Method for filtering an image and associated metrology apparatus

Abstract

A method for making metrology measurements on a region of a substrate comprising at least a portion of a target structure is disclosed. The method comprises receiving radiation information representing a portion of radiation scattered by the region; and using a filter in the Fourier domain for removing or suppressing at least a portion of the received radiation information that is not related to radiation that has been scattered by the target structure for obtaining filtered radiation information for the metrology measurement, wherein a characteristic of the filter is based on target information related to the target structure.

Description

[0001] Cross-references to related applications

[0002] This application claims priority to U.S. Application 62 / 901,938, filed September 18, 2019, and European Application 19203613.5, filed October 16, 2019, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This invention relates to a method for determining the characteristics of a structure on a substrate. The invention also relates to a measuring or inspection apparatus for determining the characteristics of a structure on a substrate. Background Technology

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

[0005] To project a pattern onto a substrate, photolithography equipment can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the feature that can be formed on the substrate. Typical wavelengths currently used are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. Photolithography equipment using extreme ultraviolet (EUV) radiation with wavelengths in the range of 4 to 20 nm (e.g., 6.7 nm or 13.5 nm) can be used to form smaller features on the substrate compared to photolithography equipment using radiation with a wavelength of, for example, about 193 nm.

[0006] Low-k1 lithography can be used to process features with dimensions smaller than the classical resolution limit of a lithography apparatus. In this process, the resolution formula can be expressed as CD = k1 × λ / NA, where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection optics in the lithography apparatus, CD is the “critical size” (typically the smallest feature size printed, but in this example, half-pitch), and k1 is an empirical resolution factor. Generally, the smaller k1 is, the more difficult it is to reproduce a pattern on the substrate that is similar in shape and size to that planned by the circuit designer to achieve specific electrical functions and performance. To overcome these difficulties, complex fine-tuning steps can be applied to the lithography projection apparatus and / or design layout. These include, but are not limited to, optimizing the numerical aperture (NA), customizing the illumination scheme, using phase-shifting patterning apparatus, various optimizations of the design layout such as optical proximity correction (OPC, sometimes also called “optical and process correction”), or other methods generally defined as “resolution enhancement techniques” (RET). Alternatively, a tight control loop for controlling the stability of the lithography apparatus can be used to improve the reproduction of the pattern under low k1 conditions.

[0007] During the manufacturing process, it is necessary to inspect the manufactured structure and / or measure its characteristics. Suitable inspection and measurement equipment is known in the art. One such known measuring device is a scatterometer, such as a dark-field scatterometer.

[0008] Embodiments of photolithography equipment and scattering apparatus are described in patent applications EP2019 / 063028, US2010 / 0328655A1, and US2006 / 0066855A1. The cited documents are incorporated herein by reference in their entirety.

[0009] When measurements are performed using a measuring device, crosstalk from structures adjacent to the target being measured (e.g., product structures) can affect the measurement. Summary of the Invention

[0010] The aim is to provide an effective and efficient solution for inspecting or measuring equipment to help minimize measurement crosstalk between structures.

[0011] Examples are disclosed in the claims and in the detailed description.

[0012] A method is provided for performing a measurement on a region of a substrate including at least a portion of a target structure. The method includes receiving radiation information representing at least a portion of radiation scattered by the region, and using a filter in the Fourier domain to remove or suppress at least a portion of the received radiation information that is unrelated to radiation scattered by the target structure, to obtain filtered radiation information for the measurement, wherein the characteristics of the filter are based on target information associated with the target structure.

[0013] According to a second aspect, a measurement apparatus is provided for performing measurements on a region of a substrate including at least a portion of a target structure. The measurement apparatus is configured to: i) receive radiation information representing at least a portion of radiation scattered by the region; and ii) use a filter in the Fourier domain to remove or suppress at least a portion of the received radiation information that is unrelated to radiation already scattered by the target structure, to obtain filtered radiation information for the measurement. The characteristics of the filter are based on target information associated with the target structure.

[0014] Also provided is a computer program product including instructions that, when executed at a processor, cause the method for performing a measurement on a region of a substrate including at least a portion of a target structure. The method includes receiving radiation information representing at least a portion of radiation scattered by the region, and using a filter in the Fourier domain to remove or suppress at least a portion of the received radiation information that is unrelated to radiation already scattered by the target structure, to obtain filtered radiation information for the measurement, wherein the characteristics of the filter are based on target information associated with the target structure. Attached Figure Description

[0015] Embodiments will now be described by way of example only, with reference to the accompanying illustrative drawings, in which:

[0016] - Figure 1 A schematic schematic diagram depicting a photolithography apparatus;

[0017] - Figure 2 A schematic schematic diagram depicting a photolithography unit;

[0018] - Figure 3 A schematic representation depicting overall photolithography, illustrating the collaboration between three key technologies used to optimize semiconductor manufacturing;

[0019] - Figure 4 Schematic diagram of a scattering device;

[0020] - Figure 5 include: Figure 5 (a) Schematic diagram of a dark-field scattering instrument. Figure 5 (b) Details of the diffraction spectrum of the target structure. Figure 5 (c) a second pair of irradiation apertures providing other irradiation modes, and (d) a third pair of irradiation apertures;

[0021] - Figure 6 A simplified schematic representation of the measuring tool is shown;

[0022] - Figure 7 A flowchart illustrating the method used for measurement is shown;

[0023] - Figure 8 It is shown that: Figure 8 (a) An example of crosstalk shown schematically, and the structure of the measurement target, and Figure 8 (b) Example of pupil image;

[0024] - Figure 9 An embodiment of the said composite electric field is shown, comprising: Figure 9 (a) Intensity image, Figure 9 (b) Phase image, and Figure 9 (c) shows a graph of the phase gradient, which corresponds to the dashed line in (b);

[0025] - Figure 10 The intensity cross-sections of the target spot and non-target spot on the pupil plane are shown in graphical form;

[0026] - Figure 11 This illustrates an image at the image plane and an embodiment of region of interest (ROI) selection;

[0027] - Figure 12 Simulation results are shown for one embodiment used to compare the effects of having and not having a filter, wherein Figure 12 (a) and Figure 12 (d) are the intensity images before and after filtering, respectively; Figure 12 (b) and Figure 12 (e) are pupil images before and after filtering, respectively; Figure 12 (c) and Figure 12 (f) Plot the overlapping sensitivity curves for filtered and unfiltered conditions, respectively;

[0028] - Figure 13 This is a flowchart describing the optimization steps of the pupil region of interest according to an embodiment of the present invention;

[0029] - Figure 14 It is shown that: Figure 14 (a) An irradiation and detection arrangement according to an embodiment of the invention, which can be used in a method for digitally separating diffraction orders, and Figure 14(b) A flowchart describing a method for digitally separating diffraction orders according to an embodiment of the present invention; and

[0030] - Figure 15 It is a block diagram illustrating a computer system. Detailed Implementation

[0031] In this document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic and particle radiation, including ultraviolet radiation (e.g., wavelengths of 365 nm, 248 nm, 193 nm, 157 nm, or 126 nm), EUV (extreme ultraviolet radiation, such as having wavelengths in the range of about 5 nm to 100 nm), X-ray radiation, electron beam radiation, and other particle radiation.

[0032] The terms “mask,” “mask,” or “patterning apparatus” as used herein can be broadly interpreted to refer to a general patterning apparatus that can be used to impart a patterned cross-section to an incident radiation beam, corresponding to a pattern to be created in a target portion of the substrate. The term “optical valve” can also be used in this context. Examples of such patterning apparatuses, in addition to classic masks (transmission or reflection masks, binary masks, phase-shift masks, hybrid masks, etc.), include programmable mirror arrays and programmable LCD arrays.

[0033] Figure 1 A lithography apparatus LA is schematically depicted. The lithography apparatus LA includes: an irradiation system (also called an irradiator) IL configured to modulate a radiation beam B (e.g., UV radiation, DUV radiation, EUV radiation, or X-ray radiation); a mask support (e.g., a mask stage) T configured to support a patterning apparatus (e.g., a mask) MA and connected to a configuration for accurately positioning the patterning apparatus MA according to specific parameters; a substrate support (e.g., a wafer stage) WT configured to hold a substrate (e.g., a wafer coated with resist) W and connected to a second positioning device PW configured to accurately position the substrate support according to specific parameters; and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted by the radiation beam B by the patterning apparatus MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0034] In operation, the irradiation system IL receives a radiation beam from the radiation source SO, for example, via a beam delivery system BD. The irradiation system IL may include various types of optical components, such as refractive, reflective, diffractive, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof, for guiding, shaping, and / or controlling the radiation. The irradiator IL can be used to adjust the radiation beam B to have a desired spatial and angular intensity distribution in its cross-section at the plane of the pattern forming apparatus MA.

[0035] As used herein, the term "projection system" PS should be interpreted broadly to encompass all types of projection systems, including refractive, reflective, diffractive, refractive-reflective, variable, magnetic, electromagnetic, and / or electrostatic optical systems, or any combination thereof, as appropriate, to the exposure radiation used, and / or other factors such as the use of immersion liquids or vacuum. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0036] The lithography apparatus LA can be of the type in which at least a portion of the substrate can be covered by a liquid (e.g., water) with a relatively high refractive index to fill the space between the projection system PS and the substrate W; this is also known as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference in its entirety.

[0037] Photolithography equipment LA can also be of the type with two or more substrate supports WT (also known as "dual platforms"). In such "multi-platform" machines, the substrate supports WT can be used in parallel, and / or a step to prepare the substrate W for subsequent exposure can be performed on a substrate W located on one of the substrate supports WT, while another substrate W on another substrate support WT is used to expose patterns on other substrates W.

[0038] In addition to the substrate support WT, the lithography apparatus LA may include a measurement platform. The measurement platform is arranged to hold sensors and / or cleaning devices. The sensors may be arranged to measure properties of the projection system PS or the radiation beam B. The measurement platform may hold multiple sensors. The cleaning devices may be arranged to clean a portion of the lithography apparatus, such as a portion of the projection system PS or a portion of a system providing immersion liquid. The measurement platform may move below the projection system PS as the substrate support WT moves away from the projection system PS.

[0039] In operation, a radiation beam B is incident on a pattern forming apparatus (e.g., a mask MA) held on a mask support T and patterned by a pattern (design layout) present on the pattern forming apparatus MA. After passing through the mask MA, the radiation beam B is passed through a projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of a second positioning device PW and a position measurement system IF, the substrate support WT can be accurately moved, for example, to position different target portions C in a focused and aligned position within the path of the radiation beam B. Similarly, a first positioning device PM and possibly another position sensor ( Figure 1 (Not explicitly depicted) can be used to accurately position the patterning apparatus MA relative to the path of the radiation beam B. Mask alignment marks M1, M2 and substrate alignment marks P1, P2 can be used to align the patterning apparatus MA and the substrate W. Although the substrate alignment marks P1, P2, as shown, occupy dedicated target portions, they can be located in the space between the target portions. When the substrate alignment marks P1, P2 are located between the target portions C, they are referred to as scribe alignment marks.

[0040] like Figure 2 As shown, the lithography apparatus LA can form part of a lithography unit LC (sometimes also called a lithography cell or (lithography) cluster), which typically also includes equipment for performing pre-exposure and post-exposure processes on a substrate W. Conventionally, this equipment includes a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chiller CH, and a baking plate BK, for example, for regulating the temperature of the substrate W, and for regulating the solvent in the resist layer. A substrate transport device or robot RO picks up the substrate W from input port I / O1 and output port I / O2, moves them between different process devices, and transfers the substrate W to the loading stage LB of the lithography apparatus LA. The apparatus in the lithography cell, also generally referred to as a coating and developing system or track, is typically under the control of a coating and developing system control unit or track control unit TCU, which itself can be controlled by a management and control system SCS, which can also control the lithography apparatus LA, for example, via a lithography control unit LACU.

