Determining a fringe height reduction factor for an interference pattern

Abstract

One embodiment provides a method comprising obtaining a Fourier transform of an interference pattern, obtaining a central band component from the Fourier transform of the interference pattern, obtaining a sideband component from the Fourier transform of the interference pattern, obtaining a central band inverse Fourier transform image by performing an inverse Fourier transform of the central band component, obtaining a sideband inverse Fourier transform image by performing an inverse Fourier transform of the sideband component, and determining a fringe height reduction factor, the fringe height reduction factor corresponding to an interference fringe height reduction due to motion blur in the interference pattern, based on a comparison between the sideband inverse Fourier transform image and the central band inverse Fourier transform image.

Description

DETERMINING A FRINGE HEIGHT REDUCTION FACTOR FOR AN INTERFERENCE PATTERNCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 712,678 which was filed on October 28, 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present disclosure relates to metrology such as digital holographic microscopy and in particular high speed dark field digital holographic microscopy and in relation to metrology applications in the manufacture of integrated circuits.BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern (also often referred to as “design layout” or “design”) at 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 on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] Low-ki lithography may be used to process features with dimensions smaller than the classical resolution limit of a lithographic apparatus. In such process, the resolution formula may be expressed as CD = k₁×λ / NA, where X is the wavelength of radiation employed, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the “critical dimension” (generally the smallest feature size printed, but in this case half-pitch) and k₁ is an empirical resolution factor. In general, the smaller k₁ the more difficult it becomes to reproduce the pattern on the substrate that resembles the shape and dimensions planned by a circuit designer in order to achieve particular electrical functionality and performance. To overcome these difficulties, sophisticated fine-tuning steps may be applied to the lithographic projection apparatus and / or design layout. These include, for example, but not limited to, optimization of NA, customized illumination schemes, use of phase shifting patterning devices, various optimization 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, tight control loops for controlling a stability of the lithographic apparatus may be used to improve reproduction of the pattern at low k1.

[0006] During the manufacturing process there is a need to inspect the manufactured structures and / or to measure characteristics of the manufactured structures. Suitable inspection and metrology apparatuses are known in the art. One of the known metrology apparatuses is a dark field holographic microscope.

[0007] In some instances, an image obtained using holographic microscopy may be subject to blurring, such as due to movement of optical elements and / or the substrate (e.g. caused by vibrations). In addition, in some cases, it may be desirable to acquire multiple images of a given structure, e.g. to compensate for the limited full well capacity of the camera(s) used for imaging.SUMMARY

[0008] It is therefore desirable to mitigate the effects of blurring of images obtained by a metrology apparatus.

[0009] In a first aspect of the disclosure, there is provided a method (e.g. a method for determining a fringe height reduction factor) comprising: obtaining a Fourier transform of an interference pattern, obtaining a central band component from the Fourier transform of the interference pattern, obtaining a sideband component from the Fourier transform of the interference pattern, obtaining a central inverse Fourier transform image by performing an inverse Fourier transform of the central band component, obtaining a sideband inverse Fourier transform image by performing an inverse Fourier transform of the sideband component, and determining a fringe height reduction factor, the fringe height reduction factor corresponding to an interference fringe height reduction due to motion blur in the interference pattern, based on a comparison between the sideband inverse Fourier transform image and the central band inverse Fourier transform image.

[0010] In a second aspect of the disclosure, there is provided a non-transitory computer readable medium comprising instructions which, when executed by a processor of a computer device, cause the computing device to carry out operations comprising: obtaining a Fourier transform of an interference pattern, obtaining a central band component from a Fourier transform of the interference pattern, obtaining a sideband component from the Fourier transform of the interference pattern, obtaining a central band inverse Fourier transform image by performing an inverse Fourier transform of the central band component, obtaining a sideband inverse Fourier transform image by performing an inverse Fourier transform of the sideband component, and determining a fringe height reduction factor, the fringe height reduction factor corresponding to an interference fringe height reduction due to motion blur in the interference pattern, based on a comparison between the sideband inverse Fourier transform image and the central band inverse Fourier transform image.

[0011] In a third aspect of the disclosure, there is provided an apparatus (e.g. an exposure apparatus) comprising: an imaging device configured to capture an interference pattern produced by radiationdiffracted by a target on a substrate, and a processor, the processor configured to: obtain a Fourier transform of the interference pattern, obtain a central band component from the Fourier transform of the interference pattern, obtain a sideband component from the Fourier transform of the interference pattern, perform an inverse Fourier transform of the central band component to produce a central band inverse Fourier transform image, perform an inverse Fourier transform image of the sideband component to produce a sideband inverse Fourier transform image, and determine a fringe height reduction factor based on a comparison between the sideband inverse Fourier transform image and the central band inverse Fourier transform image, the fringe height reduction factor corresponding to an interference fringe height reduction in the interference pattern due to motion of the target and / or the apparatus.

[0012] It will be understood that the sideband component may be a first sideband component, and that, in some examples, a second sideband component may be obtained from the Fourier transform of the interference pattern. The first sideband component and the second sideband component may correspond, for example, to a positive sideband component and a negative sideband component, respectively. The sideband inverse Fourier transform image described above may be a first sideband inverse Fourier transform image. A second sideband inverse Fourier transform image may be obtained by performing an inverse Fourier transform of the second sideband component. In some examples, the fringe height reduction factor may be a first fringe height reduction factor. The first fringe height reduction factor may be determined based on a comparison between the central band inverse Fourier transform image and one or both of the first and second sideband inverse Fourier transform images. In some examples a second fringe height reduction factor may be determined based on a comparison between one or both of the first and second sideband inverse Fourier transform images.

[0013] In some examples, there may be more than two sideband components. More generally, a plurality of sideband components may be obtained from the Fourier transform of the interference pattern, and a plurality of sideband inverse Fourier transform images may be determined by performing inverse Fourier transforms on each sideband component.

[0014] In some examples, one or more fringe height reduction factors may be determined based on a comparison between the central band inverse Fourier transform image and the sideband inverse Fourier transform image, or based on a comparison between the central band inverse Fourier transform image and the plurality of sideband inverse Fourier transform images described above.

[0015] In some examples, the interference pattern is obtained by measurement, e.g. by an imaging device (such as a camera) of a suitable metrology apparatus (for example a holographic metrology apparatus such as a dark field holographic microscope) as described herein, and may then be provided to a processor to perform the Fourier transform as described herein. In some examples, the interference pattern is retrieved from memory. For example, the interference pattern may be obtained using an appropriate apparatus, for example a holographic metrology apparatus such as a dark field holographic microscope (e.g. via an imaging device of such an apparatus). The obtained interference pattern maythen be saved to a memory (e.g. a memory of a computer device connected to the apparatus, in particular to the imaging device). The saved interference pattern may subsequently be retrieved from the memory for processing in accordance with the present disclosure.

