Measurement method and associated measurement device

Abstract

A metrology method is disclosed. The method comprises: irradiating a target comprising one or more sub-targets on a substrate with underfilled irradiation such that an irradiation beam profile underfills each of the one or more sub-targets; capturing scattered radiation resulting from the irradiation performed on the target; imaging the scattered radiation at a detection image plane to obtain an image; and determining a parameter of interest from the imaged scattered radiation.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to European application 22189687.1 filed on August 10, 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 a metrology device which can be used, for example, to determine properties of a structure 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 integrated circuit (IC) manufacturing. 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 a process, the resolution formula can be expressed as CD = k1 × λ / NA, where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the "critical dimension" (usually the smallest feature size printed, but in this case it is the half pitch), and k1 is an empirical resolution factor. In general, the smaller the k1, the more difficult it becomes to reproduce a pattern on a substrate that 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 NA, customized illumination schemes, use of phase-shifted patterning devices, various optimizations of the design layout such as optical proximity correction (OPC, sometimes also referred to as "optical and process correction") in the design layout, or other methods generally defined as "resolution enhancement techniques" (RET). Alternatively, a 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 desirable to frequently measure the resulting structure, 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 used to refer 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. Overfilled metrology 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] These edge effects need to be mitigated. 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: irradiating a target including one or more sub-targets on a substrate using underfilled irradiation so that the irradiation beam profile underfills each of the one or more sub-targets; capturing scattered radiation generated by the irradiation of the target; imaging the scattered radiation at a detection image plane to obtain an image; and determining a parameter of interest based on the imaged scattered radiation.

[0013] In a second aspect of the present invention, a measurement method is provided, comprising: irradiating a target including a plurality of sub-targets on a substrate by simultaneously irradiating each sub-target using a configured illumination beam profile, wherein the configured illumination beam profile is configured to individually irradiate each of the plurality of sub-targets; capturing scattered radiation generated by the irradiation of the target; and determining a parameter of interest based on the scattered radiation.

[0014] The present invention further provides a measuring device, which is operable to perform the method according to the first aspect or the second aspect.

[0015] These and other aspects and advantages of the apparatus and methods disclosed herein will be understood 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 to optimize semiconductor manufacturing;

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

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

[0022] Figure 6 Profiling a known form of multi-grating (or multi-sub-target) target and a measurement spot on a substrate for use in known overfill metrology methods;

[0023] Figure 7 Depicted in Figure 5 (a) obtained from the scatterometer Figure 6 an image of the target; and

[0024] Figure 8 Depiction Figure 6 Profile of a measurement spot on a target and a substrate configured according to an embodiment. DETAILED DESCRIPTION

[0025] 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 EUV (extreme ultraviolet radiation, e.g., having a wavelength in the range of about 5 nm to 100 nm).

[0026] The terms "reticle", "mask" or "patterning device" as used herein may be broadly interpreted 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.

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

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

[0029] 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 or for other factors such as the use of an 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.

[0030] The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid, 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, incorporated herein by reference.

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

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

[0033] In operation, a radiation beam B is incident on a patterning device (e.g. a mask) MA held on a mask support MT and is patterned by a pattern (design layout) present on the patterning device MA. Having traversed the mask MA, the radiation beam B passes through a projection system PS which focuses the beam on a target portion C of the substrate W. With the aid of a second positioner PW and a position measurement system IF, the substrate support WT can be accurately moved, for example, in order to position different target portions C in the path of the radiation beam in a focused and aligned position. Similarly, the first positioner PM and possibly a further position sensor (the further position sensor not being in position) are arranged in a manner such that the radiation beam is positioned on a projection system PS. Figure 1 The patterning device MA may be accurately positioned relative to the path of the radiation beam B using the mask alignment marks M1, M2 and the substrate alignment marks P1, P2. Although the substrate alignment marks P1, P2 occupy dedicated target portions as illustrated, they may be located in spaces between target portions. When the substrate alignment marks P1, P2 are located between target portions C, they are referred to as scribe line alignment marks.

[0034] like Figure 2As shown in , the lithography apparatus LA may form part of a lithography cell LC (sometimes also referred to as a lithocell or (lithography) cluster) which often also includes equipment for performing pre-exposure processes 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 (e.g. for regulating the temperature of the substrate W, for example for regulating the solvent in the resist layer). A substrate transport device or robot RO picks up the substrate W from the input / output ports I / O1, I / O2, moves the substrate between the different process equipment and transfers the substrate W to a feed table LB of the lithography apparatus LA. The devices in the lithography cell, which are often also collectively 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.

[0035] In order to correctly and consistently expose the 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 an error is detected, especially if the inspection is performed before other substrates W of the same batch or lot are yet to be exposed or processed, adjustments may be made, for example, to the exposure of subsequent substrates or other processing steps to be performed on the substrates W.

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

[0037] Typically, the patterning process in the lithographic apparatus LA is one of the most important steps in the process that requires a high degree of accuracy in the dimensioning and placement of structures on the substrate W. In order to ensure this high accuracy, three systems can be combined in a so-called "holistic" control environment, such as Figure 3One of these systems is a lithographic apparatus LA, schematically depicted in FIG, which is (in practice) connected to a metrology tool MET (a second system) and to a computer system CL (a third system). The key to this "holistic" environment is to optimize the cooperation between these three systems to enhance the overall process window and provide a tight control loop to ensure that the patterning performed by the lithographic 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 which process parameter variations in the lithographic process or patterning process are allowed.

