Measurement method and associated measurement device

Abstract

A metrology method is disclosed. The method comprises obtaining measurement data regarding measurements made on at least one target using two or more different illumination profiles; and determining a respective value of a parameter of interest of a parameter of interest for each of the two or more different illumination profiles from the measurement data. The described method: determines a respective measurement parameter deviation value for each of the two or more different illumination profiles from the measurement data, the measurement parameter deviation value describing a deviation of a measurement parameter from a measurement parameter value of a region of interest attributable to the target or a sub-target thereof; determining a relationship between a parameter value of interest and the measured parameter deviation value for the target; and determining one or both of the corrected parameter value of interest and the preferred illumination profile according to the relationship.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to European application 22189256.5 filed on August 8, 2022, and the entire contents of this European application are incorporated herein by reference. Technical Field

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

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

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

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

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

[0008] In some metrology methods, the measured target is "overfilled"; that is, the measurement target is smaller than the measurement spot. This has benefits over more conventional "underfilled" measurements, where the measurement spot is smaller than the target. Overfilled metrology enables smaller targets, and also enables simultaneous acquisition of multiple different sub-targets or target pads. It also enables the use of algorithms that enable selection of specific target areas (e.g., pixel mapping).

[0009] However, overfill measurements are sensitive to edge effects. Edge effects often appear as lighter (or darker) areas along one or more edges of one or more of the pads; these lighter areas also affect the area of ​​interest of the sub-target being considered.

[0010] It is desirable to mitigate these edge effects. Summary of the invention

[0011] Embodiments of the invention are disclosed in the claims and in the detailed description.

[0012] In a first aspect of the present invention, a measurement method is provided, comprising: obtaining measurement data regarding measurements of at least one target using two or more different illumination profiles; determining, based on the measurement data, a corresponding parameter value of interest for a parameter of interest for each of the two or more different illumination profiles; determining, based on the measurement data, a corresponding measurement parameter deviation value for each of the two or more different illumination profiles, the measurement parameter deviation value describing a deviation of a measurement parameter relative to a measurement parameter value of a region of interest attributable to the target or a sub-target thereof; determining, for the target, a relationship between the parameter value of interest and the measurement parameter deviation value; and determining, based on the relationship, one or both of a corrected parameter value of interest and a preferred illumination profile.

[0013] According to a second aspect of the present invention there is provided a computer program comprising program instructions operable to perform the method of the first aspect when run on a suitable device.

[0014] The invention still further provides a processing arrangement and a measurement device comprising the computer program of the second aspect.

[0015] These and other aspects and advantages of the apparatus and methods disclosed herein will be apparent from a consideration of the following description of exemplary embodiments and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0017] Figure 1 A schematic overview diagram depicting a lithographic apparatus;

[0018] Figure 2 A schematic overview diagram depicting a lithography unit;

[0019] Figure 3 A schematic diagram depicting overall lithography showing the collaboration between three key technologies used to optimize semiconductor manufacturing;

[0020] Figure 4 is a schematic illustration of a scatterometry device;

[0021] Figure 5 comprising (a) a schematic diagram of a dark field scatterometer for use in measuring a target in accordance with an embodiment of the present invention using a first pair of illumination apertures, (b) details of a diffraction spectrum of a target grating for a given illumination direction, (c) a second pair of illumination apertures providing an alternative illumination mode when using the scatterometer for diffraction-based overlay measurements, and (d) a third pair of illumination apertures combining the first pair of apertures with the second pair of apertures;

[0022] Figure 6 A flow chart of a measurement method according to an embodiment is shown. DETAILED DESCRIPTION

[0023] In this document, the terms "radiation" and "beam" are used to cover all types of electromagnetic radiation, including ultraviolet radiation (e.g., having a wavelength of 365 nm, 248 nm, 193 nm, 157 nm, or 126 nm) and extreme ultraviolet radiation (EUV, e.g., having a wavelength in the range of about 5 nm to 100 nm).

[0024] The terms "mask", "mask" or "patterning device" as used herein may be broadly interpreted as referring to a general patterning device that can be used to impart a patterned cross-section to an incident radiation beam, the patterned cross-section corresponding to the pattern to be produced in a target portion of the substrate. In such a context, the term "light valve" may also be used. In addition to classical masks (transmissive or reflective, binary, phase-shifting, hybrid, etc.), other examples of such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0025] Figure 1A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA comprises an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g. UV radiation, DUV radiation or EUV radiation), a mask support (e.g. mask stage) MT configured to support a patterning device (e.g. mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to certain parameters, a substrate support (e.g. wafer stage) WT configured to hold a substrate (e.g. a wafer coated with resist) W and connected to a second positioner PW configured to accurately position the substrate support according to certain parameters, and a projection system (e.g. refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0026] In operation, the illumination system IL receives a radiation beam from a radiation source SO, for example via a beam delivery system BD. The illumination system IL may include various types of optical elements for directing, shaping and / or controlling the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical elements, or any combination thereof. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0027] The term "projection system" PS as used herein should be broadly interpreted as covering various types of projection systems suitable for the exposure radiation used and / or for other factors such as the use of immersion liquid or the use of a vacuum, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems or any combination thereof. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0028] The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid, such as water, having a relatively high refractive index, so as 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.

