Improved broadband radiation generation in hollow core optical fibers

Abstract

A radiation source assembly and method for generating broadband radiation by spectral broadening. The assembly includes a pump source configured to emit modulated pump radiation at one or more wavelengths. The assembly also includes an optical fiber configured to receive the modulated pump radiation emitted by the pump source, the optical fiber including a hollow core extending along a portion of the length of the optical fiber. The hollow core is configured to guide the received radiation during propagation through the optical fiber. The radiation emitted by the pump source includes first radiation at a pump wavelength, and the pump source is configured to modulate the first radiation to excite spectral broadening in the optical fiber.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP application 19198064.8 filed on September 18, 2019 and EP application 19202163.2 filed on October 9, 2019, which are incorporated herein by reference in their entirety. Technical Field

[0003] The present invention relates to methods, assemblies and apparatus for generating broadband radiation by spectral broadening. In a specific exemplary arrangement, the present invention relates to exciting spectral broadening of radiation within a hollow core optical fiber. Background Art

[0004] A lithographic apparatus is a machine configured to apply a desired pattern to a substrate. A lithographic apparatus may be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may project a pattern (also commonly referred to as a "design layout" or "design"), for example, at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) disposed on a substrate (e.g., a wafer).

[0005] In order to project a pattern onto a substrate, a lithographic apparatus can use electromagnetic radiation. The wavelength of the radiation determines the minimum size of a feature that can be formed on a substrate. Typical wavelengths currently used are 365nm (i-line), 248nm, 193nm, and 13.5nm. Compared to lithographic apparatus using radiation with a wavelength of, for example, 193nm, lithographic apparatus using extreme ultraviolet (EUV) radiation with a wavelength in the range of 4 to 20nm (e.g., 6.7nm or 13.5nm) can be used to form smaller features on a substrate.

[0006] Low-k 1 Photolithography can be used to process features whose size is smaller than the classical resolution limit of the lithographic equipment. In this process, the resolution formula can be expressed as CD = k 1 × λ / NA, where λ is the wavelength of radiation employed, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the “critical dimension” (usually the smallest feature size printed, but in this case it is the half pitch), and k 1 is the empirical resolution factor. Usually, k 1The smaller it is, the more difficult it is to reproduce on the substrate a pattern that is similar in shape and size to that planned by the circuit designer to achieve specific electrical functionality and performance. To overcome these difficulties, complex fine-tuning steps can be applied to the lithography projection equipment and / or the design layout. For example, these include but are not limited to optimization of NA, customized illumination options, use of phase-shifting pattern forming devices, various optimizations of the design layout such as optical proximity correction (OPC, sometimes also called "optical and process correction") in the design layout, or other methods generally defined as "resolution enhancement technology" (RET). Alternatively, a tight control loop for controlling the stability of the lithography equipment can be used to improve the reproduction of patterns at low k1.

[0007] In the field of lithography, many different measurement systems can be used to obtain information about lithographic exposure patterns. Such measurement systems can use radiation to interrogate lithographically patterned substrates, such as electromagnetic radiation. Due to the small size of patterned features, the properties of the radiation used to inspect and / or measure the substrate may affect the type of information obtained as part of the measurement. The radiation properties that may affect the measurement results may include, for example, the size and shape of the radiation beam, the intensity of the radiation, and the (multiple) wavelengths of the radiation. For example, the wavelength may affect the smallest feature that can be distinguished in the measurement. Different materials and / or features may also have wavelength-dependent reactions (e.g., due to absorption, transmission, interference effects, etc.). Therefore, it may be desirable to be able to provide high-quality radiation of multiple wavelengths to inspect the substrate. This can be achieved, for example, by providing broadband radiation. Summary of the invention

[0008] According to one aspect of the present disclosure, a radiation source assembly for generating broadband radiation by spectral broadening is provided. The assembly includes: a pump source configured to emit modulated pump radiation at one or more wavelengths; and an optical fiber configured to receive the modulated pump radiation emitted by the pump source. The optical fiber includes a hollow core extending along at least a portion of the length of the optical fiber, and the hollow core is configured to guide the received radiation during propagation through the optical fiber. The radiation emitted by the pump source includes a first radiation at a pump wavelength, and the pump source is configured to modulate the first radiation to excite spectral broadening in the optical fiber.

[0009] Optionally, the pump source may further include: a first radiation source configured to generate the first radiation; and / or a second radiation source configured to generate the second radiation at a seed wavelength to modulate the first radiation.

[0010] Optionally, the first radiation and the second radiation may be mixed to modulate the first radiation by spatially overlapping at least a portion of the first radiation with at least a portion of the second radiation to produce modulated pump radiation.

[0011] Optionally, mixing the first radiation and the second radiation may include coherently overlapping at least a portion of the first radiation with at least a portion of the second radiation.

[0012] Optionally, the seed wavelength and the pump wavelength may satisfy phase matching conditions for four-wave mixing.

[0013] Optionally, the intensity of the seed radiation may not exceed 1%, 2%, 5%, 10% or 15% of the intensity of the modulated pump radiation.

[0014] Optionally, the pump source may be configured to modulate the first radiation in the time domain.

[0015] Optionally, the first radiation source may be configured to emit a first radiation pulse, and to modulate the first radiation pulse when an intensity of the first radiation is above a threshold intensity.

[0016] Optionally, the hollow core of the optical fiber may contain a fluid for exciting spectral broadening.

[0017] Optionally, the assembly may further comprise a reservoir for containing a fluid. The reservoir and the optical fiber may be configured to provide the fluid to the hollow core of the optical fiber.

[0018] Alternatively, the fluid may comprise a gas mixture including an inert gas.

[0019] Alternatively, the fluid may comprise a gas mixture including a molecular gas.

[0020] Optionally, the length of the optical fiber may be in the range of 10 cm to 40 cm.

[0021] Alternatively, the optical fiber may be a photonic crystal fiber.

[0022] Alternatively, a photonic crystal fiber may comprise a single ring microstructure surrounding a hollow core of the fiber.

[0023] Alternatively, a single ring structure may include a plurality of capillaries surrounding a hollow core.

[0024] Alternatively, the diameter of the hollow core may be in the range of 25 μm to 35 μm or 30 μm.

[0025] Alternatively, the broadband radiation may comprise supercontinuum radiation.

[0026] According to another aspect of the present disclosure, a method for generating broadband radiation by spectral broadening is provided. The method includes: emitting modulated pump radiation at one or more wavelengths by a pump source, the emitted modulated pump radiation including first radiation at the pump wavelength. The method also includes: modulating the first radiation by the pump source to excite spectral broadening in an optical fiber, and receiving the modulated pump radiation emitted by the pump source by the optical fiber. The optical fiber includes a hollow core extending along at least a portion of the length of the optical fiber, and the hollow core is configured to guide the received radiation during propagation through the optical fiber.

[0027] Optionally, the method may further include: generating the first radiation by the first radiation source and / or generating the second radiation seed radiation by the second radiation source at a seed wavelength to modulate the first radiation.

[0028] Optionally, the spectral broadening of the modulated pump radiation may include a four wave mixing process.

[0029] According to another aspect of the present disclosure, there is provided a measurement device comprising the radiation source assembly stated above.

[0030] According to another aspect of the present disclosure, there is provided an inspection device comprising the radiation source assembly stated above.

