Gas mixture for hollow-core optical fiber for generating broadband radiation

Abstract

A hollow core optical fiber and source assembly for broadband generation, wherein the hollow core of the hollow core optical fiber is filled with a gas component comprising a working gas. The optical fiber is configured to receive pulsed pump radiation at an input of the hollow-core optical fiber, a pulsed power of the pulsed pump power exceeding an ionization threshold of the gas component. The optical fiber is configured to confine and direct the pulsed pump radiation through the optical fiber such that it interacts with the working gas to generate broadband radiation by non-linear widening of the pulsed pump radiation. The gas component includes a hydrogen component less than 1% of the total gas component in the hollow fiber.

Description

[0001] Cross-references to related applications

[0002] This application claims priority to EP application 23165303.1, filed on March 30, 2023, and EP application 23191688.3, ​​filed on August 16, 2023, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This invention relates to a hollow-core optical fiber and source assembly for broadband generation. In particular, it relates to a hollow-core optical fiber configured to include a gas component for nonlinear broadening. Background Technology

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

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

[0006] Low-ki lithography can be used to process features with dimensions smaller than the classical resolution limit of a lithographic apparatus. In such process, the resolution formula can be expressed as CD = k1*lambda / NA, where lambda is the wavelength of radiation employed, NA is the numerical aperture of projection optics in the lithographic apparatus, CD is the “critical dimension” (generally the smallest feature size printed, but in this case the half-pitch), and ki is an empirical resolution factor. In general, the smaller ki the more difficult it becomes to reproduce the pattern on the substrate that resembles the shape and dimensions planned by a circuit designer in order to achieve particular electrical functionality and performance. To overcome these difficulties, sophisticated fine-tuning steps can be applied to the lithographic 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 forms of design layout optimization 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 lithographic apparatus can be used to improve the reproduction of patterns at low ki.

[0007] In the field of lithography, many measurement systems can be used within lithographic apparatuses and outside lithographic apparatuses. Generally, such measurement systems can use a radiation source to irradiate a target with radiation and can use a detection system operable to measure at least one property of a portion of the incident radiation scattered from the target. An example of a measurement system outside a lithographic apparatus is an inspection apparatus or a metrology apparatus, which can be used to determine properties of a pattern previously transferred to a substrate by the lithographic apparatus. Such an external inspection apparatus can for example include a scatterometer. Examples of measurement systems that can be provided within a lithographic apparatus include topography measurement systems (also referred to as level sensors), position measurement systems (e.g. interferometers) for determining the position of reticle or wafer stages, and alignment sensors for determining the position of alignment marks. These measurement devices can use electromagnetic radiation to perform measurements.

[0008] Some measurement systems can use radiation having different wavelength ranges to perform one or more measurements. This can be achieved by, for example, providing a broadband radiation source, such as a supercontinuum radiation source. A supercontinuum radiation source can include a hollow-core fiber in which broadband radiation is generated by spectral broadening of received input radiation, which can generally be referred to as pump radiation. The spectral broadening process can rely on nonlinear effects, for example the interaction of the pump radiation with a gas composition / gas mixture that exhibits a pronounced nonlinear response.

[0009] Described herein are assemblies, apparatuses, and methods for providing broadband radiation with improved output and / or lifetime. SUMMARY

[0010] According to a first aspect of the current disclosure, there is provided an air core optical fiber for broadband generation, wherein a hollow core of the air core optical fiber is filled with a gas composition comprising a working gas, wherein the optical fiber is configured to: receive pulsed pump radiation at an input end of the air core optical fiber, the pulsed pump power having a pulse power that exceeds an ionization threshold of the gas composition; and confine and guide the pulsed pump radiation through the optical fiber such that the pulsed pump radiation interacts with the working gas to generate broadband radiation by nonlinear broadening of the pulsed pump radiation; and wherein the gas composition comprises a hydrogen component, the hydrogen component being less than 1% of the total gas composition in the air core optical fiber. BRIEF DESCRIPTION OF DRAWINGS

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

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

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

[0014] Figure 3 A schematic representation of the overall lithography is depicted, representing the cooperation between three key technologies to optimize semiconductor manufacturing;

[0015] Figure 4 A schematic overview of a scatterometer metrology tool is depicted;

[0016] Figure 5 A schematic overview of a level sensor metrology tool is depicted;

[0017] Figure 6 A schematic overview of an alignment sensor metrology tool is depicted;

[0018] Figure 7 (a) depicts an example plot of the output power of a source assembly comprising a high purity gas comprising 50% helium and 50% working gas as a function of time;

[0019] Figure 7 (b) depicts an example plot of the output power spectral density as a function of time after a sudden breakdown event;

[0020] Figure 8(a) depicts a schematic representation of a cross-section of an example air core optical fiber that can be used for broadband generation; and

[0021] Figure 8(b) depicts a schematic representation of an air core optical fiber in a radiation source assembly. DETAILED DESCRIPTION

[0022] In the present document, the terms“radiation” and“beam” are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 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).

[0023] <reticle>

[0024] The term“reticle”,“mask” or“patterning device” as employed herein can be broadly interpreted to refer to a generic patterning device that can be used in processes of patterning a beam of radiation to give it a spatially varying intensity pattern in its cross-section, according to at least one pattern embodied on the patterning device. The term“optical mask” can also be used in this context. Examples of other such patterning devices include a programmable mirror array and a programmable LCD array.

[0025] 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 radiation 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 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.

[0026] 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 radiation. The illuminator IL can be used to adjust the beam B to have a desired spatial and angular intensity distribution in its cross-section at a plane of the patterning device MA.

[0027] The term“projection system” PS used herein should be interpreted as broadly encompassing any type of projection system, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, and / or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein can be considered as synonymous with the more general term“projection system” PS.

[0028] The lithographic apparatus LA can be of a type that includes a liquid immersion facility, in which at least a portion of the substrate W 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 referred to as immersion lithography. Further details of immersion techniques are given in US 6952253, incorporated herein by reference.

[0029] The lithographic apparatus LA can also be of a type that includes two or more substrate supports WT (also referred to as“dual stage”). In such“multiple stage” machines the substrate supports WT can be used in parallel, and / or steps can be performed simultaneously on two or more substrates W located on the substrate supports WT, to prepare the substrates W for later exposure. That is, by providing two or more substrate supports WT, two or more substrates W can be prepared for exposure at the same time. This can result in

[0030] In addition to the substrate support WT, the lithographic apparatus LA can 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 lithographic apparatus, for example a part of the projection system PS or a part of a system that provides an immersion liquid. The measurement platform can be movable under the projection system PS when the substrate support WT is away from the projection system PS.

[0031] 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 (design layout) present on the patterning device MA. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which projects the beam onto a target portion C of the substrate W. By means of the second positioner PW and the position measurement system IF, the substrate support WT can be accurately moved, for example, so as to position different target portions C in the path of the radiation beam B, in a Figure 1The mask alignment marks M1, M2 and the substrate alignment marks P1, P2 can be used to accurately position the patterning device MA relative to the path of the radiation beam B. The mask alignment marks M1, M2 and the substrate alignment marks P1, P2 can be used to align the patterning device MA and the substrate W. Although the illustrated substrate alignment marks P1, P2 occupy dedicated target portions, they can be located in spaces between target portions. When the substrate alignment marks P1, P2 are located between target portions C, these are referred to as cross-line alignment marks.

