diffraction order and the amplitude of the reference radiation (e.g., in this context, the first partial electric field may include such a relative electric field described by such a combination or ratio); and determining the second partial electric field may include determining a combination of the second partial electric field and the reference electric field based on the amplitude of the second portion of the diffraction order and the amplitude of the reference radiation (e.g., in this context, the second partial electric field may include such a relative electric field described by such a combination or ratio). Such a method may of course include measuring the target and the reference target to obtain the partial electric field and the reference electric field.

[0105] Note that, as an alternative to the ratio of the two electric fields, the relevant parameter may comprise the product of the measured electric field and a conjugate reference electric field. Such a product will have the same phase (but not magnitude) as the ratio of the measured electric field and the reference electric field. The relative acquisition expressions Eq (4) and Eq (5) enable the use of a reference that can be used to resolve the phase wrapping problem by using a reference that has a wrapping phase profile that is comparable / close to the wrapping phase profile of the target being measured. It should also be noted that the derived electric field acquisition expressions have a slight advantage over the equivalent phase acquisition expressions in that they partially resolve the (potential) phase wrapping problem.

[0106] The algorithm proposed herein performs calculations at the sensor plane (i.e., where the image intensity is measured). At the sensor plane, the numerical aperture is typically low due to the large optical magnification required to match the camera pixel size to the measurement target size and illumination wavelength combination. This low numerical aperture is an advantage because it simplifies / enables full vector electric field phase acquisition, rather than just scalar electric field phase acquisition.

[0107] The method proposed herein may enable handling of limited bandwidth (i.e., partial spectral coherence) and / or (some) partial spatial coherence, since this will result in an average phase or average relative phase of the target being measured. Such an average may be a weighted average, the weights of which may depend on the measurement conditions.

[0108] Assuming a dark field imaging detection optics configuration, only a few simple optics are needed to implement the concepts disclosed herein, typically only two masks or equivalent, which enable phase measurements at soft X-ray (SXR) wavelengths. In this context, SXR wavelengths may include wavelengths in the range of 0.1 nm to 100 nm, 0.1 nm to 50 nm, 1 nm to 50 nm, 1 nm to 30 nm, 5 nm to 30 nm, or 10 nm to 20 nm.

[0109] The proposed method disclosed herein enables (computationally) access to a field plane, which allows suppressing the environment of the metrology target via computational masking / apodization.

[0110] The proposed method disclosed herein may be beneficial for profilometry because it provides additional information, for example to resolve multi-scattering in and around the metrology target.

[0111] Figure 8is a flow chart of the proposed method according to an exemplary embodiment. At step 800, a target is measured and diffraction orders are captured by an objective lens. A mask or other spatial modulation arrangement is located in the pupil plane, which blocks (for example) the first half of the diffraction orders (in terms of pupil area) so that only the second half of the diffraction orders are detected on the detector. The edge of the mask can be located at the electric field origin of the diffraction orders.

[0112] At step 810, the target may be measured again using a complementary mask or other spatial modulation arrangement that blocks, for example, the second half of the diffraction orders (in terms of pupil area) so that only the first half of the diffraction orders are detected on the detector. Steps 800 and 810 may be combined into a single acquisition, for example, using a wedge or mirror for simultaneous imaging.

[0113] The detected intensity values ​​for each detected image from steps 800 and 810 can be converted to magnitude values ​​(e.g., by square root) at step 820, and this used to determine the partial electric field for each corresponding half of the diffraction order using, for example, equation Eq (2) or (e.g., where multiple wavelengths are used) equation Eq (3) or (e.g., where a reference target is used) the relative partial electric field using equation Eq (5).

[0114] At step 830, the partial electric fields may be combined, eg, summed, to obtain a complete electric field description for the diffraction order.

