Metrology method and related metrology device

Abstract

A metrology method is disclosed that includes obtaining measurement data for measurements of at least one target with two or more different illumination profiles, and obtaining a respective parameter-of-interest value for a parameter of interest for each of the two or more different illumination profiles. The method described includes determining, from the measurement data, a respective measured parameter deviation value for each of the two or more different illumination profiles, the measured parameter deviation value representing a deviation of the measured parameter from a measured parameter value attributable to a region of interest of the target or a sub-target thereof, determining, for the target, a relationship between the parameter-of-interest value and the measured parameter deviation value, and determining, from the relationship, one or both of a modified parameter-of-interest value and a preferred illumination profile.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to European Application No. 22189256.5, filed August 8, 2022, which is incorporated herein by reference in its entirety.

[0002] The present invention relates to metrology methods and devices that can be used, for example, to determine features of structures on a substrate. [Background technology]

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus can, for example, project a pattern (often referred to as a "design layout" or "design") in a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).

[0004] To project a pattern onto a substrate, a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features 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. Lithographic apparatus using extreme ultraviolet (EUV) radiation having a wavelength in the range of 4 nm to 20 nm, for example 6.7 nm or 13.5 nm, can be used to form smaller features on a substrate than lithographic apparatus using radiation with a wavelength of, for example, 193 nm.

[0005] Low k1 lithography can be used to process features with dimensions smaller than the typical resolution limit of a lithographic apparatus. In such processes, the resolution equation 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 to be printed, in this case half-pitch), and k1 is an empirical resolution factor. Generally, the smaller k1, the more difficult it is to reproduce on a substrate a pattern resembling the shape and dimensions intended by a circuit designer to achieve a particular electrical function and performance. To overcome these difficulties, sophisticated fine-tuning processes can be applied to the lithographic projection apparatus and / or the design layout. These include, but are not limited to, various optimizations of the design layout such as NA optimization, customized illumination schemes, use of phase-shifting patterning devices, optical proximity correction (OPC, sometimes also called "optical and process correction") in the design layout, or other methods commonly defined as "resolution enhancement techniques" (RET). Alternatively, tight control loops may be used to control the stability of the lithography apparatus to improve pattern repeatability at low k1.

[0006]

[0006] In lithographic processes, it is frequently desirable to make measurements of the structures produced, for example, for process control and verification. Various tools are known for making such measurements, including, for example, scanning electron microscopes and various forms of metrology tools (e.g., scatterometers, etc.). General terms for such tools can be metrology tools or inspection tools.

[0007] In some metrology methods, the measurement object is "overfilled," i.e., the metrology object is smaller than the measurement spot. This has advantages compared to more common "underfilled" measurements, where the measurement spot is smaller than the target. Overfilled metrology allows for smaller targets and also makes it possible to simultaneously acquire many different sub-targets or target pads. It also makes it possible to use algorithms (such as pixel mapping) that allow for the selection of specific target areas.

[0008] However, overfilled metrology is susceptible to edge effects, which often manifest as brighter (or darker) areas along one or more edges of one or more pads, and these brighter areas also affect the region of interest of the sub-target under consideration.

[0009]

[0009] It is desirable to reduce such edge effects. Summary of the Invention

[0010]

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

[0011]

[0011] In a first aspect of the present invention, there is provided a metrology method comprising obtaining measurement data for measurement of at least one target using two or more different illumination profiles, determining from the measurement data a parameter of interest value for a parameter of interest for each of the two or more different illumination profiles, determining from the measurement data a measurement parameter deviation value for each of the two or more different illumination profiles, the measurement parameter deviation value representing the deviation of the measurement parameter from the measurement parameter value attributable to a region of interest of the target or its sub-target, determining a relationship between the parameter of interest value and the measurement parameter deviation value for the target, and determining from the relationship one or both of a corrected parameter of interest value and a preferred illumination profile.

[0012] 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 executed on a suitable device.

[0013] The present invention further provides a processing arrangement and metrology device comprising the computer program of the second aspect.

[0014]

[0014] These and other aspects and advantages of the devices and methods disclosed in this specification will be understood by considering the following description of exemplary embodiments and drawings of those embodiments. [Brief explanation of the drawings]

[0015]

[0015] Some embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which: [Figure 1] 1 depicts a schematic diagram of a lithographic apparatus; [Figure 2] 1 shows a schematic diagram of a lithographic cell. [Figure 3]1 shows a schematic representation of holistic lithography, illustrating the collaboration between three major technologies to optimize semiconductor manufacturing. [Figure 4] FIG. 1 is a schematic diagram of a scatterometry apparatus; [Figure 5] (a) is a schematic diagram of a dark field scatterometer used in performing target measurements according to an embodiment of the present invention using a first illumination aperture pair; (b) is a detailed view of the diffraction spectrum of the target grating for illumination in any direction; (c) shows a second illumination aperture pair that provides another illumination mode when using the scatterometer for diffraction-based overlay measurements; and (d) shows a third illumination aperture pair that combines the first and second aperture pairs. [Figure 6] FIG. 1 is a flow diagram of a metrology method according to one embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0016]

[0016] In this specification, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g., having a wavelength of 365 nm, 248 nm, 193 nm, 157 nm, or 126 nm) and EUV (extreme ultraviolet radiation, e.g., having a wavelength in the range of about 5 to 100 nm).

