Hollow-core fiber-based broadband radiation generator with extended fiber service life

Abstract

Disclosed is an optical component for a broadband light source device, the optical component being configured to generate 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 and at least one second gas, the at least one first gas being configured for the generation of the broadband radiation, and the at least one second gas comprising helium or consisting of helium.

Description

[0001] Cross-reference to related applications

[0002] This application claims priority to European Application No. 20184730.8, filed on Jul. 8, 2020, and European Application No. 21167961.8, filed on Apr. 13, 2021, the entire contents of which are incorporated herein by reference. Technical field

[0003] The present invention relates to a broadband radiation generator based on hollow-core optical fiber, and more particularly to such a broadband radiation generator related to metrology applications in integrated circuit manufacturing. Background art

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

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

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

[0007] Metrology tools are used in many aspects of the IC manufacturing process, such as an alignment tool for properly positioning the substrate before exposure, a leveling tool for measuring the surface topography of the substrate, and a focus control and scatterometry-based tool for inspecting / measuring the exposed and / or etched product, for example, 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 being used for such metrology applications. It would be desirable to improve the current devices for broadband radiation generation. Summary of the Invention

[0008] In a first aspect of the present invention, there is provided an optical component for a broadband light source device, the optical component being configured to generate broadband radiation when receiving pump radiation, and the optical component comprising: a hollow-core photonic crystal fiber (HC-PCF); and a gas mixture that fills the HC-PCF, wherein the gas mixture comprises a mixture of at least one first gas and at least one second gas, the at least one first gas being configured for the generation of the broadband radiation, and the at least one second gas comprising helium or consisting of helium, for thermally regulating the gas mixture and / or suppressing acoustic waves in the gas mixture.

[0009] In a second aspect of the present invention, there is provided a broadband light source device, the broadband light source device comprising the optical component as described in the first aspect and further comprising a pump radiation source for generating the pump radiation.

[0010] In a third aspect, there is provided a metrology tool for inspecting a substrate, the metrology tool including a broadband light source device as described in the second aspect.

[0011] In a fourth aspect of the present invention, there is provided a method for configuring optical components for a source device configured to generate a broadband radiation output, the method including: selecting a hollow-core photonic crystal fiber (HC-PCF) and a gas mixture filling the HC-PCF, the gas mixture including a first gas and a second gas, the first gas being for generating the broadband radiation, the second gas including 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 the following: 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 THE DRAWINGS

[0012] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic diagrams, in which:

[0013] - Figure 1 A schematic overview of a lithographic apparatus is depicted;

[0014] - Figure 2 A schematic overview of a lithography cell is depicted;

[0015] - Figure 3 A schematic representation of overall lithography is depicted, the schematic representation of overall lithography showing the cooperation between three key technologies for optimizing semiconductor manufacturing;

[0016] - Figure 4 A schematic overview of a scatterometry apparatus, which may include a radiation source, acting as a metrology device according to an embodiment of the present invention is depicted;

[0017] - Figure 5 A schematic overview of a level sensor device, which may include a radiation source, according to an embodiment of the present invention is depicted;

[0018] - Figure 6 A schematic overview of an alignment sensor device, which may include a radiation source, according to an embodiment of the present invention is depicted;

[0019] - Figure 7 The transverse cross-sections of two HC-PCF designs for white light generation are schematically depicted, the HC-PCF designs including (a) a Kagome design and (b) a single-ring design;

[0020] - Figure 8Two cross-sectional images taken at two different positions of an HC-PCF with a single-ring design operating in pure krypton gas are shown: (a) a cross-sectional image taken at the fiber end; (b) a cross-sectional image taken at a position several centimeters from the same fiber end.

[0021] - Figure 9 Schematically depicts a broadband light source device for an HC-PCF based on the filling gas;

[0022] - Figure 10 (a) to Figure 10 (c) schematically depict examples of optical components in three different configurations;

[0023] - Figure 11 A graph illustrating the thermal conductivity of several binary mixtures of atomic gases as a function of molecular weight; and

[0024] - Figure 12 Curves showing simplified simulations of thermal relaxation in an HC-PCF for two different thermal diffusivities. Detailed Description

[0025] In this document, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g., having wavelengths of 365 nm, 248 nm, 193 nm, 157 nm, or 126 nm) and EUV (extreme ultraviolet radiation, e.g., having wavelengths in the range of about 5 nm to 100 nm).

[0026] As used herein, the terms "reticle", "mask", or "patterning device" can be broadly interpreted to refer to a general patterning device that can be used to impart a patterned cross-section to an incident radiation beam, which corresponds to a pattern to be created in a target portion of a substrate. In this context, the term "light valve" can also be used. Examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays in addition to typical masks (transmission or reflection type, binary type, phase-shift type, hybrid type, etc.).

[0027] Figure 1Schematically depict a lithographic apparatus LA. The lithographic apparatus LA includes: an illumination system (also referred to as an illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation); a mask support (e.g., a mask table) MT configured to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to certain parameters; a substrate support (e.g., a wafer table) WT configured to hold a substrate (e.g., a 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 the pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g., including one or more dies) of the substrate W.

[0028] In operation, the illumination system IL receives the radiation beam from a radiation source SO, e.g., via a beam delivery system BD. The illumination system IL may include various types of optical components for guiding, shaping, and / or controlling the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in a plane of the patterning device MA in its cross-section.

[0029] The term "projection system" PS as used herein should be broadly interpreted to cover various types of projection systems suitable for the exposure radiation used and / or for other factors such as the use of an immersion liquid or the use of a vacuum, including refractive, reflective, catadioptric, synthetic, magnetic, electromagnetic, and / or electrostatic optical systems or any combination thereof. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0030] The lithographic apparatus LA may be of the 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) to fill the space between the projection system PS and the substrate W - this is also referred to as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0031] The lithographic apparatus LA may also be of the type having two or more substrate supports WT (also known as "dual platforms"). In such a "multi-platform" machine, the substrate supports WT may be used in parallel, and / or steps for preparing a substrate W for a subsequent exposure of the substrate W located on one of the substrate supports WT may be carried out while another substrate W on another substrate support WT is used for exposing a pattern on another substrate W.

[0032] In addition to the substrate support WT, the lithographic apparatus LA may also include a measurement stage. The measurement stage is configured to hold sensors and / or cleaning devices. The sensors may be arranged to measure properties of the projection system PS or of the radiation beam B. The measurement stage may hold a plurality of sensors. The cleaning device may be arranged to clean a part of the lithographic apparatus, for example, a part of the projection system PS or a part of the system providing the immersion liquid. The measurement stage may move under the projection system PS when the substrate support WT is moved away from the projection system PS.

[0033] In operation, the radiation beam B is incident on a patterning device (e.g., a mask) MA held on a mask support MT and is patterned by a pattern (design layout) present on the patterning device MA. After having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. By means of the second positioner PW and the position measurement system IF, the substrate support WT can be accurately moved, for example, in order to position different target portions C in the path of the radiation beam B in a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (the other position sensor is not explicitly depicted in Figure 1 ), can be used to accurately position the patterning device MA relative to the path of the radiation beam B. Mask alignment marks M1, M2 and substrate alignment marks P1, P2 can be used to align the patterning device MA with the substrate W. Although the substrate alignment marks P1, P2 as illustrated occupy dedicated target portions, they may be located in the spaces between the target portions. When the substrate alignment marks P1, P2 are located between the target portions C, these substrate alignment marks are referred to as scribe alignment marks.

[0034] As Figure 2As shown, the lithography apparatus LA can form part of a lithography cell LC (sometimes also referred to as a lithography cell or (lithography) cluster), which typically also includes equipment for performing pre-exposure and post-exposure processes on the substrate W. Conventionally, this equipment includes a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH and a bake plate BK for, for example, regulating the temperature of the substrate W (e.g., for regulating the solvent in the resist layer). A substrate handler or robot RO picks up the substrate W from the input / output ports I / O1, I / O2, moves the substrate W between different process equipment, and transfers the substrate W to the feed table LB of the lithography apparatus LA. Devices that are generally also collectively referred to as a coating and developing system in the lithography cell are typically under the control of a coating and developing system control unit TCU, which itself can be controlled by a management control system SCS, which can also control the lithography apparatus LA, for example, via a lithography control unit LACU.

[0035] To correctly and consistently expose the substrate W exposed by the lithography apparatus LA, it is necessary to inspect the substrate to measure the properties of the patterned structure, such as the overlay error between subsequent layers, line thickness, critical dimension (CD), etc. For this purpose, an inspection tool (not shown) can be included in the lithography cell LC. If an error is detected, the exposure of subsequent substrates or other processing steps to be performed on the substrate W can be adjusted, for example, especially in the case where inspections are carried out before other substrates W in the same batch or lot are still to be exposed or processed.