[0115] For example, steps 800 to 830 may be performed for each of a pair of complementary diffraction orders from the target (e.g., +1 and -1 diffraction orders, although higher order pairs may be used). Parameters of interest may then be determined from the difference or imbalance in the measured fields (amplitude and / or phase) of the respective diffraction orders in the pair of diffraction orders. Depending on the target type, the parameters of interest may include, for example, overlap, focus, or edge placement errors. The proposed measurement technique is a dark field measurement, such that the zero-order beam is blocked or diverted to another portion of the detector (or other detector) for separate analysis. The method may be performed for each diffraction order in a pair of diffraction orders simultaneously (e.g., for +1 and -1 diffraction orders simultaneously), for example, where each of these diffraction orders is imaged simultaneously (temporally, but spatially separated). Such arrangements are known (e.g., using wedges or mirrors to divert the diffraction orders to respective different portions of the detector or different detectors) and are described in many of the references cited above.

[0116] In the above, it is stated that the method can be implemented so that only a single acquisition is required. A detailed example for implementing such a single acquisition implementation will now be described. The arrangement will be described in the context of a single acquisition to measure each of two perpendicular directions of the substrate plane. However, it will be appreciated that the method can be used for measurements in only a single direction, in which case only a single pair of illumination spots and a single pair of sub-targets will be required.

[0117] Fig. 9 is a schematic diagram of the arrangement proposed in this embodiment. The figure shows that the illumination is introduced via the illumination pupil aperture IPS, passes through the first lens system L1 and the beam splitter BS, and then reaches the objective lens OB. The objective lens OB focuses the illumination onto the target T on the wafer (or substrate) S. The radiation scattered by the target T is captured via the objective lens OB and guided by the beam splitter BS into the detection branch, which includes the second lens system L2, the detection pupil aperture (DPS), the third lens system L3, the beam steering optical device or wedge element W (for example, a four-quadrant wedge element) and the final lens L4. The final lens L4 focuses the scattered and separated illumination onto the detector DET. Each pupil plane is labeled p and each field plane is labeled f. The actual lens systems may differ in number and complexity from the simplified schematic diagram shown.

[0118] Also shown are pupil plane images (k x xk y ). The target T at the substrate (field) plane is shown to include a first pair of x-oriented sub-targets and a second pair of y-oriented sub-targets. As is well known, each pair of sub-targets may include a first sub-target having a first overlapping bias and a second sub-target having a second overlapping bias; the first bias and the second bias may be equal in magnitude and opposite in direction, although this is not a requirement for extracting overlap). In addition, a detection image DI at the detection (field) plane is shown.

[0119] As illustrated by the illumination pupil plane IPP representation, such an embodiment uses four illumination spots ILL (or zones), including a first pair on a first opposite side of the illumination pupil, and a second pair on a second opposite side of the illumination pupil; for example, such that the four illumination spots are equally angularly spaced (90 degrees of separation between adjacent spots). The four illumination spots may be positioned nominally at 0, 90, 180, and 270 degrees relative to one of a plurality of raster directions of the target (e.g., an x-raster).

[0120] The illumination NA, INA, as defined by the illumination pupil stop IPS can be larger than the detection NA, DNA. In this way, a peripheral (e.g., annular, although the actual shape is not important) illumination zone can be defined between the outer range of the illumination NA, INA and the zone corresponding to the detection NA, DNA, wherein the illumination spot ILL is located within such peripheral illumination zone. This arrangement ensures that the detection pupil stop DPS of the detection optics will block the zeroth order and all + / - 1st orders, rather than a single 1st order per illumination spot (the blocked orders are shown in gray). In this way, from each of the four illumination spots ILL, the corresponding different higher diffraction orders or first diffraction orders -1Y, +1Y, -1X, +1X will be captured within the detection NA, DNA, and will propagate to the wedge / mask plane WP. If the detection NA is larger than the illumination NA, an additional smaller detection pupil stop can be added to the detection branch.

[0121] A quadruple wedge element may be located in the wedge / mask plane WP. The wedge element has two functions: the first function is to split each of the + / -1 orders captured within the detection NA into two halves (or two parts), and the second function is to direct the radiation incident on each quadrant of the detection pupil to a corresponding different region of the detection plane including a detector (e.g., camera) DET so that they can be imaged simultaneously in the detection image DI. In this way, the wedge element roughly acts as a mask to impose the desired modulation conditions so that the above-mentioned Cramer-Krony relationship can be used for subsequent phase acquisition, electric field acquisition, and / or field ratio acquisition.