[0038] The computer system CL can use (part of) the design layout to be patterned to predict which resolution enhancement techniques to use and perform computational lithography simulations and calculations to determine which mask layouts and lithography equipment settings achieve the maximum overall process window for the patterning process (in Figure 3 Typically, 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 there may be defects due to, for example, suboptimal processing (e.g., in the process window in which the lithographic apparatus LA is currently operating). Figure 3 ) is depicted by an arrow pointing to “0” in the second scale SC2.

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

[0040] During lithography, it is desirable to frequently measure the structures produced, 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 typically rely on the supply of dedicated metrology targets, such as underfilled targets (targets in the form of simple gratings or overlapping gratings in different layers, which are large enough so that the measurement beam produces a spot smaller than the grating) or overfilled targets (whereby the irradiation spot partially or completely encompasses the target). In addition, the use of metrology tools (e.g., angle-resolved scatterometers that irradiate 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.

[0041] Scatterometers are multifunctional 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, the measurement being generally referred to as pupil-based measurement, or by having a sensor in the image plane or a plane conjugated to the image plane, in which case the measurement is generally referred to as image-based or field-based measurement. Such scatterometers and related measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP1,628,164A, which are incorporated herein by reference in their entirety. The 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.

[0042] Figure 4 A 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 produces 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 known and some parameters are assumed from knowledge of the process by which the structure was manufactured, so that only a few parameters of the structure are determined from the scatterometry data. Such a scatterometer can be configured as a normal incidence scatterometer or an oblique incidence scatterometer.

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

[0044] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, radiation emitted by a radiation source is directed onto a target and reflected or scattered radiation from the target is directed 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.

[0045] 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 of each polarization state. Such a metrology device emits polarized light (such as linear, circular 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.

[0046] In one embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the overlap of two misaligned gratings or periodic structures by measuring the reflection spectrum and / or an asymmetry in the detection configuration (the asymmetry being related to the extent of the overlap). The two (usually superimposed) grating structures may be applied in two different layers (not necessarily continuous layers) and may be formed at approximately the same position on the wafer. The scatterometer may have a symmetrical detection configuration such as described in co-owned patent application EP1,628,164A, so that any asymmetry is clearly discernible. This provides a direct way to measure misalignment in the gratings. Further examples of measuring the overlay error between two layers comprising the periodic structure via the asymmetry of the targeted periodic structure may be found in PCT patent application publication number WO2011 / 012624 or 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 (or alternatively by scanning electron microscopy) as described in US Patent Application US2011-0249244, which is incorporated herein by reference in its entirety.

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

[0048] The overall measurement quality of the lithography parameters using a specific target is at least partially determined by the measurement options used to measure such lithography parameters. The term "substrate measurement options" may include one or more parameters of the measurement itself, one or more parameters of the one or more patterns measured, or both one or more parameters of the measurement itself and one or more parameters of the one or more patterns measured. For example, if the measurement used in the substrate measurement options is a diffraction-based optical measurement, one or more of the parameters measured may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, etc. One of the criteria for selecting the measurement options may be, for example, 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.

[0049] Figure 5 (a) presents an embodiment of a metrology apparatus and more specifically a dark field scatterometer. Figure 5(b) shows in more detail the target T and the diffracted rays of the measuring radiation used to irradiate the target. The metrology apparatus shown is of the type known as a dark field metrology apparatus. The metrology apparatus may be a separate device or incorporated in a lithography apparatus LA, for example at a measuring station or in a lithography cell LC. An optical axis having several branches running through the apparatus is indicated by a dashed line O. In such an apparatus, light emitted by a source 11, for example 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 of a 4F arrangement. Different lens arrangements may be used as long as the lens arrangement still provides an image of the substrate onto the detector and at the same time allows access to an intermediate pupil plane for spatial frequency filtering. Thus, the angular range of the radiation incident on the substrate may be selected by defining the spatial intensity distribution in a plane presenting the spatial spectrum of the substrate plane, referred to herein as the (conjugate) pupil plane. In particular, this selection can be made by inserting an aperture plate 13 of a suitable form between lens 12 and lens 14 in the plane of the back-projected image of the objective lens pupil plane. In the illustrated example, the aperture plate 13 has different forms (labeled as 13N and 13S), thereby allowing different illumination modes to be selected. The illumination system in this example forms an off-axis illumination mode. In a first illumination mode, the aperture plate 13N provides off-axis from a direction designated as "north" for descriptive purposes only. In a second illumination mode, the aperture plate 13S is used to provide similar illumination, but provides illumination from the opposite direction labeled "south". Other illumination modes are possible by using different apertures. Because any unnecessary light outside the desired illumination mode will interfere with the desired measurement signal, the rest of the pupil plane is ideally darker.