[0017]

[0017] As used herein, the terms "reticle," "mask," or "patterning device" 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, in accordance with the pattern to be created in a target portion of a substrate. The term "light valve" may also be used in this context. Besides typical masks (transmissive or reflective; binary, phase-shifting, hybrid, etc.), other examples of such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0018] 1 schematically depicts a lithographic apparatus LA comprising: an illumination system IL (also referred to as an illuminator) configured to condition a radiation beam B (e.g. UV, DUV or EUV radiation), a mask support (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and coupled to a first positioner PM configured to accurately position the patterning device MA according to certain parameters, a substrate support (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and coupled to a second positioner PW configured to accurately position the substrate support according to certain parameters, and a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0019]

[0019] In operation, the illumination system IL receives a radiation beam from the radiation source SO, for example via the beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical components, or any combination thereof, for directing, shaping and / or controlling the radiation. The illuminator IL is used to condition the radiation beam B so that it has a desired spatial and angular intensity distribution in its cross-section in the plane of the patterning device MA.

[0020]

[0020] The term "projection system" PS as used herein should be interpreted broadly to encompass various types of projection systems, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used and / or other factors such as the use of an immersion liquid or a vacuum. Any use of the term "projection lens" herein should be considered as synonymous with the more general term "projection system" PS.

[0021]

[0021] The lithographic apparatus LA may be of a type in which at least a portion of the substrate is covered by a liquid having a relatively high refractive index (e.g. water) so as to fill a space between the projection system PS and the substrate W, which is also known as immersion lithography. Further information about immersion techniques can be found in US Patent No. 6,952,253, which is incorporated herein by reference.

[0022] The lithographic apparatus LA may be of a type that includes two or more substrate supports WT (also known as "dual stage" machines). In such a "multi-stage" machine, the substrate supports WT can be used in parallel, and / or a substrate W placed on one substrate support WT can be prepared for a next exposure of that substrate W while another substrate W on another substrate support WT is used to expose a pattern onto that other substrate W.

[0023] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement stage. The measurement stage is arranged to hold a sensor and / or a cleaning device. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold multiple sensors. The cleaning device may be arranged to clean part of the lithographic apparatus, for example part of the projection system PS or part of the system for supplying immersion liquid. The measurement stage may be moved below the projection system PS when the substrate support WT is spaced apart from the projection system PS.

[0024] In operation, a radiation beam B is incident on a patterning device (e.g., mask) MA, which is held on a mask support MT, and is patterned according to a pattern (design layout) present on the patterning device MA. Having passed through the mask MA, the radiation beam B passes through a projection system PS, which focuses the beam onto a target portion C of a substrate W. The substrate support WT can be accurately moved using the second positioner PW and the position measurement system IF to position different target portions C, for example, at aligned and focused positions in the path of the radiation beam B. Similarly, the patterning device MA can be accurately positioned relative to the path of the radiation beam B using the first positioner PM and possibly further position sensors (not explicitly shown in Figure 1). The patterning device MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks P1, P2 as illustrated occupy dedicated target portions, they may be located in spaces between the target portions. When the substrate alignment marks P1, P2 are located between target portions C, they are known as scribe-lane alignment marks.

[0025] As shown in Figure 2, the lithographic apparatus LA may form part of a lithographic cell LC, also called a lithocell or (litho)cluster, which often also includes apparatus for performing pre-exposure and post-exposure processes on a substrate W. Conventionally, these apparatus include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, and chill plates CH and bake plates BK for adjusting the temperature of the substrate W, e.g., the solvent in the resist layer. A substrate handler or robot RO picks up the substrate W from input / output ports I / O1, I / O2, moves it between the different process tools, and delivers it to a loading bay LB of the lithographic apparatus LA. These devices of the lithocell are often collectively referred to as a track and are typically under the control of a track control unit TCU, which itself may be controlled by a supervisory control system SCS, which may also control the lithographic apparatus LA, e.g., via a lithography control unit LACU.

[0026] To ensure that substrates W exposed by lithographic apparatus LA are exposed accurately and consistently, it is desirable to inspect the substrates and measure properties of the patterned structures, such as overlay error between subsequent layers, linewidth, critical dimension (CD), etc. For this purpose, the lithocell LC may include an inspection tool (not shown). If an error is detected, particularly if the inspection is performed at a stage when other substrates W of the same batch or lot have not yet been exposed or processed, adjustments may be made, for example, to the exposure of subsequent substrates or to other processing steps performed on that substrate W.

[0027] Inspection apparatus, sometimes referred to as metrology apparatus, are used to determine the properties of substrates W, in particular how the properties of different substrates W vary, or how properties associated with different layers of the same substrate W vary from layer to layer. Inspection apparatus may alternatively be constructed to identify defects on substrates W, and such inspection apparatus may, for example, be part of a lithocell LC, integrated into a lithography apparatus LA, or be a stand-alone device. The inspection apparatus may measure properties related to a latent image (an image in a resist layer after exposure), or a semi-latent image (an image in a resist layer after a post-exposure bake step PEB), or a developed resist image (from which exposed or unexposed parts of the resist have been removed), or even an etched image (an image after a pattern transfer step such as etching).

