WO2025051488A1

WO2025051488A1 - Substrate parameter measurement

Info

Publication number: WO2025051488A1
Application number: PCT/EP2024/072552
Authority: WO
Inventors: Han-Kwang Nienhuys; Hugo Augustinus Joseph Cramer
Original assignee: ASML Netherlands BV
Current assignee: ASML Netherlands BV
Priority date: 2023-09-05
Filing date: 2024-08-09
Publication date: 2025-03-13
Anticipated expiration: 2026-03-05
Also published as: TW202518158A

Abstract

A method of measuring a parameter of a structure of a substrate comprises directing a beam of an emitted radiation onto the structure and thereby generating a diffracted radiation due to diffraction by the structure. The emitted radiation has a spectrum comprising a plurality of peaks. The method further comprises using an imaging sensor to detect the diffracted radiation; using knowledge regarding the spectrum and a prior of the diffraction efficiency to obtain an estimate of diffraction efficiency as a function of wavelength; and using the estimated diffraction efficiency to measure the parameter of the structure.

Description

SUBSTRATE PARAMETER MEASUREMENT CROSS REFERENCE TO RELATED APPLICATIONS [0001] This application claims priority of EP application 23195434.8 which was filed on 2023-Sep-05 and of EP application 24182209.7 which was filed on 2024-Jun-14 and whom are incorporated herein in their entirety by reference. FIELD [0002] The present invention relates to a method of measuring a parameter of a structure of a substrate. 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-k₁ 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 λ 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. Confidential [0006] In lithographic processes, as well as other manufacturing processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay, the accuracy of alignment of two layers in a device. Recently, various forms of scatterometers have been developed for use in the lithographic field. [0007] The manufacturing processes may be for example lithography, etching, deposition, chemical mechanical planarization, oxidation, ion implantation, diffusion or a combination of two or more of them. [0008] Examples of known scatterometers often rely on provision of dedicated metrology targets. For example, a method may require a target structure in the form of a simple grating that is large enough that a measurement beam generates a spot that is smaller than the grating (i.e., the grating is underfilled). In so-called reconstruction methods, properties of the grating can be calculated by simulating interaction of scattered radiation with a mathematical model of the target structure. Parameters of the model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from the real target structure. [0009] 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. Diffraction-based overlay metrology using dark-field imaging of the diffraction orders enables overlay measurements on smaller targets. These targets can be smaller than the illumination spot and may be surrounded by product structures on a wafer. Examples of dark field imaging metrology can be found in numerous published patent applications, such as for example US2011102753A1 and US20120044470A. Multiple gratings can be measured in one image, using a composite grating target. The known scatterometers tend to use light in the visible or near-infrared (IR) wave range, which requires the pitch of the grating to be much coarser than the actual product structures whose properties are actually of interest. Such product features may be defined using deep ultraviolet (DUV), extreme ultraviolet (EUV) or X-ray radiation having far shorter wavelengths. Unfortunately, such wavelengths are not normally available or usable for metrology. [00010] On the other hand, the dimensions of modern product structures are so small that they cannot be imaged by optical metrology techniques. Small features include for example those formed by multiple patterning processes, and/or pitch-multiplication. Hence, targets used for high-volume metrology often use features that are much larger than the products whose overlay errors or critical dimensions are the property of interest. The measurement results are only indirectly related to the dimensions of the real product structures, and may be inaccurate because the metrology target does not suffer the same distortions under optical projection in the lithographic apparatus, and/or different processing in other steps of the manufacturing process. While scanning electron microscopy (SEM) is able to resolve these modern product structures directly, SEM is much more time consuming than Confidential optical measurements. Moreover, electrons are not able to penetrate through thick process layers, which makes them less suitable for metrology applications. Other techniques, such as measuring electrical properties using contact pads is also known, but it provides only indirect evidence of the true product structure. [00011] By decreasing the wavelength of the radiation used during metrology it is possible to resolve smaller structures, to increase sensitivity to structural variations of the structures and/or penetrate further into the product structures. One such method of generating suitably high frequency radiation (e.g. hard X-ray, soft X-ray and/or EUV radiation) may be using a pump radiation (e.g., infrared IR radiation) to excite a generating/target medium, thereby generating an emitted radiation, optionally a high harmonic generation (HHG) comprising high frequency radiation. [00012] The radiation used during metrology may have a broadband spectrum, for example a spectrum which extends over 10nm or more. Where this is the case, a given position on a detector may receive diffracted radiation which has different wavelengths and different diffraction orders. This may introduce inaccuracies into analysis of the structure using the diffracted radiation. SUMMARY [00013] According to a first aspect of the invention there is provided a method of measuring a parameter of a structure of a substrate, the method comprising directing a beam of a radiation onto the structure, the radiation having a spectrum comprising a plurality of peaks, using an imaging sensor to detect radiation which is diffracted from the structure, identifying peaks in the detected diffracted radiation, identifying peaks which are compound peaks comprising radiation having two different wavelengths, based upon knowledge of the spectrum and knowledge of a pitch of the structure, and using the remaining peaks to measure the parameter of the structure. [00014] According to a second aspect of the invention there is provided a method of determining a property of a structure on a substrate based on a measured data, wherein the measured data is obtained based on a measurement of a scattered radiation scattered by the structure, wherein the scattered radiation is generated by radiating the structure with an incident radiation, wherein the incident radiation is generated by radiating a target medium with a pump radiation, wherein the determining is based on knowledge of a property of the pump radiation. [00015] Advantageously, embodiments of the invention are able to remove ambiguity regarding the wavelength of a peak of intensity measured by a detector. This may allow for more accurate parameter measurement. [00016] The remaining peaks may be single wavelength peaks which comprise a single wavelength, or comprise a single wavelength and a negligible amount of a second wavelength. [00017] The beam of radiation may have an acute angle of incidence upon the structure. [00018] The structure may be periodic. [00019] The periodic structure may be a grating. Confidential [00020] The parameter of the structure may be measured by constructing a model of the structure. [00021] The method may use second, third and fourth diffraction orders. [00022] According to a third aspect of the invention there is provided a metrology system configured to measure a parameter of a structure of a substrate, the metrology system comprising a pump radiation and a target medium, the pump radiation being configured to direct a pump radiation beam into the target medium and thereby causing the target medium to emit radiation, the radiation having a spectrum comprising a plurality of peaks, optics configured to direct a beam of the radiation onto the structure, an imaging sensor configured to detect radiation which is diffracted from the structure, and a processor configured to identify peaks in the detected diffracted radiation, identify peaks which are compound peaks comprising radiation having two different wavelengths, based upon knowledge of the spectrum and knowledge of a pitch of the structure, and use the remaining peaks to measure the parameter of the structure. [00023] Advantageously, embodiments of the invention are able to remove ambiguity regarding the wavelength of a peak of intensity measured by a detector. This may allow for more accurate parameter measurement. [00024] The processor may be further configured to add interpolated peaks to the remaining peaks before measuring the parameter of the substrate. [00025] The remaining peaks may be single wavelength peaks which comprise a single wavelength, or comprise a single wavelength and a negligible amount of a second wavelength. [00026] The optics are configured to direct the beam of radiation onto the structure with an acute angle of incidence. [00027] The processor may be configured to measure the parameter of the structure by constructing a model of the structure. [00028] The processor may use second, third and fourth diffraction orders. [00029] According to a fourth aspect of the invention there is provided a method of measuring a parameter of a structure of a substrate, the method comprising: directing a beam of an emitted radiation onto the structure and thereby generating a diffracted radiation due to diffraction by the structure, wherein the emitted radiation having a spectrum comprising a plurality of peaks; using an imaging sensor to detect the diffracted radiation; using knowledge regarding the spectrum and a prior of the diffraction efficiency to obtain an estimate of diffraction efficiency as a function of wavelength; and using the estimated diffraction efficiency to measure the parameter of the structure. [00030] Optionally, the method also comprises using knowledge regarding blurring of the beam of emitted radiation to obtain the estimate of diffraction efficiency as a function of wavelength. [00031] Optionally, the prior of the diffraction efficiency is a prior for a second derivative of the diffraction efficiency as a function of wavelength. [00032] Optionally, the method also comprises generating the estimate of the diffracted radiation incident at the imaging sensor. Confidential [00033] Optionally, the method comprises minimizing a difference between the estimated diffracted radiation incident at the imaging sensor and the detected diffracted radiation to obtain the estimate of diffraction efficiency as a function of wavelength. [00034] Optionally, the structure is a periodic structure with a pitch. [00035] Optionally, the method comprises using knowledge regarding the pitch of the periodic structure to obtain the estimate of diffraction efficiency as a function of wavelength. [00036] Optionally, the step of using the estimated diffraction efficiency to measure the parameter of the structure comprises using the estimated diffraction efficiency as input to a simulation. [00037] Optionally, the emitted radiation is a broadband radiation. [00038] Optionally, the diffracted radiation incident at the imaging sensor comprises at least partly overlapping diffraction orders on surface of the imaging sensor. [00039] According to a fifth aspect of the invention there is provided a method for estimating diffraction efficiency of a structure of a substrate, the method comprising: directing a beam of an emitted radiation onto the structure and thereby generating a diffracted radiation due to diffraction by the structure, wherein the emitted radiation having a spectrum comprising a plurality of peaks, and wherein the diffracted illumination comprises multiple diffraction orders; using an imaging sensor to detect the diffracted radiation, wherein the diffracted radiation incident at the imaging sensor comprises at least partly overlapping diffraction orders on surface of the imaging sensor; and separating the at least partly overlapping orders using knowledge regarding the spectrum and a prior of the diffraction efficiency. [00040] Optionally, the emitted radiation is a broadband radiation. [00041] Optionally, the structure is a periodic structure. [00042] The method may also use a prior for the diffraction efficiency. [00043] The radiation emitted by the target medium may be broadband radiation. [00044] The diffracted radiation that is incident upon the sensor may comprise at least partly overlapping diffraction orders. [00045] According to a sixth aspect of the invention, there is provided a method of measuring an illumination spectrum of an illumination beam used by a metrology system, the method comprising directing a pump radiation beam into a target medium and thereby causing the target medium to emit radiation, the emitted radiation having a spectrum comprising a plurality of peaks and being used by the metrology system as an illumination beam having an illumination spectrum, directing the illumination beam onto a grating having a known diffraction efficiency and thereby generating diffracted radiation, using an imaging sensor to detect the diffracted radiation as a spectral power function, expressing the spectral power function in terms of wavelength multiplied by diffraction order, the spectral power function being a multiplication of the illumination spectrum and the known diffraction efficiency, and using the expressed spectral power function and the measured spectral power function to determine the illumination spectrum. Confidential [00046] The effect of a focus error and/or a diffraction-limited spot size may be removed from the measured spectral power function, or the effect of a focus error and/or a diffraction-limited spot size on the spectral power function is reduced. [00047] Removing or reducing the effect of a focus error and/or a diffraction limited spot size may comprise applying a deconvolution and or an integral transform. [00048] A prior probability for smoothness of the illumination spectrum may be used when removing or reducing the effect of a focus error and/or a diffraction-limited spot size from the spectral power function. [00049] The illumination spectrum may be expressed in terms of a signal response of the imaging sensor. [00050] According to a seventh aspect of the invention there is provided a non-transitory computer program product comprising machine-readable instructions therein, the instructions, upon execution by a computer system, configured to cause the computer system to at least cause performance of any of the methods mentioned above. [00051] Features of different aspects of the invention may be combined together. BRIEF DESCRIPTION OF THE DRAWINGS [00052] Embodiments 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; - Figure 2 depicts a schematic overview of a lithographic cell; - Figure 3 depicts a schematic representation of holistic lithography, representing a cooperation between three key technologies to optimize semiconductor manufacturing; - Figure 4 schematically illustrates a scatterometry apparatus; - Figure 5 schematically illustrates a transmissive scatterometry apparatus; - Figure 6 depicts a schematic representation of a metrology apparatus in which EUV and/or SXR radiation is used; - Figure 7 depicts a simplified schematic drawing of an illumination source; - Figure 8 schematically depicts a soft X-ray spectrum output by the scatterometry apparatus; - Figure 9 schematically depicts different diffraction orders that are generated by a grating when broadband spectrum radiation is incident upon the grating; - Figure 10 schematically depicts the intensity of different diffraction orders that are incident on the detector of the scatterometry apparatus for a broadband spectrum comprising a series of peaks; - Figure 11 schematically depicts a method according to an embodiment of the disclosure of selecting peaks of diffracted radiation for analysis; - Figure 12 comprises a flow diagram of steps in a method of reconstructing a grating according to an embodiment of the disclosure; - Figure 13 is a series of graphs which demonstrate operation of a method according to an Confidential embodiment of the invention; and - Figure 14 is a series of graphs which demonstrate operation of the same method with incident broadband spectrum radiation including some divergence. - Figure 15 schematically depicts an apparatus which measures a spectrum of a radiation beam; and - Figure 16 is a series of graphs which demonstrate operation of a method according to an embodiment of the invention. DETAILED DESCRIPTION [0052] In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation and particle radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm), EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range of about 5-100 nm), X-ray radiation, electron beam radiation and other particle radiation. [0053] 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. [0054] 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, EUV radiation or X-ray radiation), a mask support (e.g., a mask table) T 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. [0055] 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, diffractive, 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. [0056] The term “projection system” PS used herein should be broadly interpreted as encompassing various types of projection system, including refractive, reflective, diffractive, catadioptric, anamorphic, Confidential 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. [0057] 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 incorporated herein by reference in its entirety. [0058] 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. [0059] 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. [0060] In operation, the radiation beam B is incident on the patterning device, e.g. mask, MA which is held on the mask support T, 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 may 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 located in spaces between target portions. Substrate alignment marks P1, P2 are known as scribe-lane alignment marks when these are located between the target portions C. [0061] 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 Confidential 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, may be 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. [0062] 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 may be 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 or close to 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 or close to an 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, incorporated herein by reference in their entirety. Aforementioned scatterometers may measure gratings using light from hard X-ray (HXR), soft X-ray (SXR), extreme ultraviolet (EUV), visible to near- infrared (IR) and IR wavelength range. In case that the radiation is hard X-ray or soft X-ray, the aforementioned scatterometers may optionally be a small-angle X-ray scattering metrology tool. [0063] 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), shape of structures, etc. For this purpose, inspection tools and/or metrology 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. [0064] 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 Confidential 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). [0065] 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. [0066] 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, transmitted 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. [0067] In a third embodiment, the scatterometer MT is an ellipsometric scatterometer. The ellipsometric scatterometer allows for determining parameters of a lithographic process by measuring scattered or transmitted 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 incorporated herein by reference in their entirety. [0068] 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 (maybe at least partly 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 overlay error between the two layers containing periodic structures as target 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, incorporated herein by reference in its entirety. Confidential [0069] 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, incorporated herein by reference 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. [0070] A metrology target may be an ensemble of composite gratings, formed by a lithographic process, mostly in resist, but also after other manufacturing process, etch process for example. The pitch and line-width of the structures in the gratings may strongly depend on the measurement optics (in particular the NA of the optics) 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 resemble 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. [0071] Overall measurement quality of a lithographic parameter using a specific 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 US2016-0161863 and published US patent application US 2016/0370717A1 incorporated herein by reference in its entirety. [0072] The patterning process in a lithographic apparatus LA may be 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” Confidential control environment as schematically depicted in Fig. 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 to ensure 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) – maybe within which the process parameters in the lithographic process or patterning process are allowed to vary. [0073] 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 Fig. 3 by the double arrow in the first scale SC1). The resolution enhancement techniques may be 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 MET) to predict whether defects may be present due to e.g. sub-optimal processing (depicted in Fig.3 by the arrow pointing “0” in the second scale SC2). [0074] 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 Fig. 3 by the multiple arrows in the third scale SC3). [0075] Many different forms of metrology tools MT for measuring structures created using lithographic pattering apparatus can be provided. Metrology tools MT may use electromagnetic radiation to interrogate a structure. Properties of the radiation (e.g. wavelength, bandwidth, power) can affect different measurement characteristics of the tool, with shorter wavelengths generally allowing for increased resolution. Radiation wavelength has an effect on the resolution the metrology tool can achieve. Therefore, in order to be able to measure structures with features having small dimensions, metrology tools MT with short wavelength radiation sources are preferred. [0076] Another way in which radiation wavelength can affect measurement characteristics is penetration depth, and the transparency/opacity of materials to be inspected at the radiation wavelength. Depending on the opacity and/or penetration depth, radiation can be used for measurements in transmission or reflection. The type of measurement can affect whether information is obtained about the surface and/or the bulk interior of a structure/substrate. Therefore, penetration depth and opacity are another element to be taken into account when selecting radiation wavelength for a metrology tool. [0077] In order to achieve higher resolution for measurement of lithographically patterned structures, metrology tools MT with short wavelengths are preferred. This may include wavelengths shorter than Confidential visible wavelengths, for example in the UV, EUV, and X-ray portions of the electromagnetic spectrum. Hard X-ray methods such as Transmitted Small Angle X-ray Scattering (TSAXS) make use of the high resolution and high penetration depth of hard X-rays and may therefore operate in transmission. Soft X-rays and EUV, on the other hand, do not penetrate the target as far but may induce a rich optical response in the material to be probed. This may be due the optical properties of many semiconductor materials, and due to the structures being comparable in size to the probing wavelength. As a result, EUV and/or soft X-ray metrology tools MT may operate in reflection, for example by imaging, or by analysing diffraction patterns from, a lithographically patterned structure. [0078] For hard X-ray, soft X-ray and EUV radiations, applications in high volume manufacturing (HVM) applications may be limited due to a lack of available high-brilliance radiation sources at the required wavelengths. In the case of hard X-rays, commonly used sources in industrial applications include X-ray tubes. X-ray tubes, including advanced X-ray tubes for example based on liquid metal anodes or rotating anodes, may be relatively affordable and compact, but may lack brilliance required for HVM applications. High brilliance X-ray sources such as Synchrotron Light Sources (SLSs) and X- ray Free Electron Lasers (XFELs) currently exist, but their size (>100m) and high cost (multi-100- million euro), makes them prohibitively large and expensive for metrology applications. Similarly, there is a lack of availability of sufficiently bright EUV and soft X-ray radiation sources. [0079] One example of a metrology apparatus, such as a scatterometer, is depicted in Figure 4. It may comprise a broadband (e.g. white light) radiation projector 2 comprising optics which project radiation 5 onto a substrate W. The reflected or scattered radiation 10 is passed to a spectrometer detector 4, which measures a spectrum 6 (i.e. a measurement of intensity I as a function of wavelength λ) of the specular reflected radiation. From this data, the structure or profile 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 4. 