[0041] During photolithography, it is desirable to frequently measure the created structure, for example, for process control and verification. The tools used to perform these measurements are generally referred to as measurement tools (MTs). Different types of MTs used for such measurements are well-known, including scanning electron microscopes (SEMs) or various forms of scatterometer MTs. A scatterometer is a versatile instrument that allows for the measurement of parameters of the photolithography process via a sensor located in the pupil of the scatterometer objective or in a plane conjugate to the pupil; these measurements are typically referred to as pupil-based measurements. Alternatively, it allows for the measurement of parameters of the photolithography process via a sensor located in the image plane or a plane conjugate to the image plane; in this case, the measurements are typically referred to as image-based or field-based measurements. Such scatterers and associated 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 aforementioned scatterers can measure gratings using light from soft X-rays, extreme ultraviolet, and visible to near-IR wavelengths.

[0042] To ensure correct and consistent exposure of the substrate W by the lithography apparatus LA, it is necessary to inspect the substrate to measure properties of the patterned structure, such as overlap error between subsequent layers, line thickness, critical dimension (CD), etc. For this purpose, inspection tools and / or measurement tools (not shown) may be included in the lithography unit LC. If errors are detected, adjustments can be made to the exposure of subsequent substrates or other processing steps to be performed on the substrate W, especially if inspection is performed on other substrates W in the same batch or group before exposure or processing.

[0043] Inspection equipment, also known as metrology equipment, is used to determine the properties of a substrate W, particularly how the properties of different substrates W vary, or how the properties associated with different layers of the same substrate W change between layers. The inspection equipment may alternatively be configured to identify defects on the substrate W, and may, for example, be part of a photolithography unit (LC), or may be integrated into a photolithography apparatus (LA), or may even be a separate device. The inspection equipment can measure properties on latent images (images in a resist layer after exposure), semi-latent images (images in a resist layer after a post-exposure baking (PEB) step), or developed resist images (where the exposed or unexposed portions of the resist have been removed), or even on etched images (after a pattern transfer step such as etching).

[0044] In the first embodiment, the scatterer MT is an angle-resolved scatterer. In such a scatterer, a reconstruction method can be applied to the measured signal to reconstruct or calculate the properties of the grating. This reconstruction can, for example, be produced by simulating the interaction between the scattered radiation and the target structure using a mathematical model 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 that observed from a real target.

[0045] In the second embodiment, the scatterer MT is a spectroscopic scatterer MT. In such a spectroscopic scatterer MT, radiation emitted by a radiation source is directed to a target, and reflected or scattered radiation from the target is directed to a spectroscopic detector, which measures the spectrum of the specularly reflected radiation (i.e., the intensity as a wavelength). Based on this data, the structure or profile of the detected spectrum of the target's generation can be reconstructed, for example, through rigorous coupled-wave analysis and nonlinear regression or by comparison with a simulated spectral library.

[0046] In the third embodiment, the scatterer MT is an elliptic scatterer. An elliptic scatterer allows for the determination of parameters of the photolithography process by measuring the scattered radiation for each polarization state. This measurement device emits polarized light (such as linear, circular, or elliptically polarized light) by using, for example, a suitable polarization filter in the illumination section of the measurement device. A source suitable for the measurement device can also provide polarized radiation. Various embodiments of existing elliptic scatterers 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.

[0047] In one embodiment of a scattering instrument (MT), the scattering instrument MT is adapted to measure the overlap of two misaligned gratings or periodic structures by measuring the reflectance spectrum and / or detecting an asymmetry in the configuration (the asymmetry being related to the extent of overlap). Two (typically, stacked) grating structures can be applied to two different layers (not necessarily consecutive layers), and the two grating structures can be formed at substantially the same location on the wafer. The scattering instrument can have a symmetrical detection configuration, such as that described, for example, in the commonly owned patent application EP 1,628,164 A, such that any asymmetry is clearly distinguishable. This provides a simple way to measure misalignment in gratings. Further examples of measuring overlap error between two layers containing a periodic structure as a target via the asymmetry of the periodic structure can be found in PCT patent application publication WO 2011 / 012624 or U.S. patent application US20160161863, which are incorporated herein by reference in their entirety.

[0048] Other parameters of interest may be focus and dose. Focus and dose can be determined simultaneously by scattering measurements (or alternatively by scanning electron microscopy) as described in U.S. patent application US2011-0249244, which is incorporated herein by reference in its entirety. A single structure with a unique combination of critical dimensions and sidewall angle measurements for each point in the focus energy matrix (FEM—also known as the focus exposure matrix) can be used. If these unique combinations of critical dimensions and sidewall angles are available, the focus and dose values ​​can be uniquely determined based on these measurements.

[0049] The measurement target can be an assembly of composite gratings formed primarily in a resist by a photolithography process and also formed, for example, after an etching process. Typically, the spacing and linewidth of the structures in the grating are largely dependent on the measurement optics (specifically, the NA of the optics) to capture the diffraction order from the measurement target. As previously indicated, the diffraction signal can be used to determine the shift (also called "overlap") between two layers or to reconstruct at least a portion of the original grating, such as that produced by the photolithography process. This reconstruction can be used to provide guidance on the quality of the photolithography process and can be used to control at least a portion of the photolithography process. The target can have smaller sub-segments configured to mimic the dimensions of functional portions of a design layout in the target. Due to these sub-segments, the target will behave more similarly to the functional portions of the design layout, making the overall process parameter measurements better resemble the functional portions of the design layout. The target can be measured in an underfill mode or an overfill mode. In the underfill mode, the measurement beam produces a spot smaller than the overall target. In the overfill mode, the measurement beam produces a spot larger than the entire target. In such an overfilled mode, different targets may be measured simultaneously, thus determining different processing parameters at the same time.

[0050] The overall measurement quality of lithography parameters performed for a specific target is determined at least in part by the measurement scheme used to measure such lithography parameters. The term "substrate measurement scheme" can include measuring one or more parameters of itself, one or more parameters of one or more patterns being measured, or both. For example, if the measurement used in the substrate measurement scheme is a diffraction-based optical measurement, one or more of the measured parameters can include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, and so on. One of the criteria used to select the measurement scheme can be, for example, the sensitivity of one of the measurement parameters to handling variations. Further examples are described in U.S. Patent Application US2016-0161863 and U.S. Patent Publication US2016 / 0370717A1, which are incorporated herein by reference in their entirety.

[0051] Typically, the patterning process in a photolithography (LA) apparatus is one of the most critical steps in the process, requiring highly accurate dimensional determination and placement of structures on the substrate W. To ensure this high accuracy, such as... Figure 3 The diagram schematically depicts three systems that can be combined within a so-called "holistic" control environment. One system is the lithography apparatus LA, which is (in effect) connected to the metrology tool MET (the second system) and to the computer system CL (the third system). The key to this "holistic" environment is optimizing the coordination 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 a range of process parameters (e.g., dose, focus, coverage) within which a particular manufacturing process produces a defined result (e.g., a functional semiconductor device), typically allowing variations in process parameters during the lithography or patterning process within the defined result.

[0052] The computer system CL can use a portion of the design layout to be patterned to predict which resolution enhancement technique will be used, and perform computational lithography simulations and calculations to determine which mask layouts and lithography equipment settings achieve the maximum total process window (within) of the patterning process. Figure 3 (Depicted by a double arrow in the first scale SC1). Typically, resolution enhancement techniques are arranged to match the patterning possibilities of the lithography apparatus LA. The computer system CL can also be used to detect where the lithography apparatus LA is currently operating within the process window (e.g., using input from the metrology tool MET) to predict whether defects may exist due to, for example, suboptimal processing (in...). Figure 3 (This is depicted by the arrow pointing to "0" in the second scale SC2).

[0053] The measurement tool MET can provide input to the computer system CL for accurate simulation and prediction, and can also provide feedback to the lithography equipment LA to identify possible drift, for example, during the calibration or standardization of the lithography equipment LA. Figure 3 (This is depicted by multiple arrows in the third scale SC3).

[0054] During photolithography, frequent measurements of the created structure are required, for example, for process control and verification. Various tools are known for performing these measurements, including scanning electron microscopes or various forms of metrology equipment such as scatterometers. Examples of known scatterometers often rely on the setup of a dedicated metrology target, such as an underfilled target (a target in the form of stacked gratings or simple gratings in different layers, large enough that the measurement beam produces a spot smaller than the grating) or an overfilled target (whereby the illumination spot partially or completely contains the target). Additionally, the use of metrology tools (e.g., an angle-resolved scatterometer illuminating an underfilled target such as a grating) allows for the use of so-called reconstruction methods, where the properties of the grating can be calculated by simulating the interaction between the scattered radiation and a mathematical model of the target structure and comparing the simulation results with the measured results. The parameters of the model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from a real target.

[0055] A scatterometer is a versatile instrument that allows for the measurement of parameters of a photolithography process via a sensor located in the pupil of the scatterometer objective or in a plane conjugate to the pupil; such measurements are typically referred to as pupil-based measurements. Alternatively, it allows for the measurement of parameters of the photolithography process via a sensor located in the image plane or in a plane conjugate to the image plane; in this case, the measurements are typically referred to as image-based or field-based measurements. Such scatterometers and associated 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 aforementioned scatterometers can measure multiple targets from multiple gratings within a single image using light from soft X-rays, extreme ultraviolet, and the visible to near-IR wavelength range.

[0056] exist Figure 4The image depicts a measurement device such as a scattering instrument. It includes a broadband (e.g., white light) radiation projector 2 that projects radiation 5 onto a substrate W. Reflected or scattered radiation 10 is transmitted to a spectrometer detector 4, which measures the spectrum of the spectroscopically reflected radiation (i.e., a measurement of intensity as a function of wavelength). Based on this data, the measurement can be performed by a processing unit PU, for example, through rigorous coupled-wave analysis and nonlinear regression, or through methods such as... Figure 4 The structure or profile of the detected spectrum is reconstructed by comparing it with a library of simulated spectra, as shown at the bottom. Typically, for reconstruction, the general form of the structure is known, and some parameters are assumed based on knowledge of the process used to manufacture the structure, leaving only a few parameters of the structure to be determined from the scattering measurement data. This scatterometer can be configured as a normal-incident scatterometer or an oblique-incident scatterometer.

[0057] Figure 5 (a) An embodiment presenting a measurement device, and more particularly a dark field scattering instrument. Figure 5 (b) illustrates the target T and the diffracted rays of the measurement radiation used to illuminate the target in more detail. The illustrated measurement device belongs to the type known as a dark-field measurement device. The measurement device can be a stand-alone device or can be incorporated into a lithography apparatus LA (e.g., in a measurement station) or included in a lithography unit LC. The dashed line O indicates the optical axis having several branches running through the device. In this device, light emitted by source 11 (e.g., a xenon lamp) is guided onto the substrate W by an optical system including lenses 12, 14 and objective lens 16 via beam splitter 15. These lenses are arranged in a double sequence of 4F arrangement. Different lens arrangements can be used as long as it still provides an image of the substrate to the detector while allowing proximity to the intermediate pupil plane for spatial frequency filtering. Thus, the range of angles at which the radiation is incident on the substrate can be selected by defining the spatial intensity distribution in the plane presenting the spatial spectrum of the substrate plane (here referred to as the (conjugate) pupil plane). Specifically, this can be accomplished by inserting a suitable aperture plate 13 between lenses 12 and 14 in the plane of the back-projected image of the objective lens pupil plane. In the illustrated example, the aperture plate 13 has different forms, labeled 13N and 13S, to allow for the selection of different illumination modes. The illumination system in this example forms an off-axis illumination mode. In the first illumination mode, for ease of description only, aperture plate 13N provides off-axis (illumination) from a direction designated "North". In the second illumination mode, aperture plate 13S is used to provide similar illumination, but from the opposite direction designated "South". Other illumination modes are possible by using different apertures. It is desirable that the rest of the pupil plane be dark, because any unwanted light outside the desired illumination mode could potentially interfere with the desired measurement signal.