[0036] An inspection device, which can also be referred to as a metrology device, is used to determine the properties of the substrate W, and specifically, how the properties of different substrates W vary or how the properties associated with different layers of the same substrate W vary from layer to layer. The inspection device can alternatively be configured to identify defects on the substrate W and can, for example, be part of the lithography cell LC, or can be integrated into the lithography apparatus LA, or can even be a stand-alone device. The inspection device can measure the properties on a latent image (the image in the resist layer after exposure), or a semi-latent image (the image in the resist layer after a post-exposure bake step PEB), or a developed resist image (where the exposed or unexposed portions of the resist have been removed), or even an etched image (after a pattern transfer step such as etching).

[0037] Generally, the patterning process in the lithography apparatus LA is one of the most decisive steps in the process, which requires a high degree of accuracy in the dimensional calibration and placement of the structures on the substrate W. To ensure this high accuracy, three systems can be combined in a so-called "integrated" control environment, as Figure 3Schematically depicted. One of these systems is a lithographic apparatus LA, which is (virtually) connected to a metrology tool MT (second system) and to a computer system CL (third system). The key to this “integrated” environment lies in optimizing the cooperation between these three systems to enhance the overall process window and provide a tight control loop, thus ensuring that the patterning performed by the lithographic apparatus LA remains within the process window. The process window defines a range of process parameters (e.g., dose, focus, overlay), within which a particular manufacturing process yields a defined result (e.g., a functional semiconductor device) - typically within this defined result, process parameter variations in the lithography process or patterning process are allowed.

[0038] The computer system CL can use (a portion of) the design layout to be patterned to predict which resolution enhancement techniques to use and perform computational lithography simulations and calculations to determine which mask layout and lithographic apparatus settings achieve the maximum overall process window for the patterning process (depicted by the double arrow in Figure 3 the first scale SC1). Typically, the resolution enhancement techniques are configured to match the patterning capabilities of the lithographic apparatus LA. The computer system CL can also be used to detect where the lithographic apparatus LA is currently operating within the process window (e.g., using input from the metrology tool MT) to predict whether defects may be present due to, for example, sub-optimal processing (depicted by the arrow pointing to “0” in Figure 3 the second scale SC2).

[0039] The metrology tool MT can provide input to the computer system CL for accurate simulation and prediction, and can provide feedback to the lithographic apparatus LA to identify possible drifts in, for example, the calibration state of the lithographic apparatus LA (depicted by the multiple arrows in Figure 3 the third scale SC3).

[0040] In a lithography process, it is desirable to frequently measure the structures created, 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 measurement of parameters of a lithography process by having a sensor in a pupil or a plane conjugate to the pupil of the objective of the scatterometer (this measurement is commonly referred to as pupil-based measurement), or by having a sensor in an image plane or a plane conjugate to the image plane, in which case the measurement is commonly referred to as image or field-based measurement. Such scatterometers and related measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP1,628,164A, which are incorporated herein by reference in their entirety. The aforementioned scatterometers can use radiation from soft x-rays and light visible in the near IR wavelength range to measure gratings.

[0041] 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 be caused, for example, by simulating the interaction of scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measured results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to the diffraction pattern observed from the real target.

[0042] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In this spectroscopic scatterometer MT, radiation emitted by a radiation source is directed onto a target, and the reflected or scattered radiation from the target is directed onto a spectrometer detector that measures the spectrum of the specularly reflected radiation (i.e., a measurement of intensity as a function of wavelength). From this data, the structure or profile of the target that produced the detected spectrum can be reconstructed, for example, by rigorous coupled-wave analysis and non-linear regression or by comparison with a simulated spectral library.

[0043] In a third embodiment, the scatterometer MT is an ellipsometric scatterometer. An ellipsometric scatterometer allows for the determination of parameters of a lithography process by measuring scattered radiation for each polarization state. This metrology device emits polarized light (such as linear, circular, or elliptical) by using, for example, a suitable polarization filter in the illumination section of the metrology device. Sources suitable for the metrology device can also provide polarized radiation. Various embodiments of existing ellipsometric scatterometers are described in U.S. Patent Applications 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,449, 12 / 920,968, 12 / 922,587, 13 / 000,229, 13 / 033,135, 13 / 533,110, and 13 / 891,410, which are incorporated herein by reference in their entirety.

[0044] In Figure 4 FIG. depicts a metrology device such as a scatterometer. It includes a broadband (white light) radiation projector 2 that projects radiation onto a substrate W. The reflected or scattered radiation is transmitted to a spectrometer detector 4 that measures the spectrum 6 of the specularly reflected radiation (i.e., an intensity measurement that varies with wavelength). From this data, the structure or profile 8 that produced 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 simulated spectral library as shown at the bottom of Figure 3 ). Generally, for reconstruction, the general form of the structure is known, and some parameters are assumed based on knowledge of the process used to fabricate 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.

[0045] The overall measurement quality of lithography parameters via measurement of a metrology target is at least partially determined by the measurement option scheme used to measure such lithography parameters. The term "substrate measurement option scheme" can include one or more parameters of the measurement itself, one or more parameters of one or more patterns being measured, or both. For example, if the measurement used in the substrate measurement option scheme is a diffraction-based optical measurement, one or more of the measured parameters can include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, etc. One of the criteria for selecting a measurement option scheme can be, for example, the sensitivity of one of the measurement parameters to process variations. More examples are described in U.S. Patent Application US2016 / 0161863 and published U.S. Patent Application US 2016 / 0370717A1, which are incorporated herein by reference in their entirety.

[0046] Another type of metrology tool for IC manufacturing is a topography measurement system, a flatness sensor, or a height sensor. Such tools can be integrated into 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 indicating the height of the substrate varying according to position on the substrate. This height map can then be used to correct the position of the substrate during transfer of a pattern onto the substrate so as 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 general dimension from a plane to the substrate (also referred to as the Z-axis). Typically, the flatness 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 flatness or height sensor generates height measurements at various positions across the substrate.

[0047] Figure 5 Examples of flatness or height sensors LS known in the art are schematically shown in, which illustrate only the operating principle. In this example, the flatness sensor includes an optical system, which includes a projection unit LSP and a detection unit LSD. The projection unit LSP includes a radiation source LSO that provides a radiation beam LSB, which is imparted 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 unpolarized, pulsed or continuous (such as a polarized or unpolarized laser beam). The radiation source LSO can include a plurality of radiation sources having different colors or wavelength ranges, such as a plurality of LEDs. The radiation source LSO of the flatness sensor LS is not limited to visible light radiation, but can additionally or alternatively cover UV and / or IR radiation and any wavelength range suitable for reflection from the surface of the substrate.

[0048] The projection grating PGR is a periodic grating including a periodic structure that generates a radiation beam BE1 having a periodically varying intensity. The radiation beam BE1 having a periodically varying intensity is directed towards a measurement position MLO on the substrate W having an angle of incidence ANG with respect to an axis (Z-axis) perpendicular to the substrate surface, the angle of incidence ANG being between 0 degrees and 90 degrees, typically between 70 degrees and 80 degrees. At the measurement position MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by the arrow BE2) and directed towards the detection unit LSD.

[0049] To determine the height level at the measurement position MLO, the level sensor further includes a detection system, which includes a detection grating DGR, a detector DET, and a processing unit (not shown) for processing the output signal of the detector DET. The detection grating DGR can be the same as the projection grating PGR. The detector DET generates a detector output signal, which indicates the received light, for example, indicates the intensity of the received light, such as a light detector; or represents the spatial distribution of the received intensity, such as a camera. The detector DET can include any combination of one or more detector types.

[0050] By means of triangulation techniques, the height level at the measurement position MLO can be determined. The detected height level is generally related to the signal intensity measured by the detector DET, which has a periodicity particularly dependent on the design of the projection grating PGR and the (tilted) incident angle ANG.

[0051] The projection unit LSP and / or the detection unit LSD can include other optical components (not shown) along the path of the patterned radiation beam between the projection grating PGR and the detection grating DGR, such as lenses and / or mirrors.

[0052] In an embodiment, the detection grating DGR can be omitted, and the detector DET can be placed at the position where the detection grating DGR is located. This configuration provides a more direct detection of the image of the projection grating PGR.

[0053] To effectively cover 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 generating a measurement area MLO or an array of light spots that cover a larger measurement range.

[0054] Various height sensors of a general type are disclosed, for example, in US7265364 and US7646471, both of which are incorporated by reference. A height sensor that uses UV radiation instead of visible or infrared radiation is disclosed in US2010233600A1, which is incorporated by reference. In WO2016102127A1, which is incorporated by reference, a compact height sensor that uses a multi-element detector to detect and distinguish the position of the grating image without the need for a detection grating is described.

[0055] Another type of metrology tool used in IC manufacturing is an alignment sensor. Thus, a key aspect of the performance of a lithographic apparatus is the ability to place the applied pattern relative to features placed in a previous layer (by the same or a different lithographic apparatus) properly and accurately. For this purpose, the substrate is provided with one or more sets of marks or targets. Each mark is a structure whose position can later be measured using a position sensor, typically an optical position sensor. The position sensor may be referred to as an “alignment sensor” and the marks may be referred to as “alignment marks”.

