Radiation source based on hollow core fiber

Abstract

A broadband radiation device, comprising an HC-PCF, the HC-PCF comprising: a hollow core extending along the length of the HC-PCF for confining, in use, a working medium at a pressure; an input operable to receive pulsed pump radiation; and an output, the output being operable to emit broadband output radiation resulting from a spectral broadening of the pulsed pump radiation within the working medium confined within the HC-PCF; wherein the HC-PCF is divided into a first section and a second section; wherein the first section comprises the input end and at least a portion of the first section comprises a curved portion and / or one or more coiled portions, and the second section comprises the output end and is substantially straight; and wherein, within the first section and the second section, the spectral broadening is dominated by a self-phase modulation process and a different non-linear optical process, respectively.

Description

[0001] Cross Reference to Related Applications

[0002] This application claims priority to European application 23161377.9, filed on March 13, 2023, and the entire contents of which are incorporated herein by reference. TECHNICAL FIELD

[0003] The present invention relates to a broadband radiation source based on a hollow core fiber, and in particular to such a broadband radiation source in relation to metrology applications in the manufacture of integrated circuits. BACKGROUND

[0004] A lithography apparatus is a machine constructed to apply a desired pattern onto a substrate. The lithography apparatus can be used, for example, in the manufacture of integrated circuits (ICs). The lithography apparatus can, for example, project a pattern (which is also often referred to as a “design layout” or “design”) at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).

[0005] To project a pattern on the substrate, the lithography apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm. A lithography apparatus that uses extreme ultraviolet (EUV) radiation, having a wavelength within a range of 4-20 nm (for example 6.7 nm or 13.5 nm) can be used to form smaller features on a substrate than a lithography apparatus that uses, for example, radiation with a wavelength of 193 nm.

[0006] Low-ki lithography can be used to process features with dimensions smaller than the classical resolution limit of a lithographic apparatus, which is determined by k-i times the wavelength of the radiation employed, times a reduction factor of the projection optics. In such a process, the resolution formula can be expressed as CD = k-i x lambda / NA, where lambda is the wavelength of the radiation employed, NA is the numerical aperture of the projection optics, CD is the "critical dimension" (generally the smallest feature size that is typically used to characterize the resolution of the apparatus but in this case it is the half pitch), and ki is an empirical resolution factor. In general, the smaller ki the more difficult it becomes to reproduce the shape and dimensions planned by a circuit designer in the substrate. To overcome these difficulties, sophisticated fine-tuning steps can be applied to the lithography projection apparatus and / or the design layout. These include, for example, but are not limited to, optimization of NA, customized illumination schemes, use of phase- shift patterning devices, various optimization of the design layout such as optical proximity correction (OPC, sometimes also referred to as "optical and process correction") in the design layout, or other methods generally defined as "resolution enhancement techniques" (RET). Alternatively, tight control loops used to control the stability of the lithography apparatus can be used to improve the reproduction of the pattern at low ki.

[0007] Metrology tools are used in many aspects of the IC manufacturing process, for example as alignment tools for properly positioning substrates prior to exposure, leveling tools for measuring the surface topography of substrates, focus control and scatterometry based tools for inspecting / measuring exposed and / or etched products, for example in process control. In each case, a radiation source is needed. For various reasons including measurement robustness and accuracy, broadband or white light radiation sources are increasingly used for such metrology applications. It would be desirable to improve existing apparatus for broadband radiation generation. SUMMARY

[0008] In a first aspect of the application there is provided a broadband radiation device comprising a hollow core photonic crystal fibre HC-PCF, the HC-PCF comprising: a hollow core extending along a length of the HC-PCF for, in use, confining a working medium at a pressure; an input end operable to receive pulsed pump radiation; and an output end operable to emit broadband output radiation resulting from spectral broadening of the pulsed pump radiation within the working medium confined within the HC-PCF; wherein the HC-PCF is divided into a first section extending over a first portion of the length of the HC-PCF and a second section extending over a second portion of the length of the HC-PCF, wherein the first section comprises the input end and at least a portion of the first section comprises a bend and / or one or more windings, and the second section comprises the output end and is substantially straight; and wherein the HC-PCF is configured such that, within the first section, the spectral broadening occurs primarily via a self-phase modulation process, and within the second section, the spectral broadening occurs primarily via a nonlinear optical process different from a self-phase modulation process.

[0009] In a second aspect of the application there is provided a method for generating broadband output radiation, comprising: receiving pulsed pump radiation at an input end of a hollow core photonic crystal fibre HC-PCF, the HC-PCF having a hollow core confining a working medium at a pressure; and emitting broadband output radiation at an output end of the HC-PCF, the broadband output radiation resulting from spectral broadening of the pulsed pump radiation within the working medium confined within the HC-PCF; wherein the HC-PCF is divided into a first section extending over a first portion of the length of the HC-PCF and a second section extending over a second portion of the length of the HC-PCF, wherein the first section comprises the input end and at least a portion of the first section comprises a bend and / or one or more windings, and the second section comprises the output end and is substantially straight; and wherein the HC-PCF is configured such that, within the first section, the spectral broadening occurs primarily via a self-phase modulation process, and within the second section, the spectral broadening occurs primarily via a nonlinear optical process different from a self-phase modulation process.

[0010] Other aspects of the application include metrology devices comprising the broadband radiation device of the first aspect. BRIEF DESCRIPTION OF DRAWINGS

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

[0012] - Figure 1 A schematic schematic diagram depicting a photolithography apparatus;

[0013] - Figure 2 A schematic schematic diagram depicting a lithography system including an EUV radiation source and EUV lithography equipment or scanner;

[0014] - Figure 3 A schematic schematic diagram depicting a photolithography unit;

[0015] - Figure 4 A schematic representation depicting overall photolithography, illustrating the collaboration between three key technologies for optimizing semiconductor manufacturing;

[0016] - Figure 5 A schematic diagram depicting a scattering measurement apparatus, which may include a radiation source and serve as a measuring device, according to an embodiment of the present invention;

[0017] - Figure 6 A schematic schematic diagram depicting a horizontal sensor device that may include a radiation source, according to an embodiment of the present invention;

[0018] - Figure 7 A schematic schematic diagram depicting an alignment sensor device that may include a radiation source according to an embodiment of the present invention;

[0019] - Figure 8 It is a schematic cross-sectional view of a hollow core optical fiber in which a portion of the radiation source according to the embodiment can be formed in a transverse plane (i.e., perpendicular to the axis of the optical fiber).

[0020] - Figure 9 A schematic representation of a radiation source according to an embodiment for providing broadband output radiation;

[0021] - Figure 10 (a) and Figure 10 (b) A schematic depiction of a cross-section of an example hollow-core photonic crystal fiber (HC-PCF) design for supercontinuum generation;

[0022] Figure 11(a) shows a wavelength λ plot for simulation relative to the normalized position NP along the fiber length, the wavelength λ plot describing, for example, in... Figure 9 The spectral evolution of the pulse of input radiation within the HC-PCF of the radiation source is shown in the figure, based on modulation instability.

[0023] Figure 11(b) shows a wavelength λ plot for simulation relative to the normalized position NP along the fiber length, the wavelength λ plot describing, for example, in... Figure 9spectral evolution of a pulse of input radiation within the HC-PCF of the radiation source shown in

[0024] - Figure 12 (a) shows a plot of bandwidth expansion / increase rate BWR versus normalized position NP along the length of the optical fiber for the simulation, the BWR plot describing how the rate of increase of the spectral bandwidth of a pulse of input radiation varies as the pulse propagates along the HC-PCF of the radiation source shown in Figure 9

[0025] - Figure 12 (b) shows a plot of bandwidth expansion / increase rate BWR versus normalized position NP along the length of the optical fiber for the simulation, the BWR plot describing how the rate of increase of the spectral bandwidth of a pulse of input radiation varies as the pulse propagates along the HC-PCF of the radiation source shown in Figure 9

[0026] - Figure 13 (a) to Figure 13 (i) shows a set of different simulation plots, where each plot illustrates the numerically simulated bend loss BL of the HC-PCF (shown in, for example, Figure 8

[0027] - Figures 14(a) to 14(e) schematically depict five embodiments of a broadband radiation source RDS in which at least a portion of an initial or first section of the HC-PCF is bent or coiled, and the HC-PCF is at least partially enclosed in one or more gas chambers configured in different ways for the different embodiments; and

[0028] - Figure 15 a block diagram depicting a computer system for controlling a broadband radiation source. DETAILED DESCRIPTION

[0029] In this document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. having a wavelength of about 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range of about 5-100 nm).

[0030] ​​​The terms "reticle", "mask" or "patterning device" as employed herein can be broadly interpreted as referring to any patterning device that can be used to impart a pattern to a beam of radiation, which can be used to produce a patterned beam of radiation onto a substrate at a target portion of the substrate. In such contexts, the term "light valve" can also be used. Examples of other such patterning devices include a programmable mirror array and a programmable LCD array.

[0031] Figure 1 A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA includes an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g., UV, DUV or EUV radiation), a mask support (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters, a substrate support (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support in accordance with certain parameters, and a projection system (e.g., a refractive, catadioptric, or catoptric projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., including one or more dies) of the substrate W.

[0032] In operation, the illumination system IL receives a radiation beam from a radiation source SO, e.g., via a beam delivery system BD. The illumination system IL can include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof, for directing, shaping, and / or controlling the radiation. The illuminator IL can be used to adjust the angular intensity distribution of the radiation beam B, e.g., to have a desired spatial and angular intensity distribution at the plane of the patterning device MA.

[0033] The term "projection system" PS used herein should be broadly interpreted as encompassing various types of projection system, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic systems, or any combination thereof, as appropriate for the exposure radiation being used, and / or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein can be considered as synonymous with the more general term "projection system" PS.

[0034] The lithography apparatus LA can be of a type wherein at least a portion of the substrate can be covered by a liquid having a relatively high refractive index (e.g., water) to fill a space between the projection system PS and the substrate W, also termed an immersion lithography. More information on immersion techniques is given in US 6952253, incorporated herein by reference.

[0035] The lithography apparatus LA can also be of a type having two or more substrate supports WT (also termed "dual stage"). In such "multi-stage" machines the substrate supports WT can be used in parallel, and / or a substrate W on one of the substrate supports WT can be prepared for a next exposure of the substrate W, while another substrate W on another substrate support WT is being exposed to the patterned beam B.

[0036] In addition to the substrate support WT, the lithography apparatus LA can also include a measurement platform. The measurement platform is arranged to hold a sensor and / or a cleaning device. The sensor can be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement platform can hold multiple sensors. The cleaning device can be arranged to clean a part of the lithography apparatus, for example a part of the projection system PS or a part of the system providing the immersion liquid. The measurement platform can be movable under the projection system PS when the substrate support WT is distanced from the projection system PS.

[0037] In operation, the radiation beam B is incident on the patterning device (e.g., mask) MA held on the mask support MT and patterned by the pattern present on the patterning device MA. Having traversed the mask MA, the radiation beam B passes through the projection system PS which focuses the beam onto a target portion C of the substrate W. By means of the second positioners PW and position measurement system IF, the substrate support WT can be accurately moved, for example, so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioners PM and possibly another position sensor (not explicitly depicted in Figure 1 The patterning device MA and the substrate W can be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. While the substrate alignment marks P1, P2 as illustrated occupy a dedicated target portion, they can also be located in spaces between target portions. When the substrate alignment marks P1, P2 are located between target portions C, they are referred to as scribe-lane alignment marks.

[0038] Figure 2A lithographic system is shown comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.

[0039] The illumination system IL is configured to condition the EUV radiation beam B before it enters the patterning device MA. In addition, the illumination system IL can include a facetted field mirror device 10 and a facetted pupil mirror device 11. The facetted field mirror device 10 and the facetted pupil mirror device 11 collectively provide the EUV radiation beam B having a desired cross-sectional shape and a desired intensity distribution. The illumination system IL can also include other mirrors or devices in addition to, or instead of, the facetted field mirror device 10 and the facetted pupil mirror device 11.

[0040] After being so conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For this purpose, the projection system PS can include a plurality of mirrors 13, 14 configured to project the patterned EUV radiation beam B’ onto a substrate W held by the substrate table WT. The projection system PS can apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image having features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 can be applied. Although the projection system PS is illustrated in Figure 2 as having only two mirrors 13, 14, the projection system PS can include a different number of mirrors (e.g., six or eight mirrors).