[0029] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also known as a "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or a step of preparing a substrate W for subsequent exposure may be performed on a substrate W located on one of the substrate supports WT while another substrate W on another substrate support WT is being used to expose a pattern on another substrate W.

[0030] In addition to the substrate support WT, the lithographic apparatus LA may also comprise a measurement platform. The measurement platform is arranged to hold sensors and / or cleaning means. The sensors may be arranged to measure properties of the projection system PS or properties of the radiation beam B. The measurement platform may hold a plurality of sensors. The cleaning means may be arranged to clean parts of the lithographic apparatus, e.g. parts of the projection system PS or parts of the system for providing immersion liquid. The measurement platform may be moved under the projection system PS when the substrate support WT is away from the projection system PS.

[0031] In operation, a radiation beam B is incident on a patterning device (e.g. a mask) MA held on a mask support MT and is patterned with a pattern (design layout) present on the patterning device MA. Having traversed the mask MA, the radiation beam B passes through a projection system PS which focuses the beam onto a target portion C of the substrate W. With the aid of a second positioner PW and a position measurement system IF, the substrate support WT can be accurately moved, for example in order to position different target portions C in a focused and aligned position in the path of the radiation beam B. Similarly, the first positioner PM and possibly another position sensor (which is not in the Figure 1 The patterning device MA (depicted explicitly in FIG. 1 ) can be used to accurately position the patterning device MA relative to the path of the radiation beam B. The mask alignment marks M1, M2 and the substrate alignment marks P1, P2 can be used to align the patterning device MA and the substrate W. Although the substrate alignment marks P1, P2 occupy dedicated target portions as illustrated, they can be located in the space between target portions. When the substrate alignment marks P1, P2 are located between target portions C, they are called scribe line alignment marks.

[0032] like Figure 2 As shown in FIG. 1 , the lithography apparatus LA may form part of a lithography cell LC (sometimes also referred to as a lithocell or (lithography) cluster), which often also includes equipment for performing pre-exposure and post-exposure processes on a substrate W. Conventionally, these equipment include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH and a bake plate BK for regulating the temperature of the substrate W (e.g. for regulating the solvent in the resist layer). A substrate transport device or robot RO picks up the substrate W from the input / output ports I / O1, I / O2, moves the substrate W between the different process equipment and transfers the substrate W to a feed station LB of the lithography apparatus LA. The devices in the lithography cell, which are also generally referred to as a track or coating and developing system, are usually under the control of a track or coating and developing system control unit TCU, which itself may be controlled by a management control system SCS, which may also control the lithography apparatus LA, for example, via a lithography control unit LACU.

[0033] In order to correctly and consistently expose a substrate W exposed by the lithographic apparatus LA, it is desirable to inspect the substrate to measure properties of the patterned structure, such as overlay errors between subsequent layers, line thickness, critical dimensions (CD), etc. For this purpose, an inspection tool (not shown) may be included in the lithography cell LC. If errors are detected, then exposure of subsequent substrates or other processing steps to be performed on the substrate W may, for example, be adjusted, particularly if the inspection is performed before other substrates W of the same batch or lot are yet to be exposed or processed.

[0034] The inspection apparatus, which may also be referred to as a metrology apparatus, is used to determine properties of the substrate W, and in particular, to determine how properties of different substrates W vary or how properties associated with different layers of the same substrate W vary from layer to layer. The inspection apparatus is alternatively configured to identify defects on the substrate W, and may for example be part of the lithography cell LC, or may be integrated into the lithography apparatus LA, or may even be a separate device. The inspection apparatus may measure properties relating to a latent image (the image in the resist layer after exposure), or a semi-latent image (the image in the resist layer after a post-exposure bake step PEB), or a developed resist image (wherein exposed or unexposed parts of the resist have been removed), or even an etched image (after a pattern transfer step such as etching).

[0035] The patterning process in the lithography apparatus LA is usually one of the most important steps in the process, which requires high accuracy in the dimensioning and placement of the structures on the substrate W. To ensure this high accuracy, the three systems can be combined in a so-called "holistic" control environment, such as Figure 3 . One of these systems is the lithography apparatus LA, which is (in practice) connected to a metrology tool MET (a second system) and to a computer system CL (a third system). The key to such an "integrated" environment is to optimize the cooperation between these three systems to enhance the overall process window and provide a tight control loop to ensure that the patterning performed by the lithography apparatus LA remains within the process window. The process window defines a range of process parameters (e.g. dose, focus, overlay) within which a particular manufacturing process produces a defined result (e.g. a functional semiconductor device) - typically within the defined result, variations in process parameters of the lithography process or patterning process are allowed.