[0058] like Figure 5 As shown in (b), the target T is positioned such that the substrate W is perpendicular to the optical axis O of the objective lens 16. The substrate W may be supported by a support (not shown). The ray I of the measurement radiation incident on the target T from an angle offset from axis O produces a zeroth-order ray (solid line O) and two first-order rays (dotted line indicates +1 order, and double-dotted line indicates -1 order). It should be noted that for an overfilled small target, these rays are just one of many parallel rays covering the area of ​​the substrate that includes the measurement target T and other features. Due to the finite width of the aperture in plate 13 (necessary for allowing a useful amount of light), the incident ray I will actually occupy an angular range, and the diffracted rays O and +1 / -1 will spread out slightly. Depending on the point spread function of the small target, each order +1 and -1 will further spread out over an angular range, rather than a single ideal ray as illustrated. Note that the grating spacing and illumination angle of the target can be designed or adjusted so that the first-order rays entering the objective lens are nearly aligned with the central optical axis. Figure 5 (a) and Figure 5 The rays shown in (b) are displayed slightly off-axis, simply to make them easier to distinguish in the figure.

[0059] At least the 0th and +1st orders diffracted by the target T on the substrate W are collected by objective lens 16 and guided back by beam splitter 15. Return to Figure 5 (a) Both the first and second illumination modes are illustrated by apertures labeled North (N) and South (S) that are opposite each other in the diametrical direction. When the incident ray I of the measured radiation comes from the north side of the optical axis, that is, when the first illumination mode is applied using aperture plate 13N, the +1st order diffraction ray labeled +1 (N) enters the objective lens 16. In contrast, when the second illumination mode is applied using aperture plate 13S, the -1st order diffraction ray (labeled -1 (S)) is the ray entering the lens 16.

[0060] The second beam splitter 17 divides the diffracted beam into two measurement branches. In the first measurement branch, the optical system 18 uses the zeroth-order and first-order diffracted beams to form the diffraction spectrum (pupil plane image) of the target on the first sensor 19 (e.g., a CCD or CMOS sensor). Each diffraction order hits a different point on the sensor, allowing image processing to compare and contrast multiple orders. The pupil plane image captured by the sensor 19 can be used for focusing measurement devices and / or normalizing the intensity measurements of the first-order beam. The pupil plane image can also be used for many measurement purposes such as reconstruction.

[0061] In the second measurement branch, optical systems 20 and 22 form an image of the target T on sensor 23 (e.g., a CCD or CMOS sensor). In this second measurement branch, aperture stop 21 is positioned in a plane conjugate to the pupil plane. Aperture stop 21 acts to block the zeroth-order diffraction beam, ensuring that the image of the target formed on sensor 23 is formed only by either the -1st or +1st order beam. The image captured by sensors 19 and 23 is output to a processor PU that processes the image; the function of the processor PU will depend on the specific type of measurement being performed. It should be noted that the term "image" is used here in a broad sense. If only one of the -1st and +1st orders is present, then an image of the grating lines will not be formed.

[0062] Figure 5 The specific form of the aperture plate 13 and field stop 21 shown is merely an example. In another embodiment, coaxial illumination of the target is used, and an aperture stop with an off-axis aperture is used to transmit approximately only one first-order diffracted beam to the sensor. In still other embodiments, second-order, third-order, and higher-order beams can be used in the measurement instead of the first-order beam or in addition to the first-order beam. Figure 5 (Not shown in the image).

[0063] To enable the measurement of radiation to be adapted to these different types of measurements, the orifice plate 13 may include a plurality of orifice patterns formed around a disk, which is rotated to bring the desired pattern into place. It should be noted that the orifice plates 13N or 13S are only used for measuring gratings oriented in one direction (X or Y, depending on the setup). To measure orthogonal gratings, rotations of the target by 90° and 270° can be implemented. Figure 5 (c) and Figure 5 Different orifice plates are shown in (d). The use of these and many other variations and applications of the device are described in the previously disclosed applications mentioned above.

[0064] Figure 5The imaging optics of the measurement tools of the type shown are extremely complex, with low tolerance and high aberration requirements. To relax these requirements, computational imaging (CI) techniques have been described in published patent applications such as WO2019068459A1, which are incorporated herein by reference in their entirety, or EP18158745.2, filed February 27, 2018. In CI, the high-quality and high-NA imaging optics of a typical measurement device are replaced by a simple single lens, for example, a simple single lens that creates a relatively distorted and / or aberrated image of the measured target on an image sensor. A near-perfect image of the measured target can be obtained by direct phase-resolved measurement (e.g., holographic microscopy), or by phase recovery methods or phase acquisition methods (recovering / acquiring phase only from intensity measurements), or by a combination thereof, when the complete field (both intensity and phase) of light on the sensor is known. In phase-resolved measurements, the complete electric field on the sensor can be observed using holographic settings and knowledge applied about the imaging system. Phase acquisition methods can utilize phase acquisition algorithms and prior knowledge related to the imaging system and / or the measurement target. Examples of suitable phase acquisition methods have been described in the aforementioned published patent application WO2019068459A1.

[0065] Figure 5 The measurement tools of the type shown can use partially spatially coherent (or even incoherent) illumination. In sensors such as those based on computational imaging (CI) as described above, it is proposed that (at least nearly) fully spatially coherent illumination can be used. However, spatially coherent radiation has the drawback of increased crosstalk between the non-target structure (e.g., product structure) and the measured target. Without any mitigation strategies, this increased crosstalk leads to reduced process variation robustness.

[0066] To understand this crosstalk problem, it should be understood that fully spatially coherent illumination results in a large effective point spread function (i.e., the point spread function is large and decays slowly, and additionally, the amplitude of all fields increases, but the intensity decreases). This increases crosstalk from neighboring customer structures to targets (e.g., overlapping, focused, or critical-size CD targets).

[0067] To address this crosstalk problem, a spatially coherent illumination scheme is proposed for performing measurements. In this context, "spatial coherence" means complete spatial coherence or near-complete spatial coherence, where near-complete spatial coherence describes the level of coherence, or degree of coherence, sufficient to reduce crosstalk between the target, target segments, or from neighboring structures to the target (segment). The resulting image (first image) from this spatially coherent measurement is then convolved using a suitable Fourier transform to obtain a re-image equivalent to the image that would have been obtained using an alternative desired illumination scheme. The desired illumination scheme could be, for example, such as... Figure 5 The illustration shows current illumination schemes used on tools (e.g., partially spatial coherent radiation) or, for example, spatial incoherent radiation. Suitable Fourier transforms include Fourier transforms of the cross-intensity functions describing alternative (desired) illumination schemes.

[0068] Therefore, it will be described how to re-image a partially coherent intensity image as a convolution between an intensity image formed by single-plane wave illumination (a spatially coherent first image) and a Fourier transform of the incident cross-intensity function for the desired illumination scheme. For this convolution to be effective, the measurement should be performed in a paraxial state, or the measurement should first be computationally propagated from a non-paraxial state to a paraxial state (e.g., by computationally introducing a suitable magnification during such propagation).

[0069] Figure 6 A simplified schematic representation of the measuring tool 600 is shown. Depending on the situation, relative to... Figure 4 or Figure 5 One or more features of the described measurement tool may also be present in the measurement tool 600. In the embodiment, such an arrangement is proposed for use in conjunction with a computational imaging-based sensor as described above. However, as will be described, this is not necessary.

[0070] The measurement tool 600 includes a sensor 601, also called a detector and sometimes referred to as a camera. In one embodiment, the sensor 601 is an optical sensor, or optical detector. Optionally, the sensor 601 is located at the image plane of an imaging optics system, and is referred to as an image plane sensor or detector. The sensor 601 is positioned within the detection optics system, which may also include a lens 606 and optionally a focusing lens 603. In an exemplary arrangement, the sensor 601 may be a CMOS camera. The detector 601 can detect radiation information (or radiation signal) 605. In some embodiments, the radiation information 605 is used to indicate diffraction components.

[0071] Incident radiation 609 is directed from one direction onto a region 608 of the substrate 612 that includes at least a portion of the target structure 610. In this application, the region 608 may be referred to as the irradiation region. In an embodiment, the radiation information 605 is information about radiation scattered or diffracted by the target structure 610, representing a portion of the radiation scattered by the region. In an embodiment, the radiation information 605 is collected via a lens 606 and optionally focused onto the sensor 601 by a focusing lens 603.

[0072] In an embodiment, the diffraction component of the radiation information 605 may include one or more non-zero order diffraction components scattered from the target structure 610, such as only a first-order diffraction component, or one or more first-, second-, third-, or higher-order non-zero order diffraction components.

[0073] The incident radiation 609 can be fully spatially coherent or partially spatially coherent. Optionally, the incident radiation 609 is a highly collimated spatially coherent beam. Figure 6 Line 602 is shown to indicate the center line of the imaging optics including lenses 603 and 606.

[0074] Lens 603 may be a focusing lens to focus the radiation information 605 onto the detector 601. Reflected radiation 607 is obtained by reflecting the incident radiation 609 onto the irradiation area 608.

[0075] The target 610 diffracts incident radiation 609 into one or more non-zero order diffraction components, which may be radiation information 605. Optionally, the radiation information 605 may be a first diffraction order. The radiation information 605 may be collected by a lens 606. The system may be physically constrained in terms of the numerical aperture (NA) 604 of the imaging optics, which determines which diffraction orders from the target structure 610 are collected by the imaging optics.

[0076] The pupil plane 614 can be located anywhere between the lens 606 and the focusing lens 603 and is perpendicular to the center line of the imaging optics.

[0077] Figure 7 A flowchart illustrating a method for measuring 700 is shown. Method 700 can be fully or partially implemented using the measuring instruments discussed above (e.g., Figure 6 Measurement tool 600 or Figure 5The method 700 is performed in the measuring device described in scenario (a). The method 700 may include a first step 701 and a second step 703. In the first step 701, the radiation information 605 is received. The radiation information 605 may represent a portion of the radiation scattered by the irradiation region 608 of the substrate 612. The method may also include a step 711 of providing incident radiation emitted by a radiation source on region 608. The step 701 of receiving the radiation information may include a first additional step 707 of detecting the radiation information 605 using a detector 601 and / or a second additional step 708 of obtaining a composite electric field from the received radiation information by a phase acquisition method. In an embodiment, the step 701 of receiving the radiation information may include a third additional step 709 of optionally obtaining a phase signal including information about the target structure 610 via data transmission.

[0078] The second step 703, also known as the filtering step, may use a filter in the Fourier domain to remove or suppress at least a portion of the received radiation information 605 that is unrelated to the radiation already scattered by the target structure 610, in order to obtain filtered radiation information for measurement. Most or all of the removed or suppressed information is at least a portion of the received diffraction components, which may be the radiation information 605 that is not scattered by the target structure 610. The goal is to retain most or all of the information from the target structure 610 while removing some or all of the information related to regions 608 that are not part of the target structure 610. Therefore, crosstalk between radiation scattered by structures outside the target structure 610 (such as, for example, product structures) and radiation scattered by the target structure 610 is reduced. The result of the filtering step 703 may be to obtain filtered radiation information that can be used for measurement. The filtering step 703 may include another Fourier transform step 715, performing a Fourier transform on the composite field obtained in the step 701 of receiving the radiation information. The filtering step 703 may include an additional pupil imaging step 717, which obtains the pupil image of the received radiation information 605 in the Fourier domain.

[0079] The method 700 may further include a third step 705 in which the filtered radiometric information obtained in step 703 is subsequently used to perform an inverse Fourier transform to recalculate the filtered image.

[0080] The phase acquisition method used in the first step 701 can be a direct phase acquisition measurement or an indirect phase acquisition measurement. Direct phase acquisition measurement can be holographic or transverse shearing interferometry. Indirect phase acquisition measurement can use a phase acquisition algorithm. For the use of a phase acquisition algorithm, diversity is required; it can be a variation of wavelength, focal length, or numerical aperture (NA). Optionally, the phase acquisition algorithm is a ptychography algorithm. Optionally, the phase acquisition algorithm is the Gerchberg-Saxton algorithm.