[0032] As Figure 2 indicated, the lithography apparatus LA can form part of a litho cell LC, which is sometimes also referred to as a litho cell or (litho) cluster, which typically also includes devices to perform pre-exposure and post-exposure processes on the substrate W. These include, conventionally, a spin coater SC for depositing a coating of resist on the substrate W, a developing device DE for developing the exposed resist, a chill plate CH and a bake plate BK, e.g., for adjusting the temperature of the substrate W, e.g., for adjusting a solvent in the resist layer. A substrate handler or robot RO picks up the substrates W from input / output ports I / O1, I / O2, moves them between the different process devices, and delivers the substrates W to a load table LB of the lithography apparatus LA. The devices in the litho cell (which are also generally collectively referred to as tracks) are typically controlled by a track control unit TCU, which itself can be controlled by a supervisory system SCS, which can also control the lithography apparatus LA, e.g., via a lithography control unit LACU.

[0033] In order to have the substrates W exposed by the lithography apparatus LA exposed correctly and consistently, it is desirable to inspect 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 litho cell LC. If errors are detected, adjustments can be made, e.g., to the exposure of subsequent substrates or to other processing steps to be performed on the substrate W, particularly if the inspection is completed before other substrates W of the same batch or lot are still to be exposed or processed.

[0034] An inspection apparatus (which can also be referred to as a metrology apparatus) is used to determine properties of the substrates W, particularly 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 be, for example, part of the litho cell LC, or can be integrated into the lithography apparatus LA, or can be a stand-alone device. The inspection apparatus can measure properties on a latent image (image in the exposed resist layer), or a semi-latent image (image in the resist layer after an post-exposure bake step PEB), or a developed resist image (where exposed or unexposed parts of the resist have been removed), or even an etched image (after a pattern transfer step such as etching).

[0035] Typically, the patterning process in the lithography apparatus LA is one of the most critical steps in the process, requiring high accuracy of the size formation and placement of structures on the substrate W. To ensure this high accuracy, three systems can be combined into a so-called “holistic” control environment, as schematically depicted in Figure 3 One of these systems is the lithography apparatus LA, which is (in effect) connected to a metrology tool MT (second system) and a computer system CL (third system). The key of this “holistic” environment is to optimize the cooperation between these three systems to enhance the overall process window, and to provide a tight control loop to ensure that the patterning performed by the lithography apparatus LA stays within the process window. The process window defines a range of process parameters (e.g. dose, focus, overlay) within which a particular manufacturing process will produce a defined outcome (e.g. functional semiconductor devices), typically within which process parameters in the lithography or patterning process are allowed to vary.

[0036] The computer system CL can use (part of) the design layout to be patterned to predict resolution enhancement techniques to be used, 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 (depicted in Figure 3 by the double arrow in the first scale SCI). Typically, 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 there can be a defect due to, for example, sub-optimal processing (depicted in Figure 3 by the arrow pointing to “0” in the second scale SC2).

[0037] 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, for example in the calibration state of the lithography apparatus LA (depicted in Figure 3 by the multiple arrows in the third scale SC3). Different types of metrology tools MT for measuring one or more properties related to the lithography apparatus and / or the substrate to be patterned will now be described.

[0038] During lithography processes, it is desirable to frequently measure the created structures, for example to perform process control and verification. The tools that perform such measurements are typically called metrology tools MT. Different types of metrology tools MT for performing such measurements are known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. Scatterometers are versatile instruments that allow measuring parameters of a lithography process by placing a sensor in the pupil or in a conjugate plane of the pupil of the scatterometer objective (these measurements are often called pupil-based measurements) or by placing a sensor in an image plane or in a plane conjugate to an image plane (in this case, these measurements are often called image- or field-based measurements). Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP 1,628,164A, which are all incorporated herein by reference. The above-mentioned scatterometers can measure gratings using light from the soft x-ray and visible for near IR wavelength ranges.

[0039] 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. For example, such a reconstruction can be performed by 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 that observed from the real target.

[0040] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, radiation emitted by a radiation source is directed onto the target and the reflected or scattered radiation from the target is directed to a spectrometer detector that measures the spectrum of the specular reflected radiation (i.e. 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.

[0041] In a third embodiment, scatterometer MT is an ellipsometric scatterometer. Ellipsometric scatterometers allow for determining parameters of a lithographic process by measuring the scattered radiation for each polarization state. Such a metrology apparatus emits polarized light, such as linear, circular or elliptical, by using for example appropriate polarization filters in the illumination section of the metrology apparatus. A source suitable for the metrology apparatus can also provide polarized radiation. Various embodiments of existing ellipsometric 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 all incorporated herein by reference.

[0042] In one embodiment of scatterometer MT, scatterometer MT is suitable for measuring the overlay of two misaligned gratings or periodic structures by measuring the reflection spectrum and / or asymmetry in the detection configuration, which is related to the degree of the overlay. The two (typically overlapping) grating structures can be applied in two different layers (not necessarily consecutive layers) and can be formed at substantially the same location on the wafer. The scatterometer can have a symmetric detection configuration as described for example in commonly owned patent application EP 1,628,164 A, so that any asymmetry is clearly distinguishable. This provides a direct way of measuring misalignment in gratings. Other examples for measuring the overlay error between two layers containing periodic structures when the target passes through the asymmetry measurement of the periodic structure can be found in PCT patent application publication number WO 2011 / 012624 or U.S. patent application US 20160161863, which are all incorporated herein by reference.

[0043] Other parameters of interest can be focus and dose. Focus and dose can be determined simultaneously by scatterometry (or alternatively by scanning electron microscopy) as described in U.S. patent application US2011-0249244, which is incorporated herein by reference in its entirety. A single structure can be used that has a unique combination of critical dimension and sidewall angle measurements for each point in a focus energy matrix (FEM, also known as focus exposure matrix). If these unique combinations of critical dimension and sidewall angle are available, the focus and dose values can be uniquely determined from these measurements.

[0044] The metrology target can be the whole of a composite grating, formed by a lithographic process, primarily in resist, but also possibly 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, in particular the NA of the optics, in order to be able to capture the diffraction orders from the metrology target. As indicated earlier, the diffraction signal 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 resulting from the lithographic process. This 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 a smaller sub-segment, which is configured to mimic the dimensions of a functional part of the design layout in the target. Due to this sub-segment, the behavior of the target will be more similar to the functional part of the design layout, such 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 that is smaller than the overall target. In the overfill mode, the measurement beam generates a spot that is larger than the overall target. In such overfill mode, it can also be possible to measure different targets simultaneously, thereby determining different process parameters simultaneously.

[0045] The overall measurement quality of a lithographic parameter using a specific target is at least partially determined by the measurement scheme used to measure the lithographic parameter. The term'substrate measurement scheme' can include one or more parameters of the measurement itself, one or more parameters of the pattern being measured, or both. For example, if the measurement used in the substrate measurement scheme is a diffraction-based optical measurement, the one or more parameters of the measurement can include the wavelength of the radiation, the polarization of the radiation, the radiation incidence angle with respect to the substrate, the radiation orientation with respect to the pattern on the substrate, etc. One of the criteria for selecting a measurement scheme can be, for example, 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 by reference.

[0046] A metrology apparatus, such as scatterometer SM1, is depicted in Figure 4 The metrology apparatus comprises a broadband (white light) radiation projector 2 that projects radiation onto a substrate 6. The reflected or scattered radiation is passed to a spectrometer detector 4 that measures the spectrum 10 of the specular reflected radiation (i.e. the intensity In1 as a function of wavelength λ). From this data, e.g. by rigorous coupled wave analysis and non-linear regression or by comparison with Figure 4The structure or profile of the detected spectrum can be reconstructed by the processing unit PU, typically in comparison with a library of simulated spectra shown in the bottom part. Generally, for the reconstruction, the general form of the structure is known and some parameters are assumed by the knowledge of the process by which the structure was made, only a few parameters of the structure have to be determined from the scatterometry data. Such scatterometers can be configured as normal-incidence scatterometers or oblique-incidence scatterometers.

