Supercontinuum radiation source

Abstract

A broadband radiation source for generating output broadband radiation is disclosed, comprising a plurality of supercontinuum generating stages arranged in series, each of the supercontinuum generating stages comprising a respective non-linear generating element. The plurality of supercontinuum generation stages includes at least a first supercontinuum generation stage and a second supercontinuum generation stage, the second supercontinuum generation stage after the first supercontinuum generation stage in the series. A damage tolerance of a first non-linear generating element included within the first supercontinuum generating stage is greater than a damage tolerance of at least a second non-linear generating element included within the second supercontinuum generating stage.

Description

[0001] Cross-reference to related applications

[0002] This application claims priority to European Application No. 22211917.4, filed on Dec. 07, 2022, the entire content of which is incorporated herein by reference. Technical field

[0003] The present invention relates to an ultra - continuum or broadband radiation source, and more particularly to such a broadband radiation source for metrology applications in integrated circuit manufacturing. Background art

[0004] A lithographic apparatus is a machine configured to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). For example, a lithographic apparatus can project a pattern (also often referred to as a "design layout" or "design") present on a patterning device (e.g., a mask) onto a layer of radiation - sensitive material (resist) provided 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 this radiation determines the minimum size of the features that can be formed on the substrate. Typical wavelengths currently in use are 365 nm (i - line), 248 nm, 193 nm, and 13.5 nm. Compared to a lithographic apparatus using radiation having a wavelength of, for example, 193 nm, a lithographic apparatus using extreme ultraviolet (EUV) radiation having a wavelength in the range of 4 nm to 20 nm (e.g., 6.7 nm or 13.5 nm) can be used to form smaller features on a substrate.

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

[0007] Metrology tools are used in many aspects of the IC manufacturing process, such as alignment tools for properly positioning a substrate before exposure, leveling tools for measuring the surface topography of the substrate, or tools for checking / measuring an exposed and / or etched product based on focus control and scatterometry measurements, for example in process control. In each case, a radiation source is required. For different reasons, including measurement robustness i.e. stability, and accuracy, broadband or white light radiation sources are increasingly being used for these metrology applications. Improvements to current devices for broadband radiation generation would be desirable. SUMMARY OF THE INVENTION

[0008] In a first aspect of the invention, there is provided a broadband radiation source for generating an output broadband radiation, comprising a plurality of supercontinuum generation stages arranged in series, each said supercontinuum generation stage comprising a respective non-linear generation element; wherein said plurality of supercontinuum generation stages comprises at least a first supercontinuum generation stage and a second supercontinuum generation stage, in said series, said second supercontinuum generation stage being after said first supercontinuum generation stage; and wherein the damage tolerance of a first non-linear generation element comprised in said first supercontinuum generation stage is greater than the damage tolerance of at least a second non-linear generation element comprised in said second supercontinuum generation stage.

[0009] In a second aspect of the invention, there is provided a broadband radiation source for generating output broadband radiation, comprising a plurality of supercontinuum generation stages arranged in series, each said supercontinuum generation stage comprising a respective non-linear generation element; wherein said plurality of supercontinuum generation stages comprises at least a first supercontinuum generation stage and a second supercontinuum generation stage, in said series, said second supercontinuum generation stage being after said first supercontinuum generation stage; and wherein the optical non-linearity of a first non-linear generation element comprised in said first supercontinuum generation stage is lower than the optical non-linearity of at least a second non-linear generation element comprised in said second supercontinuum generation stage.

[0010] Other aspects of the invention include a measuring device, said measuring device comprising a broadband radiation source device of the first aspect or the second aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0012] Figure 1 A schematic overview of a lithographic apparatus is depicted;

[0013] Figure 2 A schematic overview of a lithographic cell is depicted;

[0014] Figure 3 A schematic representation of overall lithography is depicted, which represents the collaboration between three key technologies for optimizing semiconductor manufacturing;

[0015] Figure 4 A schematic overview of a scatterometry apparatus, which may comprise a radiation source according to an embodiment of the invention, used as a measuring device is depicted;

[0016] Figure 5 A schematic overview of a level sensor device, which may comprise a radiation source according to an embodiment of the invention is depicted;

[0017] Figure 6 A schematic overview of an alignment sensor device, which may comprise a radiation source according to an embodiment of the invention is depicted;

[0018] Figure 7 Is a schematic cross-sectional view of a hollow-core optical fiber, which may form part of a radiation source according to an embodiment in a transverse plane (i.e., perpendicular to the axis of the optical fiber);

[0019] Figure 8 A schematic representation of a known radiation source for providing broadband output radiation is depicted;

[0020] Figure 9 (a) and Figure 9(b) Schematically depicts a transverse cross-section of an example of a hollow-core photonic crystal fiber (HC-PCF) design for supercontinuum generation;

[0021] Figure 10 Schematically illustrates a broadband radiation source according to a first embodiment;

[0022] Figure 11 Schematically illustrates a broadband radiation source according to a first embodiment;

[0023] Figure 12 Schematically illustrates a broadband radiation source according to a second embodiment;

[0024] Figure 13 Schematically illustrates a broadband radiation source according to a third embodiment;

[0025] Figure 14 Schematically illustrates a broadband radiation source according to a fourth embodiment;

[0026] Figure 15 Schematically illustrates a broadband radiation source according to a fifth embodiment; and

[0027] Figure 16 Depicts a block diagram of a computer system for controlling a broadband radiation source. Detailed Description

[0028] In this document, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g., having wavelengths of 365 nm, 248 nm, 193 nm, 157 nm, or 126 nm) and EUV (extreme ultraviolet radiation, e.g., having wavelengths in the range of about 5 nm to 100 nm).

[0029] As used herein, the term "reticle", "mask", or "patterning device" can be broadly interpreted to refer to a general patterning device that can be used to impart a patterned cross-section to an incident radiation beam, the patterned cross-section corresponding to a pattern to be created in a target portion of a substrate. In such a context, the term "light valve" can also be used. Examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays in addition to classical masks (transmission or reflection, binary, phase-shifting, hybrid, etc.).

[0030] Figure 1Schematically depict a lithographic apparatus LA. The lithographic apparatus LA includes: 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) MT configured to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to certain parameters; a substrate support (e.g., a wafer table) WT configured to hold a substrate (e.g., a wafer coated with resist) 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., including one or more dies) of the substrate W.

[0031] In operation, the illumination system IL receives the 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 for guiding, shaping, and / or controlling the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components or any combination thereof. The illuminator IL may be used to condition the radiation beam B such that it has a desired spatial and angular intensity distribution in a cross-section at the plane of the patterning device MA.

[0032] The term "projection system" PS as used herein should be broadly interpreted to cover various types of projection systems suitable for the exposure radiation used and / or for other factors such as the use of an immersion liquid or the use of a vacuum, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic, and / or electrostatic optical systems or any combination thereof. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0033] The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid, e.g., water, having a relatively high refractive index, so as to fill the space between the projection system PS and the substrate W - this is also referred to as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0034] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also known as "dual platforms"). In such a "multi-platform" machine, the substrate supports WT may be used in parallel, and / or steps for preparing a substrate W for a subsequent exposure of the substrate W located on one of the substrate supports WT may be carried out while another substrate W on another substrate support WT is used for exposing a pattern on another substrate W.

[0035] In addition to the substrate support WT, the lithographic apparatus LA may also include a measurement stage. The measurement stage is arranged to hold sensors and / or cleaning devices. The sensors may be arranged to measure properties of the projection system PS or of the radiation beam B. The measurement stage may hold a plurality of sensors. The cleaning device may be arranged to clean parts of the lithographic apparatus, for example parts of the projection system PS or parts of the system providing the immersion liquid. The measurement stage may move below the projection system PS when the substrate support WT is moved away from the projection system PS.

[0036] In operation, the radiation beam B is incident on a patterning device (e.g., a mask MA) held on a mask support MT and is patterned by a pattern (design layout) present on the patterning device MA. After having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. By means of the second positioner PW and the 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 a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in Figure 1 are used to accurately position the patterning device MA relative to the path of the radiation beam B. Mask alignment marks M1, M2 and substrate alignment marks P1, P2 may be used to align the patterning device MA and the substrate W. Although the illustrated substrate alignment marks P1, P2 occupy dedicated target portions, they may be located in the spaces between the target portions. When the substrate alignment marks P1, P2 are located between the target portions C, these substrate alignment marks are referred to as scribe alignment marks.

[0037] As Figure 2As shown, the lithographic apparatus LA can form part of a lithographic cell LC (sometimes also referred to as a litho cell or (lithographic) cluster), which often also includes equipment for performing pre-exposure and post-exposure processes on the substrate W. Conventionally, such equipment includes 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 adjusting the temperature of the substrate W (e.g., for adjusting the solvent in the resist layer). A substrate transfer device or robot RO picks up the substrate W from the input / output ports I / O1, I / O2, moves the substrate W between different process equipment and transfers the substrate W to the feed table LB of the lithographic apparatus LA. The devices in the lithographic cell that are often collectively referred to as an automated resist coating and developing system are typically under the control of an automated resist coating and developing system control unit TCU, which itself may be controlled by a management control system SCS that can also control the lithographic apparatus LA, for example, via a lithography control unit LACU.

[0038] To correctly and consistently expose the substrate W exposed by the lithographic apparatus LA, it is desirable to inspect the substrate to measure the properties of the patterned structures, such as overlay errors between subsequent layers, line thickness, critical dimension (CD), etc. For this purpose, an inspection tool (not shown) can be included in the lithographic cell LC. If an error is detected, the exposure of subsequent substrates or other processing steps to be performed on the substrate W can be adjusted, for example, especially in cases where inspections are carried out before the other substrates W in the same batch or lot are still to be exposed or processed.

