Method for producing photonic crystal fiber

Abstract

A method of manufacturing an optical fiber including an outer sheath and a plurality of capillaries defining a hollow core is disclosed. The method comprises the following steps: obtaining an optical fiber manufacturing intermediate product; and stretching an optical fiber from the optical fiber manufacturing intermediate product. The stretching is such that the fiber-level capillary wall thickness of each capillary in the stretched optical fiber is less than 200 nm and the tensile force applied during the stretching is greater than 20 MPa.

Description

[0001] Cross-reference to related applications

[0002] This application claims priority to European Application No. 22207947.7, filed on Nov. 17, 2022, the entire content of which is incorporated herein by reference. Technical field

[0003] The present invention relates to a method of producing photonic crystal fibers, in particular hollow-core photonic crystal fibers, and to their use in the manufacture of integrated circuits. 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 (often 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] In order to project a pattern onto a substrate, a lithographic apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the features that can be formed on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. Compared to a lithographic apparatus using radiation having a wavelength of, for example, 193 nm, a lithographic apparatus using extreme ultraviolet (EUV) radiation having a wavelength in the range of 4 nm to 20 nm (e.g., 6.7 nm or 13.5 nm) can be used to form smaller features on a substrate.

[0006] Low-k1 lithography can be used to process features smaller than the classical resolution limit of a lithographic apparatus. In such a process, the resolution formula can be expressed as CD = k1×λ / NA, where λ is the wavelength of the radiation employed, 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, the smaller k1 is, the more difficult it becomes to reproduce on the substrate a pattern 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 the NA, customized illumination schemes, use of phase-shifting patterning devices, various optimizations of the design layout such as optical proximity correction (OPC, sometimes also referred to as "optical and process correction") in the design layout, or other methods generally defined as "resolution enhancement techniques" (RET). Alternatively, a strict control loop for controlling the stability of the lithographic apparatus can be used to improve the reproduction of patterns at low k1.

[0007] Metrology tools are used in many aspects of the IC manufacturing process, such as alignment tools for properly positioning the substrate prior to exposure, leveling tools for measuring the surface topography of the substrate, and focus control and scatterometry tools 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 current devices for broadband radiation generation. Among other techniques for white light generation, photonic crystal fibers (PCFs) can be used to convert narrow-band input radiation into broadband output radiation via one or more nonlinear optical processes. The output performance of a PCF-based radiation source is strongly dependent on the quality and properties of the PCF, which are mainly determined by the fiber manufacturing process. Currently, PCFs are drawn in a fiber drawing tower that draws the fiber in a continuous manner. Current fiber manufacturing methods often result in a change in one or more dimensions of the internal structure of the PCF that is greater than desired. Such a change can result in a sub-optimal structure of the drawn PCF, and thus a radiation source using such a PCF to generate broadband radiation will output light with undesired characteristics. Accordingly, an object of the present invention is to provide an improved method for producing PCFs. Summary of the Invention

[0008] According to a first aspect of the present invention, there is provided a method of manufacturing an optical fiber, the optical fiber comprising an outer sheath and a plurality of capillaries defining a hollow core, the method comprising: obtaining an optical fiber manufacturing intermediate product; and drawing the optical fiber from the optical fiber manufacturing intermediate product, wherein the drawing causes the fiber-grade capillary wall thickness of each capillary in the drawn optical fiber to be less than 200 nm; and wherein the tensile force applied during the drawing is greater than 20 MPa.

[0009] Other aspects of the present invention include a metrology device comprising a radiation source device, the radiation source device comprising an optical fiber produced by the method of the first aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0011] Figure 1 A schematic overview of a lithographic apparatus is depicted;

[0012] Figure 2 A schematic overview of a lithographic cell is depicted;

[0013] Figure 3 A schematic representation of overall lithography is depicted, which represents the collaboration between three key technologies for optimizing semiconductor manufacturing;

[0014] Figure 4 A schematic overview of a scatterometry device used as a metrology device that may include a radiation source according to an embodiment of the present invention is depicted;

[0015] Figure 5 A schematic overview of a level sensor device that may include a radiation source according to an embodiment of the present invention is depicted;

[0016] Figure 6 A schematic overview of an alignment sensor device that may include a radiation source according to an embodiment of the present invention is depicted;

[0017] Figure 7 Is a schematic cross-sectional view of a hollow core optical fiber that may form part of a radiation source according to an embodiment in a transverse plane (i.e., perpendicular to the axis of the optical fiber);

[0018] Figure 8 A schematic representation of a radiation source for providing broadband output radiation according to an embodiment is depicted;

[0019] Figure 9 (a) and Figure 9 (b) schematically depict cross-sections of examples of hollow core photonic crystal fiber (HC-PCF) designs for supercontinuum generation;