[0031] According to another aspect of the present disclosure, there is provided a lithographic apparatus comprising the radiation source assembly set out above. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0033] - Figure 1 depicts a schematic overview of a lithography apparatus;

[0034] - Figure 2 A schematic overview of a lithography unit is depicted;

[0035] - Figure 3 Depicted is a schematic representation of overall lithography showing the collaboration between three key technologies to optimize semiconductor manufacturing;

[0036] - Figure 4 A schematic representation of a scatterometer is depicted;

[0037] - Figure 5 depicts a schematic representation of a level sensor;

[0038] - Figure 6 depicts a schematic representation of an alignment sensor;

[0039] - Figure 7 depicts a schematic representation of radiation source components for generating broadband radiation;

[0040] - Figure 8 A flow chart depicting steps in a method of generating broadband radiation by spectral broadening of modulated radiation;

[0041] - Fig. 9 (a) depicts the envelope of the radiation pulse;

[0042] - Fig. 9 (b) depicts the envelope of the modulated radiation pulse in the time domain;

[0043] - Fig.10 (a) depicts a graph showing the spectral broadening as a function of the propagation length in the absence of modulated pump radiation;

[0044] - Fig.10 (b) depicts a graph showing spectral broadening as a function of the propagation length of the modulated pump radiation; and

[0045] - Fig.11 A schematic representation of a cross section of a hollow core photonic crystal fiber is depicted. DETAILED DESCRIPTION

[0046] In this document, the terms "radiation" and "beam" are used to cover all types of electromagnetic radiation, including ultraviolet radiation (e.g., wavelengths of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultraviolet radiation, e.g., wavelengths in the range of about 5 to 100 nm).

[0047] The terms "reticle", "mask" or "patterning device" as used herein may be broadly interpreted as referring to a general patterning device that can be used to impart a patterned cross-section to an incoming radiation beam, corresponding to the pattern to be created in a target portion of a substrate. In this context, the term "light valve" may also be used. In addition to classical masks (transmissive or reflective, binary, phase-shifting, hybrid, etc.), other examples of such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0048] Figure 1A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA comprises an illumination system (also referred to as an illuminator) IL configured to condition a radiation beam B (e.g. UV radiation, DUV radiation or EUV radiation), a mask support (e.g. a mask table) T constructed to support a patterning device (e.g. a mask) MA and connected to a first positioner PM (configured to accurately position the patterning device MA according to certain parameters), a substrate support (e.g. a wafer stage) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW (configured to accurately position the substrate support according to certain parameters), and a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0049] In operation, the illumination system IL receives a radiation beam from a radiation source SO (e.g., via a beam delivery system BD). The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical components, or any combination thereof, for directing, shaping and / or controlling the radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0050] The term "projection system" PS as used herein should be broadly interpreted as covering various types of projection systems, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems or any combination thereof, as appropriate for the exposure radiation used and / or other factors (such as the use of immersion liquid or the use of a vacuum). Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0051] The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, such as water, to fill the space between the projection system PS and the substrate W, which is also known as immersion lithography. More information on immersion technology is given in US6952253, which is incorporated herein by reference.

[0052] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also referred to as "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or steps for preparing a substrate W for subsequent exposure may be performed on a substrate W located on one of the substrate supports WT, while another substrate W on another substrate support WT is used to expose a pattern on another substrate W.

[0053] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measuring table. The measuring table is arranged to hold sensors and / or cleaning means. The sensors may be arranged to measure properties of the projection system PS or properties of the radiation beam B. The measuring table may hold a plurality of sensors. The cleaning means may be arranged to clean a part of the lithographic apparatus, e.g. a part of the projection system PS or a part of a system for providing immersion liquid. The measuring table may be moved under the projection system PS when the substrate support WT is away from the projection system PS.

[0054] In operation, a radiation beam B is incident on a patterning device (e.g. a mask MA) which is held on a mask support T and which is patterned by a pattern (design layout) present on the patterning device MA. After traversing the mask MA, the radiation beam B passes through a projection system PS which focuses the beam onto a target portion C of the substrate W. With the aid of a second positioner PW and a position measurement system IF, the substrate support WT can be accurately moved, for example in order to position different target portions C in the path of the radiation beam B at focused and aligned positions. Similarly, the first positioner PM and possibly another position sensor (not shown) are also provided. Figure 1 The patterning device MA (depicted explicitly in FIG. 1 ) can be used to accurately position the patterning device MA relative to the path of the radiation beam B. The patterning device MA and the substrate W can be aligned using the mask alignment marks M1, M2 and the substrate alignment marks P1, P2. Although the illustrated substrate alignment marks P1, P2 occupy dedicated target portions, they can be located in the space between target portions. When the substrate alignment marks P1, P2 are located between target portions C, these are referred to as scribe lane alignment marks.

[0055] like Figure 2 As shown, the lithography apparatus LA may form part of a lithography cell LC, which is sometimes also referred to as a lithography cell or (lithography cell) cluster, which typically also includes equipment for performing pre-exposure and post-exposure processes on a substrate W. Conventionally, these include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH and a bake plate BK, for example, for regulating the temperature of the substrate W (e.g. for regulating the solvent in the resist layer). A substrate handler or robot RO picks up substrates W from input / output ports I / O1, I / O2, moves them between the different process equipment, and delivers the substrates W to a feed station LB of the lithography apparatus LA. The devices in the lithography cell (often also collectively referred to as tracks) are typically controlled by a track control unit TCU, which itself may be controlled by a supervisory control system SCS, which may also control the lithography apparatus LA, for example via a lithography control unit LACU.

[0056] In order for substrates W exposed by lithographic apparatus LA to be correctly and consistently exposed, it is desirable to inspect the substrates to measure properties of the patterned structures, such as overlay errors between subsequent layers, line thickness, critical dimensions (CD), etc. For this purpose, an inspection tool (not shown) may be included in lithography cell LC. If errors are detected, for example, adjustments may be made to the exposure of subsequent substrates or other processing steps to be performed on substrate W, particularly if the inspection is done before other substrates W of the same batch or lot are still to be exposed or processed.

[0057] An inspection apparatus (which may also be referred to as a metrology apparatus) is used to determine properties of a substrate W, in particular how properties vary from one substrate W to another, or how properties associated with different layers of the same substrate W vary from layer to layer. The inspection apparatus may alternatively be configured to identify defects on the substrate W, and may for example be part of the lithography cell LC, or may be integrated into the lithography apparatus LA, or may even be a stand-alone device. The inspection apparatus may measure properties on a latent image (the image in the resist layer after exposure) or a semi-latent image (the image in the resist layer after a post-exposure bake step PEB), or on a developed resist image (where exposed or unexposed parts of the resist have been removed), or even on an etched image (after a pattern transfer step such as etching).

[0058] Typically, the patterning process in the lithography apparatus LA is one of the most critical steps in the process, which requires high accuracy in dimensioning and placement of structures on the substrate W. To ensure this high accuracy, the three systems can be combined into a so-called "holistic" control environment, such as Figure 3 Schematically depicted in . One of these systems is the lithography apparatus LA, which is (in practice) connected to a metrology tool MET (a second system) and a computer system CL (a third system). The key to this "holistic" environment is to optimize the cooperation between these three systems to enhance the overall process window, and to provide a tight control loop to ensure that the patterning performed by the lithography apparatus LA stays within the process window. The process window defines the range of process parameters (e.g. dose, focus, overlay) within which a specific manufacturing process will produce a defined result (e.g. a functional semiconductor device), and typically within which process parameters of a lithography process or patterning process are allowed to vary.