[0039] An inspection device, which can also be referred to as a metrology device, is used to determine the properties of the substrate W and, specifically, how the properties of different substrates W vary or how the properties associated with different layers of the same substrate W vary from layer to layer. The inspection device is alternatively configured to identify defects on the substrate W and can be, for example, part of the lithographic cell LC, or can be integrated into the lithographic apparatus LA, or can even be a separate device. The inspection device can measure the 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 a developed resist image (where the exposed or unexposed portions of the resist have been removed), or even an etched image (after a pattern transfer step such as etching).

[0040] Typically, the patterning process in the lithographic apparatus LA is one of the most important steps in the processing, which requires high accuracy in the sizing and placement of the structures on the substrate W. To ensure such high accuracy, three systems can be combined in a so-called "integrated" control environment, as Figure 3Schematically depicted, one of these systems is a lithographic apparatus LA, which is (in practice) connected to a metrology tool MT (second system) and to a computer system CL (third system). The key to such an "integrated" environment lies in optimizing the collaboration between these three systems to enhance the overall process window and provide a tight control loop, thereby ensuring that the patterning performed by the lithographic apparatus LA remains within the process window. The process window defines a range of process parameters (such as dose, focus, overlay), within which a specific manufacturing process yields a defined result (such as a functional semiconductor device) - typically within which variations in the process parameters during the lithographic or patterning process are allowed.

[0041] The computer system CL can use (a part of) the design layout to be patterned to predict which resolution enhancement techniques to use and perform computational lithography simulations and calculations to determine which mask layout and lithographic apparatus settings achieve the maximum overall process window for the patterning process (depicted by the double arrows in the first scale SC1 in Figure 3 . Typically, the resolution enhancement techniques are arranged to match the patterning capabilities of the lithographic apparatus LA. The computer system CL can also be used to detect where within the process window the lithographic apparatus LA is currently operating (e.g., using an input from the metrology tool MT) to predict whether defects may be present due to, for example, suboptimal processing (depicted by the arrow pointing to "0" in the second scale SC2 in Figure 3 .

[0042] The metrology tool MT can provide an input to the computer system CL to enable accurate simulations and predictions, and can provide feedback to the lithographic apparatus LA to identify possible drifts, for example, in the calibration state of the lithographic apparatus LA (depicted by the multiple arrows in the third scale SC3 in Figure 3 .

[0043] During the lithography process, it is desirable to frequently measure the created structures, for example, for process control and verification. Tools for performing such measurements are commonly referred to as metrology tools MT. Different types of metrology tools MT for performing such measurements are well known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. A scatterometer is a versatile instrument that allows for the measurement of lithography process parameters by having a sensor in the pupil or a plane conjugate to the pupil of the scatterometer's objective lens (the measurement is commonly referred to as pupil-based measurement), or by having a sensor in the image plane or a plane conjugate to the image plane, in which case the measurement is commonly referred to as image- or field-based measurement. Patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032, or EP1,628,164A, which are incorporated herein by reference in their entirety, further describe such scatterometers and associated measurement techniques. The aforementioned scatterometers can use radiation from soft x-rays and light visible in the near-IR wavelength range to measure gratings.

[0044] In a first embodiment, the scatterometer MT is an angular-resolved scatterometer. In such a scatterometer, a reconstruction method can be applied to the measured signal to reconstruct or calculate the properties of the grating. Such reconstruction can be caused, for example, by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measured results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to the diffraction pattern observed from the real target.

[0045] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, the radiation emitted by a radiation source is directed onto a target and the reflected or scattered radiation from the target is directed onto a spectrometer detector, which measures the spectrum of the specularly reflected radiation (i.e., the measurement of the intensity as a function of wavelength). Based on such data, the structure or profile of the target that produced the detected spectrum can be reconstructed, for example, by rigorous coupled-wave analysis and non-linear regression or by comparison with a simulated spectral library.

[0046] In a third embodiment, the scatterometer MT is an ellipsometric scatterometer. An ellipsometric scatterometer allows the determination of parameters of a lithography process by measuring scattered radiation for each polarization state. Such metrology equipment emits polarized light (such as linear, circular or elliptical) by using, for example, a suitable polarization filter in the illumination section of the metrology equipment. A source suitable for the metrology equipment can also provide polarized radiation. 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, describe various embodiments of existing ellipsometric scatterometers.

[0047] In one embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the overlap of two misaligned gratings or periodic structures by measuring the reflection spectrum and / or detecting an asymmetry in the detection configuration (the asymmetry being related to the range of overlap). Two (usually superimposed) grating structures can be applied in two different layers (not necessarily consecutive layers) and can be formed at substantially the same location on the wafer. The scatterometer can have a symmetric detection configuration, such as that described in co-owned patent application EP1,628,164A, so that any asymmetry can be clearly distinguished. This provides a direct way to measure misalignment in the grating. Other examples of measuring the overlap error between two layers including periodic structures when the target is measured via the asymmetry of the periodic structure can be found in PCT Patent Application Publication No. WO2011 / 012624 or U.S. Patent Application US 20160161863, which are incorporated herein by reference in their entirety.

[0048] Other parameters of interest can be focus and dose. Focus and dose can 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 with a unique combination of critical dimension and sidewall angle measurements for each point in a focus energy matrix (FEM - also known as a focus exposure matrix) can be used. If these unique combinations of critical dimension and sidewall angle can be obtained, the focus and dose values can be uniquely determined from these measurements.

[0049] The measurement target can be the totality of a composite grating formed mainly in a resist by a lithography process and also formed, for example, after an etching process. Generally, the pitch and linewidth of the structures in the grating depend largely on the measurement optics (especially the NA of the optics) to be able to capture the diffraction orders from the measurement target. As previously indicated, the diffraction signal can be used to determine the shift (also referred to as "overlay") between two layers or can be used to reconstruct at least a portion of the original grating produced by the lithography process. Such reconstruction can be used to provide quality guidance for the lithography process and can be used to control at least a portion of the lithography process. The target can have a smaller sub-segment configured to mimic the dimensions of the functional portion of the design layout in the target. Due to such a sub-segment, the target will behave more like the functional portion of the design layout, such that the overall process parameter measurement is preferably similar to the functional portion of the design layout. The target can be measured in an underfill mode or in an overfill mode. In the underfill mode, the measurement beam produces a spot smaller than the overall target. In the overfill mode, the measurement beam produces a spot larger than the overall target. In such an overfill mode, it may also be possible to measure different targets simultaneously and thus determine different process parameters at the same time.

[0050] The overall measurement quality of the lithography parameters using a particular target is determined at least in part by the measurement option scheme used to measure such lithography parameters. The term "substrate measurement option scheme" can include one or more parameters of the measurement itself, one or more parameters of one or more patterns being measured, or both. For example, if the measurement used in the substrate measurement option scheme is a diffraction-based optical measurement, one or more of the measured parameters can include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the direction of the radiation relative to the pattern on the substrate, etc. One of the criteria for selecting the measurement option scheme can 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.

[0051] Figure 4 depicts a metrology device, such as a scatterometer. The measurement device includes a broadband (white light) radiation projector 2 that projects radiation onto a substrate 6. The reflected or scattered radiation is passed to a spectrometer detector 4 that measures the spectrum 10 of the specularly reflected radiation (i.e., the measurement of intensity as a function of wavelength). From such data, the processing unit PU can reconstruct the structure or profile that caused the detected spectrum, for example, by rigorous coupled-wave analysis and non-linear regression, or by comparison with Figure 3Comparison of the simulated spectral libraries shown at the bottom. Generally, for reconstruction, the general form of the structure is known and some parameters are assumed from knowledge of the process used to fabricate the structure, leaving only a few parameters of the structure to be determined from the scattering measurement data. Such a scatterometer can be configured as a normal incidence scatterometer or an oblique incidence scatterometer.

[0052] The overall measurement quality of lithography parameters of a metrology target measured via measurement is at least partially determined by the measurement selection scheme used to measure such lithography parameters. The term "substrate measurement selection scheme" can include one or more parameters of the measurement itself, one or more parameters of one or more patterns being measured, or both. For example, if the measurement used in the substrate measurement selection scheme is a diffraction-based optical measurement, one or more of the parameters measured can include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, etc. One of the criteria for selecting a measurement selection scheme can 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.

[0053] Another type of metrology tool for IC manufacturing is a topography measurement system, a level sensor, or a height sensor. Such a tool can be integrated into a lithography apparatus for measuring the topography of the top surface of a substrate (or wafer). A map of the topography of the substrate, also known as a height map, can be generated from these measurements indicating the height of the substrate varying with position on the substrate. Such a height map can then be used to correct the position of the substrate during transfer of the pattern onto the substrate in order to provide a spatial image of the patterning device at the proper focus position on the substrate. It should be understood that "height" in this context generally refers to the dimension from a plane to the substrate (also known as the Z-axis). Generally, a level or height sensor performs measurements at a fixed location (relative to its own optical system), and relative movement between the substrate and the optical system of the level or height sensor causes height measurements at locations across the substrate.

[0054] Figure 5 An example of a level or height sensor LS known in the art is schematically shown in Figure 5Only the operating principle is illustrated. In this example, the level sensor includes an optical system, which includes a projection unit LSP and a detection unit LSD. The projection unit LSP includes a radiation source LSO that provides a radiation beam LSB, which is imparted by the projection grating PGR of the projection unit LSP. The radiation source LSO can be, for example, a narrowband or broadband light 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 multiple radiation sources having different colors or wavelength ranges, such as multiple 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 surface of the substrate.

[0055] The projection grating PGR is a periodic grating including a periodic structure that generates a radiation beam BE1 having a periodically varying intensity. The radiation beam BE1 having a periodically varying intensity is directed toward a measurement site MLO on the substrate W that has an incident angle ANG with respect to an axis (Z-axis) perpendicular to the surface of the incident substrate, where the incident angle ANG is between 0 degrees and 90 degrees, typically between 70 degrees and 80 degrees. At the measurement site MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by the arrow BE2) and directed toward the detection unit LSD.