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. [0080] A transmissive version of the example of a metrology apparatus, such as a scatterometer shown in Figure 4, is depicted in Figure 5. The transmitted radiation 11 is passed to a spectrometer detector 4, which measures a spectrum 6 as discussed for Figure 4. Such a scatterometer may be configured as a normal-incidence scatterometer or an oblique-incidence scatterometer. Optionally, the transmissive version using hard X-ray radiation with wavelength < 1nm, optionally <0.1nm, optionally <0.01nm. [0081] As an alternative to optical metrology methods, it has also been considered to use hard X-ray, soft X-rays or EUV radiation, for example radiation with at least one of the wavelength ranges: <0.01nm, <0.1nm, <1nm, between 0.01nm and 100nm, between 0.01nm and 50nm, between 1nm and Confidential 50nm, between 1nm and 20nm, between 5nm and 20nm, and between 10nm and 20nm. One example of metrology tool functioning in one of the above presented wavelength ranges is transmissive small angle X-ray scattering (T-SAXS as in US 2007224518A which content is incorporated herein by reference in its entirety). Profile (CD) measurements using T-SAXS are discussed by Lemaillet et al in “Intercomparison between optical and X-ray scatterometry measurements of FinFET structures”, Proc. of SPIE, 2013, 8681. It is noted that the use of laser produced plasma (LPP) x-ray source is described in U.S. Patent Publication No. 2019 /003988A1, and in U.S. Patent Publication No.2019 /215940A1, which are incorporated herein by reference in the entirety. Reflectometry techniques using X-rays (GI- XRS) and extreme ultraviolet (EUV) radiation at grazing incidence may be used for measuring properties of films and stacks of layers on a substrate. Within the general field of reflectometry, goniometric and/or spectroscopic techniques may be applied. In goniometry, the variation of a reflected beam with different incidence angles may be measured. Spectroscopic reflectometry, on the other hand, measures the spectrum of wavelengths reflected at a given angle (using broadband radiation). For example, EUV reflectometry has been used for inspection of mask blanks, prior to manufacture of reticles (patterning devices) for use in EUV lithography. [0082] It is possible that the range of application makes the use of wavelengths in e.g. the hard-X- rays, soft X-rays or EUV domain not sufficient. Published patent applications US 20130304424A1 and US2014019097A1 (Bakeman et al/KLA) describe hybrid metrology techniques in which measurements made using x-rays and optical measurements with wavelengths in the range 120 nm and 2000 nm are combined together to obtain a measurement of a parameter such as CD. A CD measurement is obtained by coupling an x-ray mathematical model and an optical mathematical model through one or more common. The contents of the cited US patent applications are incorporated herein by reference in their entirety. [0083] Figure 6 depicts a schematic representation of a metrology apparatus 302 in which the aforementioned radiation may be used to measure parameters of structures on a substrate. The metrology apparatus 302 presented in Figure 6 may be suitable for the hard X-ray, soft X-rays and/or EUV domain. [0084] Figure 6 illustrates a schematic physical arrangement of a metrology apparatus 302 comprising a spectroscopic scatterometer using hard X-ray, Soft X-Ray and/or EUV radiation optionally in grazing incidence, purely by way of example. An alternative form of inspection apparatus might be provided in the form of an angle-resolved scatterometer, which may use radiation in normal or near-normal incidence similar to the conventional scatterometers operating at longer wavelengths, and which may also use radiation with direction being greater than 1° or 2° from a direction parallel to the substrate. An alternative form of inspection apparatus might be provided in the form of a transmissive scatterometer, to which the configuration in Figure 5 applies. [0085] Inspection apparatus 302 comprises a radiation source or called illumination source 310, illumination system 312, substrate support 316, detection systems 318, 398 and metrology processing Confidential unit (MPU) 320. [0086] An illumination source 310 in this example is for a generation of EUV, hard X-ray or soft X- ray radiation. The illumination source 310 may be based on HHG techniques as shown in Figure 6, and it may also be other types of illumination sources, e.g. liquid metal jet source, inverse Compton scattering (ICS) source, plasma channel source, magnetic undulator source, free electron laser (FEL) source, compact storage ring source, electrical discharge produced plasma source, soft X-ray laser source, rotating anode source, solid anode source, particle accelerator source, microfocus source, or laser produced plasma source. [0087] The HHG source, as well as other types of sources, may have a gas target and be a gas jet/nozzle source, a capillary/fiber source or a gas cell source. The HHG source, as well as other types of sources, may have a solid or liquid target. Although HHG source with gas target is described in the text below, it will be appreciated that the invention is not limited to HHG source with gas target and may be used in HHG source with solid or liquid target and other types of source with any target. The gas target, solid target and liquid target may be referred as generating/target medium. [0088] For the example of HHG source, as shown in Figure 6, main components of the radiation source are a pump radiation source 330 operable to emit the pump radiation and a gas delivery system 332. Optionally the pump radiation source 330 is a laser, optionally the pump radiation source 330 is a pulsed high-power infrared or optical laser. The pump radiation source 330 may be, for example, a fiber-based laser with an optical amplifier, producing pulses of infrared radiation that may last for example less than 1 ns (1 nanosecond) per pulse, with a pulse repetition rate up to several megahertz, as required. The wavelength of the infrared radiation may be in the range 200nm to 10µm, for example in the region of 1 μm (1 micron). Optionally, the laser pulses are delivered as a first pump radiation 340 to the gas delivery system 332, where in the gas a portion of the radiation is converted to higher frequencies than the first radiation into an emitted radiation 342. A gas supply 334 supplies a suitable gas to the gas delivery system 332, where it is optionally ionized by an electric source 336. The gas delivery system 332 may be a cut tube. [0089] A gas provided by the gas delivery system 332 defines a gas target, which may be a gas flow or a static volume. The gas may be for example air, Neon (Ne), Helium (He), Nitrogen (N₂), Oxygen (O₂), Argon (Ar), Krypton (Kr), Xenon (Xe), Carbon dioxide and the combination of them. These may be selectable options within the same apparatus. The emitted radiation may contain multiple wavelengths. An emission divergence angle of the emitted radiation may be wavelength dependent. Different wavelengths will, for example, provide different levels of contrast when imaging structure of different materials. For inspection of metal structures or silicon structures, for example, different wavelengths may be selected to those used for imaging features of (carbon-based) resist, or for detecting contamination of such different materials. One or more filtering devices 344 may be provided. For example a filter such as a thin membrane of Aluminum (Al) or Zirconium (Zr) may serve to cut the fundamental IR radiation from passing further into the inspection apparatus. A grating (not shown) may Confidential be provided to select one or more specific wavelengths from among those generated. Optionally the illumination source comprises a space that is configured to be evacuated and the gas delivery system is configured to provide a gas target in the space. Optionally some or all of the beam path may be contained within a vacuum environment, bearing in mind that SXR and/or EUV radiation is absorbed when traveling in air. The various components of radiation source 310 and illumination optics 312 may be adjustable to implement different metrology ‘recipes’ within the same apparatus. For example different wavelengths and/or polarization may be made selectable. [0090] Depending on the materials of the structure under inspection, different wavelengths may offer a desired level of penetration into lower layers. For resolving the smallest device features and defects among the smallest device features, then a short wavelength is likely to be preferred. For example, one or more wavelengths in the range 0.01-20 nm or optionally in the range 1-10 nm or optionally in the range 10-20 nm may be chosen. Wavelengths shorter than 5 nm may suffer from very low critical angle when reflecting off materials of interest in semiconductor manufacture. Therefore to choose a wavelength greater than 5 nm may provide stronger signals at higher angles of incidence. On the other hand, if the inspection task is for detecting the presence of a certain material, for example to detect contamination, then wavelengths up to 50 nm could be useful. [0091] From the radiation source 310, the filtered beam 342 may enter an inspection chamber 350 where the substrate W including a structure of interest is held for inspection at a measurement position by substrate support 316. The structure of interest is labeled S. Optionally the atmosphere within inspection chamber 350 may be maintained near vacuum by vacuum pump 352, so that SXR and/or EUV radiation may pass with-out undue attenuation through the atmosphere. The Illumination system 312 has the function of focusing the radiation into a focused beam 356, and may comprise for example a two-dimensionally curved mirror, or a series of one-dimensionally curved mirrors, as described in published US patent application US2017/0184981A1 (which content is incorporated herein by reference in its entirety), mentioned above. The focusing is performed to achieve a round or elliptical spot S under 10 μm in diameter, when projected onto the structure of interest. Substrate support 316 comprises for example an X-Y translation stage and a rotation stage, by which any part of the substrate W may be brought to the focal point of beam to in a desired orientation. Thus the radiation spot S is formed on the structure of interest. Alternatively, or additionally, substrate support 316 comprises for example a tilting stage that may tilt the substrate W at a certain angle to control the angle of incidence of the focused beam on the structure of interest Ta. [0092] Optionally, the illumination system 312 provides a reference beam of radiation to a reference detector 314 which may be configured to measure a spectrum and/or intensities of different wavelengths in the filtered beam 342. The reference detector 314 may be configured to generate a signal 315 that is provided to processor 320 and the filter may comprise information about the spectrum of the filtered beam 342 and/or the intensities of the different wavelengths in the filtered beam. [0093] Reflected radiation 360 is captured by detector 318 and a spectrum is provided to processor Confidential 320 for use in calculating a property of the target structure Ta. The illumination system 312 and detection system 318 thus form an inspection apparatus. This inspection apparatus may comprise a hard X-ray, soft X-ray and/or EUV spectroscopic reflectometer of the kind described in US2016282282A1 which content is incorporated herein by reference in its entirety. [0094] If the target Ta has a certain periodicity, the radiation of the focused beam 356 may be partially diffracted as well. The diffracted radiation 397 follows another path at well-defined angles with respect to the angle of incidence then the reflected radiation 360. In Figure 6, the drawn diffracted radiation 397 is drawn in a schematic manner and diffracted radiation 397 may follow many other paths than the drawn paths. The inspection apparatus 302 may also comprise further detection systems 398 that detect and/or image at least a portion of the diffracted radiation 397. In Figure 6 a single further detection system 398 is drawn, but embodiments of the inspection apparatus 302 may also comprise more than one further detection system 398 that are arranged at different position to detect and/or image diffracted radiation 397 at a plurality of diffraction directions. In other words, the (higher) diffraction orders of the focused radiation beam that impinges on the target Ta are detected and/or imaged by one or more further detection systems 398. The one or more detection systems 398 generates a signal 399 that is provided to the metrology processor 320. The signal 399 may include information of the diffracted light 397 and/or may include images obtained from the diffracted light 397. [0095] To aid the alignment and focusing of the spot S with desired product structures, inspection apparatus 302 may also provide auxiliary optics using auxiliary radiation under control of metrology processor 320. Metrology processor 320 may also communicate with a position controller 372 which operates the translation stage, rotation and/or tilting stages. Processor 320 receives highly accurate feedback on the position and orientation of the substrate, via sensors. Sensors 374 may include interferometers, for example, which may give accuracy in the region of picometers. In the operation of the inspection apparatus 302, spectrum data 382 captured by detection system 318 is delivered to metrology processing unit 320. [0096] As mentioned an alternative form of inspection apparatus uses hard X-ray, soft X-ray and/or EUV radiation optionally at normal incidence or near-normal incidence, for example to perform diffraction-based measurements of asymmetry. Another alternative form of inspection apparatus uses hard X-ray, soft X-ray and/or EUV radiation with direction being greater than 1° or 2° from a direction parallel to the substrate. Both types of inspection apparatus could be provided in a hybrid metrology system. Performance parameters to be measured may include overlay (OVL), critical dimension (CD), focus of the lithography apparatus while the lithography apparatus printed the target structure, coherent diffraction imaging (CDI) and at-resolution overlay (ARO) metrology. The hard X-ray, soft X-ray and/or EUV radiation may for example have wavelengths less than 100 nm, for example using radiation in the range 5-30 nm, of optionally in the range from 10 nm to 20 nm. The radiation may be narrowband or broadband in character. The radiation may have discrete peaks in a specific wavelength band or may have a more continuous character. Confidential [0097] Like the optical scatterometer used in today’s production facilities, the inspection apparatus 302 may be used to measure structures within the resist material treated within the litho cell (After Develop Inspection or ADI), and/or to measure structures after they have been formed in harder material (After Etch Inspection or AEI). For example, substrates may be inspected using the inspection apparatus 302 after they have been processed by a developing apparatus, etching apparatus, annealing apparatus and/or other apparatus. [0098] Metrology tools MT, including but not limited to the scatterometers mentioned above, may use radiation from a radiation source to perform a measurement. The radiation used by a metrology tool MT may be electromagnetic radiation. The radiation may be optical radiation, for example radiation in the infrared, visible, and/or ultraviolet parts of the electromagnetic spectrum. Metrology tools MT may use radiation to measure or inspect properties and aspects of a substrate, for example a lithographically exposed pattern on a semiconductor substrate. The type and quality of the measurement may depend on several properties of the radiation used by the metrology tool MT. For example, the resolution of an electromagnetic measurement may depend on the wavelength of the radiation, with smaller wavelengths able to measure smaller features, e.g. due to the diffraction limit. In order to measure features with small dimensions, it may be preferable to use radiation with a short wavelength, for example EUV, hard X- ray (HXR) and/or Soft X-Ray (SXR) radiation, to perform measurements. In order to perform metrology at a particular wavelength or wavelength range, the metrology tool MT requires access to a source providing radiation at that/those wavelength(s). Different types of sources exist for providing different wavelengths of radiation. Depending on the wavelength(s) provided by a source, different types of radiation generation methods may be used. For extreme ultraviolet (EUV) radiation (e.g.1 nm to 100 nm), and/or soft X-ray (SXR) radiation (e.g. 0.1 nm to 10 nm), a source may use HHG or any other types of sources mentioned above to obtain radiation at the desired wavelength(s). [0099] Figure 7 shows a simplified schematic drawing of an embodiment 600 of an illumination source 310, which may be the illumination source for HHG. One or more of the features of the illumination source in the metrology tool described with respect to Figures 6 may also be present in the illumination source 600 as appropriate. The illumination source 600 comprises a chamber 601 and is configured to receive a pump radiation 611 with a propagation direction which is indicated by an arrow. The pump radiation 611 shown here is an example of the pump radiation 340 from the pump radiation source 330, as shown in Figure 6. The pump radiation 611 may be directed into the chamber 601 through the radiation input 605, which maybe a viewport, optionally made of fused silica or a comparable material. The pump radiation 611 may have a Gaussian or hollow, for example annular, transversal cross-sectional profile and may be incident, optionally focused, on a gas flow 615, which has a flow direction indicated by a second arrow, within the chamber 601. The gas flow 615 comprises a small volume called gas volume or gas target (for example several cubic mm) of a particular gas (for example, air, Neon (Ne), Helium (He), Nitrogen (N₂), Oxygen (O₂), Argon (Ar), Krypton (Kr), Xenon (Xe), Carbon dioxide and the combination of them.) in which the gas pressure is above a certain value. The Confidential gas flow 615 may be a steady flow. Other media, such as metallic plasmas (e.g. aluminium plasma) may also be used. [00100] The gas delivery system of the illumination source 600 is configured to provide the gas flow 615. The illumination source 600 is configured to provide the pump radiation 611 in the gas flow 615 to drive the generation of emitted radiation 613. The region where at least a majority of the emitted radiation 613 is generated is called an interaction region. The interaction region may vary from several tens of micrometers (for tightly focused pump radiation) to several mm or cm (for moderately focused pump radiation) or even up to a few meters (for extremely loosely focused pump radiation). The gas delivery system is configured to provide the gas target for generating the emitted radiation at the interaction region of the gas target, and optionally the illumination source is configured to receive the pump radiation and to provide the pump radiation at the interactive region. Optionally, the gas flow 615 is provided by the gas delivery system into an evacuated or nearly evacuated space. The gas delivery system may comprise a gas nozzle 609, as shown in Figure 6, which comprises an opening 617 in an exit plane of the gas nozzle 609. The gas flow 615 is provided from the opening 617. The gas catcher is for confining the gas flow 615 in a certain volume by extracting residual gas flow and maintaining a vacuum or near vacuum atmosphere inside the chamber 601. Optionally the gas nozzle 609 may be made of thick-walled tube and/or high thermo-conductivity materials to avoid thermo deformation due to the high-power pump radiation 611. [00101] The dimensions of the gas nozzle 609 may conceivably also be used in scaled-up or scaled- down versions ranging from micrometer-sized nozzles to meter-sized nozzles. This wide range of dimensioning comes from the fact that the setup may be scaled such that the intensity of the pump radiation at the gas flow ends up in the particular range which may be beneficial for the emitted radiation, which requires different dimensioning for different pump radiation energies, which may be a pulse laser and pulse energies can vary from tens of microjoules to joules. Optionally, the gas nozzle 609 has a thicker wall to reduce nozzle deformation caused by the thermal expansion effect, which may be detected by e.g. a camera. The gas nozzle with thicker wall may produce a stable gas volume with reduced variation. Optionally, the illumination source comprises a gas catcher which is close to the gas nozzle to maintain the pressure of the chamber 601. [00102] Due to interaction of the pump radiation 611 with the gas atoms of the gas flow 615, the gas flow 615 will convert part of the pump radiation 611 into the emitted radiation 613, which may be an example of the emitted radiation 342 shown in Figure 6. The central axes of the emitted radiation 613 may be collinear with the central axes of the incident pump radiation 611. The emitted radiation 613 may have a wavelength in X-ray or EUV range, wherein the wavelength is in a range from 0.01 nm to 100 nm, optionally from 0.1 nm to 100 nm, optionally from 1 nm to 100 nm, optionally from 1 nm to 50 nm, or optionally from 10 nm to 20 nm. [00103] In operation the emitted radiation 613 beam may pass through a radiation output 607 and may be subsequently manipulated and directed by an illumination system 603, which may be an example of Confidential the illumination system 312 in Figure 6, to a substrate to be inspected for metrology measurements. The emitted radiation 613 may be guided, optionally focused, to a structure on the substrate. [00104] Because air (and in fact any gas) heavily absorbs SXR or EUV radiation, the volume between the gas flow 615 and the wafer to be inspected may be evacuated or nearly evacuated. Since the central axes of the emitted radiation 613 may be collinear with the central axes of the incident pump radiation 611, the pump radiation 611 may need to be blocked to prevent it passing through the radiation output 607 and entering the illumination system 603. This may be done by incorporating a filtering device 344 shown in Figure 6 into the radiation output 607, which is placed in the emitted beam path and that is opaque or nearly opaque to the pump radiation (e.g. opaque or nearly opaque to infrared or visible light) but at least partially transparent to the emitted radiation beam. The filter may be manufactured using zirconium or multiple materials combined in multiple layers. The filter may be a hollow, optionally an annular, block when the pump radiation 611 has a hollow, optionally an annular, transversal cross- sectional profile. Optionally, the filter is non-perpendicular and non-parallel to propagation direction of the emitted radiation beam to have efficient pump radiation filtering. Optionally, the filtering device 344 comprise a hollow block and a thin membrane filter such as an Aluminum (Al) or Zirconium (Zr) membrane filter. Optionally, the filtering device 344 may also comprise mirrors that efficiently reflect the emitted radiation but poorly reflect the pump radiation, or comprise a wire mesh that efficiently transmits the emitted radiation but poorly transmits the pump radiation. [00105] Described herein are methods, apparatuses, and assemblies to obtain emitted radiation optionally at a high harmonic frequency of pump radiation. The radiation generated through the process, optionally the HHG which uses non-linear effects to generate radiation optionally at a harmonic frequency of provided pump radiation, may be provided as radiation in metrology tools MT for inspection and/or measurement of substrates. If the pump radiation comprises short pulses (i.e. few- cycle) then the generated radiation is not necessarily exactly at harmonics of the pump radiation frequency. The substrates may be lithographically patterned substrates. The radiation obtained through the process may also be provided in a lithographic apparatus LA, and/or a lithographic cell LC. The pump radiation may be pulsed radiation, which may provide high peak intensities for short bursts of time. [00106] The pump radiation 611 may comprise radiation with one or more wavelengths higher than the one or more wavelengths of the emitted radiation. The pump radiation may comprise infrared radiation. The pump radiation may comprise radiation with wavelength(s) in the range of 500 nm to 1500 nm. The pump radiation may comprise radiation with wavelength(s) in the range of 800 nm to 1300 nm. The pump radiation may comprise radiation with wavelength(s) in the range of 900 nm to 1300 nm. The pump radiation may be pulsed radiation. Pulsed pump radiation may comprise pulses with a duration in the femtosecond range. [00107] For some embodiments, the emitted radiation, optionally the high harmonic radiation, may comprise one or more harmonics of the pump radiation wavelength(s). The emitted radiation may Confidential comprise wavelengths in the extreme ultraviolet, soft X-Ray, and/or hard X-Ray part of the electromagnetic spectrum. The emitted radiation 613 may comprise wavelengths in one or more of the ranges of less than 1nm, less than 0.1nm, less than 0.01nm, 0.01 nm to 100 nm, 0.1 nm to 100 nm, 0.1 nm to 50 nm, 1 nm to 50 nm and 10 nm to 20 nm.613 [00108] Radiation, such as high harmonic radiation described above, may be provided as source radiation in a metrology tool MT. The metrology tool MT may use the source radiation to perform measurements on a substrate exposed by a lithographic apparatus. The measurements may be for determining one or more parameters of a structure on the substrate. Using radiation at shorter wavelengths, for example at EUV, SXR and/or HXR wavelengths as comprised in the wavelength ranges described above, may allow for smaller features of a structure to be resolved by the metrology tool, compared to using longer wavelengths (e.g. visible radiation, infrared radiation). Radiation with shorter wavelengths, such as EUVSXR and/or HXR radiation, may also penetrate deeper into a material such as a patterned substrate, meaning that metrology of deeper layers on the substrate is possible. These deeper layers may not be accessible by radiation with longer wavelengths. [00109] In a metrology tool MT, source radiation may be emitted from a radiation source and directed onto a target structure (or other structure) on a substrate. The source radiation may comprise EUVSXR and/or HXR radiation. The target structure may reflect, transmit and/or diffract the source radiation incident on the target structure. The metrology tool MT may comprise one or more sensors for detecting diffracted radiation. For example, a metrology tool MT may comprise detectors for detecting the positive (+1st) and negative (-1st) first diffraction orders. The metrology tool MT may also measure the specular reflected or transmitted radiation (0th order diffracted radiation). Further sensors for metrology may be present in the metrology tool MT, for example to measure further diffraction orders (e.g. higher diffraction orders). [00110] In an example lithographic metrology application, the HHG generated radiation may be focused onto a target on the substrate using an optical column, which may be referred to as an illuminator, which transfers the radiation from the HHG source to the target. The HHG radiation may then be reflected from the target, detected and processed, for example to measure and/or infer properties of the target. [00111] Gas target HHG configurations may be broadly divided into three separate categories: gas jets, gas cell and gas capillaries. Figure 7 depicts an example gas jet configuration in which as gas volume is introduced into a drive radiation laser beam. In a gas jet configuration, interaction of the drive radiation with solid parts is kept to a minimum. The gas volume may for example comprise a gas stream perpendicular to the drive radiation beam, with the gas volume enclosed inside a gas cell. In a gas capillary setup, the dimensions of the capillary structure holding the gas are small in a lateral direction such that it significantly influences the propagation of the drive radiation laser beam. The capillary structure may for example be a hollow-core fibre, wherein the hollow core is configured to hold the gas. [00112] A gas jet HHG configuration may offer a relative freedom to shape a spatial profile of the Confidential drive radiation beam in the far field, as it is not confined by the restrictions imposed by the gas capillary structure. Gas jet configurations may also have less stringent alignment tolerances. On the other hand, a gas capillary may provide an increased interaction zone of the drive radiation and the gaseous medium, which may optimise the HHG process. [00113] In order to use the HHG radiation, for example in a metrology application, it is separated from the drive radiation downstream of the gas target. The separation of the HHG and drive radiation may be different for the gas jet and gas capillary configurations. In both cases, the drive radiation rejection scheme can comprise a metal transmissive filter for filtering out any remaining drive radiation from the short wavelength radiation. However, before such a filter can be used, the intensity of the drive radiation should be reduced significantly from its intensity at the gas target, in order to avoid damage to the filter. The methods that can be used for this intensity reduction differ for the gas jet and capillary configurations. For a gas jet HHG, due to the relative freedom of the shape and spatial profile (which may also be referred to as a spatial distribution, and/or spatial frequencies) of the drive radiation beam focussed onto the gas target, this can be engineered such that in the far field it has a low intensity along the directions where the short wavelength radiation propagates. This spatial separation in the far field means an aperture may be used to block the drive radiation and lower its intensity [00114] In contrast, in a gas capillary structure, the spatial profile of the beam as it passes through the gaseous/solid medium may be largely dictated by the capillary. The spatial profile of the drive radiation may be determined by the shape and material of the capillary structure. For example, in the case of a hollow-core fiber being used as a capillary structure, the shape and materials of the fiber structure determine which modes of drive radiation are supported for propagation through the fiber. For most standard fibres, the supported propagating modes lead to a spatial profile where the high intensity of the drive radiation overlaps with the high intensity of the HHG radiation. For example, the drive radiation intensity may be centred, in a Gaussian or close-to-Gaussian profile in the far field. [00115] Although specific reference is made to HHG, it will be appreciated that the invention, where the context allows, may be practiced with all radiation sources. In one embodiment, the radiation source is a laser produced plasma (LPP) source as mentioned above, for hard X-ray, soft X-ray, EUV, DUV as well as visible illumination generation. In one embodiment, the radiation source is one of liquid metal jet source, inverse Compton scattering (ICS) source, plasma channel source, magnetic undulator source, free electron laser (FEL) source, compact storage ring source, electrical discharge produced plasma source, rotating anode source, solid anode source, particle accelerator source, and microfocus source. [00116] As explained further above, when the HHG process is used to generate a radiation beam used by a metrology system (e.g. the radiation beam 5 depicted in Figure 4), the radiation beam consists of harmonics of the pump radiation wavelength. In one example, the pump radiation wavelength is 1030 nm. The spectrum λ_h of the radiation beam generated by the high harmonic process may have peaks at wavelengths: Confidential ^^_^ ൌ ^ఒబ ^ Equation 1

