Radiation source

Abstract

A radiation source comprising: an electron source configured to generate pulses of electrons; a laser source configured to generate a laser beam, the laser source and the electron source being arranged such that the pulses of electrons collide with the laser beam at an interaction point to generate output radiation; and an optomechanical component configured for adjusting an angle of incidence between the laser beam and the pulses of electrons at the interaction point.

Description

RADIATION SOURCECROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 25152428.6 which was filed on 17 January 2025, and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a radiation source, in particular a radiation source comprising an electron source and a laser source.BACKGROUND

[0003] In semiconductor device manufacturing processes, apparatuses can be used to process and / or assess objects such as a semiconductor substrate or a patterning device. An example of such an apparatus is an exposure apparatus used to generate interactions between a radiation, for example an electromagnetic radiation or a charged-particle radiation, with a substrate. Other examples of such apparatuses are a deposition apparatus, an etching apparatus, and a die-bonding apparatus. A lithographic apparatus and an assessment apparatus are examples of an exposure apparatus. A lithographic apparatus may use an electromagnetic radiation or a charged-particle radiation to form a pattern onto a layer of radiation-sensitive material, for example a layer of photo resist, provided on a substrate. Examples of a lithographic apparatus are a deep ultraviolet (DUV) lithographic apparatus and an extreme ultraviolet (EUV) lithographic apparatus. An assessment apparatus may form interaction products resulted from interaction of an electromagnetic radiation or charged-particle radiation with a sample such as a substrate. An inspection apparatus and a metrology apparatus are examples of such assessment apparatuses and may be used to detect particles or measure a critical dimension, for example.

[0004] 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).

[0005] 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.

[0006] Low-ki lithography may be used to process features with dimensions smaller than the classical resolution limit of a lithographic apparatus. In such process, the resolution formula may be expressed as CD = kjxX / NA, where I 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 ki is an empirical resolution factor. In general, the smaller ki 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 ki .

[0007] 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.

[0008] 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.

[0009] Examples of known scatterometers often rely on provision of dedicated metrology targets. For example, a method may require a target 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.

[0010] 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 andUS20120044470A. 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.

[0011] 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 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.

[0012] 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.

[0013] There remains a need for a radiation source that is capable of producing high brilliance, high frequency radiation, and that is suitable for applications in high volume manufacturing in the semiconductor industry.SUMMARY

[0014] The present disclosure provides a tunable radiation source comprising an electron source, a laser source, and an optomechanical component. The optomechanical component enables adjustment of an angle of incidence between a laser beam generated by the laser source, and pulses of electrons generated by the electron source. The energy (i.e. wavelength) of output radiation, generated by inverse Compton scattering (ICS), can be tuned by adjusting the angle of incidence. The radiation source according to the present disclosure may be particularly suitable for the generation of high brilliance X-ray radiation (including hard X-ray (HXR) radiation), while remaining compact and relatively low cost,and therefore suitable for semiconductor high volume manufacturing. It will be appreciated that the radiation source described herein may additionally, or alternatively, be suitable for generation of other output radiation wavelengths, including soft X-ray (SXR), extreme ultraviolet (EUV), deep ultraviolet (DUV), visible, and near-infrared (IR) radiation.

[0015] The radiation source described herein may additionally enable adjusting the angle of incidence by adjustment of a single component of the system, without requiring realignment of any other optical components (such as individual mirrors) or altering the optical path (and therefore the focusing) of the laser beam, thereby providing a robust and repeatable way to tune the output radiation wavelength.

[0016] Described herein is a radiation source comprising: an electron source configured to generate pulses of electrons, and a laser source configured to generate a laser beam. The laser source and the electron source are arranged such that the pulses of electrons collide with the laser beam at an interaction point to generate output radiation. The radiation source further comprises an optomechanical component for adjusting an angle of incidence between the laser beam and the pulses of electrons at the interaction point.

[0017] As described herein, adjusting the angle of incidence between the laser beam and the pulses of electrons enables tuning of the wavelength of the output radiation.

[0018] The optomechanical component may be configured to guide the laser beam within a finite rotatable plane. A rotation axis of the finite rotatable plane may pass through the interaction point. The optomechanical component may be configured such that the laser beam enters and exits the finite rotatable plane along the rotation axis.

[0019] Advantageously, the angle of incidence (and therefore the wavelength of the output radiation) may be adjusted by adjusting a single angle within the system forming the radiation source. For example, the finite rotatable plane may be formed by a plurality of optical components (e.g. mirrors) mounted on a rotation stage such that the angle of incidence can be adjusted simply by rotating the rotation stage.

[0020] In some examples, an exit path of the laser beam that is along the rotation axis may additionally enable controlled guidance of the laser beam (which may be a high power beam of hundreds of watts to kilowatts) safely towards a beam dump.

[0021] The laser beam may comprise laser pulses, i.e. the laser source may comprise a pulsed laser.

[0022] In some examples, the electron source comprises a burst-mode linear inverse Compton scattering source.