[0047] In lithographic processes, it is desirable to frequently measure the created structures, for example to make process control and verification. Various tools for making such measurements are known, including a scanning electron microscope or various forms of metrology apparatus, such as a scatterometer. Examples of known scatterometers generally rely on the provision of dedicated metrology targets, such as underfilled targets (targets in the form of simple gratings or overlapping gratings in different layers, large enough for the measurement beam to generate a spot smaller than the grating) or overfilled targets (where the illumination spot partially or completely contains the target). Further, the use of metrology tools (e.g. an angular resolved scatterometer illuminating an underfilled target, such as a grating) allows the use of so-called reconstruction methods, where the properties of the grating can be calculated by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulated results with the measured results. The parameters of the model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from the real target.

[0048] Scatterometers are versatile instruments that allow the measurement of parameters of a lithographic process by placing a sensor in the pupil of the objective of the scatterometer or in a conjugate plane of the pupil (these measurements are often referred to as pupil-based measurements) or by placing a sensor in the image plane or in a plane conjugate to the image plane (in this case, these measurements are often referred to as image- or field-based measurements). Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP 1,628,164A, which are all incorporated herein by reference. In one image, the above-described scatterometer can measure multiple targets from multiple gratings using light from the soft x-ray and visible for the near IR wave range.

[0049] A topography measurement system, level sensor or height sensor that can be integrated in a lithographic apparatus is arranged to measure the topography of the top surface of a substrate (or wafer). A map of the topography of the substrate (also referred to as a height map) can be generated from these measurements, which indicates 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 the transfer of a pattern onto the substrate, in order to provide a spatial image of the patterning device in a position of proper focus on the substrate. It is to be understood that in this context, "height" refers to the dimension that is apparent out of the plane for the substrate (also referred to as the Z-axis). Typically, a level or height sensor performs measurements at a fixed position (relative to its own optical system), and the relative movement between the substrate and the optical system of the level or height sensor results in height measurements at positions over the entire substrate.

[0050] An example of a level or height sensor LS known in the art is schematically shown in Figure 5 , and Figure 5 Only the operating principle 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 that provides a radiation beam LSB that is imparted with a periodic structure by a projection grating PGR of the projection unit LSP. The radiation source LSO can be, for example, a narrowband or broadband radiation source (such as a supercontinuum light source), polarized or non-polarized, pulsed or continuous, such as a polarized or non-polarized laser beam. The radiation source LSO can comprise multiple radiation sources with different colors or wavelength ranges, such as multiple LEDs. The radiation source LSO of the level sensor LS is not limited to visible radiation, but can additionally or alternatively encompass UV and / or IR radiation and any wavelength range suitable for reflection from a substrate surface.

[0051] The projection grating PGR is a periodic grating comprising a periodic structure that results in a radiation beam BE1 with a periodically varying intensity. The intensity periodically varying radiation beam BE1 is directed towards a measurement location MLO on a substrate W that has an angle of incidence ANG with respect to an axis normal to the incident substrate surface (Z-axis) that is between 0 and 90 degrees, typically between 70 and 80 degrees. At the measurement location MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and directed towards the detection unit LSD.

[0052] 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 (e.g. indicative of the intensity of the received light) such as a photodetector or representative of the spatial distribution of the received intensity such as a camera. The detector DET can comprise any combination of one or more detector types.

[0053] By means of triangulation techniques, the height level at the measurement location MLO can be determined. The detected height level is typically related to the signal intensity measured by the detector DET, which signal intensity has a periodicity which depends, inter alia, on the design of the projection grating PGR and the (inclined) angle of incidence ANG.

[0054] The projection unit LSP and / or the detection unit LSD can comprise further optical elements such as lenses and / or mirrors along the path (not shown) of the patterned radiation beam between the projection grating PGR and the detection grating DGR.

[0055] In embodiments, the detection grating DGR can be omitted and the detector DET can be placed at the location where the detection grating DGR is located. Such a configuration provides a more direct detection of the image of the projection grating PGR.

[0056] For efficiently covering 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 a measurement region MLO or an array of spots covering a larger measurement range.

[0057] Various height sensors of the general type are disclosed in US7265364 and US7646471, both incorporated by reference. A height sensor using UV radiation instead of visible or infrared radiation is disclosed in US2010233600A1, incorporated by reference. In WO2016102127A1, incorporated by reference, a compact height sensor is described which uses a multi-element detector to detect and identify the position of a grating image without the need for a detection grating.

[0058] The position measurement system PMS can comprise any type of sensor suitable to determine a position of the substrate support WT. The position measurement system PMS can comprise any type of sensor suitable to determine a position of the mask support MT. The sensor can be an optical sensor, such as an interferometer or an encoder. The position measurement system PMS can comprise a combination of an interferometer and an encoder. The sensor can be another type of sensor, such as a magnetic sensor, a capacitive sensor, or an inductive sensor. The position measurement system PMS can determine a position relative to a reference, e.g. the metrology frame MF or the projection system PS. The position measurement system PMS can determine a position of the substrate table WT and / or the mask support MT by measuring a position or by measuring a time derivative of a position, such as a velocity or an acceleration.

[0059] The position measurement system PMS can comprise an encoder system. Encoder systems are known, e.g. from US patent application US2007 / 0058173A1, filed on September 7, 2006, which is incorporated herewith by reference. The encoder system comprises an encoder head, a grating, and a sensor. The encoder system can receive a primary radiation beam and a secondary radiation beam. Both the primary radiation beam and the secondary radiation beam originate from the same radiation beam, i.e. an original radiation beam. At least one of the primary radiation beam and the secondary radiation beam is generated by diffracting the original radiation beam with the grating. If both the primary radiation beam and the secondary radiation beam are generated by diffracting the original radiation beam with the grating, the primary radiation beam needs to have a different diffraction order than the secondary radiation beam. Different diffraction orders are, e.g., +1 order, -1 order, +2 order, and -2 order. The encoder system optically combines the primary radiation beam and the secondary radiation beam into a combined radiation beam. The sensor in the encoder head determines a phase or a phase difference of the combined radiation beam. The sensor generates a signal based on the phase or the phase difference. The signal is representative of a position of the encoder head relative to the grating. One of the encoder head and the grating can be arranged on the substrate structure WT. The other one of the encoder head and the grating can be arranged on the metrology frame MF or the base frame BF. For example, multiple encoder heads are arranged on the metrology frame MF, while the grating is arranged on a top surface of the substrate support WT. In another example, the grating is arranged on a bottom surface of the substrate support WT, and the encoder head is arranged below the substrate support WT.

[0060] The position measurement system PMS can comprise an interferometer system. Interferometer systems are known, for example, from US patent US 6,020,964, filed on July 13, 1998, which is incorporated herein by reference. The interferometer system can comprise a beam splitter, a mirror, a reference mirror and a sensor. A radiation beam is split by the beam splitter into a reference beam and a measurement beam. The measurement beam propagates to the mirror and is reflected by the mirror back to the beam splitter. The reference beam propagates to the reference mirror and is reflected by the reference mirror back to the beam splitter. At the beam splitter, the measurement beam and the reference beam are combined into a combined radiation beam. The combined radiation beam is incident on the sensor. The sensor determines a phase or a frequency of the combined radiation beam. The sensor generates a signal based on the phase or the frequency. The signal is representative of a displacement of the mirror. In embodiments, the mirror is connected to the substrate support WT. The reference mirror can be connected to the metrology frame MF. In embodiments, the measurement beam and the reference beam are combined into the combined radiation beam by additional optical components instead of the beam splitter.