[0056] A lithographic apparatus may include one or more (e.g., multiple) alignment sensors that can be used to accurately measure the position of alignment marks provided on a substrate. The alignment (or position) sensor may use optical phenomena such as diffraction and interference to obtain position information from alignment marks formed on the substrate. An example of an alignment sensor used in current lithographic apparatuses is based on a self - referencing interferometer as described in US6961116. Various enhancements and modifications of the position sensor have been developed, such as those disclosed in US2015261097A1. The content of all these publications is incorporated herein by reference.

[0057] Figure 6 FIG. 7 is a schematic block diagram of an embodiment of a known alignment sensor AS, such as that described in US6961116 and incorporated by reference. A radiation source RSO provides a radiation beam RB having one or more wavelengths, which is steered by steering optics onto a mark, such as a mark AM located on a substrate W, as an illumination spot SP. In this example, the steering optics includes a spot mirror SM and an objective lens OL. The diameter of the illumination spot SP illuminating the mark AM may be slightly smaller than the width of the mark itself.

[0058] The radiation diffracted by the alignment mark AM (in this example via the objective lens OL) is collimated into an information - carrying beam IB. The term “diffraction” is intended to include zero - order diffraction from the mark (zero - order diffraction may be referred to as reflection). A self - referencing interferometer SRI of the type disclosed, for example, in US6961116 mentioned above interferes the beam IB with itself, and then the beam is received by a light detector PD. Additional optics (not shown) may be included to provide separate beams in the case where the radiation source RSO produces more than one wavelength. The light detector may be a single element, or it may include multiple pixels as required. The light detector may include a sensor array.

[0059] The steering optics, which includes the spot mirror SM in this example, may also be used to block the zero - order radiation reflected from the mark, such that the information - carrying beam IB includes only higher - order diffracted radiation from the mark AM (although this is not necessary for the measurement, but it improves the signal - to - noise ratio).

[0060] The intensity signal SI is supplied to the processing unit PU. By combining the optical processing in the block SRI and the computational processing in the unit PU, the values of the X and Y positions of the output substrate relative to the reference frame are output.

[0061] A single measurement of the type described only fixes the position of the mark within a certain range corresponding to one pitch of the mark. A coarser measurement technique is used in combination with this measurement to identify which period of the sine wave is the period containing the marked position. The same process at coarser and / or finer levels can be repeated at different wavelengths for improving accuracy and / or for robustly detecting the mark, independent of the material on which the mark is made and the materials provided above and / or below the mark. The wavelengths can be optically multiplexed and demultiplexed to process the wavelengths simultaneously, and / or the wavelengths can be multiplexed by time sharing or frequency division.

[0062] In this example, the alignment sensor and the light spot SP remain stationary while the substrate W moves. The alignment sensor can thus be stably and accurately mounted to the reference frame while effectively scanning the mark AM in a direction opposite to the movement direction of the substrate W. During this movement, the substrate W is controlled by mounting the substrate W on a substrate support and the 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 an embodiment, one or more (alignment) marks are provided on the substrate support. Measuring the position of the mark provided on the substrate support allows calibration of the position of the substrate support as determined by the position sensor (e.g., relative to the frame to which the alignment system is connected). Measuring the position of the alignment mark provided on the substrate allows determination of the position of the substrate relative to the substrate support.

[0063] For optical semiconductor metrology, inspection applications are often preferred, such as in any of the foregoing metrology tools, where a bright light source that outputs coherent radiation simultaneously covers a wide wavelength range (e.g., from UV to IR). This broadband light source helps improve 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 changes (e.g., changing the light source to have a specific wavelength). Allowing the wavelength to be optimized for a specific application also means that the accuracy of the measurement can be further improved.

[0064] Gas lasers that emit multiple wavelengths simultaneously based on gas discharge effects can be used in these applications. However, inherent problems associated with gas lasers, such as high-intensity instability and low spatial incoherence, may make them unsuitable. Alternatively, the outputs from multiple lasers (e.g., solid-state lasers) with different wavelengths can be spatially combined in the optical path of a metrology or inspection system to provide a multi-wavelength source. The increasing complexity and high implementation cost with the number of desired wavelengths have prevented this solution from being widely used. In contrast, fiber-based broadband or white-light lasers (also known as supercontinuum lasers) are capable of emitting radiation with high spatial coherence and broad spectral coverage (e.g., from UV to IR) and are thus a very attractive and practical option.

[0065] Hollow-core photonic crystal fiber (HC-PCF) is a special type of fiber that includes a central hollow core region and an internal cladding structure surrounding the hollow, both of which extend along the entire fiber axis. The light guiding mechanism is enabled by the internal cladding waveguide structure, which can include, for example, thin-walled glass elements. Thus, the radiation is mainly confined to the hollow interior and propagates along the fiber in the form of a transverse core mode.

[0066] Multiple types of HC-PCF can be engineered, each based on different physical guiding mechanisms. Two such types of HC-PCF include: hollow-core photonic bandgap fiber (HC-PBF) and hollow-core antiresonant reflecting fiber (HC-ARF).

[0067] HC-PCF includes a hollow channel filled with a fluid such that it has the resulting desired properties for various light guiding applications; for example, high-power beam delivery using HC-PBF and gas-based white-light generation (or supercontinuum generation) using HC-ARF. Details regarding the design and manufacture of HC-PCF can be found in U.S. Patent US2004175085 (for HC-PBF) and European Patent Application EP3136143A1 (for HC-ARF), which are incorporated herein by reference. HC-PBF is configured to provide low-loss but narrow-bandwidth light guiding via a photonic bandgap effect established by the cladding structure surrounding the central hollow core. However, HC-ARF is engineered to significantly broaden the transmission bandwidth via antiresonant reflection of light from the cladding.

[0068] Figure 7 Two well-known types of HC-ARF are depicted in cross-sectional view. Figure 7 (a) shows a Kagome fiber that includes a Kagome lattice structure. Figure 7 (b) shows a single-ring or spiral fiber, where the hollow core region is formed and surrounded by layers of non-touching rings.

[0069] For gas-based white light generation, the HC-ARF can be included in a gas cell designed to operate, for example, at pressures up to several tens of bar (e.g., between 3 bar and 100 bar). When pumped by an ultrashort pump laser pulse with a sufficiently large peak power, the gas-filled HC-ARF can act as an optical frequency converter. The frequency conversion from the ultrashort pump laser pulse to a broadband laser pulse is achieved by the complex interplay of dispersion and nonlinear optical processes within the gas-filled gas. The converted laser pulse is mainly confined to the hollow core in the form of a transverse core mode and is 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 an internal cladding waveguide structure and experience strong attenuation during its propagation along the fiber. The core region and the cladding region of the HC-ARF can be configured such that higher-order core modes are phase-matched to higher-order cladding modes. In this way, the higher-order core modes can resonantly couple to the subsequently attenuated or suppressed higher-order cladding modes. Thus, low-loss and efficient single transverse mode transmission can be obtained over a wide spectral range.

[0070] The spatio-temporal transmission characteristics (e.g., its spectral amplitude and phase) of the laser pulse propagating along the HC-PCF can be changed and tuned by adjusting the pump laser parameters, the gas filling 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 filling parameters can include one or more of the following: gas type, gas pressure, and gas temperature.

[0071] The filling 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 including 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 filling gas, the nonlinear optical process can include modulation instability (MI), optical soliton splitting, 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 filling gas can be tuned by changing the gas chamber pressure, the generated broadband pulse dynamics and related spectral broadening characteristics can be adjusted to optimize frequency conversion. The generated broadband laser output can cover wavelengths from UV (e.g., <400 nm) to IR (e.g., >800 nm).

[0072] It has been found that existing HC-PCF-based broadband sources tend to face fiber service life issues and may not be able to operate after a short period of operation time. In the case where an HC-PCF-based light source is used in a metrology tool (e.g., a scatterometer), the unexpected and early failure of the HC-PCF means that all light sources will need to be removed from the tool for repair or replacement. After installing a replacement light source into the same metrology tool, a complete optical alignment of the laser beam and other necessary corrections and characterizations need to be performed again. The entire process not only increases costs 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 ways to improve / extend its service life.

[0073] Currently, several failure mechanisms have been identified. The first failure mechanism is fiber contamination usually induced by hydrocarbons deposited on one or both fiber ends. There are cleaning methods that can be used to address this contamination.

[0074] The second failure mechanism is fiber overheating. This problem is mainly caused by the fact that existing HC-PCF-based light sources do not incorporate any effective thermal 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 gas species and recombination dynamics in the plasma. This heat generated during operation will increase 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) as illustrated), the fiber geometry does not allow access and efficient cooling of the surface of the inner cladding waveguide structure. In the absence of effective heat dissipation, the heat generated will accumulate in the fiber, especially when the fiber is pumped / driven with high repetition rate pulses, until the fiber overheats and eventually fails. The inner cladding waveguide structure (e.g., such asFigure 7 (b) The surface overheating of the glass or silica waveguide described in (b) will also trigger unwanted chemical reactions, which may produce exhaust gas and thus cause pollution. Therefore, the overheating problem adversely affects the service life of the optical fiber and imposes limitations on the further development of HC-PCF-based light sources, such as power and / or repetition rate scaling.