[0122] The wedge element projects each quadrant of the pupil onto a different portion of the detector / camera, as in Fig. 9 . It can also be seen that, even though each wedge element quadrant has two halves of two different orders incident on it, the two different orders of each quadrant always have different directions (i.e., each quadrant includes one y-direction portion or half-order and one x-direction portion or half-order). It is therefore possible to separate these signals from each other in the field plane (i.e., the camera image) by spatially filtering the x-direction half-order (from the x-subtarget) from the y-direction half-order (from the y-subtarget). This means that the x-direction half-order and the y-direction half-order can be used separately for phase acquisition in the pupil plane, wherein the results are then merged.

[0123] It should also be noted that due to manufacturing and optical safety margins, the multiple boundaries separating the quadrants may include a limited range (or "dead zone"), which will result in some diffraction order radiation being blocked. In order to be able to properly reconstruct the diffraction orders, the illumination spot can be extended to be significantly wider than this "dead zone" so that the diffraction orders are fully extended outside it.

[0124] In this way, a method for full field measurement (e.g., where the phase is acquired) can be implemented in a single acquisition (i.e., a single shot). An additional benefit is that no programmable mask is required, which simplifies the implementation of the method. It will also be appreciated by those skilled in the art that in the current embodiment, a change in the wavelength of the light source will shift the order along the multiple boundaries separating the quadrants. Therefore, no realignment is required when the wavelength is changed.

[0125] Fig.10 1 is a block diagram illustrating a computer system 1100 that can assist in implementing the methods and processes disclosed herein. The computer system 1100 includes a bus 1102 or other communication mechanism for communicating information, and a processor 1104 (or multiple processors 1104 and 1105) coupled to the bus 1102 for processing information. The computer system 1100 also includes a main memory 1106 (such as a random access memory (RAM) or other dynamic storage) coupled to the bus 1102 for storing information and instructions to be executed by the processor 1104. The main memory 1106 can also be used to store temporary variables or other intermediate information during the execution of instructions by the processor 1104. The computer system 1100 also includes a read-only memory (ROM) 1108 or other static storage device coupled to the bus 1102 for storing static information and instructions for the processor 1104. A storage device 1110, such as a magnetic disk or optical disk, is provided and coupled to the bus 1102 for storing information and instructions.

[0126] The computer system 1100 can be coupled by bus 1102 to a display 1112 such as a cathode ray tube (CRT) or a flat panel or touch panel display to display information to a computer user. An input device 1114 (including alphanumeric keys and other keys) is coupled to bus 1102 for communicating information and command selections to processor 1104. Another type of user input device is a cursor control 1116, such as a mouse, trackball, or cursor direction keys, for communicating direction information and command selections to processor 1104 and for controlling cursor movement on display 1112. Such input devices typically have two degrees of freedom on two axes, a first axis (e.g., x) and a second axis (e.g., y), which allows the device to specify a position in a plane. A touch panel (screen) display can also be used as an input device.

[0127] One or more of the methods described herein may be performed by the computer system 1100 in response to the processor 1104 executing one or more sequences of one or more instructions included in the main memory 1106. Such instructions may be read into the main memory 1106 from another computer-readable medium, such as the storage device 1110. Executing the sequence of instructions included in the main memory 1106 causes the processor 1104 to perform the processing steps described herein. One or more processors of a multi-processing arrangement may also be employed to execute the sequence of instructions included in the main memory 1106. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions. Therefore, the description herein is not limited to any specific combination of hardware circuitry and software.

[0128] As used herein, the term "computer-readable medium" refers to any medium that participates in providing instructions to the processor 1104 for execution. Such media can take many forms, including but not limited to non-volatile media, volatile media, and transmission media. Non-volatile media include, for example, optical or magnetic disks, such as storage device 1110. Volatile media include volatile memory, such as main memory 1106. Transmission media include coaxial cables; copper wires and optical fibers, including the wires that make up bus 1102. Transmission media can also take the form of sound waves or light waves (such as those generated during radio frequency (RF) and infrared (IR) data communications). Common forms of computer-readable media include, for example, floppy disks, floppy disks, hard disks, magnetic tape, any other magnetic media, CD-ROMs, DVDs, any other optical media, punch cards, paper tape, any other physical media with hole patterns, RAM, PROM, and EPROM, FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described below, or any other medium that can be read by a computer.

