Broadband radiation generator based on hollow-core fiber with extended fiber lifetime

Abstract

An optical component for a broadband light source device is disclosed, the optical component configured for generating a broadband output upon receiving pump radiation, and the optical component comprising a hollow-core photonic crystal fiber (HC-PCF); and a gas mixture filling the HC-PCF wherein the gas mixture comprises a mixture of at least one first gas configured for the generation of the broadband radiation and at least one second gas comprising or consisting of helium.

Description

[0001] This application is a divisional application of patent application No. 2021800480100 with a filing date of 5 January 2023 in the China National Phase, for the invention entitled “Broadband radiation generator based on hollow-core fiber with extended fiber lifetime” of applicant “ASML NETHERLANDS B.V.”, which entered the China National Phase on 5 January 2023, with a filing date of 11 June 2021, and international application number PCT / EP2021 / 065798.

[0002] Cross Reference to Related Applications

[0003] This application claims priority to European application 20184730.8 filed on 8 July 2020 and European application 21167961.8 filed on 13 April 2021, the contents of which are incorporated herein by reference in their entirety. TECHNICAL FIELD

[0004] The present invention relates to a broadband radiation generator based on hollow-core fiber, and in particular to such a broadband radiation generator in relation to metrology applications in integrated circuit manufacturing. BACKGROUND

[0005] 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 can also be 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).

[0006] To project the 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. Lithography apparatuses are used to form smaller and smaller features on the substrate, as the technology advances. This allows more and more circuitry to be incorporated into a given area, increasing the performance of the ICs. However, as the features made using the lithography apparatuses get smaller, it is increasingly difficult to make them accurately. This is because the projection of the pattern by the lithography apparatus is subject to variations in the radiation, the lithography apparatus and the substrate. These variations can result in distortions in the pattern being projected on the substrate. Such distortions can result in variations in the size and / or shape of features made in the substrate. These variations can result in defective devices, which can result in the rejection of an IC. It is therefore desirable to make the lithography apparatus as accurate as possible.

[0007] 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 the wavelength of the radiation used. In such a process, the resolution formula is given by CD = kixl / NA, where l is the wavelength of the radiation used, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the "critical dimension" (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 shape and dimensions planned by a circuit designer in the lithographic apparatus. 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 optimizations of the design layout such as optical proximity correction (OPC, sometimes also referred to as "optical and process correction") in the design layout, or other methods generally defined as "resolution enhancement techniques" (RET). Alternatively, tight control loops used to control the stability of the lithography apparatus can be used to improve reproduction of patterns at low ki.

[0008] Metrology tools are used in many aspects of the IC manufacturing process, for example as alignment tools for proper positioning of substrates prior to exposure, leveling tools for measuring the surface topography of substrates, e.g. focus control and scatterometry based tools for inspecting / measuring the exposed and / or etched product in process control. In each case a radiation source is required. For various reasons including measurement stability and accuracy, broadband or white light radiation sources are increasingly used for such metrology applications. It would be desirable to make improvements to current arrangements for broadband radiation generation. SUMMARY

[0009] In a first aspect of the application, there is provided an optical component for a broadband light source arrangement, the optical component being configured for generating broadband radiation upon receiving pump radiation, and comprising: a hollow core photonic crystal fiber (HC-PCF); and a gas mixture filling the HC-PCF, wherein the gas mixture comprises a mixture of at least one first gas configured for the generation of the broadband radiation and at least one second gas comprising or consisting of helium for thermal regulation of the gas mixture and / or suppression of acoustic waves in the gas mixture.

[0010] In a second aspect of the application, there is provided a broadband light source arrangement comprising the optical component of the first aspect and further comprising a pump radiation source for generating the pump radiation.

[0011] In a third aspect, there is provided a metrology tool for inspection of a substrate, the metrology tool comprising a broadband light source arrangement as described in the second aspect.

[0012] In a fourth aspect of the application, there is provided a method for configuring an optical component for a source arrangement, the source arrangement being configured for generating a broadband radiation output, the method comprising: selecting a hollow core photonic crystal fiber (HC-PCF) and a gas mixture to fill the HC-PCF, the gas mixture comprising a first gas and a second gas, the first gas being for generating the broadband radiation, the second gas comprising helium; and determining an optimized molar fraction of helium present in the gas mixture, wherein the optimized molar fraction of helium is based on one or more of: increasing the thermal conductivity of the gas mixture; increasing the thermal diffusivity of the gas mixture; or selecting a desired heat transfer mechanism. BRIEF DESCRIPTION OF DRAWINGS

[0013] Embodiments of the application will now be described, by way of example only, with reference to the accompanying schematic drawings in which:

[0014] - Figure 1 depicts a schematic overview of a lithography apparatus;

[0015] - Figure 2 depicts a schematic overview of a lithography cell;

[0016] - Figure 3 depicts a schematic representation of holistic lithography, the schematic representation of holistic lithography representing a cooperation between three key technologies for optimizing semiconductor manufacturing;

[0017] - Figure 4 depicts a schematic overview of a scatterometry apparatus that can comprise a radiation source for use as a metrology arrangement according to embodiments of the application;

[0018] - Figure 5 depicts a schematic overview of a level sensor apparatus that can comprise a radiation source according to embodiments of the application;

[0019] - Figure 6 depicts a schematic overview of an alignment sensor apparatus that can comprise a radiation source according to embodiments of the application;

[0020] - Figure 7 depicts schematically a lateral cross-section of two HC-PCF designs for white light generation, the HC-PCF designs comprising (a) a Kagome design and (b) a single ring design;

[0021] - Figure 8Two cross-sectional images taken at two different locations of an HC-PCF with a single-ring design operated in pure krypton gas: (a) cross-sectional image taken at the fiber end; (b) cross-sectional image taken at a location a few centimeters from the same fiber end.

[0022] - Figure 9 Schematic depiction of a broadband light source apparatus based on a gas-filled HC-PCF;

[0023] - Figure 10 (a) to Figure 10 (c) schematically depicts an example of optical components in three different configurations;

[0024] - Figure 11 Graphs illustrating the thermal conductivity of several binary mixtures of atomic gases as a function of molecular weight; and

[0025] - Figure 12 Graphs showing the results of a simplified simulation of thermal relaxation in an HC-PCF for two different thermal diffusivities. DETAILED DESCRIPTION

[0026] 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).

[0027] The term“reticle”,“mask” or“patterning device” as used herein can be broadly interpreted to include any patterning device that can be used to impart a pattern to an incoming radiation beam, including a mask or a reticle, which are examples of patterning devices. In general, a mask is a transmissive patterning device while a reticle is a reflective patterning device. However, in the context of the present application, the terms“mask” and“reticle” are used interchangeably. In some instances, the terms“mask” and“reticle” can also be used to refer to a programmable mirror array and a programmable LCD array as the patterning device.

[0028] Figure 1A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA comprises an illumination system (also called illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation), a mask support (e.g., mask table) MT configured to support a patterning device (e.g., mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to certain parameters, a substrate support (e.g., wafer stage) WT configured to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support according to certain parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0029] In operation, the illumination system IL receives a radiation beam from a radiation source SO, for example, via a beam delivery system BD. The illumination system IL may include various types of optical components for directing, shaping, and / or controlling the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0030] The term "projection system" PS as used herein should be broadly interpreted as covering various types of projection systems, including refractive, reflective, catadioptric, synthetic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as appropriate to the exposure radiation used and / or other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein should be considered synonymous with the more general term "projection system" PS.

[0031] The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index (e.g. water) so as to fill the space between the projection system PS and the substrate W - this is also known as immersion lithography. More information on immersion techniques is given in US Pat. No. 6,952,253, which is incorporated herein by reference.

[0032] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also known as a "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or a step of preparing a substrate W for subsequent exposure may be performed on a substrate W on one of the substrate supports WT while another substrate W on another substrate support WT is being used to expose a pattern on the other substrate W.

[0033] In addition to the substrate support WT, the lithography apparatus LA can include a measurement platform. The measurement platform is configured 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 that provides the 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.

[0034] 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 a 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 focuses the beam onto a target portion C of the substrate W. By means of the second positioner PW and the position measurement system IF, the substrate support WT can be accurately moved in relation to the radiation beam B, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and possibly the other position sensor (not explicitly depicted in Figure 1 the drawings) can be used to accurately position the patterning device MA in relation to the path of the radiation beam B. The patterning device MA and the substrate W can be aligned using the mask alignment marks M1, M2 and the substrate alignment marks P1, P2. Although the substrate alignment marks P1, P2 occupy dedicated target portions as explained, they can be located in spaces between target portions. When the substrate alignment marks P1, P2 are located between target portions C, these substrate alignment marks are referred to as scribe-lane alignment marks.

[0035] As Figure 2As 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 typically also includes apparatus for pre-exposure and post-exposure processing of the substrate W. Conventionally, these include a spin coater SC for depositing a coating of photoresist on the substrate W, a developing apparatus DE for developing the exposed photoresist, and an chill plate CH and a bake plate BK, for example, for adjusting the temperature of the substrate W (for example, to adjust the temperature of the photoresist layer). A substrate handler or robot RO picks up substrates W from input / output ports I / O1, I / O2, moves the substrates W between different process apparatus and delivers the substrates W to a load deck LB of the lithography apparatus LA. The apparatuses in the lithocell, often collectively referred to as a track system, are typically under the control of a track system control unit TCU, which itself can be controlled by a supervisory control system SCS, which can also control the lithography apparatus LA, for example via a lithography control unit LACU.