[0041] The substrate W can include a previously formed pattern. In such cases, the lithographic apparatus LA aligns the image formed by the patterned EUV radiation beam B’ with the pattern previously formed on the substrate W.

[0042] A relative vacuum, i.e., a small amount of gas (e.g., hydrogen) at a pressure well below atmospheric pressure, can be provided in the radiation source SO, in the illumination system IL and / or in the projection system PS.

[0043] The radiation source SO can be a laser-produced plasma (LPP) source, a discharge-produced plasma (DPP) source, a free electron laser (FEL) or any other radiation source capable of generating EUV radiation.

[0044] As Figure 3As shown in Figure 1, the lithography apparatus LA can form part of a lithocell LC (sometimes also referred to as a litho cell or litho cluster), which also typically includes apparatus for performing pre-exposure and post-exposure processes on the substrate W. Conventionally, these apparatus include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH and a bake plate BK, for example, for adjusting the temperature of the substrate W (for example, for adjusting the solvent in the resist layer). A substrate handling device or robot RO picks up substrates W from input / output ports I / O1, I / O2, moves the substrates between the different process apparatus, and delivers the substrates W to a load deck LB of the lithography apparatus LA. The apparatus in the lithocell, which are often collectively referred to as a track or track and develop system, are typically under the control of a track or track and develop system control unit TCU, which itself can be controlled by a management control system SCS, which can also control the lithography apparatus LA, for example, via a lithography control unit LACU.

[0045] In order to expose substrates W by the lithography apparatus LA correctly and consistently, it is desirable to detect the substrates to measure properties of the patterned structures, such as overlay errors between subsequent layers, line thicknesses, critical dimensions (CD), etc. For this purpose, an inspection tool (not shown) can be included in the lithocell LC. If errors are detected, especially in the case where inspection is performed before other substrates W of the same batch or lot are still to be exposed or processed, adjustments can be made, for example, to the exposure of subsequent substrates or to other processing steps to be performed on the substrates W.

[0046] The inspection apparatus, which can also be referred to as a metrology apparatus, is used to determine properties of the substrates W, and in particular, how properties of different substrates W vary or how properties associated with different layers of the same substrate W vary from layer to layer. The inspection apparatus can alternatively be configured to identify defects on the substrates W and can for example be part of the lithocell LC, or can be integrated into the lithography apparatus LA, or even be a separate device. The inspection apparatus can measure properties on a latent image (image in a resist layer after exposure), or on a semi-latent image (image in a resist layer after a post-exposure bake step PEB), or on a developed resist image (where either the exposed or unexposed parts of the resist have been removed), or even on an etched image (after a pattern transfer step such as etching).

[0047] Typically, the patterning process in the lithography apparatus LA is one of the most critical steps in the process that requires high accuracy of the sizing and placement of structures on the substrate W. To ensure such high accuracy, three systems are combined in the lithography apparatus LA Figure 4These systems are one system, the lithography apparatus LA, which is (in effect) connected to a metrology tool MT (second system) and to a computer system CL (third system). The key to this "holistic" environment is to optimize the cooperation between these three systems to enhance the overall process window and to provide a tight control loop to ensure that the patterning performed by the lithography apparatus LA lies within the process window. The process window defines a range of process parameters (e.g., dose, focus, overlay) within which a certain manufacturing process results in a defined outcome (e.g., functional semiconductor devices) - typically, within the range of process parameters, variations in the process parameters in the lithography or patterning process are allowed.

[0048] The computer system CL can use (parts of) the design layout to be patterned to predict which resolution enhancement techniques to use and to perform computational lithography simulations and calculations to determine which mask layouts and lithography apparatus settings achieve the largest overall process window for the patterning process (in Figure 4 depicted by the double arrow in the first scale SC1). Typically, the resolution enhancement techniques are arranged to match the patterning possibilities of the lithography apparatus LA. The computer system CL can also be used to detect where the lithography apparatus LA is currently operating within the process window (e.g., using input from the metrology tool MT) to predict whether defects can exist due to, for example, sub-optimal processing (in Figure 4 depicted by the arrow pointing to "0" in the second scale SC2).

[0049] The metrology tool MT can provide input to the computer system CL to enable accurate simulations and predictions, and can provide feedback to the lithography apparatus LA to identify possible drifts in, for example, the calibration status of the lithography apparatus LA (in Figure 4 depicted by the multiple arrows in the third scale SC3).

[0050] In a lithographic process, it is desirable to frequently measure the resulting structure, for example for process control and verification. The tools used for such measurements are commonly referred to as metrology tools MT. Different types of metrology apparatuses MT for making such measurements are well known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. Scatterometers are versatile instruments allowing to measure parameters of the lithographic process by having a sensor in a pupil or a plane conjugate to the pupil of the objective of the scatterometer, the measurement being commonly referred to as pupil-based measurement, or by having a sensor in an 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. Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP 1,628,164A, incorporated herein in their entirety by reference. The aforementioned scatterometers can use light from the soft x-ray and visible to near IR wavelength range to measure gratings.

[0051] In a first embodiment, the scatterometer MT is an angle resolved scatterometer. In such a scatterometer, a reconstruction method can be applied to the measurement signal to reconstruct or calculate properties of the grating. This reconstruction can for example result from simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulated 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.

[0052] 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 the target and the reflected or scattered radiation from the target is directed to a spectrometer detector which measures the spectrum of the specular reflected radiation (i.e. measures the intensity as a function of wavelength). From this 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 library of simulated spectra.

[0053] In a third embodiment, scatterometer MT is an ellipsometry scatterometer. Ellipsometry scatterometers allow for determining parameters of a lithographic process by measuring the scattered radiation for each polarization state. Such metrology apparatuses emit polarized light, such as linear, circular or elliptical, by using, for example, appropriate polarization filters in the illumination section of the metrology apparatus. The source adapted to the metrology apparatus can also provide polarized radiation. Various embodiments of existing ellipsometry scatterometers are described in U.S. Patent Applications 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,449, 12 / 920,968, 12 / 922,587, 13 / 000,229, 13 / 033,135, 13 / 533,110, and 13 / 891,410, which are incorporated herein by reference in their entirety.

[0054] In one embodiment of scatterometer MT, scatterometer MT is adapted to measure the overlay of two misaligned gratings or periodic structures by measuring the reflection spectrum and / or detecting asymmetry in the configuration, which is related to the range of the overlay. The two (typically superimposed) grating structures can be applied in two different layers (not necessarily consecutive layers) and can be formed in substantially the same position on the wafer. The scatterometer can have a symmetric detection configuration as described, for example, in commonly owned patent application EP 1,628,164 A, so that any asymmetry can be clearly identified. This provides a direct way of measuring misalignment in the gratings. Further examples of measuring overlay error between two layers comprising periodic structures as a target measured via asymmetry in the periodic structures can be found in PCT patent application publication number WO 2011 / 012624 or U.S. patent application number US 20160161863, which are incorporated herein by reference in their entirety.

[0055] Other parameters of interest can be focus and dose. Focus and dose can be determined simultaneously by scatterometry as described in U.S. patent application US2011-0249244, incorporated herein by reference in its entirety, or alternatively by scanning electron microscopy. A single structure with a unique combination of critical dimension and side wall angle measurements for each point in the focus energy matrix (FEM, also known as focus exposure matrix) can be used. If these unique combinations of critical dimension and side wall angle can be obtained, then the focus and dose values can be uniquely determined from these measurements.

[0056] The metrology target can be a set of composite gratings formed primarily in resist by a lithographic process and also after e.g. an etching process. Typically, the pitch and line width of the structures in the grating are largely dependent on the measurement optics, especially the NA of the optics, to be able to capture the diffraction orders from the metrology target. As already indicated earlier, the diffraction signal can be used to determine the shift between two layers, also referred to as "overlay", or can be used to reconstruct at least part of the original grating as produced by the lithographic process. Such reconstruction can be used to provide guidance on the quality of the lithographic process and can be used to control at least part of the lithographic process. The target can have smaller sub-segments configured to mimic the dimensions of the functional part of the design layout in the target. Due to such sub-segments, the target will behave more similar to the functional part of the design layout, so that the overall process parameter measurement better resembles the functional part of the design layout. The target can be measured in an underfill mode or in an overfill mode. In the underfill mode, the measurement beam generates a spot smaller than the overall target. In the overfill mode, the measurement beam generates a spot larger than the overall target. In such overfill mode, it is also possible to measure different targets simultaneously, thus determining different process parameters simultaneously.

[0057] The overall measurement quality of a lithographic parameter using a particular target is determined at least partly by the measurement recipe used to measure such lithographic parameter. The term "substrate measurement recipe" can include one or more parameters of the measurement itself, one or more parameters of the pattern measured, or both. For example, if the measurement used in the substrate measurement recipe is a diffraction-based optical measurement, one or more of the parameters of the measurement 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 used to select the measurement recipe can for example be the sensitivity of one of the measurement parameters to process variations. More examples are described in US patent application US2016-0161863 and published US patent application US 2016 / 0370717 Al, which are incorporated herein in their entirety.

[0058] Figure 5 A metrology apparatus such as a scatterometer is depicted. The scatterometer comprises a broadband (white light) radiation projector 2 which projects radiation onto a substrate W. The reflected or scattered radiation passes to a spectrometer detector 4 which measures the spectrum 6 of the specular reflected radiation (i.e. a measurement of intensity as a function of wavelength). From this data, a structure or profile 8 of the detected spectrum can be generated by a processing unit PU, for example by rigorous coupled wave analysis and non-linear regression, or by comparison with Figure 5reconstructed from a comparison of the simulated spectral library shown at the bottom. Typically, for reconstruction, the general form of the structure is known and some parameters are assumed from knowledge of the process used to make the structure, leaving only a few parameters of the structure to be determined from the scatterometry data. Such a scatterometer can be configured as a normal-incidence scatterometer or an oblique-incidence scatterometer.

[0059] The overall measurement quality of measuring lithographic parameters of a metrology target is at least partially determined by the measurement recipe used to measure such lithographic parameters. The term "substrate measurement recipe" can include one or more parameters of the measurement itself, one or more parameters of the pattern(s) measured, or both. For example, if the measurement used in the substrate measurement recipe is a diffraction-based optical measurement, one or more of the parameters of the measurement 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 used to select the measurement recipe can be, for example, the sensitivity of one of the measurement parameters to process variations. More examples are described in U.S. Patent Application US 2016 / 0161863 and published U.S. Patent Application US 2016 / 0370717 Al, which are incorporated herein by reference in their entirety.

[0060] Another type of metrology tool used in IC manufacturing is a topography measurement system, level sensor, or height sensor. Such a tool can be integrated in a lithographic apparatus for measuring the topography of the top surface of a substrate (or wafer). A topography map (also referred to as a height map) of a substrate can be generated from these measurements indicating the height of the substrate as a function of position on the substrate. This height map can then be used to correct the position of the substrate during transfer of a pattern onto the substrate in order to provide a spatial image of the patterning device at the correct focus position on the substrate. It will be appreciated that "height" in this context refers generally to the dimension out-of-plane to the substrate (also referred to as the Z-axis). Typically, 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.

[0061] Figure 6 An example of a level or height sensor LS known in the art is schematically shown in Fig. 1, Figure 6Only the principle of operation is illustrated. In this example, the level sensor comprises an optical system comprising a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO providing a radiation beam LSB which is endowed with a 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 polarized or non-polarized, pulsed or continuous supercontinuum light source, such as a polarized or non-polarized laser beam. The radiation source LSO can comprise a plurality of radiation sources with different colors or wavelength ranges, such as a plurality of LEDs. The radiation source LSO of the level sensor LS is not limited to visible light radiation, but can additionally or alternatively encompass UV and / or IR radiation and any wavelength range suitable for reflection from the surface of the substrate.

[0062] The projection grating PGR is a periodic grating comprising a periodic structure which generates a radiation beam BE1 with a periodically varying intensity. The radiation beam BE1 with the periodically varying intensity is directed towards a measurement location MLO on the substrate W with an angle of incidence ANG between 0 and 90 degrees, typically between 70 and 80 degrees, with respect to an axis (Z-axis) perpendicular to the surface of the incident substrate. At the measurement location MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by the arrow BE2) and directed towards the detection unit LSD.

[0063] For determining the height level at the measurement location MLO, the level sensor further comprises a detection system comprising a detection grating DGR, a detector DET and a processing unit (not shown) for processing the output signal of the detector DET. The detection grating DGR can be identical to the projection grating PGR. The detector DET generates a detector output signal which is indicative of the received light, for example indicative of the intensity of the received light, such as a photodetector, or representing the spatial distribution of the received intensity, such as a camera. The detector DET can comprise any combination of one or more detector types.