[0061] In the manufacture of complex devices, it is common to perform many lithographic patterning steps to form functional features in successive layers on the substrate. A key aspect of the performance of a lithographic apparatus is therefore the ability to place the applied pattern correctly and accurately with respect to features laid down earlier (by the same or a different lithographic apparatus) in previous layers. For this purpose, the substrate is provided with one or more sets of marks. Each mark is a structure whose position can be measured at a later time 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”. Marks can also be referred to as metrology targets.

[0062] A lithographic 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 lithographic apparatuses is based on a self-referencing interferometer as described in US 6961116. Various enhancements and modifications to the position sensor have been developed, for example as disclosed in US2015261097A1. All of these disclosures are incorporated herein by reference.

[0063] The marks or alignment marks can comprise a series of bars formed on or in a layer provided on the substrate, or (directly) in the substrate. The bars can be regularly spaced apart and act as grating lines, such that the marks can be considered as diffractive gratings with a well-known spatial period (pitch). Depending on the orientation of these grating lines, the marks can be designed to allow measurement of position along the X-axis or along the Y-axis, which is oriented substantially perpendicular to the X-axis. Marks comprising bars arranged at +45 degrees and / or -45 degrees with respect to the X-axis and Y-axis allow combined X and Y measurement using the techniques described in US2009 / 195768A (which is incorporated herein by reference).

[0064] The alignment sensor optically scans each mark with a radiation spot to obtain a signal that varies periodically, such as a sinusoidal wave. The phase of this signal is analyzed to determine the position of the mark relative to the alignment sensor, and thus the position of the substrate relative to the alignment sensor, which in turn is fixed relative to a reference frame of the lithography apparatus. So-called coarse and fine marks related to different (coarse and fine) mark sizes can be provided, such that the alignment sensor can distinguish between different cycles of the periodic signal as well as the precise position (phase) within a cycle. Marks of different pitches can also be used for this purpose.

[0065] Measuring the position of the marks can also provide information about the deformation of the substrate on which the marks are provided, for example in the form of a wafer grid. Deformation of the substrate can occur when the substrate is exposed to radiation, for example by electrostatically clamping the substrate to the substrate table and / or heating the substrate.

[0066] Figure 6 is a schematic block diagram of an embodiment of a known alignment sensor AS such as described for example in US6961116 (which is incorporated herein by reference). A radiation source RSO provides a beam RB of one or more wavelengths, which is diverted by turning optics onto a mark, such as a mark AM located on a substrate W, as an illumination spot SP. In this example, the turning optics comprise a spot mirror SM and an objective lens OL. The mark AM is illuminated with an illumination spot SP that can have a diameter that is slightly smaller than the width of the mark itself.

[0067] Radiation diffracted by the mark AM is collimated (in this example via the objective lens OL) into the information-bearing beam IB. The term "diffracted" is intended to include zeroth order diffraction (which can be referred to as reflection) from the mark. The beam IB is interfered with itself by a self-referencing interferometer SRI (of the type disclosed in the above-mentioned US 6961116, for example). Thereafter the beam is received by a photodetector PD. In the case that more than one wavelength is created by the radiation source RSO, additional optics (not shown) can be included to provide separate beams. The photodetector can be a single element, if desired, or it can comprise several pixels. The photodetector can comprise a sensor array.

[0068] The turning optics, which in this example comprise a spot mirror SM, can also be used to block zeroth order radiation reflected from the mark, so that the information-bearing beam IB comprises only high order diffracted radiation from the mark AM (this is not essential to the measurement, but improves the signal-to-noise ratio).

[0069] The intensity signal SI is supplied to a processing unit PU. By a combination of optical processing in the block SRI and computational processing in the unit PU, values of the X position and Y position of the substrate relative to the reference frame are output.

[0070] A single measurement of the illustrated type fixes the position of the mark only within a certain range corresponding to one period of the mark. A coarse measurement technique is used in conjunction with such a measurement to identify which period of the sinusoidal wave contains the marked position. The same procedure can be repeated at different wavelengths, at a coarser and / or finer level, in order to improve accuracy and / or to robustly detect the mark, irrespective of the material in which the mark is made and the material above and / or below the position at which the mark is provided. The wavelengths can be optically multiplexed and demultiplexed, in order to be processed simultaneously, and / or they can be time- or frequency-division multiplexed.

[0071] In this example, the alignment sensor and the spot SP remain stationary, while the substrate W is moved. The alignment sensor can thus be rigidly and accurately mounted to the 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 controlled in this movement by its mounting on a substrate support and a substrate positioning system (which controls 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. Measuring the position of the marks provided on the substrate support allows the position of the substrate support, as determined by the position sensor, to be calibrated (e.g. relative to the frame to which the alignment system is connected). Measuring the position of the alignment marks provided on the substrate allows the position of the substrate relative to the substrate support to be determined.

[0072] Metrology tools and / or inspection tools (also referred to as measurement tools) such as described above typically use radiation to obtain measurement data. Different types of radiation can be used depending on the measurement target and the properties to be measured. One different property of radiation is the wavelength(s) used to obtain the measurement values, as different wavelengths can provide different information about the measurement target. Some measurement tools can use broadband radiation (such as supercontinuum radiation) to measure using broadband radiation, or can be able to tune and select the measurement wavelength(s) to be used. Depending on the range of output wavelengths and the properties of the broadband source, a differential method can be used to obtain the broadband radiation. In some implementations for generating broadband radiation, nonlinear effects can be used to broaden input radiation (also referred to as pump radiation) of a narrow wavelength range. Different known setups and methods exist to implement the nonlinear broadening. Typically, these methods rely on confinement of the pump radiation to achieve the high intensities required to experience significant nonlinear effects.

[0073] Known methods for nonlinear broadening of confined radiation include confining laser pump radiation within an optical fiber to generate broadband radiation. The laser can be an ultra-short pulsed laser (e.g. picosecond to femtosecond pulses). The nonlinear propagation dynamics of such radiation within the optical fiber can lead to broadband radiation generation due to soliton self-compression and / or modulation instability. This can be used, for example, to generate supercontinuum radiation spanning from IR to UV wavelength ranges.

[0074] The above-described methods can be used to generate broadband radiation by using nonlinear broadening of laser pulses (e.g. femtosecond laser pulses) propagating along a gas-filled hollow-core photonic crystal fiber. The gas mixture (which can also be referred to as a gas composition) filling the fiber can comprise one or more components, with one component being a working gas. The working gas can be a gas that exhibits significant nonlinear effects when interacting with high intensity radiation. The performance and lifetime of the light source can depend on the composition of the gas mixture filling the fiber. A first issue that can affect the lifetime of the fiber is damage / degradation caused by impurities in the gas. To address this issue, high purity gas components are required to prevent the fiber from gradually becoming invisible. Invisible can be understood as damage to the fiber by contamination and impurity accumulation, gradually blocking the radiation and / or by radiation scattering. Impurities can for example include oxygen O2.