[0036] In order to expose substrates W exposed by the lithography apparatus LA correctly and consistently, it is necessary to inspect the substrates to measure properties of the patterned structures, such as overlay error between subsequent layers, line thickness, critical dimension (CD), etc. For this purpose, an inspection tool (not shown) can be included in the lithocell LC. If errors are detected, the exposure of subsequent substrates can be adjusted, for example, or other processing steps to be performed on the substrates W, especially if the inspection takes place before other substrates W of the same batch or lot are still to be exposed or processed.

[0037] An 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 can even be a stand-alone device. The inspection apparatus can measure properties on a latent image (the image in the photoresist layer after exposure), or on a semi-latent image (the image in the photoresist layer after a post-exposure bake step PEB), or on a developed photoresist image (where either the exposed or the unexposed parts of the photoresist have been removed), or even on an etched image (after a pattern transfer step such as etching).

[0038] Typically, the patterning process in the lithography apparatus LA is one of the most critical steps in the process, requiring a high degree of accuracy in the sizing and placement of structures on the substrate W. To ensure this high degree of accuracy, three systems are combined in a so-called "holistic" control environment, as shown in Figure 2. Figure 3One of these systems is schematically depicted. One of these systems is a lithography apparatus LA which is (in effect) connected to a metrology tool MT (a second system) and to a computer system CL (a third system). The key to this "overall" 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 certain manufacturing process yields a defined result (e.g., a functional semiconductor device) - typically within which variations in the process parameters in the lithography or patterning process are allowed.

[0039] The computer system CL can use (part of) the design layout to be patterned to predict which resolution enhancement techniques to use and to perform computational lithography simulations and calculations to determine which mask layout and lithography apparatus settings achieve the largest overall process window for the patterning process (in Figure 3 depicted by the double arrow in the first scale SCI). Typically, the resolution enhancement techniques are configured 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 3 depicted by the arrow pointing to "0" in the second scale SC2).

[0040] 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 3 depicted by the multiple arrows in the third scale SC3).

[0041] In lithography processes, it is desirable to frequently measure the created structures, e.g. for process control and verification. Tools for making such measurements are commonly referred to as metrology tools MT. Different types of metrology tools MT for making such measurements are known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. A scatterometer is a versatile instrument that allows measuring parameters of a lithography process either by having a sensor in the pupil or in a plane conjugate to the pupil of the objective of the scatterometer (the measurement is commonly referred to as pupil-based measurement), or by having a sensor in the image plane or in 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 related measurement techniques are further described in patent application US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP 1,628,164A, incorporated herein in its entirety by reference. The aforementioned scatterometers can use light from the soft x-ray and visible to near IR wavelength range to measure gratings.

[0042] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In this scatterometer, a reconstruction method can be applied to the measured 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.

[0043] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In this spectroscopic scatterometer MT, the radiation emitted by the radiation source is directed onto the target and the reflected or scattered radiation from the target is directed onto a spectrometer detector that measures the spectrum of the specular reflected radiation (i.e. a measurement of 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, e.g. by rigorous coupled wave analysis and nonlinear regression or by comparison with a library of simulated spectra.

[0044] In a third embodiment, the scatterometer MT is an ellipsometry scatterometer. Ellipsometry scatterometers allow determining parameters of a lithography process by measuring the scattered radiation for each of the polarization states. 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. The source suitable for the metrology apparatus can also provide polarized radiation. Various embodiments of existing ellipsometry scatterometers are described in U.S. Patent Application Nos. 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.

[0045] In Figure 4 A metrology apparatus such as a scatterometer is depicted in Fig. 1. It comprises a broadband (white light) radiation projector 2 which projects radiation onto a substrate W. The reflected or scattered radiation is passed to a spectrometer detector 4 which measures the spectrum 6 (i.e. the intensity measurement as a function of wavelength) of the specularly reflected radiation. From this data, the structure or profile 8 which gave rise to the detected spectrum can be reconstructed by a processing unit PU (e.g. by rigorous coupled wave analysis and non-linear regression, or by comparison with a library of simulated spectra as shown at the bottom of Fig. 1). Generally, for the 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. This scatterometer can be configured as a normal-incidence scatterometer or an oblique-incidence scatterometer. Figure 3

[0046] The overall measurement quality of measuring a lithographic parameter of a metrology target is determined at least in part by the measurement recipe used to measure this 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 measured parameters can include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the 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. Further 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.

[0047] ​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 map of the topography of the substrate (also referred to as a height map) can be generated from these measurements indicative of 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 an aerial image of the patterning device at the correct focus position on the substrate. It will be appreciated that "height" in this context refers to the generalized dimension of the plane to 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 relative movement between the substrate and the optical system of the level or height sensor produces height measurements at various positions across the substrate.

[0048] Figure 5 An example of a level or height sensor LS known in the art is schematically illustrated in Fig. 1, which illustrates the principle of operation only. 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 imparted 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 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 light radiation but can additionally or alternatively encompass UV and / or IR radiation and any wavelength range suitable for reflection from the surface of a substrate.

[0049] The projection grating PGR is a periodic grating comprising a periodic structure which produces a radiation beam BE1 with a periodically varying intensity. The radiation beam BE1 with a periodically varying intensity is directed towards a measurement location MLO on the substrate W having an angle of incidence ANG with respect to an axis normal to the surface of the substrate of incidence (Z-axis) which 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.

[0050] 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 photo detector; or indicative of the spatial distribution of the received intensity, such as a video camera. The detector DET can comprise any combination of one or more detector types.

[0051] 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 as measured by the detector DET, which signal intensity has a periodicity which depends, among others, on the design of the projection grating PGR and the (inclined) angle of incidence ANG.

[0052] The projection unit LSP and / or the detection unit LSD can comprise further optical components, 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.

[0053] In an embodiment, the detection grating DGR can be omitted, and the detector DET can be placed at the location where the detection grating DGR is located. This configuration provides a more direct detection of the image of the projection grating PGR.

[0054] For efficiently covering the surface of the substrate W, the level sensor LS can be arranged to project an array of measurement beams BE1 onto the surface of the substrate W, thereby creating an array of measurement regions MLO or light spots covering a larger measurement range.

[0055] Various height sensors of the general type are disclosed, e.g. in US7265364 and US7646471, both incorporated by reference. In US2010233600A1, incorporated by reference, a height sensor is disclosed which uses UV radiation instead of visible or infrared radiation. In WO2016102127A1, incorporated by reference, a compact height sensor is described which uses a multi-element detector to detect and discriminate the position of a grating image without the need of a detection grating.

[0056] Another type of metrology tool used in IC manufacturing is an alignment sensor. Thus, a key aspect of the performance of a lithographic apparatus can be to place the applied pattern correctly and accurately with respect to features placed in a previous layer (by the same apparatus or a different lithographic apparatus). For this purpose, the substrate is provided with one or more sets of marks or targets. Each mark is a structure whose position can later be measured using a position sensor, typically an optical position sensor. The position sensor can be referred to as an "alignment sensor", and the marks can be referred to as "alignment marks".

[0057] A lithographic apparatus can include one or more (e.g., multiple) alignment sensors that can be used to accurately measure the position of alignment marks disposed on a substrate. 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 of position sensors have been developed, for example as disclosed in US 2015261097 Al. The contents of all these publications are incorporated herein by reference.

[0058] Figure 6 A schematic block diagram of an embodiment of a known alignment sensor AS such as described in US 6961116 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 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 diameter of the illumination spot SP illuminating the mark AM can be slightly smaller than the width of the mark itself.

[0059] Radiation diffracted by the alignment mark AM (in this example via the objective lens OL) is collimated as 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). The self-referencing interferometer SRI of the type disclosed in US 6961116 mentioned above, for example, interferes the beam IB with itself, which is then received by a light detector PD. Additional optics (not shown) can be included to provide separate beams in the case that more than one wavelength is produced by the radiation source RSO. The light detector can be a single element, or it can comprise multiple pixels as required. The light detector can comprise a sensor array.

[0060] The turning optics, which in this example comprise the spot mirror SM, 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 (although this is not essential for the measurement, it improves the signal to noise ratio).

[0061] The intensity signal SI is supplied to the 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 the Y position of the substrate relative to the reference frame are output.

[0062] A single measurement of the type illustrated only fixes the position of the mark within a certain range corresponding to one period of the mark. A coarser measurement technique is used in conjunction with the measurement to identify which period of the sinusoidal wave contains the marked position. The same process at a coarser and / or finer level can be repeated at different wavelengths for improving accuracy and / or for robustly detecting the mark, regardless of the material in which the mark is made and the materials above and / or below in which the mark is provided. The wavelengths can be optically multiplexed and demultiplexed so as to be processed simultaneously, and / or the wavelengths can be multiplexed by time-sharing or frequency-division.

[0063] In this example, the alignment sensor and the light spot SP remain stationary, while the substrate W is moved. The alignment sensor can thus be robustly 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. In this movement, the substrate W is controlled by mounting the substrate W 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 as determined by the position sensor, e.g. relative to a frame to which the alignment system is connected. 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.

[0064] For optical semiconductor metrology, inspection applications are often preferred, such as in any of the aforementioned metrology tools, a bright light source outputting coherent radiation simultaneously covers a wide wavelength range (e.g. from UV to IR). This wideband light source can contribute to improving the flexibility and robustness of the application by allowing optical inspection of wafers with different material properties in the same setup / system without any hardware change (e.g. changing the light source so as to have a specific wavelength). Allowing to optimize the wavelength for a specific application also means that the accuracy of the measurement can be further improved.

[0065] Gas lasers based on the gas discharge effect to emit multiple wavelengths simultaneously can be used in these applications. However, the inherent problems associated with gas lasers such as high intensity instability and low spatial coherence can make them unsuitable. Alternatively, outputs from multiple lasers (e.g., solid-state lasers) with different wavelengths can be spatially combined into the optical path of the metrology or inspection system in order to provide a multi-wavelength source. The increasing complexity and high implementation cost with the number of desired wavelengths prevents this solution from being widely used. In contrast, fiber-based broadband or white light lasers (also known as supercontinuum lasers) are able to emit radiation with high spatial coherence and a broad spectral coverage (e.g., from UV to IR) and are therefore a very attractive and practical solution.