[0075] The third failure mechanism is the deformation of the inner cladding of the optical fiber. In an environment of pure atomic gas such as krypton, broadband radiation or white light generation also causes mechanical deformation of the fiber cladding structure. As described above, the white light generation process causes fiber heating and plasma generation. It has been found that plasma generation in HC-PCF under short-time profiles causes pressure shock waves due to rapid gas heating. This pressure change is proportional to the free electron density. The pressure wave propagating as a sound wave in the cross-section of the HC-PCF impinges on the thin-walled glass elements (e.g., cladding tubes) of the inner cladding waveguide structure and then excites several mechanical modes of the cladding tube in a pulsed manner, causing it to vibrate under the superposition of the excited mechanical modes. The vibration of the cladding tube heated to several hundred degrees due to white light generation causes temporary and permanent deformation of the cladding tube, and thus forms a deformed inner cladding waveguide structure.

[0076] Since the inner cladding waveguide structure is used as a light guiding mechanism, which mainly confines the radiation to the hollow interior, the deformed inner cladding waveguide structure will cause optical losses and, in some severe cases, fiber damage. Figure 8 Two cross-sectional images taken at two different positions of an HC-PCF using a single-ring design operating with krypton as the only filling gas are shown: (a) a cross-sectional image taken at the end of the optical fiber; (b) a cross-sectional image taken at a position several centimeters from the end of the optical fiber shown in (a). It can be seen that when exposed to the spatially extended region of white light generation, the optical fiber waveguide can be deformed along the optical fiber.

[0077] In addition, there are several other factors that may exacerbate the acoustic-wave-induced mechanical deformation of the fiber cladding structure (e.g., glass waveguide). First, in the case of using a working gas with low thermal conductivity, poor thermal management of the optical fiber causes overheating of the cladding tube, which makes it more sensitive to mechanical deformation.

[0078] In addition, if the repetition rate of the pump pulses is somehow increased and the amplitude of the excited mechanical modes of the cladding tube is amplified, for example, when it is close to the resonance frequency of the excited mechanical modes of the cladding tube, the impact of the generated acoustic waves on the fiber cladding structure may be significantly amplified. As described above, although the pump pulses are nonlinearly broadened to broadband or supercontinuum spectral pulses in the hollow core of the HC-PCF, acoustic waves (or pressure waves) are also excited due to the rapid release of energy into the gas and heating it. In such as Figure 7(a) and Figure 7 In the tube 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. Inside each glass tube resonator, interference can be formed between the incident acoustic wave and the backward reflected acoustic wave. Therefore, each resonator will have a resonance frequency, which is mainly determined by the geometry of the tube, for example, shape, diameter, wall thickness. When the repetition rate of the pump pulse approaches or matches the resonance frequency of the glass tube, a resonance effect will occur, which significantly enhances the amplitude of the mechanical vibration of the relevant tube.

[0079] It is known that the fiber cladding deformation problem can be at least partially alleviated by adding a certain amount of hydrogen (or deuterium) to the working gas or gas mixture. Due to safety regulations, the maximum mole fraction of hydrogen in the filling gas mixture can be about 4%. Currently, the typical mole fraction of hydrogen in the filling gas mixture containing hydrogen is about 2%.

[0080] Studies have shown that the fiber cladding deformation problem is at least partially related to radiation-induced defects (color centers) in the glass. These defects may cause local mechanical stress and ultimately 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.

[0081] In addition, mixing a small amount of hydrogen with the working gas is also considered effective in suppressing the acoustic waves generated in the HC-PCF. Studies have shown that due to the diffusion-induced acoustic wave attenuation, the gas mixture may allow the suppression of the generated acoustic waves. Both atomic gases and molecular gases can be used for mixing with common working gases (such as krypton). Since molecular gases have greater internal degrees of freedom, molecular gases are more effective for acoustic suppression than atomic gases. In other words, the vibrational relaxation and rotational relaxation in molecular gases improve the acoustic suppression ability due to the coupling between molecular degrees of freedom (such as translational degrees of freedom, rotational degrees of freedom, and vibrational degrees of freedom). Molecular gases suitable for mixing with the working gas can be, for example, methane, nitrogen, oxygen, or hydrogen.

[0082] However, the inventors have found that mixtures of atomic gases can still provide considerable acoustic suppression. Therefore, it is proposed to use mixtures of atomic gases, which improve the acoustic wave suppression relative to a single working atomic gas. These mixtures can include, for example, mixtures of inert gases, such as a mixture of krypton and helium, a mixture of krypton and neon, or a mixture of xenon and helium.

[0083] For a gas mixture with a fixed gas component concentration, the amplitude suppression of the generated acoustic wave is described by the following equation:

[0084] A(f) = A0 * e (-α(f)*z) [1]

[0085] where f is the frequency, α(f) is the frequency-dependent suppression coefficient, 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 certain repetition rate / frequency f pr it will be desirable and is proposed herein to use a gas mixture in which the peak of the suppression curve corresponding to the optimal acoustic suppression frequency matches or is substantially close to the repetition rate f of the pump laser pr in order to have maximum suppression at that frequency. For a fixed mixture, one can also tune f pr away from the resonance frequency.

[0086] Although the mixing of a small amount of hydrogen with the working gas has been proposed and used to alleviate the fiber cladding deformation problem, the present inventors now recognize that introducing hydrogen also helps to alleviate the above-mentioned thermal problem. This concept has not been recognized before.

[0087] Currently, krypton 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 mainly responsible for interacting with the input pump radiation and thus generating broadband radiation. The filling gas used in an HC-PCF-based broadband light source can include only the working gas or gas mixture. Alternatively, the filling gas can include the working gas or gas mixture and one or more other gases. The thermal conductivity of krypton is about 9.5 milliwatts per millikelvin (mW / mK) at room temperature. This low thermal conductivity results in poor thermal management in the HC-PCF. In other words, the bare HC-PCF enclosed in the gas chamber and embedded in krypton 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 in direct contact with 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 is 186.9 mW / mK, which is at least an order of magnitude higher than that of commonly used working gases (e.g., krypton). The much higher thermal conductivity helps to improve the heat dissipation of the HC-PCF and thus suppress the above-mentioned fiber overheating problem.

[0088] Although adding hydrogen to the working gas can successfully alleviate the above problems of the HC-PCF-based broadband light source, this method, on the other hand, leads to problems of surface reduction and glass growth, which are also detrimental to the performance (especially the service life) of the HC-PCF-based light source. It has been found that after operating the HC-PCF-based light source with hydrogen-containing gas mixture for several hundred hours, silica or SiOx nanostructures and porous glass mainly grow at the output end of the optical fiber. One of the main sources of silica growth is the reduction of the inner fiber surface in the presence of hydrogen plasma. The reduction and etching of silica in the presence of hydrogen plasma are known phenomena. Hydrogen ions and groups (such as atomic hydrogen) attack the inner fiber surface and cause the contact surface to reduce by converting silica to silicon, or cause the surface to be etched by generating volatile silicon monoxide. The growth of silica at the end of the optical fiber leads to a gradual blockage and loss of the output power, which will ultimately cause fiber damage and result in a short service life of the white light source. In addition to the surface reduction and glass growth problems, the 2% mole fraction of the above hydrogen is only an empirical value, and the exact amount of hydrogen required to minimize or prevent the deformation of the cladding structure remains unknown.

[0089] To solve some or all of these problems, therefore, a method is proposed to improve the service life of the HC-ARF-based broadband light source based on the filling gas. The proposed method is particularly suitable for extending the service life of the HC-PCF when operating in a gas environment for white light or supercontinuum spectrum generation. Different embodiments of the proposed method and related devices will be disclosed below. A commonality shared among the disclosed embodiments is to avoid using hydrogen exclusively to improve the thermal regulation and absorption of sound waves in the working gas, so as to limit the fiber service life limitation problems related to hydrogen. Thus, it is proposed that the filling gas used is a filling gas including hydrogen with a mole fraction not exceeding, for example, ten parts per million (ppm). For example, compared with intentionally added hydrogen, only the hydrogen included in the filling gas can be naturally occurring hydrogen (e.g., native hydrogen that can be generated by materials, hydrocarbons, the exhaust of H2O on any surface, and / or as part of the broadband generation process).

[0090] As Figure 9As described in, the broadband light source device 100 includes a pump laser 110 that outputs a series of pump pulses 111, an optical component 120 that spectrally broadens the input pump pulses, and an optical diagnostic device 130 that measures the output broadband spectrum. The optical component 120 includes 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 distribution. The gas cell 102 also includes 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 permit ultrashort pump laser pulses to enter the gas cell 102 through the window. After being coupled to the filling gas in the HC-PCF 101, the pump laser pulses 111 propagate along the fiber where they undergo significant spectral broadening. The resulting broadband laser pulses are then discharged from the gas cell 102 through the output optical window 103b and measured by the optical diagnostic device 130 (e.g., a spectrometer).

[0091] 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.