where λ₀ is the wavelength of pump radiation, and h is the harmonic number and may be an odd integer. Typically, h may be a sequence, for example h = 51, 53, 55, …, 103. In the case of a 1030nm pump beam, the radiation beam provided by HHG may have wavelengths of 10nm, 10.20nm, 10.40nm, … , 20.20nm. These wavelengths together form a spectrum, with wavelengths at either end of the spectrum having the lowest intensity and wavelengths at the centre of the spectrum having the highest intensity. In between each wavelength of the spectrum there is a portion with zero or negligible intensity, and thus the spectrum consists of a series of peaks. [00117] Figure 8 schematically depicts the spectrum of an example radiation beam generated by HHG and used by the metrology tool. The spectrum comprises a series of peaks having wavelengths which are determined according Equation 1 above. The horizontal axis is wavelength and the vertical axis of Figure 8 is normalized intensity. In the schematically depicted spectrum, the intensity drops to zero between adjacent peaks. [00118] Figure 9 schematically depicts orders of diffraction that are generated by a broadband radiation beam when that broadband radiation beam is incident upon a diffraction grating. In Figure 9, the incident broadband radiation beam does not consist of peaks but instead has a continuous spread of wavelengths. The horizontal axis of Figure 9 is sin(θ^ with respect to radiation incident radiation (which is normal to the diffraction grating). The vertical axis of Figure 9 is diffraction efficiency. Although normal incidence is referred to in connection with Figure 9, embodiments of the invention work with an acute angle of incidence (normal incidence is used here for ease of explanation). Embodiments of the invention may for example use an incidence angle of 30˚ (+/- 10˚). [00119] The angular positions of peaks of the diffraction seen in Figure 9 are determined by the following diffraction equation: ^^ ^^ ^^^ ^^^ ൌ ^{ఒబ ^} ^ . _^ Equation 2