[0036] The computer system CL can use (part of) the design layout to be patterned to predict which resolution enhancement technology to use and perform computational lithography simulations and calculations to determine which mask layout and lithography equipment settings achieve the maximum overall process window for the patterning process (in Figure 3Typically, the resolution enhancement technique is arranged to match the patterning possibilities of the lithographic apparatus LA. The computer system CL may also be used to detect where within the process window the lithographic apparatus LA is currently operating (e.g. using input from a metrology tool MET) to predict whether defects may be present (e.g. at the process window) due to suboptimal processing. Figure 3 ) is depicted by an arrow pointing to “0” in the second scale SC2.

[0037] The metrology tool MET may provide input to the computer system CL to enable accurate simulations and predictions, and may provide feedback to the lithographic apparatus LA to identify, for example, possible drifts in the calibration state of the lithographic apparatus LA (e.g., Figure 3 SC3).

[0038] During lithography, it is desirable to frequently measure the structures created, for example, for process control and verification. Various tools for making such measurements are well known, including scanning electron microscopes or various forms of metrology equipment (such as scatterometers). Examples of known scatterometers often rely on the provision of dedicated measurement targets, such as underfilled targets (targets in the form of simple gratings or superimposed gratings in different layers, which are large enough that the measurement beam produces a spot smaller than the grating) or overfilled targets (so that the illumination spot partially or completely includes the target). In addition, the use of metrology tools (e.g., angle-resolved scatterometers that illuminate underfilled targets such as gratings) allows the use of so-called reconstruction methods, in which the properties of the grating can be calculated by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulated results with the measured results. The parameters of the model are adjusted until the simulated interaction produces a diffraction pattern similar to the diffraction pattern observed from a real target.

[0039] Scatterometers are versatile instruments that allow measuring parameters of a lithographic process by having a sensor in the pupil or a plane conjugated to the pupil of the objective of the scatterometer (measurements are often referred to as pupil-based measurements), or by having a sensor in the image plane or a plane conjugated to the image plane, in which case measurements are often 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 in one image using light from soft x-rays and visible to near IR wave ranges.

[0040] Figure 4A measurement device, such as a scatterometer, is depicted in . The measurement device comprises a broadband (white light) radiation projector 2 which projects radiation 5 onto a substrate W. The reflected or scattered radiation 10 is passed to a spectrometer detector 4 which measures the spectrum 6 of the specularly reflected radiation 10 (i.e. a measurement of the intensity I as a function of the wavelength λ). From such data, the structure or profile 8 which gave rise to the detected spectrum can be reconstructed by a processing unit PU, for example by rigorous coupled wave analysis and nonlinear regression or by comparison with a library of simulated spectra. Typically, for the reconstruction, the general form of the structure is well known and some parameters are assumed from knowledge of the process used to manufacture the structure, leaving only a few parameters of the structure to be determined from the scatterometry data. Such a scatterometer can be configured as a normal incidence scatterometer or an oblique incidence scatterometer.

[0041] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, reconstruction methods can be applied to the measured signals to reconstruct or calculate the properties of the grating. Such a reconstruction can, for example, result from simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measured results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to the diffraction pattern observed from a real target.

[0042] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, radiation emitted by a radiation source is directed onto a target and reflected or scattered radiation from the target is directed onto a spectrometer detector which measures the spectrum of the specularly reflected radiation (i.e. a measurement of intensity as a function of wavelength). From such data the structure or profile of the target producing the detected spectrum can be reconstructed, for example by rigorous coupled wave analysis and nonlinear regression or by comparison with a library of simulated spectra.

[0043] In a third embodiment, the scatterometer MT is an ellipsometric scatterometer. An ellipsometric scatterometer allows the parameters of a lithography process to be determined by measuring the scattered radiation for each polarization state. Such a metrology device emits polarized light (such as linear, annular or elliptical) by using, for example, an appropriate polarization filter in the illumination section of the metrology device. A source suitable for the metrology device can also provide polarized radiation. Various embodiments of existing ellipsometric scatterometers are described in U.S. patent applications 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,449, 12 / 920,968, 12 / 922,587, 13 / 000,229, 13 / 033,135, 13 / 533,110 and 13 / 891,410, which are incorporated herein by reference in their entirety.

[0044] In one embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the overlap of two misaligned gratings or periodic structures by measuring the reflection spectrum and / or an asymmetry in the detection configuration (the asymmetry being related to the extent of the overlap). The two (usually superimposed) grating structures may be applied in two different layers (not necessarily consecutive layers) and may be formed to be in approximately the same position on the wafer. The scatterometer may have a symmetrical detection configuration as described, for example, in the commonly owned patent application EP1,628,164A so that any asymmetry can be clearly identified. This provides a straightforward way to measure misalignment in the gratings. Further examples of the measurement of the overlay error between two layers comprising a periodic structure as a target via the asymmetry of the periodic structure may be found in PCT patent application publication number WO2011 / 012624 or in US patent application number US20160161863, which are incorporated herein by reference in their entirety. Other parameters of interest may be focus and dose. Focus and dose may be determined simultaneously by scatterometry as described in US Patent Application US2011-0249244, which is incorporated herein by reference in its entirety (or alternatively by scanning electron microscopy).