[0064] By means of triangulation techniques, the height level at the measurement location MLO can be determined. The detected height level is typically related to a signal intensity measured by the detector DET which has a periodicity which depends, among others, on the design of the projection grating PGR and the (inclined) angle of incidence ANG.

[0065] The projection unit LSP and / or the detection unit LSD can comprise further 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).

[0066] In embodiments, the detection grating DGR can be omitted and the detector DET can be placed where the detection grating DGR is located. This configuration provides a more direct detection of the image of the projection grating PGR.

[0067] To effectively cover the surface of the substrate W, the level sensor LS can be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating an array of measurement regions MLO or spots covering a larger measurement range.

[0068] Various height sensors of the general type are disclosed, for example, in both US7265364 and US7646471, which are incorporated by reference. In US2010233600A1, which is incorporated by reference, a height sensor is disclosed that uses UV radiation instead of visible or infrared radiation. In WO2016102127A1, which is incorporated by reference, a compact height sensor is described that uses a multi-element detector to detect and identify the position of a grating image without the need for a detection grating.

[0069] Another type of metrology tool used in IC manufacturing is an alignment sensor. A key aspect of the performance of a lithography apparatus is therefore the ability to place the applied pattern correctly and accurately with respect to features placed in previous layers (by the same apparatus or by a different lithography apparatus). To this end, 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 can be referred to as an “alignment sensor”, and the marks can be referred to as “alignment marks”.

[0070] A lithography apparatus can include one or more (e.g., multiple) alignment sensors by which the position of alignment marks provided on a substrate can be accurately measured. Alignment (or position) sensors can use optical phenomena such as diffraction and interference to obtain position information from alignment marks formed on a substrate. An example of an alignment sensor used in current lithography apparatuses is based on a self-referencing interferometer as described in US6961116. Various enhancements and modifications of position sensors have been developed, for example as disclosed in US2015261097A1. All of these disclosures are incorporated herein by reference.

[0071] Figure 7is a schematic block diagram of an embodiment of a known alignment sensor AS such as described in, for example, US6961116 and incorporated by reference. A radiation source RSO provides a beam of radiation RB having one or more wavelengths, which is turned by turning optics as an illumination spot SP onto a mark such as a mark AM located on a substrate W. In this example, the turning optics comprise a spot mirror SM and an objective lens OL. The diameter of the illumination spot SP illuminating the mark AM can be slightly smaller than the width of the mark itself.

[0072] Radiation diffracted by the alignment mark AM (in this example via the objective lens OL) is collimated into an information-carrying beam IB. The term "diffracted" is intended to include zeroth order diffraction from the mark (zeroth order diffraction can be referred to as reflection). A self-referencing interferometer SRI of the type disclosed in, for example, US6961116 mentioned above interferes the beam IB with itself, after which the beam is received by a photodetector PD. Additional optics (not shown) can be included to provide separate beams in the case that more than one wavelength is generated by the radiation source RSO. The photodetector can be a single element, or the photodetector can comprise multiple pixels as required. The photodetector can comprise a sensor array.

[0073] The turning optics comprising the spot mirror SM in this example can also be used to block zeroth order radiation reflected from the mark, so that the information-carrying beam IB includes only higher order diffracted radiation from the mark AM (this is not essential for the measurement, but improves the signal to noise ratio).

[0074] The intensity signal SI is supplied to a processing unit PU. By combining the optical processing carried out in module SRI with the computational processing carried out in unit PU, values of the X and Y positions of the substrate relative to a reference frame are output.

[0075] A single measurement of the type illustrated fixes the position of the mark to within a certain range corresponding to one period of the mark only. Coarser measurement techniques are used in conjunction with such measurements to identify which period of the sinusoidal wave is the one that includes the position of the mark. The same process at coarser and / or finer levels can be repeated at different wavelengths for improved accuracy and / or for robust detection of the mark regardless of the material from which the mark is made and the material provided above and / or below it. The wavelengths can be optically multiplexed and demultiplexed for simultaneous processing, and / or the wavelengths can be multiplexed by time division or frequency division.

[0076] In this example, the alignment sensor and the spot SP are kept stationary, while the substrate W is moved. The alignment sensor can thus be rigidly and accurately mounted to a reference frame, while effectively scanning the mark AM in a direction opposite to the direction of movement of the substrate W. The substrate W is in this movement controlled by its mounting on a substrate support and by a substrate positioning system controlling the movement of the substrate support. A substrate support position sensor (e.g. an interferometer) measures the position of the substrate support (not shown). In embodiments, one or more (alignment) marks are provided on the substrate support. The measurement of the position of the marks provided on the substrate support allows for calibrating the position of the substrate support (e.g. relative to the frame to which the alignment system is connected) as determined by the position sensor. The measurement of the position of the alignment marks provided on the substrate allows for determining the position of the substrate relative to the substrate support.

[0077] The metrology tools MT mentioned above, such as scatterometers, topography measurement systems or position measurement systems, can use radiation originating from a radiation source to perform measurements. The properties of the radiation used by the metrology tools can influence the type and quality of measurements that can be performed. For some applications, it can be advantageous to use multiple radiation frequencies to measure a substrate, e.g. broadband radiation can be used. Multiple different frequencies can be able to propagate, irradiate a metrology target and scatter off the metrology target without or with minimal interference of other frequencies. Thus, more metrology data can be obtained simultaneously, e.g. using different frequencies. Different radiation frequencies can also be able to interrogate and discover different properties of a metrology target. Broadband radiation can be used in metrology systems MT such as e.g. level sensors, alignment mark measurement systems, scatterometry tools or inspection tools. A broadband radiation source can be a supercontinuum source.

[0078] High quality broadband radiation, e.g. supercontinuum radiation, can be difficult to generate. One method for generating broadband radiation can be to broaden high power narrowband or single frequency input radiation or pump radiation, e.g. using nonlinear higher order effects. The input radiation (which can be produced 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 small regions such that a highly localized high intensity radiation is achieved. In those regions, the radiation can interact with broadening structures and / or materials forming a nonlinear medium in order to form broadband output radiation. Different materials and / or structures can be used in the high intensity radiation regions to achieve and / or improve the radiation broadening by providing a suitable nonlinear medium.

[0079] In some embodiments, 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, which helps to confine radiation traveling through the fiber in the fiber core. The fiber core can be made of a solid material that has nonlinear properties and is capable of generating broadband radiation when high intensity pump radiation is transmitted through the fiber core. While it is possible to generate broadband radiation in a solid core photonic crystal fiber, there can be several disadvantages of using a solid material. For example, if UV radiation is generated in the solid core, such radiation can not be present in the output spectrum of the fiber, since the radiation is absorbed by most solid materials.

[0080] In some embodiments, as discussed further below, methods and apparatus for broadening input radiation can use a fiber for confining 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 radiation in the fiber. The fiber can be a hollow core photonic crystal fiber (HC-PCF), which is particularly suitable for strong confinement of radiation mainly inside the hollow core of the fiber, enabling high radiation intensities. The hollow core of the fiber can be filled with a gas, which 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, UV, and extreme UV spectra. The output radiation can consist of or include broadband radiation, which can be referred to herein as white light. Figure 9 Further discussed below, methods and apparatus for broadening input radiation can use a fiber for confining 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 radiation in the fiber. The fiber can be a hollow core photonic crystal fiber (HC-PCF), which is particularly suitable for strong confinement of radiation mainly inside the hollow core of the fiber, enabling high radiation intensities. The hollow core of the fiber can be filled with a gas, which 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, UV, and extreme UV spectra. The output radiation can consist of or include broadband radiation, which can be referred to herein as white light.

[0081] Some embodiments relate to new designs of such broadband radiation sources comprising a fiber. The fiber is a hollow core photonic crystal fiber (HC-PCF). In particular, the fiber can be a hollow core photonic crystal fiber of the type comprising an anti-resonant structure for confining radiation. Such a fiber comprising an anti-resonant structure is known in the art as an anti-resonant 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, e.g. a Kagome fiber).

[0082] A variety of types of HC-PCF can be designed, each based on a different physical guiding mechanism. Two such HC-PCFs include: hollow-core photonic band-gap fiber (HC-PBF) and hollow-core anti-resonant reflecting fiber (HC-ARF). Details of the design and manufacture of HC-PCFs can be found in US Patent US2004 / 015085A1 (for HC-PBF) and International PCT Patent Application WO2017 / 032454A1 (for hollow-core anti-resonant reflecting fiber), both of which are incorporated herein by reference. Figure 10 (a) shows a Kagome fiber comprising a Kagome lattice structure.

[0083] Reference will now be made, by way of example, to the following drawings: Figure 8 Examples of optical fibers for radiation sources are described, Figure 8 is a schematic cross-sectional view of an optical fiber OF in a transverse plane. Similar to Figure 8 further embodiments similar to actual examples of optical fibers of

[0084] The optical fiber OF comprises an elongate body that is longer in one dimension than the other two dimensions of the optical fiber OF. This longer dimension can be referred to as an axial direction, and can define an axis of the optical fiber OF. The two other dimensions define a plane that can be referred to as a transverse plane. Figure 8 A cross-section of the optical fiber OF in such a transverse plane, labeled the x-y plane (i.e. perpendicular to the axis), is shown. The transverse cross-section of the optical fiber OF can be substantially constant along the optical fiber axis.

[0085] It will be appreciated that the optical fiber OF has some degree of flexibility, and thus, generally, the direction of the axis will not be uniform along the length of the optical fiber OF. Terms such as optical axis, transverse cross-section, and similar terms will be understood to mean local optical axis, local transverse cross-section, and the like. Furthermore, where components are described as being cylindrical or tubular, these terms will be understood to encompass such shapes as can have been deformed when the optical fiber OF is bent.

[0086] The optical fiber OF can have any length, and it will be appreciated that the length of the optical fiber OF can depend on the application. The optical fiber OF can have a length between 1 cm and 10 m, for example, the optical fiber OF can have a length between 10 cm and 100 cm. In embodiments, the optical fiber OF can be a tapered optical fiber that can comprise a waist section. The waist section can comprise a tapering section in which the fiber diameter is reduced. In embodiments, the waist section can also comprise a central section with a constant diameter. In embodiments, the waist section can also comprise a thickening section in which the fiber diameter is increased to the original diameter. The length of the tapered waist section can extend from, for example, a few millimeters to several tens of centimeters. If the fiber parameters are changed during fiber drawing, it can be possible to have a tapered waist section with a longer length of, for example, several tens of meters.

[0087] The optical fiber OF comprises 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 considered to comprise a body with the hollow core HC (comprising the cladding portion and the support portion SP). The cladding portion comprises a plurality of anti-resonant elements for guiding radiation through the hollow core HC. In particular, the plurality of anti-resonant elements is arranged to mainly confine radiation propagating through the optical fiber OF inside the hollow core HC and to guide the radiation along the optical fiber OF. The hollow core HC of the optical fiber OF can be substantially disposed in a central region of the optical fiber OF, such that an axis of the optical fiber OF can also define an axis of the hollow core HC of the optical fiber OF.

[0088] The cladding portion comprises a plurality of anti-resonant elements for guiding radiation to propagate through the optical fiber OF. In particular, in such embodiments, the cladding portion comprises a single ring of six tubular capillaries CAP. Each of the tubular capillaries CAP acts as an anti-resonant element.

[0089] The capillaries CAP can also be referred to as tubes. The cross-section of the capillaries CAP can be circular, or can have another shape. Each capillary CAP comprises a substantially cylindrical wall portion WP at least partially defining the hollow core HC of the optical fiber OF and separating the hollow core HC from a capillary cavity CC. It will be appreciated that the wall portion WP can act as an anti-reflective Fabry-Perot resonator for radiation propagating 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 suitable so as to ensure that the reflection back into the hollow core HC is substantially enhanced, while the transmission into the capillary cavity CC is substantially suppressed. In some embodiments, the capillary wall portion WP can have a thickness between 0.01 pm and 10.0 pm.

[0090] It will be appreciated that, as used herein, the term "clad portion" is intended to mean the portion of the optical fiber OF that is used to guide radiation propagating through the optical fiber OF (i.e. the capillaries CAP that confine the radiation within the hollow core HC). The radiation can be confined in the form of transverse modes propagating along the fiber axis.