[0066] Hollow-core photonic crystal fibers (HC-PCF) are a special type of optical fiber that includes a central hollow core region and an inner cladding structure surrounding the hollow core, both of which extend axially along the entire fiber. Light guiding is enabled by the inner cladding waveguide structure, which can include, for example, thin-walled glass elements. Thus, radiation is mainly confined to the hollow core interior and propagates along the fiber in the form of transverse core modes.

[0067] Various types of HC-PCF can be engineered, each based on a different physical guiding mechanism. Two such HC-PCFs include: hollow-core photonic bandgap fibers (HC-PBFs) and hollow-core anti-resonant reflecting fibers (HC-ARFs).

[0068] HC-PCFs include hollow channels filled with a fluid, such that they possess resulting desirable properties for various light guiding applications; for example, high power beam delivery using HC-PBFs and gas-based white light generation (or supercontinuum generation) using HC-ARFs. Details on the design and fabrication of HC-PCFs can be found in US Patent US2004175085 (for HC-PBFs) and European Patent Application EP3136143A1 (for HC-ARFs), both of which are incorporated herein by reference. HC-PBFs are configured to provide low-loss but narrow bandwidth light guiding via the photonic bandgap effect, which is established by the cladding structure surrounding the central hollow core. However, HC-ARFs are engineered to significantly widen the transmission bandwidth via anti-resonant reflection of light from the cladding.

[0069] Figure 7 Two well-known types of HC-ARFs are depicted in cross-sectional views. Figure 7 (a) shows a Kagome fiber that includes a Kagome lattice structure. Figure 7 (b) shows a single-ring or spun fiber, in which the hollow core region is formed and surrounded by a layer of non-touching rings.

[0070] For gas-based white light generation, HC-ARF can be included within a gas cell designed to operate, for example, at pressures up to tens of bars (e.g., between 3 bars and 100 bars). When pumped by ultrashort pump laser pulses with sufficiently large peak power, the HC-ARF filled with gas can act as an optical frequency converter. The frequency conversion from the ultrashort pump laser pulses to broadband laser pulses is achieved by the complex interplay of the dispersive and nonlinear optical processes within the gas-filled interior. The converted laser pulses are confined within the hollow core primarily in the form of transverse core modes and guided to the fiber end. A portion of the radiation (e.g., higher order transverse core modes or specific wavelengths) can leak from the hollow core via the inner cladding waveguide structure and experience strong attenuation during its propagation along the fiber. The core and cladding regions of the HC-ARF can be configured such that the higher order core modes are phase-matched with the higher order cladding modes. In this way, the higher order core modes can be resonantly coupled with the subsequently attenuated or suppressed higher order cladding modes. In this way, low-loss and efficient single transverse mode transmission can be achieved over a wide spectral range.

[0071] The spatiotemporal transmission characteristics (e.g., its spectral amplitude and phase) of the laser pulses transmitted along the HC-PCF can be varied and tuned via adjusting the pump laser parameters, the gas-filled parameters, and the fiber parameters. The transmission characteristics can include one or more of the following: output power, output mode profile, output temporal profile, width of the output temporal profile (or output pulse width), output spectral profile, and bandwidth of the output spectral profile (or output spectral bandwidth). The pump laser parameters can include one or more of the following: pump wavelength, pump pulse energy, pump pulse width, pump pulse repetition rate. The fiber parameters can include one or more of the following: fiber length, size and shape of the hollow core, size and shape of the cladding structure, thickness of the wall surrounding the hollow core. The gas-filled parameters can include one or more of the following: gas type, gas pressure, and gas temperature.

[0072] The fill gas can be an atomic gas such as argon, krypton and xenon; a molecular gas such as hydrogen, deuterium and nitrogen; or a gas mixture comprising two or more different gases such as a mixture of argon and hydrogen, a mixture of xenon and deuterium, a mixture of krypton and nitrogen, or a mixture of nitrogen and hydrogen. Depending on the type of fill gas, the nonlinear optical process can include modulation instability (MI), optical soliton fission, Kerr effect, Raman effect and dispersive wave generation, the details of which are described in WO2018 / 127266A1 and US9160137B1, both of which are incorporated herein by reference. Since the dispersion of the fill gas can be tuned by changing the gas cell pressure, the generated broadband pulse dynamics and associated spectral broadening features can be adjusted in order to optimize the frequency conversion. The generated broadband laser output can cover wavelengths from UV (e.g., < 400 nm) to IR (e.g., > 800 nm).

[0073] It has been found that existing HC-PCF based broadband sources tend to face fiber lifetime issues and can not operate after a relatively short period of operational time. In the case where HC-PCF based light sources are used in metrology tools (e.g., scatterometers), the unexpected and early failure of the HC-PCF means that all light sources will need to be removed from the tool in order to be repaired or replaced. After the replacement light sources are installed to the same metrology tool, a complete optical alignment of the laser beams and other necessary corrections and characterizations need to be performed again. The entire process not only increases cost, but also causes a long system downtime. Therefore, it is particularly desirable to fully understand the failure mechanism of HC-PCF in order to find a way to improve / extend its lifetime.

[0074] Currently, several failure mechanisms have been identified. The first failure mechanism is fiber contamination, typically induced by hydrocarbons deposited on one or both fiber ends. There are cleaning methods that can be used to address this contamination.

[0075] The second failure mechanism is fiber overheating. This problem is caused by the fact that existing HC-PCF based light sources do not incorporate any effective heat management measures to maintain the operating temperature of the HC-PCF. White light generation is accompanied by ionization and heat generation via atomic collisions of the gas species and recombination dynamics in the plasma. This heat generated during operation will raise the temperature of the inner cladding surface of the fiber. However, for existing HC-PCFs (e.g., such as Figure 7 (a) or Figure 7 (b) described above), the fiber geometry does not allow access and efficient cooling of the surface of the inner cladding waveguide structure. Without effective heat dissipation, the generated heat will accumulate in the fiber, especially in the case where the fiber is pumped / driven with high repetition rate pulses, until the fiber overheats and eventually breaks down. The inner cladding waveguide structure (e.g., such asFigure 7 (b) the surface of the glass or silica conduit) will also trigger unwanted chemical reactions, which can produce exhaust and thus cause contamination. Thus, the overheating problem adversely affects the optical fiber lifetime and imposes limitations in the further development of HC-PCF based light sources, e.g. power and / or repetition rate scaling.

[0076] A third failure mechanism is the deformation of the inner cladding in the fiber. In a pure atomic gas environment such as krypton, broadband radiation or white light generation also leads to mechanical deformation of the fiber cladding structure. As mentioned above, the white light generation process leads to heating of the fiber and plasma generation. It has been found that plasma generation in HC-PCF under short time scales causes pressure shock waves due to the sharp gas heating. This pressure change is proportional to the free electron density. The pressure waves, which propagate as acoustic waves in the cross section of the HC-PCF, are incident on the thin walled glass elements of the inner cladding waveguide structure (e.g. the cladding tube) and then excite several mechanical modes of the cladding tube in a pulsed fashion, thereby causing it to vibrate under the superposition of the excited mechanical modes. The vibration of the cladding tube, which is heated to several hundred degrees due to white light generation, leads to temporary and permanent deformation of the cladding tube and thus to a deformed inner cladding waveguide structure.

[0077] Since the inner cladding waveguide structure is used as a light guiding mechanism, such that the radiation is mainly confined to the hollow core interior, a deformed inner cladding waveguide structure will lead to optical losses and in some severe cases to fiber damage. Figure 8 Two cross-sectional images taken at two different locations of a HC-PCF with a single-ring design operated with krypton gas as the only fill gas are shown: (a) a cross-sectional image taken at the fiber end; (b) a cross-sectional image taken at a location several centimeters away from the fiber end shown in (a). It can be seen that the fiber conduit can deform along the fiber when exposed to the spatially extended region of white light generation.

[0078] Furthermore, there are several other factors that can aggravate the mechanically induced deformation of the fiber cladding structure (e.g. glass conduit) by acoustic waves. First, in the case of using a working gas with low thermal conductivity, the poor thermal management of the fiber leads to (excessive) heating of the cladding tube, which makes it more susceptible to mechanical deformation.

[0079] In addition, if the repetition rate of the pump pulses somehow enhances and amplifies the amplitude of the excited mechanical modes of the cladding tube, e.g. when it is close to the resonance frequency of the excited mechanical modes of the cladding tube, the effect of the generated acoustic waves on the fiber cladding structure can be significantly amplified. As mentioned above, while the pump pulses are nonlinearly broadened to broadband or supercontinuum spectral pulses inside the hollow core of the HC-PCF, acoustic waves (or pressure waves) are also excited due to the sharp release of energy to the gas and heating it. In a pure atomic gas environment such as krypton, the acoustic waves are mainly generated by the plasma generated in the HC-PCF. In a HC-PCF with a cladding tube filled with a gas mixture, the acoustic waves are generated by the plasma generated in the HC-PCF and the gas mixture in the cladding tube. Figure 7(a) and Figure 7 In the pipe type HC-PCF of the prior art design shown in (b), at least some of the glass tubes will act as resonators under the impact of the generated acoustic waves. Within each glass tube resonator, interference can be formed between the incident acoustic wave and the back-reflected acoustic wave. Thus, each resonator will have a resonance frequency which is determined mainly by the geometry of the tube, e.g. shape, diameter, wall thickness. When the repetition rate of the pump pulses approaches or matches the resonance frequency of the glass tube, a resonance effect will occur which significantly enhances the amplitude of the mechanical vibrations of the relevant tube.