[0045] The target can be measured in an underfill mode or in an overfill mode. In the underfill mode, the measurement beam produces a spot that is smaller than the overall target. In the overfill mode, the measurement beam produces a spot that is larger than the overall target. In such an overfill mode, it is also possible to measure different targets simultaneously and thus to determine different processing parameters simultaneously.

[0046] The overall measurement quality of a lithography parameter performed using a specific target is determined at least in part by a measurement profile for measuring such lithography parameters. The term "substrate measurement profile" may include one or more parameters of the measurement itself, one or more parameters of one or more patterns measured, or both. For example, if the measurement used in the substrate measurement profile is a diffraction-based optical measurement, one or more of the measured parameters may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the direction of the radiation relative to the pattern on the substrate, and the like. One of the criteria for selecting a measurement profile may, for example, be the sensitivity of one of the measurement parameters to process variations. More examples are described in U.S. patent application US2016-0161863, which is incorporated herein by reference in its entirety, and published U.S. patent application US 2016 / 0370717A1.

[0047] Figure 5 (a) An embodiment of a measurement apparatus and more particularly a dark field scatterometer is presented. Figure 5(b) illustrates in more detail the target T and the diffracted rays of the measuring radiation used to illuminate the target. The illustrated metrology apparatus is of a type known as a dark field metrology apparatus. The metrology apparatus may be a separate device or incorporated in a lithography apparatus LA or a lithography cell LC at, for example, a measurement station. An optical axis having several branches running through the apparatus is indicated by a dotted line O. In such an apparatus, light emitted by a source 11 (e.g. a xenon lamp) is directed onto a substrate W via a beam splitter 15 by an optical system comprising lenses 12, 14 and an objective lens 16. These lenses are arranged in the form of a double sequence 4F arrangement. Different lens arrangements may be used, provided that the lens arrangement still provides an image of the substrate onto the detector and simultaneously allows access to an intermediate pupil plane for spatial frequency filtering. Thus, the angular range over which the radiation is incident on the substrate may be selected by defining the spatial intensity distribution in a plane presenting the spatial spectrum of the substrate plane (in this so-called (conjugate) pupil plane). In particular, this selection can be made by inserting an aperture plate 13 of suitable form between lenses 12 and 14 in a plane that is a back-projected image of the objective pupil plane. In the illustrated example, the aperture plate 13 has different forms (labeled 13N and 13S), allowing different illumination modes to be selected. The illumination system in this example forms an off-axis illumination mode. In a first illumination mode, the aperture plate 13N provides off-axis from a direction designated as "North" for descriptive purposes only. In a second illumination mode, the aperture plate 13S is used to provide similar illumination, but from the opposite direction labeled "South". Other illumination modes are possible by using different apertures. The rest of the pupil plane is desirably dark, because any unnecessary light outside the desired illumination mode will interfere with the desired measurement signal.

[0048] like Figure 5 As shown in (b), the target T is placed with the substrate W perpendicular to the optical axis O of the objective lens 16. The substrate W may be supported by a support (not shown). The measurement radiation ray I impinging on the target T at an angle to the axis O produces a zero-order ray (solid line 0) and two first-order rays (dash-dotted line +1 and dash-dotted line -1). It should be remembered that in the case of a small overfilled target, these rays are only one of many parallel rays covering the substrate area including the measurement target T and other features. Since the hole in the plate 13 has a finite width (required to receive a useful amount of light), the incident ray I may actually occupy an angular range, and the diffracted rays 0 and +1 / -1 will be slightly spread out. According to the point spread function of the small target, each order +1 and -1 will be further spread out over the angular range, rather than a single ideal ray as shown. It should be noted that the grating spacing and the illumination angle of the target can be designed or adjusted so that the first-order rays entering the objective are closely aligned with the central optical axis. Figure 5 (a) and Figure 3The rays illustrated in (b) are shown slightly off-axis, simply to enable them to be more easily distinguished in the illustration.

[0049] At least the 0th and +1st orders diffracted by the target T on the substrate W are collected by the objective lens 16 and directed back via the beam splitter 15. Figure 5 (a) illustrates both the first illumination mode and the second illumination mode by indicating diametrically opposed apertures labeled North (N) and South (S). When the incident ray I of the measurement radiation comes from the north side of the optical axis, that is, when the first illumination mode is applied using the aperture plate 13N, the +1 diffracted ray labeled +1 (N) enters the objective lens 16. In contrast, when the second illumination mode is applied using the aperture plate 13S, the -1 diffracted ray (labeled 1 (S)) is the diffracted ray that enters the lens 16.

[0050] The second beam splitter 17 divides the diffracted beam into two measurement branches. In the first measurement branch, the optical system 18 forms a diffraction spectrum (pupil plane image) of the target on a first sensor 19 (e.g., a CCD or CMOS sensor) using the zero-order diffracted beam and the first-order diffracted beam. Each diffraction order hits a different point on the sensor so that image processing can compare and contrast the orders. The pupil plane image captured by the sensor 19 can be used to focus the metrology device and / or normalize the intensity measurement of the first-order beam. The pupil plane image can also be used for many measurement purposes such as reconstruction.