[0129] Various forms of computer-readable media may be involved in carrying one or more sequences of one or more instructions to the processor 1104 for execution. For example, instructions may be initially carried on a disk of a remote computer. The remote computer may load the instructions into its volatile memory and send the instructions via a telephone line using a modem. A modem local to the computer system 1100 may receive data on the telephone line and convert the data into an infrared signal using an infrared transmitter. An infrared detector coupled to the bus 1102 may receive the data carried in the infrared signal and place the data on the bus 1102. The bus 1102 carries the data to the main memory 1106, from which the processor 1104 retrieves and executes the instructions. The instructions received by the main memory 1106 may optionally be stored on the storage device 1110 before or after being executed by the processor 1104.

[0130] Computer system 1100 also preferably includes a communication interface 1118 coupled to bus 1102. Communication interface 1118 provides bidirectional data communication coupled to network link 1120, which is connected to local area network 1122. For example, communication interface 1118 can be an integrated services digital network (ISDN) card or a modem for providing data communication connections to corresponding types of telephone lines. As another example, communication interface 1118 can be a LAN card for providing data communication connections to compatible local area networks (LANs). Wireless links can also be implemented. In any such embodiment, communication interface 1118 sends and receives electrical signals, electromagnetic signals, or optical signals that carry digital data streams representing various types of information.

[0131] The network link 1120 typically provides data communication through one or more networks to other data devices. For example, the network link 1120 may provide a connection through the local area network 1122 to a host computer 1124 or data equipment operated by an Internet Service Provider (ISP) 1126. The ISP 1126, in turn, provides data communication services through the global packet data communication network, now commonly referred to as the "Internet" 1128. Both the local area network 1122 and the Internet 1128 use electrical, electromagnetic or optical signals that carry digital data streams. The signals that carry the digital data to and from the computer system 1100 through the various networks and the signals on the network link 1120 and through the communication interface 1118 are exemplary forms of carrier waves that transport the information.

[0132] The computer system 1100 can send messages and receive data, including program code, through the network, network link 1120 and communication interface 1118. In the Internet example, the server 1130 can transmit the program code requested by the application program through the Internet 1128, ISP 1126, local area network 1122 and communication interface 1118. For example, one such download application program can provide one or more of the technologies described herein. The received program code can be executed by the processor 1104 when it is received, and / or stored in the storage device 1110 or other non-volatile storage for later execution. In this way, the computer system 1100 can obtain the application code in the form of a carrier wave.

[0133] Additional embodiments are disclosed in the following list of numbered aspects:

[0134] 1. A dark field measurement method, comprising:

[0135] obtaining at least first diffracted radiation data and second diffracted radiation data, the first diffracted radiation data relating to a first portion of a diffraction order and the second diffracted radiation data relating to a second portion of the diffraction order, wherein the first portion and the second portion of the diffraction order are associated with respective portions of a detection pupil plane or a conjugate plane thereof;

[0136] determining a first partial electric field based on measurement parameters of the first diffracted radiation data;

[0137] determining a second partial electric field based on the measured parameters of the second diffracted radiation data; and

[0138] The electric field of the diffraction order is determined based on the first partial electric field and the second partial electric field.

[0139] 2. A dark field metrology method according to aspect 1, wherein the electric field is determined as a function of each of two dimensions of a detection plane, at which the first part of the diffraction order and the second part of the diffraction order are detected.

[0140] 3. The dark field metrology method according to aspect 2, wherein each of the first diffracted radiation data and the second diffracted radiation data comprises two-dimensional data associated with the two dimensions of the detection plane.

[0141] 4. The dark field measurement method according to aspect 2 or 3, wherein the electric field is additionally determined as a function of a third dimension.

[0142] 5. The dark field measurement method according to aspect 4, wherein the third dimension is related to the wavelength of the diffraction order.

[0143] 6. The dark field measurement method according to any preceding aspect, wherein the first part of the diffraction order and the second part of the diffraction order include parts located on both sides of the origin of the diffraction order.