[0028] Typically, the patterning process in a lithography apparatus LA is one of the most critical steps in processing, requiring high accuracy in the dimensioning and placement of structures on a substrate W. To ensure this high accuracy, three systems may be integrated into a so-called "holistic" control environment, as shown schematically in FIG. 3. One of these systems is the lithography apparatus LA, which is (virtually) connected to a metrology tool MET (second system) and a computer system CL (third system). The key to such a "holistic" environment is optimizing the interaction between these three systems to enforce a holistic process window and provide a tight control loop to ensure that the patterning performed by the lithography apparatus LA stays within the process window. The process window defines the range of process parameters (e.g., dose, focus, overlay, etc.) within which a particular manufacturing process will produce a specified result (e.g., a functional semiconductor device, etc.), and typically, the process parameters of a lithography or patterning process can be varied within this range.

[0029] The computer system CL may use (parts of) the design layout to be patterned to predict which resolution enhancement techniques to use and perform computational lithography simulations and calculations to determine which mask layout and lithography apparatus settings maximize the overall process window of the patterning process (this is shown by the double-headed arrow at the first scale SC1 in FIG. 3). Typically, the resolution enhancement techniques are positioned to match the patterning feasibility of the lithography apparatus LA. The computer system CL may also be used to detect (e.g., using input from the metrology tool MET) where the lithography apparatus LA is currently operating within the process window, and to predict, for example, whether defects due to suboptimal processing may exist (this is shown by the arrow pointing to "0" at the second scale SC2 in FIG. 3).

[0030]

[0030] The metrology tool MET can provide input to the computer system CL to enable accurate simulation and prediction, and can also provide feedback to the lithographic apparatus LA to, for example, identify possible drifts in the calibration state of the lithographic apparatus LA (this is shown by multiple arrows at the third scale SC3 in Figure 3).

[0031]

[0031] In lithography processes, it is often desirable to measure the structures produced, for example, for process control and verification. Various tools for performing such measurements are known, including scanning electron microscopes and various forms of metrology devices (such as scatterometers). Examples of known scatterometers often rely on the provision of dedicated metrology targets, such as underfilled targets (targets in the form of a simple grating or multiple overlapping gratings of different layers, large enough that a measurement beam generates a spot smaller than the grating) or overfilled targets (targets where the illumination spot partially or completely encompasses the target). Furthermore, the use of metrology tools, such as angle-resolved scatterometers that illuminate underfilled targets (such as gratings), allows the use of so-called reconstruction methods. In reconstruction methods, the properties of the grating can be calculated by simulating the interaction of a mathematical model of the target structure with scattered radiation and comparing the simulation results with measurements. The parameters of the model are adjusted until the simulated interaction generates a diffraction pattern similar to that observed from the actual target.

[0032]

[0032] Scatterometers are versatile instruments that can measure parameters of the lithographic process by placing a sensor in the pupil of the scatterometer objective lens or in a plane conjugate thereto (this is commonly referred to as pupil-based measurement), or by placing a sensor in the image plane or in a plane conjugate thereto (this is commonly referred to as image-based or field-based measurement). Such scatterometers and related measurement techniques are further described in the following patent applications: US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP 1,628,164A, which are incorporated herein by reference in their entirety. The scatterometers described above can measure multiple targets from multiple gratings in one image using soft x-ray and light in the visible to near infrared wavelength range.

[0033] A metrology apparatus such as a scatterometer is shown in Figure 4. The apparatus comprises a broadband (white light) radiation projector 2 which projects radiation 5 onto a substrate W. Reflected or scattered radiation 10 is passed to a spectrometer detector 4 which measures the spectrum 6 (i.e., a measurement of intensity I as a function of wavelength λ) of the specularly reflected radiation 10. From this data, the structure or profile 8 giving rise to the detected spectrum can be reconstructed by a processing unit PU, for example by Rigorous Coupled Wave Analysis and non-linear regression, or by comparison with a library of simulated spectra. Typically, in reconstruction, the overall morphology of the structure is known, and some parameters are assumed from knowledge of the process which created the structure, leaving only a few parameters of the structure to be determined from the scatterometry data. Such a scatterometer may be configured as a normal-incidence scatterometer or an oblique-incidence scatterometer.

[0034] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, the properties of the grating can be reconstructed or calculated by applying a reconstruction method to the measured signal. Such a reconstruction can be obtained, for example, by simulating the interaction of a mathematical model of the target structure with the scattered radiation and comparing the simulation results with the measurement results. The parameters of this mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from the real target.

[0035]

[0035] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, radiation emitted from a radiation source is directed onto a target, and reflected or scattered radiation from the target is directed to a spectrometer detector, which measures the spectrum of the specularly reflected radiation (i.e., a measure of intensity as a function of wavelength). From this data, the structure or profile of the target giving rise to the detected spectrum can be reconstructed, for example, by rigorous coupled-wave analysis and nonlinear regression, or by comparison with a library of simulated spectra.