[0023] The radiation source may comprise an electron accelerator (e.g. a compact electron accelerator) configured to accelerate the pulses of electrons. For example, the electron accelerator may be configured to accelerate the pulses of electrons to close to the speed of light.

[0024] In some examples, the radiation source comprises a variable beam expander configured to vary a width of the laser beam at the interaction point when the angle of incidence is varied. For example, the variable beam expander may be, or may comprise, a rotatable cylindrical beam expander.

[0025] Advantageously, a variable beam expander may enable co-adjustment of the width of the laser beam at the interaction point with the changing angle of incidence, thereby maintaining a beam width (i.e. beam focus) at the interaction point that is optimal for generation of the output radiation.

[0026] In some examples, the radiation source comprises a pulse stretcher configured to vary a length of laser pulses (of the laser beam) when the angle of incidence is varied.

[0027] Advantageously, a length of the laser pulses can be adjusted using the pulse stretcher to maintain the optimal beam width (focus) at the interaction point when the angle of incidence is varied.

[0028] In some examples, the radiation source comprises an optical cavity configured to recirculate laser pulses (of the laser beam). The interaction point may be within the optical cavity.

[0029] Recirculating the laser pulses may be particularly advantageous when a high repetition rate electron source is employed in the radiation source, as recirculation of the laser pulses may result in an increased number (e.g. all) of the electron pulses interacting with a laser pulse. This may contribute to an increased brightness of the radiation source.

[0030] The optical cavity may be formed at least partly within the finite rotatable plane. In some examples, the optical cavity may be formed entirely within the finite rotatable plane.

[0031] In some examples, the output radiation comprises SXR, EUV, DUV, visible, and / or near-IR radiation.

[0032] In some examples, the output radiation comprises radiation having a wavelength of 1 nm or less.

[0033] In some examples, the output radiation comprises radiation having a wavelength in the range 1-100 nm. In some examples, the output radiation comprises radiation having a wavelength in the range 1-50 nm. In some examples, the output radiation comprises radiation having a wavelength in the range 1-20 nm.

[0034] Also described herein is an apparatus, for example a metrology apparatus or an inspection apparatus, comprising the radiation source according to the present disclosure.BRIEF DESCRIPTION OF THE DRAWINGS

[0035] 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 system comprising a lithographic apparatus and a radiation source;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 radiation including EUV, HXR, and / or, SXR radiation is used;Figure 7 depicts a simplified schematic drawing of a radiation source based on inverse Compton scattering (ICS) according to the present disclosure;Figure 8 depicts a simplified schematic drawing of a radiation source comprising an optomechanical component configured for adjusting an angle of incidence between a laser beam and pulses of electrons according to the present disclosure;Figures 9(a) and (b) depict simplified schematic drawings illustrating a change in interaction length between the laser beam and the pulses of electrons when the angle of incidence between the laser beam and the pulses of electrons is changed;Figure 10 illustrates the dependence of output radiation brilliance on the interaction angle between the laser beam and the pulses of electrons for various laser beam widths;Figure 11 depicts a simplified schematic drawing of a radiation source additionally comprising an optical cavity according to the present disclosure; andFigures 12(a) and 12(b) depict simplified schematic drawings of radiation sources wherein part of the optical cavity is formed within a finite rotatable plane, and wherein the optical cavity is formed entirely within the finite rotatable plane, respectively, according to the present disclosure.DETAILED DESCRIPTION

[0035] Before describing embodiments of the invention in detail, it is instructive to present an example environment in which embodiments of the present invention may be implemented.

[0036] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.

[0037] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[0038] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13,14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image with features that may be smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13,14 in Figure 1, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[0039] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[0040] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and / or in the projection system PS.

[0041] The radiation source SO shown in Figure 1 is, for example, of a type which may be referred to as a laser produced plasma (LPP) source. A laser system 1, which may, for example, include a CO2 laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) which is provided from, e.g., a fuel emitter 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may, for example, be in liquid form, and may, for example, be a metal or alloy. The fuel emitter 3 may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the tin at the plasma formation region 4. The deposition of laser energy into the tin creates a tin plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during deexcitation and recombination of electrons with ions of the plasma.

[0042] The EUV radiation from the plasma is collected and focused by a collector 5. Collector 5 comprises, for example, a near-normal incidence radiation collector 5 (sometimes referred to more generally as a normal-incidence radiation collector). The collector 5 may have a multilayer mirror structure which is arranged to reflect EUV radiation (e.g., EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an ellipsoidal configuration, having two focal points. A first one of the focal points may be at the plasma formation region 4, and a second one of the focal points may be at an intermediate focus 6, as discussed below.

[0043] The laser system 1 may be spatially separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser system 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and / or a beamexpander, and / or other optics. The laser system 1, the radiation source SO and the beam delivery system may together be considered to be a radiation system.

[0044] Radiation that is reflected by the collector 5 forms the EUV radiation beam B. The EUV radiation beam B is focused at intermediate focus 6 to form an image at the intermediate focus 6 of the plasma present at the plasma formation region 4. The image at the intermediate focus 6 acts as a virtual radiation source for the illumination system IL. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source SO.