[0056] To determine the height level at the measurement site MLO, the level sensor further includes a detection system, which includes a detection grating DGR, a detector DET, and a processing unit (not shown) for processing the output signal of the detector DET. The detection grating DGR can be the same as the projection grating PGR. The detector DET generates a detector output signal that indicates the received light, for example, indicates the intensity of the received light, such as a photodetector, or represents the spatial distribution of the received intensity, such as a camera. The detector DET can include any combination of one or more detector types.

[0057] By means of triangulation techniques, the height level at the measurement site MLO can be determined. The detected height level is generally related to the signal intensity measured by the detector DET, which has a periodicity that depends in particular on the design of the projection grating PGR and the (tilted) incident angle ANG.

[0058] The projection unit LSP and / or the detection unit LSD can include other optical elements, such as lenses and / or mirrors, along the path of the patterned radiation beam between the projection grating PGR and the detection grating DGR (not shown).

[0059] In an embodiment, the detection grating DGR may be omitted and the detector DET may be placed at the location where the detection grating DGR was positioned. This configuration provides a more direct detection of the image of the projection grating PGR.

[0060] To effectively cover the surface of the substrate W, the horizontal sensor LS may be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby creating an array of measurement regions MLO or spots that cover a larger measurement range.

[0061] Various height sensors of a general type are disclosed, for example, in US7265364 and US7646471, both of which are incorporated by reference. A height sensor that uses UV radiation instead of visible or infrared radiation is disclosed in US2010233600A1, which is incorporated by reference. In WO2016102127A1, which is incorporated by reference, a compact height sensor that uses a multi-element detector to detect and distinguish the position of a grating image without the need for a detection grating is described.

[0062] Another type of metrology tool used in IC manufacturing is an alignment sensor. Thus, a key aspect of the performance of a lithographic apparatus is the ability to place the applied pattern relative to features placed in a previous layer (by the same apparatus or a different lithographic apparatus) properly and accurately. For this purpose, the substrate is provided with one or more sets of marks or targets. Each mark is a structure whose position can later be measured using a position sensor, typically an optical position sensor. The position sensor may be referred to as an "alignment sensor" and the marks may be referred to as "alignment marks".

[0063] A lithographic apparatus may include one or more (e.g., a plurality of) alignment sensors that can accurately measure the position of alignment marks provided on a substrate. 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 current lithographic apparatuses is based on the self-referencing interferometer described in US6961116. Various enhancements and modifications of the position sensor have been developed, such as those disclosed in US2015261097A1. The content of all these publications is incorporated herein by reference.

[0064] Figure 6It is a schematic block diagram of an embodiment of a known alignment sensor AS described in, for example, US6961116 and incorporated by reference. A radiation source RSO provides a radiation beam RB having one or more wavelengths, which is steered by a steering optical device onto a mark (such as a mark AM located on a substrate W), as an illumination spot SP. In this example, the steering optical device includes a point mirror SM and an objective lens OL. Thus, the diameter of the illumination spot SP that illuminates the mark AM can be slightly smaller than the width of the mark itself.

[0065] The radiation diffracted by the alignment mark AM (in this example via the objective lens OL) is collimated into an information-bearing beam IB. The term "diffraction" is intended to include zero-order diffraction from the mark (which can be referred to as reflection). A self-referencing interferometer SRI of the type disclosed, for example, in the aforementioned US6961116 interferes with the beam IB itself, and then the beam is received by a photodetector PD. Additional optical devices (not shown) can be included to provide separate beams in the case where the radiation source RSO produces more than one wavelength. The photodetector can be a single element, or it can include multiple pixels as needed. The photodetector can include a sensor array.

[0066] The steering optical device including the point mirror SM in this example can also be used to block the zero-order radiation reflected from the mark, such that the information-bearing beam IB includes only higher-order diffracted radiation from the mark AM (this is not necessary for measurement, but improves the signal-to-noise ratio).

[0067] An intensity signal SI is supplied to a processing unit PU. By combining the optical processing performed in the block SRI with the computational processing performed in the unit PU, values of the X position and Y position of the substrate relative to a reference frame are output.

[0068] A single measurement of the type illustrated only fixes the position of the mark within a certain range corresponding to one pitch of the mark. A coarser measurement technique is used in combination with this measurement to identify which period of a sine wave is the period including the marked position. The same process at coarser and / or finer levels can be repeated at different wavelengths for improving accuracy and / or for robustly detecting the mark, regardless of the material on which the mark is made and the material on and / or under which the mark is disposed. The wavelengths can be optically multiplexed and demultiplexed to process the wavelengths simultaneously, and / or the wavelengths can be multiplexed by time division or frequency division.

[0069] In this example, the alignment sensor and the spot SP remain stationary while the substrate W moves. The alignment sensor can thus be firmly and accurately mounted to a reference system or reference frame while effectively scanning the mark AM in a direction opposite to the direction of movement of the substrate W. The movement of the substrate W is controlled by a substrate positioning system that mounts the substrate W to a substrate support and controls the movement of the substrate support. A substrate support position sensor (such as an interferometer) measures the position of the substrate support (not shown). In an embodiment, one or more (alignment) marks are provided on the substrate support. Measuring the position of the marks provided on the substrate support allows calibration of the position of the substrate support determined by the position sensor (e.g., relative to the frame to which the alignment system is connected). Measuring the position of the alignment marks provided on the substrate allows determination of the position of the substrate relative to the substrate support.

[0070] The measurement tool MT mentioned above (such as a scatterometer, a topography measurement system, or a position measurement system) can perform measurements using radiation from a radiation source. The nature of the radiation used by the measurement tool may affect the type and quality of the measurements that can be performed. For some applications, it may be advantageous to use multiple radiation frequencies to measure the substrate, for example, broadband radiation can be used. Multiple different frequencies may be able to propagate, irradiate, and scatter off the measurement target without interfering with other frequencies or with minimal interference. Thus, more measurement data can be obtained simultaneously using different frequencies, for example. Different radiation frequencies may also be able to interrogate and reveal different properties of the measurement target. Broadband radiation can be applicable to measurement systems MT such as level sensors, alignment mark measurement systems, scatter measurement tools, or inspection tools. A broadband radiation source can be a supercontinuum source.

[0071] High-quality broadband radiation such as supercontinuum radiation may be difficult to generate. One method for generating broadband radiation can be, for example, to utilize nonlinear high-order effects to broaden high-power narrowband or single-frequency input radiation or pump radiation. The input radiation (which can be generated using a laser) can be referred to as pump radiation. Alternatively, the input radiation can be referred to as seed radiation. To obtain high-power radiation for the broadening effect, the radiation can be confined to a small area such that strongly localized high-intensity radiation is achieved. In those areas, the radiation can interact with a broadening structure and / or a material forming a nonlinear medium to produce broadband output radiation. In the high-intensity radiation areas, different materials and / or structures can be used to achieve and / or improve radiation broadening by providing a suitable nonlinear medium.

[0072] In some implementations, broadband output radiation is generated in a photonic crystal fiber (PCF). In several embodiments, such a photonic crystal fiber has a microstructure around its fiber core that helps to confine the radiation traveling through the fiber in the fiber core. The fiber core can be made of a solid material having nonlinear properties and capable of generating broadband radiation when high-intensity pump radiation is transmitted through the fiber core. Although it is feasible to generate broadband radiation in a solid-core photonic crystal fiber, there can be several drawbacks to using a solid material. For example, if UV radiation is generated in the solid core, such radiation may not be present in the output spectrum of the fiber because the radiation is absorbed by most solid materials and causes permanent damage.

[0073] In some implementations, as discussed further below Figure 8 A method and apparatus for broadening input radiation can use a fiber for confining the input radiation and for broadening the input radiation to output broadband radiation. The fiber can be a hollow-core fiber and can include an internal structure for enabling efficient guiding and confinement of the radiation in the fiber. The fiber can be a hollow-core photonic crystal fiber (HC-PCF), which is particularly suitable for strong radiation confinement mainly inside the hollow core of the fiber, thereby achieving high radiation intensity. The hollow core of the fiber can be filled with a gas that serves as a broadening medium for broadening the input radiation. Such a fiber and gas arrangement can be used to generate a supercontinuum radiation source. The radiation input to the fiber can be electromagnetic radiation, such as radiation in one or more of the infrared, visible light, UV, and extreme UV spectra. The output radiation can consist of or include broadband radiation, which can be referred to herein as white light.

[0074] Some embodiments relate to a new design of such a broadband radiation source including a fiber. The fiber is a hollow-core photonic crystal fiber (HC-PCF). Specifically, the fiber can be of a type that includes an antiresonant structure for confining radiation. Such a fiber including an antiresonant structure is known in the art as an antiresonant fiber, a tubular fiber, a single-ring fiber, a negative-curvature fiber, or a suppressed-coupling fiber. Various different designs of such fibers are known in the art. Alternatively, the fiber can be a photonic bandgap fiber (HC-PBF, such as a Kagome fiber).

[0075] Multiple types of HC-PCF can be designed, each based on a different physical guiding mechanism. Two such HC-PCF include: hollow-core photonic bandgap fiber (HC-PBF) and hollow-core anti-resonant reflecting fiber (HC-ARF). Details of the design and manufacture of HC-PCF can be found in U.S. Patent US2004 / 015085A1 (for HC-PBF) and International PCT Patent Application WO2017 / 032454A1 (for hollow-core anti-resonant reflecting fiber), which are incorporated herein by reference. Figure 9 FIG. (a) shows a Kagome fiber including a Kagome lattice structure.

[0076] Now refer to Figure 7 an example of a fiber for use in a radiation source is described. Figure 7 FIG. is a schematic cross-sectional view of the fiber OF in a transverse plane. Other embodiments similar to the actual example of the fiber disclosed in WO2017 / 032454A1 are Figure 7 disclosed.

[0077] The fiber OF includes an elongate body, and the fiber OF is longer in one dimension compared to the other two dimensions of the fiber OF. This longer dimension can be referred to as the axial direction and can define the axis of the fiber OF. The other two dimensions define a plane that can be referred to as the transverse plane. Figure 7 FIG. shows a cross-section of the fiber OF in such a transverse plane (i.e., perpendicular to the axis) labeled as the x-y plane. The transverse cross-section of the fiber OF can be substantially constant along the fiber axis.