[0091] The support portion is generally tubular and supports the six capillaries CAP of the clad portion. The six capillaries CAP are evenly distributed around the inner surface of the inner support portion SP. The six capillaries CAP can be described as being arranged in a generally hexagonal formation.

[0092] The capillaries CAP are arranged such that each capillary is not in contact with any of the other capillaries CAP. Each of the capillaries CAP is in contact with the inner support portion SP and is spaced apart from adjacent capillaries CAP in the annular structure. This arrangement can be beneficial as it can increase the transmission bandwidth of the optical fiber OF (relative to, for example, an arrangement in which the capillaries are in contact with each other). Alternatively, in some embodiments, each of the capillaries CAP can be in contact with adjacent capillaries CAP in the annular structure.

[0093] The six capillaries CAP of the clad portion are arranged in an annular structure around the hollow core HC. The inner surface of the annular structure of the capillaries CAP 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 smallest dimension between opposing capillaries, indicated by arrow d) can be between 10 pm and 1000 pm. The diameter d of the hollow core HC can affect the mode field diameter, the impact loss, the dispersion, the modal complexity and the nonlinear properties of the hollow core fiber OF.

[0094] In this embodiment, the clad portion comprises a single annular arrangement of capillaries CAP (the capillaries CAP acting 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 does not pass through more than one capillary CAP.

[0095] It will be appreciated that other embodiments can be provided with different arrangements of anti-resonant elements. These arrangements can include arrangements with multiple rings of anti-resonant elements and arrangements with nested anti-resonant elements. Figure 10 (a) shows an embodiment of an HC-PCF with three rings of capillaries CAP that are stacked on top of each other in a radial direction. In this embodiment, each capillary CAP is in contact with other capillaries both in the same ring and in different rings. Furthermore, although Figure 8The embodiment shown in FIG. 1 includes six rings of capillaries, but in other embodiments, one or more rings including any number of anti-resonant elements (e.g., 4, 5, 6, 7, 8, 9, 10, 11, or 12 capillaries) can be disposed in the cladding portion.

[0096] Figure 10 (b) shows a modified embodiment of the HC-PCF with a single ring of tubular capillaries discussed above. In Figure 10 In the example of (b), there are two concentric rings of tubular capillaries 21. To hold the inner and outer rings of tubular capillaries 21, a support tube ST can be included in the HC-PCF. The support tube can be made of silica.

[0097] Figure 8 and Figure 10 In the example of (a) and Figure 10 The tubular capillaries of the example of (b) can have a circular cross-sectional shape. For tubular capillaries, other shapes are also possible, such as an elliptical or polygonal cross-section. Additionally, Figure 8 and Figure 10 In the example of (a) and Figure 10 The solid material of the tubular capillaries of the example of (b) can include a plastic material such as PMA, a glass such as silica, or a soft glass.

[0098] Figure 9 A radiation source RDS is depicted for providing broadband output radiation. The radiation source RDS includes a pulsed pump radiation source PRS or any other type of source capable of generating short pulses of a desired length and energy level; an optical fiber OF having a hollow core HC (e.g., of the type shown in Figure 8 Although in Figure 9 the radiation source RDS includes Figure 8 the optical fiber OF shown in

[0099] 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 to broaden the input radiation to provide output radiation ORD. The working medium WM enables broadening of the frequency range of the received input radiation IRD in order to provide broadband output radiation ORD.

[0100] The radiation source RDS further comprises a reservoir RSV. The optical fiber OF is disposed inside the reservoir RSV. The reservoir RSV can also be referred to as a housing, a container or a gas cell. The reservoir RSV is configured to contain a working medium WM. The reservoir RSV can comprise one or more features known in the art for controlling, regulating and / or monitoring the composition of the working medium WM (which can be a gas) inside the reservoir RSV. The reservoir RSV can comprise 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 proximate to the input end IE of the optical fiber OF. The first transparent window TW1 can form part of a wall of the reservoir RSV. The first transparent window TW1 can be transparent at least for the frequency of the received input radiation IRD, such that the received input radiation IRD (or at least a substantial portion of the received input radiation IRD) can be coupled into the optical fiber OF located inside the reservoir RSV. It will be appreciated that optical means (not shown) can be provided for coupling the input radiation IRD into the optical fiber OF.

[0101] The reservoir RSV comprises a second transparent window TW2 forming part of a wall of the reservoir RSV. In use, the second transparent window TW2 is positioned proximate to the output end OE of the optical fiber OF when the optical fiber OF is disposed inside the reservoir RSV. The second transparent window TW2 can be transparent at least for the frequency of the broadband output radiation ORD of the device 120.

[0102] Alternatively, in another embodiment, both opposite ends of the optical fiber OF can be placed inside different reservoirs. The optical fiber OF can comprise a first end segment configured to receive the input radiation IRD and a second end segment for outputting the broadband output radiation ORD. The first end segment can be placed inside a first reservoir containing the working medium WM. The second end segment can be placed inside a second reservoir, wherein the second reservoir can also contain the working medium WM. The function of the reservoirs can be as described above in relation to the embodiment of Fig. 1. Figure 9The first reservoir can comprise a first transparent window configured to be transparent to the input radiation IRD. The second reservoir can comprise a second transparent window configured to be transparent to the broadband output broadband radiation ORD. The first and second reservoirs can also comprise sealable openings to allow the optical fiber OF to be partially placed inside the reservoirs and partially placed outside the reservoirs, such that a gas can be sealed inside the reservoirs. The optical fiber OF can also comprise an intermediate section that is not contained inside the reservoirs. Such an arrangement using two separate gas reservoirs can be particularly convenient for embodiments in which the optical fiber OF is relatively long (e.g., when the length is greater than 1 m). It will be appreciated that for such an arrangement using two separate gas reservoirs, the two reservoirs (which can comprise 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 to provide a device for providing a working medium WM within the hollow core HC of the optical fiber OF.

[0103] 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 the frequency on the window is transmitted through the window.

[0104] Both the first TW1 and second TW2 transparent windows can form an airtight seal within the walls of the reservoir RSV, such that the working medium WM (which can be a gas) can be contained within the reservoir RSV. It will be appreciated that the gas WM can be contained within the reservoir RSV at a pressure that is different from the ambient pressure of the reservoir RSV.

[0105] The working medium WM can comprise inert gases such as argon, krypton and xenon, Raman active gases such as hydrogen, deuterium and nitrogen, or gas mixtures such as argon / hydrogen mixtures, xenon / deuterium mixtures, krypton / nitrogen mixtures, or nitrogen / hydrogen mixtures. Depending on the type of the filling gas, the nonlinear optical processes can include modulation instability (MI), optical soliton self-compression, optical soliton fission, Kerr effect, Raman effect and dispersive wave generation (DWG), the details of which are described in WO2018 / 127266A1 and US9160137B1 (both of which are hereby incorporated by reference). Since the dispersion of the filling gas can be tuned by changing the working medium WM pressure (i.e. gas cell pressure) in the reservoir RSR, the generated broadband pulse dynamics and associated spectral broadening characteristics can be adjusted in order to optimize the frequency conversion.

[0106] In one embodiment, the working medium WM can be disposed within the hollow core HC at least during reception of the input radiation IRD for generating the broadband output radiation ORD. It will be appreciated that the gas WM can be absent, in whole or in part, from the hollow core HC when the optical fiber OF is not receiving the input radiation IRD for generating the broadband output radiation.

[0107] For achieving frequency broadening, high intensity radiation can be desired. An advantage of the hollow core HC optical fiber OF is that the hollow core HC optical fiber OF can achieve higher intensity radiation by stronger spatial confinement of the radiation propagating through the optical fiber OF, thereby achieving higher localized radiation intensity. The radiation intensity inside the optical fiber OF can be higher, for example, due to high received input radiation intensity and / or due to strong spatial confinement of the radiation inside the optical fiber OF. An advantage of the hollow core optical fiber is that the hollow core optical fiber can guide radiation having a wider wavelength range than a solid core optical fiber, and in particular, the hollow core optical fiber can guide radiation in both the ultraviolet and infrared ranges.

[0108] An advantage of using the hollow core HC optical fiber OF can be that a majority of the radiation guided inside the optical fiber OF is confined in the hollow core HC. Thus, a majority of the interactions of the radiation inside the optical fiber OF are with the working medium WM 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.

[0109] 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 comprise ultrafast pulses generated, for example, by a laser.

[0110] The input radiation IRD can be coherent radiation. The input radiation IRD can be collimated radiation, which can have the advantage of facilitating and improving the efficiency of coupling the input radiation IRD into the optical fiber OF. The input radiation IRD can comprise a single frequency or a narrow range of frequencies. The input radiation IRD can be generated by a laser. Similarly, the output radiation ORD can be collimated and / or can be coherent.

[0111] The broadband range of the output radiation ORD can be a continuous range, comprising a continuous range of frequencies of the radiation. The output radiation ORD can comprise supercontinuum radiation. Continuous radiation can be beneficial for use in a number of applications, for example in metrology applications. For example, a continuous range of frequencies can be used to interrogate a large number of properties. A continuous range of frequencies can for example be used to determine and / or cancel a frequency dependence of a measured property. The supercontinuum output radiation ORD can comprise electromagnetic radiation for example in the wavelength range of 100 nm to 4000 nm. The broadband output radiation ORD frequency range can for example be 400 nm to 900 nm, 500 nm to 900 nm, or 200 nm to 2000 nm. The supercontinuum output radiation ORD can comprise white light.

[0112] The input radiation IRD provided by the pulsed pump radiation source PRS can be pulsed. The input radiation IRD can comprise electromagnetic radiation at one or more frequencies between 200 nm and 2 pm. The input radiation IRD can for example comprise electromagnetic radiation having a wavelength of 1.03 pm. The repetition rate of the pulsed radiation IRD can have an order of magnitude of 1 kHz to 100 MHz. The pulse energy can have an order of magnitude of 0.1 pJ to 100 pJ, for example 1 pJ to 10 pJ. The pulse duration of the input radiation IRD can be between 10 fs and 10 ps, for example 300 fs. The average power of the input radiation IRD can be between 100 mW and several hundred W. The average power of the input radiation IRD can for example be 20 W to 50 W.

[0113] The pulsed pump radiation source PRS can be a laser. The spatio-temporal transmission properties (e.g., its spectral amplitude and phase) of such laser pulses transmitted along the optical fiber OF can be varied and tuned by adjustment of (pump) laser parameters, working medium WM variations, and optical fiber OF parameters. The spatio-temporal transmission properties can include one or more of: output power, output mode profile, output temporal profile, width of output temporal profile (or output pulse width), output spectral profile, and bandwidth of output spectral profile (or output spectral bandwidth). The pulsed pump radiation source PRS parameters can include one or more of: pump wavelength, pump pulse energy, pump pulse width, pump pulse repetition rate. The optical fiber OF parameters can include one or more of: 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 medium WM (e.g., fill gas) parameters can include one or more of: gas type, gas pressure, and gas temperature.

[0114] The broadband output radiation ORD provided by the radiation source RDS can have an average output power of 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 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.

[0115] As described above, there are many nonlinear optical processes involved in generating broadband output radiation ORD (e.g., supercontinuum or white light). Which nonlinear optical processes have a more pronounced spectral broadening effect than others will depend on how the operating parameters are set. For example, by choosing the pump wavelength and / or optical fiber OF such that the pump pulses propagate in the normal dispersion regime (positive group velocity dispersion (GVD)) through the optical fiber, self-phase modulation (SPM) dominates the nonlinear optical processes and is responsible for the spectral expansion of the pump pulses. However, in most cases, the spectral broadening of the input radiation IRD provided by the pulsed pump radiation source PRS is driven by the dynamics of optical solitons that require the pump pulses to propagate in the optical fiber OF in the anomalous dispersion regime (negative GVD). This is because, in the anomalous dispersion regime, the effects of the Kerr nonlinearity and dispersion take actions opposite to each other. When the pulse parameters of the pump pulses launched into an optical fiber OF (e.g., HC-PCF) with anomalous dispersion do not precisely match the pulse parameters of an optical soliton, the pump pulses will evolve into optical soliton pulses with some optical soliton order and dispersive waves.