where p is the pitch of the m is the diffraction order. [00120] In Figure 9, a solid line indicates the second diffraction order m=2, a dashed line indicates the third diffraction order m=3, and a dotted line indicates the fourth diffraction order m=4. As may be seen, in Figure 9 the second diffraction order m=2 begins at smaller values of sin(θ), and the third and fourth diffraction orders are begin at larger values of sin(θ). However, there is considerable overlap between the diffraction orders. As will be understood from Figure 9, because different diffraction orders overlap with each other when the broadband radiation is used, it is problematic to determine which diffraction order contributes to a measured intensity at a given value of sin(θ). Confidential [00121] Referring to Figure 4, the detector 4 of a metrology tool detects broadband radiation which has been diffracted by a grating on the substrate W. The position at which diffracted radiation is incident upon the detector 4 is determined by the diffraction angle θ of the radiation. For relatively small angles, as is the case for metrology tools, Sin(θ) can be mapped directly to position on the detector 4. Thus, the horizontal axis of Figure 9 may be considered to be a linear position on the detector 4 (with arbitrary units). As noted above, the diffraction orders overlap with each other. The detector 4 is therefore unable to discriminate for example between the second and third diffraction orders, at a position on the detector where both orders have a significant intensity. This limits metrology which can be achieved using this method. At least some of the embodiments of the invention address this issue. [00122] One embodiment of the invention is a method of determining a property, optionally the property is a wavelength dependent property, e.g. a target response, of a structure/target on a substrate. Optionally the target response is a target diffraction efficiency spectrum. The method of determining may be based on a measured data, e.g. intensity measured by a detector. The method of determining may use a property of a pump radiation, e.g. source spectrum of the pump radiation. Optionally the method of determining may also use a property of the structure, e.g. pitch of the structure. The measured data may be obtained based on a measurement of a scattered radiation scattered by the structure, which is generated by radiating the structure with an incident radiation, and optionally the incident radiation may be generated by radiating a target medium with the pump radiation, optionally via a HHG process. [00123] Figure 10 schematically depicts diffraction orders arising from broadband radiation generated using HHG. As noted above in connection with Figure 8, the spectrum of the incident radiation beam consists of a series of peaks which are separated from one another. As a result, the diffracted radiation as incident upon the detector 4 (see Figure 4) also comprises a series of peaks. As with Figure 9, the second diffraction order m=2 is depicted using a solid line, the third diffraction order m=3 is depicted using a dashed line, and the fourth diffraction order m=4 is depicted using a dotted line. In Figure 10 the vertical axis is the illumination (or source) spectrum, for the hypothetical case where the diffraction efficiency is constant (not depending on wavelength). The units would be intensity (as is the case for Figure 11), but the vertical axis has been scaled to set 1.0 as a maximum value for each diffraction order) to allow the peaks to be more easily seen. The horizontal axis is sin(θ) or equivalently linear position on the detector with arbitrary units. For each diffraction order, the left hand side of Figure 10 shows the shortest wavelengths, with the wavelengths gradually increasing towards the right hand side of Figure 10. [00124] The first few peaks in Figure 10 are from the second diffraction order m=2. Figure 10 shows only longer wavelengths of the second diffraction order (shorter wavelengths will have a smaller sin(θ) and are not depicted). The peak intensities of successive peaks of the second diffraction order are lower as the wavelength gets longer (corresponding with a larger sin(θ)). [00125] Initial peaks of the third diffraction order m=3 have a low peak intensity at shorter wavelengths, gradually increasing to a maximum peak intensity and then decreasing again. As Confidential schematically depicted, the shorter wavelengths of the third diffraction order m=3 overlap with the longer wavelengths of the second diffraction order m=2. As schematically depicted, for some wavelengths the peaks of the second and third diffraction orders overlap considerably, whereas for other wavelengths the peaks of the second and third diffraction orders do not overlap at all or have a negligible overlap. [00126] Figure 10 also shows the fourth diffraction order m=4. Initial peaks of the fourth diffraction order have a low peak intensity at shorter wavelengths, gradually increasing to a maximum peak intensity. The intensity of peaks of the fourth diffraction order will reduce for larger values of sin(θ) but this is not shown here. As schematically depicted, for some wavelengths the peaks of the third and fourth diffraction orders overlap considerably, whereas for other wavelengths the peaks of the third and fourth diffraction orders do not overlap at all or have a negligible overlap. [00127] The horizontal axis of Figure 10 may be considered to be a linear position on the detector 4 (see Figure 4) with arbitrary units. The left hand and right hand ends of Figure 10 may correspond with left hand and right hand ends of the detector 4. Peaks of different diffraction orders are incident at different linear positions on the detector 4. In some instances, a peak of intensity comprises peaks of two diffraction orders which overlap considerably. This may be referred to as a compound peak. In other instances a peak of intensity comprises a peak of a single diffraction order (or a single diffraction order plus a negligible contribution from another diffraction order). This may be referred to as a single- wavelength peak. [00128] Figure 11 schematically depicts the intensity of radiation seen by the detector 4 when the diffracted broadband radiation of Figure 10, multiplied by the diffraction efficiency of Figure 9, is incident upon the detector. The vertical axis of Figure 11 is the intensity of radiation incident upon the detector, and the horizontal axis is the linear position on the detector (arbitrary units). When two peaks corresponding to two different diffraction orders have significant overlap, then this generates a compound peak, i.e. a peak formed from two different diffraction orders combined together. Four such compound peaks 700, 702, 704, 706 are depicted in Figure 11. [00129] Peaks of radiation on the detector which consist of the second diffraction order m=2 with a negligible contribution from the third and fourth diffraction orders are identified with a disc. Peaks of radiation on the detector which consist of the third diffraction order m=3 with negligible contributions from the second and fourth diffraction orders are identified by an upwardly pointing triangle. Peaks of the fourth diffraction order m=4 with negligible contributions from the second and third diffraction orders are identified with downwardly pointing triangles. These peaks may be referred to as single wavelength peaks. [00130] As may be seen from Figure 11, a plurality of peaks for each of the second, third and fourth diffraction orders with negligible contributions from other diffraction orders are identified. In other words, a plurality of single wavelength peaks are identified. This addresses the issue of at least partly overlapping diffraction orders explained further above. Confidential [00131] The metrology tool may be configured to omit compound peaks when performing analysis of a detected diffraction spectrum. The metrology tool may be configured to use only single wavelength peaks when performing analysis of a detected diffraction spectrum. [00132] The metrology tool may be configured to interpolate between measured single wavelength peaks and add in missing peaks. The resulting diffraction spectrum including interpolated peaks may then be used for analysis of the substrate grating that generated the diffraction pattern. [00133] In one example, a so-called reconstruction method is used to reconstruct properties of the grating that generated the detected diffraction. As explained further above, in a reconstruction method properties of the grating are calculated by simulating interaction of scattered radiation with a mathematical model of the grating structure. Parameters of the model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from the real grating (including the interpolated peaks). The reconstruction method may use knowledge of properties of the metrology tool such as the spectrum of the broadband radiation beam (watts per nm), exposure time (i.e. amount of time during which radiation is detected), the location of the detector (which may be an imaging detector). Properties of the metrology tool may be measured using a calibration process. This is a physics-based modelling approach. [00134] In a machine-learning approach, a model of the SXR tool is created. A model of a target structure with a repetitive structure of dimensions and/or optical properties can be varied. Using the model of the target structure with an electromagnetic (EM) solver, e.g., Rigorous Coupled-Wave Analysis (RCWA), one can simulate a series of target responses R(m,λ) for a given target structure pitch (or pitches in 2 directions). Using the series of target responses in the SXR tool model one can predict the corresponding series of measured intensities I(κ) for a typical source spectrum S(λ). The series of measured intensity and the corresponding, annotated, target response can then be used to train a neural network, or other machine learning model, to infer a target response R(m,λ), given the source spectrum S(λ), a measured intensity I(κ) and the target pitch(es). [00135] Advantageously, using an embodiment of the invention may provide reconstruction of a grating with an accuracy that was previously not achievable using existing methods. In particular, embodiments of the invention may be used to remove ambiguity regarding the wavelength of a peak of intensity measured by a detector. [00136] The method of the invention may be used for gratings. A grating may be a simple 1-D grating (i.e. having parallel lines). The grating may be a 2-D grating (i.e. having a unit cell repeating both in x and y directions). The 2-D grating may have a checkerboard configuration. A grating may be formed from a periodic product structure. More than one grating may be provided, for example in different layers of the substrate thereby allowing overlay measurements to be performed (overlay metrology). A grating is an example of a target structure that may be used by embodiments of the invention. Other forms of target structure which have a periodic structure may be used. The method of the invention may be used for nonperiodic structures. Confidential [00137] The wavelengths of the peaks of the broadband radiation generated by a HHG source depend upon properties of the HHG source, such as the gas which is provided, the form of the capillary structure of the source, etc. These properties do not change, and thus the wavelengths of the peaks are known when a metrology measurement is performed. If desired, a spectrometer may be used periodically to measure the wavelengths of the peaks. [00138] Metrology methods may be used to measure properties of a grating such as critical dimension and overlay. Critical dimension may vary properties of the grating; overlay may vary the position of the grating relative to a grating in a different layer of the substrate. However, the pitch of the grating itself can be expected to be substantially as designed. Thus, the pitch of the grating which is illuminated by the broadband radiation is known. Prior knowledge of the radiation spectrum provided by the HHG source, and optionally prior knowledge of the pitch of the target, allows expected positions of diffraction peaks to be calculated. This prior knowledge also allows identification of single wavelength peaks of detected radiation which are to be used in analysis and identification of compound peaks which are not to be used in analysis (as schematically depicted in Figure 11). [00139] Figure 12 schematically depicts a method according to an embodiment of the invention. In a first step 800 direct broadband radiation at a target grating and measure the intensity of diffracted radiation incident on an imaging detector as a function of position. [00140] In a second step 802, identify the positions of peaks of intensity. Identifiers are allocated to each peak, e.g. i= 0, 1, 2… [00141] In a third step 804, for each peak attempt to identify the diffraction order m and optionally the harmonic number h (HHG is used as an example here), using knowledge of the spectrum of the broadband radiation and optionally using knowledge of the grating pitch. If a single diffraction order and harmonic number is identified (the peak is a single wavelength peak) then calculate the wavelength of that peak. This provides a data point that can be used for metrology. In one embodiment, if multiple diffraction orders and harmonic numbers apply for a given peak (the peak is a compound peak), then do not use that peak for metrology. [00142] In a fourth step 806, interpolate missing data points (i.e. missing single wavelength peaks) and add these to the data. [00143] In a fifth step 808, use the data to reconstruct the grating. [00144] In the depicted embodiment, between each wavelength peak of the spectrum there is a portion with zero intensity. In some embodiments the portion between each wavelength peak may have a non- zero but negligible intensity. In such situations, the non-zero intensity may have a negligible effect upon the accuracy of the metrology method. In some embodiment, the non-zero intensity between wavelength peaks may be non-negligible. The metrology method may still be used, but the non-zero intensity between wavelength peaks may reduce the accuracy of the metrology method. [00145] Operation of the HHG illumination source 310 may be modified to minimise the intensity of radiation between wavelength peaks. For example, narrowing the spectrum of the pump radiation 330, Confidential 611 (e.g. by providing longer pump radiation pulses) may reduce the intensity of radiation between wavelength peaks. This may reduce overall power of the broadband radiation beam, but still provide an increase of the accuracy of the metrology method. The width of the wavelength peaks may be influenced by adding some chirp to pump radiation pulses. This may also reduce overall power of the broadband radiation beam. [00146] In some instances the harmonic numbers h may not be exact integers. However, the method works in the same way. [00147] An advantage of embodiments of the invention is that once the single wavelength diffraction peaks have been identified, and missing peaks have been interpolated, the resulting diffraction spectrum is independent of parameters of the metrology tool. Thus, reconstruction of a grating using the diffraction spectrum does not need to include metrology tool parameters. [00148] A further advantage of embodiments of the invention is that the reconstruction of the grating using the diffraction spectrum may be performed using physics-based analysis (based on autocorrelation functions). An example of this is described in US20230040124A1, which content is incorporated herein by reference in its entirety. This avoids incorrect outcomes that may otherwise arise due to nuisance parameters if a non-physics based analysis were to be used. [00149] In some instances, there may be some divergence of the broadband radiation beam that is incident upon a target. This divergence will tend to blur the signal that is received at the detector 4. With reference to Figure 11 as an example, blurring due to divergence of the broadband radiation beam widens the peaks. A user of the metrology system might not receive information about gratings on lithographic substrates that are being measured. These issues may make it difficult to implement the machine-learning approach described further above to reconstruct properties of a grating. An alternative approach is now described. [00150] The alternative approach estimates the diffraction efficiency R using knowledge of the spectrum ^^_^of the broadband radiation beam (which may be referred to as the illumination spectrum ^^_^) and optionally knowledge of blurring B and. This knowledge of the of the blurring B and of the spectrum ^^_^ may be obtained using sensors present in the metrology system. [00151] The spectrum ^^_^ may originate from a high-harmonic-generation process and may comprise odd harmonics of a pump laser wavelength (as in equation 1), or it may comprise both odd and even harmonics. [00152] In one example, the estimation of the diffraction efficiency R also uses two priors (a prior is an assumed probability distribution for a parameter). The first prior represents the fact that the diffraction efficiency as a function of wavelength R_λ is smooth, that is: it has a relatively low second derivative; non-smooth functions ^^_ఒ have a lower probability than smooth ones. The second prior represents the fact that R_λ is close to a reasonable value R⁽⁰⁾ , for example 10^-4 or 10^-3 at wavelengths which make up the broadband radiation beam. The pitch of the measured target may also be known. In Confidential some embodiments the estimation may be performed without the first prior (i.e. the prior relating to second derivative of R_λ). [00153] The signal that is measured by the detector 4 may be referred to as a spectral flux. The spectral flux on the detector plane may be mapped to pupil space. If the target is at the origin and a detector pixel is at coordinates ^ ^^, ^^, ^^^, with ^ ^^, ^^^ in the plane of the target, then the pupil coordinates of that pixel are ^ ^^/ ^^, ^^/ ^^^, where ^^^ଶ ൌ ^^^ଶ ^ ^^^ଶ ^ ^^^ଶ. The spectral flux may be considered to be an intensity ^^^Δ ^^^ as a function of pupil-space displacement Δ ^^. Pupil-space displacement in this context means the offset of the between the pupil coordinates of the 0^th-order diffraction and the coordinates of the observed intensity. For a given diffraction order ^^, we have Δ ^^ ൌ ^^ ^^/ ^^, where ^^ is the pitch (period) of the target pattern. [00154] The illumination spectral flux, being the known spectrum of the broadband radiation source, may be represented as ^^_ఒ^ ^^^. This can be converted to pupil space, being the signal that would be seen at the detector 4 for perfect diffraction by the target and with no blurring, as: ^_{^ ^ ^^ ^} ^{^^ ^^ Δ ^^} ^_{,Δ ^^ ൌ} _{^^ ^^ఒ ൬} _{^^ ^ . ^equation 3^}