[0036] In a third embodiment, the scatterometer MT is an elliptically polarized scatterometer. An elliptically polarized scatterometer allows for determining parameters of a lithography process by measuring scattered radiation for each polarization state. Such a metrology apparatus emits polarized light (e.g., linear, circular, elliptical, etc.), for example, by using appropriate polarizing filters in the illumination section of the metrology apparatus. A source suitable for the metrology apparatus may provide polarized radiation. Various embodiments of existing ellipsometric scatterometers are described in the following 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 entireties.

[0037]

[0037] In one embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the overlay of two misaligned gratings or periodic structures by measuring asymmetries in the reflectance spectra and / or detection configurations (such asymmetries being related to the degree of overlay). The two (usually partially overlapping) grating structures may be applied to two different layers (not necessarily consecutive layers) and may be formed at substantially the same location on the wafer. The scatterometer may have a symmetric detection configuration such that any asymmetry is clearly distinguishable, as described, for example, in co-owned patent application EP 1,628,164 A. This provides a straightforward method for measuring grating misalignment. Further examples for measuring the overlay error between two layers containing periodic structures by measuring the target through the asymmetry of the periodic structures are described in PCT Patent Application Publication No. WO 2011 / 012624 or U.S. Patent Application No. US 20160161863, each of which is incorporated herein by reference in its entirety. Other parameters of interest may be focus and dose, which may be determined simultaneously by scatterometry (or alternatively by scanning electron microscopy) as described in U.S. Patent Application US2011-0249244, which is incorporated herein by reference in its entirety.

[0038]

[0038] The target may be measured in underfill mode or overfill mode. In underfill mode, the measurement beam generates a spot that is smaller than the entire target. In overfill mode, the measurement beam generates a spot that is larger than the entire target. In such overfill mode, it may be possible to measure different targets simultaneously, thereby simultaneously determining different process parameters.

[0039] The overall measurement quality of a lithography parameter using a particular target depends, at least in part, on the measurement recipe used to measure this lithography parameter. The term "substrate measurement recipe" may include one or more parameters of the measurement itself, one or more parameters of one or more patterns to be measured, or both. For example, if the measurement used in the substrate measurement recipe is a diffraction-based optical measurement, the one or more parameters of the measurement may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation to the substrate, the orientation of the radiation relative to the pattern on the substrate, etc. One criterion for selecting a measurement recipe may be, for example, the sensitivity of one measurement parameter to process variations. Further examples are described in U.S. Patent Application No. US2016-0161863 and U.S. Patent Application Publication No. US2016 / 0370717A1, which are incorporated herein by reference in their entireties.

[0040] FIG. 5( a) shows an embodiment of a metrology apparatus, more specifically a dark-field scatterometer. FIG. 5( b) shows in more detail a target T and the diffracted beams of measurement radiation used to illuminate the target. The metrology apparatus shown is of a type known as a dark-field metrology apparatus. This metrology apparatus may be a stand-alone device or may be integrated into a lithography apparatus LA, for example in a measurement station or in a lithographic cell LC. The optical axis with several branches throughout the apparatus is represented by a dotted line O. In this apparatus, light emitted from a source 11 (e.g., a xenon lamp) is directed onto a substrate W via a beam splitter 15 by an optical system comprising lenses 12, 14 and an objective lens 16. These lenses are arranged in a double sequence in a 4F configuration. Different lens configurations can be used, as long as they provide an image of the substrate on the detector while at the same time allowing access to an intermediate pupil plane for spatial frequency filtering. Therefore, by defining the spatial intensity distribution in a plane (referred to herein as the (conjugate) pupil plane) that gives the spatial spectrum of the substrate plane, the angular range over which the radiation is incident on the substrate can be selected. This can be done, in particular, by inserting an aperture plate 13 of an appropriate shape in a plane that is a back-projected image of the objective lens pupil plane, between lenses 12 and 14. In the illustrated example, aperture plates 13 have different shapes, labeled 13N and 13S, which allow for the selection of different illumination modes. The illustrated illumination system forms an off-axis illumination mode. In the first illumination mode, aperture plate 13N provides off-axis illumination from a direction designated "north" for convenience of explanation. In the second illumination mode, aperture plate 13S is used to provide similar illumination from the opposite direction, labeled "south." Other illumination modes are possible using different apertures. It is desirable for the remainder of the pupil plane to be dark, since unwanted light outside the desired illumination mode would interfere with the desired measurement signal.

[0041] As shown in FIG. 5(b), the target T is mounted with the substrate W perpendicular to the optical axis O of the objective lens 16. The substrate W may be supported by a support (not shown). A measurement radiation beam I incident on the target T at an off-axis O angle results in a zeroth order ray (solid line 0) and two first order rays (dashed-dotted lines +1 and dashed-dotted lines −1). It should be noted that for an overfilled small target, these rays are only one of many parallel rays covering the area of the substrate containing the metrology target T and other features. Because the aperture in the plate 13 has a finite width (necessary to admit a significant amount of light), the incident beam I will actually occupy a range of angles, and the diffracted beams 0 and +1 / −1 will be somewhat diffused. According to the point spread function of the small target, the +1 and −1 orders will be further diffused over a range of angles and will not be a single ideal beam as shown. Here, the grating pitch and 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. The rays shown in Figures 5(a) and 3(b) are depicted as being somewhat off-axis simply to make them easier to distinguish in the figures.