[0081] Figure 8 (a) A schematic representation of an example of crosstalk between a non-target structure NT (with structural size) on the left and a measurement target structure on the right is shown to illustrate crosstalk example 801. Figure 8 (b) shows an example 802 of the pupil image, which illustrates a filtering scheme in the Fourier domain for suppressing the crosstalk mentioned above.

[0082] exist Figure 8 In the crosstalk example 801 shown in (a), the non-target structure NT does not belong to the overlapping target structure embodiment 810, which shows an example of target structure 610, and the non-target structure NT is at least partially in Figure 6 Within the irradiation area 608. In one embodiment, the non-target structure NT is a product structure, and other target structures or any other structures exist on the substrate 612. At least the non-target structure NT is one or more structures that do not need to be measured in the measurement using the method 700. Optionally, the non-target structure NT can be a periodic structure with spacing. Optionally, the non-target structure NT can include multiple spacings. There may be a distance d between the non-target structure NT and the overlapping target structure 810. Optionally, the distance d is greater than 1 μm. Optionally, the distance d is within 1 μm to 10 μm. It should be noted that Figure 8 The overlapping target structure 810 shown in (a) is merely an example, and the actual target structure may have different designs. Figure 8 (a) and the crosstalk example 801 discussed above imply certain locations of the overlapping target structure 810 and at least one non-target structure NT. The locations within the irradiation area 608 are merely examples.

[0083] The purpose of applying measurement to the target structure 610 is to determine the characteristics of the target structure 610, wherein optionally, the characteristics of the target structure 610 are one of the overlap, critical dimensions, and focusing of the photolithography apparatus, while the photolithography apparatus prints the target structure 610. Figure 8In the example shown in (a), one example of the target structure 610, the overlapping target structure 810, is defined by four structures X in the plane of the substrate 612 that define the overlap. +d Y +d X -d and Y -d Configuration. Optionally, the overlapping target structure 810 may include two layers located in two different layers of the substrate 612 and stacked on top of each other. X and Y are symbolic representations of two perpendicular directions, + and - represent two opposite directions, and d is the offset displacement between the two layers in the + or - direction in the plane of the substrate 612. The overlapping target can be used to measure the drift between the two layers in both the X and Y directions, where the drift refers to the corresponding overlap value of the two layers in the X and Y directions, respectively. The measurement is as described above and will not be described in detail here. Optionally, the four structures X +d Y +d X -d and Y -d It is a periodic structure with the same spacing (target structure spacing P), optionally a grating. The non-target structure NT in region 608 may not have the same spacing as the overlapping target structure 810, and there may be a Fourier domain separation between the first part of the received radiation information scattered by the overlapping target structure 810 and the second part of the received radiation information not scattered by the overlapping target structure 810. Figure 8 In the crosstalk example 801 shown in (a), the grating of the non-target structure NT has a structure X in the X direction that is identical to that of the overlapping target structure 810. +d and X -d The gratings are oriented in the same direction. In this embodiment, crosstalk in the X direction is severe.

[0084] It has been recognized that crosstalk (optional optical crosstalk) between the non-target structure NT and the overlapping target structure 810 is a concern because it can disrupt the overlap measurement. In fully spatially coherent imaging systems, this type of crosstalk can be very severe due to the wide point spread function, and it can be particularly dangerous for overlap signal extraction. The crosstalk can become even more severe when the distance d becomes smaller and / or when the non-target structure spacing is closer to the target structure spacing P. The inventors have recognized that the embodiments discussed can reduce the crosstalk by suppressing information from the non-target structure NT, thereby reducing the influence of the non-target structure NT on the received radiation information obtained from the measurement target structure 610.

[0085] In this embodiment, the composite electric field diffracted by the non-target structure NT will interfere with the composite electric field diffracted by the target structure 610 and introduce spatial perturbation into the image of the target image. This is referred to as crosstalk, or product optical crosstalk. These perturbations can introduce errors into measurement. The magnitude of this error can depend on the proximity of the non-target structure NT to the target structure 610, and more specifically, on the similarity of steep discontinuities (e.g., the edges of the non-target structure NT) and the spacing between the non-target structure NT and the target structure 610.

[0086] exist Figure 8 (b) shows the pupil image embodiment 802 (which is as follows) Figure 6 In the pupil plane 614 shown, the incident radiation 609 and the reflected radiation 607 correspond to the incident radiation spot 815 and the reflected radiation spot 809 in the pupil plane, respectively. The region in the pupil plane based on the incident radiation spot 815 and the reflected radiation spot 809 corresponds to the illumination region 803. The NA region 811 may be defined by the NA 604 of the imaging optics of the detection subsystem of the measurement tool / equipment, which can determine which diffraction orders from the overlapping target structure 810 are collected by the imaging optics. The radiation information 605 propagating to the sensor 601 may include the radiation within the NA region 811. In this embodiment, the illumination region 803 is larger than the NA region 811 (this is merely an example), while in practice, the sizes of these two regions in the pupil plane are based on precise design. Alternatively, the illumination region 803 may be equal to or smaller than the NA region 811. The two axes are indicated as normalized angular frequencies Fx and Fy in two perpendicular directions in the pupil plane.

[0087] The target spot 807 indicates a portion of the received radiation information related to the radiation scattered by the target structure 610, while at least one non-target spot 813 indicates a portion of the received radiation information unrelated to the radiation scattered by the target structure. In the pupil plane, radiation scattered from the overlapping target structure 810 can form the target spot 807, while radiation scattered from the non-target structure NT can form the at least one non-target spot 813 (optionally a product spot), or multiple non-target spots 813 (optionally multiple product spots). For a fixed incident radiation wavelength λ and a fixed incident radiation direction, the position of the spot in the pupil plane (which is the center frequency of the spot in the Fourier domain, or the pupil coordinate) is determined by the spacing of the corresponding structures as shown by the non-target spots 813.

[0088] An example of a filter 805 in the Fourier domain is used to remove or suppress a portion of the received radiation information outside the illustrated annular region, which is largely unscattered by the target structure 610, to obtain filtered radiation information for the measurement. Optionally, the filter 805 is an amplitude filter. Optionally, the filter 805 is an algorithm in the software domain. Optionally, the filter 805 is a hardware filter. The characteristics of the filter 805 may be based on target information associated with the target structure 610, optionally including the center frequency NA of the filter. x,y and the width NA of the filter in the Fourier domain w At least one of them. Optionally, the target information includes the target structure size A and the phase gradient slope k. x At least one of the following. Optionally, the target information includes the spacing of the target structure. This will be discussed in detail in the following description.

[0089] In one embodiment, such as Figure 7 As shown, step 701 of receiving radiation information includes a third additional step 709, which optionally involves obtaining target information from the phase signal via data transmission. The target information includes the target structure size A and the phase gradient slope k. x .

[0090] In such Figure 8 (b) shows, and in the pupil image embodiment 802 discussed above, a location of the target spot 807 and the at least one non-target spot 813 is implied. The portion within the NA region 811 is merely illustrative. Depending on the spacing of the target structures 610 and the spacing of the non-target structures NT, for example, the target spot 807 may be located on the left side of the NA region 811 and the at least one non-target spot 813 may be located on the right side. In another example, the non-target structures NT have different spacings, and several non-target spots 813 may be located on the left side of the NA region 811, and some or a few of the non-target spots may be located on the right side of the NA region 811, while the target spot may be located in the central portion of the NA region 811. In the example, the target spot 807 and the non-target spot 813 are drawn as ring-shaped, relatively small dots. In practical examples, spots 807, 813 may have different shapes and / or their sizes may be larger. This depends on the precise shape of the structure on the substrate.

[0091] The following steps are examples of methods that can be applied to remove or suppress the effects of the mentioned crosstalk: 1) Optionally, a phase signal is obtained via data transmission, or a phase acquisition is performed on the image received by the detector 601 as a technique used in computational imaging to obtain a composite field. 2) Access to the Fourier domain (pupil) is obtained by performing a Fourier transform on the composite field, wherein filtering is optionally possible via selection of a region of interest (ROI) in the pupil plane. 3) The crosstalk from the non-target structure NT can be at least partially removed or suppressed by using a filter (optionally a software filter or a hardware filter) in a binary or apodization manner, optionally provided that the ratio of the signal strength from the non-target structure NT to the signal strength from the target structure 610 is reduced by the filtering action. And optionally, 4) The image is recalculated by an inverse Fourier transform.

[0092] Figure 9 An embodiment of the composite electric field of at least one diffraction order in the case of the detector or camera being horizontal is shown, illustrating: (a) image 910, which shows the intensity of the composite electric field, representing its amplitude; (b) image 920, which shows the phase of the composite electric field; and (c) graph 930, which shows the phase gradient to the equivalent NA region, corresponding to the dashed line in (b). The coordinate axes are shown. Figure 9 (a) and Figure 9 (b) The X and Y directions. Figure 9 In (c), the x-axis represents the relative position Pos in the irradiation area on the wafer, and the y-axis represents the phase value Pha. Figure 9 It can also be regarded as Figure 8 An embodiment of the schematic diagram in (a).

[0093] exist Figure 9 In (a), the three white structures 911, 912, and 913 in the intensity image with a black background are non-target structure 911 (which could be a product structure) and the first measurement target X. +d 912 and the second measurement target X -d 913. The non-target structure 911 may be periodic in the X direction, where the X direction is the measurement target X. +d and X -d In the same direction, such as Figure 8 As shown in (a). When measuring the characteristics of the target in the X direction, the two X-direction structures X +d With X -d Crosstalk between them can be severe. Figure 9In (b), the three structures 921, 922, and 923 are surrounded by a striped background. Structures 921, 922, and 923 correspond to... Figure 9 Structures 911, 912, and 913 of the intensity image in (a).

[0094] Figure 9 The line in (c) illustrates the target structure X. +d The phase gradient along the X-direction of 922. The phase value is shown as the phase from the measurement target structure X, based on the relative position (shown as the X-axis, Pos) in the irradiated area on the wafer. +d The Y-axis of the radiation information of a target structure in 922 (which was chosen as an example) is Pha.

[0095] The slope of line 931 between points 933 and 935 is the slope of the target structure X. +d The phase slope k along the X direction x When the target structure X +d When the spacing is constant, the phase slope k x For the target structure X +d The phase gradient is a constant value for all grating lines within the NA. The phase gradient can be mapped to a specific NA region. x The specific NA position NA x It is the filter 805 based on the phase slope k x The center frequency in the Fourier domain is calculated by the following formula, where the center frequency can also be the center frequency of the target spot 807 of the target structure 610:

[0096]

[0097] The method described above is applicable to both the X and Y directions for calculating the NA portion of the filter 805. x,y ,in

[0098]

[0099] In one embodiment, the filter can be designed to be positioned in the pupil plane at NA. x,y Width NA w The form of an amplitude filter. The width NA of the filter 805. w Both the shape and the width NA of the filter 805 have a significant impact on image quality and overlap sensitivity, as will be shown in the following description. w It can be inversely proportional to the target structural dimension A and is given by the following expression:

[0100] NA w∝λ / A, or

[0101] NA w =C*λ / A

[0102] Where C is a constant that can be selected such that the filter retains most of the target spot 807 in the pupil plane (shown in FIG (8)) while removing or suppressing other spots outside the filter 805, such as the non-target spot 813.