[00155] For a diffraction efficiency ^^_ఒ^ ^^, ^^^ of the target, the signal that will be seen at the detector 4 is ^_{^^Δ ^^^ ൌ ^^ ^^^Δ ^^^ ∗ ^ ^^} ^{^^ Δ ^^} ^_{^ ^^,Δ ^^^ ⋅ ^^ఒ ൬ ^^,} _{^^ ^^^ , ^equation 4^}

where ^^^Δ ^^^ is a blur kernel and ∗ is the convolution operator. The blur kernel ^^^Δ ^^^ is determined based on blurring as measured by the metrology system. In some instances blurring may be ignored (e.g. if the blurring is sufficiently small that it will not significantly affect the metrology output). [00156] In one example, the estimation of ^^_ఒ may be a minimization problem. Let ^^{^}^^ ^^,Δ ^^^ be the right- hand side of equation 4. Then, ଶ^ଶ ^_{^ ^ ^^, ^^^ ൌ argmin ^ฮ ^^ െ} ^{^ ଶ ଶ ^^ ^^ ଶ ^^^ ଶ} ఒ_{^^ฮ ^ ^ ^^^ ^ ^ ^ ^^ ^ฮ ^^ െ ^^ ฮ ൡ , equation 5} _{ோ ^^ ^^}