[0078] It should be understood that the fiber OF has a certain degree of flexibility, and thus, generally, the direction of the axis will not be uniform along the length of the fiber OF. Terms such as the optical axis, transverse cross-section, etc. should be understood to mean the local optical axis, local transverse cross-section, etc. In addition, in cases where a component is described as being cylindrical or tubular, these terms will be understood to cover such shapes that may have been deformed when the fiber OF is bent.

[0079] The fiber OF can have any length and it will be understood that the length of the fiber OF can depend on the application. The fiber OF can have a length between 1 cm and 10 m. For example, the fiber OF can have a length between 10 cm and 100 cm.

[0080] Optical fiber OF includes: a hollow core HC; a cladding portion surrounding the hollow core HC; and a support portion SP surrounding and supporting the cladding portion. The optical fiber OF can be regarded as including a body (including the cladding portion and the support portion SP) having a hollow core HC. The cladding portion includes a plurality of anti-resonant elements for guiding radiation through the hollow core HC. Specifically, the plurality of anti-resonant elements are arranged to confine the radiation mainly propagating inside the hollow core HC through the optical fiber OF and to guide the radiation along the optical fiber OF. The hollow core HC of the optical fiber OF can be generally disposed in the central region of the optical fiber OF such that the axis of the optical fiber OF can also define the axis of the hollow core HC of the optical fiber OF.

[0081] The cladding portion includes a plurality of anti-resonant elements for guiding radiation to propagate through the optical fiber OF. Specifically, in this embodiment, the cladding portion includes a single ring of six tubular capillaries CAP. Each of the tubular capillaries CAP serves as an anti-resonant element. It should be understood that these anti-resonant elements can have different cross-sections, such as an elliptical cross-section or a nested circular cross-section, where a smaller diameter circular tube is located within a larger diameter circular tube.

[0082] The capillary CAP can also be referred to as a tube. The cross-section of the capillary CAP can be circular or can have another shape. Each capillary CAP includes a generally cylindrical wall portion WP that at least partially defines the hollow core HC of the optical fiber OF and separates the hollow core HC from the capillary cavity CC. It should be understood that the wall portion WP can serve as an anti-reflection Fabry - Perot resonator for the radiation that propagates through the hollow core HC (and the radiation can be incident on the wall portion WP at a grazing angle of incidence). The thickness of the wall portion WP can be appropriate to ensure substantially enhanced reflection back into the hollow core HC while substantially suppressing transmission into the capillary cavity CC. In some embodiments, the capillary wall portion WP can have a thickness between 0.01 µm and 10.0 µm.

[0083] It should be understood that as used herein, the term "cladding portion" is intended to mean the portion of the optical fiber OF that guides the radiation propagating through the optical fiber OF (i.e., the capillary CAP that confines the radiation within the hollow core HC). The radiation can be confined in the form of a transverse mode propagating along the optical fiber axis.

[0084] The support portion is generally tubular and supports the six capillary capillaries CAP of the cladding portion. The six capillary capillaries CAP are uniformly distributed around the inner surface of the inner support portion SP. The six capillary capillaries CAP can be described as being arranged in a generally hexagonal form.

[0085] The capillary CAPs are arranged such that each capillary does not contact any of the other capillary CAPs. Each of the capillary CAPs contacts the inner support portion SP and is spaced apart from adjacent capillary CAPs in the annular structure. Such an arrangement can be beneficial as it can increase the transmission bandwidth of the optical fiber OF (relative to an arrangement where, for example, the capillaries are in contact with each other). Alternatively, in some embodiments, each of the capillary CAPs can contact adjacent capillary CAPs in the annular structure.

[0086] Six capillary CAPs of the cladding portion are arranged in a ring structure around the hollow core HC. The inner surface of the ring structure of the capillary CAPs at least partially defines the hollow core HC of the optical fiber OF. The diameter d of the hollow core HC (which can be defined as the minimum dimension between opposing capillaries, indicated by the arrow d) can be between 10 µm and 1000 µm. The diameter d of the hollow core HC can affect the mode field diameter, impact loss, dispersion, modal multiplicity i.e., mode complexity, and non - linear properties of the optical fiber OF in the hollow core HC.

[0087] In such an embodiment, the cladding portion includes a single - ring arrangement of capillary CAPs (which act as anti - resonant elements). Thus, a line in any radial direction from the center of the hollow core HC to the outside of the optical fiber OF passes through no more than one capillary CAP.

[0088] It should be understood that other embodiments can be provided with different arrangements of anti - resonant elements. These arrangements can include arrangements having multiple rings of anti - resonant elements and arrangements having nested anti - resonant elements. Figure 9 (a) shows an embodiment of an HC - PCF having three rings of capillary CAPs, with the rings stacked on top of each other in the radial direction. In such an embodiment, each capillary CAP contacts other capillaries both within the same ring and in different rings. Additionally, although Figure 7 the embodiment shown includes a ring of six capillaries, in other embodiments, one or more rings including any number of anti - resonant elements (such as 4, 5, 6, 7, 8, 9, 10, 11, or 12 capillaries) can be provided in the cladding portion.

[0089] Figure 9 (b) shows a modified embodiment of the HC - PCF having a single ring of tubular capillaries discussed above. In Figure 9 (b)'s example, there are two coaxial rings of tubular capillaries 21. To hold the inner and outer rings of the tubular capillaries 21, a support tube ST can be included in the HC - PCF. The support tube can be made of silica.

[0090] Figure 7 and Figure 9 (a) and Figure 9The exemplary tubular capillary of (b) may have a circular cross-sectional shape. For the tubular capillary, other shapes are also possible, such as an elliptical or polygonal cross-section. Additionally, Figure 7 and Figure 9 (a) and Figure 9 the solid material of the exemplary tubular capillary of (b) may include a plastic material (such as PMA), glass (such as silica or soft glass).

[0091] Figure 8 Depicts a known radiation source RDS for providing broadband output radiation. The radiation source RDS may include a pulsed pump radiation source PRS, a continuous wave source, or any type of source capable of generating short pulses having a desired length and energy level; an optical fiber OF having a hollow core HC (for example, of the type shown in Figure 7 ); and a working medium WM (such as a gas) disposed within the hollow core HC. Although in Figure 8 the radiation source RDS includes the optical fiber OF shown in Figure 7 , in alternative embodiments, other types of hollow core HC optical fibers OF may be used.

[0092] The pulsed pump radiation source PRS is configured to provide input radiation IRD. The hollow core HC of the optical fiber OF is arranged to receive the input radiation IRD from the pulsed pump radiation source PRS and broaden the input radiation IRD to provide output radiation ORD. The working medium WM enables broadening the frequency range of the received input radiation IRD to provide broadband output radiation ORD.

[0093] The radiation source RDS further includes a reservoir RSV. The optical fiber OF is disposed inside the reservoir RSV. The reservoir RSV may also be referred to as a housing, container, or gas chamber. The reservoir RSV is configured to contain the working medium WM. The reservoir RSV may include one or more features known in the art for controlling, regulating, and / or monitoring the composition of the working medium WM (which may be a gas or a liquid) inside the reservoir RSV. The reservoir RSV may include a first transparent window TW1. In use, the optical fiber OF is disposed inside the reservoir RSV such that the first transparent window TW1 is positioned close to the input end IE of the optical fiber OF. The first transparent window TW1 may form part of the wall of the reservoir RSV. The first transparent window TW1 may be transparent at least for the frequency of the received input radiation, such that the received input radiation IRD (or at least a major part thereof) can be coupled into the optical fiber OF located inside the reservoir RSV. It should be understood that optical devices (not shown) may be provided for coupling the input radiation IRD into the optical fiber OF.

[0094] The reservoir RSV includes a second transparent window TW2 that forms part of the wall of the reservoir RSV. In use, when the optical fiber OF is disposed inside the reservoir RSV, the second transparent window TW2 is positioned close to the output end OE of the optical fiber OF. The second transparent window TW2 can be transparent at least for the frequencies of the broadband output radiation ORD of the device 120.

[0095] Alternatively, in another embodiment, two opposite ends of the optical fiber OF can be placed inside different reservoirs. The optical fiber OF can include a first end section configured to receive the input radiation IRD, and a second end section for outputting the broadband output radiation ORD. The first end section can be placed inside a first reservoir including a working medium WM. The second end section can be placed inside a second reservoir, where the second reservoir can also include the working medium WM. The functions of the reservoirs can be as described above with respect to Figure 8 what has been described. The first reservoir can include a first transparent window configured to be transparent to the input radiation IRD. The second reservoir can include a second transparent window configured to be transparent to the broadband output broadband radiation ORD. The first reservoir and the second reservoir can also include sealable openings to allow the optical fiber OF to be partially placed inside the reservoir and partially outside the reservoir such that a gas can be sealed inside the reservoir. The optical fiber OF can also include an intermediate section that is not contained inside the reservoir. Such an arrangement using two separate gas reservoirs can be particularly convenient for embodiments where the optical fiber OF is relatively long (e.g., when the length exceeds 1 m). It should be understood that for such an arrangement using two separate gas reservoirs, the two reservoirs (which can include one or more features known in the art for controlling, regulating, and / or monitoring the composition of the gas inside the two reservoirs) can be considered as devices for providing the working medium WM inside the hollow core HC of the optical fiber OF.

[0096] In this context, a window can be transparent to a frequency if at least 50%, 75%, 85%, 90%, 95%, or 99% of the incident radiation at that frequency on the window passes through the window.

[0097] Both the first transparent window TW1 and the second transparent window TW2 can form an airtight seal within the wall of the reservoir RSV such that a working medium WM (which can be a gas) can be included inside the reservoir RSV. It should be understood that the gas WM can be contained inside the reservoir RSV at a pressure different from the ambient pressure of the reservoir RSV.