[0042] At least the 0th and +1st orders diffracted by the target T on the substrate W are collected by the objective lens 16 and returned through the beam splitter 15. Returning to FIG. 5( a), both the first and second illumination modes are illustrated with diametrically opposite apertures labeled north (N) and south (S). When the incident ray I of measurement radiation comes from the north side of the optical axis, i.e., when the first illumination mode is applied using aperture plate 13N, the +1 diffracted ray, labeled +1(N), enters the objective lens 16. Conversely, when the second illumination mode is applied using aperture plate 13S, the −1 diffracted ray (labeled 1(S)) enters the lens 16.

[0043]

[0043] A second beam splitter 17 splits the diffracted beam into two measurement branches. In the first measurement branch, an optical system 18 uses the zeroth and first diffraction order beams to form a diffraction spectrum (pupil plane image) of the target on a first sensor 19 (e.g., a CCD or CMOS sensor). Because each diffraction order strikes a different point on the sensor, image processing allows each order to be compared and contrasted. The pupil plane image captured by sensor 19 can be used to focus the metrology device and / or to normalize the intensity measurements of the first order beam. The pupil plane image can also be used for many measurement purposes, such as reconstruction.

[0044] In the second measurement branch, the optical system 20, 22 forms an image of the target T on a sensor 23 (e.g., a CCD or CMOS sensor). In the second measurement branch, an aperture stop 21 is provided in a plane conjugate with the pupil plane. The aperture stop 21 has the function of blocking the zeroth order diffracted beam so that the image of the target formed on the sensor 23 is formed only from the −1st or +1st order beam. The images captured by the sensors 19 and 23 are output to a processor PU, which processes the images. The function of the processor PU depends on the type of measurement being performed. Note that the term “image” is used in a broad sense here. Thus, an image of the grating lines will not be formed if only the −1st and +1st orders are present.

[0045]

[0045] The particular configuration of aperture plate 13 and field stop 21 shown in Figure 5 is merely exemplary. In another embodiment of the invention, on-axis illumination of the target is used, and an aperture stop with an off-axis aperture is used to pass substantially only the first diffracted order light to the sensor. In yet another embodiment, second, third, and even higher order beams (not shown in Figure 5) can be used for measurements instead of, or in addition to, the first order beam.

[0046]

[0046] To make the measurement radiation adaptable to these different types of measurements, the aperture plate 13 may comprise a number of aperture patterns formed around a disk that is rotated to arrange the desired pattern. Note that aperture plates 13N or 13S can only be used to measure gratings oriented in one direction (X or Y depending on the setup). For measurements of orthogonal gratings, 90° and 270° rotations of the target can be performed. Figures 5(c) and 5(d) show different aperture plates. The use of these devices, as well as many other variations and applications, are described in the above-mentioned published applications.

[0047] As an alternative to a scatterometer, the metrology device may comprise a holographic microscope, such as a digital holographic microscope or a digital dark-field holographic microscope. Such devices are disclosed, for example, in WO2021121733A1, which is incorporated herein by reference.

[0048] One known metrology method that can be performed using a metrology tool such as that shown in FIG. 5( a) is known as diffraction-based overlay (DBO) or microdiffraction-based overlay (μDBO). Such μDBO techniques use the imaging branch of the metrology tool (the branch through detector 23) to determine structure asymmetry based on the asymmetry in intensity or diffraction efficiency, or the difference in intensity or diffraction efficiency, between the first and second diffraction orders of a complementary diffraction order pair (typically, such pairs are complementary first diffraction order pairs, i.e., the first diffraction order may include the +1 order and the second diffraction order may include the −1 order, as shown in FIG. 5( b)). Therefore, it should be noted that in this context, the terms “first” and “second” do not refer to the diffraction orders, but are simply used to distinguish between the two diffraction orders that make up a complementary pair; the first and second diffraction orders may be the +2 and −2 diffraction orders or a higher complementary diffraction order pair. The zeroth order (specular radiation) is typically blocked or directed elsewhere (e.g., to another part of the detector for monitoring purposes) and is not used in μDBO metrology. The primary "image" used to estimate parameters of interest is formed only from the higher (e.g., 1st) diffraction orders. Asymmetries in the structure can be used to estimate parameters of interest such as overlay and focus, depending on the target design.

[0049]

[0049] A measurement parameter (e.g., intensity, amplitude, or diffraction efficiency) can be determined from the captured μDBO camera image by locating the target and integrating a particular region of interest (ROI) within the camera image, e.g., giving a single measurement parameter value for each sub-target. However, the measurement image may show significant measurement parameter deviations from the average value for the entire region of interest at one or more edges of one or more sub-targets of the imaged target. This measurement parameter deviation is sometimes referred to as an edge effect. Often, this manifests as a region of higher intensity (or related parameter) at the edge of the target, but it may also manifest as a region of lower intensity. In either case, this edge effect affects the intensity / measurement parameter within the region of interest and therefore influences the estimation of the parameter of interest (e.g., overlay, etc.) from the measurement parameter. As targets become smaller (e.g., 5 μm 2 In particular, edge effects from the surrounding environment can be problematic.