Confidential where Λ and ^^ are regularization parameters to adjust the relative importance of minimizing the corresponding term and ‖ ^^^ ^^^‖^ଶ ൌ ^ ^ ^^^ ^^^^^ଶ ^^ ^^, equation 6 where the integral is taken over the relevant range of ^^.Figure 13 is a demonstration of how the above method works (generated via a simulation). The uppermost plot in Figure 13 schematically depicts diffraction orders at the detector 4 arising from broadband radiation generated using HHG. As noted above, the spectrum of the incident radiation beam consists of a series of peaks which are separated from one another. The diffracted radiation at detector 4 also comprises a series of peaks. The second diffraction order m=2, third diffraction order m=3 and fourth diffraction order m=4 are each labelled. As can be seen, each diffraction order comprises a series of peaks. The third diffraction order overlaps with the second and fourth diffraction orders. [00157] The middle plot depicts the simulated intensity of radiation incident at the detector 4 after diffraction by a target, which in this example is a grating with a pitch of 100nm. [00158] The lowermost plot depicts estimated diffraction efficiency for each of the three diffraction orders. The estimated diffraction efficiency determined using the above method is indicated using a solid line for the second diffraction order, a dashed line for the third diffraction order, and a dashed and dotted line for the fourth diffraction order. Because Figure 13 is a simulation, the actual diffraction efficiency known from the simulation is known. The actual diffraction efficiency for each order is depicted using a dotted line. It can be seen that the estimated diffraction efficiency closely follows the actual diffraction efficiency. [00159] Figure 14 shows the same three plots as Figure 13, the being the spectrum of radiation at the detector (uppermost plot), intensity of radiation at the detector (middle plot), and estimated diffraction efficiency (lowermost plot). In the simulation of Figure 14, the incident radiation beam has a 2 mrad beam divergence at 100 nm pitch, whereas in the simulation of Figure 13 the incident radiation beam does not diverge. [00160] In the lowermost plot of Figure 14, the estimated diffraction efficiency is indicated using a solid line for the second diffraction order, a dashed line for the third diffraction order, and a dashed and dotted line for the fourth diffraction order. The actual diffraction efficiency for each order is depicted using a dotted line. Despite the presence of blurring in the incident radiation beam, the estimated diffraction efficiency closely follows the actual diffraction efficiency. This demonstrates that the above method works well in the presence of blurring due to a diverging incident radiation beam. [00161] Implementation details of an embodiment of the method are now described. The ‘reasonable’ value ^^^{^^^} ^ can be taken to have different values for each diffraction order. In Figures 13 and 14, the Confidential inferred value (i.e. the value estimated using the method) takes this value at the edges of the range. [00162] An implementation may reparametrize with ^^ ൌ Δ ^^ ⋅ ^^, . That is, ^^, ^^, and ^^ are all expressed as functions of ^^. This is convenient because then the minimization algorithm doesn’t need to know the pitch of the target, but it is not necessary for the general algorithm to parametrize spectra in terms of ^^ . For example, it would be straightforward to do it in terms of Δ ^^. ^^ may equivalently be expressed as m.λ. [00163] For the regularization parameters in equation 5, ^^ may be small so that the term which includes ^^ in equation 5 is negligible wherever there is substantial intensity ^^. Λ may be sufficiently large that the peaks of the illumination spectrum don’t start showing up in the inferred diffraction efficiency ^^. [00164] The least-squares optimization can be done using standard library routines for least-squares solving a matrix-vector equation ^^ ^^ ൌ ^^ for ^^, given a large, (sparse) matrix. The optimization may be highly efficient. If the signal ^^ is represented by ^^ points and there are ^^ diffraction orders, then the vector ^^ will have ^^ ^^ (diffraction efficiency ^^ at ^^ values of ^^, for each diffraction order), the matrix ^^ will have shape ^^ ൈ ^^ ^^ with ^^ ൌ ^2 ^^ ^ 1^ ^^ െ 2 ^^, and ^^ will be a vector with ^^ elements. [00165] Each row in the matrix ^^ will represent an equation with the corresponding element of ^^ as the right-hand side of the equation. [00166] A set of ^^ rows in matrix A represent equation 2. If there is no blurring, each of these rows have ^^ nonzero values representing illumination values; the corresponding element in ^^ is the corresponding observed signal value ^^^ ^^^. If there is blurring, there will be a few more nonzero values to couple this row to nearby wavelengths ( ^^ values). [00167] Another ^^^ ^^ െ 2^ rows represent the equations Λ_^ ^^^ଶ ^^/ ^^ ^^ ൌ 0 for each discrete ^^ value, where the second derivative is approximated as ^_{^ᇱᇱ^ ^^^ ^} ^{^^^ ^^ െ 1^ ^ ^^^ ^^ ^ 1^ െ 2 ^^^ ^^^} ^_{^ଶ equation 7}

where ^^ is the step size ^^. [00168] Another ^^ ^^ rows represent the equations ^^ ^^_క^ ^^, ^^^ ൌ ^^ ^^^{^^^} ^ . More rows could be added to the matrix ^^ to represent additional equations. [00169] When the matrix equation is solved, the ^^ vector may contain the estimated/inferred diffraction-efficiency spectra in the ^^ domain, which can be converted to a representation in wavelength for further analysis. [00170] The above method relates to a diffraction grating target with a pitch that extends in one direction. This may be referred to as a 1D-periodic target. The method may also be used for a target which has pitches extending in two directions (which may be referred to as a 2D-periodic target). Confidential For a 2D-periodic target characterized by x- and y-pitches ^^_௫ and ^^_௬, diffraction orders are identified as ^ ^^_௫ , ^^_௬^. The algorithm is applied on groups of diffraction orders with potential confusion, for e_{xample the group ^} ^{^} _2,െ1 ^{^} _, ^{^} _4,െ2 ^{^} _, ^{^} _6,െ4 ^{^} _{^ or more generally, groups of orders ^ ^^ ^^} ^{^^^ ^^^} ௫_{, ^^ ^^௬ ^} where ^^ ^ 1 and in this example ^^^{^^^} ൌ 2, ^^^{^^^} ௫_௬ ൌ െ1. [00171] The effective pitch is

ଶ^{ଶ ି^} ^_൭ ^{^ ^^^} ^_൭ ^{^^^^^} ^_{^ ൌ} ^{^௫} _^ ^௬ _{^ ^ equation 8}

[00172] The above efficiency values even when noisy signals are used as the input for the method. [00173] Optionally, the smoothness parameter Λ can be made a function of ^^ . This is beneficial because the target response tends to be quasiperiodic in 1/ ^^ but not in ^^ or ^^. Around wavelengths where the target response is known to be not so smooth, it can be reduced. This may be done at around ^^ ൌ 12.4 nm, where the optical properties of silicon change abruptly. [00174] The above method invention may use methods known in the field of machine learning. However, the embodiment of the method described here it isn’t strictly machine learning since there is no training involved. A small number of simulations, e.g. one or two, may be performed to obtain values for the two regularization parameters Λ and ^^. Λ and ^^ may be estimated based on prior knowledge. [00175] The above method may use a machine-learning approach, for example with the parameters Λ_୫, ^^, ^^^{^^^} ^ being optimized using training data (simulated measurements). Far less training data will be if a neural network is to generate a model of the SXR tool (as described further above).

At least partly of the above mentioned steps of the above mentioned method may be used for estimating diffraction efficiency of a structure of a substrate. In one embodiment, there is a method for estimating diffraction efficiency comprising directing a beam of an emitted radiation onto the structure and thereby generating diffracted radiation due to diffraction by the structure. Optionally the emitted radiation having a spectrum comprising a plurality of peaks, and optionally the diffracted illumination comprises multiple diffraction orders. The method for estimating diffraction efficiency further comprises using an imaging sensor to detect the diffracted radiation. Optionally the diffracted radiation incident at the imaging sensor comprises at least partly overlapping diffraction orders on surface of the imaging sensor. The method for estimating diffraction efficiency further comprises separating the at least partly overlapping orders using knowledge regarding the spectrum and a prior of the diffraction efficiency. [00177] Optionally the step of using the estimated diffraction efficiency to measure the parameter of the structure of the above method comprises using the estimated diffraction efficiency as input to a Confidential simulation. Optionally the simulation is a model-based reconstruction. [00178] The above method allows calculation of target autocorrelation functions using a full measured signal instead of only using non-overlapping orders (which may be referred to as compound orders). [00179] The above method is robust. It does not separate handling of zero values in the illumination spectrum. It does not have Fourier-transform-related artifacts near the edge of the signal. [00180] The term broadband radiation may be interpreted as meaning radiation having a spectrum where the ratio of the longest and shortest wavelengths exceeds 1.4. The spectrum may comprise a series of peaks. [00181] As explained further above, embodiments of the invention may use knowledge of the spectrum ^^^ ^^^ of the incident radiation beam (which may be referred to as the illumination spectrum ^^^ ^^^). The illumination spectrum may be expressed in terms of wavelength or wavenumber (number of wavelengths per unit distance). [00182] Figure 15 schematically depicts an example of part of a metrology apparatus according to the present disclosure. The part of the metrology apparatus illustrated in Figure 15 may correspond to part of the scatterometer illustrated in Figure 6. A radiation beam 901 is incident on a metrology target on a substrate W (e.g. a wafer). The radiation beam 901 may for example be broadband radiation and may include radiation in the hard X-ray (HXR), soft X-ray (SXR), extreme ultraviolet (EUV), visible to near-infrared (IR) and/or IR wavelength range. The radiation beam 901 may be generated by the radiation source SO, 310 illustrated in e.g. Figure 6 and described further above. The radiation beam may be referred to as an illumination beam. [00183] Diffraction from the metrology target on the substrate W onto a detector 905 (e.g. an array sensor 905) is captured. The diffraction efficiency of the target for the various wavelength components can be measured and translated into estimates for parameters of interest such as overlay and critical dimension of the substrate W. [00184] As shown in Figure 15, the illumination beam 901 may be directed towards the substrate W via a mirror 902 (e.g. a toroid mirror). The illumination beam 901 used for the measurement may be monitored using an imaging sensor 903 (e.g. which may be referred to as an illumination monitoring sensor). For example, part of the illumination beam 901 (which may be referred to as measurement illumination) may be diffracted onto the illumination monitoring sensor 903 by a transmissive diffraction grating 904. The diffraction efficiency of the transmissive diffraction grating 904 may be known (e.g. via a calibration measurement). The output from the illumination monitoring sensor 903 may be used to determine the spectrum ^^^ ^^^ of the illumination beam (which may be referred to as the illumination spectrum ^^^ ^^^) using a method described further below. [00185] In an alternative arrangement (not illustrated), the illumination bean 901 may be reflected from a fiducial diffraction grating onto an imaging sensor 905 which is configured to perform metrology measurements. The diffraction efficiency of the fiducial diffraction grating may be known (e.g. via a Confidential calibration measurement). The output from the imaging sensor 905 may be used to determine the illumination spectrum ^^^ ^^^ (e.g. using the method described below). Other forms of grating may be used to measure the illumination spectrum. [00186] An issue associated with measuring the illumination spectrum ^^^ ^^^ is that the illumination spectrum may include wavelength components which range over more than an octave (that is there is more than a factor of 2 between the longest wavelength and the shortest wavelength). For example, the illumination spectrum may range from 8 nm to 22 nm. The position of the illumination beam 901, transmissive diffraction grating 904, and the illumination monitoring sensor 903 may be known accurately. However, there is a fundamental problem, being that the 2nd diffraction order of a given wavelength λ will overlap with the 1st diffraction order of wavelength 2 ^^. For example, the 2nd order of 10 nm and the 1st order of 20 nm overlap on the illumination monitoring sensor 903. This is problematic in that the overlap means that the illumination spectrum ^^^ ^^^ cannot be measured in a straightforward manner using the illumination monitoring sensor 903. [00187] This problem is addressed by an embodiment of the invention. The term ^^ ൌ ^^ ^^ is used by the method, in which ^^ is the diffraction-order number ( ^^ ൌ 1,2, … ^ and ^^ is the wavelength. ^^ is the same term that is used further above in connection with other methods. [00188] As noted above, there is overlap at the illumination monitoring sensor 903 of different diffraction orders, meaning that an output signal from the illumination monitoring sensor 903 is not a measurement of the illumination spectrum ^^^ ^^^. What is measured may instead be referred to as a spectral power function ^^^ ^^^, and includes contributions from overlapping diffraction orders. The spectral power function ^^^ ^^^ as measured by the illumination monitoring sensor 903 may be expressed in units ADU nm^-1s^-1, where ADU is “Analog-Digital unit” (units of sensor output, without accounting for the wavelength-dependent sensitivity of the sensor). The output signal from the illumination monitoring sensor 903 may be converted to the spectral power function ^^^ ^^^ using knowledge of the pitch of the transmissive diffraction grating 904 and the separation between the transmissive diffraction grating and the illumination monitoring sensor. [00189] The diffraction efficiency of the transmissive diffraction grating 904 is known (this may be previously determined using a calibration method). The spectral power function ^^^ ^^^ has been measured using the illumination monitoring sensor 903. Taken together these can be used to calculate the illumination spectrum S(λ). [00190] Let the true incident spectrum, i.e. the illumination spectrum, be ^^^ ^^^, also in units ADU nm- ¹s^-1. This spectrum is not known yet. Let the diffraction efficiency of the transmissive diffraction grating 904 be ^^^ ^^, ^^^, which depends on the wavelength and the discrete diffraction order number. This is dimensionless with values between zero and one. The diffraction efficiency of the transmissive diffraction grating 904 is known. Confidential [00191] Let a point-spread function (“kernel”) be ^^^ ^^, ^^^, to represent for example a focus error and the diffraction-limited spot size on the illumination sensor 903. [00192] The spectral power function ^^^ ^^^ is described the equation: ^_{^^ ^^^ ൌ ^} ^{1 ^^} ^_{^ ^ ^^ ൬ ^^,} _{^^^ ^^ ൬} ^{^^} ^_{^^൨ ∗ ^^^ ^^, ^^^ Equation 9}