[0050] like Figure 5 As shown in (b), the target T is placed with the substrate W perpendicular to the optical axis O of the objective lens 16. The substrate W may be supported by a support (not shown). A measurement radiation ray I 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 area of ​​the substrate including the measurement target T and other features. Since the hole in the plate 13 has a finite width (necessary to receive a useful amount of light), the incident ray I will actually occupy an angular range, and the diffracted rays 0 and +1 / -1 will be slightly spread out. Depending on the point spread function of the small target, each order +1 and -1 will be further spread out over the angular range, rather than a single ideal ray as shown. It should be noted that the grating pitch and the illumination angle of the target can be designed or adjusted so that the first-order rays entering the objective are closely aligned with the central optical axis. Figure 5 (a) and Figure 5 The rays illustrated in (b) are shown slightly off-axis simply to enable them to be more easily distinguished in the drawing.

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

[0052] The second beam splitter 17 divides the diffraction beam into two measurement branches. In the first measurement branch, the optical system 18 uses the zero-order diffraction beam and the first-order diffraction beam to form a diffraction spectrum (pupil plane image) of the target on a first sensor 19 (e.g., a CCD or CMOS sensor). Each diffraction order hits a different point on the sensor so that image processing can compare and contrast several orders. The pupil plane image captured by the sensor 19 can be used to focus the metrology device and / or normalize the intensity measurement of the first-order beam. The pupil plane image can also be used for various measurement purposes such as reconstruction.

[0053] 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 to the pupil plane. The aperture stop 21 serves to block the zero-order diffraction beam so that the image of the target formed on the sensor 23 is formed only by the -1 or +1 order beams. The image captured by the sensors 19 and 23 is output to a processor PU that processes the image, the function of which will depend on the specific type of measurement being performed. It should be noted that the term "image" is used in a broad sense. Thus, if only one of the -1 order and the +1 order is present, an image of such a grating line will not be formed.

[0054] Figure 5 The specific forms of aperture plate 13 and field stop 21 shown in 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).

[0055] In order that the measuring radiation can be adapted for these different types of measurements, the aperture plate 13 may include a plurality 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). For measuring orthogonal gratings, target rotations of up to 90° and 270° may be implemented. Figure 5 (c) and Figure 5 A different orifice plate is shown in (d). The use of these orifice plates and many other variations and applications of the apparatus are described in the previously published applications referenced above.

[0056] 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.

[0057] Figure 6 An overlapping target or composite overlapping target is depicted according to known actual formation on a substrate. The overlapping target in this example includes four sub-targets (e.g., gratings) 32 to 35, which are closely positioned together so that the sub-targets will all be within a measurement spot 31 formed by a measurement radiation illumination beam of a measurement device (i.e., the target is filled). Therefore, the four sub-overlapping targets are all irradiated simultaneously and imaged simultaneously on the sensor 23. In the example for overlap measurement, the sub-targets 32 to 35 are themselves composite structures formed by overlapping gratings patterned in different layers of a semiconductor device formed on a substrate W. The sub-targets 32 to 35 can have overlap offsets that are biased differently to facilitate the measurement of overlap between layers in which different parts of the composite sub-targets are formed. This will be referred to below. Figure 7 To explain the meaning of overlapping bias. Sub-targets 32 to 35 can also be different in their orientation, as shown, so that the incident radiation is diffracted in the X direction and the Y direction. In an example, sub-targets 32 and 34 are X-direction sub-targets with biases of +d, -d, respectively. Sub-targets 33 and 35 are Y-direction sub-targets with offsets of +d and -d, respectively. Individual images of these sub-targets can be identified in the image captured by sensor 23. This is just an example of overlapping targets. Overlapping targets can include more or less than four sub-targets.

[0058] Figure 7 Shown in the use of Figure 5 (d) In the case of orifice plate 13NW or 13SE Figure 5 (a) Used in equipment Figure 6An example of an image that can be formed on and detected by the sensor 23 due to overlapping targets. Although the pupil plane image sensor 19 cannot distinguish between different individual sub-targets 32 to 35, the image sensor 23 can perform such a distinction. The shaded area 40 represents the field of the image on the sensor, within which the illumination spot 31 on the substrate is imaged into the corresponding circular area 41. Within such a circular area, the rectangular areas 42 to 45 represent the images of the small overlapping target sub-targets 32 to 35. If the overlapping targets are located in the product area, product features are also visible in the periphery of this image field. The image processor and controller PU use pattern recognition to process these images to identify the individual images 42 to 45 of the sub-targets 32 to 35. In this way, the images do not have to be aligned very accurately at specific locations within the sensor frame, which greatly improves the throughput of the measuring device as a whole.

[0059] Once individual images of an overlapping target have been identified, the intensities of those individual images can be measured, for example, by averaging or summing selected pixel intensity values ​​within the identified regions. The intensities and / or other properties of the images can be compared to each other. These results can be combined to measure different parameters of the lithography process. Overlay performance is an important example of such a parameter.