[0092] Figure 10 (a) to Figure 10 (c) schematically depict three known configurations of the optical components 120, 120’, 120”. Figure 10 (a) illustrates the 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 within a number of (e.g., three) sub-cells 102a, 102b, 102c that are interconnected by using a suitable sealing mechanism 105. The pressure-tight connection ensures that all the sub-cells 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 respectively included within two separate gas cells 102d, 102e, while the central portion 101b of the fiber acting as a fluid connection is included outside the gas cell.

[0093] Note that Figure 10 (a) to Figure 10(c) The configurations of the optical components 120, 120', 120" illustrated are merely three examples. Many other different configurations are equally applicable. For example, in some embodiments, the optical component 120 may not use a single gas chamber 102 or multiple sub-chambers 102a to 102e to form a gas environment that at least partially encloses the HC-PCF 101. Instead, the HC-PCF 101 can first be filled with a filling gas and then sealed, for example, by attaching mirrors to each of the two ends of the optical fiber. In this way, the filling gas is retained within the optical fiber (e.g., hollow core and cladding structure) without the need for a separate gas chamber. The two mirrors can be configured in the same manner in terms of the allowable transmission band as the Figure 9 mirrors illustrated in

[0094] In an embodiment, the filling gas of the HC-PCF-based broadband light source can be a gas mixture comprising or consisting of: helium and another gas or gas mixture. In an embodiment, helium can act as an alternative gas to hydrogen, which is currently used to mitigate the problems of cladding structure deformation and optical fiber overheating. Similar to hydrogen, helium has a high thermal conductivity, i.e., 156.7 mW / mK, and can thus help improve the thermal management of the HC-PCF 101. Although different molecular gases with high thermal conductivity can also be used for the same purpose (described in the following embodiments) to replace hydrogen, atomic gases are preferred because, for example, the fact that molecular gases such as hydrogen, oxygen, and H2O will decompose after exposure to light and become ions and groups that subsequently attack the glass and cause surface reduction problems.

[0095] The second gas or gas mixture can act 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 problem. Note that, in order to provide effective shock absorption or acoustic suppression properties, the two or more atomic gases of the filling gas mixture should have sufficiently different atomic weights required for diffusion-assisted acoustic wave attenuation.

[0096] In some embodiments, the filling gas mixture can comprise or consist of: helium and a second atomic gas for white light generation. The second atomic gas can be, for example, krypton, neon, argon, or xenon. In other embodiments, the filling gas mixture can comprise or consist of: helium and a second atomic gas mixture. The second atomic gas mixture for white light generation can include at least two other different atomic gases, such as krypton and neon, krypton and argon, or krypton and xenon.

[0097] In various embodiments, the filling gas mixture may comprise or consist of: helium, a second atomic gas or gas mixture, and a molecular gas or gas mixture. The second atomic gas or gas mixture may be used as the working gas or gas mixture for white light generation. Since molecular gases have greater internal degrees of freedom compared to atomic gases, when mixed with atomic gas mixtures containing helium such as those in the foregoing embodiments, the molecular gases can further enhance the acoustic suppression ability. In some embodiments, the filling gas mixture may comprise helium, a second atomic gas or gas mixture, and a molecular gas. For this purpose, the molecular gas may be, for example, nitrogen, oxygen, or H2O. In other embodiments, the filling gas mixture may comprise: helium, a second atomic gas or second atomic gas mixture, and a molecular gas mixture. The molecular gas mixture may comprise, for example, two or more of nitrogen, oxygen, or H2O.

[0098] In some embodiments, the filling gas mixture may comprise or consist of: helium and a different gas or gas mixture. Here, the helium can be used to mitigate the problems of overheating of the optical fiber and deformation of the cladding structure. The different gas or gas mixture used as the working gas or gas mixture can be carefully selected to determine one or more desired properties (e.g., spectral range and spectral profile) for broadband radiation generation in a light source based on the filling gas in the HC-PCF. For example, by appropriately 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 the infrared (IR) region. In cases where a broadband UV spectrum is preferred, the filling gas mixture may comprise or consist of: for example, helium and a molecular gas or a gas mixture including nitrogen, or an atomic gas such as argon or krypton. However, in cases where a broadband IR spectrum is preferred, the filling gas mixture may comprise or consist of: for example, helium and xenon.

[0099] In some embodiments, the concentration or mole fraction of helium in the filling gas mixture containing helium can be optimized such that a balance is obtained between heat conduction for releasing excess heat from the optical fiber and convection. In other words, the problems of cladding structure deformation and overheating of the optical fiber can be mitigated by optimizing the helium concentration of the filling gas mixture. The selection of the filling gas mixture may depend on factors such as one or more of the following: the output characteristics of the light source (e.g., power, spectrum), the acceptable total gas pressure, and / or the preferred heat dissipation method. Adding helium to the filling gas mixture can enable a heat dissipation mechanism by increasing the thermal conductivity and thermal diffusivity of the filling gas mixture.

[0100] As described above, helium can be used as an alternative gas to hydrogen due to its high thermal conductivity and applicability for acoustic suppression to mitigate the problems of cladding structure deformation and optical fiber overheating. It is known that the thermal conductivity of a gas mixture containing helium varies with the helium concentration. Figure 11 is a curve illustrating the thermal conductivity of several binary mixtures of atomic gases as a function of molecular weight. In Figure 11 's curve, the thermal conductivity (κ) of each gas mixture is characterized at a pressure of 20 bar (or 2.0 megapascals (MPa) as indicated in the figure) for two different temperatures, namely, 1200 K and 400 K. The molecular weight (MW) of each gas mixture is the average of the respective 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 figure 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 in which the thermal conductivity of a binary mixture composed of the two atomic gases represented by the two hollow circles varies with the molecular weight of the gas mixture at a temperature of 400 K. Similarly, each dashed line connects a pair of two hollow squares and indicates the range in which the thermal conductivity of a binary mixture composed of the two atomic gases represented by the two hollow squares varies with the molecular weight of the gas mixture at a temperature of 1200 K.

[0101] The solid circles represent the thermal conductivity of some exemplary gas mixtures that can be used as the filling gas mixture for a light source of a HC-PCF based on a filling gas. Two gas mixtures represented by two solid circles on the solid line, namely, a first mixture composed of 50% mole fraction of xenon and 50% mole fraction of helium (or Xe / He 50 / 50) and a second mixture composed of 80% mole fraction of krypton and 20% mole fraction of helium (or Kr / He 80 / 20), can be used in a HC-PCF based broadband 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 conductivity of the exemplary gas mixtures in the figure is 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 operating under different conditions (e.g., different total gas pressures and / or different temperatures).

[0102] As is clearly visible in the figure, for any filling 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 the HC-PCF-based radiation source 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 for the working gas. The total gas pressure is defined as the sum of the partial pressures of the constituent gases of the filling gas mixture. Therefore, it is necessary to achieve a balance between the thermal conductivity and the optical nonlinearity when determining the optimal concentration (or mole fraction) of helium in the filling gas mixture.

[0103] Referring to Figure 11 , different filling gas mixture configurations can be classified into, for example, three main groups based on the thermal conductivity of the corresponding working gas. The first group may include filling gas mixtures with high thermal conductivity. In the first group, each filling gas mixture may have a thermal conductivity, for example, between 75% and 100% of the thermal conductivity of pure helium at a given temperature and pressure. The second group may include filling gas mixtures with medium thermal conductivity. In the second group, each filling gas mixture may have a thermal conductivity, for example, between 35% and 75% of the thermal conductivity of pure helium at a given temperature and pressure. The third group may include filling gas mixtures with low thermal conductivity. In the third group, each filling gas mixture may have a thermal conductivity, for example, between 0% and 35% of the thermal conductivity of pure helium at a given temperature and pressure.

[0104] For each filling gas mixture in the first group, the working gas (e.g., xenon) may have a higher degree of ionization, and the resulting overheating problem may have a higher severity level. Therefore, the working gas may 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 filling gas mixture may 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 filling gas mixture has a very high helium concentration and thus allows for high thermal conductivity. These filling 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).

[0105] For each of the second set of filling gas mixtures, the working gas (e.g., argon or krypton) can have a lower ionization degree, and the resulting overheating problem may be less severe compared to the first set of filling gas mixtures. Thus, a lower helium concentration can be used for filling gas mixtures containing helium. For example, the filling gas mixture can consist of 50% mole fraction of krypton and 50% mole fraction of helium, or 70% mole fraction of argon and 30% mole fraction of helium. This has the advantage of maintaining the total working pressure of the gas mixture at an easily manageable level (e.g., below 50 bar).

[0106] For each of the third set of filling gas mixtures, the ionization degree of the working gas (e.g., argon or neon) can be similar to or lower than that of the second set. By comparison, the filling gas mixtures of the third set can have a lower helium concentration compared to those of the second set and thus have a lower thermal conductivity. Similar to the first set, the filling gas mixtures of the second and third sets can be operated at a higher total gas pressure (e.g., 50 bar to 70 bar) in order to maintain the desired partial pressure of the corresponding working gas.