[0045] Although Figure 1 depicts the radiation source SO as a laser produced plasma (LPP) source, any suitable source such as a discharge produced plasma (DPP) source, high harmonic generation (HHG) source or a free electron laser (FEL) may be used to generate EUV radiation.

[0046] As shown in Figure 2 the lithographic apparatus LA may form part of a lithographic cell LC, also sometimes referred to as a lithocell or (litho)cluster, which often also includes apparatus to perform pre- and post-exposure processes on a substrate W. Conventionally these include spin coaters SC to deposit resist layers, developers DE to develop exposed resist, chill plates CH and bake plates BK, e.g. for conditioning the temperature of substrates W e.g. for conditioning solvents in the resist layers. A substrate handler, or robot, RO picks up substrates W from input / output ports I / Ol, 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.

[0047] 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 nearinfrared (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.

[0048] 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.

[0049] An inspection apparatus, which may also be referred to as a metrology apparatus, is used to determine properties of the substrates W, and in particular, how properties of different substrates W vary or how properties associated with different layers of the same substrate W vary from layer to layer. The inspection apparatus may alternatively be constructed to identify defects on the substrate W and may, for example, be part of the lithocell LC, or may be integrated into the lithographic apparatus LA, or may even be a stand-alone device. The inspection apparatus may measure the properties on a latent image (image in a resist layer after the exposure), or on a semi-latent image (image in a resist layer after a post-exposure bake step PEB), or on a developed resist image (in which the exposed or unexposed parts of the resist have been removed), or even on an etched image (after a pattern transfer step such as etching).

[0050] 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.

[0051] 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.

[0052] 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 aredescribed 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.

[0053] 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 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 EP 1,628, 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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” 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.

[0058] 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 SCI). 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).

[0059] 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).

[0060] 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.

[0061] 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.

[0062] 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 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.

[0063] 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.

[0064] 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 which projects 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 I) of the specular reflected radiation. From this data, the structure or profile 8 giving rise to the detected spectrum may bereconstructed 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.

[0065] 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 < Inm, optionally <0.1 nm, optionally <0.0 Inm.

[0066] 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, <lnm, between O.Olnm and lOOnm, between O.Olnm and 50nm, between Inm and 50nm, between Inm and 20nm, between 5nm and 20nm, and between lOnm 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.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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 unit (MPU) 320.

[0071] An illumination source 310 in this example is for a generation of EUV, hard X-ray or soft X-ray radiation. In the example illustrated in Figure 6, the illumination source 310 is based on high harmonic generation (HHG) techniques. However, the illumination source 310 may also be based on 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.

[0072] 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.

[0073] For the example of a 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 10pm, for example in the region of 1 pm (1 micron). Optionally, the laser pulses are delivered as a first pump radiation 340to the gas delivery system 332, wherein 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.

[0074] 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 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.

[0075] As discussed above, an alternative illumination source 310 may comprise an inverse Compton scattering (ICS) source (such that the emitted beam 342, also referred to as output radiation, is generated at least partly by ICS, e.g. rather than HHG). In some examples, an ICS source may enable generation of radiation having higher brilliance at shorter wavelengths (e.g. including HXR wavelengths) than can be achieved by HHG, without requiring large facilities such as SLSs or XFELs that would be impractical for HVM applications.

[0076] The present disclosure provides a tunable radiation source based on ICS. The tunable radiation source according to the present disclosure is suitable for use as the illumination source 310 in the example of Figure 6.

[0077] 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 awavelength 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.

[0078] From the radiation source 310, the output radiation 342 (which may be a filtered beam) 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 T. Optionally the atmosphere within inspection chamber 350 may be maintained near vacuum by vacuum pump 352, so that HXR, 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 / 0184981 Al (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 pm 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 T.

[0079] 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.

[0080] Reflected radiation 360 is captured by detector 318 and a spectrum is provided to processor 320 for use in calculating a property of the target structure T. 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.

[0081] 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 ofthe 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.

[0082] 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.

[0083] 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 monochromatic, narrowband or broadband in character. The radiation may have discrete peaks in a specific wavelength band or may have a more continuous character.

[0084] 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.

[0085] 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 anelectromagnetic 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. As described above, ICS, HHG, and / or other types of sources may be used to obtain radiation of the desired wavelength(s) including extreme ultraviolet (EUV) radiation (e.g. 1 nm to 100 nm), soft X-ray (SXR) radiation (e.g. 0.1 nm to 10 nm), and / or hard X-ray (HXR) radiation (e.g. radiation with wavelength < Inm, optionally <0. Inm, optionally <0.01nm).