where ∗ is the operator over ^^. [00193] In some cases, ^^^ ^^, ^^^ may be similar to a Dirac delta function. Where this is the case the ^^^ ^^, ^^^ term of Equation 9 may be omitted. What remains is a system of equations with the values of ^^^ ^^^ different ^^ values as unknowns. For example, if the wavelength range of ^^ is known to be the 8-22 nm range and ^^ is captured in the range 8-22 nm as well, in steps of 0.01 nm (1400 points), then there will be 1400 equations with 1400 unknowns. Other step sizes may be used. Other wavelength ranges may be used. These equations are solved, using the measured spectral power function ^^^ ^^^ and the known diffraction efficiency ^^ ^ ^^, ^క ^^, to obtain the illumination spectrum ^^^ ^^^. [00194] In general, the illumination spectrum S(λ) may obtained using a very sparse system of N

equations with N unknowns. [00195] In some cases, the kernel ^^^ ^^, ^^^ may be fixed (and not a Dirac delta function). That is, the kernel may be the same for all values of ^^ and for each diffraction order m. Where this is the case a deconvolution may be used to eliminate K. One deconvolution method which could be used is a Wiener deconvolution, which applies an optimized lowpass filter to ^^ before carrying out the deconvolution. What remains is a system of equations with the values of ^^^ ^^^ at different ^^ values as unknowns. These equations are solved to obtain the illumination spectrum ^^^ ^^^. [00196] In some cases, there may be a different kernel ^^^ ^^, ^^^ for each diffraction order m, for example, ^^^{^}1, ^^^{^} ് ^^^2, ξ^ (i.e. the Kernel for second order may be different to the Kernel

for first diffraction). In such cases, a prior may be which assumes smoothness of the

illumination spectrum ^^^{^} ^^^{^}. In one example, the entire right-hand side of Equation 9 is written out as 1400 equations with 1400 unknowns. The smoothness prior may be described using 1398 equations which favor solutions that are continuous and generally smooth (that is, a prior probability on smoothness is assumed). If ^^^{^} ^^^{^} is discretized as ^^^ ^^^, then those equations (one for each ^^) would be of the form ^^_^^ ^^^ ^^ ^ 1^ ^ ^^^ ^^ െ 1^ െ 2 ^^^ ^^^^ ൌ 0 Equation 10 where ^^_^ ^ 0 is a regularization parameter that can be tuned based on the signal-to-noise ratio. An system of equations (Equations 9 and 10) remains, and a least-squares fitting method Confidential (or other fitting method) can be used to obtain the illumination spectrum ^^^ ^^^. Optionally, ^^_^ may be made dependent on ^^ based on knowledge of what a typical spectrum will look like (for ^^_^

may be made smaller near expected locations of peaks in the spectrum). In general, a prior

for spectrum smoothness may be used when deconvolving K. [00197] The number of equations referred to above is merely an example, and other numbers of equations may be used. One equation, being the right hand side of Equation 9, per measured value of ^^^ ^^^ may be used. The number of smoothness equations (e.g. Equation 10 or equivalent) may be two fewer, given that the smoothness equations relate to differences between adjacent ^^^ ^^^ values. In general, there may be N equations according to Equation 9 and N-2 equations according to Equation 10 (or a different representation of the smoothness prior). In this example the smoothness prior replaces the Weiner deconvolution. [00198] In some cases, there may be a generic integral kernel ^^^ ^^, ^^, ^^^ᇱ^. In such cases a smoothness prior may be used in the manner described above.

[00199] A smoothness prior may be used in cases where the ^^^ ^^, ^^^ is fixed. A smoothness prior may be used in cases where the kernel ^^^ ^^, ^^^ is a Dirac delta function. [00200] When a smoothness prior is used, an overdetermined system of 2N-2 equations and N unknowns is obtained. A least-squares fitting method (or other fitting method) can be used to obtain the illumination spectrum ^^^ ^^^ using this overdetermined system of equations. [00201] In some cases, the diffraction efficiency ^^^ ^^, ^^^ may be zero or near-zero for some order- wavelength combinations. For such combinations, any value of ^^^ ^^, ^^^ would be equivalent in Equation 9. To create a more well-behaved function ^^, equations of the form: ^_{^ଶ ^^} ^{^} _^^ ^{^} _{ൌ 0 Equation 11} may be added, where ^^_ଶ is another regularization parameter. This regularization will favor solutions with small values for ^^^ ^^, ^^). [00202] If the point-spread function is not constant over ^^, the convolution in Equation 9 may be replaced by a discretized integral transform with a kernel of the form ^^^ ^^, ^^, ^^^ᇱ^ , discretized as ^^^ ^^, ^^, ^^^ , (where j can be negative). The equations ^ ^^ ൌ 1 … ^^^ will be