[0107] Additional exemplary filling gas mixtures are provided in Table 1. Note that although the table below only lists binary atomic gas mixtures (i.e., consisting of two different atomic gases), this does not imply the inapplicability of other types of gas mixtures, such as, for example, filling gas mixtures including more than two atomic gases or consisting of more than two atomic gases, or filling gas mixtures including one or more atomic gases and one or more molecular gases, or consisting of one or more atomic gases and one or more molecular gases.

[0108] Table 1

[0109]

[0110] As shown in Table 1, each exemplary filling gas mixture consists of helium and another heavier atomic gas. The mole fractions 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, such as Ar and He, Kr and He, or Xe and He, a preferred range of the mole fraction of helium is provided. For example, for the gas mixture of xenon and helium, the mole fraction of helium is preferably equal to or higher than 50%.

[0111] The thermal diffusivity is a measure of how quickly a thermal gradient generated by a pulse at the center of the optical fiber (i.e., at the point of maximum intensity and ionization) smooths out or diffuses in the system (e.g., the HC - PCF filled with gas), and can be expressed as follows:

[0112]

[0113] where κ indicates the thermal conductivity of the filling gas mixture, ρ indicates the density of the filling gas mixture, and C p indicates the heat capacity of the filling gas mixture. To prevent the hot gas from reaching a high steady-state temperature under the action of subsequent laser pulses, it is desirable to reduce the time scale of the relaxation of the thermal gradient to be shorter than the pulse separation time (i.e., the reciprocal of the laser repetition rate). According to Equation [2], the increase in the thermal conductivity of the filling gas mixture due to the addition of more helium also results in an increase in the thermal diffusivity. In addition to using a filling gas mixture with a high conductivity, a higher thermal diffusivity can be obtained by configuring the filling gas mixture to have a lower density and / or a lower heat capacity. Figure 12 is a curve showing a simplified simulation of thermal relaxation in an optical fiber for two different thermal diffusivities. As shown in the figure, the normalized peak temperature (NPT) of the optical fiber (e.g., HC-PCF 101), and the horizontal axis of the figure is time. The figure determines that: compared to a filling gas mixture with a lower thermal diffusivity (e.g., α = 10), a filling gas mixture with a higher thermal diffusivity (e.g., α = 0.5) results in much faster (e.g., twice as fast) thermal relaxation in the optical fiber (e.g., HC-PCF 101).

[0114] As described above, the selection of the filling 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 molecular excitation within the material without overall motion; however, convective heat transfer occurs due to the overall motion of a fluid (e.g., the filling gas) relative to the boundary. In the case of using a heavy atom gas as the working gas (e.g., Kr or Xe) for generating broadband radiation in an HC-PCF, it is preferred that the thermal relaxation results in less convection or momentum transfer of the heavy working gas. The reason is that the convection of the heavy working gas can generate an air flow in the optical fiber due to the formation of a temperature gradient inside and outside or across the cross-section of the optical fiber. The flow of the hot heavy working gas may cause the pump pulse to experience a change in the refractive index of the medium through which it travels (the working gas). Local changes in the refractive index profile may in turn lead to radiation output instability (e.g., power instability, spectral instability). Adding helium to the heavy working gas may allow heat transfer to occur mainly via the movement of 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 does not affect the output of the light source and only helps to transfer the excess heat generated in the optical fiber.

[0115] Note that the above description for selecting an optimal filling gas mixture for alleviating the cladding structure deformation and fiber overheating problems is equally applicable to the case where the filling gas mixture includes helium and a working gas mixture including two or more atomic gases (instead of a single atomic gas as described in the exemplary gas mixtures above), or to the case where the filling gas mixture includes helium and a working gas mixture including one or more atomic gases and one or more molecular gases. In any case, the filling gas mixture can be optimally selected by considering its thermal conductivity, thermal diffusivity, and heat transfer mechanism simultaneously.

[0116] According to Equation [1], the intensity of the acoustic suppression of the pressure shock wave depends on the pump pulse repetition rate. Thus, in an embodiment, the filling gas mixture containing helium can be configured such that the optimal acoustic suppression frequency matches or is substantially close to the repetition rate of the pump pulse. In some embodiments, the repetition rate of the pump laser can be tuned to match or be substantially close to the optimal acoustic suppression frequency of the filling gas or gas mixture. In other embodiments, a suitable filling gas or gas mixture having an optimal acoustic suppression frequency that matches or is substantially close to the repetition rate of the pump pulse can be selected.

[0117] The pump gas frequency matching feature can be used alone or in combination with any of the foregoing embodiments to maximize the acoustic suppression intensity of the corresponding filling gas mixture. For example, in an embodiment, a filling gas mixture including helium and nitrogen or consisting of helium and nitrogen can have an optimal acoustic suppression frequency that precisely matches the repetition rate of the pump pulse. Thus, a light source based on HC-PCF with the filling gas can simultaneously generate broadband UV radiation (determined by nitrogen) and have an extended fiber service life, where the effects from fiber overheating (e.g., alleviated by using helium) and cladding tube deformation (alleviated by using a mixture of nitrogen and helium together with the pump gas frequency matching feature) are minimized.

[0118] Since the acoustic resonance effect induced by the cladding structure (e.g., the cladding tube) may amplify the influence of the acoustic wave, it is also preferably ensured that the pump pulse repetition rate is different from one or more resonance frequencies of the excited resonance modes within the cladding structure (e.g., the cladding tube) of the HC-PCF 101. In some embodiments, the repetition rate of the pump laser can be adjustable so as to avoid the resonance effect. 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 resonance modes within the cladding structure of the HC-PCF 101 after being impacted by the acoustic waves. In some embodiments, preventing the excitation of resonance modes within the cladding structure can be achieved by carefully selecting the appropriate gas composition of the filling gas mixture and / or by carefully selecting the HC-PCF 101 with an appropriate fiber geometry.

[0119] Similar to the pump gas frequency matching feature, the above-mentioned pump cladding anti-resonance feature can be used alone or in combination with any of the foregoing embodiments. For example, in different embodiments, the pump cladding anti-resonance feature can be applied to the above embodiments, in which the filling gas includes helium and nitrogen or consists of helium and nitrogen and the optimal acoustic suppression frequency matches the pump pulse repetition rate. This embodiment can ensure that the maximized acoustic suppression intensity is not impaired by the resonance effect of the cladding tube.

[0120] In some embodiments, a certain thermal management measure can be adopted to improve the heat dissipation of the gas chamber 102. The gas chamber 102 with improved heat dissipation can facilitate the effective removal of the heat transferred from the HC-PCF 101 to the gas chamber 102 via the filling gas or gas mixture containing helium. In some embodiments, the gas chamber 102 can be connected to an efficient cooling system that can quickly remove heat from the gas chamber body. In different embodiments, the gas chamber 102 can be constructed using a material with high thermal conductivity. In addition, the improved gas chamber 102 with higher heat dissipation can be used in combination with any of the foregoing embodiments.

[0121] In all the above embodiments, the filling gas mixture contains a certain amount of helium. Compared with the filling gas mixture containing hydrogen, the filling gas mixture containing helium not only alleviates the problems of fiber overheating and cladding structure deformation, but also prevents the surface reduction problem that would otherwise occur when using the filling gas mixture containing hydrogen. It should be understood that even in the case where hydrogen is not intentionally introduced, the filling gas or gas mixture may still include trace amounts of hydrogen, which may come from any surface and / or exhaust gas, hydrocarbons, or H2O generated as part of the broadband generation process. However, these residual or naturally occurring hydrogen is not sufficient to act to alleviate any of the foregoing problems or cause glass surface reduction.

[0122] In an embodiment, the filling gas mixture may consist of 50% by mole fraction of helium and 50% by mole fraction of krypton. However, other mole fractions of helium may also be applicable. The mole fraction of helium may be 2% or greater than 2% of the filling gas mixture; for example, equal to or greater than 10% or 50% of the filling gas mixture. For example, the mole fraction of helium may 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 precisely, the mole fraction of helium may be 2%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, or 80% of the filling gas mixture. In the above, "comprising" describes including these gases, but does not necessarily exclude other gases, and "consisting of" describes including only these gases.

[0123] Although helium (or other inert gases) is a very useful gas medium and can alleviate both the problems of overheating of the optical fiber and deformation of the cladding structure, some embodiments may use a filling 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 dissipation chamber feature may be applied to a filling gas or gas mixture that does not include helium. In an embodiment, all of the above three features are used together with a filling gas mixture that does not include helium. Since the pump gas frequency matching feature and the pump tube anti-resonance feature can alleviate the problem of cladding tube deformation, and the heat dissipation chamber feature can alleviate the problem of overheating of the optical fiber, the service life of the optical fiber can still be extended even when helium is not used. The embodiment can further prevent the problem of surface reduction by avoiding the use of a large amount of hydrogen in the filling gas mixture.

[0124] Thus, a method for optimizing the repetition rate of 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, and a method for making the optimal acoustic suppression frequency of the gas medium substantially match the repetition rate of the pump radiation generated by the pump radiation source are also contemplated. Optimization or matching can be achieved, for example, by appropriately tuning the pump radiation parameters, optimizing the gas mixture, or both.