[0059] The computer system CL can use (a portion of) the design layout to be patterned to predict the resolution enhancement technique to be used, and perform computational lithography simulations and calculations to determine which mask layouts and lithography equipment settings achieve the maximum overall process window for the patterning process (in Figure 3Typically, the resolution enhancement technique is arranged to match the patterning possibilities of the lithographic apparatus LA. The computer system CL may also be used to detect where within the process window the lithographic apparatus LA is currently operating (e.g. using input from a metrology tool MT) to predict whether defects may be present (e.g. due to suboptimal processing). Figure 3 0 in the figure).

[0060] 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 drift, for example in the calibration status of the lithographic apparatus LA (in Figure 3 (depicted by multiple arrows in the third scale SC3).

[0061] During the lithographic process, it is desirable to frequently measure the structures created, for example for process control and verification. The tool that performs such measurements is typically referred to as a measurement tool MT. Different types of measurement tools MT for performing such measurements are known, including scanning electron microscopes or various forms of scatterometer measurement tools MT. Scatterometers are versatile instruments that allow measurement of parameters of the lithographic process by placing a sensor in the pupil of a scatterometer objective or in a plane conjugate to the pupil (these measurements are generally referred to as pupil-based measurements) or by placing a sensor in an image plane or in a plane conjugate to the image plane (in this case, these measurements are generally referred to as image-based or field-based measurements). Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP1,628,164A, which are incorporated herein by reference in their entirety. The scatterometer described above can measure the grating using light from soft x-rays and visible to the near IR wavelength range.

[0062] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, reconstruction methods may be applied to the measured signals to reconstruct or calculate the properties of the grating. For example, such a reconstruction may be performed by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measured results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from an actual target.

[0063] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, radiation emitted by a radiation source is directed onto a target and reflected or scattered radiation from the target is directed to a spectrometer detector which measures the spectrum of the specularly reflected radiation (i.e. measures the intensity as a function of wavelength). From this data, the structure or profile of the target which produced the detected spectrum can be reconstructed, for example by rigorous coupled wave analysis and nonlinear regression or by comparison with a library of simulated spectra.

[0064] In a third embodiment, the scatterometer MT is an ellipsometer. An ellipsometer allows the parameters of a lithography process to be determined by measuring the scattered radiation for each polarization state. Such a metrology device emits polarized light (such as linear, circular or elliptical) by using, for example, appropriate polarization filters in the illumination section of the metrology device. A source suitable for the metrology device may also provide polarized radiation. Various embodiments of existing ellipsometers are described in U.S. patent applications 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,449, 12 / 920,968, 12 / 922,587, 13 / 000,229, 13 / 033,135, 13 / 533,110 and 13 / 891,410, which are incorporated herein by reference in their entirety.

[0065] Examples of known scatterometers typically rely on the provision of dedicated metrology targets, such as underfilled targets (targets, in the form of simple gratings or overlapping gratings in different layers, are large enough to allow the measurement beam to generate a spot smaller than the grating) or overfilled targets (where the illumination spot partially or completely encompasses the target). Further, the use of metrology tools (e.g., angle-resolved scatterometers that illuminate underfilled targets, such as gratings) allows the use of so-called reconstruction methods, where the properties of the grating can be calculated by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measurement results. The parameters of the model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from an actual target.

[0066] In one embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the overlap of two misaligned gratings or periodic structures by measuring a reflection spectrum and / or an asymmetry in the detection configuration, the asymmetry being related to the degree of the overlap. The two (usually overlapping) grating structures may be applied in two different layers (not necessarily continuous layers) and may be formed at substantially the same position on the wafer. The scatterometer may have a symmetrical detection configuration such as described in co-owned patent application EP1,628,164A, so that any asymmetry is clearly distinguishable. This provides a direct way of measuring misalignment in the gratings. Other examples for measuring the overlay error between two layers comprising a periodic structure when the target passes through the asymmetry measurement of the periodic structure may be found in PCT patent application publication number WO 2011 / 012624 or in US patent application US 20160161863, which are incorporated herein by reference in their entirety.

[0067] 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 U.S. Patent Application US2011-0249244, which is incorporated herein by reference in its entirety. A single structure may be used that has a unique combination of critical dimension and sidewall angle measurements for each point in the focus energy matrix (FEM, also called focus exposure matrix). If these unique combinations of critical dimensions and sidewall angles are available, focus and dose values ​​may be uniquely determined from these measurements.

[0068] The metrology target may be the entirety of a composite grating, which is formed by a lithography process, mainly in a resist, but may also be formed after, for example, an etching process. Typically, the pitch and line width of the structures in the grating are largely determined by the measurement optics (particularly the NA of the optics) in order to be able to capture the diffraction orders from the metrology target. As indicated earlier, the diffraction signal may be used to determine the shift between two layers (also called "overlap"), or may be used to reconstruct at least part of the original grating produced by the lithography process. This reconstruction may be used to provide guidance on the quality of the lithography process, and may be used to control at least a part of the lithography process. The target may have a smaller subdivision, which is configured to mimic the size of a functional part of the design layout in the target. Due to this subdivision, the behavior of the target will be more similar to the functional part of the design layout, so that the overall process parameter measurement better resembles the functional part of the design layout. The target may be measured in an underfill mode or in an overfill mode. In the underfill mode, the measurement beam generates a spot that is smaller than the overall target. In the overfill mode, the measurement beam generates a spot that is larger than the overall target. In this overfill mode, it is also possible to measure different targets simultaneously, thereby determining different process parameters simultaneously.

[0069] The overall measurement quality of a lithography parameter using a specific target is determined at least in part by the measurement profile used to measure the lithography parameter. The term "substrate measurement profile" may include one or more parameters of the measurement itself, one or more parameters of the one or more patterns being measured, or both. For example, if the measurement used in the substrate measurement profile is a diffraction-based optical measurement, the one or more parameters measured may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, and the like. One of the criteria for selecting a measurement profile may be, for example, the sensitivity of one of the measurement parameters to process variations. More examples are described in U.S. patent application US2016-0161863 and published U.S. patent application US 2016 / 0370717A1, which are incorporated herein by reference in their entirety.

[0070] Measurement equipment (such as scatterometer SM1) is used Figure 4 It comprises a broadband (white light) radiation projector 2 which projects radiation onto a substrate 6. The reflected or scattered radiation is passed to a spectrometer detector 4 which measures the spectrum 10 of the specularly reflected radiation (i.e. the intensity INT as a function of wavelength λ). From this data, for example, by rigorous coupled wave analysis and nonlinear regression or by comparing Figure 4 The structure or profile that produced the detected spectrum can be reconstructed by the processing unit PU for comparison with the simulated spectral library shown at the bottom. Typically, for the reconstruction, the general form of the structure is known and some parameters are assumed by knowledge of the process by which the structure was manufactured, with only a few parameters of the structure to be determined from the scatterometry data. Such a scatterometer can be configured as a normal incidence scatterometer or an oblique incidence scatterometer.

[0071] A topological measurement system, a level sensor or a height sensor that can be integrated in a lithographic apparatus is arranged to measure the topology of the top surface of a substrate (or wafer). A map of the topology of the substrate (also referred to as a height map) can be generated from these measurements, and the map indicates the height of the substrate according to the position on the substrate. The height map can then be used to correct the position of the substrate during the pattern transfer to the substrate so as to provide a spatial image of the pattern forming device in an appropriate focus position on the substrate. It is to be understood that in this context, "height" refers to a dimension (also referred to as the Z axis) that is clearly outside the plane for the substrate. Typically, a level or height sensor performs measurement at a fixed position (relative to its own optical system), and the relative movement between the substrate and the optical system of the level or height sensor causes height measurement to be performed at a position on the entire substrate.