[0086] Figure 7 shows a simplified schematic drawing of an example of a radiation source 700 based on ICS, which may be suitable for use as at least part of an illumination source 310 as described above in relation to any of Figures 1 to 6. The radiation source 700 comprises an electron source 702. The electron source 702 may comprise a photoemission source, in which pulses (or bunches) of electrons are emitted from a cathode. The electron source 702 may be, or may comprise, a burst-mode electron source, for example the electron source 702 may comprise a thermionic emitter in combination with radiofrequency (RF) cavities to provide bursts of electron bunches. One example of an electron source that may be suitable in some implementation according to the present disclosure is described in W. F. Toonen et al., “Gigahertz repetition rate thermionic electron gun concept”, Phys. Rev. Accel. Beams 22, 123401 (published 11 December 2019), the contents of which are hereby incorporated by reference.

[0087] The electron source 702 generates pulses of electrons 710, represented by ellipses in Figure 7. The pulses of electrons 710 may comprise bunches of electrons. The generated pulses (or bunches) of electrons may also be referred to herein as an electron beam.

[0088] Optionally, the radiation source 700 may comprise an electron accelerator 704 (e.g. a compact electron accelerator) configured to accelerate the pulses of electrons 710, for example to close to the speed of light. An example of a suitable (compact) electron accelerator 704 is a linear radiofrequency (RF) accelerator, comprising a plurality of metal cavities into which a resonant RF wave is fed. The resulting large electric field in the cavities accelerates the electrons. It may be particularly desirable for the RF accelerator to operate at a RF frequency that is as high as possible, because the length of the RF accelerator scales inversely proportionally to the RF frequency. In an example, a state-of-the art RF accelerator may operate at a RF frequency of up to around 12 GHz. An alternative example of a suitable (compact) electron accelerator 704 is a laser-wakefield accelerator, which operates by creating a large electric field in a plasma channel.

[0089] The radiation source 700 further comprises a laser source 706, e.g. a pulsed laser, configured to generate a laser beam 708. The laser beam 708 (also referred to as pump radiation) may comprise radiation having a wavelength, or wavelengths, longer than the wavelength(s) of the output radiation.The laser beam 708 may comprise infrared radiation. The laser beam 708 may comprise radiation with wavelength(s) in the range of 500 nm to 1500 nm. The laser beam 708 may comprise radiation with wavelength(s) in the range of 800 nm to 1300 nm. The laser beam 708 may comprise radiation with wavelength(s) in the range of 900 nm to 1300 nm. In some examples, the laser beam 708 may comprise radiation with wavelength(s) of less than 500 nm. For example, the laser beam 708 may comprise radiation with wavelength(s) in the range of 200 nm to 500 nm. In some examples, the laser beam 708 comprises radiation having a wavelength of around 800 nm. In some examples, the laser beam 708 comprises radiation having a wavelength corresponding to a harmonic of 800 nm radiation, for example around 400 nm (i.e. second harmonic), or around 266 nm (third harmonic). In some examples, the laser beam 708 comprises radiation having a wavelength corresponding to a harmonic of another laser wavelength, e.g. 1030 nm, or 1064 nm. The laser beam 708 may be a pulsed laser beam, that is the laser beam 708 may comprise pulsed radiation. The pulsed radiation may comprise pulses with a duration in the femtosecond range.

[0090] The electron source 702 and the laser source 706 are arranged such that the pulses of electrons 710 collide with the laser beam 708 at an interaction point 712. The laser beam 708 is sufficiently intense, and the pulses of electrons are travelling sufficiently fast (i.e. close to the speed of light) that the laser photons that bounce off of the counter-propagating pulses of electrons 710 are converted into higher energy photons (e.g. X-ray photons or EUV photons) due to the relativistic Doppler effect. The higher energy photons constitute a narrow beam of output radiation 714 travelling in the same direction as the pulses of electrons 710. The wavelength of the output radiation 714 is varied by varying an angle of incidence 0 between the laser beam 708 and the pulses of electrons 710 at the interaction point 712. In examples, the output radiation 714 may have a wavelength in the X-ray (e.g. SXR and / or HXR) or EUV range, wherein the wavelength of the output radiation 714 may be 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, optionally from 1 nm to 20 nm, or optionally from 10 nm to 20 nm.

[0091] In operation, the output radiation 714 may be subsequently manipulated and directed by an illumination system, such as the illumination system 312 in Figure 6, to a substrate to be inspected for metrology measurements. The output radiation 714 may be guided, optionally focused, to a structure on the substrate.

[0092] Because air (and in fact any gas) heavily absorbs SXR or EUV radiation, the volume between the radiation source 700, 310 may be evacuated or nearly evacuated. Depending on the arrangement of the system, the laser beam 708 may need to be blocked to prevent it from entering the illumination system 312. This may be done by incorporating a filtering device 344 shown in Figure 6 into the radiation output, which is placed in the emitted beam path and that is opaque or nearly opaque to the wavelength(s) of the laser beam 708 (e.g. opaque or nearly opaque to infrared or visible light) but at least partially transparent to the output radiation 714.

[0093] The output radiation 714 may be provided as radiation in inspection tools or metrology tools MT for inspection and / or measurement of substrates. The substrates may be lithographically patterned substrates. The output radiation 714 may also be provided in a lithographic apparatus LA, and / or a lithographic cell LC.