[0103] The phase acquisition composite field of the overlapping target structure 810 (e.g., shown in...) Figure 9 (a) and Figure 9 (b) The image provides at least a portion of the characteristics of the filter 805 required to remove or suppress the portion of the received radiation information not scattered by the target structure 610 in order to obtain filtered radiation information for the measurement. Optionally, for the proposed method using the filter in the Fourier domain, the spacing of the non-target structures is sufficiently different from the target spacing. This condition is met in a variety of practical situations where the target spacing can be approximately 100 nm, while the non-target structure (optionally a product structure) is at least an order of magnitude smaller. Optionally, in the pupil plane, the separation between the center frequency of the target spot 807 and the center frequency of the non-target spot (optionally the product spot 813) is greater than 0.2 NA for efficient filtering. It should be noted that under this condition, the solution is particularly attractive for non-target structures located very close to the measurement target, because the solution can operate in a pupil space where the non-target and target signals can be easily separated based on the phase acquisition composite field as shown above. This solution is unaffected by uncalibrated aberrations in the pupil plane because, although these aberrations cause displacement or deformation effects in the field plane (which is the Fourier transform pair of the pupil plane and the plane of the image or detector), the linear phase gradient of the composite field is related to the location of the target spot in the pupil plane, defined by its λ / p value. It may be more difficult to use this method to filter non-target structures with spacing very similar to or even the same as the target, and therefore its crosstalk effects will be greater.

[0104] Figure 10 The intensity cross-sections of the target spot 1013 and the non-target spot 1011 on the pupil plane are shown in Figure 1000. The target spot 1013 may be formed by radiation scattered from the target structure 610, and the non-target spot 1011 may be formed by radiation scattered from the non-target structure NT. The X-axis indicates the normalized angular frequency Fx, or pupil coordinate, in one direction of the pupil plane. Figure 8 In the schematic image of (b), the Y-axis indicates the relative intensity I. The different lines of the target spot 1013 and the non-target spot 1011 represent directions perpendicular to the X-axis in the pupil plane (e.g., Figure 8 (b) shows different strength sections in the direction Fy. Figure 8 The system hardware aperture or system hardware aperture gate in NA region 811 in (b) indicates three filters that are referred to as rectangular filter 1003, super-Gaussian filter 1005 and Sonine filter 1007 respectively, based on their different cross-sectional shapes or transmission functions.

[0105] Figure 10 The simulation shown includes a pair of X-target structures placed diagonally within a typical overlapping target, such as Figure 8 X in (a) +d and X -d As shown. The X-target structures have the same overlap bias value and opposite bias directions along the grating direction, which in this embodiment is the X-direction. The non-target structure can be positioned adjacent to and optionally away from the overlapping target on the left side by 1 μm, the overlapping target also being simulated as an X-grating with a fixed spacing. Due to the processing steps performed on the substrate W, the target has additional overlap that may also exist in the non-target structure. In the simulation embodiment, the measurement target has a spacing of 770 nm, while the non-target structure has a varying spacing, which will be discussed later. These values ​​are selected such that the non-target signal sweeps across the entire pupil, while the target signal is located. The wafer is illuminated with incident radiation of wavelength λ = 570 nm. In this example, the inventors analyzed the effects of three different shapes of the filter, while in practice the filter can have any shape based on the degree to which crosstalk effects can be removed or suppressed. The cross-sectional shapes, or transfer functions, of the rectangular filter 1003, the super-Gaussian filter 1005, and the Sonine filter 1007 can be described by the following equations:

[0106]

[0107]

[0108]

[0109] Where r is the radial coordinate linked to NA via r = NA·f, and f is Figure 6 The focal length of lens 606 shown is given. Therefore, in the Fourier domain, r0 = NA. x,y ·f is the center frequency NA corresponding to the filter.x,y The pupil coordinates, and r w =NA w •f corresponds to the width NA of the filter in the Fourier domain. w Parameters p and β are shape parameters that control the roll-off, or attenuation, of the filter. Optionally, the filter's transfer function is such that it is flat in the middle and gently rolls off, or attenuates, at its edges. In the inventors' simulations, the center frequency NA of the filter is... x,y =0.2N and the width of the filter is NA w It is ±0.175NA.

[0110] Figure 11 This is one embodiment of an image 1100 showing the intensity of the combined electric field at at least one diffraction order at the image plane, and similar images have been shown in... Figure 9 In (a), the three structures 1111, 1112, and 1113 in the intensity image with a black background are non-target structures 1111 and the first measurement target X. +d 1112 and the second measurement target X -d 1113. The multiple rectangles drawn in structures 1112 and 1113 are selections of the region of interest (ROI) of the measurement target used to vary the crop ratio. The crop ratio is the ratio of the ROI box (shown as the rectangle in both X structures 1112 and 1113) to the size of the measurement target structure, or the size of the measured X target structure (optionally X). +d 1112 or X -d The relative size of the ROI (size 1113) compared to the target structure size. When the clipping ratio is equal to 1, it means that the size of the ROI box is the same as the measured target structure size. Figure 11 An example of ROI that can be used to calculate the average strength on the target structure is illustrated, wherein the shear ratio is optionally increased from 0.1 to 1.

[0111] The rectangular ROI was chosen to conform to the structural definition because the simulation was performed under the assumption of no aberration sensors, and therefore the image was free of distortion or aberration. The filter's effect on visual image quality is to blur structural edges (due to resolution loss) and suppress ringing features, mathematically known as the Gibbs phenomenon. Therefore, the size of the ROI box is a good parameter for evaluating this method because it highlights the sensitivity of the average intensity to local ringing (smaller ROIs) and edge blurring (larger ROIs).

[0112] Figure 12 Simulation results are shown for an embodiment comparing the effects of having and not having a filter. Figure 12 The left column, which includes an intensity image (a), a pupil image (b), and an overlap sensitivity plot (c), shows an example of a simulation before filtering. In the embodiment described, a filter is used that... Figure 12 (e) is indicated as 2217, and the filtered simulation is shown in Figure 12 The right-hand column includes the intensity image (d), the pupil image (e), and the overlap sensitivity plot (f). The filtering is used in the pupil plane or in the Fourier domain to suppress or remove information outside the circle while preserving information inside the circle.

[0113] Figure 12 (b) and Figure 12 (e) shows pupil images 1204 and 2204, respectively, in the Fourier domain, illustrating the pupil images before and after filtering. The filtering suppresses the crosstalk mentioned above, such as... Figure 8 As illustrated in (b). Target spots 1216 and 2216 can be formed by radiation scattered from the measured target structure, as shown in... Figure 8 (b) is illustrated as 807. The non-target spot 1215 (optionally a product spot) may be formed by radiation scattered from the non-target structure NT. Figure 12 (b) NA region 1214 and Figure 12 2214 in (e) can be defined by NA 604 of the imaging optics simulated by the measurement tool / equipment's detection system, as in Figure 8 (b) is illustrated as 811. It should be noted that the pupil image is obtained by calculating the Fourier transform of the detector image. Before filtering, there is a non-target spot 1215, although it is almost invisible after filtering using the filter. Figure 12 (e) is illustrated as circle 2217. Optionally, the filter is a super-Gaussian filter, which may be flat in the middle and gently roll off at its edges, i.e., attenuate.

[0114] As mentioned above, the super-Gaussian filter can be designed to soften, i.e., smooth, the edges of the filter while maintaining a flat central region, such as... Figure 10 As shown, it can help suppress ringing features in the image of the structure on the detector due to the relative suppression of the sidelobes, i.e., the sidelobes, of the effective point spread function associated with the filter.

[0115] Figure 12 (a) and Figure 12 (d) are intensity images 1202 and 2202 of the composite electric field at the image plane, and as shown in Figure 1202. Figure 9 (a) and Figure 11Similarly, intensity images 1202 and 2202 correspond to pupil images 1204 and 2204, respectively. Figure 12 (a) and Figure 12 (d) can be images of the same incident radiation information, one filtered and one unfiltered, where the incident radiation information can be radiation of at least one diffraction order. Figure 12 In (a), the three white structures 1211, 1212, and 1213 in the intensity image with a dark background can be non-target structure 1211 (optionally a product structure) and the first measurement target X. +d 1212 and the second measurement target X -d 1213. In this simulation example, the distance d between the non-target structure and the target structure is 1 μm. For this embodiment, after filtering, the non-target structure is almost invisible in the intensity image, while responding to... Figure 12 In (a), the images of the two measurement targets 2212 and 2213 (1212 and 1213) are still visible, as shown in the image. Figure 12 As shown in (d), the filtering results in a loss of resolution that is visible when edges in the detector image are blurred. However, this may not be a problem when the information used in the measurement tool is based on, for example, the average intensity within a region of a ROI.

[0116] Figure 12 (c) and Figure 12 (f) Shows overlap sensitivity curves 1206 and 2206 corresponding to 1204 and 2204 respectively, which are overlap errors as a function of the clipping ratio with and without filtering. The X-axis indicates the clipping ratio, while the Y-axis indicates the overlap error. In this example, the overlap error is defined as the difference between the set overlap value and the value obtained from the simulated measurement, which can be calculated based on the intensity asymmetry on the target, given ROI. NA of 0.4 is used in this simulation example. The black curves 1218 or 2218 around the Y=0 line show the overlap error in the absence of any adjacent non-target structures (optionally product structures) (i.e., no crosstalk). It can be seen that the overlap error curve is less sensitive to the ROI when filtered compared to when no filter is used. It can also be seen that the overlap error decreases after filtering and the overlap error is roughly similar to the overlap error that would exist in the absence of non-target structures (optionally product structures) (e.g., as line 2218).

[0117] In the simulation example above, the measurement target has a spacing of 770 nm, while the adjacent non-target structures have spacings varying from 450 nm to 650 nm in 50 nm increments. Although only results from a specific non-target structure spacing of 500 nm are shown, all other non-target structure spacings have been simulated. This conclusion applies when the spacing of the non-target structures is significantly different from the target spacing. If the spacing of the non-target structures is close to the target spacing, it may not be sufficiently removed by some filters and may still be present in the filtered detector image. In these cases, overlap sensitivity may increase with varying ROIs, indicating poor crosstalk suppression.

[0118] When the spacing between the non-target structures is sufficiently different from the target spacing, for example, when the spacing between the non-target structures is 450 nm, 500 nm, or 550 nm, the overlap error may be less sensitive to the ROI when using some filters, such as the rectangular filter 1003, compared to when no filter is used. When the spacing between the non-target structures is not sufficiently different from the target spacing, for example, when the spacing between the non-target structures is 600 nm or 650 nm, the signal from the non-target structures may not be sufficiently removed by the filter, meaning that at least a portion of the non-target signal falls inside the filter and is exposed, i.e., appears in the detector.

[0119] When using filters, such as filters with a transfer function that is flat in the middle and gradually rolls off at its edges (optionally a super-Gaussian filter 1005 or a Sonine filter 1007), the overlap sensitivity is less than when using the rectangular filter 1003 when the non-target structure spacing is close to the target spacing, or when the non-target structure spacing is not sufficiently different from the target spacing, for example, when the non-target structure spacing is 600 nm or 650 nm. The closeness or tightness of the overlap error curve to the overlap error curve that would appear without non-target structures (optionally product structures) (e.g., as line 2218) indicates the effectiveness of crosstalk removal or suppression / reduction.

[0120] Compared to, for example, using a rectangular filter, using a filter that rolls off gently at its edges (optionally a super-Gaussian filter) may result in a slight increase in the main lobe of the point spread function, leading to a smaller loss of resolution. By design, the detector and pupil images can look similar to those using a rectangular filter. The key difference is that when the spacing of the non-target structures differs significantly from the target spacing, the overlap error may be significantly less sensitive to the ROI. Even when the spacing of the non-target structures is close to the target spacing, the overlap error can still be smaller than, for example, the overlap error when using the rectangular filter. The closeness or tightness of the overlap error curve to the black reference curve demonstrates the effectiveness of crosstalk mitigation using the super-Gaussian filter.

[0121] In the simulations mentioned above, a super-Gaussian filter is used to remove or suppress at least a portion of the received radiation information that is not related to the radiation already scattered by the target structure. However, other types of filters can also be used, optionally rectangular filters and Sonine filters. The characteristics of the filter are based on target information associated with the target structure. The characteristics of the filter can also be optimized and determined in advance through simulation.