[0072] Examples of level or height sensors LS known in the art are Figure 5, which only illustrates the operating principle. In this example, the level sensor comprises an optical system comprising a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO, which provides a radiation beam LSB imparted by a projection grating PGR of the projection unit LSP. The radiation source LSO can be, for example, a narrowband or broadband radiation source (such as a supercontinuum light source), polarized or non-polarized, pulsed or continuous, such as a polarized or non-polarized laser beam. The radiation source LSO can include a plurality of radiation sources with different colors or wavelength ranges, such as a plurality of LEDs. The radiation source LSO of the level sensor LS is not limited to visible radiation, but can additionally or alternatively cover UV and / or IR radiation and any wavelength range suitable for reflection from the substrate surface.

[0073] The projection grating PGR is a periodic grating including a periodic structure, which causes the radiation beam BE1 to have a periodically varying intensity. The radiation beam BE1 with periodically varying intensity is directed to a measurement location MLO on a substrate W having an incident angle ANG with respect to an axis (Z axis) perpendicular to the incident substrate surface, the incident angle ANG being between 0 and 90 degrees, typically between 70 and 80 degrees. At the measurement location MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and directed to a detection unit LSD.

[0074] In order to determine the height level at the measurement location MLO, the level sensor further comprises a detection system comprising a detection grating DGR, a detector DET and a processing unit (not shown) for processing an output signal of the detector DET. The detection grating DGR may be identical to the projection grating PGR. The detector DET generates a detector output signal which is indicative of the received light (e.g. indicating the intensity of the received light) (such as a photodetector) or which represents a spatial distribution of the received intensity (such as a camera). The detector DET may comprise any combination of one or more detector types.

[0075] By means of triangulation techniques the height level at the measurement location MLO can be determined.The detected height level is typically related to the signal intensity measured by the detector DET, which has a periodicity that depends inter alia on the design of the projection grating PGR and the (oblique) angle of incidence ANG.

[0076] The projection unit LSP and / or the detection unit LSD may comprise further optical elements such as lenses and / or mirrors along the path (not shown) of the patterned radiation beam between the projection grating PGR and the detection grating DGR.

[0077] In an embodiment, the detection grating DGR may be omitted and the detector DET may be placed where the detection grating DGR is located. This configuration provides a more direct detection of the image of the projection grating PGR.

[0078] In order to effectively cover the surface of the substrate W, the level sensor LS may be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating an array of measurement areas MLO or spots covering a larger measurement range.

[0079] For example, various height sensors of the general type are disclosed in US7265364 and US7646471, both of which are incorporated by reference. A height sensor using UV radiation rather than visible or infrared radiation is disclosed in US2010233600A1, which is incorporated by reference. In WO2016102127A1 (which is incorporated by reference), a compact height sensor is described that uses a multi-element detector to detect and identify the position of a grating image without the need for a detection grating.

[0080] In the manufacture of complex devices, typically many lithographic patterning steps are performed to form functional features in successive layers on a substrate. Therefore, a key aspect of the performance of a lithographic apparatus is the ability to correctly and accurately place the applied pattern with respect to features laid down in previous layers (either by the same apparatus or by a different lithographic apparatus). For this purpose, the substrate is provided with one or more sets of marks. Each mark is a structure whose position can be measured later using a position sensor (typically an optical position sensor). The position sensor may be referred to as an "alignment sensor" and the mark may be referred to as an "alignment mark".

[0081] The lithographic apparatus may include one or more (e.g., multiple) alignment sensors, by which the positions of alignment marks disposed on the substrate can be accurately measured. The alignment (or position) sensor may use optical phenomena, such as diffraction and interference, to obtain position information from alignment marks formed on the substrate. An example of an alignment sensor used in a current lithographic apparatus is based on a self-referencing interferometer described in US6961116. For example, as disclosed in US2015261097A1, various enhancements and modifications of position sensors have been developed. The contents of all of these publications are incorporated herein by reference.

[0082] The mark or alignment mark may comprise a series of bars formed on or in a layer disposed on a substrate, or formed (directly) in the substrate. The bars may be regularly spaced and act as grating lines, so that the mark may be considered as a diffraction grating with a well-known spatial period (pitch). Depending on the orientation of these grating lines, the mark may be designed to allow measurement of a position along the X-axis or along the Y-axis (which is oriented substantially perpendicular to the X-axis). A mark comprising bars arranged at +45 degrees and / or -45 degrees relative to the X-axis and the Y-axis allows combined X and Y measurements to be performed using the techniques described in US2009 / 195768A (which is incorporated by reference).

[0083] The alignment sensor optically scans each mark with a radiation spot to obtain a periodically varying signal such as a sine wave. The phase of this signal is analyzed to determine the position of the mark relative to the alignment sensor, and thus the position of the substrate relative to the alignment sensor, which in turn is fixed relative to the reference frame of the lithographic apparatus. So-called coarse and fine marks associated with different (coarse and fine) mark sizes can be provided so that the alignment sensor can distinguish different cycles of the periodic signal and the exact position (phase) within a cycle. Marks of different pitches can also be used for this purpose.

[0084] Measuring the position of the mark may also provide information about deformation of the substrate on which the mark is located, for example in the form of a wafer grid.Deformation of the substrate may occur, for example, by electrostatically clamping the substrate to a substrate table and / or heating the substrate when the substrate is exposed to radiation.

[0085] Figure 6 is a schematic block diagram of an embodiment of a known alignment sensor AS, such as described, for example, in US6961116 (which is incorporated by reference). A radiation source RSO provides a radiation beam RB at one or more wavelengths, which is transferred by transfer optics onto a mark, such as a mark AM located on a substrate W, as an illumination spot SP. In this example, the transfer optics comprise a spot reflector SM and an objective lens OL. The diameter of the illumination spot SP, at which the mark AM is illuminated, may be slightly smaller than the width of the mark itself.

[0086] Radiation diffracted by the mark AM is collimated (in this example, via the objective lens OL) into an information-carrying beam IB. The term "diffraction" is intended to include zero-order diffraction from the mark (which may be referred to as reflection). A self-referencing interferometer SRI (for example of the type disclosed in US6961116 mentioned above) causes the beam IB to interfere with itself, after which the beam is received by the photodetector PD. In the event that more than one wavelength is created by the radiation source RSO, additional optics (not shown) may be included to provide separate beams. If desired, the photodetector may be a single element, or it may comprise several pixels. The photodetector may comprise an array of sensors.

[0087] The transfer optics, which in this example comprise a spot mirror SM, can also be used to block zero-order radiation reflected from the mark so that the information-carrying beam IB comprises only higher-order diffracted radiation from the mark AM (which is not essential for the measurement but improves the signal-to-noise ratio).

[0088] The intensity signal SI is supplied to the processing unit PU. By a combination of optical processing in block SRI and computational processing in unit PU, values ​​of the X and Y position on the substrate relative to the frame are output.

[0089] A single measurement of the type illustrated only fixes the position of the mark within a specific range corresponding to one pitch of the mark. Coarse measurement techniques are used in conjunction with such measurements to identify which period of the sine wave is the period that contains the marked position. The same process can be repeated at different wavelengths at a coarser and / or finer level to increase accuracy and / or robustly detect the mark regardless of the material the mark is made of and the material on and / or below the location where the mark is placed. The wavelengths can be optically multiplexed and demultiplexed so as to be processed simultaneously, and / or they can be multiplexed by time or frequency division.