[0080] It is known that the problem of fiber cladding deformation can be at least partially alleviated by adding an amount of hydrogen (or deuterium) to the working gas or gas mixture. Due to safety regulations, the maximum mole fraction of hydrogen in the fill gas mixture can be about 4%. Currently, the typical mole fraction of hydrogen in the fill gas mixture containing hydrogen is about 2%.

[0081] Studies have shown that the problem of fiber cladding deformation is at least partially related to radiation-induced defects (color centers) in the glass. These defects can cause local mechanical stress and eventual deformation of thin glass structures, such as fiber cladding structures, when formed by exposing the glass to UV photons. Hydrogen and deuterium are known to be suitable for restoring these defects in the glass matrix.

[0082] Furthermore, mixing small amounts of hydrogen with the working gas is also believed to be effective in suppressing the generated acoustic waves in the HC-PCF. Studies have shown that the gas mixture can allow suppression of the generated acoustic waves due to diffusion- induced acoustic wave attenuation. Both atomic and molecular gases can be used for mixing with the commonly used working gas, e.g. krypton. Since molecular gases have more internal degrees of freedom, molecular gases are more effective than atomic gases for acoustic suppression. In other words, the vibrational and rotational relaxation in molecular gases improves the acoustic suppression capability due to the coupling between the molecular degrees of freedom, e.g. translational, rotational and vibrational degrees of freedom. Molecular gases suitable for mixing with the working gas can be, for example, methane, nitrogen, oxygen or hydrogen.

[0083] However, the inventors have found that mixtures of atomic gases can still provide considerable acoustic suppression. Therefore, it is proposed to use atomic gas mixtures which improve the acoustic wave suppression relative to a single working atomic gas. These mixtures can include, for example, noble gas mixtures, such as mixtures of krypton and helium, krypton and neon, or xenon and helium.

[0084] For a gas mixture with fixed concentrations of the gas components, the amplitude suppression of the generated acoustic wave is described by the following equation:

[0085] [1]

[0086] wherein, is the frequency, is a frequency dependent suppression factor, and z is the propagation distance from the source of the acoustic wave (e.g. the middle of the fiber core). Since the acoustic wave is continuously generated by a series of pump pulses at a repetition rate / frequency it would be desirable and is proposed herein to use a gas mixture where the peak of the suppression curve corresponding to the optimal acoustic suppression frequency matches or is substantially close to the repetition rate of the pump laser so as to have maximum suppression at this frequency. For a fixed mixture one can also tune away from the resonance frequency.

[0087] Although a small amount of hydrogen gas mixed with the working gas is proposed and used to alleviate the problem of fiber cladding deformation, the present inventors now realize that the introduction of hydrogen gas also helps to alleviate the above-mentioned thermal problem. This concept has not been realized before.

[0088] Currently, krypton gas is often used as the working gas in existing HC-PCF based broadband light sources. Note that the working gas or gas mixture refers to the gas or gas mixture that is primarily responsible for interacting with the input pump radiation and thus generating the broadband radiation. The fill gas used in HC-PCF based broadband light sources can include only the working gas or gas mixture. Or alternatively, the fill gas can include the working gas or gas mixture and one or more other gases. The thermal conductivity of krypton gas is about 9.5 milliwatts per millikelvin (mW / mK) at room temperature. This low thermal conductivity leads to poor thermal management in the HC-PCF. In other words, the bare HC-PCF enclosed in the gas cell and embedded in krypton gas cannot effectively dissipate the heat generated during white light generation. This is especially true and important for the inner surface of the fiber cladding structure that is directly exposed to the broadband radiation. Since low thermal conductivity is a characteristic of most gaseous media, the above-mentioned thermal problem is a common problem for all those gases. The thermal conductivity of hydrogen gas is 186.9 mW / mK, which is at least one order of magnitude higher than commonly used working gases (e.g. krypton gas). The much higher thermal conductivity helps to improve the heat dissipation of the HC-PCF and thus suppresses the above-mentioned fiber overheating problem.

[0089] Although the addition of hydrogen gas to the working gas can successfully alleviate the above-mentioned problems of HC-PCF based broadband light sources, this approach leads on the other hand to surface reduction and glass growth problems which are also detrimental to the performance of HC-PCF based light sources, especially the lifetime. It has been found that after a few hundreds of hours of operation of HC-PCF based light sources with hydrogen gas containing gas mixtures, silica or SiOx nanostructures and loose glass mainly grow at the output end of the optical fiber. One of the main sources of silica growth is the reduction of the internal fiber surface in the presence of hydrogen plasma. The reduction and etching of silica in the presence of hydrogen plasma is a known phenomenon. Hydrogen ions and radicals such as atomic hydrogen attack the internal fiber surface and cause the reduction of the contact surface by converting silica into silicon, or the etching of the surface by producing volatile silicon monoxide. The growth of silica at the end of the optical fiber leads to a progressive blockage and loss of output power which will eventually lead to the fiber damage and to a short lifetime of the white light source. In addition to the surface reduction and glass growth problems, the 2% molar fraction of hydrogen gas mentioned above is only an empirical value and the precise amount of hydrogen gas needed to minimize or prevent the cladding structure deformation remains unknown.

[0090] To solve some or all of these problems, a method for improving the lifetime of a HC-ARF based broadband light source using a filling gas is thus proposed. The proposed method is especially suitable for lengthening the lifetime of HC-PCF when operated in a gas environment for white light or supercontinuum generation. Different embodiments of the proposed method and related devices will be disclosed hereafter. One commonality shared between the disclosed embodiments is to avoid the exclusive use of hydrogen gas for improving the thermal regulation and the absorption of acoustic waves within the working gas in order to limit the fiber lifetime limiting problems related to hydrogen gas. Thus, the filling gas used is one comprising a molar fraction of hydrogen gas not exceeding for example ten parts per million (ppm). For example, the only hydrogen gas included in the filling gas can be a naturally occurring hydrogen gas (for example, native hydrogen gas that can be produced by the materials, hydrocarbons, outgassing of H2O on any surface, and / or as part of the broadband generation process) compared to the intentionally added hydrogen gas.

[0091] As Figure 9As explained above, the broadband light source device 100 comprises a pump laser 110 outputting a series of pump pulses 111, an optical component 120 spectrally broadening the input pump pulses, and an optical diagnostic device 130 measuring the output broadband spectrum. The optical component 120 comprises an HC-PCF (e.g. HC-ARF) 101 having a specific fiber length and a gas cell 102 filled with a filling gas or gas mixture under a specific pressure or having a pressure profile. The gas cell 102 further comprises an input optical window 103a and an output optical window 103b positioned at respective ends of the gas cell 102. The input optical window 103a is operable to admit ultra-short pump laser pulses into the gas cell 102 via the window. After coupling into the HC-PCF 101 filled with the filling gas, the pump laser pulses 111 propagate along the fiber where they experience a significant spectral broadening. The resulting broadband laser pulses are then expelled from the gas cell 102 via the output optical window 103b and measured by the optical diagnostic device 130 (e.g. a spectrometer).

[0092] To fill the HC-PCF 101 with the filling gas, the gas cell 102 can be in communication with a pressurized gas source or reservoir (not shown). The walls of the gas cell 102 and the inner surfaces of the windows 103a, 103b enclose a cavity. The axis of the gas cell is parallel to the axis of the HC-PCF 101.

[0093] Figure 10 (a) to Figure 10 (c) schematically depicts three known configurations of the optical component 120, 120', 120". Figure 10 (a) illustrates a first configuration in which the entire HC-PCF 101 is included within a single gas cell 102. Figure 10 (b) illustrates an alternative configuration in which the entire HC-PCF 101 is included in several (e.g. three) sub-chambers 102a, 102b, 102c which are interconnected by using appropriate sealing mechanisms 105. Pressure tight connections ensure that all sub-chambers reach the same pressure desired for white light generation. Figure 10 (c) illustrates another configuration in which two fiber ends 101a, 101c of the HC-PCF 101 are included in two separate gas cells 102d, 102e, respectively, while the central portion 101b of the fiber acting as a fluidic connection is included outside the gas cells.

[0094] Note that, Figure 10 (a) to Figure 10The configurations of optical components 120, 120', 120" described in (c) are merely three examples. Many other different configurations are equally applicable. For example, in some embodiments, optical component 120 can not use a single gas cell 102 or multiple sub-chambers 102a-102e to form a gas environment that at least partially encloses HC-PCF 101. Instead, HC-PCF 101 can be first filled with a fill gas and then sealed, for example, by attaching a mirror to each of the two ends of the optical fiber. In this way, the fill gas is held within the optical fiber (e.g., the hollow core and cladding structure) without the need for a separate gas cell. The two mirrors can be configured in the same way as Figure 9 mirrors described in (c).

[0095] In embodiments, the fill gas of the HC-PCF based broadband light source can be a gas mixture comprising or consisting of helium gas and another gas or gas mixture. In embodiments, helium gas can serve as a replacement gas for hydrogen gas, which is currently used to mitigate cladding structure deformation and optical fiber overheating issues. Similar to hydrogen gas, helium gas has a high thermal conductivity, i.e., 156.7 mW / mK, and can thus help improve the thermal management of HC-PCF 101. Although a different molecular gas with a high thermal conductivity can also be used to replace hydrogen gas for the same purpose (described in the following embodiments), atomic gases are preferred due to, for example, the fact that molecular gases such as hydrogen, oxygen, and H2O will decompose after being exposed to light and become ions and radicals that subsequently attack the glass and cause surface reduction issues.