[0075] Another issue that can affect the lifetime and performance of the optical fiber can be thermal damage. The presence of high intensity radiation means that local high temperatures can occur when the radiation interacts with the gas, which in turn can heat the optical fiber material. To address the risk of thermal damage, the thermal conductivity of the gas composition can be increased by adding a gas with good thermal conductivity. Good thermal conductivity can be understood as a thermal conductivity that is much higher than the thermal conductivity of the other components of the gas mixture, such that the average thermal composition of the gas mixture is increased. In some examples, good thermal conductivity can mean that the addition of a gas with good thermal conductivity achieves a thermal conductivity of the gas mixture such that no thermal damage occurs under normal operation of the broadband fiber source. The gas added to increase the thermal conductivity can be a light gas (a gas with low weight particles), such as helium or neon. The light gas can be added in an amount ranging from 10% to 50% of the gas mixture. The addition of a light gas can ensure good thermal conductivity and cooling in the fiber system, which can guarantee the structural integrity of the hollow core fiber.

[0076] Two lifetime-extending features of the gas mixture are described above: a pure gas (low levels of impurities) and the addition of a light gas to increase thermal conductivity. The advantages of these features mean that it is desirable to implement both features in a broadband radiation generating fiber setup. The inventors have found that the application of a light gas and the high purity of the gas mixture in turn leads to sudden and unpredictable breakdowns of the broadband radiation generation process, resulting in a sharp reduction in the broadband radiation emission of the source, as shown in Figure 7 (a) and 7(b). In Figure 7 (a), the graph 700 depicts the output power of an example broadband source with a high purity gas mixture comprising 50% helium and 50% working gas (krypton) as a function of time. In the initial start-up phase, a gradual increase 702 in the output power of the broadband source is shown. Once the output power ramps up to full operation, a steady state output condition can be achieved in 704, where a constant output power is provided by the source for a constant input pump power. At 706, an unexpected breakdown of the source operation occurs, which means that the output power is reduced for the same input pump power. This can be the result of a fiber damage. In Figure 7 (b), these effects are shown in more detail, depicting an example graph of the output power spectral density as a function of wavelength. The arrow 708 shows the decay of the output spectral power over time after the sudden breakdown event 706. The spectral changes during the decay can continue until almost all of the broadband radiation output is lost. Although the breakdown is shown in Figure 7 (a) to occur during the steady operation 704 of the source, it can also occur during the ramp-up phase 702.

[0077] This sudden breakdown occurrence is unexpected and its origin is unknown. Within the present disclosure, the inventors provide an explanation for the cause of this phenomenon and provide a solution to solve it.

[0078] An explanation for the sudden breakdown occurrence of the broadband source setup described herein is disclosed herein. During their research, the inventors realized that a stable plasma is formed within the hollow-core fiber. Due to the high intensity of the pump input pulses within the hollow-core fiber, each pulse can ionize a small fraction of the gas mixture, forming a plasma. This ionization can be the result of tunneling ionization. The plasma can be ionized while the laser pulse is present. The presence of a stable plasma depends on the ionization rate of the working gas, which is caused by the pulses passing through said working gas. In case the ionization rate is high enough, the amount of ionized gas will be large enough to survive the period between subsequent pulses, where the working gas does not ionize. The ionization rate can be high enough in the configuration we researched when the energy of the laser pulse exceeds a value of 2 uJ. The configuration researched uses a hollow-core fiber with a diameter of about 30 um and laser pulses with a duration of 300 fs and a wavelength of about 1 um. In this case, the corresponding peak power will be 80 W and the corresponding peak power density within the hollow-core fiber is about 2.5 TW / cm2. It cannot be excluded that for other configurations (other hollow-core fiber configurations, different pump laser repetition rates, wavelengths, pulse durations and gas mixture properties) ionization can occur at lower values of the corresponding peak power density, e.g. for a peak power density of 1 TW / cm2, the gas mixture can already have a significant ionization, e.g. corresponding to a pulse energy of about 1 uJ and a pulse peak power of 30 W.

[0079] Between laser pulses, the plasma can decay / recombine. If the plasma population decays / recombines fast enough between subsequent pulses, no plasma build-up occurs. However, if the plasma decays / recombines too slowly and some plasma remains at the time of arrival of the subsequent laser pulse. The remaining free electrons can be accelerated by the next (and subsequent) laser pulse(s) and further ionize the neutral atoms. This can lead to an exponential growth of the plasma density within the fiber. This plasma formation can eventually lead to a strong absorption of the pump laser radiation and the generated broadband radiation. The inventors determined that plasma build-up can be the root cause of the sudden breakdown of broadband generation using a hollow-core fiber containing a high purity gas mixture.

[0080] The rate of plasma decay can be strongly dependent on the composition and purity of the gas mixture present within the optical fiber. In particular, it has been determined that gas mixtures including light gases (e.g., helium or neon gas can be added to increase thermal conductivity) can be more susceptible to slow plasma decay, and thus more likely to result in plasma breakdown. The gas mixtures described herein including helium can be particularly susceptible to plasma breakdown. In addition, certain contaminants (e.g., molecular gases) can accelerate plasma decay, which can reduce the likelihood of plasma breakdown. High rates of plasma decay can be referred to as plasma quenching, which can also prevent the buildup of critical high free electron densities.

[0081] The above observations regarding the effect of gas composition on plasma formation can explain why sudden breakdown is less likely to be observed in gas mixtures of lower purity and / or gas mixtures that do not include light gases. Hollow core optical fibers and source assemblies that can have extended lifetimes when operated as part of a broadband radiation generation source will now be described.

[0082] A hollow core optical fiber for broadband generation is provided herein. FIG. 8(a) depicts a cross-section of an example hollow core optical fiber 800 that can be used for broadband radiation generation. FIG. 8(b) depicts the hollow core optical fiber 800 in a radiation source assembly 850. The hollow core optical fiber 800 has a hollow core 802 filled with a gas composition including a working gas. The hollow core optical fiber 800 is configured to receive pulsed pump radiation 810 at an input end 812 of the hollow core optical fiber 800. The pulsed power of the pulsed pump power exceeds an ionization threshold of the gas composition. The hollow core optical fiber 800 is further configured to confine and guide the pulsed pump radiation through the optical fiber such that it interacts with the working gas to generate broadband radiation by nonlinear broadening of the pulsed pump radiation. The broadened radiation can be provided as broadband output radiation 820 at an output end 814 of the hollow core optical fiber 800. During propagation, the radiation can be confined within the hollow core 802 of the hollow core optical fiber 800. The gas composition includes a hydrogen component that is less than 1% of the total gas composition in the hollow core optical fiber 800.

[0083] An advantage of the above-described hollow core optical fiber 800 can be that the lifetime of the optical fiber can be maintained during broadband radiation generation. This can be due to the hydrogen component of the gas composition quenching the formation of plasma, thereby inhibiting the formation of a stable plasma. As a result, damage that can be caused by a plasma within the hollow core of the optical fiber can be avoided. The optical fiber 800 can include a cladding 804 surrounding the hollow core. The cladding can include an anti-resonant element 806 configured to confine radiation within the hollow core 802. If a stable plasma is formed within the optical fiber, the cladding 804 (and in particular the anti-resonant element) can be at risk of thermal damage. The presence of the gas composition provided in the present disclosure can prevent the occurrence of such stable plasma formation.

[0084] The hydrogen component can be provided in a range of 0.001% to 1% of the gas component. The hydrogen component can be provided in a range of 0.01% to 1% of the gas component. Having the hydrogen component present in a low percentage of the gas component can still achieve the plasma quenching effect. If the amount of hydrogen component present in the gas mixture becomes too high, this can cause safety issues as the gas mixture can become flammable. The presence of a larger amount of hydrogen component in the gas mixture can also increase the risk of impurity contamination. It is therefore desirable to keep the percentage of hydrogen component below an upper threshold value.