[0090] In this example, the alignment sensor and the spot SP remain stationary while the substrate W is moving. The alignment sensor can therefore be rigidly and accurately mounted to the frame while effectively scanning the mark AM in a direction opposite to the direction of movement of the substrate W. The substrate W is controlled in this movement by being mounted on a substrate support and a substrate positioning system (controlling the movement of the substrate support). A substrate support position sensor (e.g., an interferometer) measures the position of a substrate support (not shown). In an embodiment, one or more (alignment) marks are provided on the substrate support. Measuring the position of the mark provided on the substrate support allows the position of the substrate support determined by the position sensor to be calibrated (e.g., relative to the frame to which the alignment system is connected). Measuring the position of the alignment mark provided on the substrate allows the position of the substrate relative to the substrate support to be determined.

[0091] The measurement tool MT (such as the above-mentioned scatterometer, level sensor and alignment sensor) can use radiation to perform measurements. The radiation can be electromagnetic radiation. The radiation can be optical radiation, for example, including wavelengths in the infrared, visible and / or ultraviolet parts of the electromagnetic spectrum. The radiation may include wavelengths in the extreme ultraviolet EUV (for example, 1nm to 100nm) and / or soft X-ray SXR (for example, 0.1nm to 10nm) parts of the electromagnetic spectrum. The measurement tool MT may include or be connected to a radiation source. The type and quality of the measurement performed by the measurement tool MT may be affected by the properties of the radiation used. Different types of radiation may be provided by different types of sources. Some sources may provide radiation at a single wavelength or a small wavelength range. Other sources may be suitable for providing broadband radiation. For different types of radiation, the effects and techniques for generating and providing radiation may be different. For example, a source providing broadband radiation may use spectral broadening of narrowband or single-wavelength radiation. In order to provide a high-quality broadband radiation source, it may be desirable to have a high level of control over the radiation output by the source. Increased control can allow the properties of the radiation to be controlled and set so that the quality and control of the resulting measurement can be improved. Described herein are methods and assemblies for providing an improved broadband radiation source.

[0092] Broadband radiation can be generated using nonlinear processes. Nonlinear processes may require high radiation intensities to be effectively excited. This can be achieved, for example, by coupling high intensity radiation into the optical fiber. Inside the core, a strong local radiation intensity can be obtained. The optical fiber can be a photonic crystal fiber, which can achieve strong confinement of radiation, for example within the core. This may help to provide local high intensity radiation. Nonlinear processes may also require a nonlinear medium in which the nonlinear process can occur. This can be, for example, a nonlinear crystal or a nonlinear fluid, such as a nonlinear gas or gas mixture. The nonlinear medium can be provided inside the optical fiber. The optical fiber can be a hollow core optical fiber, such as a hollow core photonic crystal fiber. A nonlinear medium (such as a nonlinear fluid) can be provided inside the hollow core. The high intensity radiation can then be strongly confined within the hollow core of the optical fiber, allowing the high intensity radiation to interact with the nonlinear medium to generate broadband radiation.

[0093] Depending on the dispersion properties of the fiber and the (multiple) wavelengths provided in the fiber, different phenomena may contribute to the generation of broadband radiation. Examples of mechanisms in which broadband generation can operate include self-phase modulation (SPM) and modulation instability (MI). For example, both SPM and MI may appear in supercontinuum generation in hollow core fibers. Some spectral broadening processes (such as, for example, SPM) involve the interaction of radiation with nonlinear media. The interaction of high-intensity radiation with nonlinear media may lead to the occurrence of nonlinear effects, such as dispersion experienced due to the Kerr effect leading to broadening of the radiation pulse. It may lead to spectral broadening of the radiation pulse. Other spectral broadening processes (such as modulation instability and four-wave mixing) may be due to the effects of mixing two or more radiation wavelengths in a nonlinear medium. The broadening of the radiation wavelength range may be due to deviations in the radiation pulse or the modulation is enhanced by the nonlinear Kerr effect. Modulation instability may lead to spectral broadening of the modulated radiation, thereby contributing to the generation of broadband radiation.

[0094] Modulation instabilities may result in the generation of broadband radiation. However, MI is a stochastic process. It may rely on noise causing modulation of the received radiation to allow modulation instabilities to occur. Therefore, after a certain amount of time, such as after a certain propagation distance through an optical fiber, the amount of spectral broadening caused by modulation instabilities may be difficult to predict. This may make the process less reliable, which may require increasing the interaction time / propagation length to increase the probability of the expected amount of spectral broadening occurring. This may make the system inefficient.

[0095] Components, devices, and methods are described herein for broadening received radiation by seeding a modulation instability process. Figure 7 A radiation source assembly 100 for generating broadband radiation by spectral broadening is depicted. The assembly includes a pump source 102 configured to emit radiation at one or more wavelengths. The pump source 102 may include a first radiation source 104 configured to emit first radiation 106 at a pump wavelength. The pump wavelength may be referred to herein as a first wavelength, and the first radiation may be referred to as pump radiation.

[0096] The pump source 102 may be configured to modulate the first radiation 106 to form a modulated pump radiation 108. The modulated pump radiation 108 may be output by the pump source 102. The modulated pump radiation 108 may be coupled into an optical fiber 110, which includes a hollow core 112 configured to guide the radiation coupled into the optical fiber 110. The modulated pump radiation 108 coupled into the optical fiber 110 may propagate through the optical fiber 110. Inside the optical fiber 110, spectral broadening of the modulated pump radiation 108 may occur to form broadband radiation 114. The broadband radiation 114 may be coupled out of the optical fiber 110. The modulation properties of the modulated pump radiation 108 coupled into the optical fiber 110 may excite the spectral broadening inside the optical fiber 110. The radiation source assembly 100 may also include an optical assembly 116 for coupling the modulated pump radiation 108 into the optical fiber 110. The optical assembly 116 may, for example, include a focusing lens or a focusing objective.

[0097] Figure 8 A flow chart depicting the steps of a method for forming broadband radiation 114 by spectral broadening. In step 802, first radiation 106 is provided to a pump source 102 at one or more wavelengths. The first radiation 106 may be provided by the first radiation source 104. In step 804, the first radiation is modulated by the pump source 102. The modulated pump radiation 108 is emitted from the pump source 102. In step 806, the modulated pump radiation 108 may be coupled into an optical fiber 110, which is a hollow core fiber, and is therefore received by the optical fiber 110. Inside the optical fiber 110, in step 808 of the method, the modulated pump radiation 108 is spectrally broadened to form the broadband radiation 114. The spectral broadening process may include modulation instability and / or four wave mixing (FWM) processes to generate additional wavelengths from one or more wavelengths input from the source 102 to the optical fiber 110.

[0098] An advantage of providing modulated pump radiation 108 to the optical fiber 110 to broaden the wavelength range is that broadband radiation 114 can be generated faster and more reliably than broadband radiation generated using unmodulated pump radiation. This may be because the modulation of the first radiation 106 can act as a seed for the modulation instability process. Instead of relying on the random introduction of noise to the first radiation 106 to initiate modulation instability, the modulation introduced to the first radiation can increase the likelihood and consistency of the modulation instability process starting. This can increase the stability and / or predictability of the broadband generation process. The components and methods described herein can reduce the interaction time / propagation length required to generate broadband radiation compared to embodiments that do not use modulated pump radiation.

[0099] The pump source 102 may include a second radiation source 118 configured to generate a second radiation 120 at a seed wavelength. The first radiation 106 and the second radiation 120 may be mixed, which may cause modulation of the first radiation 106 to produce modulated pump radiation 108. The pump source 102 may include two radiation sources 104, 118 for generating radiation at a first wavelength and a second wavelength, respectively. The two sources 104, 118 may be, for example, two separate laser sources. In an exemplary arrangement, the pump source 102 may receive the first radiation 106 and the second radiation 120 from external first and second radiation sources.