^^^ ^^^ ൌ ^ ^^^ ^^, ^^, ^^^ ^^^ ^^, ^^ െ ^^^ ^^^ ^^ െ ^^^ Equation 12 ^^

[00203] In a error a limited spot size, as represented by a Kernel, may be removed from the spectral power function, or the effect on the spectral power function may be reduced. The deconvolution and the integral transform may be considered to be examples of Confidential removing (or reducing) the effect of a focus error and a diffraction-limited spot size from the spectral power function. The use of a smoothness prior may be considered to be examples of removing (or reducing) the effect of a focus error and a diffraction-limited spot size from the spectral power function. [00204] An example which demonstrates operation of the above method is depicted in Figure 16 (generated via simulation). The depicted example uses a made-up diffraction efficiency of the transmissive diffraction grating 904 rather than a measured diffraction efficiency to demonstrate the method. The first graph in Figure 16 is an example of the observed signal at the illumination monitoring sensor 903 (i.e. the spectral power function ^^^ ^^^ as measured by the illumination monitoring sensor). The second graph in Figure 16 is the illumination spectrum ^^^ ^^^ which is calculated using the above method, starting from the measured spectral power function. The second graph includes error values, and it can be seen that the errors are relatively small. [00205] The illumination spectrum ^^^ ^^^ may be used during metrology measurements. The illumination spectrum ^^^ ^^^ may vary over time. Consequently, the illumination spectrum may be calculated on a continuous basis using measurements made by the illumination monitoring sensor 903. Alternatively, the illumination spectrum may be calculated periodically using measurements made by the illumination monitoring sensor 903. [00206] The third graph shows the diffraction efficiency of the transmissive grating 904 that was used in the calculation of the illumination spectrum ^^^ ^^^. As noted above, this is made-up for the simulation. However, the diffraction efficiency of the transmissive grating 904 may be measured via a calibration measurement. As noted further above, other forms of grating may be used. [00207] The calculated illumination spectrum ^^^ ^^^ is in units ADU nm^-1s^-1 (units of sensor output, without accounting for the wavelength-dependent sensitivity of the sensor). Since the overlapping wavelengths have been resolved in the calculated illumination spectrum ^^^ ^^^, the illumination spectrum can be divided by the sensor sensitivity spectrum (wavelength-dependent value in ADU/J units – measured via a calibration measurement) to obtain a spectrum in W/nm units. [00208] If the illumination monitoring sensor 903 and the sensor 905 used to measure light after diffraction from the substrate W (see Figure 15) have identical sensitivity spectra, then the calculated illumination spectrum may be kept in ADU-based units. That is, because the response is the same for both sensors 903, 905, conversion into W/nm units is redundant. The illumination spectrum may be expressed in other units, such as electrical charge or electron count rather than in ADU. [00209] The method of calculating the illumination spectrum ^^^ ^^^ may be performed by a controller. [00210] An embodiment may include a computer program containing one or more sequences of machine-readable instructions describing a method of optical metrology and/or a method of analyzing a measurement to obtain information about a lithographic process. An embodiment may comprise computer code containing one or more sequences of machine-readable instructions or data describing the method. This computer program or code may be executed for example within unit MPU in the Confidential apparatus of Figure 6 and/or the control unit CL of Figure 3. There may also be provided a data storage medium (e.g., semiconductor memory, magnetic or optical disk, etc.) having such a computer program or code stored therein. Where an existing metrology apparatus, for example of the type shown in Figure 6, is already in production and/or in use, an embodiment of the invention can be implemented by the provision of an updated computer program product for causing a processor to perform one or more of the methods described herein. The computer program or code may optionally be arranged to control the optical system, substrate support and the like to perform a method of measuring a parameter of the lithographic process on a suitable plurality of targets. The computer program or code can update the lithographic and/or metrology recipe for measurement of further substrates. The computer program or code may be arranged to control (directly or indirectly) the lithographic apparatus for the patterning and processing of further substrates. [00211] The illumination source may be provided in for example a metrology apparatus MT, an inspection apparatus, a lithographic apparatus LA, and/or a lithographic cell LC. [00212] The properties of the emitted radiation used to perform a measurement may affect the quality of the obtained measurement. For example, the shape and size of a transverse beam profile (cross- section) of the radiation beam, the intensity of the radiation, the power spectral density of the radiation etc., may affect the measurement performed by the radiation. It is therefore beneficial to have a source providing radiation that has properties resulting in high quality measurements. [00213] Further embodiments are disclosed in the subsequent numbered clauses: 1. A method of measuring a parameter of a structure of a substrate, the method comprising: directing a beam of a radiation onto the structure, wherein the radiation having a spectrum comprising a plurality of peaks; using an imaging sensor to detect radiation which is diffracted from the structure; identifying peaks in the detected diffracted radiation; identifying peaks which are compound peaks comprising radiation having two different wavelengths, based upon knowledge of the spectrum and knowledge of a pitch of the structure; and using the remaining peaks to measure the parameter of the structure. 2. The method of clause 1, further comprising adding interpolated peaks to the remaining peaks before measuring the parameter of the substrate. 3. The method of clause 1 or clause 2, wherein the remaining peaks are single wavelength peaks which comprise a single wavelength, or comprise a single wavelength and a negligible amount of a second wavelength. 4. The method of any preceding clause, wherein the beam of radiation has an acute angle of incidence upon the structure. 5. The method of any preceding clause, wherein the structure is periodic. 6. The method of clause 5, wherein the periodic structure is a grating. Confidential 7. The method of any preceding clause, wherein the parameter of the structure is measured by constructing a model of the structure. 8. The method of any preceding clause, wherein the method uses second, third and fourth diffraction orders. 9. A metrology system configured to measure a parameter of a structure of a substrate, the metrology system comprising: a source to emit a radiation, the radiation having a spectrum comprising a plurality of peaks; optics configured to direct a beam of the radiation onto the structure; an imaging sensor configured to detect radiation which is diffracted from the structure; and a processor configured to: identify peaks in the detected diffracted radiation; identify peaks which are compound peaks comprising radiation having two different wavelengths, based upon knowledge of the spectrum and knowledge of a pitch of the structure; and use the remaining peaks to measure the parameter of the structure. 10. The metrology system of clause 9, wherein the processor is further configured to add interpolated peaks to the remaining peaks before measuring the parameter of the substrate. 11. The metrology system of clause 9 or clause 10, wherein the remaining peaks are single wavelength peaks which comprise a single wavelength, or comprise a single wavelength and a negligible amount of a second wavelength. 12. The metrology system of any of clauses 9 to 11, wherein the optics are configured to direct the beam of radiation onto the structure with an acute angle of incidence. 13. The metrology system of any of clauses 9 to 12, wherein the processor is configured to measure the parameter of the structure by constructing a model of the structure. 14. The metrology system of any of clauses 9 to 13, wherein the processor uses second, third and fourth diffraction orders. 15. A method of determining a property of a structure on a substrate based on a measured data, wherein the measured data is obtained based on a measurement of a scattered radiation scattered by the structure, wherein the scattered radiation is generated by radiating the structure with an incident radiation, wherein the incident radiation is generated by radiating a target medium with a pump radiation, wherein the determining is based on knowledge of a property of the pump radiation. 16. The method of clause 15, wherein the incident radiation is generated by radiating the target medium with the pump radiation via a nonlinear process. 17. The method of clause 16, wherein the nonlinear process is a high harmonic generation. 18. The method of any of clauses 15 to 17, wherein the property of a pump radiation is wavelength. 19. The method of any of clauses 15 to 18, wherein the scattered radiation is a broadband radiation. Confidential 20. The method of any of clauses 15 to 19, wherein the scattered radiation have a wavelength within 10-20nm wavelength range. 21. The method of any of clauses 15 to 20, wherein the property of the structure is wavelength dependent. 22. The method of any of clauses 15 to 21, wherein the structure is a periodic structure with a pitch and the measured data is obtained based on the measurement of a diffracted radiation diffracted by the structure. 23. The method of clause 22, wherein the wavelength dependent property is diffraction efficiency related. 24. The method of any of clauses 15 to 23, wherein the incident radiation has a spectrum comprising a plurality of peaks. 25. The method of clause 24, the method further comprising identifying peaks in the scattered radiation. 26. The method of clause 24 or 25, further comprising identifying peaks which are compound peaks comprising radiation having two different wavelengths, based upon knowledge of the spectrum. 27. The method of clause 25 or 26 when referring to clause 22, wherein identifying peaks which are compound peaks comprising radiation having two different wavelengths is based upon knowledge of the pitch of the structure. 28. A method of measuring a parameter of a structure of a substrate, the method comprising: directing a beam of an emitted radiation onto the structure and thereby generating diffracted radiation due to diffraction by the structure, wherein the emitted radiation having a spectrum comprising a plurality of peaks; using an imaging sensor to detect the diffracted radiation; using knowledge regarding the spectrum and a prior of the diffraction efficiency to obtain an estimate of diffraction efficiency as a function of wavelength; and using the estimated diffraction efficiency to measure the parameter of the structure. 29. The method of clause 28, wherein the method comprises using knowledge regarding blurring of the beam of emitted radiation to obtain the estimate of diffraction efficiency as a function of wavelength. 30. The method of clause 28 or 29, wherein the prior of the diffraction efficiency is a prior for a second derivative of the diffraction efficiency as a function of wavelength. 31. The method of any of clauses 28 to 30, wherein the method comprises generating the estimate of the diffracted radiation incident at the imaging sensor. 32. The method of any of clauses 28 to 31, wherein the method comprises minimizing a difference between the estimated diffracted radiation incident at the imaging sensor and the detected diffracted radiation to obtain the estimate of diffraction efficiency as a function of wavelength. 33. The method of any of clauses 28 to 32, wherein the structure is a periodic structure with a pitch. Confidential 34. The method of clause 33, wherein the method comprises using knowledge regarding the pitch of the periodic structure to obtain the estimate of diffraction efficiency as a function of wavelength. 35. The method of any of clauses 28 to 34, wherein the step of using the estimated diffraction efficiency to measure the parameter of the structure comprises using the estimated diffraction efficiency as input to a simulation. 36. The method of any of clauses 28 to 35, wherein the emitted radiation is a broadband radiation. 37. The method of any of clauses 28 to 36, wherein the diffracted radiation incident at the imaging sensor comprises at least partly overlapping diffraction orders on surface of the imaging sensor. 38. A method for estimating diffraction efficiency of a structure of a substrate, the method comprising: directing a beam of an emitted radiation onto the structure and thereby generating diffracted radiation due to diffraction by the structure, wherein the emitted radiation having a spectrum comprising a plurality of peaks, and wherein the diffracted illumination comprises multiple diffraction orders; using an imaging sensor to detect the diffracted radiation, wherein the diffracted radiation incident at the imaging sensor comprises at least partly overlapping diffraction orders on surface of the imaging sensor; and separating the at least partly overlapping orders using knowledge regarding the spectrum and a prior of the diffraction efficiency. 39. The method of clause 38, wherein the emitted radiation is a broadband radiation. 40. The method of clause 38 or 39, wherein the structure is a periodic structure. 41. A method of measuring an illumination spectrum of an illumination beam used by a metrology system, the method comprising: directing a pump radiation beam into a target medium and thereby causing the target medium to emit radiation, the emitted radiation having a spectrum comprising a plurality of peaks and being used by the metrology system as an illumination beam having an illumination spectrum; directing the illumination beam onto a grating having a known diffraction efficiency and thereby generating diffracted radiation; using an imaging sensor to detect the diffracted radiation as a spectral power function; expressing the spectral power function in terms of wavelength multiplied by diffraction order, the spectral power function being a multiplication of the illumination spectrum and the known diffraction efficiency; and using the expressed spectral power function and the measured spectral power function to determine the illumination spectrum. 42. The method of clause 41, wherein the effect of a focus error and/or a diffraction-limited spot size is removed from the measured spectral power function, or the effect of a focus error and/or a diffraction- limited spot size on the spectral power function is reduced. 43. The method of clause 42, wherein removing or reducing the effect of a focus error and/or a Confidential diffraction limited spot size comprises applying a deconvolution and or an integral transform. 44. The method of clause 42, wherein a prior probability for smoothness of the illumination spectrum is used when removing or reducing the effect of a focus error and/or a diffraction-limited spot size from the spectral power function. 45. The method of any of clauses 41 to 44, wherein the illumination spectrum is expressed in terms of a signal response of the imaging sensor. 46. A metrology apparatus comprising a controller, wherein the controller is configured to cause performance of the method of any of clauses 1 to 8 and 15 to 45. 47. A non-transitory computer program product comprising machine-readable instructions therein, the instructions, upon execution by a computer system, configured to cause the computer system to at least cause performance of the method of any of clauses 1 to 8 and 15 to 45. [00214] 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. [00215] Although specific reference may be made in this text to embodiments in the context of a lithographic apparatus, embodiments may be used in other apparatus. Embodiments 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 apparatuses may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions. [00216] Although specific reference may be made in this text to embodiments in the context of an inspection or metrology apparatus, embodiments may be used in other apparatus. Embodiments may form part of a mask inspection apparatus, a lithographic apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). The term “metrology apparatus” (or “inspection apparatus”) may also refer to an inspection apparatus or an inspection system (or a metrology apparatus or a metrology system). E.g. the inspection apparatus that comprises an embodiment may be used to detect defects of a substrate or defects of structures on a substrate. 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. [00217] Although specific reference may have been made above to the use of embodiments in the context of optical lithography, it will be appreciated that the invention, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography. [00218] While the targets or target structures (more generally structures on a substrate) described above are metrology target structures specifically designed and formed for the purposes of measurement, Confidential in other embodiments, properties of interest may be measured on one or more structures which are functional parts of devices formed on the substrate. Many devices have regular, grating-like structures. The terms structure, target grating and target structure as used herein do not require that the structure has been provided specifically for the measurement being performed. Further, pitch of the metrology targets 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 dimension of typical non-target structures optionally product structures made by lithographic process in the target portions C. In practice the lines and/or spaces of the overlay gratings within the target structures may be made to include smaller structures similar in dimension to the non-target structures. [00219] While specific embodiments have been described above, it will be appreciated that the invention 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 invention as described without departing from the scope of the claims set out below. [00220] 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 invention 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 invention 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. [00221] Although specific reference is made to HXR, SXR and EUV electromagnetic radiations, it will be appreciated that the invention, where the context allows, may be practiced with all electromagnetic radiations, including radio waves, microwaves, infrared, (visible) light, ultraviolet, X- rays, and gamma rays. [00222] Additional objects, advantages and features of the present invention are set forth in this specification, and in part will become apparent to those skilled in the art on examination of the following, or may be learned by practice of the invention. The inventions disclosed in this application are not limited to any particular set of, or combination of, objects, advantages and features. It is contemplated that various combinations of the stated objects, advantages and features make up the inventions disclosed in this application. Confidential

Claims

CLAIMS 1. A method of measuring a parameter of a structure of a substrate, the method comprising: directing a beam of an emitted radiation onto the structure and thereby generating a diffracted radiation due to diffraction by the structure, wherein the emitted radiation having a spectrum comprising a plurality of peaks; using an imaging sensor to detect the diffracted radiation; using knowledge regarding the spectrum and a prior of the diffraction efficiency to obtain an estimate of diffraction efficiency as a function of wavelength; and using the estimated diffraction efficiency to measure the parameter of the structure.

2. The method of claim 1, wherein the method comprises using knowledge regarding blurring of the beam of emitted radiation to obtain the estimate of diffraction efficiency as a function of wavelength.

3. The method of claim 1 or 2, wherein the prior of the diffraction efficiency is a prior for a second derivative of the diffraction efficiency as a function of wavelength.

4. The method of any preceding claims, wherein the method comprises generating the estimate of the diffracted radiation incident at the imaging sensor.

5. The method of any preceding claims, wherein the method comprises minimizing a difference between the estimated diffracted radiation incident at the imaging sensor and the detected diffracted radiation to obtain the estimate of diffraction efficiency as a function of wavelength.

6. The method of any preceding claims, wherein the structure is a periodic structure with a pitch.

7. The method of claim 6, wherein the method comprises using knowledge regarding the pitch of the periodic structure to obtain the estimate of diffraction efficiency as a function of wavelength.

8. The method of any preceding claims, wherein the step of using the estimated diffraction efficiency to measure the parameter of the structure comprises using the estimated diffraction efficiency as input to a simulation.

9. The method of any preceding claims, wherein the emitted radiation is a broadband radiation. Confidential

10. The method of any preceding claims, wherein the diffracted radiation incident at the imaging sensor comprises at least partly overlapping diffraction orders on surface of the imaging sensor.

11. A method for estimating diffraction efficiency of a structure of a substrate, the method comprising: directing a beam of an emitted radiation onto the structure and thereby generating a diffracted radiation due to diffraction by the structure, wherein the emitted radiation having a spectrum comprising a plurality of peaks, and wherein the diffracted illumination comprises multiple diffraction orders; using an imaging sensor to detect the diffracted radiation, wherein the diffracted radiation incident at the imaging sensor comprises at least partly overlapping diffraction orders on surface of the imaging sensor; and separating the at least partly overlapping orders using knowledge regarding the spectrum and a prior of the diffraction efficiency.

12. The method of claim 11, wherein the emitted radiation is a broadband radiation.

13. The method of claim 11 or 12, wherein the structure is a periodic structure.

14. A metrology apparatus comprising a controller, wherein the controller is configured to cause performance of the method of any of claims 1 to 13.

15. A non-transitory computer program product comprising machine-readable instructions therein, the instructions, upon execution by a computer system, configured to cause the computer system to at least cause performance of the method of any of claims 1 to 13. Confidential