[0051] In the second measurement branch, the optical system 20, 22 forms an image of the target T on a sensor 23, such as a CCD or CMOS sensor. In the second measurement branch, an aperture stop 21 is provided in a plane conjugate to the pupil plane. The aperture stop 21 serves to block the zero-order diffraction beam so that the image of the target formed on the sensor 23 is formed by the -1 or +1 order beam. The image captured by the sensors 19 and 23 is output to a processor PU that processes the image, the function of which will depend on the specific type of measurement being performed. It should be noted that the term "image" is used in a broad sense. Thus, if only one of the -1 order and the +1 order is present, no image of the grating lines will be formed.

[0052] Figure 5 The specific forms of aperture plate 13 and field stop 21 shown in are examples only. In another embodiment of the invention, coaxial illumination of the target is used, and an aperture stop with an off-axis hole is used to pass substantially only one first-order diffracted light to the sensor. In still other embodiments, instead of or in addition to the first-order beam, second-order beams, third-order beams and higher-order beams ( Figure 5 not shown).

[0053] To allow the measurement radiation to be adapted for these different types of measurements, the aperture plate 13 may include a number of aperture patterns formed around a disk that is rotated to bring the desired pattern into position. It should be noted that the aperture plates 13N or 13S may only be used to measure gratings oriented in one direction (X or Y depending on the setting). To measure orthogonal gratings, the target may be rotated 90° and 270°. Figure 5 (c) and Figure 5 Different orifice plates are shown in (d). The use of these orifice plates as well as many other variations and applications of the apparatus are described in the previously published applications mentioned above.

[0054] As an alternative to a scatterometer, the measurement device may include a holographic microscope, such as a digital holographic microscope or a digital dark field holographic microscope. Such a device is disclosed in, for example, WO2021121733A1, which is incorporated herein by reference.

[0055] You can use Figure 5 A known metrology method performed by the metrology tool illustrated in (a) is known as diffraction-based overlay (DBO) or micro-diffraction-based overlay (µDBO). Such µDBO techniques use the imaging branch of the metrology tool (the branch through the detector 23) and determine the structural asymmetry based on the intensity or diffraction efficiency asymmetry or intensity or diffraction efficiency difference between the first diffraction order and the second diffraction order of a pair of complementary pairs of diffraction orders (usually the first diffraction order of the complementary pair, i.e., the first diffraction order may include the +1 order and the second diffraction order may include the -1 order, as illustrated in FIG. 5(b)). Thus, in this context, the terms "first" and "second" do not refer to the diffraction order and are only used to distinguish between the two diffraction orders of the complementary pair, noting that the first diffraction order and the second diffraction order may be the +2 and -2 diffraction orders, or a higher complementary pair of diffraction orders. The zeroth order (specular radiation) is usually blocked or shunted, i.e. diverted, elsewhere (e.g. to another part of the detector for monitoring purposes); it is not used in the µDBO measurement. The main "image" for the inference of the parameter of interest is formed only by the higher (e.g. first) diffraction orders. The asymmetry of the structure can be used to infer parameters of interest, such as overlay or focal length, depending on the target design.

[0056] The measurement parameter (e.g., intensity, amplitude, or diffraction efficiency) can be determined from the captured µDBO camera image by finding the target position and integrating a region of interest (ROI) within the captured µDBO camera image, for example, to provide a single value of the measurement parameter for each sub-target. However, the measurement image may show that at one or more edges of one or more sub-targets of the imaged target, there is a considerable deviation of the measurement parameter from the average value within the region of interest. This measurement parameter deviation may be referred to as an edge effect. In many cases, it manifests as a region with higher intensity (or related parameter) at the edge of the target, but it may also manifest as a region with lower intensity. In either case, this edge effect affects the intensity / measurement parameter within the region of interest and therefore affects the parameter of interest (e.g., overlap) inferred from the measurement parameter. As targets become smaller (e.g., 5µm square and smaller), the edge effect becomes more significant and more difficult to handle using known methods. In addition, edge effects from the environment may also cause problems.

[0057] Therefore, in order to solve the problem of edge effect, a method is proposed, comprising: obtaining measurement data about measurement of a target using two or more different illumination profiles; determining a corresponding parameter value of interest for the parameter of interest for each of the two or more different illumination profiles; determining a corresponding measurement parameter deviation value for each of the two or more different illumination profiles, the measurement parameter deviation value describing a (e.g., maximum or minimum (extreme value)) deviation of the measurement parameter relative to the measurement parameter value of the region of interest attributed to the target; determining a relationship between the parameter value of interest and the measurement parameter deviation value for the target; and determining one or both of a corrected parameter value of interest and a preferred illumination profile based on the relationship.

[0058] The method may comprise determining the preferred illumination profile as an illumination profile that minimizes deviations in the measurement parameter of the target.