[0100] The first radiation source 104 may generate and / or provide the first radiation 106. The second radiation 120 may be generated and / or provided by the second source 118. The second radiation may include a second wavelength different from the first wavelength. The modulated pump radiation 108 output by the pump source 102 may primarily include radiation passed through the first radiation source 104 at the first wavelength (also referred to as the pump wavelength). This may be because the intensity of the first radiation 106 output by the first radiation source 104 may be significantly greater than the intensity of the second radiation 118 output by the second radiation source 118. The intensity of the second radiation 120 may not exceed 1%, 2%, 5%, 10%, or 15% of the intensity of the first radiation 106. When the first radiation 106 and the second radiation 120 are mixed, the second radiation 120 may modulate at least a portion of the first radiation 106 in the time domain. Such modulation may form a seed for exciting modulation instabilities for spectral broadening.

[0101] Thus, the second radiation 120 may be referred to as seed radiation, the second wavelength may be referred to as a seed wavelength, and the second radiation source 118 may be referred to as a seed source. The seed radiation 120 may be a second harmonic of the first radiation 106. In this case, the second source 118 may provide an output formed by second harmonic generation of some of the first radiation 106. The second radiation 120 may also be obtained as self-phase modulated radiation obtained from the first radiation 106. In this case, the second source 118 may provide an environment to achieve self-phase modulation of the first radiation 106, and output the resulting second radiation 120. Figure 8 In the flowchart of FIG. 804 , an optional step may be included next to the step 802 of providing the first radiation 106. The optional step may include providing the second radiation 120 at the seed wavelength in the pump source 102. In step 804, modulating the first radiation 106 to obtain modulated pump radiation 108 may be obtained by mixing the first radiation 106 and the second radiation 120.

[0102] The first radiation 106 and the second radiation 120 can be mixed by spatially overlapping the first radiation and the second radiation. Mixing the first radiation 106 and the second radiation 120 can also include coherently overlapping the first radiation 106 and the second radiation 120. This can be achieved using an optical device 122, which can include a dichroic mirror, for example. Overlapping the radiation of the two wavelengths can cause the radiation to interact when certain conditions are met. The conditions can be related to the phase relationship of the mixed first radiation and the second radiation, for example. These conditions can impose requirements on the relationship between the pump wavelength and the seed wavelength and the environmental properties of the assembly 100 (e.g., the refractive index of the medium, the fiber geometry, etc.). Fig. 9 The radiation pulse envelope in the time domain is depicted. Fig. 9 (a) depicts a pulse of first radiation 106 generated by the first source 104 before it is modulated. The envelope of the radiation represents the intensity of the pulse over time. The shape of the envelope may be substantially smooth. Fig. 9 (b) depicts a modulated radiation pulse 108. At least a portion of the pulse envelope may have a modulated intensity. The pump source 102 may be configured to modulate the pump radiation 108 at an intensity above a threshold intensity. Fig. 9 In (b), the peak of the pulse envelope is shown as being modulated. This may be caused by the interaction of the pump radiation 106 with the seed radiation 120.

[0103] In order to understand which conditions may be met to excite modulation instabilities, it may be useful to consider the relationship between the mixing of pump radiation and seed radiation in both the time domain and the frequency domain. In the time domain, mixing high-intensity pump radiation and low-intensity seed radiation may result in deviations from the smooth shape of the envelope of the pump radiation pulse in the time domain. This is the pump radiation modulation represented in the time domain. In order to understand how this modulation may excite spectral broadening and under what conditions, the frequency domain may provide a better picture. In the frequency domain, the spectral broadening caused by modulation instabilities may be viewed as a wave mixing process. The mixing of pump radiation and seed radiation may, for example, provide a trigger for a four-wave mixing process. Four-wave mixing (FWM) may rely on third-order nonlinearity. In FWM, frequency f1 may mix and interact with frequency f2 and generate more frequencies, such as f3 and f4, where the frequencies satisfy the condition f1+f2=f3+f4. For example, FWM may also occur for frequencies where 2*f1=f3+f4. Once more frequencies are generated, these frequencies may be phase-matched with other wavelengths, thereby generating radiation at an increased number of wavelengths. This can lead to the creation of broadband radiation generation. Other nonlinear processes can also contribute to broadband radiation generation.

[0104] In order for frequencies to be generated through a wave mixing process, phase matching conditions between the radiation frequencies may have to be met or approximately met. The phase of the radiation may depend on the frequency / wavelength of the radiation and the refractive index experienced by the radiation. Therefore, the dispersive properties of the environment in which the radiation is present in the assembly 100 may affect the phase matching conditions that may need to be met or approximately met. Specifically, phase matching conditions may have to be met inside the hollow core fiber 110. As described herein, pump wavelengths and seed wavelengths that meet the phase matching conditions inside the optical fiber 110 can be mixed to achieve a wave mixing process, such as a four-wave mixing process.

[0105] If radiation at the seed wavelength and radiation at the pump wavelength are mixed and they are phase matched inside the optical fiber 110, four wave mixing can be stimulated to achieve spectral broadening in the optical fiber. The FWM process can be stimulated by initiating the wave mixing process by some radiation already present at phase matched frequencies. The mixed first radiation and second radiation can, for example, be provided as modulated pump radiation 108, as described above. Mixing radiation at the seed wavelength and the pump wavelength can stimulate the FWM process, which can speed up the time for the process to start and occur. This stimulation of the wave mixing process can increase the stability and conversion efficiency of the process. This in turn can reduce the time required to achieve spectral broadening of the radiation to form broadband radiation 114.

[0106] The phase matching condition can be fully satisfied for exciting the four wave mixing process. This can be achieved, for example, by tuning the seed wavelength to phase match the pump frequency and the seed frequency. For this purpose, a tunable seed source can be provided in the component 100. However, the process may also be successful for frequencies that are close to but not completely satisfy the FWM phase matching condition. The advantage of the frequency combination that approximately satisfies the phase matching condition may be that precise tuning is not required, because the approximate match also results in spectral broadening. A tunable seed source may not be required, which can keep the cost of the component 100 low. Another advantage of the approximate phase matching condition may be that the radiation can be provided at a convenient wavelength, such as a wavelength that is easily available from a radiation source. Examples of wavelengths provided by readily available sources include wavelengths of 1550nm, 1030nm, and wavelengths in the range of 700 to 800nm. If the phase matching condition is more closely met, the spectral broadening may be more effective and / or faster. In an example embodiment, a pump wavelength of 1030nm can be used. The combination of 1030 nm pump radiation and seed radiation of example wavelengths of 1550 nm, 1200 nm, and 750 nm can all result in spectral broadening of the excitation compared to an embodiment without seed radiation. However, if the seed wavelength is too close to the pump wavelength (e.g., a pump wavelength of 1030 nm and a seed wavelength of 1064 nm), the effect of the seed radiation may be limited. The relationship between the pump wavelength and the seed wavelength can be tuned to optimize the effectiveness of exciting modulation instabilities and resulting spectral broadening.

[0107] Spectral broadening may rely on nonlinear processes. High radiation intensity may be required to obtain efficiency of the nonlinear process. In order to achieve high radiation intensity of the modulated pump radiation 108, the modulated pump radiation 108 may be coupled into an optical fiber 110. The optical fiber 110 may confine the radiation to a small volume, thereby creating a local high intensity. At least a portion of the modulated pump radiation 108 coupled into the hollow core optical fiber 110 may be confined within the hollow core 112 of the optical fiber 110. In some instances, at least 80% of the radiation coupled into the optical fiber may be confined within the hollow core. In some instances, at least 85% of the radiation coupled into the optical fiber may be confined within the hollow core. In some instances, substantially all of the radiation coupled into the optical fiber may be confined within the hollow core.