[0094] The output radiation 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 MT, compared to using longer wavelengths (e.g. visible radiation, infrared radiation). Radiation with shorter wavelengths, such as EUV, SXR 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, e.g. visible light.

[0095] 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 EUV, SXR 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 (+1 st) and negative (-1st) first diffraction orders. The metrology tool MT may also measure the specular reflected or transmitted radiation (Oth 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).

[0096] 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.

[0097] As well as HHG and ICS sources, other types of radiation sources may be practiced for hard X-ray, soft X-ray, EUV, DUV as well as visible illumination generation. For example, a radiation source may include one or more of a LPP source, a liquid metal jet source, a plasma channel source, a magnetic undulator source, a free electron laser (FEL) source, a compact storage ring source, an electrical discharge produced plasma source, a rotating anode source, a solid anode source, a particle accelerator source, and a microfocus source.

[0098] A shortcoming of many lab-based HXR sources (such as X-ray tubes) is that they only operate at specific wavelengths (or equivalently, energies) set by the atomic properties of the material used as an anode. For many sources, changing the wavelength of the output radiation requires physically exchanging (at least part of) the source. Nevertheless, energy-tunability of a radiation source (includinga HXR or EUV source) can be very desirable for semiconductor manufacturing process, e.g. for semiconductor metrology. For example, not only the geometry of the semiconductor structures may be probed, but also their chemical composition may be determined by measuring their relative HXR absorption or refractive index as function of HXR energy. Another example is the so-called HXR standing wave technique, where a reflecting HXR beam can be made selectively sensitive to a layer at a particular depth in the metrology target, which is dependent on the HXR energy used.

[0099] The present disclosure therefore provides a tunable radiation source. As described above, for an ICS radiation source 700, the wavelength of the output radiation 714, which can include HXR radiation, can be tuned by varying the angle of incidence 0 between the laser beam 708 and the pulses of electrons 710 at the interaction point 712 (see Figure 7). In general, for ICS sources such as the radiation source 700 illustrated in Figure 7, the energy of the output radiation, h(Ox, is given bywhere h(O0is the photon energy of the laser beam 708, fl = V I C is the electron velocity divided by the speed of light, and 0 is the angle of incidence of the laser beam 708 with respect to the pulses of electrons 710. Because the pulses of electrons are generally accelerated to nearly the speed of light, fl is very close to unity and therefore Equation (1) yields an output radiation energy that is significantly higher (e.g. by 4 orders of magnitude) than the photon energy of the laser.

[0100] From Equation (1), there are theoretically three ways to adjust the energy (wavelength) of the output radiation from an ICS source: change the electron beam velocity fl , change the photon energy tl , (i.e. the wavelength of the laser beam), or change the angle of incidence 0.

[0101] In practice, it may be difficult to change the electron beam velocity, because the transport, acceleration, focusing, and beam quality optimization of the electron beam are usually specific to a particular electron beam velocity. Changing the electron beam velocity may therefore reqiure retuning of all of the electron optics in the beamline, which may be impractical and difficult to do in a robust and repetitive way. In addition, the ICS radiation source is preferably operated with electron velocities as close to the speed of light as possible (i.e. 1 — / ? « 1 ), as the solid angle of the radiation cone, fl = is the relativistic factor of the electron beam. Therefore, the closer the electronbeam velocity is to the speed of light, the larger the directionality, and therefore the higher the brilliance of the output radiation.

[0102] In addition, changing the photon energy of the laser beam is generally also difficult due to the limited tunability of laser sources, and because the optics used to manipulate, transport, and focus the laser beam are usually optimized for a limited range of wavelengths.

[0103] Therefore, as discussed above, it is preferable to adjust the wavelength of the output radiation by adjusting the angle of incidence 0 between the laser beam and the electron beam (pulses of electrons). This means that the electron beam can be operated at the highest possible velocity (and therefore the output radiation brilliance can be optimized).

[0104] The present disclosure provides a flexible optical setup in which the angle of incidence can be freely and quickly adjusted, without the need for cumbersome realignment of multiple optical elements every time the angle of incidence is changed, and without the needs to rotate the whole laser source around the interaction point.

[0105] Figure 8 schematically illustrates an example of part of a radiation source 800 according to the present disclosure. In Figure 8, the electron source and the radiation source have been omitted for clarity.

[0106] As shown in Figure 8, the radiation source 800 comprises an optomechanical component 822 configured for adjusting the angle of incidence 0 between the laser beam 808 and the pulses of electrons 810. The optomechanical component 822 may comprise a plurality of mirrors or other optical elements (not shown in Figure 8 for clarity) to guide the laser beam 808 through the optomechanical component 822 in a finite rotatable plane, such that the angle of incidence 0 at the interaction point 812 is varied by rotating the optomechanical component 822 (i.e. rotating the finite rotatable plane). For example, the plurality of mirrors or other optical elements forming the optomechanical component 822 may be mounted on a rotation stage 824. It will be understood that, in some embodiments, the “optomechanical component” described herein may refer collectively to the plurality of mirrors (or other optical elements) and the rotation stage 824.