[0098] The working medium WM may include: noble gases such as argon, krypton, and xenon; Raman active gases such as hydrogen, deuterium, and nitrogen; gas mixtures such as argon / hydrogen mixtures, xenon / deuterium mixtures, krypton / nitrogen mixtures; mixtures of molecular gases such as nitrogen / hydrogen mixtures; mixtures of atomic gases such as argon / helium, krypton / helium, xenon / helium; or ternary gas mixtures such as argon / helium / hydrogen, or krypton / helium / hydrogen. Depending on the thermodynamic conditions of the RSV, such as the type of fill gas, its pressure and temperature, and depending on the laser conditions, such as pulse duration, energy, and wavelength, the nonlinear optical processes may include modulation instability (MI), optical soliton self-compression, optical soliton splitting, Kerr effect, Raman effect, and dispersion wave generation (DWG), details of each of which are described in WO2018 / 127266A1 and US9160137B1 (both of which are hereby incorporated by reference). Since the foregoing parameters can be tuned, the generated broadband pulse dynamics and associated spectral broadening characteristics can be adjusted to optimize frequency conversion. For example, the dispersion of the gas-filled hollow core fiber, which is a key factor defining the interaction range of the laser pulse with the WM, can be tuned by varying the working medium WM pressure (i.e., the gas chamber pressure) in the reservoir RSV.

[0099] In one embodiment, the working medium WM may be disposed within the hollow core HC at least during receipt of the input radiation IRD for generating the broadband output radiation ORD. It should be understood that when the optical fiber OF is not receiving the input radiation IRD for generating the broadband output radiation, the gas WM may be wholly or partially absent from the hollow core HC.

[0100] To achieve frequency broadening, high-intensity radiation may be desirable. The advantage of the optical fiber OF with the hollow core HC is that high-intensity radiation can be achieved by the strong spatial confinement of the radiation propagating through the optical fiber OF, thereby achieving a highly localized radiation intensity. The radiation intensity inside the optical fiber OF may be high, for example, due to a high received input radiation intensity and / or due to the strong spatial confinement of the radiation inside the optical fiber OF. The advantage of the hollow core optical fiber is that it can guide radiation with a wider wavelength range compared to a solid core optical fiber, and specifically, the hollow core optical fiber can guide radiation in the ultraviolet and infrared ranges in the spectral regions that fused silica (the typical glass used in optical fibers) is positively absorbing.

[0101] An advantage of using the optical fiber OF with the hollow core HC may be that most of the radiation guided inside the optical fiber OF is confined within the hollow core HC. Thus, most of the interaction of the radiation inside the optical fiber OF is with the working medium WM, which is disposed inside the hollow core HC of the optical fiber OF. As a result, the broadening effect of the working medium WM on the radiation can be increased.

[0102] The received input radiation IRD can be electromagnetic radiation. The input radiation IRD can be received as pulsed radiation. For example, the input radiation IRD can include, for example, ultrafast pulses generated by a laser.

[0103] The input radiation IRD can be radiation that is coherent in time and / or in space. The input radiation IRD can be collimated radiation, the advantage of which can be to facilitate and increase the efficiency of coupling the input radiation IRD into the optical fiber OF. The input radiation IRD can include a single frequency or a narrow frequency range. The input radiation IRD can be generated by a laser. Similarly, the output radiation ORD can be collimated and / or can be correlated in time and / or in space.

[0104] The broadband range of the output radiation ORD can be a continuous range, including a continuous range of radiation frequencies. The output radiation ORD can include supercontinuum radiation. The continuous radiation can be beneficial for use in a number of applications, such as in metrology applications. For example, the continuous frequency range can be used to interrogate a large number of properties. The continuous frequency range can be used, for example, to determine and / or eliminate the frequency dependence of the measured property and / or to enable rapid selection and / or switching among different frequencies using a spectral filtering device. The supercontinuum output radiation ORD can include, for example, electromagnetic radiation in the wavelength range from 100 nm to 4000 nm. The broadband output radiation ORD frequency range can be, for example, 400 nm to 900 nm, 500 nm to 900 nm, or 200 nm to 2000 nm. The supercontinuum output radiation ORD can include white light.

[0105] The input radiation IRD provided by the pulsed pump radiation source PRS (or other input radiation source) can be pulsed. The input radiation IRD can include electromagnetic radiation of one or more frequencies between 200 nm and 3 µm. The input radiation IRD can include, for example, electromagnetic radiation having a wavelength of 1.03 µm. The repetition rate of the pulsed radiation IRD can be on the order of 1 kHz to 100 MHz. The pulse energy can be on the order of 1 nJ to 100 µJ (e.g., 1 µJ to 10 µJ). The pulse duration of the input radiation IRD can be between 1 fs and 10 ps, for example 300 fs. The average power of the input radiation IRD can be between 100 mW and several 100 W. The average power of the input radiation IRD can be, for example, 20 to 50 W.

[0106] The pulsed pump radiation source PRS can be a laser. The spatio-temporal transmission characteristics (e.g., its spectral amplitude and phase) of such laser pulses transmitted along the optical fiber OF can be changed and tuned by adjusting the (pump) laser parameters, the working component WM variations, and the optical fiber OF parameters. The spatio-temporal transmission characteristics can include one or more of the following: output power, output mode profile, output temporal profile, width of the output temporal profile (or output pulse width), output spectral profile, and bandwidth of the output spectral profile (or output spectral bandwidth). The input radiation source IRS parameters can include one or more of the following: pump wavelength, pump pulse energy, pump pulse width, pump pulse repetition rate. The optical fiber OF parameters can include one or more of the following: fiber length, size and shape of the hollow core HC, size and shape of the capillary, thickness of the wall of the capillary surrounding the hollow core HC. The working component WM (e.g., filling gas) parameters can include one or more of the following: gas type, gas pressure, and gas temperature, or gas composition and / or partial pressure in the case where WM is a mixture of different gases.

[0107] The broadband output radiation ORD provided by the radiation source RDS can have an average output power of at least 100 mW. The average output power can be at least 1 W. The average output power can be at least 5 W. The average output power can be at least 10 W. The broadband output radiation ORD can be pulsed broadband output radiation ORD. The broadband output radiation ORD can be continuous wave CW broadband output radiation ORD. The broadband output radiation ORD can have a power spectral density in the entire wavelength band of the output radiation of at least 0.01 mW / nm. The power spectral density in the entire wavelength band of the broadband output radiation can be at least 3 mW / nm.

[0108] As described above, there are many nonlinear optical processes that involve generating broadband output radiation ORD (e.g., supercontinuum or white light). Which nonlinear optical processes have a more significant spectral broadening effect compared to other processes will depend on how the operating parameters are set. For example, by selecting the pump wavelength and / or the optical fiber OF such that the pump pulse propagates through the fiber in the normal dispersion region (positive group velocity dispersion (GVD)), self-phase modulation is the dominant nonlinear optical process and is responsible for the spectral broadening of the pump pulse. However, in most cases, the spectral broadening of the input radiation IRD provided by the pulsed pump radiation source PRS is driven by soliton dynamics that require the pump pulse to propagate in an optical fiber OF in the anomalous dispersion region (negative GVD). This is because in the anomalous dispersion region, the effects of Kerr nonlinearity and dispersion act in opposite directions, causing the pulse to maintain and / or enhance its peak intensity. When the pulse parameters of the pump pulse launched into an optical fiber OF with anomalous dispersion (e.g., HC-PCF) do not precisely match the pulse parameters of the soliton, the pump pulse will evolve into a soliton pulse with a certain soliton order and dispersion wave.

[0109] As is well known, soliton self-compression and modulation instability are two main mechanisms for spectral broadening in soliton-driven broadband radiation generation. The difference between the two mechanisms is that the soliton self-compression process is associated with a low soliton order, while the modulation instability process is associated with a high soliton order. The soliton order N of the pulsed input radiation IRD is a convenient parameter that can be used to distinguish the conditions under which spectral broadening is dominated by modulation instability from the conditions under which spectral broadening is dominated by soliton self-compression. The soliton order N of the pulsed input radiation IRD is given as follows:

[0110] (1)

[0111] where γ is the nonlinear phase (or nonlinear parameter); P p is the pump peak power of the pulsed input radiation IRD; τ is the pump pulse duration of the pulsed input radiation IRD; and β2 is the group velocity dispersion of the nonlinear element (e.g., optical fiber and working medium WM).

[0112] When the spectral broadening is usually dominated by modulation instability, while when the spectral broadening is usually dominated by soliton self-compression.

[0113] Some known broadband radiation sources use the following arrangement, which produces spectral broadening of pulsed pump radiation, but in which the parameters of the pulsed pump radiation, the optical fiber, and the working medium are configured to allow modulation instability to produce spectral broadening. There are various reasons why modulation instability is used to produce the said spectral broadening. First, it is known that modulation instability produces broadband radiation with a relatively flat intensity-wavelength distribution, provided that a sufficient number of pulses are averaged. Such a broadband radiation source can be called a white light radiation source (due to the relatively flat spectral intensity distribution). Second, a relatively low-cost laser source can be used as the pump radiation source to achieve modulation instability.

[0114] On the other hand, in the system of optical soliton self-compression, the input pump pulse undergoes compression in the time domain, which is accompanied by an increase in the spectral width. After optical soliton self-compression, the compressed pulse undergoes optical soliton fission, in which the pulse splits into multiple optical solitons. Such optical soliton fission causes temporal broadening of the radiation pulse and spectral shift.

[0115] To summarize the above, for example, Figure 8 Regarding the principle behind the said radiation source of, for example, the present technique for supercontinuum (SC) generation depends on the interaction of the driving laser radiation with the nonlinear element in order to increase the spectral bandwidth. In such a scenario, the peak intensity of the laser radiation is high enough such that it undergoes multiple optical nonlinear effects, thereby broadening its spectrum. Generally, the wavelength of the laser pulse is chosen such that it falls within the normal or anomalous dispersion range of the nonlinear element. In the former case, the broadening is controlled by self-phase modulation (SPM), while in the latter case, the spectral broadening is controlled by optical soliton dynamics. Alternatively, the nonlinear element can be designed to provide pure normal dispersion, such as an all-normal dispersion (ANDi) fiber. In such a case, the broadening is controlled by SPM and optical wave breaking.