[0144] 7. A dark field measurement method according to any of the preceding aspects, wherein each of the first diffraction radiation data and the second diffraction radiation data comprises intensity data, and the method comprises converting the intensity data into the measurement parameters of the first diffraction radiation data and the measurement parameters of the second diffraction radiation data, in each case, the measurement parameters comprising amplitude.

[0145] 8. A dark field measurement method according to any preceding aspect, comprising determining a parameter of interest based on the determined electric field.

[0146] 9. The method of clause 8, wherein the parameter of interest is overlay, focus or edge placement error.

[0147] 10. The dark field measurement method according to any of the preceding aspects, wherein determining the electric field comprises adding the first portion of the electric field and the second portion of the electric field together.

[0148] 11. A dark field metrology method according to any preceding aspect, wherein the first diffracted radiation data and the second diffracted radiation data are related to measurements of a structure on a substrate.

[0149] 12. The dark field measurement method according to aspect 11, further comprising obtaining reference radiation data related to a reference radiation, wherein the reference radiation is related to a measurement of a reference structure; wherein:

[0150] Determining the first partial electric field comprises determining a combination of the first partial electric field and a reference electric field of the reference radiation based on measurement parameters of the first diffracted radiation data and measurement parameters of the reference radiation; and

[0151] Said determining a second partial electric field comprises determining a combination of said second partial electric field and said reference electric field from said measured parameters of said second diffracted radiation data and said measured parameters of said reference radiation.

[0152] 13. A dark field measurement method according to any of the preceding aspects, wherein the step of obtaining the second diffraction radiation data includes estimating the second diffraction radiation data to be zero, and the step of determining the second portion of the electric field and / or related parameters includes determining the second portion of the electric field and / or related parameters to be zero.

[0153] 14. A dark field metrology method according to any preceding aspect, comprising illuminating a structure located on a substrate to obtain said diffraction orders; and

[0154] The diffraction orders are spatially filtered and / or spatially separated at the detection pupil plane to obtain the first portion of the diffraction orders and the second portion of the diffraction orders.

[0155] 15. The dark field metrology method according to any one of clauses 1 to 12, comprising illuminating a structure located on a substrate to obtain the diffraction orders.

[0156] 16. The dark field measurement method according to aspect 15, comprising:

[0157] spatially filtering and / or spatially separating the diffraction orders at the detection pupil plane to obtain the first portion of the diffraction orders and the second portion of the diffraction orders; and

[0158] The first portion of the diffraction orders and the second portion of the diffraction orders are detected separately to obtain the first diffracted radiation data and the second diffracted radiation data.

[0159] 17. The dark field metrology method of clause 16, comprising measuring the first portion of the diffraction orders and the second portion of the diffraction orders simultaneously.

[0160] 18. The dark field metrology method of clause 17, comprising using beam steering optics to steer the first portion of the diffraction orders and the second portion of the diffraction orders to be detected separately.

[0161] 19. A dark field metrology method according to clause 17 or 18, comprising simultaneously measuring the first portion of the diffraction orders and the second portion of the diffraction orders for each of:

[0162] Higher diffraction orders from at least one first-direction sub-target measured using illumination from the first direction, higher diffraction orders from at least one first-direction sub-target measured using illumination from the second direction, higher diffraction orders from at least one second-direction sub-target measured using illumination from the first direction, and higher diffraction orders from at least one second-direction sub-target measured using illumination from the second direction.

[0163] 20. The dark field metrology method according to clause 19, wherein the at least one first sub-target and the at least one second sub-target are illuminated simultaneously in four different directions spaced 90 degrees apart.

[0164] 21. The dark field metrology method of clause 16, comprising sequentially measuring the first portion of the diffraction orders and the second portion of the diffraction orders.

[0165] 22. The dark field metrology method of clause 21, comprising spatially filtering each of the first portion of diffraction orders and the second portion of diffraction orders using respective complementary spatial filters.

[0166] 23. The dark field metrology method of clause 21, comprising configuring a spatial light modulator for each of the first portion of the diffraction orders and the second portion of the diffraction orders.