[0108] The hollow core 112 of the optical fiber 110 may include a fluid for enabling and / or causing spectral broadening when the radiation interacts with the fluid. The fluid may be a nonlinear medium. The fluid may be a gas. The gas may be a pure gas or a gas mixture. The mixture may include hydrogen (H 2 The gas mixture may include an inert gas (e.g., Ar, He, Kr, Xe, Ne). The gas may include a molecular gas (e.g., N 2). The optical fiber 110 can be positioned in a resealable reservoir, which can be suitable for containing a fluid. The reservoir can, for example, have one or more gas supplies for providing and controlling a gas mixture inside the hollow core 112 of the optical fiber 110.

[0109] The hollow core 112 of the optical fiber 110 can produce an optical fiber 110 with a flat dispersion curve, for example compared to a solid core optical fiber. An advantage of having a flat dispersion curve can be that the phase matching condition can be approximated for a wider range of wavelengths compared to a setup with a larger dispersion variation. This can be because the wavelength-dependent refractive index value of the optical fiber affects the phase matching condition. If the refractive index of a larger wavelength range around the wavelength that meets the phase matching condition has a similar value, it means that the phase matching condition can be approximated for this larger wavelength range. This can lead to a wider and / or more efficiently generated wavelength range, for example using four-wave mixing.

[0110] The length of the optical fiber 110 required to obtain spectral broadening may be determined by the amount of interaction time / interaction length required to spectrally broaden the modulated pump radiation 108 to form the broadband radiation 114 . Fig.10 A graph is depicted of the wavelength λ of the generated radiation as a function of the position L along the length of the hollow core optical fiber 110. The grey scale depicts the energy spectral density ESD in decibels dB. By modulating the modulated pump radiation 108 coupled into the optical fiber 110 to excite modulation instabilities, the required length can be reduced. Fig.10 (a) depicts a graph of spectral broadening without providing modulated radiation. For this embodiment, the length of optical fiber 110 required to obtain broadband radiation over the range of interest (e.g., 400 nm to 2000 nm) may be 40 cm or longer. Fig.10 As can be seen from the graph of (a), a longer propagation length is required before the spectrum broadening extends to a wider wavelength range. At propagation length 1100, depicting a 12 cm propagation length along the fiber 110, the amount of broadening achieved in the non-modulated implementation is limited to a slow broadening of wavelengths on either side of the pump wavelength. After propagation length 1100, broadening of the spectrum to a broadband range begins due to modulation instability / four wave mixing. The skilled person will appreciate that the propagation length 1100 before modulation instability broadening occurs can be random and is not limited to 12 cm. Fig.10(b) Spectral broadening, in which the spectrum is broadened to cover a wider wavelength range from a shorter propagation length. Several sidebands of the pump radiation are simultaneously generated and broadened from earlier points along the fiber 110. By providing the first radiation 106 modulated by the phase-matched second radiation 120, the length of the fiber 110 can be reduced to less than 40 cm. For example, the length can be 10 cm. The length of the optical fiber 110 in the assembly 100 can be up to 10 cm; up to 20 cm; up to 30 cm; up to 40 cm; or in the range of 10 cm to 40 cm. For example, the seed wavelength provided to the assembly 100 can be tuned to reduce the propagation length required for broadband generation.

[0111] The optical fiber 110 may be a photonic crystal fiber. An advantage of using a photonic crystal fiber may be that it may achieve strong confinement of radiation within the hollow core. This may help achieve high radiation intensity to improve the efficiency of the nonlinear spectral broadening process. The photonic crystal fiber may include a plurality of microstructures forming a photonic crystal for confining radiation to the hollow core 112 of the optical fiber 110. The optical fiber 110 has an elongated dimension along which the radiation is intended to propagate. A cross section of the optical fiber perpendicular to the elongated dimension may include a distribution of microstructures around the hollow core. This distribution may form a photonic crystal structure for guiding and confining radiation. The distribution in the cross section may be substantially constant along the elongated dimension of the optical fiber 110. Fig.11 A schematic representation of a cross-section of a hollow core optical fiber 110 including a plurality of microstructures 124 is depicted. The microstructure may include a single ring of capillaries 124 surrounding a hollow core 112 of the optical fiber 110. The capillaries themselves may have a hollow core 126. The capillaries 124 may be spatially separated so that the edges of adjacent microstructures do not contact each other. The capillaries may be referred to as anti-resonant elements (AREs). The diameter of the hollow core 112 of the optical fiber 110 may be in the range of 25 μm to 35 μm. The hollow core 112 may, for example, have a diameter of 30 μm. More information about the hollow core photonic crystal fiber 110 for the assembly 100 may be found in WO 2017 / 032454 A1, which is incorporated by reference.

[0112] The broadband radiation 114 generated by the source assembly 100 may include supercontinuum radiation. The supercontinuum radiation may include radiation in a continuous range of wavelengths. The broadband radiation may include radiation in a range of 400nm to 2000nm. The supercontinuum radiation may include a continuous range of wavelengths at least in the range of 400nm to 2000nm.

[0113] Further embodiments are disclosed in the subsequent numbered clauses:

[0114] 1. A radiation source assembly for generating broadband radiation by spectral broadening, the assembly comprising:

[0115] a pump source configured to emit modulated pump radiation at one or more wavelengths;

[0116] an optical fiber configured to receive modulated pump radiation emitted by a pump source and comprising a hollow core extending along at least a portion of the length of the optical fiber, the hollow core being configured to guide the received radiation during propagation through the optical fiber,

[0117] Wherein the radiation emitted by the pump source comprises first radiation at a pump wavelength, and wherein the pump source is configured to modulate the first radiation to stimulate spectral broadening in the optical fiber.

[0118] 2. The assembly of clause 1, wherein the pump source further comprises: a first radiation source configured to generate the first radiation; and / or a second radiation source configured to generate second radiation at a seed wavelength to modulate the first radiation.

[0119] 3. An assembly according to clause 2, wherein the first radiation and the second radiation are mixed to modulate the first radiation by spatially overlapping at least a portion of the first radiation with at least a portion of the second radiation to produce modulated pump radiation.

[0120] 4. The assembly of clause 3, wherein mixing the first radiation and the second radiation comprises: coherently overlapping at least a portion of the first radiation with at least a portion of the second radiation.

[0121] 5. An assembly according to any of clauses 2 to 4, wherein the seed wavelength and the pump wavelength satisfy phase matching conditions for four-wave mixing.

[0122] 6. An assembly according to any of clauses 2 to 5, wherein the intensity of the seed radiation does not exceed 1%, 2%, 5%, 10% or 15% of the intensity of the modulated pump radiation.

[0123] 7. An assembly according to any preceding clause, wherein the pump source is configured to modulate the first radiation in the time domain.

[0124] 8. An assembly according to clause 7, wherein the first radiation source is configured to emit a first radiation pulse, and when an intensity of the first radiation is above a threshold intensity, modulate the first radiation pulse.

[0125] 9. An assembly according to any preceding clause, wherein the hollow core of the optical fiber contains a fluid for exciting spectral broadening.

[0126] 10. The assembly of clause 9, further comprising a reservoir for containing a fluid, the reservoir and the optical fiber being configured to provide the fluid to the hollow core of the optical fiber.

[0127] 11. An assembly according to clause 9 or 10, wherein the fluid comprises a gas mixture comprising an inert gas.