[0059] The step of determining the relationship may comprise fitting a (eg linear) model relating the parameter value of interest to the measured parameter deviation values.

[0060] The measurement parameter deviation value may be determined based on a region of the detected measurement image outside the region of interest, for example at one or more edges of each object or sub-object.

[0061] The method may include determining a corrected parameter value of interest as a parameter value of interest corresponding to a measured parameter bias value of zero based on the relationship.

[0062] The target may include a pair of sub-targets for each of one or more measurement directions (wherein at least one of the pair has an intentional, i.e., deliberate bias such that there is a bias difference between the sub-target pairs), wherein the parameter of interest is determined based on the pair of sub-targets. As is known, each pair of sub-targets may include biases of equal magnitude and opposite directions. In the case where the target includes sub-targets in each measurement direction, the method may be performed in each measurement direction (e.g., where the measurement direction relates to one of the two perpendicular directions of the substrate plane, the sub-targets are typically referred to as X and Y sub-targets). In the case where the target includes two or more sub-targets, the measurement parameter deviation value of each target may optionally include the maximum magnitude measurement parameter deviation value on different sub-targets. Another alternative may include using the average value of the measurement parameter deviation values ​​on the sub-targets. In this way, the sub-target used to determine the actual measurement parameter deviation is unimportant.

[0063] The method may be performed online for each target; for example, each target may be illuminated using the two or more different illumination profiles to obtain measurement data, and a corrected and / or preferred illumination profile may be determined for each target.

[0064] The number of illumination profiles used per target may be, for example, two, more than two, more than three, or more than five.

[0065] Different illumination profiles may include different illumination shapes in the illumination pupil plane, ie corresponding to different ranges or groups of illumination angles.

[0066] Figure 6 is a flow chart illustrating a method according to an embodiment. Figure 6 (a) shows a number of merely exemplary illumination profiles 600 to 630 or illumination apertures that may be used for the methods disclosed herein. The number of different illumination profiles used and / or their shapes may differ from those illustrated. Thus, the size, shape within the illumination pupil plane and / or position of the shape of any of the illumination profiles 600 to 630 may differ significantly.

[0067] Figure 6(b) shows four plots 635 to 650 of a measured parameter (e.g., intensity) versus target or sub-target position (herein in one dimension) for four irradiation profiles 600, 605, 620, 630. In each case, a representative (e.g., average) measured parameter value is determined over a region of interest ROI, within which there are only small variations in the measured parameter. Also shown is a measured parameter deviation value PD, which is the difference between the maximum or minimum measured parameter value within the ROI and the average value, or the maximum or minimum measured parameter value within the ROI divided by the average value. Such a step of determining a measured parameter deviation value PD can be performed for all irradiation profiles 600 to 630.

[0068] The method may further comprise determining a measure of a value of interest (e.g., an overlap value) for the measurement data associated with each of the illumination profiles 600 to 630, respectively. This may be done in any known manner for calculating a parameter of interest from the measurement data (e.g., intensity data), such as from an intensity / measurement parameter asymmetry or an intensity / measurement parameter difference for a pair of complementary diffraction orders (e.g., +1 and -1 orders).

[0069] Since a single overlap value can be determined from two images (e.g. from +1 order and -1 order), the measurement parameter deviation value can be determined from either image (or for example an average measurement parameter deviation value can be determined from two images). In an embodiment, the measurement parameter deviation value with the largest magnitude can be selected. Such an approach can also be used when the overlap value is determined from four images (e.g. from +1 order and -1 order from two differently offset sub-targets), for example the maximum magnitude of the measurement parameter deviation values ​​from the four images can be selected. However, this is not necessary.

[0070] Figure 6 (c) is a plot of parameter deviation of interest PD versus inferred parameter of interest (e.g., overlap) OV. The plot includes (in this particular example) seven points, each corresponding to a respective one of the illumination profiles 600 to 630. A linear regression or model 655 can be fit to such data. From such model 655, an overlap (or other parameter of interest) value OVcor corresponding to a zero parameter deviation PD of interest can be determined and used as the corrected parameter / overlap value of interest.

[0071] Alternatively or additionally, the model 655 may be used to identify a preferred illumination profile for a parameter deviation of interest, such as an illumination profile corresponding to, very close to, or closest to zero.

[0072] The concepts disclosed herein are disclosed in the context of overlay measurements. However, the methods are not limited thereto. For example, a target may be formed with a focus-based asymmetry (i.e., an asymmetry that depends on the actual scanner focus used to expose the target). The methods disclosed herein may also be applied to focus measurements based on these targets (e.g., diffraction-based focus DBF or micro-diffraction-based focus µDBF), in which case the determined model will have measurement parameter deviations relative to the inferred focus. Similarly, other target types and measurement techniques may be used, such as continuous diffraction-based overlay (cDBO) targets, and corresponding cDBO measurement techniques.