[0167] 24. The dark field metrology method according to clause 14 or any one of clauses 16 to 24, wherein the spatial filtering comprises apodization in the pupil plane.

[0168] 25. A dark field metrology method according to any one of clauses 14 to 24, comprising illuminating the structure using soft X-ray wavelength illumination.

[0169] 26. A computer program comprising program instructions operable to perform the method according to any one of clauses 1 to 13 when run on a suitable device.

[0170] 27. A non-transitory computer program carrier comprising a computer program according to clause 26.

[0171] 28. A processing arrangement comprising:

[0172] a computer program carrier comprising a computer program according to clause 27; and

[0173] A processor is operable to execute the computer program.

[0174] 29. A dark field metrology device operable to perform a method according to any one of clauses 14 to 25.

[0175] 30. A dark field measurement device, comprising:

[0176] an illumination arrangement for illuminating a structure on a substrate so as to generate at least one diffraction order;

[0177] a detection arrangement for detecting the at least one diffraction order or a portion thereof;

[0178] a spatial filter arrangement located in a detection pupil plane of the dark field metrology device or a conjugate plane thereof, the spatial filter arrangement being operable to separately transmit at least a first portion of the diffraction orders and a second portion of the diffraction orders such that the first portion and the second portion are separately detected on the detection arrangement; and

[0179] A processor operable to:

[0180] determining a first partial electric field and / or related parameters based on measured parameters of first diffracted radiation data associated with said first portion of said diffraction orders;

[0181] determining a second partial electric field and / or related parameters based on measured parameters of second diffracted radiation data associated with said second portion of said diffraction orders; and

[0182] The electric field of the diffraction order is determined based on at least the first partial electric field and the second partial electric field.

[0183] 31. A dark field metrology apparatus according to clause 30, wherein the processor is operable to determine the electric field as a function of each of two dimensions of a detection plane at which the first portion of the diffraction orders and the second portion of the diffraction orders are detected.

[0184] 32. A dark field metrology apparatus according to clause 31, wherein each of the first diffracted radiation data and the second diffracted radiation data comprises two-dimensional data relating to the two dimensions of the detection plane.

[0185] 33. A dark field metrology apparatus according to clause 31 or 32, wherein the processor is operable to determine the electric field as a function of a third dimension.

[0186] 34. A dark field metrology apparatus according to clause 33, wherein the third dimension is related to the wavelength of the diffraction order.

[0187] 35. A dark field metrology apparatus according to any one of clauses 30 to 34, wherein an edge of the spatial filter arrangement is located at the origin of the diffraction order in the detection pupil plane or a conjugate plane thereof.

[0188] 36. A dark field measurement device according to any one of aspects 30 to 35, wherein each of the first diffraction radiation data and the second diffraction radiation data includes intensity data, and the processor is capable of operating to convert the intensity data into the measurement parameters of the first diffraction radiation data and the measurement parameters of the second diffraction radiation data, in each case, the measurement parameters include amplitude.

[0189] 37. A dark field metrology apparatus according to any one of clauses 30 to 36, the processor being operable to determine a parameter of interest from the determined electric field.

[0190] 38. A dark field metrology apparatus according to clause 37, wherein the parameter of interest is overlay, focus or edge placement error.

[0191] 39. A dark field metrology apparatus according to any one of clauses 30 to 38, wherein the processor is operable to determine the electric field by summing the first portion of the electric field and the second portion of the electric field together.

[0192] 40. A dark field measurement device according to any one of aspects 30 to 39, wherein the spatial filter arrangement includes a beam steering optical device, which is used to steering the first part of the diffraction order to the first part of the detection arrangement, and steering the second part of the diffraction order to the second part of the detection arrangement.

[0193] 41. A dark field metrology apparatus according to clause 40, wherein the beam steering optics comprises one or both of a mirror, an optical wedge or an optical wedge element.

[0194] 42. A dark field metrology apparatus according to clause 40 or 41, wherein the beam steering optics is operable to simultaneously separate and steer the first portion of the diffraction orders and the second portion of the diffraction orders for each of:

[0195] Higher diffraction orders from at least one first-direction sub-target measured using illumination from the first direction, higher diffraction orders from at least one first-direction sub-target measured using illumination from the second direction, higher diffraction orders from at least one second-direction sub-target measured using illumination from the first direction, and higher diffraction orders from at least one second-direction sub-target measured using illumination from the second direction.