[0060] You can use Figure 5 One 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 are based on the analysis of a pair of complementary diffraction orders (usually a pair of complementary first diffraction orders, i.e., the first diffraction order may include the +1 order and the second diffraction order may include the -1 order, as Figure 5 The asymmetry in the structure is determined by the intensity or diffraction efficiency asymmetry or intensity or diffraction efficiency difference between the first and second diffraction orders (illustrated in (b)). Thus, in this context, the terms "first" and "second" do not refer to the diffraction order and are simply used to distinguish two diffraction orders of a complementary pair, noting that the first and second diffraction orders can be +2 and -2 diffraction orders, or a pair of higher complementary diffraction orders. The zeroth order (mirror radiation) is typically blocked or shunted elsewhere (e.g., to another part of the detector for monitoring purposes); the zeroth order is not used in μDBO measurements. The main "image" used for the inference of the parameters 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 overlap or focal length, depending on the target design.

[0061] A measurement parameter (e.g., intensity, amplitude, or diffraction efficiency) can be determined from a captured μDBO camera image by finding the target location and integrating a region of interest (ROI) within the 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, the measurement parameter deviates considerably from the average value of the region of interest. Such deviations in the measurement parameter may be referred to as edge effects. In many cases, edge effects appear as regions with higher intensity (or related parameters) at the edge of the target, but edge effects may also appear as regions with lower intensity. In either case, such edge effects may affect the intensity / measurement parameter within the region of interest, and therefore affect the parameters of interest (e.g., overlap) inferred from the measurement parameter. As targets become smaller (e.g., 5 μm square and smaller), edge effects become more significant and more difficult to handle with known methods. In addition, edge effects from the environment may also present problems.

[0062] The reason edge effects are a problem is that many metrology techniques such as μDBO or its focal metrology equivalent μDBF (where the target has an asymmetry that relies on scanner focus rather than on overlap) use overfill metrology where the entire target (which may be a composite target consisting of multiple individual sub-targets) is small and fully contained within the measurement spot used to measure the target. This is described above. Figure 6 In overfill measurement, the target edge, surrounding structures and other sub-targets are captured and imaged in each measurement image. As the target becomes smaller, the ratio of target diffraction to unwanted edge effect diffraction becomes smaller, and the ratio becomes more difficult to find a "clean" (i.e., containing only appropriate parameter information of interest) portion of the target pad.

[0063] Overfill measurement has the advantage of being fast; it allows multiple sub-targets to be measured simultaneously, where a region of interest ROI is selected from each sub-target based on the image. More complex algorithms for selecting portions of the imaged target are also possible, such as pixel mapping. These benefits in speed and image portion selection are not possible with underfill measurement techniques, which use a measurement spot smaller than each sub-target to underfill each sub-target continuously and individually. On the other hand, underfill measurement can provide the advantage of being unaffected by edge effects, because edges and surrounding structures or other sub-targets are not captured in the measurement spot and therefore do not affect the image. However, because making the spot small enough to underfill the target presents its own challenges and limitations, this is only true when performed perfectly.

[0064] As already described, overfilled metrology methods such as μDBO and μDBF typically capture images at an image plane, or a plane conjugate to the image plane, of the metrology tool's imaging optics. Each of these images is typically formed from a single diffraction order (e.g., the +1 and -1 diffraction orders), but images of two or more diffraction orders may be simultaneously imaged (i.e., separated at the detection image plane) over different areas in a single composite image. This is in contrast to underfilled metrology methods, which produce an "image," i.e., an angularly resolved image of the target, at the pupil plane (or a plane conjugate to the pupil plane) of the metrology tool's imaging optics. Reference Figure 5 For a metrology tool of the type illustrated in (a), overfill metrology may use an imaging branch of the type comprising detector 23 at the image plane, and underfill metrology may use an imaging branch of the type comprising detector 19 at the pupil plane (or Fourier plane).

[0065] A metrology method is proposed, comprising: irradiating a target on a substrate using underfilled illumination; capturing scattered radiation resulting from the irradiation of the target; imaging the scattered radiation at an image plane; and determining a parameter of interest from the imaged scattered radiation.

[0066] The first step may include irradiating the target so as to optimize the illumination beam profile (e.g., a measurement spot) on the target to irradiate one or more target regions corresponding to one or more regions of interest on a detected image at a detection image plane, while minimizing irradiation of obstructing structures on the substrate (e.g., target edges and / or surrounding structures) located outside the one or more target regions.

[0067] A metrology method is also proposed, comprising: irradiating a target including a plurality of sub-targets on a substrate by simultaneously irradiating each sub-target using a configured illumination beam profile, the configured illumination beam profile being configured to individually irradiate each of the plurality of sub-targets; capturing scattered radiation resulting from the irradiation of the target; and determining a parameter of interest from the scattered radiation. Such an embodiment may capture scattered radiation at an image plane and / or at a pupil plane.

[0068] In an embodiment, the target comprises two or more sub-targets, the irradiation step comprises irradiating each sub-target simultaneously using underfill irradiation, and the capture step comprises capturing scattered radiation generated by the irradiation of each sub-target simultaneously. In this way, the scattered radiation from each target will be imaged at the image plane simultaneously.

[0069] A method for performing an irradiation step, for example, for optimizing the profile of the irradiation beam onto the target will now be described. The optimization may be such that nothing that will not be measured is irradiated (e.g., the target edge or surrounding structures are not irradiated). However, the measurement spot should not be too small, because a measurement spot that is too small may result in excessive sensitivity to the spot position on the target. Thus, the optimization may maximize the area of ​​each sub-target that is irradiated, or at least ensure that a sufficiently large area of ​​each sub-target is irradiated. Such a measurement spot should be positioned very precisely to avoid colliding with any target edge. Thus, a tolerance may be added when determining the spot size to provide a certain latitude in the placement of the measurement spot, but this further reduces the spot size.