[0096] The second gas or gas mixture can serve as a working gas for white light generation. The second gas or gas mixture can be an atomic gas or an atomic gas mixture or a molecular gas or a molecular gas mixture. As described above, a mixture of two or more atomic gases can help suppress the generated acoustic waves and thus mitigate the cladding structure deformation issue. Note that, in order to provide effective shock absorption or acoustic suppression properties, the two or more atomic gases of the fill gas mixture should have sufficiently different atomic weights needed to diffuse and assist acoustic wave attenuation.

[0097] In some embodiments, the fill gas mixture can comprise or consist of helium gas and a second atomic gas for white light generation. The second atomic gas can be, for example, krypton gas, neon gas, argon gas, or xenon gas. In other embodiments, the fill gas mixture can comprise or consist of helium gas and a second atomic gas mixture. The second atomic gas mixture for white light generation can comprise at least two other different atomic gases, for example, krypton gas and neon gas, krypton gas and argon gas, or krypton gas and xenon gas.

[0098] In different embodiments, the fill gas mixture can comprise or consist of: helium gas, a second atomic gas or gas mixture, and a molecular gas or gas mixture. The second atomic gas or gas mixture can be used as the working gas or gas mixture for white light generation. Since a molecular gas has more internal degrees of freedom than an atomic gas, the molecular gas can further enhance the acoustic suppression capability when mixed with an atomic gas mixture containing helium gas such as those in the aforementioned embodiments. In some embodiments, the fill gas mixture can comprise helium gas, a second atomic gas or gas mixture, and a molecular gas. For this purpose, the molecular gas can be, for example, nitrogen, oxygen, or H2O. In other embodiments, the fill gas mixture can comprise: helium gas, a second atomic gas or a second atomic gas mixture, and a molecular gas mixture. The molecular gas mixture can comprise, for example, two or more of nitrogen, oxygen, or H2O.

[0099] In some embodiments, the fill gas mixture can comprise or consist of: helium gas and a different gas or gas mixture. Here, the helium gas can be used to alleviate the fiber overheating problem and the cladding structure deformation problem. The different gas or gas mixture used as the working gas or gas mixture can be carefully chosen to determine one or more desired properties (e.g., spectral range and spectral profile) for the broadband radiation generation in the fill gas based HC-PCF light source. For example, by properly selecting the type of gas or the composition of the gas mixture, it is possible to generate broadband radiation mainly in the ultraviolet (UV) region or in the infrared (IR) region. In cases where a broadband UV spectrum is preferred, the fill gas mixture can comprise or consist of, for example, helium gas and a molecular gas or a gas mixture comprising nitrogen, or, for example, an atomic gas such as argon or krypton. However, in cases where a broadband IR spectrum is preferred, the fill gas mixture can comprise or consist of, for example, helium gas and xenon.

[0100] In some embodiments, the helium gas concentration or mole fraction in the fill gas mixture containing helium gas can be optimized such that a balance is achieved between the heat conduction and convection for releasing excess heat from the optical fiber. In other words, the cladding structure deformation and fiber overheating problems can be alleviated by optimizing the helium gas concentration of the fill gas mixture. The selection of the fill gas mixture can depend on factors such as one or more of the following: the output characteristics (e.g., power, spectrum) of the light source, the acceptable total gas pressure, and / or the preferred heat dissipation method. The addition of helium gas to the fill gas mixture can enable the heat dissipation mechanism by increasing the thermal conductivity and the thermal diffusivity of the fill gas mixture.

[0101] As described above, helium gas can be used as a replacement gas for hydrogen gas due to its high thermal conductivity and suitability for acoustic suppression to mitigate cladding structure deformation and optical fiber overheating issues. It is known that the thermal conductivity of a gas mixture comprising helium gas varies with the concentration of helium gas. Figure 11 is a plot illustrating the thermal conductivity of several binary mixtures of atomic gases as a function of molecular weight. In Figure 11 In the plot of, the thermal conductivity (κ) of each gas mixture is characterized at a pressure of 20 bar (or 2.0 megapascal (MPa) as indicated in the plot) for two different temperatures, i.e., 1200 K and 400 K. The molecular weight (MW) of each gas mixture is the average of the individual molecular weights weighted by their respective percentages. Figure 11 Each of the hollow circles in represents the thermal conductivity of an atomic gas (e.g., helium (He), neon (Ne), argon (Ar), krypton (Kr), or xenon (Xe)) at a temperature of 400 K; each of the hollow squares in the plot represents the thermal conductivity of an atomic gas (e.g., He, Ne, Ar, Kr, or Xe) at a temperature of 1200 K. Each solid line connects a pair of two hollow circles and indicates the range of the thermal conductivity of a binary mixture consisting of the two atomic gases represented by the two hollow circles as a function of the molecular weight of the gas mixture at a temperature of 400 K. Likewise, each dashed line connects a pair of two hollow squares and indicates the range of the thermal conductivity of a binary mixture consisting of the two atomic gases represented by the two hollow squares as a function of the molecular weight of the gas mixture at a temperature of 1200 K.

[0102] The solid circles represent the thermal conductivities of some exemplary gas mixtures that can be used as the fill gas mixture for the light source of a HC-PCF based fill gas based HC-PCF. Two gas mixtures, i.e., a first mixture consisting of 50% molar fraction of xenon and 50% molar fraction of helium (or Xe / He 50 / 50) and a second mixture consisting of 80% molar fraction of krypton and 20% molar fraction of helium (or Kr / He 80 / 20), as represented by the two solid circles on the solid line, can be used in a HC-PCF based wideband radiation source operating at a high repetition rate (e.g., 1 MHz, 2.5 MHz, 5 MHz, 10 MHz, 20 MHz, 30 MHz, 40 MHz, or 80 MHz). Note that the thermal conductivities of the exemplary gas mixtures in the plot are for a given temperature and total gas pressure (e.g., 400 K and 20 bar). Those exemplary gas mixtures will have different thermal conductivities when operated under different conditions (e.g., different total gas pressure and / or different temperature).

[0103] It is apparent from the figures that for any fill gas mixture containing helium, increasing the helium concentration (or mole fraction) of the gas mixture decreases the molecular weight and increases the thermal conductivity. At the same time, increasing the helium concentration of the gas mixture decreases the partial pressure of the working gas (e.g., Ne, Ar, Kr, or Xe). Since the output performance of HC-PCF-based radiation sources depends on the partial pressure of the working gas (and thus, on the optical nonlinearity of the working gas), it is desirable to increase the total gas pressure in order to maintain the same partial pressure of the working gas. The total gas pressure is defined as the sum of the partial pressures of the constituent gases of the fill gas mixture. Therefore, a balance between thermal conductivity and optical nonlinearity needs to be achieved when determining the optimal concentration (or mole fraction) of helium in the fill gas mixture.

[0104] Referring to Figure 11 Different fill gas mixtures can be classified into, for example, three main groups based on the thermal conductivity of the corresponding working gas. The first group can include fill gas mixtures with high thermal conductivity. In the first group, each fill gas mixture can have a thermal conductivity at a given temperature and pressure that is, for example, between 75% and 100% of the thermal conductivity of pure helium. The second group can include fill gas mixtures with moderate thermal conductivity. In the second group, each fill gas mixture can have a thermal conductivity at a given temperature and pressure that is, for example, between 35% and 75% of the thermal conductivity of pure helium. The third group can include fill gas mixtures with low thermal conductivity. In the third group, each fill gas mixture can have a thermal conductivity at a given temperature and pressure that is, for example, between 0% and 35% of the thermal conductivity of pure helium.

[0105] For each fill gas mixture of the first group, the working gas (e.g., xenon) can have a higher degree of ionization, and the resulting overheating problem can have a higher level of severity. Therefore, the working gas can be mixed with a higher mole fraction of helium in order to achieve the higher thermal conductivity required to mitigate the overheating problem. For example, the fill gas mixture can consist of 10% mole fraction of xenon and 90% mole fraction of helium, or 20% mole fraction of xenon and 80% mole fraction of helium. This fill gas mixture has a very high concentration of helium and thus allows for a high thermal conductivity. These fill gas mixtures can be operated at a higher total gas pressure (e.g., 40 bar) in order to maintain the desired partial pressure of the working gas (e.g., xenon).

[0106] For each of the second group of fill gas mixtures, the working gas (e.g., argon or krypton) can have a lower degree of ionization, and the resulting overheating problem can be less severe than for the first group of fill gas mixtures. Thus, a lower helium concentration can be used for fill gas mixtures that include helium. For example, the fill gas mixture can consist of 50% molar fraction of krypton and 50% molar fraction of helium, or 70% molar fraction of argon and 30% molar fraction of helium. This has the advantage of maintaining the total working pressure of the gas mixture to a level that is easy to handle (e.g., below 50 bar).

[0107] For each of the third group of fill gas mixtures, the degree of ionization of the working gas (e.g., argon or neon) can be similar to or lower than that of the second group. By comparison, the third group of fill gas mixtures can have a lower concentration of helium, and thus a lower thermal conductivity, than those of the second group. Similar to the first group, the fill gas mixtures of the second and third groups can be operated at a higher total gas pressure (e.g., 50-70 bar) in order to maintain a desired partial pressure of the corresponding working gas.

[0108] Additional example fill gas mixtures are provided in Table 1. Note that although the following table lists only binary atomic gas mixtures (i.e., consisting of two different atomic gases), this is not meant to exclude the applicability of other types of gas mixtures, such as, for example, fill gas mixtures that include or consist of more than two atomic gases, or that include or consist of one or more atomic gases and one or more molecular gases.