[0085] The hydrogen component can include hydrogen gas. The hydrogen component can include at least one isotope of hydrogen gas. The isotope of hydrogen gas can include at least one of deuterium and tritium.

[0086] The gas component can further include a cooling gas. The cooling gas can be a light gas configured to increase the thermal conductivity of the gas component. The cooling gas can include at least one of helium gas He or neon gas Ne. In a specific example, the cooling gas can be helium gas. The cooling gas can constitute 20% to 50% of the gas component.

[0087] The impurity concentration of the gas component as a whole can be below 0.001% (1000 ppm). The impurities can include one or more of oxygen and H2O. The low impurity concentration can be achieved by providing the gas component in an environment that is capable of being kept pure. The walls of the environment can for example have a low impurity permeation rate. The permeation rate can be low enough to achieve the desired impurity concentration. The definition of the permeation rate of a material includes properties that are specific to a particular setup (e.g. surface area) and the optical fiber and source assembly described herein do not specify these properties. Therefore, no specific definition of permeation is provided in this context.

[0088] The working gas can include at least one of argon gas Ar, krypton gas Kr and xenon gas Xe. Depending on the type(s) of working gas in the gas component, the nonlinear optical process can include modulation instability (MI), soliton self-compression, soliton fission, Kerr effect, Raman effect and dispersive wave generation, the details of which are described in WO2018 / 127266A1 and US9160137B1 (both incorporated herein by reference). Other properties (e.g. the pressure of the gas mixture within the optical fiber) can influence the nonlinear broadening effect.

[0089] The elements of the source assembly 850 depicted in FIG. 8(b) will now be described in more detail. The radiation source assembly 850 can include a pulsed pump input assembly 830 for providing pump radiation to the source assembly 850. The pump input assembly 830 can be configured to provide input radiation 810, also referred to as pump radiation, to the hollow core fiber 800. The hollow core 802 of the hollow core fiber 800 can be arranged to receive the input radiation 810 from the pulsed pump radiation source and broaden it to provide output radiation 820. The gas mixture can cause the frequency range of the received input radiation 810 to be broadened, providing broadband output radiation 820. The pump input assembly 830 can be configured to receive radiation from an external source, or can include a pump radiation source, such as a laser or any other type of source capable of generating short pulse radiation having a desired length and energy level.

[0090] The hollow core 802 of the hollow core fiber 800 can be filled with a gas mixture. In some implementations, a hollow core fiber 800 filled with a gas mixture can be provided. In other implementations, the hollow core fiber 800 can be disposed in a gas chamber 840, also referred to as an enclosure, container, or reservoir. The gas chamber 840 can be configured to provide the gas mixture to the hollow core fiber 800 when the source assembly 850 is in use. An advantage of providing a filled hollow core fiber can be that a simpler setup can be achieved, as there is no need to provide a setup to provide the gas mixture to the fiber. An advantage of providing a gas chamber 840 configuration can be that the composition of the gas mixture can be more easily tuned / changed. Although in FIG. 8(b) the radiation source assembly 850 is shown as including a gas chamber, alternative implementations including a filled fiber (without a gas chamber) can also be considered. Although in FIG. 8(b) the radiation source 850 includes the fiber 800 shown in FIG. 8(a), in alternative embodiments other types of hollow core fibers can be used.

[0091] In some implementations, the radiation source assembly 850 can include a gas cell 840 for providing a gas mixture within the hollow core optical fiber 800. The hollow core optical fiber 800 can be disposed within a reservoir within the gas cell 840. The gas cell 840 can be configured to provide and contain the gas mixture. The gas cell can include one or more features known in the art for controlling, regulating, and / or monitoring the composition of the gas mixture. In use, the hollow core optical fiber 800 can be disposed within the gas cell 840 such that a first transparent window is located proximate the input end 812 of the optical fiber 800. The first transparent window can be transparent at least to the frequency of the received input radiation such that the received output radiation 810 (or at least a substantial portion thereof) can be coupled into the hollow core 802 of the optical fiber 800 located within the gas cell 840. It will be appreciated that optical devices (not shown) can be provided for coupling the input radiation 810 into the optical fiber 800. In use, the output end 814 of the hollow core optical fiber 800 can be proximate a second transparent window. The second transparent window can be transparent at least to the frequency of the broadband output radiation 820 of the radiation source 850. Alternatively, in another embodiment, the two opposite ends 812, 814 of the hollow core optical fiber 800 can be placed within different reservoirs of the gas cell 840. This arrangement using two separate gas reservoirs can be particularly convenient for embodiments in which the hollow core optical fiber 800 is relatively long (e.g., when the length is greater than 1 m). In this context, a window can be transparent to a frequency if at least 50%, 75%, 85%, 90%, 95%, or 99% of incident radiation of that frequency on the window is transmitted through the window.

[0092] In implementations in which the hollow core optical fiber 800 has been filled with a gas mixture, a gas cell 840 can not be required. However, the elements described above in relation to the gas cell can still be present in the radiation source assembly (e.g., transparent windows, coupling optics). The filled hollow core optical fiber 800 can be closed at either end of the hollow core 802 to contain the gas mixture within the hollow core. The material used to close the hollow core of the optical fiber can be the same material as the cladding of the optical fiber 800. The closed end of the optical fiber can be transparent to one or more frequencies of radiation being input to / from the optical fiber 800.

[0093] To achieve frequency broadening, high intensity radiation can be desirable. The advantage of having a hollow core optical fiber is that it can achieve high intensity radiation by strong spatial confinement of the radiation propagating through the optical fiber 800, thereby achieving high localized radiation intensity. The radiation intensity within the optical fiber can be high, for example due to high received input radiation intensity and / or due to strong spatial confinement of the radiation within the hollow core optical fiber. The hollow core optical fiber can confine and guide a large portion of the radiation within the hollow core 802 of the optical fiber. The hollow core optical fiber 800 can be able to guide radiation in a wider wavelength range than a solid core optical fiber, in particular the hollow core optical fiber can guide radiation in the ultraviolet and infrared range.

[0094] The example hollow core optical fiber 800 shown in Figs. 8(a) and 8(b) will now be described in more detail Figure 8(a) and 8(b) The optical fiber 800 can comprise an elongated body defining a length of the optical fiber. The length of the optical fiber is the dimension that is longer compared to the other two dimensions of the optical fiber. This longer dimension can be referred to as the axial direction and can define an axis of the hollow core optical fiber. As shown in Fig. 8(a), the two other dimensions of the optical fiber define a plane that can be referred to as the transverse plane. Fig. 8(a) shows a cross-section of the hollow core optical fiber 800 in this transverse plane, i.e. perpendicular to the axis. The cross-section of the hollow core optical fiber 800 can be substantially constant along the optical fiber axis.

[0095] It is to be understood that the hollow core optical fiber 800 has a degree of flexibility, and as such, the direction of the axis is generally not uniform along the length of the optical fiber 800. Terms such as optical axis, cross-section, etc. are to be understood as referring to a local optical axis, a local cross-section, etc. Furthermore, where components are described as being cylindrical or tubular, these terms are to be understood as encompassing such shapes that can have been deformed as the hollow core optical fiber 800 is bent.