[0050]

[0050] Therefore, the proposed method to address the problem of edge effects includes obtaining measurement data for measurements of a target using two or more different illumination profiles, determining a respective parameter of interest value for a parameter of interest for each of the two or more different illumination profiles, determining a respective measured parameter deviation value for each of the two or more different illumination profiles, wherein the measured parameter deviation value represents the deviation (e.g., maximum or minimum (extreme value)) of the measured parameter relative to the measured parameter value attributable to the region of interest of the target, determining a relationship between the parameter of interest value and the measured parameter deviation value for the target, and determining from this relationship one or both of a modified parameter of interest value and a preferred illumination profile.

[0051]

[0051] The method may include determining the preferred illumination profile as the illumination profile that results in the smallest deviation of the measurement parameter of the target.

[0052]

[0052] The step of determining the relationship may include fitting a model (eg, a linear model) relating the parameter of interest values to the values of the measured parameter deviations.

[0053]

[0053] Measurement parameter deviation values may be determined from areas where measurement images are detected outside the region of interest, for example at one or more edges of each target or sub-target.

[0054]

[0054] The method may include determining the corrected parameter value of interest as the parameter value of interest corresponding to a value of zero for the measured parameter deviation in accordance with the above relationship.

[0055] The target may include a pair of sub-targets for each of one or more measurement directions (at least one of the pairs may be intentionally biased to create a bias difference between the sub-target pairs), from which the parameter of interest may be determined. As is known, the sub-targets of each pair may include equal and opposite biases. If the target includes a sub-target for each measurement direction, the method may be performed for each measurement direction (e.g., if the measurement direction is related to one of two orthogonal directions in the substrate plane, the sub-targets are typically referred to as an X sub-target and a Y sub-target). If the target includes two or more sub-targets, the measurement parameter deviation value for each target may optionally include the largest measurement parameter deviation value among the different sub-targets. Another alternative may include using an average of the measurement parameter deviation values among the sub-targets. In this manner, the sub-target used to determine the actual measurement parameter deviation is not important.

[0056]

[0056] This method may be performed inline for each target, for example by illuminating each target with two or more different illumination profiles as described above to obtain measurement data and determine a corrected illumination profile and / or a preferred illumination profile for each target.

[0057]

[0057] The number of illumination profiles used per target may be, for example, two, or more than two, or more than three, or more than five.

[0058]

[0058] Different illumination profiles may include different illumination shapes in the illumination pupil plane, i.e., may correspond to different ranges or groups of illumination angles.

[0059]

[0059] Figure 6 is a flow diagram illustrating a method according to one embodiment. Figure 6(a) shows, by way of example only, a number of illumination profiles 600-630 or illumination apertures that can be used in the methods disclosed herein. The number of different illumination profiles used and / or their shapes may differ from those shown. Thus, the shape of any of the illumination profiles 600-630 may vary significantly in size, shape, and / or position within the illumination pupil plane.

[0060]

[0060] Figure 6(b) shows four plots 635-650 illustrating the relationship between a measurement parameter (e.g., intensity) and a target or sub-target position (here, one-dimensional) for four illumination profiles 600, 605, 620, 630. In each case, a representative (e.g., average) measurement parameter value is determined for a region of interest (ROI) where the measurement parameter variation remains within a small range. Also shown is a measurement parameter deviation value PD, which is the difference between the maximum or minimum measurement parameter value and the average value within the ROI, or the maximum or minimum measurement parameter value divided by the average value within the ROI. This step of determining the measurement parameter deviation value PD can be performed for all illumination profiles 600-630.

[0061] The method may further include separately determining a measurement of interest (e.g., an overlay value) for the measurement data for each illumination profile 600-630. This may be done by any known method for calculating the parameter of interest from the measurement data (e.g., intensity data), such as from an asymmetry or difference in the intensities / measurement parameters of a pair of complementary diffraction orders (e.g., +1 and −1 orders).

[0062]

[0062] Because a single overlay value can be determined from two images (e.g., +1 and -1 orders), the value of the measured parameter deviation may be determined from either image (or from both images, e.g., an average). In one embodiment, the largest measured parameter deviation value may be selected. Such an approach may also be used when the overlay value is determined from four images (e.g., +1 and -1 order images from two sub-targets with different biases), e.g., the largest measured parameter deviation value from the four images may be selected. However, this is not required.

[0063] FIG. 6(c) plots the estimated parameter of interest (e.g., overlay) OV versus the parameter of interest deviation PD. Such a plot includes seven points (in this particular example), one for each illumination profile 600-630. A linear regression or model 655 can be fitted to this data. According to this model 655, the overlay (or other parameter of interest) value OV corresponding to a parameter of interest deviation PD of zero is: cor can be determined, which can be used as the modified parameter of interest / overlay value.