[0070] In a first embodiment, a spot size selector is proposed that can be dynamically controlled to configure the illumination so that only the portion of the target corresponding to the region of interest (ROI) on the resulting image is irradiated. An algorithm can be used to perform ROI identification and control the illumination configuration based on the ROI identification. This ROI identification and illumination configuration can be performed online for at least a subset of the measured targets.

[0071] In an embodiment, the method may include irradiating the target in an overfilling manner, i.e., such that the complete spot irradiates the entire target including the edges. A pattern recognition step may then be performed to identify the location of the ROI. The illumination may then be configured so that any illumination that irradiates the target area that does not correspond to the ROI on the image may be turned off or blocked, so that only the target area corresponding to the ROI is irradiated. This is only an example of an illumination strategy. For certain targets, layers, and environments, edge effects at the boundaries of sub-targets may not be a problem or a small problem; alternatively, for example, very "bright" features (as imaged) at a distance that are slightly further away may be a much more significant problem. In such a case, it may be preferred to irradiate the complete pad (e.g., including one or more edges) rather than very bright features. This will eliminate crosstalk from problem features, but relax the strict positioning specifications that may otherwise be required by the proposed method. Thus, the illumination may also be configured to irradiate only areas that do not cause undesirable crosstalk.

[0072] Once the illumination beam profile has been appropriately configured or optimized, a (e.g., underfill) measurement may be performed using the configured illumination beam profile. Although the method may be performed on a single ROI target (e.g., a target that is not divided into or includes separate sub-targets), in terms of speed and throughput, it may be preferable to measure targets that include multiple sub-targets (e.g., such as Figure 6), where the sub-targets are measured together and simultaneously. Thus, the configured illumination beam profile can be configured to illuminate different sub-targets individually without illuminating their edges or any adjacent structures. The resulting scattered illumination can then be imaged at an image plane, for example, such that different sub-targets for each of the diffraction orders of a complementary pair (e.g., +1 and -1 diffraction orders) are individually imaged at different regions of the detection image plane.

[0073] In an alternative embodiment, it is possible to tune the relative intensity of the (e.g., underfill) illuminated area per subtarget. This can include optimizing the relative intensities to further reduce crosstalk (e.g., by changing the relative intensity between the X and Y subtargets of the target). Such a scenario can achieve an increase in the dynamic range of the system, which in turn can measure two directions simultaneously, even if one direction is much brighter under normal, uniform, single-spot illumination (i.e., the brighter direction does not saturate the camera, or cause some residual intra-target leakage). In an extreme variation of such an approach, the X and Y subtargets can be irradiated separately; for example, if this is optimal for intra-target crosstalk, and if the X and Y subtargets are to be measured with separate profiles in any case.

[0074] Figure 8 1 illustrates the measurement of a target (such as a typical μDBO target) measured using a measurement illumination configured according to the concepts disclosed herein. Figure 6 The only difference compared to the example of FIG. 1 is that the measurement illumination no longer comprises a single measurement spot 31 covering the entire target, but the measurement illumination comprises four configured measurement spots 81, each of which is configured to correspond to a region of interest (e.g., reference region 31) of each sub-target 32 ​​to 34. Figure 7 The ROI in the image above corresponds to the corresponding target area.

[0075] In the case where the illuminator, illumination device or spot size selector is a programmable illuminator or a pixel-based illuminator (e.g., an illumination device using a spatial light modulator (SLM) or grating light valve (GLV) technology), such control can be performed on a (for example) per-pixel basis via the illumination pixel. GLV technology is sold by Silicon Light Machines (SLM) and described in US6947613B. Such an illuminator or illumination device can be located, for example, in the illumination image plane (i.e., the image plane within the illumination branch of the metrology tool).

[0076] This method enables underfill metrology to be performed on multiple targets or sub-targets simultaneously, thus preventing any stray light from irradiating the environment and edges (both inside and outside the target), while not requiring very accurate illumination alignment of the targets.

[0077] The first step of the proposed method can be roughly performed, for example, during pattern recognition / ROI identification, using very low integration times. For example, this can be done for each target. For example, in the case where all areas not corresponding to the ROI are identified from one acquisition and are turned off, the illumination configuration can be performed in a single step. Alternatively, the illumination configuration can be performed iteratively, wherein each iteration checks (or continuously images and checks) the image for edge effects and / or other undesirable effects until all target areas or pixels that do not irradiate the target area corresponding to the image ROI are turned off. As an alternative (or additional) to such feedback-based illumination calibration, a feed-forward approach can be implemented, in which each successive measurement is used to update the expected illumination configuration (irradiated pixels) for the next measurement; compared to current overfilling metrology techniques, such an approach can result in zero throughput impact after measuring the first target. If the target positioning within the image / detector coordinate system is sufficiently predictable, another way in which the acquisition time can be improved is by performing an initial calibration of the illumination configuration for the target.