[0116] This method can produce a very non-uniform power spectral density (PSD). For example, the PSD from the pump laser may drop by two to three orders of magnitude. This is an indication of the low conversion efficiency from the driving laser pulse to the supercontinuum region of interest, which can be, for example, 100 nm to 1200 nm, 300 nm to 1000 nm, or 400 nm to 900 nm. The same is true for systems based on modulation instability (MI), which is a result of the random nature of the supercontinuum generation dynamics based on higher-order optical solitons and the broadening process starting from noise.

[0117] It is known to use multi-color pumping to expand the supercontinuum spectrum and / or increase its PSD at the short-wavelength edge of the supercontinuum. A typical arrangement may include using the fundamental wave of the pump laser and its (e.g., second-order) harmonic to pump the nonlinear element. However, this method has drawbacks. For example, there is a large spectral gap between the fundamental frequency of the pump laser and its second harmonic, where there is no substantially strong radiation provided to the nonlinear element. Additionally, the fundamental wave and its harmonic are in different dispersion ranges of the nonlinear medium. This causes the laser radiation in normal dispersion to broaden rapidly over time, losing its peak intensity, and subsequently resulting in reduced spectral broadening. Moreover, several laser systems or harmonic generation stages may be required to implement this method. Thus, for high-power applications, harmonic generation in high-power laser systems is extremely challenging and technically expensive.

[0118] In single-frequency or multi-color generation, the supercontinuum is the result of the interaction of a laser pulse (or multiple laser pulses) with only a single nonlinear element stage. Thus, the spectral broadening is limited to the interaction range of only one nonlinear element and is restricted by its nonlinear properties and propagation dynamics. For example, the spectral shape in optical soliton self-compression (SSC) is determined by the phase-matching conditions for the dispersive wave via gas pressure, core diameter, etc. After such a broadening stage, the PSD will remain as provided by the nonlinear interaction.

[0119] The nonlinear medium is pumped with a laser pulse to achieve a high enough peak intensity to initiate nonlinear optical processes such as SSC or MI. However, recent advances in supercontinuum generation and nonlinear optical media have enabled the generation of a supercontinuum with a high peak intensity, which can also be sufficient to initiate nonlinear optical processes in subsequent nonlinear elements / media. For example, a high-energy, high-peak-intensity supercontinuum can be generated in an air-filled hollow-core fiber (e.g., HC-PCF) via the MI or SSC process.

[0120] Thus, it is proposed to pump a nonlinear medium / element with a supercontinuum rather than a laser pulse to generate broadband radiation. The supercontinuum should be strong enough and / or wide enough to drive further nonlinear optical phenomena in the optical nonlinear medium. In the context of the present disclosure, the supercontinuum includes radiation (e.g., originating from a laser) that subsequently undergoes at least one spectral broadening step via (or within) a nonlinear medium. The required intensity level may be related to the optical nonlinearity and optical damage of the subsequent nonlinear element stage.

[0121] The proposed radiation source includes two or more consecutive supercontinuum generation stages, each of the supercontinuum generation stages includes a respective non-linear generation element, and wherein the damage tolerance of the first non-linear generation element included in the first supercontinuum generation stage is greater than the damage tolerance of at least a second non-linear generation element included in the second supercontinuum generation stage, and wherein the second supercontinuum generation stage is immediately after the first supercontinuum generation stage.

[0122] The first supercontinuum generation stage may be arranged to generate a first supercontinuum and provide the first supercontinuum to the second supercontinuum generation stage to generate a second supercontinuum. The second supercontinuum generation stage may be the final supercontinuum generation stage, and the second supercontinuum serves as the output broadband radiation. Alternatively, in the case where the second supercontinuum generation stage is not the final supercontinuum generation stage, the second supercontinuum may be provided to the immediately following supercontinuum generation stage. Thus, each successive supercontinuum generation stage before the final supercontinuum generation stage may generate a supercontinuum for the next supercontinuum generation stage.

[0123] The radiation source may include a radiation source arranged to provide laser radiation to the first supercontinuum generation stage to generate the first supercontinuum.

[0124] In an embodiment, in the case where there are more than two consecutive supercontinuum generation stages, each successive supercontinuum generation stage may have a lower damage tolerance than its previous supercontinuum generation stage (i.e., the stages may be arranged in the order of their damage tolerances: from higher tolerance to lower tolerance). However, in other embodiments, the second supercontinuum generation stage may have a lower damage tolerance than the first supercontinuum generation stage, while the successive supercontinuum generation stages after the second stage may have any damage tolerance; for example, for the successive supercontinuum generation stages after the second stage, the damage tolerance may remain constant or become greater.

[0125] The damage tolerance may be evaluated for each respective non-linear generation element included in each supercontinuum generation stage according to a damage threshold.

[0126] There is also disclosed a radiation source, wherein the optical non-linearity of the first non-linear generation element included in the first supercontinuum generation stage is lower than the optical non-linearity of at least a second non-linear generation element included in the second supercontinuum generation stage.

[0127] In an embodiment, there may be free-space coupling between at least the first supercontinuum generation stage and the second supercontinuum generation stage. In an embodiment, there may be free-space coupling between each pair of the two or more consecutive supercontinuum generation stages.

[0128] In an embodiment, the first non-linear generating element may comprise a hollow-core optical fiber (e.g., HC-PCF). Alternatively or additionally, the second non-linear generating element may comprise a solid non-linear generating medium. The solid non-linear generating medium may comprise a solid-core optical fiber (e.g., SC-PCF), or a (synthetic) crystal (e.g., BBO crystal or PPLN crystal).

[0129] In the context of the present disclosure, the non-linear generating element may comprise a non-linear generating medium (e.g., wherein the non-linear generating medium is a solid element), or the non-linear generating element may comprise an element for confining the non-linear generating medium (e.g., wherein the non-linear generating medium is a gas, e.g., the non-linear generating element may be a hollow-core optical fiber). It will be appreciated that in either case, any non-linear generating medium may also be a liquid.

[0130] Figure 10 Schematically illustrates the concept according to a general embodiment. A driving laser DL generates driving radiation DR, which is received by a first supercontinuum generation stage SCGS1. The first supercontinuum generation stage comprises a first non-linear generating element having a first damage threshold. The first supercontinuum generation stage generates a first supercontinuum SC1, which is received by the next successive supercontinuum generation stage or second supercontinuum generation stage, the second supercontinuum generation stage comprising a second non-linear generating element having a second damage threshold less than the first damage threshold. The second supercontinuum generation stage may be the final supercontinuum generation stage SCGSn (i.e., n = 2), such that the supercontinuum output from such a supercontinuum generation stage comprises broadband output radiation SC out . Alternatively, there may be any number of intermediate supercontinuum generation stages (i.e., n > 2) between the first supercontinuum generation stage SCG1 and the final supercontinuum generation stage SCGSn, each intermediate supercontinuum generation stage receiving a supercontinuum from the immediately preceding supercontinuum generation stage and generating a supercontinuum for the immediately succeeding supercontinuum generation stage, the penultimate stage generating a supercontinuum for pumping the final supercontinuum generation stage SC n-1 . For each successive supercontinuum generation stage after the second supercontinuum generation stage (i.e., from the input to the output of the radiation source), the damage threshold of the non-linear generating element of each supercontinuum generation stage may also be reduced (or at least not increased).

[0131] Thus, relative to the optical nonlinearity of the relevant stage, each supercontinuum generated (at least before the final stage) is strong enough to drive further nonlinear optical phenomena, such as mixing and spectral broadening, in the next supercontinuum generation stage, thereby generating new enhanced supercontinuum radiation at the output of each successive supercontinuum generation stage. In such a context, enhancement can be described as having improved PSD flatness (e.g., a flatter PSD) and increased spectral coverage, especially in the supercontinuum region of interest, and / or improved PSD values.

[0132] Multiple specific examples of the proposed broadband radiation source will now be described, each of which is Figure 10 an embodiment of the broadband radiation source depicted in

[0133] Figure 11 shows an arrangement in which (at least) the first supercontinuum generation stage includes an air-filled hollow-core fiber HCF (e.g., HC-PCF). Such a hollow-core fiber HCF can generate a supercontinuum from drive radiation DR (e.g., emitted by a drive laser DL) via MI and / or SSC. In the case of MI, the generated first supercontinuum radiation SC1 has a fine temporal structure consisting of a distinct (e.g., approximately 10 femtosecond) temporal structure. The supercontinuum radiation SC1 can be received by the next supercontinuum generation stage including a solid-core fiber SCF (or other solid nonlinear medium) (such as SC-PCF). The solid-core fiber SCF can generate an output supercontinuum SC for another supercontinuum generation stage out (as depicted) or another supercontinuum (as previously mentioned, any number of such stages can exist). Since the damage threshold of the air-filled hollow-core fiber is higher than that of the solid-core PCF, the peak intensity of the spikes of the supercontinuum SC1 is high enough to drive nonlinear broadening in the second nonlinear element (such as the solid-core PCF).

[0134] In an embodiment, the first supercontinuum radiation SC1 can be coupled into the solid-core fiber SCF via free-space optics. Alternatively, the hollow-core fiber HCF can be spliced to the solid-core fiber SCF, rendering the free-space optics redundant.

[0135] In another embodiment, an MI- or SSC-based supercontinuum can drive further nonlinear effects in another gas. The following stages benefit from the fine temporal structure of the MI-based supercontinuum or the short pulses of the SSC supercontinuum in order to drive further nonlinear processes.