[0064]

[0064] Alternatively or additionally, model 655 may be used to identify a preferred lighting profile, for example, a lighting profile that corresponds to zero deviation of the parameter of interest, a lighting profile that is very close to this, or a lighting profile that is closest to this.

[0065] The concepts disclosed herein are described in terms of overlay metrology. However, the methods are not so limited. For example, targets may be formed with focus-based asymmetry (i.e., asymmetry that depends on the focus of the scanner actually used to expose the target). The methods disclosed herein are equally applicable to focus metrology based on such targets (e.g., diffraction-based focus DBF or microdiffraction-based focus μDBF), in which case the determined model will be one that represents the relationship of the measurement parameter deviation to the estimated focus. Similarly, other target types and metrology techniques may be used, such as continuous diffraction-based overlay (cDBO) targets and corresponding cDBO measurement techniques.

[0066] To measure each target with a different illumination profile, an illumination mode selector (IMS) can be used. IMS is a known illumination selection method in which various fixed apertures are arranged on an aperture wheel and can be selectively switched or rotated into the illumination beam path as needed. Faster switching between multiple illumination profiles can be achieved by other illumination technologies, such as by using programmable illumination (e.g., as described in US6947613B) implemented using grating light valve (GLV) technology such as that sold by Silicon Light Machines (SLM), or by using any suitable spatial light modulation technology (e.g., in the illumination pupil plane). For example, when performing the proposed method inline, it can be beneficial to quickly switch between different illumination profiles (and optionally different colors). Therefore, implementing different illuminations can be achieved in many different ways. The illumination profile can be defined or imposed in the illumination pupil plane (e.g., plane 13 shown in FIG. 5(a)).

[0067]

[0067] Further embodiments of the present invention are described in the clauses listed below. 1. A metrology method comprising: acquiring measurement data for measurements of at least one target using two or more different illumination profiles; determining a respective parameter-of-interest value for the parameter of interest for each of two or more different illumination profiles from the measurement data; determining, from the measurement data, for each of two or more different illumination profiles, a respective measurement parameter deviation value, the measurement parameter deviation value representing a deviation of a measurement parameter relative to a measurement parameter value attributable to a region of interest of the target or sub-target thereof; determining a relationship between a parameter value of interest and a value of a measured parameter deviation for said target; determining from the relationship one or both of a modified parameter-of-interest value and a preferred illumination profile; A method comprising: 2. The method of clause 1, comprising determining the preferred illumination profile as the illumination profile corresponding to the smallest measurement parameter deviation of said target. 3. The method of clause 1 or 2, wherein the step of determining the relationship comprises fitting a model relating values of the parameter of interest to values of the measured parameter deviations. 4. The method of clause 3, wherein the model comprises a linear model. 5. A method according to any of the preceding clauses, wherein the value of the measurement parameter deviation represents an extreme value of the measurement parameter deviation relative to the measurement parameter value attributable to the region of interest of said target or its sub-targets. 6. A method according to any preceding clause, comprising determining the corrected parameter value of interest as the parameter value of interest corresponding to a value of zero of the measured parameter deviation in accordance with the above relationship. 7. The method of any preceding clause, wherein the value of the measurement parameter deviation is determined from an area where the measurement image is detected outside the area of interest. 8. The method of any preceding clause, wherein the value of the measurement parameter deviation is determined from one or more edges of the target or one or more edges of one or more sub-targets of the target. 9. A method according to any preceding clause, wherein the target comprises one or more sub-targets for each measurement direction, and the method is performed for each measurement direction such that modified parameter values of interest and / or preferred illumination profiles for each measurement direction are obtained. 10. The method of any preceding clause, wherein the number of lighting profiles used per target is greater than three. 11. The method according to any preceding clause, wherein the number of lighting profiles used per target is greater than five. 12. A method according to any preceding clause, wherein the measured parameter value attributable to a region of interest of said target or a sub-target thereof comprises an average measured parameter value of the region of interest. 13. The method of any preceding clause, wherein the measurement parameter is intensity, diffraction efficiency, or amplitude. 14. The method of any preceding clause, wherein the parameter of interest is overlay or focus. 15. A method according to any preceding clause, wherein the measured parameter deviation of the target comprises the largest measured parameter deviation value across different sub-targets of the target, and / or the largest measured parameter deviation value across different diffraction orders scattered by the target. 16. A method according to any preceding clause, comprising measuring said at least one target to obtain measurement data. 17. A method according to clause 16, comprising carrying out said method in-line as part of a lithography process. 18. The method of clause 17, further comprising exposing said at least one target onto a substrate, performing said measuring step, and using the corrected parameter values of interest, or parameter values of interest corresponding to a preferred illumination profile, when correcting a subsequent exposure step for a subsequent substrate. 19. A computer program comprising program instructions operable to perform the method according to any of clauses 1 to 12 when the computer program is run on a suitable device. 20. A non-transitory computer program carrier containing a computer program as defined in clause 19. 21. A computer program carrier containing a computer program according to clause 20; a processor operable to execute the computer program; a processing configuration, 22. A metrology device comprising a processing arrangement according to clause 21. 23. A metrology device according to clause 22, including imaging optics for capturing scattered radiation from the target, and a detector for detecting the scattered radiation to obtain a measured image of the target. 24. A metrology device according to clause 22 or 23 which is a scatterometer. 25. A metrology device according to clause 22 or 23, which is a dark-field holographic microscope. 26. A metrology device according to any one of clauses 22 to 25, operable to perform the method according to clause 16 or 17. 27. A lithocell comprising a metrology device according to any one of clauses 22 to 26 and a lithographic apparatus. 28. A lithocell according to clause 27, operable to carry out the method according to clause 18.