[0016] The fringe height reduction factor(s) may be used to determine a characteristic of the target, such as overlay. For example, the fringe height reduction factor(s) may be used to determine (e.g. quantify) an impact of motion blur on the interference pattern. This information may enable a more accurate measurement of the characteristic (e.g. overlay) to be determined. In some examples, the fringe height reduction factor(s) may be used to compensate the fringe height reduction in the interference pattern due to motion of the target and / or the apparatus (e.g. the imaging device).

[0017] Embodiments of the disclosure described herein advantageously enable the effect of motion blur (e.g. due to vibrations in a metrology apparatus and / or the target) to be determined computationally without the need for any additional hardware. For example, the impact of vibrations on the measurement can be determined simply by obtaining one or more calibration images and processing those calibration images according to the method(s) described herein.BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 depicts a schematic overview of a lithographic apparatus according to embodiments of the disclosure.Figure 2 depicts a schematic overview of a lithographic cell according to embodiments of the disclosure.Figure 3 depicts a schematic representation of holistic lithography, representing a cooperation between three key technologies to optimize semiconductor manufacturing according to embodiments of the disclosure.Figure 4 depicts a schematic overview of a scatterometry apparatus used as a metrology device, which may comprise a dark field digital holographic microscope according to embodiments of the disclosure according to embodiments of the disclosure.Figure 5 depicts a schematic overview of a dark field digital holographic microscope, adapted for use in lithographic process metrology, according to embodiments of the disclosure.Figure 6 depicts a schematic overview of an imaging branch of a dark field digital holographic microscope according to embodiments of the disclosureFigure 7 schematically illustrates fringe height reduction in an interference pattern due to motion blur according to embodiments of the disclosure.Figure 8 is a flow diagram of a method of obtaining a central band inverse Fourier transform image and sideband inverse Fourier transform images according to embodiments of the disclosure.Figure 9 shows a relationship between a central band inverse Fourier transform image and sideband inverse Fourier transform images according to embodiments of the disclosure.Figure 10 shows determination of fringe height reduction factors by error optimization according to embodiments of the disclosure.Figure 11 illustrates the variation in measured fringe height reduction factors determined from multiple measurements according to embodiments of the disclosure.Figure 12 is a block diagram of an example of a method according to embodiments of the disclosure.Figure 13 depicts a block diagram of a computer system for implementing the methods described herein according to embodiments of the disclosure.DETAILED DESCRIPTION

[0019] In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range of about 5-100 nm).

[0020] The term “reticle”, “mask” or “patterning device” as employed in this text may be broadly interpreted as referring to a generic patterning device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Besides the classic mask (transmissive or reflective, binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include a programmable mirror array and a programmable LCD array.

[0021] Figure 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation or EUV radiation), a mask support (e.g., a mask table) MT constructed 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 in accordance with certain parameters, a substrate support (e.g., a wafer table) WT constructed 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 in accordance with 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 patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0022] In operation, the illumination system IL receives a radiation beam from a radiation source SO, e.g. via a 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 radiation.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 a plane of the patterning device MA.

[0023] The term “projection system” PS used herein should be broadly interpreted as encompassing various types of projection system, 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 for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system” PS.

[0024] The lithographic apparatus LA may be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system PS and the substrate W - which is also referred to as immersion lithography. More information on immersion techniques is given in US6952253, which is hereby incorporated herein by reference in its entirety.

[0025] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also named “dual stage”). In such “multiple stage” machine, the substrate supports WT may be used in parallel, and / or steps in preparation of a subsequent exposure of the substrate W may be carried out on the substrate W located on one of the substrate support WT while another substrate W on the other substrate support WT is being used for exposing a pattern on the other substrate W.

[0026] 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 a part of the projection system PS or a part of a system that provides the immersion liquid. The measurement stage may move beneath the projection system PS when the substrate support WT is away from the projection system PS.

[0027] In operation, the radiation beam B is incident on the patterning device, e.g. mask, MA which is held on the mask support MT, and is patterned by the pattern (design layout) present on patterning device MA. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and a position measurement system IF, the substrate support WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B at a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in Figure 1) may be used to accurately position the patterning device MA with respect to the path of the radiation beam B. 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 locatedin spaces between target portions. Substrate alignment marks Pl, P2 are known as scribe-lane alignment marks when these are located between the target portions C.

[0028] As shown in Figure 2 the lithographic apparatus LA may form part of a lithographic cell LC, also sometimes referred to as a lithocell or (litho)cluster, which often also includes apparatus to perform pre- and post-exposure processes on a substrate W. Conventionally these include spin coaters SC to deposit resist layers, developers DE to develop exposed resist, chill plates CH and bake plates BK, e.g. for conditioning the temperature of substrates W e.g. for conditioning solvents in the resist layers. A substrate handler, or robot, RO picks up substrates W from input / output ports I / O1, I / O2, moves them between the different process apparatus and delivers the substrates W to the loading bay LB of the lithographic apparatus LA. The devices in the lithocell, which are often also collectively referred to as the track, are typically under the control of a track control unit TCU that in itself may be controlled by a supervisory control system SCS, which may also control the lithographic apparatus LA, e.g. via lithography control unit LACU.

[0029] In order for the substrates W exposed by the lithographic apparatus LA to be exposed correctly and consistently, it is desirable to inspect substrates to measure properties of patterned structures, such as overlay errors between subsequent layers, line thicknesses, critical dimensions (CD), etc. For this purpose, inspection tools (not shown) may be included in the lithocell LC. If errors are detected, adjustments, for example, may be made to exposures of subsequent substrates or to other processing steps that are to be performed on the substrates W, especially if the inspection is done before other substrates W of the same batch or lot are still to be exposed or processed.

[0030] An inspection apparatus, which may also be referred to as a metrology apparatus, is used to determine properties of the substrates W, and in particular, how properties of different substrates W vary or how properties associated with different layers of the same substrate W vary from layer to layer. The inspection apparatus may alternatively be constructed to identify defects on the substrate W and may, for example, be part of the lithocell LC, or may be integrated into the lithographic apparatus LA, or may even be a stand-alone device. The inspection apparatus may measure the properties on a latent image (image in a resist layer after the exposure), or on a semi -latent image (image in a resist layer after a post-exposure bake step PEB), or on a developed resist image (in which 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).

[0031] Typically the patterning process in a lithographic apparatus LA is one of the most critical steps in the processing which requires high accuracy of dimensioning and placement of structures on the substrate W. To ensure this high accuracy, three systems may be combined in a so called “holistic” control environment as schematically depicted in Figure 3. One of these systems is the lithographic apparatus LA which is (virtually) connected to a metrology tool MT (a second system) and to a computer system CL (a third system). The key of such “holistic” environment is to optimize the cooperation between these three systems to enhance the overall process window and provide tight control loops toensure that the patterning performed by the lithographic apparatus LA stays within a process window. The process window defines a range of process parameters (e.g. dose, focus, overlay) within which a specific manufacturing process yields a defined result (e.g. a functional semiconductor device) -typically within which the process parameters in the lithographic process or patterning process are allowed to vary.