[0136] It has been observed that there is a large PSD drop between the spectral dispersion wave and the SPM-broadened spectrum of the first supercontinuum radiation SC1. This can lead to the output radiation SC outSome measurement applications are not suitable. To solve this problem, it is proposed to reduce the pressure of the gas (nonlinear medium) inside the hollow-core fiber HCF to below 100 bar. This step is optional and shifts the dispersive wave to a shorter wavelength. The first supercontinuum radiation is then coupled into the solid-core fiber SCF. In such a case, the spectrum of the supercontinuum SC1 includes the normal and anomalous dispersion ranges of the solid-core fiber, but this is not a necessary condition. As the pulse propagates along the solid-core fiber, the dispersive wave redshifts, thus filling the PSD dip. Additionally, the remaining part of the SPM-broadened pump undergoes four-wave mixing (FWM), and its energy is converted to other wavelengths on both the short-wavelength side and the long-wavelength side, thereby further smoothing the spectrum. These combined effects improve the PSD of the output radiation SC out of.

[0137] Figure 12 and Figure 13 Each illustrates an arrangement in which the first supercontinuum SC1 is generated via filamentation in a transparent solid or a filamentation element FE. Generating a supercontinuum via filamentation in a solid core is a good way to fabricate a compact, alignment-free, robust, i.e., solid-state broadband light source at low cost. As already described, it is desirable to improve the spectral flatness and increase the spectral coverage of the output supercontinuum SC out . The damage threshold of the proposed bulk media (e.g., the first nonlinear medium) (e.g., sapphire, fused silica, or a laser crystal such as KGW or YAG) for the filamentation element FE is quite high, e.g., up to J / cm for femtosecond pulses 2 flux level. Thus, these media can be used for (at least) the first supercontinuum generation stage and then for one or more additional supercontinuum generation stages, which include corresponding nonlinear elements with lower damage thresholds. For example, these nonlinear elements can include one or more of the solid-core fibers SCF (depicted as in Figure 12 ) or the mixing stage FMS (depicted as in Figure 13 ) to increase the bandwidth of the supercontinuum and smooth its PSD.

[0138] The mixing stage FMS can include a synthetic crystal, such as periodically poled lithium niobate (PPLN). The mixing stage can support broadband phase matching. For example, this can be achieved via quasi-phase-matched broadband second-harmonic generation. Other techniques for broadband phase matching, such as angularly dispersive supercontinuum, can also be used.

[0139] Figure 14FIG. illustrates another embodiment, in which supercontinuum generation is performed via stepwise broadening in a series of two or more (e.g., solid core) optical fibers having a core diameter that decreases for each successive stage. In the particular example shown, three such stages are provided, including a first solid core optical fiber SCF1 having a first diameter, a second solid core optical fiber SCF2 having a second diameter less than the first diameter, and a third solid core optical fiber SCF3 having a third diameter less than the second diameter. Since the laser-induced damage threshold (LIDT) in an optical fiber having a core diameter scales with i.e., , the damage threshold decreases for each successive nonlinear element or solid core optical fiber SCF1, SCF2, SCF3. This arrangement enables the utilization of the various dispersion profiles provided by different optical fibers having different core diameters. Optionally, the material constituting the solid core optical fiber may be different for one or more of the optical fibers, thus also utilizing the various dispersion profiles provided by different core materials.

[0140] The coupling between different optical fibers may all be via free space optics, may all be spliced or joined, or a combination of both (i.e., the first pair SCF1, SCF2 are free space coupled and the second pair SCF2, SCF3 are spliced or vice versa). When spliced, a connector or adapter may be provided between the optical fibers for mode filling the diameter of each successive optical fiber.

[0141] Figure 15 FIG. illustrates another embodiment, in which at least one supercontinuum generation stage after the first supercontinuum generation stage SCGS1 includes a crystal such as a barium borate (BBO) crystal BBO as the nonlinear medium. For example, the BBO crystal may include a cut angle θ c = 22° under type I phase matching. Such a BBO crystal enhances the output broadband radiation characteristics in the visible light frequency range. Notably, with a single BBO crystal used in this way, a large portion of the IR region of the output broadband radiation can be converted to visible light. In such an embodiment, the first supercontinuum generation stage SCGS1 may include, for example, a hollow core optical fiber (e.g., HC-PCF).

[0142] It can be appreciated that Figures 11 to 15 FIG. shows a non-exhaustive number of different arrangements possible in the context of the disclosure herein and the general arrangement of Figure 10 . Other arrangements are possible within the scope of the present disclosure.

[0143] In an embodiment, the feedback loop can be configured to control (e.g., adjust) the input radiation (e.g., in terms of power) based on a performance metric (e.g., the measured PSD) for each supercontinuum level; for example, this operation can be performed to prevent optical damage.

[0144] Figure 16 FIG. is a block diagram of a computer system 1600 that can assist in implementing the methods and processes disclosed herein. The computer system 1600 includes a bus 1602 or other communication mechanism for communicating information, and a processor 1604 (or processors 1604 and 1605) coupled to the bus 1602 for processing information. The computer system 1600 also includes a main memory 1606 coupled to the bus 1602 for storing information and instructions to be executed by the processor 1604, such as random access memory (RAM) or other dynamic storage. The main memory 1606 can also be used to store temporary variables or other intermediate information during the execution of instructions to be executed by the processor 1604. The computer system 1600 further includes a read-only memory (ROM) 1608 or other static storage device coupled to the bus 1602 for storing static information and instructions for the processor 1604. A storage device 1610, such as a magnetic disk or optical disk, is provided and the storage device 1610 is coupled to the bus 1602 for storing information and instructions.

[0145] The computer system 1600 can be coupled by the bus 1602 to a display 1612 for displaying information to a computer user, such as a cathode ray tube (CRT), flat panel display, or touch panel display. An input device 1614 including alphanumeric keys and other keys is coupled to the bus 1602 for communicating information and command selections to the processor 1604. Another type of user input device is a cursor control 1616 for communicating direction information and command selections to the processor 1604 and for controlling the movement of a cursor on the display 1612, such as a mouse, trackball, or cursor direction keys. Such input devices typically have two degrees of freedom in two axes, namely a first axis (e.g., x) and a second axis (e.g., y), thereby allowing the device to specify a position in a plane. A touch panel (screen) display can also be used as an input device.

[0146] One or more of the methods described herein can be performed by computer system 1600 in response to execution of one or more sequences of one or more instructions included in main memory 1606 by processor 1604. Such instructions can be read into main memory 1606 from another computer-readable medium, such as storage device 1610. Execution of the instruction sequences included in main memory 1606 causes processor 1604 to perform the process steps described herein. The instruction sequences included in main memory 1606 can also be executed using one or more processors in a multiprocessing arrangement. In alternative embodiments, hardwired circuitry can be used in place of or in combination with software instructions. Accordingly, the description herein is not limited to any specific combination of hardware circuitry and software.

[0147] The term "computer-readable medium" as used herein refers to any medium that participates in providing instructions to processor 1604 for execution. Such a medium can take many forms, including but not limited to non-volatile media, volatile media, and transmission media. Non-volatile media includes, for example, optical or magnetic disks, such as storage device 1610. Volatile media includes volatile memory, such as main memory 1606. Transmission media includes coaxial cables, copper wire, and fiber optics, including the wires that comprise bus 1602. Transmission media can also take the form of acoustic or light waves, such as those generated during radio frequency (RF) and in infrared (IR) data communications. Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, hard disk, magnetic tape, any other magnetic medium, a CD-ROM, a DVD, any other optical medium, punch cards, paper tape, any other physical medium with patterns of holes, a RAM, a PROM, and an EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described hereinafter, or any other medium from which a computer can read.

[0148] Various forms of computer-readable media can be involved in carrying one or more sequences of one or more instructions to processor 1604 for execution. For example, the instructions can initially be carried on a magnetic disk of a remote computer. The remote computer can load the instructions into its volatile memory and send the instructions over a telephone line using a modem. A modem local to computer system 1600 can receive the data on the telephone line and convert the data into an infrared signal using an infrared transmitter. An infrared detector coupled to bus 1602 can receive the data carried in the infrared signal and place the data on bus 1602. Bus 1602 carries the data to main memory 1606, from which processor 1604 retrieves and executes the instructions. The instructions received by main memory 1606 can optionally be stored on storage device 1610 either before or after execution by processor 1604.

[0149] The computer system 1600 also preferably includes a communication interface 1618 coupled to the bus 1602. The communication interface 1618 provides a two-way data communication coupling to a network link 1620 that is connected to a local area network 1622. For example, the communication interface 1618 can be an integrated services digital network (ISDN) card or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, the communication interface 1618 can be a local area network (LAN) card that provides a data communication connection to a compatible LAN. A wireless link can also be implemented. In any such implementation, the communication interface 1618 transmits and receives electrical, electromagnetic, or optical signals that carry digital data streams representing various types of information.

[0150] The network link 1620 generally provides data communication through one or more networks to other data devices. For example, the network link 1620 can provide a connection to a main computer 1624 or to a data device operated by an Internet service provider (ISP) 1626 through the local area network 1622. The ISP 1626 in turn provides data communication services through the global packet data communication network, now commonly referred to as the "Internet" 1628. Both the local area network 1622 and the Internet 1628 use electrical, electromagnetic, or optical signals that carry digital data streams. Signals through the various networks and signals on the network link 1620 and through the communication interface 1618 are exemplary forms of carrier waves that convey information, where the signals carry digital data to and from the computer system 1600.

[0151] The computer system 1600 can send messages and receive data, including program code, from the network, the network link 1620, and the communication interface 1618. In the Internet example, a server 1630 can transmit the requested code for an application through the Internet 1628, the ISP 1626, the local area network 1622, and the communication interface 1618. For example, one such downloaded application can provide one or more of the techniques described herein. The received code can be executed by the processor 1604 when it is received, and / or stored in the storage device 1610 or other non-volatile storage for later execution. In this way, the computer system 1600 can obtain application code in the form of a carrier wave.