[0109] Table 1

[0110] Gas mixture Mole fraction of gas-1 Mole fraction of gas-2 Ar (gas-1) and He (gas-2) 100 – X X (X > 10%) Kr (gas-1) and He (gas-2) 100 – X X (X > 20%) Xe (gas-1) and He (gas-2) 100 – X X (X > 50%)

[0111] As shown in Table 1, each example fill gas mixture consists of helium and another heavier atomic gas. The molar fraction of helium (gas-2) and the heavier atomic gas (gas-1) are denoted as X and 100-X, respectively. For each binary atomic gas mixture in the table, e.g., Ar and He, Kr and He, or Xe and He, a preferred range of the molar fraction of helium is provided. For example, for a gas mixture of xenon and helium, the molar fraction of helium is preferably equal to or higher than 50%.

[0112] The thermal diffusivity is a measure of how quickly a thermal gradient produced by one pulse at the center of an optical fiber (i.e., at the point of maximum intensity and ionization) is smoothed out or diffused in the system (e.g., HC-PCF of fill gas), and can be expressed as follows:

[0113] [2]

[0114] where k indicates the thermal conductivity of the fill gas mixture, p indicates the density of the fill gas mixture, and C p indicates the heat capacity of the fill gas mixture. To prevent the hot gas from reaching a high steady-state temperature under the action of a subsequent laser pulse, it is desirable to reduce the timescale of the relaxation of the thermal gradient to be shorter than the pulse separation time (i.e., the inverse of the laser repetition rate). According to equation [2], the thermal conductivity of the fill gas mixture, which increases with more helium, also leads to an increase in the thermal diffusivity. In addition to using a fill gas mixture with a high electrical conductivity, a higher thermal diffusivity can be obtained by configuring the fill gas mixture to have a lower density and / or a lower heat capacity. Figure 12 is a plot showing a simplified simulation of the thermal relaxation in an optical fiber for two different thermal diffusivities. As shown in the plot, the normalized peak temperature (NPT) of the optical fiber (e.g., HC-PCF 101), and the horizontal axis of the plot is time. The plot determines that a fill gas mixture with a higher thermal diffusivity (e.g., a = 0.5) leads to a much faster (e.g., twice as fast) thermal relaxation in the optical fiber (e.g., HC-PCF 101) compared to a fill gas mixture with a lower thermal diffusivity (e.g., a = 10).

[0115] As described above, the selection of the fill gas mixture can also depend on the preferred heat transfer mechanism, such as convective heat transfer or conductive heat transfer. Conductive heat transfer occurs by means of molecular excitation within a material without bulk motion; however, convective heat transfer occurs due to bulk motion of a fluid (e.g., the fill gas) relative to a boundary. In the case where a heavy atom gas is used as the working gas (e.g., Kr or Xe) for the generation of broadband radiation in an HC-PCF, it is preferred that the thermal relaxation produces little convection or momentum transfer of the heavy working gas. The reason is that convection of the heavy working gas can create gas flow in the optical fiber due to the formation of temperature gradients inside and outside of or across the cross-section of the fiber. The flow of the hot heavy working gas can cause the pump pulse to experience a varying refractive index of the medium it travels (working gas) through. The local variations in the refractive index profile can in turn lead to radiation output instabilities (e.g., power instability, spectral instability). The addition of helium to the heavy working gas can allow heat transfer to occur primarily via the movement of the light helium atoms, which can move freely through the heavy gas atoms in a manner similar to heat transfer in metals via the movement of free light electrons relative to heavy ions. Due to its low optical nonlinearity, helium gas does not affect the output of the light source and only helps to transfer the excess heat generated in the optical fiber.

[0116] Note that the above description for selecting an optimal fill gas mixture for mitigating cladding structure deformation and fiber overheating issues applies equally to the case where the fill gas mixture includes helium and a working gas mixture that includes two or more atomic gases (rather than a single atomic gas as described in the example gas mixtures above), or to the case where the fill gas mixture includes helium and a working gas mixture that includes one or more atomic gases and one or more molecular gases. In any case, the fill gas mixture can be optimally selected by considering its thermal conductivity, thermal diffusivity, and heat transfer mechanisms simultaneously.

[0117] According to equation [1], the strength of acoustic suppression of pressure shockwaves depends on the pump pulse repetition rate. Thus, in embodiments, the fill gas mixture including helium can be configured such that the optimal acoustic suppression frequency matches or substantially approaches the repetition rate of the pump pulses. In some embodiments, the repetition rate of the pump laser can be tuned to match or substantially approach the optimal acoustic suppression frequency of the fill gas or gas mixture. In other embodiments, a suitable fill gas or gas mixture can be selected that has an optimal acoustic suppression frequency that matches or substantially approaches the pump pulse repetition rate.

[0118] The pump gas frequency matching feature can be used individually or in combination with any of the foregoing embodiments in order to maximize the strength of acoustic suppression of the corresponding fill gas mixture. For example, in embodiments, a fill gas mixture including or consisting of helium and nitrogen can have an optimal acoustic suppression frequency that precisely matches the repetition rate of the pump pulses. As such, a light source based on a HC-PCF filled with the gas can simultaneously produce broadband UV radiation (determined by the nitrogen) and have an extended fiber lifetime, where the effects of fiber overheating (e.g., mitigated by using helium) and cladding tube deformation (mitigated by using a mixture of nitrogen and helium along with the pump gas frequency matching feature) issues are minimized.

[0119] Since the acoustic resonance effects induced by the cladding structure (e.g., cladding tube) can amplify the effects of the acoustic waves, it is also preferable to ensure that the pump pulse repetition rate is different from one or more resonance frequencies of excited resonant modes within the cladding structure (e.g., cladding tube) of the HC-PCF 101. In some embodiments, the repetition rate of the pump laser can be adjustable so that the resonance effects can be avoided. Alternatively, the acoustic impedance of the HC-PCF 101 (as seen by the acoustic waves generated during white light generation) can be arranged to substantially prevent the excitation of resonant modes within the cladding structure of the HC-PCF 101 after impingement by the acoustic waves. In some embodiments, the prevention of the excitation of resonant modes within the cladding structure can be achieved by careful selection of the appropriate gas composition of the fill gas mixture and / or by careful selection of the HC-PCF 101 having a suitable fiber geometry.

[0120] Similar to the pump gas frequency matching feature, the above-described pump cladding anti-resonance feature can be used alone or in combination with any of the preceding embodiments. For example, in different embodiments, the pump cladding anti-resonance feature can be applied to the above embodiments in which the fill gas includes or consists of helium and nitrogen and the optimal acoustic suppression frequency matches the pump pulse repetition rate. This embodiment can ensure that the maximum acoustic suppression strength is not compromised by the resonance effects of the cladding tube.

[0121] In some embodiments, certain thermal management measures can be employed to improve the heat dissipation of the gas cell 102. A gas cell 102 with improved heat dissipation can facilitate the efficient removal of heat transferred from the HC-PCF 101 to the gas cell 102 via the fill gas or gas mixture containing helium. In some embodiments, the gas cell 102 can be connected to a high-efficiency cooling system that can quickly remove heat from the gas cell body. In different embodiments, the gas cell 102 can be constructed with materials having high thermal conductivity. Furthermore, the improved gas cell 102 with higher heat dissipation can be used in combination with any of the preceding embodiments.

[0122] In all of the above embodiments, the fill gas mixture contains an amount of helium. Compared to a fill gas mixture containing hydrogen, a fill gas mixture containing helium not only alleviates the problems of fiber overheating and cladding structure deformation, but also prevents the surface reduction problem that otherwise arises when using a fill gas mixture containing hydrogen. It should be appreciated that even without intentional introduction of hydrogen, the fill gas or gas mixture can still include trace amounts of hydrogen that can come from exhaust, hydrocarbons, or H20 on any surfaces and / or generated as part of the broadband generation process. However, these residual or naturally occurring hydrogen gases are not sufficient to act to alleviate any of the foregoing problems or cause glass surface reduction.

[0123] In embodiments, the fill gas mixture can consist of 50% by mole fraction of helium and 50% by mole fraction of krypton. However, other mole fractions of helium can also be applicable. The mole fraction of helium can be 2% or more than 2% of the fill gas mixture; for example, equal to or greater than 10% or 50% of the fill gas mixture. For example, the mole fraction of helium can be between 10% and 90%, between 20% and 80%, between 30% and 70%, between 40% and 60%, between 45% and 55%, or between 55% and 65%; and more particularly, the mole fraction of helium can be 2%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, or 80% of the fill gas mixture. In the foregoing, "comprising" is described to include the recited gases, but does not necessarily exclude other gases, and "consisting of is described to include only the recited gases.

[0124] While helium (or other noble gas) is a very useful gas medium and can alleviate both fiber overheating and cladding structure deformation problems, some embodiments can use a fill gas mixture that does not include helium. In some embodiments, one or more of the pump gas frequency matching feature, the pump cladding anti-resonance feature, and the heat sink gas cell feature can be applied to a fill gas or gas mixture that does not include helium. In embodiments, all three of the above features are used with a fill gas mixture that does not include helium. Since the pump gas frequency matching feature and the pump tube anti-resonance feature can alleviate cladding tube deformation problems, and the heat sink gas cell feature can alleviate fiber overheating problems, fiber service life can still be extended even when helium is not used. Embodiments can further prevent surface reduction problems by avoiding the use of large amounts of hydrogen in the fill gas mixture.

[0125] Thus, also envisaged is a method for optimizing the repetition rate of the pump radiation so that the repetition rate of the pump radiation does not match any of the acoustic resonance frequencies of the inner cladding structure of the HC-PCF, and a method for substantially matching the optimal acoustic suppression frequency of the gas medium to the repetition rate of the pump radiation generated by the pump radiation source. The optimization or matching can be achieved, for example, by suitably tuning the pump radiation parameters, optimizing the gas mixture, or both.