[0107] The rotation axis of the finite rotatable plane (represented in Figure 8 by the dash-dotted line) passes through the interaction point 812, and the laser beam 808 enters and exits the finite rotatable plane of the optomechanical component 822 along the rotation axis of the finite rotatable plane (e.g. the laser beam 808 may be guided into and away from the optomechanical component 822, respectively, by a pair of fixed mirrors 821). The laser beam 808 is guided by the optomechanical component 822 along an S-shaped path through the interaction point 812, and the S-shaped path is rotatable around the rotation axis. In this way, the angle of incidence 0 (and therefore the wavelength of the output radiation 814) can be adjusted freely by just one rotation state, without the need to adjust the alignment of any individual mirror(s) or other optical component(s). This means that the radiation source 800 can always be operated at an optimally high electron energy, resulting in an optimally high brilliance of the output radiation 814, regardless of the energy (wavelength) of the output radiation 814. In addition, the length of the optical path between the upstream laser and the interaction point 812 is not altered by the rotation. This is important because the laser beam 808 is preferably focused at the interaction point 812 for optimal ICS operation. This focusing may be achieved by inserting a transmissive or reflective focusing optic or a combination of transmitting and / or reflective focusing optics (not shown in Figure 8) at an appropriate distance upstream of the interaction point 812. Any change in distance between the focusingoptic(s) and the interaction point 812 would lead to violation of the focusing requirement. Optionally, one or more of any of the mirrors forming the optomechanical component 822, and / or the fixed mirrors 821, that are upstream of the interaction point 812 may be curved and thereby function as the focusing optic(s).

[0108] The angle of incidence 0 may be adjusted continuously all the way between 0 and 180 degrees (in some cases, there may be some excluded angles due to mechanical supports of optical components). From Equation (1), this leads to an output radiation 814 tunability between a minimum output radiation energy h(Ox minand a maximum output radiation energy given bywhere Ukinis the kinetic energy of the electron beam and me2= 0.511 MeV is the rest energy of the electron. For a typical Ukln= 50 MeV electron beam and 1000 nm laser beam, h(x maxis around 50 keV, which is typically well above the X-ray energies of interest for semiconductor metrology (silicon becomes transparent between 20 and 25 keV, and above). Therefore, the radiation source 800 according to the present disclosure may permit output energy 814 tunability essentially throughout the relevant part of the X-ray spectrum for semiconductor metrology, and beyond.

[0109] The ICS conversion efficiency may depend on the angle of incidence 0. This dependency is due to the length of the interaction region along which the electron beam interacts with the laser beam, which changes as a function of the angle of incidence 0. This is illustrated in Figures 9(a) and 9(b). An angle of incidence 0 between the laser beam 908 and the electron beam 910 that is close to 90 degrees, as shown in Figure 9(b), results in a shorter interaction length 930 than an angle of incidence 0 that is closer to 0 degrees, as shown in Figure 9(a). The shorter interaction length results in a reduced efficiency of output radiation generation. To compensate for this, the laser beam width at the focus in the plane of the laser beam 908 and the electron beam 910 (i.e. the plane of the drawing in Figures 9(a) and 9(b)) is preferably increased when operating the radiation source at large angles of incidence. This may be achieved by reducing the laser beam size that is incident on the focusing optic. On the other hand, at the same time such looser focusing also leads to a lowered laser intensity, which tends to reduce the output radiation generation efficiency. Therefore, in the direction perpendicular to the plane formed by the electron beam and the part of the laser beam passing through the interaction point, the laser beam width is preferably kept small regardless of the incidence angle to limit the loss of laser intensity. Nonetheless, the optimal beam size at the focus is the result of a tradeoff between the opposing effects of interaction length and laser intensity. This tradeoff is illustrated in Figure 10, which shows the variation in output radiation brilliance (“X-ray brightness”) with the angle of incidence 0 for different widths of the laser beam in the plane of the electron beam and the laser beam (for all cases shown inFigure 10, the width of the laser beam in the direction perpendicular to the plane formed by the electron beam and the part of the laser beam passing through the interaction point is 5 pm). As shown in Figure 10, small angles of incidence require a tightly focused laser beam while large angles of incidence require a more loosely focused laser beam for optimal operation. The optimal in-plane laser width Woptis given as a function of angle of incidence 0 bywhere cr is the temporal pulse length of the laser beam.