[0116] It is well known that optical soliton self-compression and modulation instability are two main mechanisms for spectral broadening in optical soliton-driven broadband radiation generation. The distinction between the two mechanisms is that the optical soliton self-compression process is associated with low optical soliton order, while the modulation instability process is associated with high optical soliton order. The optical soliton order N of the pulsed input radiation IRD is a convenient parameter that can be used to distinguish between conditions under which spectral broadening is dominated by modulation instability from conditions under which spectral broadening is dominated by optical soliton self-compression. The optical soliton order N of the pulsed input radiation IRD is given by:

[0117] (1)

[0118] where is a material-dependent nonlinear parameter describing the nonlinear phase change intensity per watt of optical power; is the pump peak power of the pulsed input radiation IRD; is the pump pulse duration of the pulsed input radiation IRD defined as the time interval in which the instantaneous optical power normalized to its maximum value is greater than 1 / e 2 ; and is the group-velocity dispersion of the working medium WM.

[0119] When spectral broadening is typically dominated by modulation instability (MI), while when spectral broadening is typically dominated by optical soliton self-compression (SSC).

[0120] Some known broadband radiation sources use an arrangement that produces spectral broadening of the pulsed input radiation but in which the parameters of the pulsed input radiation IRD, the optical fiber, and the working medium are configured to allow MI to produce spectral broadening. There are multiple reasons for MI to be used to produce spectral broadening. First, the MI process is known to produce broadband radiation with a relatively flat intensity wavelength distribution if a sufficient number of pulses are averaged. Such a broadband radiation source can be referred to as a white light radiation source (due to the relatively flat spectral intensity distribution). Second, the MI process can be implemented using a relatively economical laser source as the pump radiation source PRS.

[0121] On the other hand, in the regime of SSC, the input pump pulse undergoes compression in the time domain, which is accompanied by an increase in the width of the spectrum. After SSC, the compressed pulse of optical soliton order N0 undergoes optical soliton fission, in which the pulse splits into multiple optical solitons of order N1< N0. This optical soliton fission causes temporal broadening of the radiation pulse and spectral modulation with wavelength due to interference of the multiple optical solitons.

[0122] Compared to a noise-fed MI system, the broadband radiation generated by SSC will have virtually no frame-by-frame variation. This is because in the SSC process, moderate nonlinearity and anomalous dispersion cooperate to gradually and smoothly compress the duration of the input pump pulse along the fiber, whereas in the MI process, due to strong amplification caused by noise, the input pump pulse is decomposed into a random sub-pulse sequence (“optical soliton showers”). Thus, advantageously, in the SSC system, each individual compressed pulse is characterized by having a substantially identical broadband spectrum. In contrast, modulation instability systems would require integration over many pulses to obtain a broadband spectrum and reduce their inherent frame-by-frame variation. The intensity noise of the broadband output radiation ORD generated from an SSC system (e.g., P. Uebel, S. Bauerschmidt, Y. Ni, patent application EP3796080A1 (2019) incorporated herein by reference) can be several (e.g., two) orders of magnitude lower than the intensity noise of the broadband output radiation ORD generated from an MI system and is generally limited by the noise of the pump radiation source PRS.

[0123] As can be seen from equation (1), the optical soliton order of the pulsed input radiation IRD is related to the pulse duration of the pulsed input radiation IRD. Proportional. Therefore, in the generally existing technology arrangement where optical soliton self-compression dominates, the pulse duration of the input pulsed radiation IRD is typically... The pulse duration is reduced to approximately 30 fs or less. To achieve this arrangement, a pre-compressed high-power femtosecond fiber laser is typically used (e.g., a fiber-based chirped pulse amplification (FCPA) system, which typically supplies the pulse duration). >100 fs input pulsed radiation (IRD) or Ti:sapphire amplifiers (providing inherently short pulses without pre-compression) are used as pulsed pump radiation sources (PRS). On the one hand, in the case of fiber-based FCPA, pre-compression requires additional components for nonlinear spectral broadening and (one or more) components for time compression to the negative group velocity dispersion (GVD) duration of 30 fs or less. Since its interface with subsequent spectral broadening stages is typically free space, additional components may be needed for beam stabilization, resulting in a larger, more cost-intensive configuration. On the other hand, the thermal constraint of the crystalline medium limits the average power of Ti:sapphire amplifiers to typically less than 5 W, and for applications in optical metrology tools (MT), the repetition rate is undesirably low (at the kHz level).

[0124] As mentioned above, for both the MI and SSC processes, spectral broadening is driven by optical soliton dynamics and requires an anomalous dispersive environment. Figures 11(a) and 11(b) illustrate descriptions of (e.g., in...)Figure 9 Two exemplary simulations of the MI-based spectral evolution and the SSC-based spectral evolution of the pulse of input radiation IRD within the HC-PCF of the radiation source RDS (as illustrated in Fig. 11 (a) and Fig. 11 (b)). As can be seen in Fig. 11 (a) and Fig. 11 (b), the spectral broadening within the initial segment of the optical fiber is mainly caused by SPM for both the MI and the SSC process, and thus is relatively weak. The length of this initial optical fiber segment can be different for different spectral broadening processes.

[0125] It is well known that the spectral bandwidth With the position along the optical fiber linearly scaled, and can be approximated as the order of magnitude, as [A. Zheltikov, “Analytical insights into self-phase modulation: beyond the basic theory”, Opt. Express 26, 17571 (2018)]:

[0126] (2)

[0127] where is the nonlinear coefficient, is the peak power of the pulse, is the pulse duration, as defined above with respect to equation (1). Thus, the initial optical fiber segment can be defined as the segment where the spectral bandwidth is sufficiently linearized. An exemplary criterion can be that the local slope of the bandwidth versus position curve should not be more than twice the slope in the SPM-dominated segment.

[0128] As Figure 12 illustrated in Fig. 11 (a) and Fig. 11 (b), the rate of bandwidth increase per optical fiber length rises from a relatively flat level of about 50 THz / m (guided by the solid line) to a value of more than 100 THz / m at a normalized position of 0.69 in the case of the SSC process. Thus, the initial optical fiber segment has a relative length of about 69% of the total optical fiber length in this particular exemplary simulation. By this definition, the typical length of the SPM-dominated optical fiber segment in the SSC process can be between 64% (for lower optical soliton orders, such as N = 7.5) and 77% (for higher optical soliton orders, such as N = 13.0) of the total optical fiber length. At constant optical soliton order, this range can be slightly different for different input pulse durations.

[0129] As Figure 12As apparent from the illustration in (b), the above definition of the SPM-dominated segment criterion also applies to the MI process, since the rate of bandwidth increase within the initial fiber segment is relatively flat (e.g., up to a normalized position of 0.48). In addition, the noise-amplification-affected further broadening of the fiber length The absolute length of the SPM-dominated segment is defined from the fiber length forward, which further broadens the segment that is strongly affected by noise amplification. This length can be estimated using the nonlinear parameter and the peak power is analytically estimated as [J. M. Dudley, et al., "Supercontinuum generation in photonic crystal fiber", Rev. Mod. Phys. 78, 1135 (2006)]:

[0130] (3)

[0131] By way of example and with reference to Fig. 11(a) and Fig. 11(b), the initial SPM-dominated segment can be considered to start at the input end of the fiber and to end at a position along the fiber at which a spectral range of less than 400 nm is generated around a central pump wavelength of 1030 nm. Thus, the length of the initial SPM-dominated fiber segment is determined to be about 48% of the total fiber length in the case of an MI-dominated process (as illustrated in Fig. 11(a)) and about 70% of the total fiber length in the case of an SSC process (as illustrated in Fig. 11(b)).

[0132] In all current configurations of a radiation source RDS based on a hollow core HC fiber OF (e.g., HC-PCF), the gas cell RSV is configured such that the fiber can be installed as straight as possible. Keeping the fiber straight avoids or minimizes fiber bending losses, which increase as the bending radius decreases and are larger for shorter optical wavelengths. The qualitative dependence of the bending loss on the geometry definition and the optical wavelength can be understood in terms of a critical bending radius The critical bending radius can be understood as an analytically computable lower limit for the bending radius. The critical bending radius is expressed as [M. H. Frosz, et al., "Analyical formulation for the bend loss in single-ring hollow-core photonic crystal fibers", Photonics Res. 5, 88-91 (2017), which is incorporated herein by reference] as:

[0133] (4)

[0134] where D indicates the fiber core diameter, d indicates the diameter of the surrounding capillaries, and l indicates the optical wavelength, indicates the first zero of the Bessel function and the angle indicates the azimuthal orientation of the fiber cross-section relative to the plane of the bend. Equation (4) shows that the minimum allowed bend radius increases significantly as the optical wavelength decreases. This relationship is confirmed by the bend loss measurement data provided in R. M. Carter, et al., "Measurement of resonant bend loss in anti-resonant hollow core optical fiber", Opt. Express 25, 20612 (2017). Therefore, the fiber bend loss can greatly affect or distort the spectrum of the broadband output radiation ORD generated in a hollow core HC fiber OF (e.g., a HC-PCF), thus possibly causing its short wavelength cutoff to shift to a higher wavelength than the shortest wavelength required by a metrology tool MT (e.g., one of the metrology tools described above), and making the spectrum practically unusable.

[0135] However, installing a hollow core HC fiber OF in a completely straight configuration is not always desirable, since the hollow core HC fiber OF results in a larger footprint of the radiation source RDS and reduces the flexibility of the routing of the optical path in industrial products (e.g., the metrology tools MT described above). This is particularly relevant in relation to SSC-based radiation sources, which require a fiber length of about 1 m, which is several times longer than the typical fiber length required by MI-based radiation sources.

[0136] The inventors have found that the spectrum in the SPM-dominated region (e.g., indicated in Figures 11(a) and 11(b)) is sufficiently narrow band in both SSC and MI processes, so that the effect of the fiber bend loss on the output performance is not significant. Figure 13 (a) toFigure 13 (i) shows a set of different simulation plots, wherein each plot illustrates the numerically simulated bend loss BL as a function of the bend radius BR of the HC-PCF (e.g., shown in Figure 8 In this exemplary set of simulation data, the center wavelength of the input radiation IRD is varied from 500 nm to 1300 nm in increments of 100 nm. Here, the bend loss is defined as the excess power loss, which is the difference between the power loss of light transmission inside the bent fiber and the power loss of light transmission inside the same but straight fiber. As Figure 13 (a) to Figure 13 As can be seen in (i), a common general trend for all wavelengths is that the bend loss of the fiber increases as the bend radius decreases. In addition, for a given (constant) bend radius, the bend loss of the fiber increases as the wavelength decreases. At larger bend radii, the spikes that appear for decreasing light wavelengths stem from the coupling of the hollow core mode with the modes located at the capillaries, resulting in the creation of additional loss peaks.

[0137] Generally, bend losses less than or equal to 0.1 dB / m are considered negligible. This translates to a 2% transmission loss over the entire fiber length compared to the case where the fiber is held straight. Such fiber bending incurs a transmission loss that is small and, therefore, does not significantly impair the performance of the HC-PCF based radiation source RDS. Thus, it is proposed herein to bend or coil at least a portion of the SPM dominated section of the HC-PCF in order to reduce the footprint of the radiation source RDS while roughly maintaining the desired characteristics (e.g., spectral width, short wavelength cutoff, spectral flatness, etc.) of the wideband output radiation ORD.

[0138] According to a first aspect of the present disclosure, there is provided a broadband radiation device comprising a hollow core photonic crystal fiber, HC-PCF, the HC-PCF comprising: a hollow core extending along a length of the HC-PCF for, in use, confining a working medium at a pressure; an input end operable to receive pulsed pump radiation; and an output end operable to emit broadband output radiation resulting from spectral broadening of the pulsed pump radiation within the working medium confined within the HC-PCF. The HC-PCF can be divided into a first section extending over a first portion of the length of the HC-PCF and a second section extending over a second portion of the length of the HC-PCF, wherein the first section comprises the input end and at least a portion of the first section comprises a bend and / or one or more windings, and the second section comprises the output end and is substantially straight. The HC-PCF can be configured such that, within the first section, the spectral broadening occurs primarily via an SPM process, and within the second section, the spectral broadening occurs primarily via a nonlinear optical process different from the self-phase modulation process. The term "primarily" or "dominated by" can indicate that the SPM-induced spectral broadening accounts for more than 50%, more than 60%, more than 70%, or more than 80% of the total spectral broadening occurring within the first section of the HC-PCF.

[0139] In embodiments, the pulsed pump radiation IRD can remain substantially narrow spectral bandwidth within the first section where the SPM-dominated spectral broadening occurs. That is, the SPM-dominated spectral broadening in the first section can be substantially weaker than the spectral broadening induced by the different nonlinear optical process in the second section. Preferably, the length of the SPM-dominated fiber section can be, for example, between 60% and 80%, between 60% and 77%, between 64% and 77%, between 64% and 74%, between 68% and 74%, or between 68% and 70% of the total fiber length.