[0032] The computer system CL may use (part of) the design layout to be patterned to predict which resolution enhancement techniques to use and to perform computational lithography simulations and calculations to determine which mask layout and lithographic apparatus settings achieve the largest overall process window of the patterning process (depicted in Figure 3 by the double arrow in the first scale SCI). Typically, the resolution enhancement techniques are 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 the metrology tool MT) to predict whether defects may be present due to e.g. sub-optimal processing (depicted in Figure 3 by the arrow pointing “0” in the second scale SC2).

[0033] The metrology tool MT may provide input to the computer system CL to enable accurate simulations and predictions, and may provide feedback to the lithographic apparatus LA to identify possible drifts, e.g. in a calibration status of the lithographic apparatus LA (depicted in Figure 3 by the multiple arrows in the third scale SC3).

[0034] In lithographic processes, it is desirable to make frequently measurements of the structures created, e.g., for process control and verification. Tools to make such measurement are typically called metrology tools MT. Different types of metrology tools MT for making such measurements are known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. Scatterometers are versatile instruments which allow measurements of the parameters of a lithographic process by having a sensor in the pupil or a conjugate plane with the pupil of the objective of the scatterometer, measurements usually referred as pupil based measurements, or by having the sensor in the image plane or a plane conjugate with the image plane, in which case the measurements are usually referred as image or field based measurements. Such scatterometers and the associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP1,628,164A, which are all incorporated herein by reference in their entireties. Aforementioned scatterometers may measure gratings using light from soft x-ray and visible to near-IR wavelength range.

[0035] In a first embodiment, the scatterometer MT is an angular resolved scatterometer. In such a scatterometer reconstruction methods may be applied to the measured signal to reconstruct or calculate properties of the grating. Such reconstruction may, for example, result from simulating interaction of scattered radiation with a mathematical model of the target structure and comparing the simulation results with those of a measurement. Parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from the real target.

[0036] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such spectroscopic scatterometer MT, the radiation emitted by a radiation source is directed onto the target and the reflected or scattered radiation from the target is directed to a spectrometer detector, which measures a spectrum (i.e. a measurement of intensity as a function of wavelength) of the specular reflected radiation. From this data, the structure or profile of the target giving rise to the detected spectrum may be reconstructed, e.g. by Rigorous Coupled Wave Analysis and non-linear regression or by comparison with a library of simulated spectra.

[0037] In a third embodiment, the scatterometer MT is a ellipsometric scatterometer. The ellipsometric scatterometer allows for determining parameters of a lithographic process by measuring scattered radiation for each polarization states. Such metrology apparatus emits polarized light (such as linear, circular, or elliptic) by using, for example, appropriate polarization filters in the illumination section of the metrology apparatus. A source suitable for the metrology apparatus may provide polarized radiation as well. Various embodiments of existing ellipsometric scatterometers are described in US 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 all incorporated herein by reference in their entireties.

[0038] 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 asymmetry in the reflected spectrum and / or the detection configuration, the asymmetry being related to the extent of the overlay. The two (typically overlapping) grating structures may be applied in two different layers (not necessarily consecutive layers), and may be formed substantially at the same position on the wafer. The scatterometer may have a symmetrical detection configuration as described e.g. in co-owned patent application EP1,628,164A, such that any asymmetry is clearly distinguishable. This provides a straightforward way to measure misalignment in gratings. Further examples for measuring overlay error between the two layers containing periodic structures as target is measured through asymmetry of the periodic structures may be found in PCT patent application publication no. WO 2011 / 012624 or US patent application US 20160161863, which are incorporated herein by reference in their entireties.

[0039] 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 by reference herein in its entirety. A single structure may be used which has a unique combination of critical dimension and sidewall angle measurements for each point in a focus energy matrix (FEM - also referred to as Focus Exposure Matrix). If these unique combinations of critical dimension and sidewall angle are available, the focus and dose values may be uniquely determined from these measurements.

[0040] A metrology target may be an ensemble of composite gratings, formed by a lithographic process, mostly in resist, but also after etch process for example. Typically the pitch and line-width of the structures in the gratings strongly depend on the measurement optics (in particular the NA of theoptics) to be able to capture diffraction orders coming from the metrology targets. As indicated earlier, the diffracted signal may be used to determine shifts between two layers (also referred to ‘overlay’) or may be used to reconstruct at least part of the original grating as produced by the lithographic process. This reconstruction may be used to provide guidance of the quality of the lithographic process and may be used to control at least part of the lithographic process. Targets may have smaller sub-segmentation which are configured to mimic dimensions of the functional part of the design layout in a target. Due to this sub-segmentation, the targets will behave more similar to the functional part of the design layout such that the overall process parameter measurements resembles the functional part of the design layout better. The targets may be measured in an underfilled mode or in an overfilled mode. In the underfilled mode, the measurement beam generates a spot that is smaller than the overall target. In the overfilled mode, the measurement beam generates a spot that is larger than the overall target. In such overfilled mode, it may also be possible to measure different targets simultaneously, thus determining different processing parameters at the same time.

[0041] A metrology apparatus, such as a scatterometer, is depicted in Figure 4. It comprises a broadband (white light) radiation projector 2 which projects radiation onto a substrate W. The reflected or scattered radiation is passed to a spectrometer detector 4, which measures a spectrum 6 (i.e. a measurement of intensity as a function of wavelength) of the specular reflected radiation. From this data, the structure or profde 8 giving rise to the detected spectrum may be reconstructed by processing unit PU, e.g. by Rigorous Coupled Wave Analysis and non-linear regression or by comparison with a library of simulated spectra as shown at the bottom of Figure 3. In general, 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 made, 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.

[0042] Overall measurement quality of a lithographic parameter via measurement of a metrology target is at least partially determined by the measurement recipe used to measure this lithographic parameter. The term “substrate measurement recipe” may include one or more parameters of the measurement itself, one or more parameters of the one or more patterns measured, or both. For example, if the measurement used in a substrate measurement recipe is a diffraction-based optical measurement, one or more of the parameters of the measurement may include the wavelength of the radiation, the polarization of the radiation, the incident angle of radiation relative to the substrate, the orientation of radiation relative to a pattern on the substrate, etc. One of the criteria to select a measurement recipe may, for example, be a sensitivity of one of the measurement parameters to processing variations. More examples are described in US patent application US20160161863 and US patent application US20160370717A1, which are incorporated herein by reference in their entireties.

[0043] In order to monitor the lithographic process, parameters of the patterned substrate are measured. Parameters may include, for example, the overlay error between successive layers formed in or on thepatterned substrate. This measurement may be performed on a product substrate and / or on a dedicated metrology target. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized tools. A fast and non-invasive form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured.