[0078] An alternative embodiment that can be implemented by a simpler illumination device (such as the spot size selector currently used) can include introducing a suitable pre-configured mask (e.g., at the illumination image plane) into the illumination, which is arranged or optimized to irradiate the target or sub-target in an underfilled manner. For example, for a target including four sub-targets of a certain size, the mask can include four similarly sized rectangular holes that accurately illuminate the sub-target area corresponding to the desired (e.g., μDBO / μDBF) ROI, while not irradiating its edges or other structures (e.g., configured as by Figure 8 ion as shown in the measurement spot 81 in FIG.

[0079] The basic approach may be the same, for example, performing an overfill measurement (e.g. using a conventional measurement spot) to check that the positioning is correct, followed by switching to the configured illumination using a mask. As previously described, this may be done only for the first measurement, wherein the positioning is corrected on subsequent measurements based on this first measurement and any other previous acquisitions (e.g. if the target positioning is sufficiently predictable as previously described).

[0080] The disadvantage of this approach compared to the pixelated illuminator (eg GLV) approach is that it is inherently less flexible; the mask or spot size selector will need to be changed for each different target. However, this approach is simpler and cheaper to implement in hardware.

[0081] Intermediate solutions such as an "aperture" or configurable hole (eg similar to a camera hole) which enable specific parameterized size variations of the spot size selector hole for different objectives are also conceivable.

[0082] Determining which pixels to turn off (e.g., at the illumination image plane) via a spot size selector or other illumination device can be implemented, for example, in a manner similar to pixel mapping. For example, a method can iteratively turn off pixels and check for performance indicators that indicate a good image (e.g., indicating a lack of edge effects or other imaging effects). For example, this can be performed until such edge effects and / or unwanted imaging effects are minimized and / or below a threshold level. This can be done online or in a setup phase to minimize throughput losses.

[0083] In an optional embodiment, the methods disclosed herein may be combined with dynamic pupil pixel selection. It will be appreciated that, to date, the above embodiments relate to illumination configuration within the image plane of the illumination branch; this is necessary to select only a portion of the target for illumination. However, illumination pupil configuration is a known method for reducing edge effects (e.g., using NA limiting apertures) and enhancing performance. Configuring the illumination pupil cannot result in selectable illumination of only a portion of the target. In addition, the NA limiting aperture results in a throughput penalty, which dynamic pupil pixel selection will mitigate. The illumination beam profile configuration and the dynamic pupil pixel configuration may be jointly optimized to minimize, for example, edge effects or other unwanted imaging effects. This may be performed, for example, based on image acquisition and evaluation in a (e.g., iterative) feedback loop until such edge effects and / or unwanted imaging effects are minimized and / or below a threshold level; for example, as just described, but both the dynamic pupil pixel selection and the illumination (image plane) pixel selection are changed within multiple iterations. As an alternative to full co-optimization, each of the optimizations may be performed in succession.

[0084] The concepts disclosed herein are primarily disclosed in the context of overlap measurements. However, the method is not limited thereto. For example, a target may be formed with an asymmetry based on focal length (i.e., an asymmetry that depends on the actual scanner focal length used to expose the target). The methods disclosed herein may also be applied to focal length measurements based on these targets (e.g., diffraction-based focal length DBF or micro-diffraction-based focal length μDBF), in which case the determined model will have deviations in the measurement parameters relative to the inferred focal length. Similarly, the concepts disclosed herein may be applied to other (e.g., overlap) target types and measurement techniques, such as continuous diffraction-based overlap (cDBO) targets, and corresponding cDBO measurement techniques, or image-based overlap (IBO) targets and corresponding IBO measurement techniques. Anything imaged in a finite box (e.g., in the field plane) may be obtained from this method.

[0085] Alternative solutions can be envisioned within the context of the present invention. For example, where each of the sub-targets is of similar size (as is typical), the illumination can be configured for a single sub-target and divided into multiple beams to illuminate the other sub-targets. For example, where the target includes four sub-targets, the illumination can be configured based on one of the sub-targets and divided into four beams to irradiate the four sub-targets simultaneously. Similarly, the illumination can be configured based on two of the sub-targets, where such configured illumination is doubled to irradiate all four sub-targets. Of course, if the target includes only two sub-targets, the single configured beam can be split into two configured beams. For example, the splitting of the configured beam can be performed by the measurement tool optics (e.g., using a pupil symmetrizer of the optical elements in some measurement tools, which replicates the pupil, rotates the pupil 180 degrees, and superimposes the pupil with the original pupil so that the pupil is symmetrical).

[0086] Further embodiments according to the invention are presented in the following numbered aspects:

[0087] 1. A measurement method, comprising:

[0088] irradiating a target on a substrate including one or more sub-targets using underfill illumination such that an illumination beam profile underfills each of the one or more sub-targets;

[0089] capturing scattered radiation resulting from said irradiating said target;

[0090] imaging the scattered radiation at a detection image plane to obtain an image; and

[0091] A parameter of interest is determined from the imaged scattered radiation.

[0092] 2. The measurement method according to aspect 1, wherein the step of irradiating the target comprises:

[0093] The illumination beam profile on the target is configured to irradiate one or more target regions corresponding to one or more regions of interest on the detected image at the detected image plane while minimizing irradiation of obstructing structures on the substrate located outside the one or more target regions.