[0126] The HC-PCF 101 used in this embodiment can employ the references made to the HC-PCF 101 of Figure 1. Figure 7HC-ARF of Kagome design or single-ring design. Alternatively, other fiber designs (not shown) such as suppressed coupling design, inner-cycloid core Kagome, and nested tube design can be used. The pump pulse duration can be selected to be greater than 100 fs, and more specifically, in the following ranges: for example, 100 fs to 100 ps, 100 fs to 30 ps, or 100 fs to 1 ps. The selected pump pulse can be 100 fs, 150 fs, 200 fs, 250 fs, 300 fs, 350 fs, 400 fs, 450 fs, 500 fs, 600 fs, 700 fs, 800 fs, 900 fs, 1 ps, 10 ps, 20 ps, 30 ps, 100 ps. The pump wavelength can be selected from the visible regime, near-IR regime, or mid-IR regime. The pump laser pulse can have a repetition rate of hundreds of hertz (Hz), thousands of hertz (kHz), or millions of hertz (MHz). Specifically, the repetition rate can be selected to be in the range of 300 kHz to 100 MHz, such as 300 kHz, 500 kHz, 1 MHz, 5 MHz, 10 MHz, 20 MHz, 30 MHz, 40 MHz.

[0127] A broadband light source apparatus configured to prolong the lifetime of HC-PCF as disclosed herein, comprising a fill gas mixture comprising helium gas.

[0128] A broadband light source apparatus configured to prolong the lifetime of HC-PCF as disclosed herein, wherein the pump pulse repetition rate and the optimal acoustic suppression frequency of the fill gas mixture match or are substantially close to each other.

[0129] A broadband light source apparatus configured to prolong the lifetime of HC-PCF as disclosed herein, comprising an optical component that can be configured to refer to Figure 10 any of the configurations.

[0130] Other embodiments are disclosed in the following numbered list of aspects:

[0131] 1. A broadband light source apparatus configured for generating a broadband output upon receiving pump radiation, the broadband light source apparatus comprising:

[0132] an optical component comprising:

[0133] a hollow core photonic crystal fiber (HC-PCF); and

[0134] a gas mixture filling the HC-PCF;

[0135] wherein the gas mixture comprises a mixture of at least one first gas configured for generating broadband radiation and at least one second gas configured for increasing the thermal conductivity of the gas mixture and / or providing acoustic suppression of a shockwave induced during the generation of broadband radiation, and further wherein the gas mixture comprises no more than ten parts per million (ppm) molar fraction of hydrogen.

[0136] 2. A broadband light source device configured for generating a broadband output upon receiving pump radiation, the broadband light source device comprising:

[0137] an optical component comprising:

[0138] a hollow core photonic crystal fiber (HC-PCF); and

[0139] a gas mixture filling the HC-PCF;

[0140] wherein the gas mixture comprises a mixture of at least one first gas configured for generating broadband radiation and at least one second gas configured for increasing the thermal conductivity of the gas mixture and / or providing acoustic suppression of a shockwave induced during the generation of broadband radiation, and further wherein the gas mixture comprises no more than 4% molar fraction of hydrogen.

[0141] 3. The broadband light source device according to any one of aspects 1 or 2, wherein the at least one second gas comprises or consists of an atomic gas.

[0142] 4. The broadband light source device according to aspect 3, wherein the at least one second gas comprises or consists of helium.

[0143] 5. The broadband light source device according to aspect 3 or 4, wherein the at least one first gas comprises or consists of an atomic gas having a larger atomic mass than the second gas.

[0144] 6. The broadband light source device according to aspect 5, wherein the at least one first gas comprises or consists of one or more of krypton, xenon, argon, neon.

[0145] 7. The broadband light source device according to any one of the preceding aspects, wherein the at least first gas comprises or consists of one or more molecular gases.

[0146] 8. The broadband light source device of any of the preceding aspects, wherein the at least one second gas comprises or consists of one or more molecular gases.

[0147] 9. The broadband light source device of aspect 8, wherein the one or more molecular gases are selected from the group consisting of: nitrogen, oxygen, H20.

[0148] 10. The broadband light source device of any of the preceding aspects, wherein the second gas constitutes at least 2% of the mole fraction of the gas mixture.

[0149] 11. The broadband light source device of any of aspects 1 to 9, wherein the second gas constitutes at least 10% of the mole fraction of the gas mixture.

[0150] 12. The broadband light source device of any of aspects 1 to 9, wherein the second gas constitutes between 30% and 70% of the mole fraction of the gas mixture.

[0151] 13. The broadband light source device of any of aspects 1 to 9, wherein the second gas constitutes between 40% and 60% of the mole fraction of the gas mixture.

[0152] 14. The broadband light source device of any of aspects 4 to 9, wherein the at least one first gas consists of xenon and the at least one second gas consists of helium, the at least one second gas constituting a mole fraction equal to or higher than 50%.

[0153] 15. The broadband light source device of aspect 14, wherein the at least one first gas consists of xenon constituting a mole fraction of 20% or less of the gas mixture and the at least one second gas consists of helium constituting a mole fraction of 80% or more of the gas mixture.

[0154] 16. The broadband light source device of aspect 14, wherein the at least one first gas consists of xenon constituting a mole fraction of 10% or less of the gas mixture and the at least one second gas consists of helium constituting a mole fraction of 90% or more of the gas mixture.

[0155] 17. The broadband light source device of any of aspects 4 to 9, wherein the at least one first gas consists of krypton and the at least one second gas consists of helium, the at least one second gas constituting a mole fraction equal to or higher than 20%.

[0156] 18. The broadband light source arrangement according to aspect 17, wherein the at least one first gas consists of krypton gas and the at least one second gas consists of helium gas, the krypton gas and the helium gas each constituting 50% ± 10% of the molar fraction of the gas mixture. 10% of the molar fraction.

[0157] 19. The broadband light source arrangement according to any one of aspects 4 to 9, wherein the at least one first gas consists of argon gas and the at least one second gas consists of helium gas, the at least one second gas constituting a molar fraction equal to or higher than 10%.

[0158] 20. The broadband light source arrangement according to aspect 19, wherein the at least one first gas consists of argon gas constituting 70% of the molar fraction or less of the gas mixture and the at least one second gas consists of helium gas constituting 30% of the molar fraction or more of the gas mixture.

[0159] 21. The broadband light source arrangement according to aspect 19, wherein the at least one first gas consists of argon gas constituting 90% of the molar fraction or less of the gas mixture and the at least one second gas consists of helium gas constituting 10% of the molar fraction or more of the gas mixture.

[0160] 22. The broadband light source arrangement according to any one of the preceding aspects, wherein the HC-PCF comprises a single-ring HC-PCF.

[0161] 23. The broadband light source arrangement according to any one of the preceding aspects, further comprising a pump radiation source for generating the pump radiation.

[0162] 24. The broadband light source arrangement according to aspect 23, wherein an optimal acoustic suppression frequency of the gas mixture substantially matches a repetition rate of the pump radiation generated by the pump radiation source.

[0163] 25. The broadband light source arrangement according to aspect 24, wherein the gas composition or gas composition of the gas mixture is configured for the matching of the optimal acoustic suppression frequency of the gas mixture and the repetition rate of the pump radiation.

[0164] 26. The broadband light source arrangement according to aspect 24 or 25, configured to tune the repetition rate of the pump radiation to match the optimal acoustic suppression frequency of the gas mixture.

[0165] 27. A broadband light source apparatus according to any of aspects 23 to 26, wherein the pump radiation source is configured such that the repetition rate of the pump radiation does not match any of the acoustic resonance frequencies of the inner cladding structure of the HC-PCF.

[0166] 28. A broadband light source apparatus according to any of the preceding aspects, wherein the inner cladding structure of the HC-PCF comprises one or more silica tubes.

[0167] 29. A broadband light source apparatus according to any of the preceding aspects, wherein the gas mixture is configured to define a spectral range of the generated broadband radiation.

[0168] 30. A broadband light source apparatus according to any of the preceding aspects, wherein the broadband output comprises a wavelength range of 200 nm to 3000 nm, or a sub-range such as 400 nm to 2000 nm within this range.

[0169] 31. A metrology apparatus comprising a broadband light source apparatus according to any of the preceding aspects.

[0170] 32. A metrology apparatus according to aspect 31, comprising a scatterometer metrology device, a level sensor or an alignment sensor.

[0171] 33. A method for configuring a source apparatus, the source apparatus being configured for generating a broadband radiation output and the source apparatus comprising:

[0172] a pump radiation source for outputting pump radiation; and

[0173] a hollow core photonic crystal fibre (HC-PCF);

[0174] a gas mixture filling the HC-PCF, wherein the method comprises:

[0175] matching an optimal acoustic suppression frequency of the gas medium substantially to a repetition rate of the pump radiation generated by the pump radiation source.

[0176] 34. A method according to aspect 33, comprising optimising a gas composition of a gas medium to match the optimal acoustic suppression frequency of the gas medium to the repetition rate of the pump radiation.

[0177] 35. A method according to aspect 33 or 34, comprising tuning the repetition rate of the pump radiation to match the optimal acoustic suppression frequency of the gas medium.

[0178] 36. A method according to any one of aspects 33 to 35, comprising optimising the repetition rate of the pump radiation such that the repetition rate does not match any of the acoustic resonance frequencies of the inner cladding structure of the HC-PCF.

[0179] 37. A method for configuring a source apparatus, the source apparatus being configured for producing a broadband radiation output and the source apparatus comprising:

[0180] a pump radiation source for outputting pump radiation; and

[0181] a hollow core photonic crystal fibre (HC-PCF);

[0182] a gas mixture filling the HC-PCF, wherein the method comprises:

[0183] optimising the repetition rate of the pump radiation such that the repetition rate of the pump radiation does not match any of the acoustic resonance frequencies of the inner cladding structure of the HC-PCF.