[0044] Examples of known scatterometers include angle-resolved scatterometers of the type described in US2006033921A1 and US2010201963A1, which are incorporated by reference herein in their entireties. The targets used by such scatterometers are relatively large, e.g., 40pm by 40pm, gratings and the measurement beam generates a spot that is smaller than the grating (i.e., the grating is underfilled). In addition to measurement of feature shapes by reconstruction, diffraction based overlay can be measured using such apparatus, as described in published patent application US2006066855A1, which is incorporated by reference herein in its entirety. Diffraction-based overlay metrology using dark field imaging of the diffraction orders enables overlay measurements on smaller targets. Examples of dark field imaging metrology can be found in international patent applications WO 2009 / 078708 and WO 2009 / 106279 which are incorporated by reference herein in their entireties. Further developments of the technique have been described in published patent publications US20110027704A, US20110043791A, US2011102753A1, US20120044470A, US20120123581A, US20130258310A, US20130271740A and WO2013178422A1, which are all incorporated by reference herein in their entities. These targets can be smaller than the illumination spot and may be surrounded by product structures on a wafer. Multiple gratings can be measured in one image, using a composite grating target. The contents of all these applications are also incorporated herein by reference.

[0045] In a diffraction-based dark field metrology device, a beam of radiation is directed onto a metrology target and one or more properties of the scattered radiation are measured so as to determine a property of interest of the target. The properties of the scattered radiation may comprise, for example, intensity at a single scattering angle (e.g., as a function of wavelength) or intensity at one or more wavelengths as a function of scattering angle.

[0046] Measurement of targets in dark field metrology may comprise, for example, measuring the a first intensity of the 1stdiffraction order I+i and a second intensity of the -1stdiffraction order I i and calculating an intensity asymmetry (A=I+i- 1 i), which is indicative of asymmetry in the target. The metrology targets may comprise one or more grating structures from which a parameter of interest may be inferred from such intensity asymmetry measurements, e.g., the targets are designed such that the asymmetry in the target varies with the parameter of interest. For example, in overlay metrology a target may comprise at least one composite grating formed by at least a pair of overlapping sub-gratings that are patterned in different layers of the semiconductor device. Asymmetry of the target will therefore be dependent on alignment of the two layers and therefore overlay. Other targets may be formed with structures which are exposed with different degrees of variation based on the focus setting used duringthe exposure; the measurement of which enabling that focus setting to be inferred back (again through intensity asymmetry).

[0047] Patent application publication US20240160151A1 and international patent application publication W02024 / 028046A1, discuss examples of metrology apparatuses including holographic microscopes. The cited documents are hereby incorporated by reference herein in their entireties. And international patent application WO2019197117A1, incorporated herein by reference in its entirety, discloses a method and metrology apparatus based on a dark field digital holographic microscope (df-DHM) to determine a characteristic, e.g., overlay, of a structure manufactured on a substrate. Such an apparatus may be used to obtain holographic images to which the methods disclosed herein may be performed to correct the holographic images for stage drift and / or other drift parameters. However, the holographic images may be obtained by any holographic microscope or metrology tool, whether dark field or bright field. For the purpose of description, aspects of Figure 3 of the international patent application WO2019197117A1 are replicated in Figure 5. Figure 5 schematically illustrates the disclosed df-DHM specifically adapted for use in lithographic process metrology according to various embodiments.

[0048] The df-DHM in Figure 5 further comprises a reference optical unit 16, 18 which is used to provide additional two reference radiation beams 51, 52 (the reference radiation). Such two reference radiation beams 51, 52 are respectively paired with two corresponding portions 41, 42 of the scattered radiation beams 31, 32 (the object radiation). The two scattered-reference beam pairs are used sequentially to form two interference patterns. Coherence control is provided by way of adjusting the relative optical path-length difference (OPD) between the two scattered-reference beams within each beam pair. However, no coherence control is available between the two beam pairs.

[0049] Due to the use of a single light source and insufficient coherence control, all four radiation beams, i.e. the first portion 41 of the scattered radiation 31, the first reference radiation 51, the second portion 42 of the scattered radiation 32 and the second reference radiation 52, are mutually coherent. If these four mutually coherent radiation beams were allowed to reach the same position of the sensor 6 at the same time, namely operating in a parallel acquisition scheme, multiple interference patterns comprising desired information containing patterns and undesired artefact-contributing patterns would overlap each other. The undesired interference patterns may be formed by interference between e.g., the portion 41 of the first scattered radiation 31 and the portion 42 of the second scattered radiation 32. Since it would be technically challenging and time consuming to completely separate the superimposed interference patterns, parallel acquisition is impractical this arrangement.

[0050] The use of a sequential acquisition scheme in the example of Figure 5 allows the full NA of the objective lens to be available for both illumination and detection. However, the system may suffer the same problem of low measurement speed due to sequential acquisition. Therefore, it is desirable to have a df-DHM capable of performing parallel acquisition such that a high measurement speed and a high design flexibility can be simultaneously obtained.

[0051] Figure 6 schematically illustrates the imaging branch of a df-DHM 600 in accordance with an embodiment. A df-DHM comprises an imaging branch and an illumination branch. In this embodiment, a metrology target 660 comprising a structure on a substrate 650 is illuminated by two illumination beams of radiation, i.e., a first illumination beam of radiation 610 and a second illumination beam of radiation 620. In an embodiment, such two illumination beams 610, 620 may simultaneously illuminate the metrology target 660.

[0052] In an embodiment, the first illumination beam 610 may be incident on the metrology target 660 at a first angle of incidence in a first direction with respect to the optical axis OA. The second illumination beam 620 may be incident on the metrology target 660 at a second angle of incidence in a second direction with respect to the optical axis OA. The first angle of incidence of the first illumination beam 610 and the second angle of incidence of the second illumination beam 620 may be substantially the same. The angle of incidence of each illumination beam may be, for example in the range of 70 degrees to 90 degrees, in the range of 50 degrees to 90 degrees, in the range of 30 degrees to 90 degrees, in the range of 6 degrees to 90 degrees. The illumination of the metrology target 660 may result in radiation being scattered from the target. In an embodiment, the first illumination beam 610 may be incident on the metrology target 660 at a first azimuthal angle, corresponding to the first direction. The second illumination beam 620 may be incident on the metrology target 660 at a second azimuthal angle, corresponding to the second direction. The first azimuthal angle of the first illumination beam 610 and the second azimuthal angle of the second illumination beam 620 may be different; e.g., opposing angles 180 degrees apart.

[0053] Depending on the structure of the metrology target 660, the scattered radiation may comprise reflected radiation, diffracted radiation or transmitted radiation. In this embodiment, the metrology target may be a diffraction-based overlay target; and each illumination beam may correspond to a scattered beam comprising at least one non-zeroth diffraction order. Each scattered beam carries information of the illuminated metrology target. For example, the first illumination beam 610 may correspond to the first scattered beam 611 comprising the positive first diffraction order +1stDF; the second illumination beam 620 may correspond to the second scattered beam 621 comprising the negative first diffraction order -1stDF. The zeroth diffraction order and other undesired diffraction orders may either be blocked by a beam blocking element (not shown) or configured to completely fall outside the NA of the objective lens 670. As a result, the df-DHM may be operated in a dark field mode. Note that, in some embodiments, one or more optical elements, e.g., a lens combination, may be used to achieve same optical effect of the objective lens 670.