[0152] Additional embodiments of the invention are set forth in the following numbered list of aspects:

[0153] 1. A broadband radiation source for generating output broadband radiation, comprising a plurality of supercontinuum generation stages arranged in series, each of the supercontinuum generation stages including a respective non-linear generation element;

[0154] Wherein the plurality of supercontinuum generation stages includes at least a first supercontinuum generation stage and a second supercontinuum generation stage, and in the series connection, the second supercontinuum generation stage is after the first supercontinuum generation stage; and

[0155] wherein a damage tolerance of a first nonlinear generation element included in the first supercontinuum generation stage is greater than a damage tolerance of at least a second nonlinear generation element included in the second supercontinuum generation stage.

[0156] 2. The broadband radiation source according to aspect 1, wherein the first supercontinuum generation stage is arranged to generate a first supercontinuum and provide the first supercontinuum to the second supercontinuum generation stage for generating a second supercontinuum.

[0157] 3. The broadband radiation source according to aspect 2, further comprising a laser radiation source arranged to provide laser radiation to the first supercontinuum generation stage for generating the first supercontinuum.

[0158] 4. The broadband radiation source according to any one of the preceding aspects, wherein the second supercontinuum generation stage is a final supercontinuum generation stage such that the output broadband radiation includes the second supercontinuum.

[0159] 5. The broadband radiation source according to aspect 1, 2 or 3, comprising one or more other supercontinuum generation stages, wherein the second supercontinuum is provided to a next successive supercontinuum generation stage among the one or more other supercontinuum generation stages, such that each successive supercontinuum generation stage before the final supercontinuum generation stage generates a supercontinuum for a next supercontinuum generation stage in the series connection, and the output broadband radiation includes the supercontinuum from the final supercontinuum generation stage.

[0160] 6. The broadband radiation source according to aspect 5, wherein each successive supercontinuum generation stage after the second supercontinuum generation stage includes a corresponding nonlinear generation element, and a damage tolerance of the corresponding nonlinear generation element is lower than or equal to a damage tolerance of the nonlinear generation element of a immediately preceding supercontinuum generation stage.

[0161] 7. The broadband radiation source according to aspect 5, wherein each successive supercontinuum generation stage includes a corresponding nonlinear generation element, and a damage tolerance of the corresponding nonlinear generation element is lower than a damage tolerance of the nonlinear generation element of a immediately preceding supercontinuum generation stage.

[0162] 8. The broadband radiation source according to any one of the preceding aspects, comprising free space coupling between at least the first supercontinuum generation stage and the second supercontinuum generation stage.

[0163] 9. The broadband radiation source according to any one of the foregoing aspects, including free-space coupling between all of the plurality of supercontinuum generation stages.

[0164] 10. The broadband radiation source according to any one of aspects 1 to 8, including fiber-based coupling between all of the plurality of supercontinuum generation stages.

[0165] 11. The broadband radiation source according to any one of aspects 1 to 8, wherein the first nonlinear element and the second nonlinear element are spliced together.

[0166] 12. The broadband radiation source according to aspect 11, wherein all of the corresponding nonlinear generation elements are spliced together.

[0167] 13. The broadband radiation source according to any one of the foregoing aspects, wherein at least the first nonlinear generation element includes a hollow-core fiber.

[0168] 14. The broadband radiation source according to aspect 13, wherein the hollow-core fiber includes a hollow-core photonic crystal fiber.

[0169] 15. The broadband radiation source according to any one of aspects 1 to 12, wherein at least the first nonlinear generation element includes a solid-core fiber.

[0170] 16. The broadband radiation source according to any one of aspects 1 to 12, wherein at least the first nonlinear generation element includes a filamentation element that is operable to generate a supercontinuum via filamentation.

[0171] 17. The broadband radiation source according to any one of the foregoing aspects, wherein at least the second nonlinear generation element includes a solid-state nonlinear generation element.

[0172] 18. The broadband radiation source according to aspect 17, wherein the solid-state nonlinear generation element includes a solid-core fiber.

[0173] 19. The broadband radiation source according to aspect 17, wherein the solid-state nonlinear generation element includes a crystal.

[0174] 20. The broadband radiation source according to aspect 19, wherein the solid-state nonlinear generation element includes a BBO crystal.

[0175] 21. The broadband radiation source according to any one of aspects 1 to 17, wherein at least the second supercontinuum generation stage includes a mixing stage.

[0176] 22. The broadband radiation source according to aspect 21, wherein the second supercontinuum generation stage supports broadband phase matching.

[0177] 23. The broadband radiation source according to aspect 21 or 22, wherein the second nonlinear generation element comprises periodically poled lithium niobate.

[0178] 24. The broadband radiation source according to any one of aspects 1 to 12, wherein each of the plurality of supercontinuum generation stages comprises a respective optical fiber, each optical fiber having a diameter, and the diameter is decreased for each successive supercontinuum generation stage.

[0179] 25. The broadband radiation source according to any of the foregoing aspects, wherein the output broadband radiation comprises wavelengths of at least between 100 nm and 1200 nm.

[0180] 26. The broadband radiation source according to any of the foregoing aspects, wherein the output broadband radiation comprises wavelengths in a range of at least between 400 nm and 900 nm.

[0181] 27. A broadband radiation source for generating output broadband radiation, comprising a plurality of supercontinuum generation stages arranged in series, each of the supercontinuum generation stages comprising a respective nonlinear generation element; wherein the plurality of supercontinuum generation stages comprises at least a first supercontinuum generation stage and a second supercontinuum generation stage, and in the series, the second supercontinuum generation stage is after the first supercontinuum generation stage; and wherein the optical nonlinearity of a first nonlinear generation element included in the first supercontinuum generation stage is lower than the optical nonlinearity of at least a second nonlinear generation element included in the second supercontinuum generation stage.

[0182] 28. A measurement device, comprising a radiation source according to any of the foregoing aspects.

[0183] 29. The measurement device according to aspect 28, comprising a scatterometer measurement device, a level sensor or an alignment sensor.

[0184] Although specific reference may be made herein 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, and the like.

[0185] Although embodiments of the invention may be specifically described herein with reference to 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 for measuring or processing an object such as a wafer (or other substrate) or a mask (or other patterning device). Such apparatus may generally be referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0186] Although the above may specifically refer to the use of embodiments of the invention in the context of optical lithography, it should be understood that the invention is not limited to optical lithography and may be used in other applications (such as imprint lithography) where the context allows.

[0187] Although specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced in other ways different from those described. The foregoing description is intended to be illustrative, not restrictive. Thus, those skilled in the art will appreciate that the described invention may be modified without departing from the scope of the claims set forth below.

Claims

1. A broadband radiation source for generating output broadband radiation, comprising a plurality of supercontinuum generation stages arranged in series, each of the supercontinuum generation stages including a respective nonlinear generation element; Among them, The plurality of supercontinuum generation stages includes at least a first supercontinuum generation stage and a second supercontinuum generation stage, and in the series, the second supercontinuum generation stage is after the first supercontinuum generation stage; And wherein, the damage tolerance of a first nonlinear generation element included in the first supercontinuum generation stage is greater than the damage tolerance of at least a second nonlinear generation element included in the second supercontinuum generation stage.

2. The broadband radiation source according to claim 1, wherein, The first supercontinuum generation stage is arranged to generate a first supercontinuum and provide the first supercontinuum to the second supercontinuum generation stage for generating a second supercontinuum.

3. The broadband radiation source according to claim 2, further comprising a laser radiation source arranged to provide laser radiation to the first supercontinuum generation stage for generating the first supercontinuum.

4. The broadband radiation source according to claim 1, wherein, The second supercontinuum generation stage is a final supercontinuum generation stage such that the output broadband radiation includes the second supercontinuum.

5. The broadband radiation source according to claim 1 further comprises one or more additional supercontinuum generation stages, wherein, The second supercontinuum is provided to the next successive supercontinuum generation stage among the one or more other supercontinuum generation stages such that each successive supercontinuum generation stage before the final supercontinuum generation stage generates a supercontinuum for the next supercontinuum generation stage in the series, and the output broadband radiation includes the supercontinuum from the final supercontinuum generation stage.

6. The broadband radiation source according to claim 5, wherein, Each successive supercontinuum generation stage after the second supercontinuum generation stage includes a respective nonlinear generation element, and the damage tolerance of the respective nonlinear generation element is lower than or equal to the damage tolerance of the nonlinear generation element of the immediately preceding supercontinuum generation stage.

7. The broadband radiation source according to claim 5, including free space coupling between at least the first supercontinuum generation stage and the second supercontinuum generation stage.

8. The broadband radiation source according to claim 1, including fiber-based coupling between the first supercontinuum generation stage and the second supercontinuum generation stage.

9. The broadband radiation source according to claim 1, wherein At least the first nonlinear generation element includes a hollow core fiber.

10. The broadband radiation source according to claim 9, wherein, The hollow core fiber is a hollow core photonic crystal fiber.

11. The broadband radiation source according to claim 1, wherein, At least the first nonlinear generation element includes a filamentation element that can be operated to generate a supercontinuum via filamentation.

12. The broadband radiation source according to claim 1, wherein, At least the second nonlinear generation element includes a solid nonlinear generation element.

13. The broadband radiation source according to claim 12, wherein, The solid nonlinear generation element includes a solid core fiber.

14. The broadband radiation source according to claim 12, wherein, The solid nonlinear generation element includes a crystal.

15. The broadband radiation source according to claim 1, wherein, At least the second supercontinuum generation stage includes a mixing stage.

16. The broadband radiation source according to claim 1, wherein, Each of the plurality of supercontinuum generation stages includes a respective optical fiber, and each optical fiber has a diameter that decreases for each successive supercontinuum generation stage.

17. A measuring device, comprising a radiation source according to claim 1.

18. The measuring device according to claim 17, including a scatterometer measuring device, a level sensor or an alignment sensor.

19. An inspection device, comprising a radiation source according to claim 1.

20. The inspection device according to claim 19, wherein the inspection device is configured to identify defects on a substrate.

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