[0125] The HC-PCF 101 used in this embodiment may employ reference to Figure 7The HC-ARF of Kagome design or single-ring design. Alternatively, other fiber designs (not shown) can be used, such as suppression coupling design, inner cycloid core Kagome, and nested tubular design. The pump pulse duration can be selected to be greater than 100 fs, and more specifically, within the following ranges: for example, 100 fs to 100 ps, 100 fs to 30 ps, or 100 fs to 1 ps. The selected pump pulses 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 state, near-IR state, or mid-IR state. The pump laser pulses can have a repetition rate of several hundred hertz (Hz), several thousand hertz (kHz), or several megahertz (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.

[0126] A broadband light source device configured to extend the service life of the HC-PCF disclosed herein, including a filling gas mixture that contains helium.

[0127] A broadband light source device configured to extend the service life of the HC-PCF disclosed herein, wherein the pump pulse repetition rate of the filling gas mixture and the optimal acoustic suppression frequency match or are substantially close to each other.

[0128] A broadband light source device configured to extend the life of the HC-PCF disclosed herein, including an optical component that can be configured with reference to Figure 10 any one of the configurations of

[0129] Other embodiments are disclosed in the following numbered list of aspects:

[0130] 1. A broadband light source device configured to generate a broadband output when receiving pump radiation, the broadband light source device including:

[0131] An optical component, the optical component including:

[0132] A hollow-core photonic crystal fiber (HC-PCF); and

[0133] A gas mixture that fills the HC-PCF;

[0134] Wherein, the gas mixture comprises a mixture of at least one first gas and at least one second gas, the at least one first gas being configured to generate broadband radiation, the at least one second gas being configured to increase the thermal conductivity of the gas mixture and / or provide acoustic suppression of shock waves induced during the generation of the broadband radiation, and wherein, in addition, the gas mixture comprises hydrogen in a mole fraction of not more than ten parts per million (ppm).

[0135] 2. A broadband light source device configured to generate a broadband output upon receiving pump radiation, the broadband light source device comprising:

[0136] An optical component, the optical component comprising:

[0137] A hollow-core photonic crystal fiber (HC-PCF); and

[0138] A gas mixture filling the HC-PCF;

[0139] Wherein, the gas mixture comprises a mixture of at least one first gas and at least one second gas, the at least one first gas being configured to generate broadband radiation, the at least one second gas being configured to increase the thermal conductivity of the gas mixture and / or provide acoustic suppression of shock waves induced during the generation of the broadband radiation, and wherein, in addition, the gas mixture comprises hydrogen in a mole fraction of not more than 4%.

[0140] 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.

[0141] 4. The broadband light source device according to aspect 3, wherein the at least one second gas comprises or consists of helium.

[0142] 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 greater atomic weight than the second gas.

[0143] 6. The broadband light source device according to aspect 5, wherein the at least one first gas comprises one or more of krypton, xenon, argon, neon or consists of one or more of krypton, xenon, argon, neon.

[0144] 7. The broadband light source device according to any one of the foregoing aspects, wherein the at least first gas comprises one or more molecular gases or consists of one or more molecular gases.

[0145] 8. The broadband light source device according to any one of the foregoing aspects, wherein the at least one second gas comprises one or more molecular gases or consists of one or more molecular gases.

[0146] 9. The broadband light source device according to aspect 8, wherein the one or more molecular gases are selected from the following range: nitrogen, oxygen, H2O.

[0147] 10. The broadband light source device according to any one of the foregoing aspects, wherein the second gas constitutes at least 2% of the molar fraction of the gas mixture.

[0148] 11. The broadband light source device according to any one of aspects 1 to 9, wherein the second gas constitutes at least 10% of the molar fraction of the gas mixture.

[0149] 12. The broadband light source device according to any one of aspects 1 to 9, wherein the second gas constitutes a molar fraction between 30% and 70% of the gas mixture.

[0150] 13. The broadband light source device according to any one of aspects 1 to 9, wherein the second gas constitutes a molar fraction between 40% and 60% of the gas mixture.

[0151] 14. The broadband light source device according to any one 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, and the at least one second gas constitutes a molar fraction equal to or higher than 50%.

[0152] 15. The broadband light source device according to aspect 14, wherein the at least one first gas consists of xenon having a molar fraction of 20% or less of the gas mixture, and the at least one second gas consists of helium having a molar fraction of 80% or more of the gas mixture.

[0153] 16. The broadband light source device according to aspect 14, wherein the at least one first gas consists of xenon having a molar fraction of 10% or less of the gas mixture, and the at least one second gas consists of helium having a molar fraction of 90% or more of the gas mixture.

[0154] 17. The broadband light source device according to any one 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, and the at least one second gas constitutes a molar fraction equal to or higher than 20%.

[0155] 18. The broadband light source device 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, and krypton gas and helium gas respectively constitute a molar fraction of 50% ± 10% and a molar fraction of 50% ± 10% of the gas mixture.

[0156] 19. The broadband light source device 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, and the at least one second gas constitutes a molar fraction equal to or higher than 10%.

[0157] 20. The broadband light source device according to aspect 19, wherein the at least one first gas consists of argon gas constituting a molar fraction of 70% or less of the gas mixture, and the at least one second gas consists of helium gas constituting a molar fraction of 30% or more of the gas mixture.

[0158] 21. The broadband light source device according to aspect 19, wherein the at least one first gas consists of argon gas constituting a molar fraction of 90% or less of the gas mixture, and the at least one second gas consists of helium gas constituting a molar fraction of 10% or more of the gas mixture.

[0159] 22. The broadband light source device according to any one of the foregoing aspects, wherein the HC-PCF includes a single-ring HC-PCF.

[0160] 23. The broadband light source device according to any one of the foregoing aspects, further comprising a pump radiation source for generating the pump radiation.

[0161] 24. The broadband light source device according to aspect 23, wherein the optimal acoustic suppression frequency of the gas mixture substantially matches the repetition rate of the pump radiation generated by the pump radiation source.

[0162] 25. The broadband light source device according to aspect 24, wherein the gas composition or gas components of the gas mixture are configured to cause the optimal acoustic suppression frequency of the gas mixture to match the repetition rate of the pump radiation.

[0163] 26. The broadband light source device according to aspect 24 or 25, wherein the broadband light source device is configured to tune the repetition rate of the pump radiation to match the optimal acoustic suppression frequency of the gas mixture.

[0164] 27. The broadband light source device according to any one 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.

[0165] 28. The broadband light source device according to any one of the foregoing aspects, wherein the inner cladding structure of the HC-PCF includes one or more silica tubes.

[0166] 29. The broadband light source device according to any one of the foregoing aspects, wherein the gas mixture is configured to define the spectral range of the generated broadband radiation.

[0167] 30. The broadband light source device according to any one of the foregoing aspects, wherein the broadband output includes a wavelength range of 200 nm to 3000 nm, or a sub-range such as 400 nm to 2000 nm within this range.

[0168] 31. A measurement device, comprising the broadband light source device according to any one of the foregoing aspects.

[0169] 32. The measurement device according to aspect 31, comprising a scatterometer measurement device, a level sensor or an alignment sensor.

[0170] 33. A method for configuring a source device, the source device being configured to generate a broadband radiation output and the source device comprising:

[0171] a pump radiation source for outputting pump radiation; and

[0172] a hollow-core photonic crystal fiber (HC-PCF);

[0173] a gas mixture filling the HC-PCF, wherein the method comprises:

[0174] 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.

[0175] 34. The method according to aspect 33, comprising: optimizing the gas composition of the gas medium so that the optimal acoustic suppression frequency of the gas medium matches the repetition rate of the pump radiation.

[0176] 35. The 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.

[0177] 36. The method according to any one of aspects 33 to 35, comprising: optimizing 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.

[0178] 37. A method for configuring a source device configured to generate a broadband radiation output, the source device comprising:

[0179] a pump radiation source for outputting pump radiation; and

[0180] a hollow-core photonic crystal fiber (HC-PCF);

[0181] a gas mixture filling the HC-PCF, wherein the method comprises:

[0182] optimizing 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.

[0183] 38. The 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 to generate broadband radiation, and the at least one second gas is configured to increase the thermal conductivity of the gas mixture and / or provide acoustic suppression of shock waves generated during the generation of the broadband radiation, and wherein the gas mixture comprises a hydrogen mole fraction of not more than ten parts per million (ppm).

[0184] 39. The method according to aspect 38, wherein the second gas comprises an atomic gas.

[0185] 40. The method according to aspect 39, wherein the second gas comprises helium.

[0186] 41. The method according to aspect 39 or 40, wherein the first gas comprises an atomic gas having a larger atomic weight than the second atomic gas.

[0187] 42. The method according to aspect 41, wherein the first atomic gas is selected from the range: krypton, xenon, argon, neon.