[0122] If the region outside the NA region has a high average amount of optical aberrations, then the optimal selection of the filter size and shape can reduce the impact of optical aberrations. For example... Figure 12 As seen in (c), the overlap error caused by crosstalk oscillates around zero depending on the clipping ratio. The amplitude of the oscillation indicates the severity of the crosstalk. The size and shape of the filter can have a significant impact on the oscillation. A smooth (i.e., apodized) filter can mitigate the oscillation and suppress the ringing features in the image, thus helping to reduce the overlap error caused by crosstalk. These advantages may occur at the cost of a loss of resolution (i.e., resolution degradation) due to the smaller aperture of the filter used. Due to the target layout, the resolution degradation can be handled and may not introduce additional measurement errors.

[0123] Embodiments that can be used to optimize the selection of the ROI in the pupil or Fourier plane will now be described, i.e., such that filtering is applied selectively (e.g., in...). Figure 7 During the filtering step 703 of the method, the pupil region is optimized compared to the previously described method. Figure 13 This is a flowchart describing this method. At step 1300, a phase-acquired image including a description of the composite electric field is obtained using phase-acquired and / or holographic methods as described, for example, based on the received radiation information. Step 1310 includes a Fourier transform step of performing a Fourier transform on the composite field obtained in step 1300 to obtain a corresponding pupil image.

[0124] As an alternative to, or in combination with, steps 1300 and 1310, an optimized pupil image can be obtained via simulation, for example, based on analysis of an ideal structure defined on a mask. In this embodiment, the pupil response to these structures can be simulated for appropriate exposure / processing conditions. This pupil response can be predicted for the target and its surrounding environment. This analysis can be performed during the setup of the layer and can be used as is or as a starting point for further fine-tuning for the selection of the pupil region on actual measurements (e.g., performing the simulation first, and then performing steps 1300 / 1310).

[0125] Step 1320 is a pupil optimization step performed to optimize the ROI within the pupil (e.g., the pupil obtained from steps 1300 / 1310, or a simulated pupil). This can include freely chosen methods (based on, for example, observational trial and error). However, such complex methods can be very time-consuming. More efficient optimization step methods can include using clustering algorithms (e.g., K-means) to identify these regions and comparing only the permutations of such regions. At step 1330, the optimized pupil for this iteration (e.g., information filtered out from the ROI) is an inverse Fourier transform performed using the filtered radiative information to recalculate the optimized filtered image for this iteration.

[0126] At step 1340, a first evaluation step may be performed to evaluate the optimized pupil used for this iteration. The evaluation may be based on one or more suitable key performance indicators (KPIs) describing the stability of the ROI, such as signal strength and / or image contrast in a frequency region with / without the expected edges. More specifically, the optimized pupil will include minimized or smaller edges within the ROI and high (e.g., maximized) contrast of the ROI relative to the remainder of the target. A second evaluation step 1350 may evaluate the intra-wafer variation of diffraction intensity in the pupil or field plane. The diffraction intensity (as discussed) may be perturbed by neighboring regions and features, and different target sites of other similar targets are expected to have different surrounding environments. Thus, minimal intra-wafer variation indicates a good ROI. It should be noted that the evaluation within this method may include any one or both of evaluation steps 1340 and 1350. The method then continues with another iteration until all candidate ROIs have been evaluated and / or the optimized ROI has been identified. The optimized ROI can then be used for measurements performed using the methods disclosed herein.

[0127] It should be understood that this optimization method comes at the cost of performing two Fourier transform steps in each iteration to transform back and forth between the field plane and the pupil plane. Therefore, it may be desirable to limit the number of iterations for each optimization (e.g., using a clustering algorithm as described) and / or the frequency of performing the full optimization. The pupil optimization can be performed, for example, in the following situations:

[0128] -Only when setting up the machine (although this may be too simplistic);

[0129] -Only when the optional configuration is set;

[0130] -At the beginning of each batch;

[0131] -At the beginning of each chip;

[0132] - Triggered by an event; for example, when an image KPI (e.g., such as the KPI already described) deviates beyond a trigger threshold. In such cases, the optimization can be performed only whenever such an event occurs, or this can be used in conjunction with pre-arranged ROI optimization at one of the frequencies described above.

[0133] This approach results in increased flexibility when applying pupil filters, and improved robustness or tolerance to crosstalk with the product structure. The method can be automated in optional setups or during each batch or per wafer operation.

[0134] To date, the methods described herein have outlined methods for filtering out non-target-related influences / perturbations based on received composite field information relevant to target measurements. Here, an extension of the basic concept is described, which separates opposite diffraction orders so that these opposite diffraction orders can be measured simultaneously in phase acquisition (computational imaging) measurements. The methods can be used alone or in combination with filtering methods for removing external non-target influences described herein.

[0135] In conventional non-computational measurements, known techniques include obtaining separate images of opposite higher diffraction orders (e.g., +1 and -1) using a "quadruple illumination aperture," which involves illumination only in two diagonally opposite quadrants or quartzes, leaving the other two quadrants or quartzes unused for detection. This illumination aperture results in simultaneous measurements in "normal illumination mode" and "complementary illumination mode" (i.e., using illumination incident simultaneously in opposite directions). An optical wedge is positioned in the pupil plane to guide the diffraction radiation and separate multiple orders, such that a portion of one diffraction order is detected in one quadrant or quartz of the multiple detection quadrants or quartzes, and a portion of the opposite diffraction order is detected in another detection quadrant or quartz.

[0136] A similar concept is proposed to be implemented digitally based on access to the reconstructed pupil obtained in the described method. The proposal includes the following steps: illuminating from two opposite directions; for example, in a “normal illumination mode” and a “complementary illumination mode”, reconstructing the pupil and separating the normal and complementary information in the pupil plane based on the pupil location. This method only works if the diffraction steps generated by the normal and complementary illumination angles are separated in the pupil (if the diffraction steps cannot be separated in the pupil based on their position, then temporally continuous acquisition / acquisition can be performed). This can be achieved by the proper selection of the illumination mode (including a limited range of incident and azimuth angles). Such illumination modes may include, for example, a “quadruple illumination aperture” as described.

[0137] This separation allows for the application of the aforementioned digital equivalent of the optical wedge in computational imaging sensor designs. Figure 14 (a) shows an exemplary arrangement of illumination pupil IlP and detection pupil DP, which shows possible locations of the +1 and -1 diffraction steps relative to the detection pupil DP (only one direction is shown). Figure 14 (b) is a flowchart describing this overview measurement method based on this concept:

[0138] Step 1400: Use computational imaging (e.g., based on...) Figure 14 (a) The radiation information obtained by the illumination and detection arrangement) reconstructs the pupil representation P describing the electric field in the pupil plane.

[0139] Step 1410: Apply two separation filters F1 and F2 to the reconstructed pupil plane to separate opposite diffraction orders (e.g., +1X and -1X signals) in the pupil, thereby providing two approximately complementary pupil images (corresponding to normal and complementary modes, respectively). Since the approximate location within the pupil for each diffraction order will be known, the filtering can be performed based on the pupil location. The separation filters can be implemented in hardware or software.

[0140] • Step 1420: Reconstruct (digitally re-image) each pupil image in pupil images Im1 and Im2 into a field plane representation.

[0141] • Step 1430: Apply an algorithm to determine the asymmetry between the +1 order and the -1 order, wherein the asymmetry can be represented by the intensity, or phase, or a combination of the electric field.

[0142] As is currently being done in diffraction-based overlap (DBO), two biased targets can be used to calibrate the sensitivity of signal asymmetry to overlap.

[0143] In cases where a large spacing between second-order or higher-order diffractions is captured on the camera, these individual orders can also be separated. For each order, the image can be processed independently to infer overlap.

[0144] When gratings with different spacings exist in the field of view, such as different spacings between X and Y or between multiple targets, pupil filtering can also be used to allow these individual spacings to be imaged individually or separately. This will reduce crosstalk between these structures.

[0145] Computational imaging eliminates the need for additional hardware to separate different orders or simultaneously generate normal / complementary modes, offering greater flexibility. Computational re-imaging also provides greater freedom in selecting separation filters, allowing for the selection of filters with hard or soft edges (e.g., to avoid diffraction effects (apodization) at those edges).

[0146] The above embodiments can be used to solve problems in computational imaging, such as crosstalk caused by coherent illumination as mentioned above. In one embodiment, the above embodiments can use a software domain to solve hardware problems, fully utilizing algorithms used in computational imaging. The above embodiments can allow for the reduction of crosstalk by removing or suppressing unwanted regions in the Fourier domain (or pupil plane). The above embodiments can be adapted to situations where the spacing between non-target structures (optionally sufficiently) differs from the target spacing. The above embodiments may not require any prior knowledge because the image obtained according to the phase acquisition method provides all the information for specifying the filter. It is expected that the above embodiments are robust relative to uncalibrated, or residual wavefront aberrations, because such aberrations may not affect the location of the non-target structures and target signals in the pupil space.

[0147] In, for example Figure 5 Measuring equipment, inspection equipment, such as Figure 1 Photolithography equipment, and / or for example Figure 2 The above-mentioned embodiments are provided in the photolithography unit.

[0148] In another embodiment, there is a computer program product including instructions that, when executed by a processor, cause the methods of the embodiments mentioned above to be performed.

[0149] Other embodiments are disclosed in subsequent numbered sections:

[0150] 1. A method for performing measurement on a region of a substrate including at least a portion of a target structure, the method comprising:

[0151] - Receive radiation information representing at least a portion of the radiation scattered by the region.

[0152] - A filter is used in the Fourier domain to remove or suppress at least a portion of the received radiation information that is unrelated to the radiation already scattered by the target structure, in order to obtain filtered radiation information for the measurement.

[0153] The characteristics of the filter are based on target information related to the target structure.

[0154] 2. The method according to aspect 1, wherein the target information includes the target structure size (A) and the phase gradient slope (k). x,y At least one of them.

[0155] 3. The method according to aspect 1 or 2, wherein the characteristics of the filter include: the center frequency (NA) of the filter in the Fourier domain. x,y ) and the width of the filter in the Fourier domain (NA) w At least one of the following.

[0156] 4. The method according to aspects 2 and 3, wherein the width of the filter is inversely proportional to the size of the target structure and is given by the following expression:

[0157] NNA w ∝λ / A

[0158] Where λ is the wavelength of the radiation.

[0159] 5. The method according to any of the foregoing aspects of references 2 and 3, wherein the center frequency of the filter in the Fourier domain is based on the phase gradient slope, wherein

[0160]

[0161] Where λ is the wavelength of the radiation.

[0162] 6. The method according to any of the foregoing aspects, wherein the method includes obtaining a composite electric field from received radiation information by a phase acquisition method, optionally the phase acquisition method being a direct phase acquisition measurement or an indirect phase acquisition measurement, optionally the indirect phase acquisition measurement using a phase acquisition algorithm.

[0163] 7. The method according to aspect 6, wherein the method includes performing a Fourier transform on the composite field.

[0164] 8. The method according to any of the foregoing aspects, wherein the method includes obtaining a phase signal including the target information.

[0165] 9. The method according to aspects 2 and 8, wherein the target information obtained from the phase signal includes the target structure size and the phase gradient slope.

[0166] 10. The method according to any of the foregoing aspects, wherein the method includes obtaining a pupil image of the radiation information received in the Fourier domain.

[0167] 11. The method according to aspect 10, wherein the method includes subsequently performing an inverse Fourier transform using the filtered radiometric information to recalculate the filtered image.

[0168] 12. The method according to any of the foregoing aspects, wherein the target information includes the spacing of the target structure.

[0169] 13. The method according to aspect 12, wherein the non-target structures in the region do not have the spacing of the target structures.

[0170] 14. The method according to any of the foregoing aspects, wherein the method includes providing incident radiation emitted by a radiation source.

[0171] 15. The method according to any of the foregoing aspects, wherein the method includes detecting the radiation information using a detector.

[0172] 16. The method according to any of the foregoing aspects, wherein the received radiation information includes information on one or more non-zero order diffraction components scattered from the target structure.