[0096] The hollow core optical fiber can have any length, and it is to be understood that the length of the optical fiber 800 can depend on the application. For example, the length of the optical fiber can be between 1 cm and 10 m. The length of the hollow core optical fiber 800 can for example be between 10 cm and 100 cm.

[0097] The hollow core fiber can include a hollow core 802 surrounded by a cladding portion 804. A support portion can be provided surrounding and supporting the cladding portion 804. The hollow core fiber 800 can be considered to include a main body (including the cladding portion and the support portion SP) with the hollow core 802. The hollow core fiber 800 can include a plurality of anti-resonant elements 806 for guiding radiation through the hollow core 802. In some implementations, the plurality of anti-resonant elements 806 can be arranged to confine radiation propagating through the fiber 800 primarily within the hollow core 802. The anti-resonant elements can guide the radiation along the fiber 800. The hollow core 802 can be substantially disposed at a central region of the fiber 802 such that an axis of the fiber 802 can also define an axis of the hollow core 802.

[0098] In some implementations, the anti-resonant elements 806 can include a plurality of capillaries. The capillaries can be arranged in a single ring of capillaries surrounding the hollow core 802. In a particular example, the cladding portion 804 can include a single ring of six tubular capillaries 806 surrounding the hollow core, where each tubular capillary can act as an anti-resonant element.

[0099] In example implementations, a hollow core fiber with a gas mixture can be provided as described herein, where the purity of the gas mixture is such that the impurity concentration is below 0.001% (100 0 ppm). The gas mixture can include a working gas for nonlinear broadening, a light gas for increasing thermal conductivity, and a hydrogen component for suppressing stable plasma generation. The purity of the gas composition, the presence of the light gas component, the working gas component, and the hydrogen component can all contribute to extending the lifetime of the hollow core fiber when the hollow core fiber is used for broadband radiation generation.

[0100] The broadband radiation can include supercontinuum radiation. The supercontinuum radiation can include radiation spanning a range from ultraviolet UV to infrared IR radiation. This can be, for example, from 100 nm to 2000 nm, from 200 nm to 2000 nm, or in the range of 200 nm to 1600 nm.

[0101] The pump radiation can be pulsed pump radiation. The pump radiation can be a single (pulsed) beam of radiation provided to the optical input of the PIC. The radiation can be in the range of 400 nm to 2000 nm, or in the range of 800 nm to 1600 nm. The pulsed radiation can include one or more specific wavelengths of radiation, such as 400 nm, 515 nm, 800 nm, 1030 nm, 1550 nm, and / or 2000 nm.

[0102] Other embodiments of the invention are disclosed in the following numbered clauses:

[0103] 1. A hollow-core optical fiber for broadband generation, wherein a hollow core of the hollow-core optical fiber is filled with a gas composition comprising a working gas, wherein the optical fiber is configured to:

[0104] receive, at an input end of the hollow-core optical fiber, pulsed pump radiation having a pulse power that exceeds an ionization threshold of the gas composition; and

[0105] confining and guiding the pulsed pump radiation through the optical fiber such that the pulsed pump radiation interacts with the working gas to generate broadband radiation by nonlinear broadening of the pulsed pump radiation; and

[0106] wherein the gas composition comprises a hydrogen component that is less than 1% of a total gas composition in the hollow-core optical fiber.

[0107] 2. The hollow-core optical fiber according to clause 1, wherein the hydrogen component is in a range of 0.001% to 1% or 0.01% to 1% of the gas composition.

[0108] 3. The hollow-core optical fiber according to clause 1 or 2, wherein the hydrogen component comprises hydrogen gas.

[0109] 4. The hollow-core optical fiber according to any of the preceding clauses, wherein the hydrogen component comprises at least one isotope of hydrogen gas.

[0110] 5. The hollow-core optical fiber according to clause 4, wherein the at least one isotope of hydrogen gas comprises at least one of deuterium and tritium.

[0111] 6. The hollow-core optical fiber according to any of the preceding clauses, wherein the working gas comprises at least one of argon, krypton, and xenon.7. The hollow-core optical fiber according to any of the preceding clauses, wherein the gas composition comprises a cooling gas.

[0112] 8. The hollow-core optical fiber according to clause 7, wherein the cooling gas comprises at least one of helium and neon.

[0113] 9. The hollow-core optical fiber according to clause 7 or 8, wherein the cooling gas is in a range of 20% to 50% of the gas composition.

[0114] 10. The hollow-core optical fiber according to any of the preceding clauses, wherein an impurity concentration of the gas composition is below 0.001%.

[0115] 11. The hollow-core optical fiber according to clause 10, wherein the impurity comprises one or more of oxygen and H2O.

[0116] 12. The hollow-core optical fiber according to any of the preceding clauses, wherein the broadband radiation comprises supercontinuum radiation.

[0117] 13. The hollow-core optical fiber according to clause 12, wherein the broadband radiation comprises wavelengths in a range of 200 nm to 2000 nm.

[0118] 14. The hollow-core fiber according to any of the preceding clauses, wherein the hollow- core fiber is a hollow-core photonic crystal fiber.

[0119] 15. The hollow-core fiber according to clause 14, wherein the hollow-core photonic crystal fiber comprises a single capillary ring around the hollow core.

[0120] 16. The hollow-core fiber according to any of the preceding clauses, wherein the broadband radiation is output at an output end of the hollow-core fiber.

[0121] 17. The hollow-core fiber according to any of the preceding clauses, wherein the broadband radiation is generated by a modulational instability of the working gas interacting with the pulsed pump radiation.

[0122] 18. The hollow-core fiber according to any of the preceding clauses, wherein the pump radiation comprises one or more wavelengths in the range of 800 nm to 2000 nm or 400 nm to 550 nm.

[0123] 19. The hollow-core fiber according to any of the preceding clauses, wherein the fiber is a filled fiber, wherein a gas component is enclosed within the filled fiber.

[0124] 20. The hollow-core fiber according to any of the preceding clauses, wherein the pulse power corresponds to a peak power density within the hollow-core fiber of greater than 1 TW / cm2.

[0125] 21. The hollow-core fiber according to any of the preceding clauses, wherein the pulse power corresponds to a peak power density within the hollow-core fiber of greater than 2.5 TW / cm2.

[0126] 22. The hollow-core fiber according to any of the preceding clauses, wherein the pulse power corresponds to a pulse peak power of greater than 30 W.

[0127] 23. The hollow-core fiber according to any of the preceding clauses, wherein the pulse power corresponds to a pulse peak power of greater than 80 W.

[0128] 24. The hollow-core fiber according to any of the preceding clauses, wherein the pulse power corresponds to a pulse energy of greater than 1 uJ.

[0129] 25. The hollow-core fiber according to any of the preceding clauses, wherein the pulse power corresponds to a pulse energy of greater than 2 uJ.

[0130] 26. A source assembly for broadband radiation generation, the source assembly comprising:

[0131] a hollow-core fiber according to any of clauses 1 to 25; and

[0132] a pump input assembly configured to provide pulsed pump radiation to the hollow core optical fiber.

[0133] 27. The source assembly of any of clauses 26, wherein the optical fiber is a filled optical fiber, wherein the gas component is enclosed within the filled optical fiber.

[0134] 28. The source assembly of any of clauses 26 to 27, wherein the source assembly further comprises a gas cell configured to supply the gas component to the hollow core optical fiber.

[0135] 29. The source assembly of clause 28, wherein the gas cell has an impurity permeation rate such that the impurity concentration of the gas component is below 0.001%.

[0136] 30. The source assembly of any of clauses 26 to 29, wherein the pump input assembly comprises a pulsed pump laser.