[0094] 3. The measurement method according to aspect 2, wherein the obstructing structure comprises one or more of the following: at least an edge of the target or the one or more sub-targets, and / or at least one surrounding structure.

[0095] 4. The measurement method according to any preceding aspect, wherein the target comprises two or more sub-targets;

[0096] The irradiating step includes: irradiating each sub-target simultaneously using underfill irradiation; and

[0097] The capturing step includes simultaneously capturing scattered radiation generated by the irradiation of each sub-target.

[0098] 5. The metrology method according to aspect 4, wherein the illumination beam profile is configured only for a single subset or an appropriate subset of the sub-targets; and

[0099] The illumination beam profile is divided into additional beams for providing illumination for all of the sub-targets.

[0100] 6. The measurement method according to aspect 4 or 5 comprises: changing the relative intensity of the radiation irradiating each corresponding sub-target.

[0101] 7. The metrology method according to any preceding aspect, wherein the method iteratively varies the illumination beam profile and checks a performance metric indicative of an acceptable image until the image is acceptable according to the performance metric.

[0102] 8. The measurement method according to clause 7, wherein the performance indicator comprises a measure of whether edge effects and / or unwanted imaging effects are present in the image.

[0103] 9. The measurement method according to any of the preceding aspects, wherein:

[0104] performing the method for a plurality of targets; and

[0105] Each successive measurement is used to update the expected illumination beam profile for the next measurement.

[0106] 10. The metrology method according to any preceding aspect, comprising: performing an initial calibration of the illumination beam profile for the target.

[0107] 11. The measurement method according to any preceding aspect, comprising: configuring the illumination beam profile at an illumination image plane.

[0108] 12. The metrology method according to any preceding aspect, comprising: configuring the illumination beam profile using a programmable illumination device.

[0109] 13. The metrology method according to clause 12, wherein the programmable illumination device comprises a spatial light modulator or an illumination device based on a grating light valve.

[0110] 14. The metrology method according to any of clauses 1 to 11, comprising configuring the illumination beam profile using a mask comprising one or more apertures optimized for the illumination of the target.

[0111] 15. The measurement method according to aspect 14, wherein the one or more holes are configurable.

[0112] 16. The measurement method according to any preceding aspect, comprising: additionally configuring the illumination pupil of the illumination at an illumination pupil plane.

[0113] 17. The metrology method according to clause 16, comprising: jointly optimizing the illumination beam profile and the illumination pupil.

[0114] 18. A measurement method, comprising:

[0115] irradiating a target on a substrate comprising a plurality of sub-targets by simultaneously irradiating each sub-target using a configured illumination beam profile, the configured illumination beam profile being configured for individually irradiating each sub-target of the plurality of sub-targets;

[0116] capturing scattered radiation resulting from said irradiating said target; and

[0117] A parameter of interest is determined from the scattered radiation.

[0118] 19. The measurement method according to aspect 18, wherein the step of irradiating the target comprises:

[0119] The illumination beam profile on the target is configured to illuminate multiple target areas while minimizing irradiation of interfering structures on the substrate located outside one or more edges of the one or more target areas and / or the sub-targets, each target area corresponding to a corresponding sub-target of the multiple sub-targets.

[0120] 20. A metrology method according to clause 18 or 19, wherein the illumination beam profile is configured only for a single subset or an appropriate subset of the sub-targets; and

[0121] The illumination beam profile is divided into additional beams to provide illumination for all of the plurality of sub-targets.

[0122] 21. A metrology method according to any of clauses 18 to 20, wherein the method iteratively varies the illumination beam profile and checks a performance metric related to the captured scattered radiation until the captured scattered radiation is acceptable according to the performance metric.

[0123] 22. A metrology method according to clause 21, wherein the performance indicator comprises whether edge effects and / or unwanted imaging effects affect the measure of the captured scattered radiation.

[0124] 23. The metrology method according to any one of aspects 18 to 22, wherein the method is performed for a plurality of targets; and

[0125] Each successive measurement is used to update the expected illumination beam profile for the next measurement.

[0126] 24. A metrology method according to any one of clauses 18 to 23, comprising: performing an initial calibration of the illumination beam profile for the target.

[0127] 25. The measurement method according to any one of aspects 18 to 24, comprising: configuring the illumination beam profile at an illumination image plane.

[0128] 26. A metrology method according to any one of aspects 18 to 25, comprising: using a programmable illumination device to configure the illumination beam profile.

[0129] 27. The metrology method according to clause 26, wherein the programmable illumination device comprises a spatial light modulator or an illumination device based on a grating light valve.

[0130] 28. A metrology method according to any of clauses 18 to 25, comprising configuring the illumination beam profile using a mask comprising one or more apertures optimized for the illumination of the target.

[0131] 29. The measurement method according to aspect 28, wherein the one or more holes are configurable.

[0132] 30. The metrology method according to any one of clauses 18 to 29, comprising: further configuring an illumination pupil of the illumination at an illumination pupil plane.

[0133] 31. The measurement method according to aspect 30 comprises: jointly optimizing the illumination beam profile and the illumination pupil.