[0140] In embodiments, the different nonlinear optical process by which the spectrally broadened pulsed input radiation IRD passes in the second section of the HC-PCF can be dominated by MI. The pulsed input radiation IRD can be configured to evolve into an optical soliton pulse having an optical soliton order of N = 30, N = 35, N = 40, or N = 45.

[0141] In embodiments, the different nonlinear optical process by which the spectrally broadened pulsed input radiation IRD passes in the second section of the HC-PCF can be dominated by SSC. The pulsed input radiation IRD can be configured to evolve into an optical soliton pulse having an optical soliton order of N = 30, N = 35, N = 40, or N = 45. ,​ , or .

[0142] In embodiments, the HC-PCF can be a single-ring HC-PCF, such as shown in Figure 8 The critical bend radius can be determined from the diameter D of the hollow core, the diameter d of the capillary, the azimuthal angle of the fiber cross-section, and the (central) wavelength of the pulsed input radiation using the above equation (4).

[0143] In embodiments, the HC-PCF can have a length, i.e. a length from the input end to the output end, which is suitably determined for generating the broadband output radiation ORD via the MI or SSC process. For the MI process, the fiber length can be, for example, between 5 cm and 1000 cm, between 10 cm and 800 cm, between 20 cm and 500 cm, between 50 cm and 200 cm, or about 500 cm, while for the SSC process, the fiber length can be about 1000 cm. In embodiments, the first section of the HC-PCF can have a similar length as the second section of the HC-PCF. This can be the case when the MI process dominates the spectral broadening of the pulsed input radiation IRD. In embodiments, the first section of the HC-PCF can be longer than the remaining sections of the HC-PCF. This can be the case when the SSC process dominates the spectral broadening of the pulsed input radiation IRD.

[0144] How the first section of the HC-PCF is coiled and / or bent can depend on the size requirements for the HC-PCF-based broadband radiation source RDS. That is, the first section of the HC-PCF can be flexibly bent or coiled to comply with certain size requirements, as long as the resulting bend or coil radius is not smaller than the critical bend radius. Alternatively, a trade-off between radiation loss and size requirements can be made; for example, if the radiation loss is not too large for a particular application, and compactness is considered more important, then a critical bend radius can be allowed to be exceeded. For example, the critical bend radius can be such that the radiation loss in the first section does not exceed 50%, does not exceed 30%, does not exceed 10%, does not exceed 5%, does not exceed 1%, does not exceed 0.5%, or does not exceed 0.1%.

[0145] In some embodiments, the HC-PCF can be bent by an angle to follow a circular, partial circular, elliptical, or partial elliptical path. Here, the bending angle can be defined as the angle between two fiber portions located at two corresponding sides of the bend. The bending angle can be, for example, at least 20 degrees, at least 45 degrees, at least 60 degrees, at least 90 degrees, at least 120 degrees, or at least 135 degrees. In some embodiments, the HC-PCF can be coiled into one or more complete circles or ellipses. In all such embodiments, the bending or coiling radius can be maintained to be greater than the critical bending radius determined by placing the relevant parameters of the HC-PCF and the pulsed input radiation IRD in the above procedure (4).

[0146] In embodiments, the broadband radiation device can further include a working medium WM confined within the hollow core, and a pulsed pump radiation source PRS arranged to generate pulsed pump radiation IRD. In embodiments, the working medium WM is configured to generate anomalous dispersion.

[0147] In embodiments, the input pulse width of the pulsed pump radiation IRD can be shorter than 5000 fs, shorter than 1000 fs, shorter than 500 fs, shorter than 300 fs, or shorter than 100 fs.

[0148] In embodiments, the broadband output radiation ORD can include a spectrum partially overlapping the range of 200 nm to 2000 nm, the range of 400 nm to 1600 nm, or the range of 500 nm to 900 nm. In embodiments, the output broadband radiation can include a spectrum having a full width half maximum (FWHM) width of at least 500 nm, at least 400 nm, at least 300 nm, or at least 200 nm. In embodiments, the broadband output radiation can include a spectrum spanning from 500 nm to 900 nm.

[0149] In embodiments, the broadband radiation device can include a first gas chamber for containing the working medium. The first gas chamber can be configured to at least partially enclose the HC-PCF. In embodiments, the broadband radiation device can further include a second gas chamber for containing the working medium. In such embodiments, the first gas chamber can be configured to enclose a first portion including the input end of the HC-PCF and operate at a first pressure, and the second gas chamber can be configured to enclose a second portion including the output end of the HC-PCF and operate at a second pressure. In embodiments, the first pressure in the first gas chamber can be different (e.g., lower) than the second gas pressure in the second gas chamber.

[0150] In an embodiment, the first gas cell can be configured to enclose a first portion comprising the input end of the HC-PCF and / or a second portion comprising the output end of the HC-PCF. In an embodiment, the first gas cell can be configured to enclose only the first section of the HC-PCF (or the SPM-dominated section). In an embodiment, the first gas cell can be configured to enclose the entire HC-PCF. In an embodiment, the first gas cell can be configured to enclose only the second section of the HC-PCF. In an embodiment, at least a portion of the first gas cell can be curved or coiled in a manner similar to the curved or coiled portion of the HC-PCF.

[0151] Figs. 14(a) to 14(e) schematically depict five embodiments of a broadband radiation source RDS in which at least a portion of the first section of the HC-PCF is curved or coiled, and the HC-PCF is at least partially enclosed in one or more gas cells which are configured in different ways for different embodiments.

[0152] Referring to Fig. 14(a), in an embodiment, at least a portion of the first section of the HC-PCF can be coiled into a generally circular ring. It will be appreciated that the optical fiber can also be coiled into other shapes (e.g., an elliptical shape). The HC-PCF can be completely enclosed by a first gas cell RSV1 which can be filled with the same working medium WM, a coiled connection tube CCT, and a second gas cell RSV2. The first gas cell RSV1 can be configured to enclose a first end portion comprising the input end of the HC-PCF. The second gas cell RSV2 can be configured to enclose a second end portion comprising the output end of the HC-PCF. The second end portion can comprise part or all of the second section of the HC-PCF. While not necessary, both the input end portion and the output end portion can be generally parallel to the propagation direction of the pulsed pump radiation IRD (or to the Z direction with reference to the local coordinate system immediately adjacent to Fig. 14(d)). The coiled connection tube CCT can have a coiled shape (e.g., a circular or elliptical shape) that is generally the same as the coiled portion of the HC-PCF, and can be configured to enclose at least the coiled portion of the HC-PCF.

[0153] The first gas cell RSV1 can have a first diameter, the second gas cell RSV2 can have a second diameter, and the coiled connection tube CCT can have a third diameter, the first, second, and third diameters all being along a transverse direction perpendicular to the fiber axis of the HC-PCF. In an embodiment, the first diameter of the first gas cell RSV1 can be similar to the second diameter of the second gas cell RSV2. The difference between the first diameter and the second diameter can be, for example, within 10% of each other, within 5% of each other, within 2% of each other, or within 1% of each other. The first diameter of the first gas cell RSV1 can be, for example, within 10% of the third diameter of the coiled connection tube CCT, within 5% of the third diameter of the coiled connection tube CCT, within 2% of the third diameter of the coiled connection tube CCT, or within 1% of the third diameter of the coiled connection tube CCT. The second diameter of the second gas cell RSV2 can be, for example, within 10% of the third diameter of the coiled connection tube CCT, within 5% of the third diameter of the coiled connection tube CCT, within 2% of the third diameter of the coiled connection tube CCT, or within 1% of the third diameter of the coiled connection tube CCT. The first diameter of the first gas cell RSV1 can be, for example, within 10% of the third diameter of the coiled connection tube CCT, within 5% of the third diameter of the coiled connection tube CCT, within 2% of the third diameter of the coiled connection tube CCT, or within 1% of the third diameter of the coiled connection tube CCT. The second diameter of the second gas cell RSV2 can be, for example, within 10% of the third diameter of the coiled connection tube CCT, within 5% of the third diameter of the coiled connection tube CCT, within 2% of the third diameter of the coiled connection tube CCT, or within 1% of the third diameter of the coiled connection tube CCT. The third diameter of the coiled connecting tube CCT can be smaller than both the first diameter of the first gas cell RSV1 and the second diameter of the second gas cell RSV2, in embodiments. The third diameter can be, for example, no more than 50% of both the first diameter and the second diameter, no more than 40% of both the first diameter and the second diameter, no more than 30% of both the first diameter and the second diameter, or no more than 20% of both the first diameter and the second diameter. The radius of curvature (or coiled radius) of the coiled connecting tube can be, for example, between 1 cm and 500 cm, between 1 cm and 400 cm, between 1 cm and 300 cm, between 1 cm and 200 cm, or between 2 cm and 100 cm, in embodiments.

[0154] Each of the first gas cell RSV1 and the second gas cell RSV2 can include an input end and an output end. The first gas cell RSV1 can further include a gas connection via which the working medium WM can be pumped into or out of the first gas cell RSV1, in embodiments. The two ends of the coiled connecting tube CCT can be connected to the output end of the first gas cell RSV1 and the input end of the second gas cell RSV2, respectively, in a sealed manner. Such an arrangement can allow the working medium WM to flow, for example, from the first gas cell RSV1 to the second gas cell RSV2 via the coiled connecting tube CCT, and thus the working medium WM can be at approximately the same pressure PI in the first gas cell RSV1, the coiled connecting tube, and the second gas cell RSV1.

[0155] The first gas cell RSV1 can include a first transparent window TW1 located at the input end of the first gas cell RSV1 and configured to substantially transmit the pulsed pump radiation IRD. The second gas cell RSV2 can include a second transparent window TW2 located at the output end of the second gas cell RSV2 and configured to substantially transmit the broadband output radiation ORD. The pulsed pump radiation IRD can be emitted from a free-space pulsed pump radiation source PRS and focused into the hollow core of the HC-PCF via a lens. Once passing through the first transparent window TW1 of the first gas cell RSV1, the pulsed pump radiation IRD can enter the hollow core of the HC-PCF via the input end of the optical fiber and can subsequently be spectrally broadened into the broadband output radiation ORD. The broadband output radiation ORD can exit the HC-PCF via the output end of the optical fiber and can eventually exit the second gas cell RSV2 via the second transparent window TW2.

[0156] With reference to Figure 14(b), in an embodiment, at least a portion of the first section of the HC-PCF can be coiled in a manner similar to the coiled fiber shown in Figure 14(a). The HC-PCF can be partially enclosed by a first gas cell RSV1’ and a second gas cell RSV2’ filled with the same working medium WM. The working medium WM in the first gas cell RSV1’ can also be different from the working medium WM in the second gas cell RSV2’. Similar to the embodiment of Figure 14(a), the first gas cell RSV1’ can be configured to enclose a first end portion including the input end of the HC-PCF, and the second gas cell RSV2’ can be configured to enclose a second end portion including the output end of the HC-PCF. The second end portion can include a portion or all of the second section of the HC-PCF. However, the main difference between the two embodiments can be that the embodiment of Figure 14(b) can not include a coiled connecting tube CCT, and thus the working medium WM can not flow between the first gas cell RSV1’ and the second gas cell RSV2’. In such an embodiment, there can be a gas-tight seal between the HC-PCF and each of the first gas cell RSV1’ and the second gas cell RSV2’. For example, the first gas cell RSV1’ can include a first gas-tight opening GTO1 at the output end of the gas cell RSV1’, through which the HC-PCF can be inserted into the first gas cell RSV1’ in a sealed manner. Similarly, the second gas cell RSV2’ can include a second gas-tight opening GTO2 at the input end of the gas cell RSV2’, through which the HC-PCF can be inserted into the second gas cell RSV2’ in a sealed manner.