[0068]

[0068] Although specific reference is made in this specification to the use of lithographic apparatus in IC manufacture, it should be understood that the lithographic apparatus described herein may have other applications, including the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0069] Although specific reference is made herein to embodiments of the invention in the context of an inspection or metrology apparatus, embodiments of the invention may also be used in other apparatus. Embodiments of the invention 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 devices). The term "metrology apparatus" may also refer to an inspection apparatus or inspection system. For example, an inspection apparatus including embodiments of the invention may be used to detect defects in a substrate or in a structure on a substrate. In such embodiments, the feature of interest of a structure on a substrate may relate 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.

[0070] Although specific reference is made to a "metrology apparatus / tool / system" or an "inspection apparatus / tool / system," these terms may refer to the same or similar types of tools, apparatus, or systems. For example, an inspection apparatus or metrology apparatus incorporating embodiments of the present invention may be used to determine characteristics of structures on a substrate or wafer. For example, an inspection apparatus or metrology apparatus incorporating embodiments of the present invention may be used to detect defects in the substrate or in structures on the substrate or wafer. In such embodiments, the characteristics of interest in the structures on the substrate may relate 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 or wafer.

[0071]

[0071] Although specific reference has been made above to the use of embodiments of the present invention in the context of optical lithography, it will be appreciated that, where circumstances permit, the present invention is not limited to optical lithography and may also be used in other applications, such as imprint lithography.

[0072] While the targets or target structures (more generally structures on a substrate) described above are metrology target structures designed and formed specifically for measurement purposes, in other embodiments the property of interest may be measured of one or more structures that are functional part of a device formed on the substrate. Many devices have regular grid-like structures. The terms structure, target grid, and target structure as used herein do not require that the structure be specifically provided for the measurement being made. Also, the pitch P of the metrology target may be close to or smaller than the resolution limit of the scatterometer optical system, but may be much larger than the dimensions of typical product features created in the target portion C by a lithographic process. In practice, the lines and / or spaces of the overlay grating in the target structure may be made to include smaller structures of similar dimensions to the product features.

[0073]

[0072] While specific embodiments of the present invention have been described above, it will be apparent that the present invention can be practiced in other ways than those described above. The foregoing description is intended to be illustrative and not limiting. Thus, it will be apparent to those skilled in the art that modifications may be made to the invention as described without departing from the scope of the appended claims.

Claims

1. 1. A metrology method comprising: acquiring measurement data for measurements of at least one target using two or more different illumination profiles; determining a respective parameter-of-interest value for a parameter of interest for each of the two or more different illumination profiles from the measurement data; determining, from the measurement data, for each of the two or more different illumination profiles, a respective measurement parameter deviation value, the measurement parameter deviation value representing a deviation of a measurement parameter relative to a measurement parameter value attributable to a region of interest of the target or sub-target thereof; determining a relationship between the parameter of interest value and the measured parameter deviation value for the target; determining from said relationship one or both of a modified parameter-of-interest value and a preferred lighting profile; A method comprising:

2. The method of claim 1 , comprising determining the preferred illumination profile as the illumination profile corresponding to the smallest deviation of the measured parameter of the target.

3. The method of claim 1 or 2, wherein the step of determining the relationship comprises fitting a model relating the parameter of interest values to the values of the measured parameter deviations.

4. The method of claim 3 , wherein the model comprises a linear model.

5. The method according to any of claims 1 to 4, wherein the measurement parameter deviation values represent extreme values of measurement parameter deviations relative to measurement parameter values resulting from regions of interest of the target or its sub-targets.

6. The method according to any one of claims 1 to 5, comprising determining the modified parameter value of interest as the parameter value of interest corresponding to a value of zero of the measured parameter deviation according to said relationship.

7. The method according to any one of claims 1 to 6, wherein the value of the measurement parameter deviation is determined from a region outside the region of interest where a measurement image is detected.

8. The method according to any of the preceding claims, wherein the value of the measurement parameter deviation is determined from one or more edges of the target or one or more of its sub-targets.

9. 9. The method according to claim 1, wherein the target comprises one or more sub-targets for each measurement direction, and wherein the method is performed for each measurement direction such that modified parameter-of-interest values and / or preferred illumination profiles are obtained for each measurement direction.

10. The method according to any one of claims 1 to 9, wherein the number of illumination profiles used per target is greater than three.

11. The method according to any preceding claim, wherein the measured parameter value attributable to a region of interest of 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 to perform the method according to any of claims 1 to 11 when the computer program is run on a suitable device.

13. a computer program carrier containing a computer program according to claim 12; a processor operable to execute the computer program; a processing configuration,

Images