[0128] 12. An assembly according to clause 9 or 10, wherein the fluid comprises a gas mixture comprising a molecular gas.

[0129] 13. An assembly according to any of the preceding clauses, wherein the length of the optical fiber is in the range of 10 cm to 40 cm.

[0130] 14. An assembly according to any preceding clause, wherein the optical fiber is a photonic crystal fiber.

[0131] 15. The assembly of clause 14, wherein the photonic crystal fiber comprises a single ring microstructure surrounding a hollow core of the fiber.

[0132] 16. The assembly of clause 15, wherein the single ring structure comprises a plurality of capillaries surrounding the hollow core.

[0133] 17. A component according to any of the preceding clauses, wherein the diameter of the hollow core is in the range 25 μm to 35 μm or is 30 μm.

[0134] 18. An assembly according to any of the preceding clauses, wherein the broadband radiation comprises supercontinuum radiation.

[0135] 19. A method for generating broadband radiation by spectral broadening, the method comprising

[0136] emitting modulated pump radiation at one or more wavelengths by a pump source, the emitted modulated pump radiation comprising first radiation at the pump wavelength;

[0137] modulating the first radiation by a pump source to stimulate spectral broadening in the optical fiber;

[0138] Modulated pump radiation emitted by a pump source is received by an optical fiber, the optical fiber including a hollow core extending along at least a portion of the length of the optical fiber, the hollow core configured to guide the received radiation during propagation through the optical fiber.

[0139] 20. The method according to clause 19, further comprising:

[0140] A first radiation is generated by a first radiation source and / or a second radiation seed radiation is generated by a second radiation source at a seed wavelength to modulate the first radiation.

[0141] 21. The method according to clause 20, wherein the spectral broadening of the modulated pump radiation comprises a four wave mixing process.

[0142] 22. A measurement device comprising the radiation source assembly of any one of clauses 1 to 18.

[0143] 23. An inspection apparatus comprising the radiation source assembly of any one of clauses 1 to 18.

[0144] 24. A lithographic apparatus comprising the radiation source assembly of any of clauses 1 to 18.

[0145] Although the use of lithographic equipment in the manufacture of ICs may be specifically cited herein, it should be understood that the lithographic equipment 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.

[0146] Although embodiments of the present invention may be specifically referenced herein in the context of lithographic equipment, embodiments of the present invention may be used in other equipment. Embodiments of the present invention may form part of a mask inspection equipment, a metrology equipment, or any equipment that measures or processes objects such as a wafer (or other substrate) or a mask (or other pattern forming device). These equipment may generally be referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0147] 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 is not limited to optical lithography and may be used in other applications, such as imprint lithography, where the context permits.

[0148] Although specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be practiced in a manner other than as described. The above description is intended to be illustrative, not restrictive. Therefore, it will be apparent to those skilled in the art that modifications may be made to the described invention without departing from the scope of the claims set forth below.

[0149] Although specific reference is made to "measurement equipment / tool / system" or "inspection equipment / tool / system," these terms may refer to the same or similar types of tools, equipment, or systems. For example, an inspection or measurement equipment including embodiments of the present invention may be used to determine characteristics of structures on a substrate or wafer. For example, an inspection equipment or measurement equipment including embodiments of the present invention may be used to detect defects in a substrate or defects in a structure on a substrate or wafer. In such embodiments, the characteristics of a structure of interest on a substrate may be related to a defect in the structure, the absence of a particular portion of the structure, or the presence of an unwanted structure on the substrate or wafer.

Claims

1. A radiation source assembly for generating broadband radiation by spectral broadening, the assembly comprising: a pump source configured to emit modulated pump radiation at one or more wavelengths; an optical fiber configured to receive the modulated pump radiation emitted by the pump source and comprising a hollow core extending along at least a portion of the length of the optical fiber, the hollow core being configured to guide the received radiation during propagation through the optical fiber, wherein the radiation emitted by the pump source comprises pulsed first radiation at a pump wavelength, and wherein the pump source is configured to modulate pulses of the first radiation to stimulate spectral broadening in the optical fiber according to variations of a signal independent of the first radiation.

2. The assembly according to claim 1, further comprising: a radiation source configured to generate said first radiation; and / or a radiation source configured to generate a signal in the form of second radiation of a seed wavelength to modulate said first radiation.

3. The assembly of claim 2, wherein the pump source comprises a first radiation source and a second radiation source, the first radiation and the second radiation being mixed to modulate the first radiation by spatially overlapping at least a portion of the first radiation with at least a portion of the second radiation to produce modulated pump radiation.

4. The assembly according to claim 3, wherein: Mixing the first radiation and the second radiation includes coherently overlapping at least a portion of the first radiation with at least a portion of the second radiation.

5. The assembly according to any one of claims 2 to 4, wherein the seed wavelength and the pump wavelength satisfy a phase matching condition for four-wave mixing.

6. The assembly of any of claims 2 to 5, wherein the intensity of the seed radiation does not exceed 1%, 2%, 5%, 10% or 15% of the intensity of the modulated pump radiation.

7. The assembly of any preceding claim, wherein the pump source is configured to modulate the first radiation in the time domain.

8. The assembly according to claim 7, wherein: The pump source is configured to modulate pulses of the first radiation when the intensity of the first radiation is above a threshold intensity.

9. An assembly according to any one of the preceding claims, wherein the hollow core of the optical fiber contains a fluid for exciting spectral broadening.

10. The assembly of claim 9, further comprising a reservoir for containing the fluid, the reservoir and the optical fiber being configured to provide the fluid to the hollow core of the optical fiber.

11. An assembly according to claim 9 or 10, wherein the fluid comprises a gas mixture comprising an inert gas.

12. An assembly according to claim 9 or 10, wherein the fluid comprises a gas mixture comprising a molecular gas.

13. An assembly according to any one of the preceding claims, wherein the length of the optical fiber is in the range of 10 cm to 40 cm.

14. An assembly according to any preceding claim, wherein the optical fiber is a photonic crystal fiber.

15. The assembly of claim 14, wherein the photonic crystal fiber comprises a single ring microstructure surrounding the hollow core of the fiber.

16. The assembly of claim 15, wherein: The single ring structure includes a plurality of capillaries surrounding the hollow core.

17. A component according to any one of the preceding claims, wherein the diameter of the hollow core is in the range of 25 to 35 μm or is 30 μm.

18. An assembly according to any preceding claim, wherein the broadband radiation comprises supercontinuum radiation.

19. A method of generating broadband radiation by spectral broadening, the method comprising: emitting modulated pump radiation at one or more wavelengths by a pump source, the emitted modulated pump radiation comprising pulsed first radiation at the pump wavelength; modulating pulses of the first radiation by the pump source according to a variation of a signal independent of the first radiation to stimulate spectral broadening in the optical fiber; The modulated pump radiation emitted by the pump source is received by the optical fiber, the optical fiber comprising a hollow core extending along at least a portion of the length of the optical fiber, the hollow core being configured to guide the received radiation during propagation through the optical fiber.

20. The method according to claim 19, further comprising: A first radiation is generated by a first radiation source and / or a second radiation seed radiation is generated by a second radiation source at a seed wavelength to modulate the first radiation.

21. The method of claim 20, wherein the spectral broadening of the modulated pump radiation comprises a four wave mixing process.

22. A measurement device comprising the radiation source assembly of any one of claims 1 to 18.

23. An inspection apparatus comprising the radiation source assembly of any one of claims 1 to 18.

24. A lithographic apparatus comprising the radiation source assembly of any one of claims 1 to 18.

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