[0054] Both scattered beams 611, 621 may be collected by objective lens 670 and subsequently refocused onto an image sensor 680. Objective lens 670 may comprise multiple lenses, and / or df-DHM 600 may comprise a lens system having two or more lenses, e.g., an objective lens and an imaging lens similar to the exemplary df-DHG of Figure 5, thereby defining a pupil plane of the objective lens between the two lenses and an image plane at the focus of the imaging lens. In this embodiment, aportion 612 of the first scattered beam 611 and a portion 622 of the second scattered beam 621 are simultaneously incident at a common position of the image sensor 680. At the same time, two reference beams of radiation, i.e. a first reference beam 630 and a second reference beam 640, are incident on the same position of the image sensor 680. Such four beams may be grouped into two pairs of scattered radiation and reference radiation. For example, the first scattered-reference beam pair may comprise the portion 612 of the first scattered beam 611 and the first reference beam 630. Likewise, the portion 622 of the second scattered-reference beam pair may comprise the second scattered beam 621 and the second reference beam 640. These two scattered-reference beam pairs may subsequently form two interference patterns (holographic images) which at least partially overlap in spatial domain.

[0055] In an embodiment, in order to separate the two at least partially, spatially overlapping interference patterns (e.g., in the spatial frequency domain), the first reference beam 630 may have a first angle of incidence with respect to the optical axis OA and the second reference beam 640 may have a second angle of incidence with respect to the optical axis OA; the first angle of incidence and the second angle of incidence being different. Alternatively or in addition, the first reference beam 630 may have a first azimuthal angle with respect to the optical axis OA and the second reference beam 640 may have a second azimuthal angle with respect to the optical axis OA; the first and second azimuthal angles being different.

[0056] In order to generate an interference pattern, the two beams of each scattered-reference beam pair should be at least partially coherent to each other, to a degree which is sufficient to form an interference pattern. Note that each scattered radiation beam may have a phase offset with respect to its corresponding illumination radiation. For example, at the image plane of the image sensor 680, such a phase offset may comprise contributions due to the optical path-length (OPD) from the metrology target 660 to the image sensor 680, and by the interaction with the metrology target.

[0057] The processing unit 690 (and the processing unit 70 in Figure 5) may be a computer system. The computer system may be equipped with an image reconstruction algorithm which is used to perform all the aforementioned tasks, comprising performing Fourier transform, extracting each individual high order spatial spectrum, performing inverse Fourier transform, calculating complex fields and determining a characteristic of the structure based on the results.

[0058] As an interferometric method for measuring characteristics such as overlay, digital holographic microscopy (DHM) (such as performed using a df-DHM as described above and illustrated in Figures 5 and 6) can be very sensitive to vibrations. For example, relative motion between any of the metrology target 660, the beams 610, 620, 630, 640, and the imaging device (image sensor 680) can affect the quality of the retrieved information by introducing motion blur due to the relative shift between the wavefronts of the scattered beams arriving at the imaging device. Figure 7 illustrates schematically that, when the interference fringes vibrating back and forth (left) are integrated over time, e.g. by the capturing of an image, the fringe height 701 is effectively reduced (right). The reduced fringe height 701 can be expressed as a fraction of the true fringe height, which may be referred to as a “fringe heightreduction factor”. For example, a fringe height reduction factor for the +1stdiffraction order. / ' i. and a fringe height reduction factor for the -1stdiffraction order, i, may be defined as 0 <f±\ < 1.

[0059] In some examples, multiple images may be obtained, and may be averaged, which may also be subject to blurring due to the vibration sensitivity described above.

[0060] According to the present disclosure, one or more fringe height reduction factors may be determined computationally from an interference pattern itself, without the need for any additional measurements or hardware.

[0061] Figure 8 illustrates an interference pattern 802, that may be obtained by DHM (e.g. from the image sensor 70, 680 of a df-DHM) or by any other suitable metrology technique. In the case of DHM, the interference pattern 802 may also be referred to as a hologram. The interference pattern 802 may be used to determine a characteristic, such as overlay, of a metrology target as described herein. According to the present disclosure, a Fourier transform FT (two-dimensional Fourier transform, e.g. a fast Fourier transform) may be performed on the interference pattern 802 to produce a Fourier transform of the interference pattern 804, also referred to as an image spectrum in the spatial frequency domain. In the example illustrated in Figure 8, the Fourier transform of the interference pattern 804 comprises a central band component CB, and two high order spatial spectra or sidebands +SB, -SB (where +SB*, -SB* are the complex conjugates of the sidebands +SB, -SB).

[0062] According to the present disclosure, inverse Fourier transforms IFTs (e.g. fast inverse Fourier transforms) are performed on each of the central band component and the sidebands +SB, -SB to obtain a central band inverse Fourier transform image 806a, and two sideband inverse Fourier transform images 806b, 806c. As shown in Figure 8, the central band inverse Fourier transform image 806a is unaffected by the vibrations and contains only amplitude information (A). However the sideband inverse Fourier transform images 806b, 806c contain both amplitude (A) and phase (P) information. The inventor has found that, rather than discarding the information from the central band as in many existing approaches to metrology, the central band inverse Fourier transform image 806a may be used to determine the fringe height reduction factor(s). f±1, discussed above, via a comparison with the sideband inverse Fourier transform images 806b, 806c.

[0063] While the examples described and illustrated herein primarily relate to a Fourier transform 804 comprising two sideband components +SB, -SB, it will be understood that the methods and examples described herein are applicable to Fourier transforms of diffraction patterns comprising one sideband component, and comprising more than two sideband components.

[0064] In the following, it is assumed that the wavefront shift due to motion (vibrations) is smaller than due to aberrations during acquisition such as defocusing. Additionally, it is assumed that the reference beam shapes remain fairly constant, and that the number of photons captured from each beam remains constant for a given exposure time (see again Figures 5 and 6). In some examples, it may be desirable to minimize shot noise and / or stray light (e.g. that reaches the image sensor 680) when obtaining the interference pattern, as the central band may be particularly sensitive to these phenomena.

[0065] Referring to Figure 6, the time-averaged intensity, °f the light received at the image sensor 680, where there is relative motion in a direction, x, between any of the image sensor 680, the illumination beams 610, 620, the reference beams 630, 640, and the metrology target 660, is given bywhere Iref±(x̄) are the contributions from the reference beams 630, 640, Iill±1(x̄) are the contributions from the illumination beams (e.g. the portions 612, 622 of the scattered illumination beams that are incident on the image sensor 680), f±1are the fringe reduction factors described herein, and <p±1(x) are phase terms. As is clear from Equation (1), the central band CB contribution (given by Iref+(x̄) + Iill+1(x̄) + Iref−(x̄) + Iill−1(x̄)) isn°t affected by the wavefront shifts (e.g. due to vibrations), while the contributions from the sidebands ±SB are affected by the wavefront shifts (e.g. due to vibrations), since they contain cosine terms.