[0184] 38. A method according to any one of aspects 33 to 37, wherein the gas mixture comprises at least one first gas and at least one second gas, wherein the at least one first gas is configured for producing broadband radiation and the at least one second gas is configured to increase the thermal conductivity of the gas mixture and / or to provide acoustic suppression of shockwaves induced during the production of broadband radiation, and further wherein the gas mixture comprises no more than ten parts per million (ppm) molar fraction of hydrogen.

[0185] 39. A method according to aspect 38, wherein the second gas comprises an atomic gas.

[0186] 40. A method according to aspect 39, wherein the second gas comprises helium.

[0187] 41. A method according to aspect 39 or 40, wherein the first gas comprises an atomic gas having a greater atomic mass than the second atomic gas.

[0188] 42. A method according to aspect 41, wherein the first atomic gas is selected from the group consisting of krypton, xenon, argon, neon.

[0189] 43. A method for configuring a source apparatus, the source apparatus being configured for producing a broadband radiation output and the source apparatus comprising:

[0190] a pump radiation source for outputting pump radiation; and

[0191] a hollow core photonic crystal fiber (HC-PCF);

[0192] a gas mixture filling the HC-PCF, wherein the method comprises:

[0193] optimizing the molar fraction of helium in the gas mixture by optimizing one or more of:

[0194] the thermal conductivity of the gas mixture;

[0195] the thermal diffusivity of the gas mixture; or

[0196] a heat transfer mechanism.

[0197] 44. The method of aspect 43, wherein the optimization of the heat transfer mechanism comprises selecting a gas or gas mixture having a heavy molecular weight for the at least one first gas.

[0198] 45. The method of aspect 44, wherein the molecular weight of the at least one first gas is at least a factor 10 heavier than the molecular weight of helium.

[0199] 46. The method of any one of aspects 44 or 45, wherein the partial pressure of each constituent gas of the gas mixture is maintained substantially simultaneously with the optimization of the molar fraction of helium.

[0200] 47. An optical component for a broadband light source arrangement, the optical component being configured for generating a broadband output upon receiving pump radiation, and the optical component comprising:

[0201] a hollow core photonic crystal fiber (HC-PCF); and

[0202] a gas mixture filling the HC-PCF, wherein the gas mixture comprises a mixture of at least one first gas configured for generating broadband radiation and at least one second gas comprising or consisting of helium.

[0203] 48. The optical component of aspect 47, wherein the at least one first gas comprises or consists of one or more of krypton, xenon, argon, neon.

[0204] 49. The optical component of aspect 47 or 48, wherein the at least first gas comprises or consists of one or more molecular gases.

[0205] 50. The optical component according to any one of aspects 47 to 49, wherein the at least one second gas comprises one or more molecular gases.

[0206] 51. The optical component according to aspect 50, wherein the one or more molecular gases are selected from the group consisting of: nitrogen, oxygen, H20.

[0207] 52. The optical component according to any one of aspects 47 to 51, wherein the second gas constitutes at least 2% of the molar fraction of the gas mixture.

[0208] 52. The optical component according to any one of aspects 47 to 51, wherein the second gas constitutes at least 10% of the molar fraction of the gas mixture.

[0209] 53. The optical component according to any one of aspects 47 to 51, wherein the second gas constitutes between 30% and 70% of the molar fraction of the gas mixture.

[0210] 54. The optical component according to any one of aspects 47 to 51, wherein the second gas constitutes between 40% and 60% of the molar fraction of the gas mixture.

[0211] 55. The optical component according to aspect 47, wherein the at least one first gas consists of xenon gas and the at least one second gas consists of helium gas, the at least one second gas constituting a molar fraction equal to or higher than 50%.

[0212] 56. The optical component according to aspect 55, wherein the at least one second gas consists of helium gas constituting a molar fraction of 80% or more of the gas mixture.

[0213] 57. The optical component according to aspect 55, wherein the at least one second gas consists of helium gas constituting a molar fraction of 90% or more of the gas mixture.

[0214] 58. The optical component according to aspect 47, wherein the at least one first gas consists of krypton gas and the at least one second gas consists of helium gas, the at least one second gas constituting a molar fraction equal to or higher than 20%.

[0215] 59. The optical component according to aspect 58, wherein the at least one first gas consists of krypton gas and the at least one second gas consists of helium gas, the krypton gas and the helium gas constituting a molar fraction of 50% ± 10% and 50% ± 10% of the gas mixture, respectively. 10% of the molar fraction, respectively.

[0216] 60. The optical component of aspect 47, wherein the at least one first gas consists of argon and the at least one second gas consists of helium, the at least one second gas comprising a mole fraction equal to or greater than 10%.

[0217] 61. The optical component of aspect 60, wherein the at least one first gas consists of argon comprising a mole fraction of 70% or less of the gas mixture and the at least one second gas consists of helium comprising a mole fraction of 30% or more of the gas mixture.

[0218] 62. The optical component of aspect 60, wherein the at least one first gas consists of argon comprising a mole fraction of 90% or less of the gas mixture and the at least one second gas consists of helium comprising a mole fraction of 10% or more of the gas mixture.

[0219] 63. The optical component of any one of aspects 47 to 62, wherein the HC-PCF is a single-ring HC-PCF.

[0220] 64. A method for configuring an optical component for a source device, the source device configured for producing a broadband radiation output, the method comprising:

[0221] selecting a hollow core photonic crystal fiber (HC-PCF) and a gas mixture, the gas mixture comprising a first gas filling the HC-PCF; and

[0222] determining an optimized mole fraction of helium gas present in the gas mixture, wherein the optimized mole fraction of helium gas is based on one or more of:

[0223] increasing a thermal conductivity of the gas mixture;

[0224] increasing a thermal diffusivity of the gas mixture; or

[0225] selecting a desired heat transfer mechanism.

[0226] 65. The method of aspect 64, further comprising selecting a gas or gas mixture having a heavy molecular weight for the first gas in order to select a desired heat transfer mechanism.

[0227] 66. The method of aspect 65, wherein the at least one first gas has a molecular weight at least a factor of 10 heavier than that of helium.

[0228] 67. A method according to either of aspects 65 or 66, wherein the partial pressure of each constituent gas of the gas mixture is maintained substantially simultaneously with the determination of the molar fraction of helium.

[0229] 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 such as liquid-crystal displays (LCDs), thin-film magnetic heads, etc.

[0230] Although specific reference can be made in this text to the use of embodiments of the application in the context of optical lithography, it should be understood that embodiments of the application can have other applications in other devices manufacture fields where fine patterns are created by the projection of radiation beams. One of the other non-limiting applications for embodiments of the application is the fabrication of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays such as liquid-crystal displays (LCDs), thin-film magnetic heads, etc.

[0231] 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, for example, imprint lithography contexts.

[0232] 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 intended to be illustrative, not restrictive. Accordingly, modifications can be made by people of skill in the art without departing from the scope of the following claims.

Claims

1. A source device configured to generate a broadband radiation output and comprising: a pump radiation source, the pump radiation source being configured to output pump radiation; Hollow-core photonic crystal fiber; and a gas mixture, the gas mixture filling the hollow core photonic crystal fiber, Therein, the optimal acoustic suppression frequency of the gas mixture is configured to match a repetition rate of the pump radiation generated by the pump radiation source.

2. The source device according to claim 1, wherein The gas composition of a gas mixture is optimized such that the optimal acoustic suppression frequency of the gas mixture matches the repetition rate of the pump radiation.

3. The source device according to claim 1 or 2, wherein: The repetition rate of the pump radiation is tuned to match the optimal acoustic suppression frequency of the gas mixture.

4. The source device according to claim 1 or 2, wherein: The repetition rate of the pump radiation is optimized such that the repetition rate of the pump radiation does not match any of the acoustic resonance frequencies of the inner cladding structure of the hollow-core photonic crystal fiber.

5. A source device configured to generate a broadband radiation output and comprising: a pump radiation source, the pump radiation source being configured to output pump radiation; Hollow-core photonic crystal fiber; and a gas mixture, the gas mixture filling the hollow core photonic crystal fiber, The repetition rate of the pump radiation is optimized so that the repetition rate of the pump radiation does not match any of the acoustic resonance frequencies of the inner cladding structure of the hollow-core photonic crystal fiber.

6. The source device according to any one of claims 1, 2 and 5, wherein: The gas mixture comprises at least one first gas and at least one second gas, wherein the at least one first gas is configured to generate broadband radiation and the at least one second gas is configured to increase thermal conductivity of the gas mixture and / or provide acoustic suppression of shock waves induced during the generation of broadband radiation, and further wherein the gas mixture comprises a mole fraction of hydrogen of no more than 10 parts per million.

7. The source device according to claim 6, wherein: The second gas includes an atomic gas.

8. The source device according to claim 7, wherein: The second gas includes helium.

9. The source device according to claim 7 or 8, wherein: The first gas includes an atomic gas having a larger atomic weight than the atomic gas of the second gas.

10. The source device according to claim 9, wherein: The atomic gas of the first gas is selected from the following range: krypton, xenon, argon, and neon.

11. The source device according to claim 6, wherein: The at least one first gas includes one or more molecular gases.

12. The source device according to claim 11, wherein: The at least one second gas includes one or more molecular gases.

13. The source device according to claim 12, wherein: The one or more molecular gases are selected from the following range: nitrogen, oxygen, H2O.

14. The source device according to claim 6, wherein: The at least one second gas comprises a mole fraction of at least 2% of the gas mixture.

15. The source device according to claim 6, wherein: The at least one second gas constitutes a mole fraction of between 40% and 60% of the gas mixture.

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