[0134] 32. A measurement method according to any preceding aspect, comprising: varying the relative intensity of the radiation irradiating each corresponding sub-target.

[0135] 33. A measurement method according to any preceding aspect, wherein the step of irradiating the target comprises: irradiating each sub-target simultaneously using underfill irradiation, so that the irradiation beam profile underfills each sub-target in the plurality of sub-targets.

[0136] 34. A method according to any preceding aspect, wherein the parameter of interest is overlap or focal length.

[0137] 35. A method according to any preceding aspect, comprising: performing the method in-line as part of a lithography process.

[0138] 36. The method according to aspect 35, further comprising:

[0139] exposing the at least one target onto a substrate;

[0140] executing the measurement method; and

[0141] The parameter value of interest is used to correct a subsequent exposure step on a subsequent substrate.

[0142] 37. A measurement device operable to perform a method according to any one of aspects 1 to 35.

[0143] 38. A metrology device according to clause 37, comprising: imaging optics for capturing the scattered radiation from the target; and a detector for detecting the scattered radiation to obtain an image of the target.

[0144] 39. The measurement device according to aspect 37 or 38, wherein the measurement device is a scatterometer.

[0145] 40. The measuring device according to aspect 37 or 38 is a dark-field holographic microscope.

[0146] 41. A lithocell comprising: a metrology apparatus according to any one of clauses 37 to 40; and a lithographic apparatus.

[0147] 42. A lithocell according to clause 41, operable to perform a method according to clause 36.

[0148] 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 patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0149] 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 undesirable structure on the substrate.

[0150] Although specific reference is made to "measurement equipment / tool / system" or "inspection equipment / tool / system", 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 a structure 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 the structure on the substrate may be related to a defect in the structure, the absence of a particular portion of the structure, or the presence of an undesirable structure on the substrate or on the wafer.

[0151] 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.

[0152] While the targets or target structures (more generally, structures on a substrate) described above are metrology target structures specifically designed and formed for the purpose of measurement, in other embodiments, the property of interest may be measured on one or more structures that are a functional part of a device formed on a substrate. Many devices have a regular grating-like structure. The terms "structure," "target grating," and "target structure" as used herein do not require that the structure be provided specifically for the measurement being performed. In addition, the pitch P of the metrology target may be close to the resolution limit of the optical system of the scatterometer or may be smaller, but may be much larger than the size of a typical product feature manufactured by the lithography process in the target portion C. In practice, the rows and / or spaces of the overlapping gratings within the target structure may be made to include smaller structures that are similar in size to the product features.

[0153] 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 rather than restrictive. Therefore, it will be appreciated by those skilled in the art that the present invention may be modified as described without departing from the scope of the claims set forth below.

Claims

1. A measurement method, comprising: irradiating a target on a substrate including one or more sub-targets using underfill illumination such that an illumination beam profile underfills each of the one or more sub-targets; capturing scattered radiation resulting from said irradiating said target; imaging the scattered radiation at a detection image plane to obtain an image; as well as A parameter of interest is determined from the imaged scattered radiation.

2. The measuring method according to claim 1, wherein: The step of irradiating the target comprises: The illumination beam profile on the target is configured to illuminate one or more target regions corresponding to one or more regions of interest on the detected image at the detected image plane while minimizing irradiation of obstructing structures on the substrate located outside the one or more target regions.

3. The measuring method according to claim 2, wherein: The obstructing structure comprises one or more of the following: at least an edge of the target or the one or more sub-targets, and / or at least one surrounding structure.

4. A measurement method according to any preceding claim, wherein: The goal includes two or more sub-goals; The irradiating step includes: irradiating each sub-target simultaneously using underfill irradiation; and The capturing step includes simultaneously capturing scattered radiation generated by the irradiation of each sub-target.

5. The measuring method according to claim 4, wherein: configuring the illumination beam profile only for a single subset or an appropriate subset of the sub-targets; and The illumination beam profile is divided into additional beams for providing illumination for all of the sub-targets.

6. The measuring method according to claim 4 or 5, comprising: The relative intensity of the radiation irradiating each corresponding sub-target is varied.

7. A measurement method according to any preceding claim, wherein: The method iteratively changes the illumination beam profile and checks for a performance metric indicative of an acceptable image until the image is acceptable according to the performance metric.

8. A measurement method according to any preceding claim, wherein: The method is performed for a plurality of targets; and each successive measurement is used to update the expected illumination beam profile for the next measurement.

9. A measurement method according to any preceding claim, comprising: An initial calibration of the illumination beam profile is performed for the target.

10. A measurement method according to any preceding claim, comprising: The illumination beam profile is configured at an illumination image plane.

11. A measurement method according to any preceding claim, comprising: The illumination beam profile is configured using a programmable illumination device.

12. The measurement method according to any one of claims 1 to 10, comprising: The illumination beam profile is configured using a mask including one or more apertures optimized for the illumination of the target.

13. The measuring method according to claim 12, wherein: The one or more holes are configurable.

14. The measurement method according to any preceding claim, further comprising: The illumination pupil is configured to be illuminated at an illumination pupil plane.

15. The measuring method according to claim 14, comprising: The illumination beam profile and the illumination pupil are jointly optimized.

Images