[0137] 31. The source assembly of clause 30, wherein the pulsed pump laser is configured to provide pulses with a pulse energy exceeding 1 uJ.

[0138] 32. The source assembly of clause 30, wherein the pulsed pump laser is configured to provide pulses with a pulse energy exceeding 2 uJ.

[0139] 33. The source assembly of clause 30, wherein the pulsed pump laser is configured to provide pulses with a pulse peak power exceeding 30 W.

[0140] 34. The source assembly of clause 30, wherein the pulsed pump laser is configured to provide pulses with a pulse peak power exceeding 80 W.

[0141] 35. A broadband radiation source comprising the hollow core optical fiber of any of clauses 1 to 25.

[0142] 36. A broadband radiation source comprising the source assembly of any of clauses 26 to 34.

[0143] 37. A metrology apparatus comprising the hollow core optical fiber of any of clauses 1 to 25.

[0144] 38. An inspection apparatus comprising the hollow core optical fiber of any of clauses 1 to 25.

[0145] 39. A lithographic apparatus comprising the hollow core optical fiber of any of clauses 1 to 25.

[0146] 40. A lithographic cell comprising the apparatus of any of clauses 37 to 39.

[0147] 41. A source assembly for broadband radiation generation, comprising:

[0148] A hollow core fiber (HCF) filled with a gas composition comprising a working gas and a hydrogen component; and a radiation source configured to provide pulsed pump radiation to an input end of the HCF, the pulsed pump radiation configured to interact with the working gas to generate broadband radiation and having a pulse power that exceeds an ionization threshold of the working gas, characterized in that the hydrogen component constitutes less than 1% of the molar percentage of the gas composition.

[0149] 42. A hollow core fiber (HCF) filled with a gas composition comprising a hydrogen component and a working gas to generate broadband radiation by nonlinear broadening upon receipt of pulsed pump radiation, characterized in that the hydrogen component constitutes less than 1% of the molar percentage of the gas composition.

[0150] 43. A method for generating broadband radiation, the method comprising:

[0151] providing a hollow core fiber (HCF) filled with a gas composition comprising a working gas and a trace hydrogen component, the hydrogen component constituting less than 1% of the molar percentage of the gas composition; and

[0152] directing pulsed pump radiation into the working gas within the HCF to generate broadband radiation.

[0153] 44. A source assembly for broadband radiation generation, comprising:

[0154] a hollow core fiber (HCF) filled with a gas composition comprising a working gas; and

[0155] a radiation source configured to provide pulsed pump radiation to an input end of the HCF, the pulsed pump radiation configured to interact with the working gas to generate broadband radiation, wherein the gas composition further comprises a trace gas to neutralize ions formed by the pulsed pump radiation on a timescale that is less than a period between successive pulses of the pump radiation.

[0156] 45. A hollow core fiber (HCF) filled with a gas composition comprising a working gas to generate broadband radiation by nonlinear broadening upon receipt of pulsed pump radiation and a trace gas to neutralize ions formed by the pulsed pump radiation on a timescale that is less than a period between successive pulses of the pulsed pump radiation.

[0157] 46. A method of generating broadband radiation, the method comprising:

[0158] providing a hollow core fiber (HCF) filled with a gas composition comprising a working gas and a trace gas to neutralize ions of the working gas formed during the generation of the broadband radiation; and directing pulsed pump radiation into the working gas within the HCF to initiate a nonlinear optical process.

[0159] 47. The source assembly according to any of clauses 26 to 34, wherein the gas composition further comprises helium for providing cooling of the gas composition.

[0160] 48. The source assembly according to any of clauses 26 to 34 or 47, wherein the working gas comprises argon.

[0161] 49. The HCF according to clause 45, wherein the gas composition further comprises helium for providing cooling of the gas composition.

[0162] 50. The hollow core fiber according to any of clauses 1 to 25, 45 or 49, wherein the working gas comprises argon.

[0163] Although specific reference can be made in this text to the use of the lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein can have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.

[0164] Although specific reference can be made in this text to the use of embodiments of the application in the manufacture of ICs, it should be understood that the embodiments of the application can have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.

[0165] Although specific reference can have been made above to the use of embodiments of the application in the context of optical lithography, it will be appreciated that the application, where the context allows, is not limited to optical lithography and can be used in other applications, for example imprint lithography.

[0166] Although specific reference can be made to "metrology apparatus / tool / system" or "inspection apparatus / tool / system", these terms can refer to the same or similar type of tool, apparatus or system. For example, an inspection or metrology apparatus including embodiments of the application can be used to determine a property of a structure on a substrate or wafer. For example, an inspection apparatus or metrology apparatus including embodiments of the application can be used to detect a defect of a substrate or a defect of a structure on a substrate or wafer. In such embodiments, the property of interest of the structure on the substrate can be related to a defect in the structure, an absence of a particular part of the structure or a presence of an unwanted structure on the substrate or wafer.

[0167] While specific embodiments of the application have been described above, it will be appreciated that the application can be practiced otherwise than as described. The description is not intended to limit the application. Accordingly, those skilled in the art will recognize that modifications can be made by using no more than routine engineering and by making modifications which are well within the scope of the application as set forth in the claims below.

Claims

1. A hollow-core optical fiber for broadband generation, wherein a hollow core of the hollow-core optical fiber is filled with a gas composition comprising a working gas, wherein the optical fiber is configured to: receiving pulsed pump radiation at an input end of the hollow core optical fiber, the pulsed pump power having a pulse power that exceeds an ionization threshold of the gaseous composition; and restrict and guide the pulsed pump radiation through the optical fiber such that the pulsed pump radiation interacts with the working gas to generate broadband radiation by nonlinear broadening of the pulsed pump radiation; and wherein the gas composition comprises a hydrogen component that is less than 1% of the total gas composition in the hollow-core optical fiber.

2. The hollow-core optical fiber of claim 1, wherein the hydrogen component is in a range of 0.01% to 1% of the gas composition.

3. The hollow-core optical fiber of claim 1, wherein the working gas comprises at least one of argon, krypton, and xenon.

4. The hollow-core optical fiber of claim 1, wherein the gas composition comprises a cooling gas comprising at least one of helium and neon.

5. The hollow-core optical fiber of claim 4, wherein the cooling gas is in a range of 20% to 50% of the gas composition.

6. The hollow-core optical fiber of claim 1, wherein the broadband radiation comprises wavelengths in a range of 200 nm to 2000 nm.

7. The hollow-core optical fiber of claim 1, wherein the hollow-core optical fiber is a hollow-core photonic crystal fiber.

8. The hollow-core optical fiber of claim 1, wherein the pulsed power corresponds to a peak power density greater than 1 TW / cm2 within the hollow-core optical fiber.

9. The hollow-core optical fiber of claim 1, wherein the pulsed power corresponds to a pulse peak power greater than 30 W.

10. The hollow-core optical fiber of claim 1, wherein the pulsed power corresponds to a pulse energy greater than 1 uJ.

11. A source assembly for broadband radiation generation, the source assembly comprising: a hollow-core optical fiber according to claim 1; and a pump input assembly configured to provide pulsed pump radiation to the hollow-core optical fiber.

12. The source assembly of claim 11, wherein the pulsed pump laser is configured to provide pulses with a pulse energy exceeding 1 uJ.

13. A broadband radiation source comprising the source assembly according to claim 11.

14. A metrology apparatus comprising the hollow-core optical fiber according to claim 1.

15. An inspection apparatus comprising the hollow-core optical fiber according to claim 1.

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