[0110] Therefore, in some examples according to the present disclosure, an optional variable beam expander 826 may be incorporated into the path of the laser beam 808 upstream of the interaction point 812, as shown in Figure 8, to enable maintenance of the optimal output radiation 814 generation conditions according to Equation 3 by appropriately co-adjusting the beam size with the angle of incidence 0 at the interaction point 812. The variable beam expander 826 should be configured to adjust the beam size in a first direction, and to keep the beam size in a second direction (perpendicular to the first direction) constant. The variable beam expander 826 may comprise a rotatable cylindrical beam expander (e.g. a cylindrical beam expander mounted on a rotation stage upstream of the optical element 822, as illustrated in Figure 8, where the rotation stage of the variable beam expander 826 may be configured to rotate with the rotation stage 824, i.e. when 0 is adjusted). In an alternative example (not shown), the variable beam expander may be positioned somewhere in the finite rotatable plane defined by the optical element 822, in the optical path of the laser beam 808. This would remove the requirement that the variable beam expander need itself be rotatable.

[0111] A further possibility for maintaining an optimal laser width at the focus, according to Equation 3, is to vary the length of the laser pulses <T in response to changes in the angle of incidence 0. For example, as shown in Figure 8, an optional pulse stretcher 828 could be included in the optical path of the laser beam 808 upstream of the interaction point 812 (e.g. upstream of the optical element 822). The length of the laser pulses <T should be appropriately co-adjusted along with the angle of incidence 0 to maintain the optimal laser width at the interaction point 812 according to Equation 3.

[0112] A further example of a radiation source 1100 according to the present disclosure is illustrated schematically in Figure 11. Similarly to the radiation source 700 illustrated in Figure 7, the radiation source 1100 illustrated in Figure 11 comprises an electron source 1102 configured to generate pulses of electrons 1110, and a laser source 1106 (e.g. a pulsed laser) configured to generate a laser beam 1108 (e.g. comprising laser pulses). The radiation source 1100 may optionally further comprise an electron accelerator 1104 (e.g. a compact electron accelerator) configured to accelerate the pulses of electrons 1110. As shown in Figure 11, the radiation source 1100 may further comprise an optical cavity 1140(e.g. formed by a plurality of mirrors and / or other optical components), the optical cavity 1140 being configured to recirculate the laser pulses of the laser beam 1108, and the interaction point 1112 may be within the optical cavity. In examples of radiation sources 1100 comprising an optical cavity 1140, the laser source 1106 may be referred to as a seed laser. High repetition rate, low energy pulses from the seed laser are coupled into and stacked in the optical cavity 1140. The result is a high-energy single laser pulse that circulates in the cavity.

[0113] The radiation source 1100 illustrated in Figure 11 and described herein may be particularly suitable when the electron source 1102 is a high repetition rate electron source, such as in a burst-mode linear ICS source, or in a circular ICS source where electron bunches are recirculated in a storage ring, such as the storage ring-based ICS source produced by Lyncean Technologies. A high repetition rate of the electron bunches may enable further increases in the brightness of the output radiation 1114, where the recirculation of laser pulses in the optical cavity 1140 ensures that every electron bunch encounters a laser pulse when passing the interaction point 1112.

[0114] Preferably, the roundtrip time of the laser pulses in the optical cavity 1140 matches the repetition rate of the electron bunches (pulses of electrons 1110), which may be around 1 GHz or more. It may be that the repetition rate of the electron bunches corresponds to the accelerated repetition rate (i.e. by an electron accelerator 1104), which may be e.g. an integer division of a radiofrequency of the electron accelerator 1104, and / or corresponds to the radiofrequency of a RF cavity of the electron source itself (e.g. where the electron source is a burst-mode electron source as described herein). For example, in some cases, it may be most optimal for the repetition rate of the electron bunches to match the RF frequency, / , of the electron accelerator 1104 and / or of the RF cavity of the electron source itself, e.g. up to around / =12 GHz. In some cases, it may be more practical for the repetition rate of the electron bunches to be a subharmonic of the RF frequency, f / N, where N is an integer, e.g.f / N = 3 GHz. In some implementations, this may mean that the optical path length per roundtrip in the optical cavity 1140 is less than a few tens of centimeters.

[0115] Figure 12(a) schematically illustrates an implementation of a radiation source 1200a comprising an optomechanical component 1222, similar to the optomechanical component 822 of the radiation source illustrated in Figure 8. As described herein in relation to other examples, the optomechanical component 1222 guides the laser beam 1208 along an optical path (e.g. an S-shaped path) within a finite rotatable plane (e.g. the optomechanical component 1222 may be mounted on a rotation stage 1224) so that the laser beam 1208 interacts with pulses of electrons 1210 (also referred to as electron bunches) at the interaction point 1212 to produce output radiation 1214, where the rotation axis of the finite rotatable plane may pass through the interaction point 1212. As described above in relation to Figure 11, the laser beam 1208 may be provided by a seed laser. The optical cavity (which has a bow-tie shape in the example of Figure 12(a)) may be formed, e.g. by a plurality of mirrors and / or other optical components, such that part (e.g. an arm) of the optical cavity comprises the rotatable part of the optical path (e.g. the optical path through the optomechanical component 1222). For example,e.g. one of the arms of the optical cavity may comprise the S-shaped optical path as shown in Figure 12(a). In the example illustrated in Figure 12(a), the optical cavity is formed by a pair of fixed plane mirrors 1223a, and a pair of fixed curved mirrors 1227a, located outside of the finite rotatable plane (i.e. outside of the optomechanical component 1222). It will be understood that other configurations of optical components are also possible for forming the optical cavity, for example employing different numbers of optical components such as mirrors, and / or different configurations of curved and / or plane mirrors, to that shown in Figure 12(a).