[0073] In order to measure each target with a different illumination profile, an illumination mode selector (IMS) may be used. IMS is a known illumination selection method, wherein different fixed apertures are arranged on an aperture wheel such that the fixed apertures can be selectively switched or rotated into the illumination beam path as required. Faster switching between illumination profiles may be obtained via other illumination techniques, for example using programmable illumination, such as using grating light valve (GLV) technology such as sold by Silicon Light Machines (SLM) as described, for example, in US6947613B or using, for example, any suitable spatial light modulation technique (e.g. implemented in the illumination pupil plane). For example, fast switching between different illumination profiles (and optionally different colors) may be beneficial when performing the proposed method online. In this way, different illuminations may be implemented in a certain number of different ways. The illumination profiles may be switched at the illumination pupil plane (e.g., at the illumination pupil plane) in a manner similar to that of the illumination profiles of FIG. 1 . Figure 5 The illumination profile is defined or imposed by the plane 13 illustrated in (a).

[0074] Further embodiments according to the invention are described in the following numbered aspects:

[0075] 1. A measurement method, comprising:

[0076] obtaining measurement data regarding measurements of at least one target using two or more different illumination profiles;

[0077] determining a respective parameter value of interest for the parameter of interest for each of the two or more different illumination profiles based on the measurement data;

[0078] determining, based on the measurement data, a respective measurement parameter deviation value for each of the two or more different illumination profiles, the measurement parameter deviation value describing a deviation of a measurement parameter relative to a measurement parameter value attributed to a region of interest of the target or a sub-target thereof;

[0079] determining a relationship between a parameter value of interest and a deviation value of the measured parameter for the target; and

[0080] One or both of a corrected parameter value of interest and a preferred illumination profile are determined based on the relationship.

[0081] 2. A method according to clause 1, comprising determining the preferred illumination profile as the illumination profile corresponding to the lowest measured parameter deviation of the target.

[0082] 3. A method according to aspect 1 or 2, wherein the step of determining the relationship includes fitting a model that relates the parameter value of interest to the measured parameter deviation value.

[0083] 4. A method according to aspect 3, wherein the model comprises a linear model.

[0084] 5. A method according to any preceding aspect, wherein the measurement parameter deviation value describes an extreme deviation of a measurement parameter relative to a measurement parameter value of a region of interest attributed to the target or a sub-target thereof.

[0085] 6. A method according to any preceding aspect, comprising determining a corrected parameter value of interest as a parameter value of interest corresponding to a zero measured parameter bias value based on the relationship.

[0086] 7. A method according to any preceding aspect, wherein the measurement parameter deviation value is determined based on a region of the detected measurement image outside the region of interest.

[0087] 8. A method according to any preceding aspect, wherein the measurement parameter deviation value is determined based on one or more edges of the target or one or more sub-targets thereof.

[0088] 9. A method according to any preceding aspect, wherein the target comprises one or more sub-targets in each measuring direction, and the method is performed in each measuring direction to obtain a corrected parameter value of interest and / or a preferred illumination profile in each measuring direction.

[0089] 10. A method according to any preceding clause, wherein the number of illumination profiles used per target is greater than three.

[0090] 11. A method according to any preceding clause, wherein the number of illumination profiles used per target is greater than five.

[0091] 12. A method according to any preceding aspect, wherein the measured parameter value of the region of interest attributed to the target or a sub-target thereof comprises an average measured parameter value of the region of interest.

[0092] 13. A method according to any preceding aspect, wherein the measured parameter is intensity, diffraction efficiency or amplitude.

[0093] 14. A method according to any preceding aspect, wherein the parameter of interest is overlay or focal length.

[0094] 15. A method according to any preceding aspect, wherein the measurement parameter deviation of the target comprises measurement parameter deviation values ​​having maximum magnitudes at different sub-targets throughout the target and / or different diffraction orders scattered by the target.

[0095] 16. A method according to any preceding aspect, comprising measuring the at least one target to obtain the measurement data.

[0096] 17. A method according to clause 16, comprising performing the method in-line as part of a lithography process.

[0097] 18. The method according to aspect 17 also includes: exposing the at least one target onto a substrate; performing the measuring step; and using the corrected parameter value of interest or the parameter value of interest corresponding to the preferred illumination profile when correcting a subsequent exposure step on a subsequent substrate.

[0098] 19. A computer program comprising program instructions operable to perform the method according to any one of aspects 1 to 12 when run on a suitable device.

[0099] 20. A non-transitory computer program carrier comprising a computer program according to clause 19.

[0100] 21. A processing arrangement comprising:

[0101] a computer program carrier comprising a computer program according to aspect 20; and

[0102] A processor is operable to execute the computer program.

[0103] 22. A metrology device comprising a processing arrangement according to clause 21.

[0104] 23. A metrology device according to clause 22, comprising: imaging optics for capturing scattered radiation from the target; and a detector for detecting scattered radiation to obtain a measured image of the target.

[0105] 24. The metrology device according to aspect 22 or 23, the metrology device being a scatterometer.

[0106] 25. The metrology device according to aspect 22 or 23, wherein the metrology device is a dark-field holographic microscope.

[0107] 26. The metrology device according to any one of aspects 22 to 25, the metrology device being operable to perform the method according to aspect 16 or 17.