[0066] Rearranging the each of the contributions in Equation (1) gives the following relationships between the central band contribution, ICB, and sideband contributions, SB±1(x), and the reference and illumination beam contributions:ICB= Iref++ Iref−+ Iill+1+ Iill−1ICB− Iref+− Iref−= Iill+1+ Iill−1(3)

[0067] Figure 9 shows the relationship 900 between the central band inverse Fourier transform image 902, and the side band inverse Fourier transform images 904, 906, as related by Equation (8) derivable from Equations (2) to (7) above:whereandcorrespond to the motion- (vibration-) affected sideband inverse Fourier transform images 904, 906 without the fringe height reduction factors, f±1.

[0068] The fringe height reduction factors can then be determined by finding the combination of f+\ and i for which the difference in (7^;+ 1 + u-i )CB extracted from each of the central band and the sidebands is the smallest. Figure 10(a) shows the determination of f+1and f−1by error optimization, where Error-min(Error) is plotted for different values of f+1and f−1, where the Error is given by:where only the region corresponding to the mask in Figure 10(d) (the region having interference of +1 or -1, from Figure 10(c)) is considered. The residual is shown in Figure 10(d).

[0069] Advantageously, the approach described above enables the fringe height reduction factors f+1and f−1to be determined without requiring any additional hardware beyond existing DHM equipment. The fringe height reduction factors may then be applied to a measurement of a characteristic of the target, such as overlay, to improve the accuracy of the DHM measurement.

[0070] Figure 11 illustrates the variation in measured fringe height reduction factors determined from multiple DHM measurements of a target (i.e. 100 images) according to the method described herein. As shown in Figure 11, both f+1(line 1104) and f−1(line 1102) remain very consistent across multiple acquired images, meaning that the approach described herein may provide a reliable measurement ofthe amount of fringe height reduction, and therefore a reliable method for correcting DHM measurements.

[0071] Figure 12 is a block diagram illustrating an example of a method 1200 according to the present disclosure.

[0072] In a step S1202, the method 1200 comprises obtaining a Fourier transform of an interference pattern. As described herein, the Fourier transform of the interference pattern may be obtained by performing a Fourier transform on an interference pattern, such as a two-dimensional Fourier transform and / or a fast Fourier transform, where the interference pattern may be obtained using a DHM (e.g. df-DHM) such as illustrated in Figures 5 and 6 and described herein. In some examples, the Fourier transform of the interference pattern may be retrieved from a memory, e.g. computer storage.

[0073] In a step S1204, the method 1200 comprises obtaining a central band component from the Fourier transform of the interference pattern.

[0074] In a step S1206, the method 1200 comprises obtaining a sideband component from the Fourier transform of the interference pattern.

[0075] As described herein, the method 1200 may comprise obtaining two or more sideband components from the Fourier transform of the interference pattern.

[0076] In a step S1208, the method 1200 comprises obtaining a central band inverse Fourier transform image by performing an inverse Fourier transform (e.g. an inverse fast Fourier transform) ofthe central band component.

[0077] In a step S 1210, the method 1200 comprises obtaining a sideband inverse Fourier transform image by performing an inverse Fourier transform (e.g. an inverse fast Fourier transform) of the sideband component.

[0078] In a step S 1212, the method 1200 comprises determining a fringe height reduction factor (e.g. ±i) based on a comparison between the sideband inverse Fourier transform image and the central band inverse Fourier transform image. For example, the sideband inverse Fourier transform image and the central band inverse Fourier transform image may be related according to Equation (8) above.

[0079] The fringe height reduction factor(s), and any other parameters of interest of the target, may be determined by a processing unit 70, 690 of df-DHM, which may comprise a computer system, and / or by a suitable computer system which may be connected to an apparatus for obtaining the interference pattern (such as a df-DHM), or which may be separate from the apparatus. In some examples, the method 1200 illustrated in Figure 12 may be implemented by a computer. For example, a non-transitory computer readable medium, e.g. a EEPROM (e.g. a flash memory) a disk, CD- or DVD-ROM, programmed memory such as read-only memory (e.g. for Firmware), may comprise instructions which, when executed by a processor of a computing device (such as the computer system illustrated in Figure 13), cause the computing device to carry out one or more of the methods described herein. The instructions may be provided on one or more carriers. For example there may be one or more nontransient memories, e.g. a EEPROM (e.g. a flash memory) a disk, CD- or DVD-ROM, programmedmemory such as read-only memory (e.g. for Firmware), one or more transient memories (e.g. RAM), and / or a data carrier(s) such as an optical or electrical signal carrier. The memory / memories may be integrated into a corresponding processing chip and / or separate to the chip. Code (and / or data) to implement embodiments of the present disclosure may comprise source, object or executable code in a conventional programming language (interpreted or compiled) such as C, or assembly code, code for setting up or controlling an ASIC (Application Specific Integrated Circuit) or FPGA (Field Programmable Gate Array), or code for a hardware description language. For example, the instructions may comprise instructions for controlling the image sensor 680 as described herein via a suitable hardware interface system.

[0080] Figure 13 is a block diagram that illustrates a computer system 1500 that may assist in implementing the methods and flows disclosed herein. As described herein, the computer system 1500 may correspond to, or form part of, the processing unit 70, 690 described herein. In some examples, the computer system 1500 may be separate from the metrology apparatus used to obtain the interference pattern. For example, the interference pattern, and / or the Fourier transform of the interference pattern, may be provided to the computer system 1500 (e.g. retrieved from a memory).

[0081] Computer system 1500 includes a bus 1502 or other communication mechanism for communicating information, and a processor 1504 (or multiple processors 1504 and 1505) coupled with bus 1502 for processing information. Computer system 1500 also includes a main memory 1506, such as a random access memory (RAM) or other dynamic storage device, coupled to bus 1502 for storing information and instructions to be executed by processor 1504. Main memory 1506 also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor 1504. Computer system 1500 further includes a read only memory (ROM) 1408 or other static storage device coupled to bus 1502 for storing static information and instructions for processor 1504. A storage device 1510, such as a magnetic disk or optical disk, is provided and coupled to bus 1502 for storing information and instructions.

[0082] Computer system 1500 may be coupled via bus 1502 to a display 1512, such as a cathode ray tube (CRT) or flat panel or touch panel display for displaying information to a computer user. An input device 1514, including alphanumeric and other keys, is coupled to bus 1502 for communicating information and command selections to processor 1504. Another type of user input device is cursor control 1516, such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor 1504 and for controlling cursor movement on display 1512. This input device typically has two degrees of freedom in two axes, a first axis (e.g., x) and a second axis (e.g., y), that allows the device to specify positions in a plane. A touch panel (screen) display may also be used as an input device.