[0157] Since there is no gas flow between the first gas cell RSV1’ and the second gas cell RSV2’, the second gas cell RSV2’ can also include a gas connection through which the same or different working medium WM can be pumped into or out of the second gas cell RSV2’. Such a configuration can allow for independent control of the pressure and / or type of working medium WM in the first gas cell RSV1’ and the second gas cell RSV2’. In an embodiment, the first gas cell RSV1’ and the second gas cell RSV2’ can be filled with the same working medium WM, and the two gas cells RSV1’, RSV2’ can be set to two different pressure levels, i.e. a first pressure P1’ and a second pressure P2’. In this way, a pressure gradient between the first pressure P1’ in the first gas cell RSV1’ and the second pressure P2’ in the second gas cell RSV2’ can be maintained, since the gas flow through the HC-PCF is negligible compared to the pipe connections between the gas cells (e.g. the embodiment shown in Figure 14(a)). Such a pressure gradient can be used to reduce the fluctuations in the in-coupling (e.g. by keeping the input end of the HC-PCF fiber at a lower pressure P1’), and can allow for optimization of the spectral range of the broadband output radiation ORD generated via the SSC process.

[0158] Referring to FIG. 14(c), in an embodiment, at least a portion of the first section of the HC-PCF can be coiled in a manner similar to the coiled fiber shown in FIG. 14(a) or FIG. 14(b). The HC-PCF can be completely enclosed by a coiled connecting tube CCT’ and a gas cell RSV2’’ filled with the same working medium WM. The coiled connecting tube CCT’ can be configured to at least enclose the first section of the HC-PCF. The gas cell RSV2’’ can be configured to enclose an end portion including the output end of the HC-PCF. The end portion of the HC-PCF can include a portion or all of the second section of the HC-PCF.

[0159] Similar to the embodiment of FIG. 14(a), the output end of the coiled connecting tube CCT’ can be connected to the input end of the gas cell RSV2’’ in a sealed manner such that the working medium WM can flow between the coiled connecting tube CCT’ and the gas cell RSV2’’. The pressure P2’’ of the working medium WM in both the coiled connecting tube CCT’ and the gas cell RSV2’’ can be controlled by the gas connection of the gas cell RSV2’’. Instead of being connected to another gas cell (e.g., the first gas cell RSV1 shown in FIG. 14(a)) in a sealed manner, the input end of the coiled connecting tube CCT’ can be connected to a fiber splice interface FSI in a sealed manner. In an embodiment, the transverse diameter of the coiled connecting tube CCT’ can be less than the diameter of the gas cell RSV2’’. The transverse diameter of the coiled connecting tube CCT’ can be no more than, for example, 50% of the transverse diameter of the gas cell RSV2’’, no more than 40% of the transverse diameter of the gas cell RSV2’’, no more than 30% of the transverse diameter of the gas cell RSV2’’, or no more than 20% of the transverse diameter of the gas cell RSV2’’.

[0160] The input end of the HC-PCF can be spliced to a transmission fiber T configured to transmit pulsed pump radiation IRD from a pulsed pump radiation source PRS to the HC-PCF. In an embodiment, the transmission fiber TF can be part of the pulsed pump radiation source PRS. The fiber splice interface FSI can be configured to hold the spliced fiber ends of the transmission fiber TF and the HC-PCF while providing a gas-tight seal to the input end of the coiled connecting tube CCT’.

[0161] While the foregoing embodiments require at least two separate gas-containing components for partially or completely enclosing the HC-PCF, the embodiments shown in FIG. 14(d) and FIG. 14(e) can use a single piece gas cell to completely enclose the HC-PCF. Referring to FIG. 14(d) and FIG. 14(e), the single piece gas cell RSV3, RSV3' can have a lateral diameter that is substantially constant along the length of the gas cell RSV3, RSV3'. The gas cell RSV3, RSV3' can include a first straight section SS1, SS1', a coiled section CS (e.g., shown in FIG. 14(d)) or a bent section BS (e.g., shown in FIG. 14(e)), and a second straight section SS2, SS2'. Similar to the foregoing embodiments, the gas cell RSV3, RSV3' can include a first transparent window TW1 configured to transmit the pulsed pump radiation IRD and a second transparent window TW2 configured to transmit the broadband output radiation ORD. The gas cell RSV3, RSV3' can also include gas connections through which the working medium WM can be pumped into or evacuated from the gas cell RSV3, RSV3', and the pressure P3 of the working medium can be controlled.

[0162] In the case of FIG. 14(d), the first section of the HC-PCF can be at least partially enclosed in the coiled section CS of the single piece gas cell RSV3. In the case of FIG. 14(e), the first section of the HC-PCF can be at least partially enclosed in the bent section CS of the single piece gas cell RSV3'. For both cases, the second section of the HC-PCF in which the spectral broadening of the pulsed pump radiation IRD is dominated by the MI or SSC process can be completely enclosed in the second straight section SS2, SS2' of the gas cell RSV3, RSV3'. In embodiments, the second straight section SS2, SS2' can also enclose a portion of the first section of the HC-PCF.

[0163] Figure 15is a block diagram that illustrates a computer system 1600 upon which a method and processes described herein can be implemented. Computer system 1600 includes a bus 1602 or other communication mechanism for communicating information, and a processor 1604 (or multiple processors 1604 and 1605) coupled with bus 1602 for processing information. Computer system 1600 also includes a main memory 1606, such as a random access memory (RAM) or other dynamic storage device, coupled to bus 1602 for storing information and instructions to be executed by processor 1604. Main memory 1606 can also be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor 1604. Computer system 1600 further includes a read only memory (ROM) 1608 or other static storage device coupled to bus 1602 for storing static information and instructions for processor 1604. A storage device 1610, such as a magnetic disk or optical disk, is provided and coupled to bus 1602 for storing information and instructions.

[0164] Computer system 1600 can be coupled via bus 1602 to a display 1612, such as a cathode ray tube (CRT) or flat panel or touch panel display for displaying information to a computer user. An input device 1614, including alphanumeric and other keys, is coupled to bus 1602 for communicating information and command selections to processor 1604. Another type of user input device is cursor control 1616, such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor 1604 and for

[0165] One or more of the methods as described herein can be performed by computer system 1600 in response to processor 1604 executing one or more sequences of one or more instructions contained in main memory 1606. Such instructions can be read into main memory 1606 from another computer-readable medium, such as storage device 1610. Execution of the sequences of instructions contained in main memory 1606 causes processor 1604 to perform the process steps described herein. One or more processors in a multi-processing arrangement can also be employed to execute the sequences of instructions contained in main memory 1606. In alternative embodiments, hard-wired circuitry can be used in place of or in combination with software instructions. Thus, the description herein is not limited to any specific combination of hardware circuitry and software.

[0166] The term "computer readable media" as used herein includes both non-transitory and transitory computer readable media. Non-transitory computer readable media include, but are not limited to portable or fixed storage devices, optical storage devices, magnetic storage devices, magneto-optical storage devices, or any other non-transitory storage medium which can be used to store and / or carry computer readable instructions. Transitory computer readable media include, but are not limited to, carrier waves, transmission signals, and any other transient media which can be used to store and / or carry computer readable instructions.

[0167] Various forms of computer readable media can be involved in carrying one or more sequences of one or more instructions to the 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 dynamic memory and send the instructions over a telephone line using a modem. A modem local to the computer system 1600 can receive the data on the telephone line and use an infra-red transmitter to convert the data to an infra-red signal. An infra-red detector coupled to the bus 1602 can receive the data carried in the infra-red signal and place the data on the bus 1602. The bus 1602 carries the data to the main memory 1606, from which the processor 1604 retrieves and executes the instructions. The instructions received by the main memory 1606 can optionally be stored on storage device 1610 either before or after execution by the processor 1604.

[0168] The computer system 1600 also preferably includes a communication interface 1618 coupled to bus 1602. The communication interface 1618 provides a two-way data communication coupling to a network link 1620 that is connected to a local 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 to provide a data communication connection to a compatible LAN. Wireless links can also be implemented. In any such implementation, the communication interface 1618 sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information.

[0169] Network link 1620 typically provides data communication through one or more networks to other data devices. For example, network link 1620 can provide a connection through local network 1622 to a host computer 1624 or to data equipment operated by an Internet Service Provider (ISP) 1626. ISP 1626 in turn provides data communication services through the worldwide packet data communication network, now commonly referred to as the "Internet" 1628. Local network 1622 and Internet 1628 both use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on network link 1620 and through communication interface 1618, which carry the digital data to and from computer system 1600, are exemplary forms of carrier waves transporting the information.

[0170] Computer system 1600 can send messages and receive data, including program code, through the network(s), network link 1620 and communication interface 1618. In the Internet example, a server 1630 might transmit a requested code for an application program through Internet 1628, ISP 1626, local network 1622 and communication interface 1618. One such downloaded application can provide for one or more of the techniques described herein, for example. The received code can be executed by processor 1604 as it is received, and / or stored in storage device 1610, or other non-volatile storage for later execution. In this manner, computer system 1600 can obtain application code in the form of a carrier wave.

[0171] Further embodiments are disclosed in the following list of numbered aspects:

[0172] 1. A broadband radiation device comprising a hollow core photonic crystal fiber (HC-PCF), the HC-PCF comprising:

[0173] a hollow core extending along a length of the HC-PCF for confining, in use, a working medium at a pressure,

[0174] an input operable to receive pulsed pump radiation; and

[0175] an output operable to emit broadband output radiation resulting from spectral broadening of the pulsed pump radiation within the working medium confined within the HC-PCF;

[0176] The HC-PCF is divided into a first segment extending over a first portion of the length of the HC-PCF and a second segment extending over a second portion of the length of the HC-PCF. The first segment includes the input terminal and at least a portion of the first segment includes a bend and / or one or more coils. The second segment includes the output terminal and is substantially straight.

[0177] The HC-PCF is configured such that, in the first segment, the spectral broadening occurs primarily via a self-phase modulation process, and in the second segment, the spectral broadening occurs primarily via a nonlinear optical process different from the self-phase modulation process.

[0178] 2. The broadband radiation device according to aspect 1, wherein the different nonlinear optical processes are dominated by modulation instability (MI) or optical soliton self-squeezing (SSC).

[0179] 3. The broadband radiation device according to aspect 1 or 2, wherein the HC-PCF is a single-ring HC-PCF.

[0180] 4. The broadband radiation device according to any of the foregoing aspects, wherein at least a portion of the first segment of the HC-PCF is coiled and / or bent such that the resulting coiling radius and / or bending radius is not less than that obtained by... The determined critical bending radius, where, d indicates the diameter of the hollow core of the HC-PCF, and d indicates the diameter of the capillary of the HC-PCF. Indicates the wavelength of the pulsed pump radiation. Indicator Bessel function The first zero point, and Indicates the azimuth angle between the cross section of the HC-PCF and the bending plane.

[0181] 5. The broadband radiating device according to any of the foregoing aspects, wherein the HC-PCF is bent such that the angle between the two optical fiber portions at two corresponding sides of the bend is at least 45 degrees.

[0182] 6. The broadband radiating device according to any of the foregoing aspects, wherein the HC-PCF is bent such that the angle between the two optical fiber portions at two corresponding sides of the bend is at least 90 degrees.

[0183] 7. The broadband radiation device according to any of the foregoing aspects, the broadband radiation device comprising a first chamber for containing the working medium, the first chamber being configured to at least partially enclose the HC-PCF.

[0184] 8. The broadband radiation device according to aspect 7, wherein the first gas cell is configured to enclose a first portion comprising the input end of the HC-PCF and / or a second portion comprising the output end of the HC-PCF.

[0185] 9. The broadband radiation device according to aspect 7 or 8, wherein the first gas cell is configured to enclose only the first section of the HC-PCF.

[0186] 10. The broadband radiation device according to aspect 7 or 8, wherein the first gas cell is configured to enclose the entire HC-PCF.

[0187] 11. The broadband radiation device according to aspect 7 or 8, wherein the first gas cell is configured to enclose only the second section of the HC-PCF.

[0188] 12. The broadband radiation device according to any one of aspects 7 to 10, wherein at least a portion of the first gas cell is curved or coiled so as to enclose a curved or coiled portion of the HC-PCF.

[0189] 13. The broadband radiation device according to aspect 9, further comprising a second gas cell for containing the working medium; the second gas cell is configured to enclose the second section and operates at a second pressure.

[0190] 14. The broadband radiation device according to aspect 13, wherein the first pressure in the first gas cell is different from the second pressure in the second gas cell.

[0191] 15. The broadband radiation device according to any preceding aspect, wherein a length between the input end and the output end of the HC-PCF is between 5 cm and 1000 cm.

[0192] 16. The broadband radiation device according to any preceding aspect, wherein a length between the input end and the output end of the HC-PCF is between 10 cm and 800 cm.

[0193] 17. The broadband radiation device according to any preceding aspect, wherein a length between the input end and the output end of the HC-PCF is between 20 cm and 500 cm.