[0196] 43. A dark field metrology apparatus according to clause 42, comprising an illumination arrangement operable to simultaneously illuminate the at least one first sub-target and the at least one second sub-target in four different directions spaced 90 degrees apart.

[0197] 44. A dark field metrology apparatus according to clause 38 or 39, wherein the beam steering optics comprises a digital micromirror device.

[0198] 45. A dark field metrology apparatus according to any of clauses 30 to 37, wherein the spatial filter arrangement is operable to sequentially transmit only one of the first portion of the diffraction orders and the second portion of the diffraction orders.

[0199] 46. ​​A dark field metrology apparatus according to clause 45, wherein the spatial filter arrangement comprises at least two complementary masks.

[0200] 47. A dark field metrology device according to clause 45, wherein the spatial filter arrangement comprises a configurable aperture or a spatial light modulator.

[0201] 48. A dark field metrology device according to any one of clauses 30 to 47; wherein the processor is operable to:

[0202] determining the first partial electric field as a combination of the first partial electric field and a reference electric field of the reference radiation, the reference radiation being associated with a measurement of a reference structure, based on measurement parameters of the first diffracted radiation data and measurement parameters of the reference radiation; and

[0203] The second partial electric field is determined as a combination of the second partial electric field and the reference electric field based on measured parameters of the second diffracted radiation data and the measured parameters of the reference radiation.

[0204] 49. A dark field metrology apparatus according to any of clauses 30 to 48, wherein the spatial filter arrangement is operable to perform apodization in the pupil plane.

[0205] 50. A dark field metrology apparatus according to any one of clauses 30 to 49, wherein the illumination arrangement is operable to produce soft X-ray wavelength illumination.

[0206] Although specific reference may be made herein to the use of lithographic apparatus in IC manufacturing, it should be understood that the lithographic apparatus described herein may have other applications. Other possible applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0207] Although specific reference may be made herein to embodiments of the invention in the context of inspection or metrology equipment, embodiments of the invention may be used in other equipment. Embodiments of the invention may form part of a mask inspection equipment, a lithography equipment, or any equipment that measures or processes an object such as a wafer (or other substrate) or a mask (or other pattern forming device). The term "metrology equipment" may also refer to an inspection equipment or a detection system. For example, an inspection equipment including embodiments of the invention may be used to detect defects in a substrate or defects in a structure on a substrate. In such embodiments, the characteristic of interest of a structure on a substrate may be related to a defect in the structure, the absence of a particular portion of the structure, or the presence of an unwanted structure on the substrate.

[0208] Although specific reference is made to "measurement equipment / tools / systems" or "inspection equipment / tools / systems," these terms may refer to the same or similar types of tools, equipment, or systems. For example, an inspection or measurement equipment including embodiments of the present invention may be used to determine characteristics of structures on a substrate or wafer. For example, an inspection equipment or measurement equipment including embodiments of the present invention may be used to detect defects in a substrate or defects in a structure on a substrate or on a wafer. In such embodiments, the characteristic of interest of a structure on a substrate may be related to a defect in the structure, the absence of a particular portion of a structure, or the presence of an unwanted structure on a substrate or on a wafer.

[0209] Although specific reference has been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention is not limited to optical lithography and may be used in other applications, such as imprint lithography, where the context permits.

[0210] Although the targets or target structures (more generally, structures on a substrate) described above are metrology target structures specifically designed and formed for the purpose of measurement, in other embodiments, the properties of interest may be measured on one or more structures that are functional parts of a device formed on a substrate. Many devices have regular grating-like structures. The terms structure, target grating, and target structure as used herein do not require that the structure be provided specifically for the measurement being performed. In addition, the pitch P of the metrology target may be close to the resolution limit of the optical system of the scatterometer or may be smaller, but may be much larger than the size of a typical product feature made by a lithography process in the target portion C. In practice, the lines and / or spaces of the overlapping gratings within the target structure may be made to include smaller structures that are similar in size to the product features.