[0083] One or more of the methods as described herein may be performed by computer system 1500 in response to processor 1504 executing one or more sequences of one or more instructions contained in main memory 1506. Such instructions may be read into main memory 1506 from another computer-readable medium, such as storage device 1510. Execution of the sequences of instructions contained in main memory 1506 causes processor 1504 to perform the process steps described herein. One or more processors in a multi-processing arrangement may also be employed to execute the sequences of instructions contained in main memory 1506. In an alternative embodiment, hard-wired circuitry may be used in place of or in combination with software instructions. Thus, the description herein is not limited to any specific combination of hardware circuitry and software.

[0084] The term “computer-readable medium” as used herein refers to any medium that participates in providing instructions to processor 1504 for execution. Such a medium may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media include, for example, optical or magnetic disks, such as storage device 1510. Volatile media include dynamic memory, such as main memory 1506. Transmission media include coaxial cables, copper wire and fiber optics, including the wires that comprise bus 1502. Transmission media can also take the form of acoustic or light waves, such as those generated during radio frequency (RF) and infrared (IR) data communications. Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, any other magnetic medium, a CD-ROM, DVD, any other optical medium, punch cards, paper tape, any other physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described hereinafter, or any other medium from which a computer can read.

[0085] Various forms of computer readable media may be involved in carrying one or more sequences of one or more instructions to processor 1504 for execution. For example, the instructions may initially be borne on a magnetic disk of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to computer system 1500 can receive the data on the telephone line and use an infrared transmitter to convert the data to an infrared signal. An infrared detector coupled to bus 1502 can receive the data carried in the infrared signal and place the data on bus 1502. Bus 1502 carries the data to main memory 1506, from which processor 1504 retrieves and executes the instructions. The instructions received by main memory 1506 may optionally be stored on storage device 1510 either before or after execution by processor 1504.

[0086] Computer system 1500 also preferably includes a communication interface 1518 coupled to bus 1502. Communication interface 1518 provides a two-way data communication coupling to a network link 1520 that is connected to a local network 1522. For example, communication interface 1518 may be an integrated services digital network (ISDN) card or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, communication interface 1518 may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In any such implementation, communication interface 1518 sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information.

[0087] Network link 1520 typically provides data communication through one or more networks to other data devices. For example, network link 1520 may provide a connection through local network 1522 to a host computer 1524 or to data equipment operated by an Internet Service Provider (ISP) 1526. ISP 1526 in turn provides data communication services through the worldwide packet data communication network, now commonly referred to as the “Internet” 1428. Local network 1522 and Internet 1528 both use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on network link 1520 and through communication interface 1518, which carry the digital data to and from computer system 1500, are exemplary forms of carrier waves transporting the information.

[0088] Computer system 1500 may send messages and receive data, including program code, through the network(s), network link 1520, and communication interface 1518. In the Internet example, a server 1430 might transmit a requested code for an application program through Internet 1528, ISP 1526, local network 1522 and communication interface 1518. One such downloaded application may provide for one or more of the techniques described herein, for example. The received code may be executed by processor 1504 as it is received, and / or stored in storage device 1510, or other non-volatile storage for later execution. In this manner, computer system 1500 may obtain application code in the form of a carrier wave.

[0089] As described herein, in some examples, the interference pattern may be retrieved from a memory and / or a network as described herein with relation to the computer system 1500, and the Fourier transform of the interference pattern may be carried out by the computer system 1500, e.g. to carry out the step S1202 of obtaining the Fourier transform of the interference pattern, as illustrated in Figure 12. In some examples, the Fourier transform may have been performed by a separate system and saved to the memory and / or a network storage, and the Fourier transform of the interference pattern may itself be by the computer system 1500, e.g. to carry out the step S1202 of obtaining the Fourier transform of the interference pattern, as illustrated in Figure 12.

[0090] In the following, further features, characteristics, and exemplary technical solutions of the present disclosure will be described in terms of clauses that may be optionally claimed in any combination:1. A method comprising:obtaining a Fourier transform of an interference pattern;obtaining a central band component from the Fourier transform of the interference pattern; obtaining a sideband component from the Fourier transform of the interference pattern; obtaining a central band inverse Fourier transform image by performing an inverse Fourier transform of the central band component;obtaining a sideband inverse Fourier transform image by performing an inverse Fourier transform of the sideband component; anddetermining a fringe height reduction factor, the fringe height reduction factor corresponding to an interference fringe height reduction due to motion blur in the interference pattern, based on a comparison between the sideband inverse Fourier transform image and the central band inverse Fourier transform image.2. The method according to clause 1, wherein the interference pattern is obtained from radiation diffracted by a target on a substrate.3. The method according to clause 1, wherein obtaining the Fourier transform of the interference pattern comprises performing a Fourier transform on the interference pattern.4. The method according to clause 1, wherein obtaining the Fourier transform of the interference pattern comprises receiving the Fourier transform of the interference pattern.5. The method according to clause 1, comprising:obtaining a second sideband component from the Fourier transform of the interference pattern; obtaining a second sideband inverse Fourier transform image by performing an inverse Fourier transform of the second sideband component; anddetermining a second fringe height reduction factor based on a comparison between the second sideband inverse Fourier transform image and the central band inverse Fourier transform image. 6. The method according to clause 2, comprising using the fringe height reduction factor and the interference pattern to determine a characteristic of the target.7. The method according to clause 6, wherein the characteristic is overlay.8. A non-transitory computer readable medium comprising instructions which, when executed by a processor of a computing device, cause the computing device to carry out operations comprising: obtaining a Fourier transform of an interference pattern;obtaining a central band component from a Fourier transform of the interference pattern; obtaining a sideband component from the Fourier transform of the interference pattern; obtaining a central band inverse Fourier transform image by performing an inverse Fourier transform of the central band component;obtaining a sideband inverse Fourier transform image by performing an inverse Fourier transform of the sideband component; anddetermining a fringe height reduction factor, the fringe height reduction factor corresponding to an interference fringe height reduction due to motion blur in the interference pattern, based on a comparison between the sideband inverse Fourier transform image and the central band inverse Fourier transform image.9. The non-transitory computer readable medium according to clause 8, wherein the operations comprise obtaining the Fourier transform of the interference pattern by performing a Fourier transform on the interference pattern.10. The non-transitory computer readable medium according to clause 8, wherein the operations comprise obtaining the Fourier transform of the interference pattern by retrieving the Fourier transform of the interference pattern from memory.11. The non-transitory computer readable medium according to clause 8, wherein the operations comprise:obtaining a second sideband component from the Fourier transform of the interference pattern; obtaining a second sideband inverse Fourier transform image by performing an inverse Fourier transform of the second sideband component; anddetermining a second fringe height reduction factor based on a comparison between the second sideband inverse Fourier transform image and the central band inverse Fourier transform image. 12. The non-transitory computer readable medium according to clause 8, wherein the interference pattern corresponds to radiation diffracted by a target on a substrate, and wherein the operations further comprise using the fringe height reduction factor and the interference pattern to determine a characteristic of the target.13. The non-transitory computer readable medium according to clause 12, wherein the characteristic is overlay.14. An exposure apparatus comprising:an imaging device configured to capture an interference pattern produced by radiation diffracted by a target on a substrate; anda processor, the processor configured to:obtain a Fourier transform of the interference pattern;obtain a central band component from the Fourier transform of the interference pattern;obtain a sideband component from the Fourier transform of the interference pattern;perform an inverse Fourier transform of the central band component to produce a central band inverse Fourier transform image;perform an inverse Fourier transform of the sideband component to produce a sideband inverse Fourier transform image; anddetermine a fringe height reduction factor based on a comparison between the sideband inverse Fourier transform image and the central band inverse Fourier transform image, the fringe height reduction factor corresponding to an interference fringe height reduction in the interference pattern due to motion of the target and / or the apparatus.15. The exposure apparatus according to clause 14, wherein the processor is configured to obtain the Fourier transform of the interference pattern by performing a Fourier transform on the interference pattern.16. The exposure apparatus according to clause 14, wherein the processor is configured to obtain the Fourier transform of the interference pattern by retrieving the Fourier transform of the interference pattern from memory.17. The exposure apparatus according to clause 14, wherein the processor is configured to: obtain a second sideband component from the Fourier transform of the interference pattern; obtain a second sideband inverse Fourier transform image by performing an inverse Fourier transform of the second sideband component; anddetermine a second fringe height reduction factor based on a comparison between the second sideband inverse Fourier transform image and the central band inverse Fourier transform image.18. The exposure apparatus according to clause 14, wherein the processor is configured to use the fringe height reduction factor and the interference pattern to determine a characteristic of the target.19. The exposure apparatus according to clause 18, wherein the characteristic is overlay.20. The exposure apparatus according to clause 14, wherein the processor is configured to use the fringe height reduction factor to compensate the fringe height reduction in the interference pattern due to motion of the target and / or the apparatus.