[0116] In some examples, it may be preferable that a larger part of the optical cavity be integrated into the rotatable part of the optical path. Figure 12(b) schematically illustrates a similar implementation of a radiation source 1200b to that illustrated in Figure 12(a). However, in the example illustrated in Figure 12(b), the optical cavity is formed entirely within the finite rotatable plane (i.e. within the optomechanical component 1222) by a pair of plane mirrors 1223b and a pair of curved mirrors 1227b that are integrated with the optomechanical component 1222. Integrating a larger part of the optical cavity within the finite rotatable plane (e.g. forming the optical cavity entirely within the rotatable plane) may facilitate fitting all of the optics required to produce the finite rotatable plane (e.g. the S-shaped path through the optomechanical component 1222) within the required optical path length (which may be a few tens of centimeters, as discussed above), which may otherwise be challenging when a large number of additional optical components is required.

[0117] It will be understood that any of the examples of radiation sources described and illustrated herein may be combined. For example, a radiation source may comprise one or more of the beam expander, pulse stretcher, electron accelerator, and optical cavity described herein.

[0118] It will be understood that any of the radiation sources described and illustrated herein (e.g. the radiation source 700 illustrated in Figure 7, the radiation source 800 illustrated in Figure 8, the radiation source 1100 illustrated in Figure 11, the radiation source 1200a illustrated in Figure 12(a), and / or the radiation source 1200b illustrated in Figure 12(b)) may be used as the radiation source for a metrology tool MT (or metrology apparatus or inspection apparatus) and / or a lithographic apparatus LA as described herein. For example, any of the radiation sources based on ICS described and illustrated herein may be used as at least part of the illumination source 310 of the metrology apparatus 302 illustrated in Figure 6, instead of or in addition to the HHG source described and illustrated in relation to Figure 6.

[0119] 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 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 programor 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.

[0120] 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.

[0121] 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 (crosssection) 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.

[0122] While specific embodiments of the invention 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.

[0123] 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.

[0124] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non- vacuum) conditions.

[0125] 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, acharacteristic 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.

[0126] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine -readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[0127] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.

[0128] It should be appreciated that the term color is used throughout this text synonymously with wavelength or spectral component and the colors may include those outside the visible band (e.g., infrared or ultraviolet wavelengths).

[0129] 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 1-100 nm), X-ray radiation, electron beam radiation and other particle radiation.

[0130] 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 ECD array.

[0131] The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components. Reflective components are likely to be used in an apparatus operating in the UV and / or EUV ranges.

[0132] 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.

Claims

CLAIMS1. A radiation source comprising:an electron source configured to generate pulses of electrons;a laser source configured to generate a laser beam, the laser source and the electron source being arranged such that the pulses of electrons collide with the laser beam at an interaction point to generate output radiation; andan optomechanical component configured for adjusting an angle of incidence between the laser beam and the pulses of electrons at the interaction point.

2. A radiation source according to claim 1 , wherein the optomechanical component is configured to guide the laser beam within a finite rotatable plane, wherein a rotation axis of the finite rotatable plane passes through the interaction point.

3. A radiation source according to claim 2, wherein the optomechanical component is configured such that the laser beam enters and exits the finite rotatable plane along the rotation axis.

4. A radiation source according to any one of the preceding claims, comprising an electron accelerator configured to accelerate the pulses of electrons.

5. A radiation source according to any one of the preceding claims, comprising a variable beam expander configured to vary a width of the laser beam at the interaction point when the angle of incidence is varied.

6. A radiation source according to claim 5, wherein the variable beam expander is a rotatable cylindrical beam expander.

7. A radiation source according to any one of the preceding claims, wherein the laser beam comprises laser pulses.

8. A radiation source according to claim 7, comprising a pulse stretcher configured to vary a length of the laser pulses when the angle of incidence is varied.

9. A radiation source according to claim 7 or 8, wherein the radiation source comprises an optical cavity configured to recirculate the laser pulses, wherein the interaction point is within the optical cavity.

10. A radiation source according to claim 9 as dependent on claim 2, wherein the optical cavity is formed at least partly within the finite rotatable plane.

11. A radiation source according to any one of the preceding claims, wherein the electron source comprises a burst-mode electron source.

12. A radiation source according to any one of the preceding claims, wherein the output radiation comprises radiation having a wavelength of 1 nm or less.

13. A radiation source according to any one of claims 1 to 12, wherein the output radiation comprises radiation having a wavelength in the range 1-100 nm, 1-50 nm, or 1-20 nm.

14. An apparatus comprising the radiation source of any one of the preceding claims.

15. An apparatus according to claim 14, wherein the apparatus is an metrology or inspection apparatus.

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