[0211] Although specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be practiced in ways other than those described. The foregoing description is intended to be illustrative, not restrictive. Therefore, it will be apparent to those skilled in the art that the present invention may be modified as described without departing from the scope of the claims set forth below.

Claims

1. A dark field measurement device, comprising: an illumination arrangement for illuminating a structure on a substrate so as to generate at least one diffraction order; a detection arrangement for detecting the at least one diffraction order or a portion thereof; a spatial filter arrangement located in a detection pupil plane of the dark field metrology device or a conjugate plane thereof, the spatial filter arrangement being operable to separately transmit at least a first portion of the diffraction orders and a second portion of the diffraction orders such that the first portion and the second portion are separately detected on the detection arrangement; as well as A processor operable to: determining a first partial electric field based on measured parameters of first diffracted radiation data associated with said first portion of said diffraction orders; determining a second partial electric field based on measured parameters of second diffracted radiation data associated with said second portion of said diffraction orders; as well as The electric field of the diffraction order is determined based on at least the first partial electric field and the second partial electric field.

2. The dark field measurement device according to claim 1, wherein: The processor is operable to determine the electric field as a function of each of two dimensions of a detection plane at which the first portion of the diffraction orders and the second portion of the diffraction orders are detected.

3. The dark field measurement device according to claim 2, wherein: Each of the first diffracted radiation data and the second diffracted radiation data comprises two-dimensional data relating to the two dimensions of the detection plane.

4. The dark field measurement device according to claim 2 or 3, wherein: The processor is operable to determine the electric field as a function of a third dimension.

5. The dark field measurement device according to claim 4, wherein: The third dimension is related to the wavelength of the diffraction order.

6. The dark field measurement device according to any one of claims 1 to 5, wherein: The edge of the spatial filter arrangement is located at the origin of the diffraction order in the detection pupil plane or a conjugate plane thereof.

7. The dark field measurement device according to any one of claims 1 to 6, wherein: Each of the first diffracted radiation data and the second diffracted radiation data comprises intensity data, and the processor is operable to convert the intensity data into the measurement parameters of the first diffracted radiation data and the measurement parameters of the second diffracted radiation data, in each case the measurement parameters comprising amplitude.

8. A dark field metrology apparatus according to any one of claims 1 to 7, the processor being operable to determine a parameter of interest from the determined electric field.

9. The dark field measurement device according to claim 8, wherein: The parameters of interest are overlay, focus or edge placement errors.

10. The dark field measurement device according to any one of claims 1 to 9, wherein: The processor is operable to determine the electric field by summing the first portion of the electric field and the second portion of the electric field together.

11. The dark field measurement device according to any one of claims 1 to 10, wherein: The spatial filter arrangement comprises beam steering optics for steering the first portion of the diffraction orders to a first part of the detection arrangement and steering the second portion of the diffraction orders to a second part of the detection arrangement.

12. The dark field measurement device according to claim 11, wherein: The beam steering optics include one or both of a mirror or an optical wedge and, optionally, the beam steering optics include a digital micromirror device.

13. The dark field measurement device according to any one of claims 1 to 10, wherein: The spatial filter arrangement is operable to sequentially transmit only one of the first portion of the diffraction orders and the second portion of the diffraction orders and, optionally, satisfies one of the following: i) the spatial filter arrangement comprises at least two complementary masks; and ii) the spatial filter arrangement comprises a configurable aperture or a spatial light modulator.

14. A dark field measurement method, comprising: obtaining at least first diffracted radiation data and second diffracted radiation data, the first diffracted radiation data relating to a first portion of a diffraction order and the second diffracted radiation data relating to a second portion of the diffraction order, wherein the first portion and the second portion of the diffraction order are associated with respective portions of a detection pupil plane or a conjugate plane thereof; determining a first partial electric field based on measurement parameters of the first diffracted radiation data; determining a second partial electric field based on the measured parameters of the second diffracted radiation data; and The electric field of the diffraction order is determined based on the first partial electric field and the second partial electric field.

15. A computer program comprising program instructions operable to perform the method according to claim 14 when run on a suitable device.

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