[0188] 43. A method for configuring a source device configured to generate a broadband radiation output, the source device comprising:

[0189] a pump radiation source for outputting pump radiation; and

[0190] Hollow-core photonic crystal fiber (HC-PCF);

[0191] A gas mixture that fills the HC-PCF, wherein the method includes:

[0192] Optimizing the molar fraction of helium in the gas mixture by optimizing one or more of the following:

[0193] The thermal conductivity of the gas mixture;

[0194] The thermal diffusivity of the gas mixture; or

[0195] The heat transfer mechanism.

[0196] 44. The method according to aspect 43, wherein the optimization of the heat transfer mechanism includes selecting a gas or gas mixture having a heavy molecular weight for the at least one first gas.

[0197] 45. The method according to aspect 44, wherein the molecular weight of the at least one first gas is at least a factor of 10 heavier than the molecular weight of helium.

[0198] 46. The method according to any one of aspects 44 or 45, wherein the partial pressure of each constituent gas of the gas mixture is substantially maintained while the molar fraction of helium is optimized.

[0199] 47. An optical component for a broadband light source device, the optical component being configured to generate a broadband output when receiving pump radiation, and the optical component includes:

[0200] Hollow-core photonic crystal fiber (HC-PCF); and

[0201] A gas mixture that fills the HC-PCF, wherein the gas mixture includes a mixture of at least one first gas and at least one second gas, the at least one first gas being configured to generate broadband radiation, and the at least one second gas includes helium or consists of helium.

[0202] 48. The optical component according to aspect 47, wherein the at least one first gas includes one or more of krypton, xenon, argon, neon or consists of one or more of krypton, xenon, argon, neon.

[0203] 49. The optical component according to any one of aspects 47 or 48, wherein the at least first gas includes one or more molecular gases or consists of one or more molecular gases.

[0204] 50. The optical component according to any one of aspects 47 to 49, wherein the at least one second gas includes one or more molecular gases.

[0205] 51. The optical component according to aspect 50, wherein the one or more molecular gases are selected from the group consisting of nitrogen, oxygen, and H2O.

[0206] 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.

[0207] 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.

[0208] 53. The optical component according to any one of aspects 47 to 51, wherein the second gas constitutes a molar fraction between 30% and 70% of the gas mixture.

[0209] 54. The optical component according to any one of aspects 47 to 51, wherein the second gas constitutes a molar fraction between 40% and 60% of the gas mixture.

[0210] 55. The optical component according to aspect 47, wherein the at least one first gas consists of xenon, and the at least one second gas consists of helium, and the at least one second gas constitutes a molar fraction equal to or higher than 50%.

[0211] 56. The optical component according to aspect 55, wherein the at least one second gas consists of helium constituting a molar fraction of 80% or more of the gas mixture.

[0212] 57. The optical component according to aspect 55, wherein the at least one second gas consists of helium constituting a molar fraction of 90% or more of the gas mixture.

[0213] 58. The optical component according to aspect 47, wherein the at least one first gas consists of krypton, and the at least one second gas consists of helium, and the at least one second gas constitutes a molar fraction equal to or higher than 20%.

[0214] 59. The optical component according to aspect 58, wherein the at least one first gas consists of krypton, and the at least one second gas consists of helium, and krypton and helium respectively constitute a molar fraction of 50% ± 10% and 50% ± 10% of the gas mixture.

[0215] 60. The optical component according to aspect 47, wherein the at least one first gas consists of argon, and the at least one second gas consists of helium, and the at least one second gas constitutes a molar fraction equal to or higher than 10%.

[0216] 61. The optical component according to aspect 60, wherein the at least one first gas consists of argon having a molar fraction of 70% or less of the gas mixture, and the at least one second gas consists of helium having a molar fraction of 30% or more of the gas mixture.

[0217] 62. The optical component according to aspect 60, wherein the at least one first gas consists of argon having a molar fraction of 90% or less of the gas mixture, and the at least one second gas consists of helium having a molar fraction of 10% or more of the gas mixture.

[0218] 63. The optical component according to any one of aspects 47 to 62, wherein the HC-PCF is a single-ring HC-PCF.

[0219] 64. A method for configuring an optical component for a source device configured to generate a broadband radiation output, the method comprising:

[0220] selecting a hollow-core photonic crystal fiber (HC-PCF) and a gas mixture, the gas mixture including a first gas filling the HC-PCF; and

[0221] 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 the following:

[0222] increasing the thermal conductivity of the gas mixture;

[0223] increasing the thermal diffusivity of the gas mixture; or

[0224] selecting a desired heat transfer mechanism.

[0225] 65. The method according to aspect 64, further comprising selecting a gas or a gas mixture having a heavy molecular weight for the first gas in order to select the desired heat transfer mechanism.

[0226] 66. The method according to aspect 65, wherein the molecular weight of the at least one first gas is at least a factor of 10 heavier than the molecular weight of helium.

[0227] 67. The method according to aspect 65 or 66, wherein the partial pressure of each constituent gas of the gas mixture is maintained substantially while the mole fraction of helium is determined.

[0228] Although specific reference may be made herein to the use of a lithographic apparatus in the manufacture of integrated circuits, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, the guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

[0229] Although embodiments of the invention may be specifically referred to herein in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatuses. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus for measuring or processing an object such as a wafer (or other substrate) or a mask (or other patterning device). These apparatuses may be collectively referred to as lithographic tools. Such lithographic tools may operate under vacuum conditions or ambient (non-vacuum) conditions.

[0230] Although the embodiments of the invention may have been specifically referred to above in the context of the use of optical lithography, it will be appreciated that, where the context allows, the invention is not limited to optical lithography and may be used in other applications (e.g., imprint lithography).

[0231] Although specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced in other ways different from those described. The above description is intended to be illustrative, not restrictive. Thus, it will be apparent to those skilled in the art that the described invention may be modified without departing from the scope of the claims set forth below.

Claims

1. An optical component for a broadband light source device, the optical component being configured to generate broadband radiation upon receiving pump radiation, and the optical component comprising: A hollow-core photonic crystal fiber (HC-PCF); And A gas mixture that fills the hollow-core photonic crystal fiber, wherein the gas mixture comprises a mixture of at least one first gas and at least one second gas, the at least one first gas being configured for the generation of the broadband radiation, the at least one second gas being configured for thermal regulation of the gas mixture and / or suppression of acoustic waves in the gas mixture, wherein the second gas comprises helium or consists of helium, and an optimized molar fraction of helium present in the gas mixture is based on one or more of the following: increasing the thermal conductivity of the gas mixture; increasing the thermal diffusivity of the gas mixture; or selecting a desired heat transfer mechanism.

2. The optical component according to claim 1, wherein, The at least one first gas comprises one or more of krypton, xenon, argon, neon or consists of one or more of krypton, xenon, argon, neon.

3. The optical component according to claim 1, wherein, The at least one second gas further comprises one or more molecular gases.

4. The optical component according to claim 3, wherein, The one or more molecular gases are selected from one or more of nitrogen (N2), oxygen (O2), H2O.

5. The optical component according to claim 1, wherein, The second gas constitutes at least 10% of the molar fraction of the gas mixture.

6. A broadband light source device comprising the optical component of claim 1 and further comprising a pump radiation source for generating the pump radiation.

7. A measuring device comprising the broadband light source device of claim 6.

8. The optical component according to claim 1, wherein, The at least one first gas consists of xenon and the at least one second gas consists of helium, and the at least one second gas constitutes a molar fraction equal to or higher than 50%.

9. The optical component according to claim 8, wherein, The at least one second gas consists of helium that constitutes 80% or more of the molar fraction of the gas mixture.

10. The optical component according to claim 1, wherein, The at least one first gas consists of krypton and the at least one second gas consists of helium, and the at least one second gas constitutes a molar fraction equal to or higher than 20%.

11. The optical component according to claim 1, wherein, The at least one first gas consists of krypton gas, and the at least one second gas consists of helium gas. Krypton gas and helium gas respectively constitute 50% ± 10% mole fraction and 50% 10% mole fraction of the gas mixture.

12. The optical component according to claim 11, wherein, The at least one first gas consists of argon and the at least one second gas consists of helium, and the at least one second gas constitutes a molar fraction equal to or higher than 10%.

13. The optical component according to claim 12, wherein, The at least one first gas consists of argon that constitutes 70% or less of the molar fraction of the gas mixture, and the at least one second gas consists of helium that constitutes 30% or more of the molar fraction of the gas mixture.

14. The optical component according to claim 12, wherein, The at least one first gas consists of argon that constitutes 90% or less of the molar fraction of the gas mixture, and the at least one second gas consists of helium that constitutes 10% or more of the molar fraction of the gas mixture.

15. A method for configuring an optical component for a source device configured to generate a broadband radiation output, the method comprising: Select a hollow-core photonic crystal fiber (HC-PCF) and a gas mixture that fills the hollow-core photonic crystal fiber, the gas mixture including a first gas for generating the broadband radiation and a second gas including helium; and Determine 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 the following: Increase the thermal conductivity of the gas mixture; Increase the thermal diffusivity of the gas mixture; or Select a desired heat transfer mechanism.

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