[0173] 17. The method according to any of the foregoing aspects, wherein the transfer function of the filter is such that it is flat in the middle and rolls off gently at its edges.

[0174] 18. The method according to any of the foregoing aspects, wherein the incident radiation is fully spatially coherent or partially spatially coherent.

[0175] 19. The method according to any of the foregoing aspects, wherein the filter is an amplitude filter.

[0176] 20. The method according to any of the foregoing aspects, wherein the filter is an algorithm in the software domain.

[0177] 21. The method according to any one of aspects 1 to 19, wherein the filter is a hardware filter.

[0178] 22. The method according to any of the foregoing aspects, wherein in the Fourier domain there exists a separation between a first portion of the received radiation information scattered by the target structure and a second portion of the received radiation information not scattered by the target structure.

[0179] 23. The method according to any of the foregoing aspects, further comprising:

[0180] - Use one or more filters in the Fourier domain to separate at least one pair of opposing higher diffraction orders within at least a portion of the radiation scattered by the region.

[0181] 24. The method according to aspect 23, wherein the radiation information relates to radiation obtained in a normal irradiation mode and in a complementary irradiation mode.

[0182] 25. The method according to aspect 23 or 24, comprising the following steps:

[0183] A digital separation filter is applied to the pupil representation reconstructed based on the radiation information, the digital separation filter being operable to separate the at least one pair of opposite diffraction orders in the pupil; and

[0184] Reconstruct the pupil image corresponding to each diffraction order in the opposite diffraction order in the field plane.

[0185] 26. The method according to aspect 25, wherein the digital separation filter is applied based on the pupil region.

[0186] 27. The method according to any one of aspects 23 to 26, comprising determining an asymmetry between opposite diffraction orders, said asymmetry including an asymmetry of intensity, phase, or a combination of intensity and phase.

[0187] 28. The method according to any one of aspects 23 to 27, wherein the radiation information includes information related to multiple pairs of opposite diffraction orders; and the method includes applying the filter to separate each of these diffraction orders.

[0188] 29. The method according to any of the foregoing aspects, comprising performing a pupil region optimization step to optimize the region in the Fourier domain that has been removed or suppressed using the filter.

[0189] 30. The method according to aspect 29, wherein the optimization step of the pupil region of interest includes iteratively performing the following:

[0190] - Reconstruct the pupil representation based on the radiation information and / or simulate the pupil representation based on mask data describing the structure on the mask;

[0191] The region of interest to be evaluated is determined, and the pupil representation is filtered based on this region of interest;

[0192] - Determine the filtered image represented by the filtered pupil; and

[0193] The filtered image is evaluated relative to the filtered image from the previous iteration.

[0194] 31. The method according to aspect 30, wherein determining the region of interest to be evaluated includes freely selecting regions or performing a clustering algorithm to identify regions for evaluation.

[0195] 32. The method according to aspect 30 or 31, wherein the evaluation step includes one or both of the following:

[0196] Assess the stability of the area of ​​interest to be assessed; and

[0197] Evaluate intra-crystal variations in diffraction intensity in the pupil or field plane.

[0198] 33. The method according to any one of aspects 29 to 32, wherein the pupil area optimization step of interest is performed at machine setup, at configuration setup, at the start of each batch, or at the start of each wafer.

[0199] 34. The method according to any one of aspects 29 to 33, wherein the pupil region optimization step of concern is performed each time a triggering event occurs.

[0200] 35. A method for performing a measurement on a region of a substrate including at least a portion of a target structure, the method comprising:

[0201] - Receive radiation information representing at least a portion of the radiation scattered by the region.

[0202] - Use one or more filters in the Fourier domain to separate at least one pair of opposing higher diffraction orders within at least a portion of the radiation scattered by the region.

[0203] 36. The method according to aspect 35, wherein the radiation information relates to radiation obtained in a normal irradiation mode and in a complementary irradiation mode.

[0204] 37. The method according to aspect 35 or 36, comprising the following steps:

[0205] A digital separation filter is applied to the pupil representation reconstructed based on the radiation information, the digital separation filter being operable to separate the at least one pair of opposite diffraction orders in the pupil; and

[0206] Reconstruct the pupil image corresponding to each diffraction order in the opposite diffraction order in the field plane.

[0207] 38. The method according to aspect 37, wherein the digital separation filter is applied based on the pupil region.

[0208] 39. The method according to any one of aspects 35 to 38, comprising determining an asymmetry between opposite diffraction orders, said asymmetry including an asymmetry of intensity, phase, or a combination of intensity and phase.

[0209] 40. The method according to any one of aspects 35 to 39, wherein the radiation information includes information related to multiple pairs of opposite diffraction orders; and the method includes applying the filter to separate each of these diffraction orders.

[0210] 41. The method according to any of the foregoing aspects, wherein the measurement determines the characteristics of the target structure.

[0211] 42. The method according to aspect 41, wherein the characteristic of the target structure is one of overlap, focal length, and critical size.

[0212] 43. A measurement apparatus for performing measurements on a region of a substrate including at least a portion of a target structure, the measurement apparatus being configured to:

[0213] - Receive radiation information representing at least a portion of the radiation scattered by the region.

[0214] - A filter is used in the Fourier domain to remove or suppress at least a portion of the received radiation information that is unrelated to the radiation already scattered by the target structure, in order to obtain filtered radiation information for the measurement.

[0215] The characteristics of the filter are based on target information related to the target structure.

[0216] 44. A measurement apparatus for performing measurement on a region of a substrate including at least a portion of a target structure, the measurement apparatus comprising a processor configured to execute instructions for performing the following steps:

[0217] - Receive radiation information representing at least a portion of the radiation scattered by the region.

[0218] - A filter is used in the Fourier domain to remove or suppress at least a portion of the received radiation information that is unrelated to the radiation already scattered by the target structure, in order to obtain filtered radiation information for the measurement.

[0219] The characteristics of the filter are based on target information related to the target structure.

[0220] 45. A measurement apparatus for performing measurements on a region of a substrate including a target structure, the measurement apparatus comprising:

[0221] - A source, said source being used to illuminate a region on the substrate.

[0222] - A detection system, the detection system being configured to receive radiation information representing at least a portion of the radiation scattered by the region.

[0223] The detection system includes a filter in the pupil plane for removing or suppressing at least a portion of the received radiation information that is unrelated to radiation already scattered by the target structure, in order to obtain filtered radiation information for the measurement.

[0224] The characteristics of the filter are based on target information related to the target structure.

[0225] 46. ​​A computer program product comprising instructions that, when executed at a processor, cause to perform the method according to any one of aspects 1 to 42.

[0226] 47. A measurement apparatus for performing measurement on a region of a substrate including at least a portion of a target structure, the measurement apparatus being configured for the method according to any one of aspects 1 to 42.

[0227] While the use of lithography equipment in IC manufacturing can be specifically mentioned in this article, it should be understood that the lithography equipment described herein can have other applications. Possible other applications include manufacturing integrated optical systems, guiding and detecting patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin-film magnetic heads, and so on.

[0228] While specific examples in the context of lithography equipment may be mentioned herein, these examples can be applied to other equipment. Examples may form part of mask inspection equipment, metrology equipment, or any equipment that measures or processes objects such as wafers (or other substrates) or masks (or other patterning apparatus). These devices are generally referred to as lithography tools. Such lithography tools can operate under vacuum conditions or ambient (non-vacuum) conditions.

[0229] While embodiments of the invention may be specifically referred to herein in the context of inspection or measurement equipment, the embodiments can be used in other equipment. The embodiments may form part of a mask inspection apparatus, a lithography apparatus, or any apparatus that measures or processes objects such as wafers (or other substrates) or masks (or other patterning apparatuses). The term "measurement equipment" or "inspection equipment" may also refer to an inspection apparatus or detection system (or a measurement apparatus or measurement system). For example, the inspection apparatus including the embodiments may be used to detect defects in a substrate or defects in a structure on a substrate. In such embodiments, the characteristics of interest in the structure on the substrate may involve defects in the structure, the absence of a specific portion of the structure, or the presence of an unwanted structure on the substrate.

[0230] While the use of the embodiments in the context of optical lithography may be specifically mentioned above, it will be understood that the invention may be used in other applications (e.g., imprint lithography) where the context permits.

[0231] While the targets or target structures described above (more generally, structures on a substrate) are measurement target structures specifically designed and formed for measurement purposes, in other embodiments, properties of interest can be measured on one or more structures that are functional parts of a device formed on the substrate. Many devices have regular grating-like structures. As used herein, the terms structure, target grating, and target structure do not require that the structure be specifically set for the measurement being performed. Furthermore, the spacing of the measurement targets can be close to or possibly smaller than the resolution limit of the scatterer's optical system, but can be much larger than the size of a typical non-target structure (optionally a product structure fabricated in the target portion C by a photolithography process). In practice, the lines and / or spaces of overlapping gratings within the target structure can be fabricated to include smaller structures similar in size to non-target features.

[0232] While specific embodiments have been described above, it should be understood that the invention can be practiced in other ways than those described. The above description is intended to be illustrative and not restrictive. Therefore, those skilled in the art will understand that modifications can be made to the described invention without departing from the scope of the claims set forth below.

[0233] While specifically referring to "measuring equipment / tools / systems" or "inspection equipment / tools / systems," these terms can refer to tools, equipment, or systems of the same or similar type. For example, inspection or measuring equipment including embodiments of the present invention can be used to determine the characteristics of structures on a substrate or wafer. For example, inspection or measuring equipment including embodiments of the present invention can be used to detect defects in a substrate, or defects in structures on a substrate or wafer. In such embodiments, the characteristics of interest concerning the structure on the substrate may relate to defects in the structure, the absence of a specific portion of the structure, or the presence of unwanted structures on the substrate or wafer.

[0234] Figure 13 This is a block diagram illustrating a computer system 1600 that can assist in implementing the methods and processes disclosed herein. The computer system 1600 includes a bus 1602 or other communication mechanism for communicating information, and a processor 1604 (or multiple processors 1604 and 1605) coupled to the bus 1602 for processing information. The computer system 1600 also includes a main memory 1606, such as random access memory (RAM) or other dynamic storage device, coupled to the bus 1602 for storing information and instructions to be executed by the processor 1604. The main memory 1606 may also be used to store temporary variables or other intermediate information during the execution of instructions to be executed by the processor 1604. The computer system 1600 also includes a read-only memory (ROM) 1608 or other static storage device coupled to the bus 1602 for storing static information and instructions for the processor 1604. A storage device 1610, such as a magnetic disk or optical disk, is provided and coupled to the bus 1602 for storing information and instructions.

[0235] Computer system 1600 can be coupled via bus 1602 to a display 1612 for displaying information to a computer user, such as a cathode ray tube (CRT), flat panel display, or touch panel display. An input device 1614, including alphanumeric keys and other keys, is coupled to bus 1602 for communicating information and command selection to processor 1604. Another type of user input device is a cursor controller 1616, such as a mouse, trackball, or cursor direction keys, for communicating directional information and command selection to processor 1604 and for controlling cursor movement on display 1612. Such input devices typically have two degrees of freedom on two axes (i.e., a first axis (e.g., x) and a second axis (e.g., y)), allowing the device to specify a position in a plane. Touch panel (screen) displays can also be used as input devices.

[0236] The computer system 1600 may perform one or more sequences of one or more instructions contained in main memory 1606 in response to processor 1604 executing one or more instructions as described herein. These instructions may be read into main memory 1606 from another computer-readable medium, such as storage device 1610. Execution of the instruction sequences contained in main memory 1606 causes processor 1604 to perform the process steps described herein. One or more processors arranged in a multiprocessor configuration may also be used to execute the instruction sequences contained in main memory 1606. In alternative embodiments, a hard-wired circuit system may be used instead of or in combination with software instructions. Therefore, the description herein is not limited to any particular combination of hardware circuitry and software.