[0108] 27. A lithocell comprising: a metrology apparatus according to any one of clauses 22 to 26; and a lithographic apparatus.

[0109] 28. A lithocell according to clause 27, operable to perform a method according to clause 18.

[0110] Although specific reference may be made herein to the use of lithographic apparatus in IC manufacturing, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection modes for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0111] Although specific reference may be made herein to embodiments of the invention in the context of inspection or metrology equipment, embodiments of the invention may be used in other equipment. Embodiments of the invention may form part of a mask inspection equipment, a lithography equipment, or any equipment that measures or processes an object such as a wafer (or other substrate) or a mask (or other pattern forming device). The term "metrology equipment" may also refer to an inspection equipment or a detection system. For example, an inspection equipment including embodiments of the invention may be used to detect defects in a substrate or defects in a structure on a substrate. In such embodiments, the characteristic of interest of a structure on a substrate may be related to a defect in the structure, the absence of a particular portion of the structure, or the presence of an unwanted structure on the substrate.

[0112] Although specific reference is made to "measurement equipment / tools / systems" or "inspection equipment / tools / systems," these terms may refer to the same or similar types of tools, equipment, or systems. For example, an inspection or measurement equipment including embodiments of the present invention may be used to determine characteristics of structures on a substrate or on a wafer. For example, an inspection equipment or measurement equipment including embodiments of the present invention may be used to detect defects in a substrate or defects in a structure on a substrate or on a wafer. In such embodiments, the characteristic of interest of a structure on a substrate may relate to a defect in the structure, the absence of a particular portion of a structure, or the presence of an unwanted structure on a substrate or on a wafer.

[0113] Although the above may have made specific reference to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention is not limited to optical lithography and may be used in other applications, such as imprint lithography, where the context permits.

[0114] Although the targets or target structures (more generally, structures on a substrate) described above are measurement target structures specifically designed and formed for the purpose of measurement, in other embodiments, the properties of interest may be measured for one or more structures that are functional parts of a device formed on a substrate. Many devices have regular grating-like structures. The terms structure, target grating, and target structure as used herein do not require that the structure be provided specifically for the measurement being performed. In addition, the pitch P of the measurement target may be close to the resolution limit of the optical system of the scatterometer or may be smaller, but may be much larger than the size of a typical product feature made by a lithography process in the target portion C. Instead, the lines and / or spaces of the overlapping gratings within the target structure may be manufactured to include smaller structures similar in size to the product features.

[0115] Although specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be practiced in other ways than those described. The above description is intended to be illustrative, not restrictive. Thus, it will be appreciated by those skilled in the art that modifications may be made to the described invention without departing from the scope of the claims set forth below.

Claims

1. A measurement method, comprising: obtaining measurement data regarding measurements of at least one target using two or more different illumination profiles; determining a respective parameter value of interest for the parameter of interest for each of the two or more different illumination profiles based on the measurement data, determining, based on the measurement data, a respective measurement parameter deviation value for each of the two or more different illumination profiles, the measurement parameter deviation value describing a deviation of a measurement parameter relative to a measurement parameter value attributed to a region of interest of the target or a sub-target thereof; Determining, for the target, a relationship between a parameter value of interest and a deviation value of the measured parameter; as well as One or both of a corrected parameter value of interest and a preferred illumination profile are determined based on the relationship. 2 . The method of claim 1 , comprising determining the preferred illumination profile as the illumination profile corresponding to the lowest measured parameter deviation of the target.

3. The method according to claim 1 or 2, wherein: The step of determining the relationship includes fitting a model that relates the parameter values ​​of interest to the measured parameter deviation values.

4. The method according to claim 3, wherein: The model comprises a linear model.

5. A method according to any preceding claim, wherein: The measured parameter deviation value describes an extreme deviation of a measured parameter relative to a measured parameter value of a region of interest attributed to the target or a sub-target thereof.

6. A method according to any preceding claim, comprising determining a corrected parameter value of interest as the parameter value of interest corresponding to a measured parameter bias value of zero in accordance with the relationship.

7. A method according to any preceding claim, wherein: The measurement parameter deviation value is determined based on a region of the detected measurement image outside the region of interest.

8. A method according to any preceding claim, wherein: The measurement parameter deviation value is determined based on one or more edges of the target or one or more sub-targets thereof.

9. A method according to any preceding claim, wherein: The target comprises one or more sub-targets in each measuring direction, and the method is performed in each measuring direction to obtain a corrected parameter value of interest and / or a preferred illumination profile in each measuring direction.

10. A method according to any preceding claim, wherein: The number of illumination profiles used per target is greater than three.

11. A method according to any preceding claim, wherein: The measured parameter value of the region of interest attributed to the target or a sub-target thereof comprises an average measured parameter value of the region of interest.

12. A computer program comprising program instructions operable, when run on a suitable device, to perform the method according to any one of claims 1 to 11.

13. A processing arrangement comprising: A computer program carrier comprising a computer program according to claim 12; and A processor is operable to execute the computer program.

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