[0194] 18. The broadband radiation device according to any preceding aspect, further comprising:

[0195] the working medium, the working medium being confined within the hollow core; and

[0196] a pulsed pump radiation source arranged to generate the pulsed pump radiation.

[0197] 19. A broadband radiation device according to aspect 18, operable such that the input pulse width of the pulsed pump radiation is shorter than 1000 fs.

[0198] 20. A broadband radiation device according to aspect 19, operable such that the input pulse width of the pulsed pump radiation is shorter than 500 fs.

[0199] 21. A broadband radiation device according to any of aspects 18 to 20, wherein the working medium is configured to generate anomalous dispersion.

[0200] 22. A broadband radiation device according to any preceding aspect, wherein the output broadband radiation comprises a spectrum partially overlapping the range 200 nm to 2000 nm.

[0201] 23. A broadband radiation device according to any preceding aspect, wherein output broadband radiation comprises a spectrum partially overlapping the range 400 nm to 1600 nm.

[0202] 24. A broadband radiation device according to any preceding aspect, wherein output broadband radiation comprises a spectrum having a full width half maximum (FWHM) width of at least 500 nm.

[0203] 25. A broadband radiation device according to any preceding aspect, wherein output broadband radiation comprises a spectrum having a FWHM width of at least 300 nm.

[0204] 26. A broadband radiation device according to any preceding aspect, wherein output broadband radiation comprises a spectrum spanning 500 nm to 900 nm.

[0205] 27. A broadband radiation device according to any preceding aspect, wherein the first portion is between 60% and 80% of the length from the input end to the output end of the HC-PCF.

[0206] 28. A broadband radiation device according to any preceding aspect, wherein the first portion has a length determined by wherein, is a material dependent non-linear parameter, and is the peak power of the pulsed pump radiation.

[0207] 29. A metrology device comprising a radiation source according to any preceding aspect.

[0208] 30. The measuring device according to aspect 29, the measuring device comprising a scattering measuring device, a level sensor, or an alignment sensor.

[0209] 31. A method for generating broadband output radiation, comprising:

[0210] Pulsed pump radiation is received at the input end of a hollow-core photonic crystal fiber (HC-PCF), wherein the hollow-core photonic crystal fiber has a hollow core that confines a working medium under pressure; and

[0211] Broadband output radiation is emitted at the output end of the HC-PCF, which is generated by the spectral broadening of the pulsed pump radiation within the working medium confined within the HC-PCF.

[0212] The HC-PCF is divided into a first segment extending over a first portion of the length of the HC-PCF and a second segment extending over a second portion of the length of the HC-PCF. The first segment includes the input terminal and at least a portion of the first segment includes a bend and / or one or more coils. The second segment includes the output terminal and is substantially straight.

[0213] The HC-PCF is configured such that, in the first segment, the spectral broadening occurs primarily via a self-phase modulation process, and in the second segment, the spectral broadening occurs primarily via a nonlinear optical process different from the self-phase modulation process.

[0214] 32. The method according to aspect 31, wherein the different nonlinear optical processes are dominated by modulation instability (MI) or optical soliton self-squeezing (SSC).

[0215] 33. The method according to aspect 31 or 32, wherein the HC-PCF is a monocyclic HC-PCF.

[0216] 34. The method according to aspect 33, wherein at least a portion of the first segment of the HC-PCF is coiled and / or bent such that the resulting coiling radius and / or bending radius is not less than that obtained by... The determined critical bending radius, where, d indicates the diameter of the hollow core of the HC-PCF, and d indicates the diameter of the capillary of the HC-PCF. Indicates the wavelength of the pulsed pump radiation. Indicator Bessel function The first zero point, and Indicates the azimuth angle between the cross section of the HC-PCF and the bending plane.

[0217] 35. The method of any one of aspects 31 to 34, wherein a length between the input end and the output end of the HC-PCF is between 5 cm and 1000 cm.

[0218] 36. The method of any one of aspects 31 to 35, wherein a length between the input end and the output end of the HC-PCF is between 10 cm and 800 cm.

[0219] 37. The method of any one of aspects 31 to 36, wherein a length between the input end and the output end of the HC-PCF is between 20 cm and 500 cm.

[0220] 38. The method of any one of aspects 31 to 37, operable such that a pulse width of the pulsed pump radiation is shorter than 1000 fs.

[0221] 39. The method of any one of aspects 31 to 38, operable such that the pulse width of the pulsed pump radiation is shorter than 500 fs.

[0222] 40. The method of any one of aspects 31 to 39, wherein the output broadband radiation comprises a spectrum partially overlapping a range of 200 nm to 2000 nm.

[0223] 41. The method of any one of aspects 31 to 40, wherein output broadband radiation comprises a spectrum partially overlapping a range of 400 nm to 1600 nm.

[0224] 42. The method of any one of aspects 31 to 41, wherein output broadband radiation comprises a spectrum having a full width half maximum (FWHM) width of at least 500 nm.

[0225] 43. The method of any one of aspects 31 to 42, wherein output broadband radiation comprises a spectrum having a FWHM width of at least 300 nm.

[0226] 44. The method of any one of aspects 31 to 43, wherein output broadband radiation comprises a spectrum spanning 500 nm to 900 nm.

[0227] While specific references can be made to the use of lithography equipment in IC manufacturing within this document, it should be understood that the lithography equipment described herein can have other applications. Possible other applications include manufacturing integrated optical systems, guiding and detecting patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin-film magnetic heads, etc.

[0228] While specific reference is made herein to embodiments of the invention within the context of a photolithography apparatus, these embodiments can be used with other apparatuses. Embodiments of the invention can form part of any apparatus that forms a mask inspection apparatus, a measurement apparatus, or measures or processes objects such as wafers (or other substrates) or masks (or other patterning apparatuses). These apparatuses are generally referred to as photolithography tools. Such photolithography tools can be used under vacuum conditions or ambient (non-vacuum) conditions.

[0229] While reference may be made specifically to the use of embodiments of the invention in the context of optical lithography, it will be understood that the invention is not limited to optical lithography and may be used in other applications, such as imprint lithography, where circumstances permit.

[0230] While references may specifically be made to "measuring equipment / tools / systems" or "inspection equipment / tools / systems," these terms can refer to the same or similar types of tools, equipment, or systems. For example, inspection or measuring equipment including embodiments of the present invention can be used to determine the characteristics of a structure on a substrate or wafer. Inspection or measuring equipment including embodiments of the present invention can be used to detect defects in a substrate or defects in a structure on a substrate or wafer. In such embodiments, the characteristics of interest in the structure on the substrate may be related to defects in the structure, the absence of a specific portion of the structure, or the presence of unwanted structures on the substrate or wafer.

[0231] While specific embodiments of the invention have been described above, it will be understood that the invention can be practiced in other ways than those described. The above description is intended to be illustrative and not restrictive. Therefore, those skilled in the art will understand that modifications can be made to the described invention without departing from the scope of the claims set forth below.

Claims

1. A broadband radiation device comprising a hollow-core photonic crystal fiber, HC-PCF, the HC-PCF comprising: a hollow core extending along a length of the HC-PCF for, in use, confining a working medium at a pressure, an input end operable to receive pulsed pump radiation; and an output end operable to emit broadband output radiation resulting from spectral broadening of the pulsed pump radiation within the working medium confined within the HC-PCF; wherein the HC-PCF is divided into a first section extending over a first portion of the length of the HC-PCF and a second section extending over a second portion of the length of the HC-PCF, wherein the first section comprises the input end and at least a portion of the first section comprises a bend and / or one or more windings, and the second section comprises the output end and is substantially straight; and wherein the HC-PCF is configured such that, within the first section, the spectral broadening occurs primarily via a self-phase modulation process, and within the second section, the spectral broadening occurs primarily via a nonlinear optical process different from the self-phase modulation process.

2. The broadband radiation device of claim 1, wherein, The different nonlinear optical process is dominated by modulation instability, MI, or optical soliton self-compression, SSC.

3. The broadband radiation device of any preceding claim, wherein, the at least one portion of the first section of the HC-PCF is wound and / or bent such that the resulting winding radius and / or bending radius is not smaller than the critical bending radius determined by wherein denotes the hollow core diameter of the HC-PCF, d denotes the capillary diameter of the HC-PCF, denotes the wavelength of the pulsed pump radiation, denotes the first zero of the Bessel function and denotes the azimuthal angle between the cross section of the HC-PCF and the bending plane.

4. The broadband radiation device of any preceding claim, wherein, The HC-PCF is bent such that an angle between two fiber portions at two corresponding sides of the bend is at least 45 degrees, or, optionally, the bend is at least 90 degrees.

5. The broadband radiation device of any preceding claim, comprising a first gas cell for containing the working medium, the first gas cell being configured to at least partially enclose the HC-PCF.

6. The broadband radiation device of claim 5, wherein, The first gas cell is configured to enclose a first portion comprising the input end of the HC-PCF and / or a second portion comprising the output end of the HC-PCF.

7. The broadband radiation device according to claim 5 or 6, wherein, The first gas cell is configured to enclose the entire HC-PCF.

8. The broadband radiation device according to any one of claims 5 to 7, wherein, At least a portion of the first gas cell is bent or wound so as to enclose a bent or wound portion of the HC-PCF.

9. The broadband radiation device of any preceding claim, wherein, A length between the input end and the output end of the HC-PCF is between 5 cm and 1000 cm.

10. The broadband radiation device of any preceding claim, further comprising: the working medium confined within the hollow core; and a pulsed pump radiation source arranged to generate the pulsed pump radiation.

11. The broadband radiation device of claim 10, operable such that an input pulse width of the pulsed pump radiation is shorter than 1000 fs, or, optionally, shorter than 500 fs.

12. The broadband radiation device of any preceding claim, wherein, The first portion is between 60% and 80% of the length of the HC-PCF from the input end to the output end.

13. The broadband radiation device of any preceding claim, wherein, The first portion has a length determined by wherein, is a material dependent non-linear parameter, and is the peak power of the pulsed pump radiation.

14. A metrology device comprising a radiation source according to any preceding claim.

15. A method for generating broadband output radiation, comprising: receiving pulsed pump radiation at an input end of a hollow-core photonic crystal fiber (HC-PCF), the HC-PCF having a hollow core that confines a working medium under a pressure; and emitting broadband output radiation at an output end of the HC-PCF, the broadband output radiation resulting from spectral broadening of the pulsed pump radiation within the working medium confined within the HC-PCF; wherein the HC-PCF is divided into a first section extending over a first portion of the length of the HC-PCF and a second section extending over a second portion of the length of the HC-PCF, wherein the first section includes the input end and at least a portion of the first section includes a bend and / or one or more windings, and the second section includes the output end and is substantially straight; and wherein the HC-PCF is configured such that, within the first section, the spectral broadening occurs primarily via a self-phase modulation process, and within the second section, the spectral broadening occurs primarily via a nonlinear optical process different from the self-phase modulation process.

16. A method for generating broadband output radiation, comprising: receiving pulsed pump radiation at an input end of a hollow-core photonic crystal fiber (HC-PCF), the HC-PCF having a hollow core that confines a working medium under a pressure; and emitting broadband output radiation at an output end of the HC-PCF, the broadband output radiation resulting from spectral broadening of the pulsed pump radiation within the working medium confined within the HC-PCF; wherein the HC-PCF is configured such that, within the HC-PCF, the spectral broadening occurs primarily via a nonlinear optical process different from a self-phase modulation process.

17. A method for generating broadband output radiation, comprising: receiving pulsed pump radiation at an input end of a hollow-core photonic crystal fiber (HC-PCF), the HC-PCF having a hollow core that confines a working medium under a pressure; and emitting broadband output radiation at an output end of the HC-PCF, the broadband output radiation resulting from spectral broadening of the pulsed pump radiation within the working medium confined within the HC-PCF; wherein the HC-PCF is configured such that, within the HC-PCF, the spectral broadening occurs primarily via a nonlinear optical process different from a self-phase modulation process.

18. A method for generating broadband output radiation, comprising: receiving pulsed pump radiation at an input end of a hollow-core photonic crystal fiber (HC-PCF), the HC-PCF having a hollow core that confines a working medium under a pressure; and emitting broadband output radiation at an output end of the HC-PCF, the broadband output radiation resulting from spectral broadening of the pulsed pump radiation within the working medium confined within the HC-PCF; wherein the HC-PCF is configured such that, within the HC-PCF, the spectral broadening occurs primarily via a nonlinear optical process different from a self-phase modulation process.

Images