[0091] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, 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.

[0092] Although specific reference may be made in this text to embodiments of the disclosure in the context of a lithographic apparatus, embodiments of the disclosure may be used in other apparatus. Embodiments of the disclosure may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

[0093] Although specific reference may have been made above to the use of embodiments of the disclosure in the context of optical lithography, it will be appreciated that the disclosure, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.

[0094] Although specific reference is made to “metrology apparatus / tool / system” or “inspection apparatus / tool / system”, these terms may refer to the same or similar types of tools, apparatuses or systems. E.g. the inspection or metrology apparatus that comprises an embodiment of the disclosure may be used to determine characteristics of structures on a substrate or on a wafer. E.g. the inspection apparatus or metrology apparatus that comprises an embodiment of the disclosure may be used to detect defects of a substrate or defects of structures on a substrate or on a wafer. In such an embodiment, a characteristic of interest of the structure on the substrate may relate to defects in the structure, the absence of a specific part of the structure, or the presence of an unwanted structure on the substrate or on the wafer.

[0095] While specific embodiments of the disclosure have been described above, it will be appreciated that the disclosure may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the disclosure as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A method comprising:obtaining a Fourier transform of an interference pattern;obtaining a central band component from the Fourier transform of the interference pattern; obtaining a sideband component from the Fourier transform of the interference pattern; obtaining a central band inverse Fourier transform image by performing an inverse Fourier transform of the central band component;obtaining a sideband inverse Fourier transform image by performing an inverse Fourier transform of the sideband component; anddetermining a fringe height reduction factor, the fringe height reduction factor corresponding to an interference fringe height reduction due to motion blur in the interference pattern, based on a comparison between the sideband inverse Fourier transform image and the central band inverse Fourier transform image.

2. The method according to claim 1, wherein the interference pattern is obtained from radiation diffracted by a target on a substrate.

3. The method according to claim 1, wherein obtaining the Fourier transform of the interference pattern comprises performing a Fourier transform on the interference pattern.

4. The method according to claim 1, wherein obtaining the Fourier transform of the interference pattern comprises receiving the Fourier transform of the interference pattern.

5. The method according to claim 1, comprising:obtaining a second sideband component from the Fourier transform of the interference pattern; obtaining a second sideband inverse Fourier transform image by performing an inverse Fourier transform of the second sideband component; anddetermining a second fringe height reduction factor based on a comparison between the second sideband inverse Fourier transform image and the central band inverse Fourier transform image.

6. The method according to claim 2, comprising using the fringe height reduction factor and the interference pattern to determine a characteristic of the target.

7. The method according to claim 6. wherein the characteristic is overlay.

8. A non-transitory computer readable medium comprising instructions which, when executed by a processor of a computing device, cause the computing device to carry out operations comprising: obtaining a Fourier transform of an interference pattern;obtaining a central band component from a Fourier transform of the interference pattern; obtaining a sideband component from the Fourier transform of the interference pattern; obtaining a central band inverse Fourier transform image by performing an inverse Fourier transform of the central band component;obtaining a sideband inverse Fourier transform image by performing an inverse Fourier transform of the sideband component; anddetermining a fringe height reduction factor, the fringe height reduction factor corresponding to an interference fringe height reduction due to motion blur in the interference pattern, based on a comparison between the sideband inverse Fourier transform image and the central band inverse Fourier transform image.

9. The non-transitory computer readable medium according to claim 8, wherein the operations comprise obtaining the Fourier transform of the interference pattern by performing a Fourier transform on the interference pattern.

10. The non-transitory computer readable medium according to claim 8, wherein the operations comprise obtaining the Fourier transform of the interference pattern by retrieving the Fourier transform of the interference pattern from memory.

11. The non-transitory computer readable medium according to claim 8, wherein the operations comprise:obtaining a second sideband component from the Fourier transform of the interference pattern; obtaining a second sideband inverse Fourier transform image by performing an inverse Fourier transform of the second sideband component; anddetermining a second fringe height reduction factor based on a comparison between the second sideband inverse Fourier transform image and the central band inverse Fourier transform image.

12. The non-transitory computer readable medium according to claim 8, wherein the interference pattern corresponds to radiation diffracted by a target on a substrate, and wherein the operations further comprise using the fringe height reduction factor and the interference pattern to determine a characteristic of the target.

13. The non-transitory computer readable medium according to claim 12, wherein the characteristic is overlay.

14. An exposure apparatus comprising:an imaging device configured to capture an interference pattern produced by radiation diffracted by a target on a substrate; anda processor, the processor configured to:obtain a Fourier transform of the interference pattern;obtain a central band component from the Fourier transform of the interference pattern;obtain a sideband component from the Fourier transform of the interference pattern;perform an inverse Fourier transform of the central band component to produce a central band inverse Fourier transform image;perform an inverse Fourier transform of the sideband component to produce a sideband inverse Fourier transform image; anddetermine a fringe height reduction factor based on a comparison between the sideband inverse Fourier transform image and the central band inverse Fourier transform image, the fringe height reduction factor corresponding to an interference fringe height reduction in the interference pattern due to motion of the target and / or the apparatus.

15. The exposure apparatus according to claim 14, wherein the processor is configured to obtain the Fourier transform of the interference pattern by performing a Fourier transform on the interference pattern.

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