[0237] As used herein, the term "computer-readable medium" refers to any medium that participates in providing instructions to processor 1604 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 discs or magnetic disks, such as storage device 1610. Volatile media include dynamic memory, such as main memory 1606. Transmission media include coaxial cables, copper wires, and optical fibers, including wires containing bus 1602. Transmission media can also take the form of sound waves or light waves, such as sound waves or light waves 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, punched cards, paper tape, any other physical media with a perforated pattern, RAM, PROMs and EPROMs, FLASH-EPROMs, any other memory chips or cartridges, carrier waves as described below, or any other media that can be read by a computer.

[0238] Various forms of computer-readable media are used to carry one or more sequences of instructions to processor 1604 for execution. For example, the instructions may initially be carried on the disk of a remote computer. The remote computer may load the instructions into its dynamic memory and transmit the instructions over a telephone line using a modem. A modem local to computer system 1600 may receive data over the telephone line and convert the data into an infrared signal using an infrared transmitter. An infrared detector coupled to bus 1602 may receive the data carried in the infrared signal and place the data on bus 1602. Bus 1602 carries the data to main memory 1606, from which processor 1604 fetches and executes instructions. Instructions received by main memory 1606 may optionally be stored on storage device 1610 before or after execution by processor 1604.

[0239] Computer system 1600 also preferably includes a communication interface 1618 coupled to bus 1602. Communication interface 1618 provides bidirectional data communication coupling with network link 1620 connected to local area network 1622. For example, communication interface 1618 may be an Integrated Services Digital Network (ISDN) card or modem to provide data communication connectivity to a corresponding type of telephone line. As another example, communication interface 1618 may be a local area network (LAN) card to provide data communication connectivity to a compatible LAN. A wireless link may also be implemented. In any such implementation, communication interface 1618 transmits and receives electrical, electromagnetic, or optical signals carrying digital data streams representing various types of information.

[0240] Network link 1620 typically provides data communication to other data devices via one or more networks. For example, network link 1620 may provide a connection to host computer 1624 or to data equipment operated by Internet Service Provider (ISP) 1626 via local area network 1622. ISP 1626 then provides data communication services via a global packet data communication network (now commonly referred to as the "Internet" 1628). Both local area network 1622 and Internet 1628 use electrical, electromagnetic, or optical signals to carry digital data streams. Signals passing through various networks and signals on network link 1620 and through communication interface 1618 (which carries digital data to and from computer system 1600) are example forms of carrier waves for transmitting information.

[0241] Computer system 1600 can send messages and receive data including process code via a network, network link 1620, and communication interface 1618. In an Internet example, server 1630 can transmit requested code for an application via the Internet 1628, ISP 1626, local area network 1622, and communication interface 1618. For example, such a downloadable application can provide for one or more of the techniques described herein. The received code can be executed by processor 1604 upon receipt and / or stored in storage device 1610 or other non-volatile storage device for later execution. In this way, computer system 1600 can obtain application code in carrier form.

Claims

1. A method for performing measurement on a region of a substrate including at least a portion of a target structure, the method comprising: - Receive radiation information at the image plane representing at least a portion of the dark-field diffraction radiation of the target structure and non-target structures scattered by the region. - Convert the received radiation information in the pupil plane or Fourier domain, - Use a filter in the pupil plane or Fourier domain to remove or suppress at least a portion of the received radiation information that is unrelated to the radiation scattered by the target structure, to obtain filtered radiation information for the measurement. in, The characteristics of the filter are based on target information related to the target structure. - Convert the filtered radiation information into image data, and - Use the image data to adjust parameters of interest in the semiconductor process.

2. The method according to claim 1, wherein the target information includes the target structure size (A) and the phase gradient slope (A). At least one of the following.

3. The method according to claim 1, wherein the characteristics of the filter include: The center frequency of the filter in the Fourier domain ( ) and the width of the filter in the Fourier domain (NA) w At least one of the following.

4. The method of claim 3, wherein the width of the filter is inversely proportional to the size of the target structure and is given by the following expression: in It is the wavelength of the radiation.

5. The method according to any one of claims 2 and 3, wherein the center frequency of the filter in the Fourier domain is based on the phase gradient slope, wherein in It is the wavelength of the radiation.

6. The method according to claim 1, wherein the method includes obtaining a composite electric field from the received radiation information by a phase acquisition method, optionally the phase acquisition method being a direct phase acquisition measurement or an indirect phase acquisition measurement, optionally the indirect phase acquisition measurement using a phase acquisition algorithm.

7. The method of claim 6, wherein the method includes performing a Fourier transform on the composite electric field.

8. The method of claim 2, wherein the method includes obtaining a phase signal including the target information.

9. The method of claim 8, wherein the target information obtained from the phase signal includes the target structure size and the phase gradient slope.

10. The method of claim 1, wherein the method includes obtaining a pupil image of the radiation information received in the Fourier domain.

11. The method of claim 10, wherein the method includes subsequently performing an inverse Fourier transform using the filtered radiometric information to recalculate the filtered image.

12. The method according to claim 1, wherein the target information includes the spacing of the target structure.

13. The method of claim 12, wherein the non-target structures in the region do not have the spacing of the target structures.

14. The method of claim 1, wherein the method includes providing incident radiation emitted by a radiation source.

15. The method of claim 1, wherein the method includes detecting the radiation information using a detector.

16. The method of claim 1, wherein the received radiation information includes information on one or more non-zero order diffraction components scattered from the target structure.

17. The method of claim 1, wherein the transfer function of the filter is such that it is flat in the middle and rolls off gently at its edges.

18. The method of claim 1, wherein the incident radiation is fully spatially coherent or partially spatially coherent.

19. The method of claim 1, wherein the filter is an amplitude filter.

20. The method of claim 1, wherein the filter is an algorithm in the software domain.

21. The method of claim 1, wherein the filter is a hardware filter.

22. The method of claim 1, wherein in the Fourier domain there exists a separation between a first portion of the received radiation information scattered by the target structure and a second portion of the received radiation information not scattered by the target structure.

23. The method according to claim 1, further comprising: - One or more filters are used in the Fourier domain to separate at least one pair of opposing higher diffraction orders within at least a portion of the radiation scattered by the region.

24. The method of claim 23, wherein the radiation information relates to radiation obtained in a normal irradiation mode and in a complementary irradiation mode.

25. The method according to claim 23 or 24, comprising the following steps: A digital separation filter is applied to the pupil representation reconstructed based on the radiation information, the digital separation filter being operable to separate the at least one pair of opposite diffraction orders in the pupil; and Reconstruct the pupil image corresponding to each diffraction order in the opposite diffraction order in the field plane.

26. The method of claim 25, wherein the digital separation filter is applied based on the pupil region.

27. The method of claim 23, further comprising determining an asymmetry between opposite diffraction orders, said asymmetry including an asymmetry of intensity, phase, or a combination of intensity and phase.

28. The method of claim 23, wherein the radiation information includes information associated with multiple pairs of opposite diffraction orders; and the method includes applying the filter to separate each of these diffraction orders.

29. The method of claim 1, further comprising performing a pupil region optimization step to optimize the region in the Fourier domain that has been removed or suppressed using the filter.

30. The method of claim 29, wherein the optimization step of the pupil region of interest comprises iterating over the following: - Reconstruct the pupil representation based on the radiation information and / or simulate the pupil representation based on mask data describing the structure on the mask; The region of interest to be evaluated is determined, and the pupil representation is filtered based on this region of interest; - Determine the filtered image represented by the filtered pupil; and The filtered image is evaluated relative to the filtered image from the previous iteration.

31. The method of claim 30, wherein determining the region of interest to be evaluated includes freely selecting regions or performing a clustering algorithm to identify regions for evaluation.

32. The method of claim 30 or 31, wherein the evaluation step comprises one or both of the following: Assess the stability of the area of ​​interest to be assessed; and Evaluate intra-crystal variations in diffraction intensity in the pupil or field plane.

33. The method of claim 29, wherein the pupil area optimization step of interest is performed at machine setup, at configuration setup, at the start of each batch, or at the start of each wafer.

34. The method of claim 29, wherein the pupil region optimization step is performed each time a triggering event occurs.

35. A method for performing a measurement on a region of a substrate including at least a portion of a target structure, the method comprising: - Receive radiation information at the image plane representing at least a portion of the dark-field diffraction radiation of the target structure and non-target structures scattered by the region. - Convert the received radiation information in the pupil plane or Fourier domain. - Using one or more filters in the pupil plane or Fourier domain to remove or suppress at least a pair of opposite higher diffraction orders within at least a portion of the radiation scattered by the region to generate image data, and - Use the image data to adjust parameters of interest in the semiconductor process.

36. The method of claim 35, wherein the radiation information relates to radiation obtained in a normal irradiation mode and a complementary irradiation mode.

37. The method according to claim 35 or 36, comprising the following steps: A digital separation filter is applied to the pupil representation reconstructed based on the radiation information, the digital separation filter being operable to separate the at least one pair of opposite diffraction orders in the pupil; and Reconstruct the pupil image corresponding to each diffraction order in the opposite diffraction order in the field plane.

38. The method of claim 37, wherein the digital separation filter is applied based on the pupil region.

39. The method of claim 35, further comprising determining an asymmetry between opposite diffraction orders, the asymmetry including an asymmetry of intensity, phase, or a combination of intensity and phase.

40. The method of claim 35, wherein the radiation information includes information associated with multiple pairs of opposite diffraction orders; and the method includes applying the filter to separate each of these diffraction orders.

41. The method of claim 35, wherein the measurement determines the characteristics of the target structure.

42. The method of claim 41, wherein the characteristic of the target structure is one of overlap, focal length, and critical size.

43. A measurement apparatus for performing measurements on a region of a substrate including at least a portion of a target structure, the measurement apparatus being configured to: - Receive radiation information at the image plane representing at least a portion of the dark-field diffraction radiation of the target structure and non-target structures scattered by the region. - Convert the received radiation information in the pupil plane or Fourier domain. - A filter is used in the pupil plane or Fourier domain to remove or suppress at least a portion of the received radiation information that is unrelated to the radiation already scattered by the target structure, in order to obtain filtered radiation information for the measurement. in, The characteristics of the filter are based on target information related to the target structure. - Convert the filtered radiation information into image data, and - Use the image data to adjust parameters of interest in the semiconductor process.

44. A measurement apparatus for performing measurement on a region of a substrate including at least a portion of a target structure, the measurement apparatus comprising a processor configured to execute instructions for performing the following steps: - Receive radiation information at the image plane representing at least a portion of the dark-field diffraction radiation of the target structure and non-target structures scattered by the region. - Convert the received radiation information in the pupil plane or Fourier domain. - A filter is used in the pupil plane or Fourier domain to remove or suppress at least a portion of the received radiation information that is unrelated to the radiation already scattered by the target structure, in order to obtain filtered radiation information for the measurement. in, The characteristics of the filter are based on target information related to the target structure. - Convert the filtered radiation information into image data, and - Use the image data to adjust parameters of interest in the semiconductor process.

45. A measurement apparatus for performing measurements on a region of a substrate including a target structure, the measurement apparatus comprising: - A source, said source being used to illuminate a region on the substrate. - A detection system for receiving, at an image plane, radiation information representing at least a portion of the dark-field diffraction radiation of the target structure and non-target structures scattered by the region. - A processor configured to convert the received radiation information in the pupil plane or Fourier domain. - The detection system includes a filter in the pupil plane or Fourier domain to remove or suppress at least a portion of the received radiation information that is unrelated to the radiation already scattered by the target structure, in order to obtain filtered radiation information for the measurement. The characteristics of the filter are based on target information related to the target structure. - The processor, configured to convert filtered radiation information into image data, and - The processor, which is configured to use the image data to adjust parameters of interest in the semiconductor process.

46. ​​A computer program product comprising instructions that, when executed at a processor, cause to perform the method according to any one of claims 1 to 42.

47. A measurement apparatus for performing measurement on a region of a substrate including at least a portion of a target structure, said measurement apparatus being configured for use with the method according to any one of claims 1 to 42.

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