[0020] Figure 10 is a flowchart depicting a method of manufacturing HC-PCF that can form the basis of the concepts and embodiments disclosed herein;

[0021] Figure 11 is an example simulation of the confinement loss of an HC-PCF (e.g., the optical fiber shown in Figure 7 ) as a function of wavelength;

[0022] Figure 12 is a graph showing the variation of the capillary outer diameter at the optical fiber level with the relative capillary outer diameter at the glass rod level for different values of the ratio Γ (capillary inner diameter / capillary outer diameter) at the glass rod level, illustrating the sensitivity of the final capillary outer diameter to changes in the glass rod level capillary outer diameter for different values of the capillary diameter ratio Γ;

[0023] Figure 13 is a graph showing the variation of the capillary outer diameter at the optical fiber level with the relative capillary wall thickness at the glass rod level for different values of the capillary diameter ratio Γ (capillary inner diameter / capillary outer diameter) at the glass rod level, illustrating the sensitivity of the final capillary outer diameter to changes in the glass rod level capillary wall thickness for different values of the capillary diameter ratio Γ;

[0024] Figure 14 is a graph showing the variation of the capillary inner diameter at the optical fiber level with the relative pressure (compared to the nominal pressure) when stretching the same glass rod into the same optical fiber at different tensile force values, illustrating the sensitivity of the final capillary inner diameter to pressure for different tensile force values; and

[0025] Figure 15 depicts a lithographic system including a lithographic apparatus and a radiation source, the lithographic system being an alternative to the Figure 1 lithographic system. Detailed Description

[0026] 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).

[0027] As used herein, the term "reticle", "mask", or "patterning device" can be broadly interpreted as referring to a general patterning device that can be used to impart a patterned cross-section to an incident radiation beam, the patterned cross-section corresponding to a pattern to be created in a target portion of a substrate. In such a context, the term "light valve" may also be used. Examples of such other patterning devices include programmable mirror arrays and programmable LCD arrays in addition to classical masks (transmission or reflection, binary, phase-shifting, hybrid, etc.).

[0028] Figure 1 A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA includes: an illumination system (also referred to as an illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation); a mask support (e.g., a mask table) MT configured to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to certain parameters; a substrate support (e.g., a wafer table) WT configured to hold a substrate (e.g., a wafer coated with resist) W and connected to a second positioner PW configured to accurately position the substrate support according to certain parameters; and a projection system (e.g., a refractive projection lens system) PS configured to project 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.

[0029] In operation, the illumination system IL receives the radiation beam from a radiation source SO via, for example, 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 its cross-section at the plane of the patterning device MA.

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

[0031] The lithographic apparatus LA can be of a type in which at least a portion of the substrate is covered by a liquid (e.g., water) having a relatively high refractive index so as to fill the space between the projection system PS and the substrate W, which is also known as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0032] The lithographic apparatus LA can also be of a type having two or more substrate supports WT (also referred to as "dual platforms"). In such a "multi-platform" machine, the substrate supports WT can be used in parallel, and / or steps of preparing a substrate W for a subsequent exposure of the substrate W located on one of the substrate supports WT can be carried out while another substrate W on another substrate support WT is used for exposing a pattern on the other substrate W.

[0033] In addition to the substrate support WT, the lithographic apparatus LA can also include a measurement platform. The measurement platform is arranged to hold sensors and / or cleaning devices. The sensors can be arranged to measure properties of the projection system PS or of the radiation beam B. The measurement platform can hold a plurality of sensors. The cleaning device can be arranged to clean parts of the lithographic apparatus, such as a part of the projection system PS or a part of the system providing the immersion liquid. The measurement platform can move under the projection system PS when the substrate support WT is moved away from the projection system PS.

[0034] 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 a second positioner PW and a position measurement system IF, the substrate support WT can be accurately moved, e.g., in order to position different target portions C in the path of the radiation beam B in a focused and aligned position. Similarly, a first positioner PM and possibly another position sensor (which is not explicitly depicted in Figure 1 are used to accurately position the patterning device MA relative to the path of the radiation beam B. Mask alignment marks M1, M2 and substrate alignment marks P1, P2 can be used to align the patterning device MA and the substrate W. Although the illustrated substrate alignment marks P1, P2 occupy dedicated target portions, the substrate alignment marks can be located in the space 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.

[0035] As Figure 2As shown, the lithographic apparatus LA can form part of a lithographic cell LC (sometimes also referred to as a lithographic cell or (lithographic) cluster), which typically also includes equipment for performing pre-exposure and post-exposure processes on the substrate W. Conventionally, these devices include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH and a bake plate BK for adjusting the temperature of the substrate W (e.g., for adjusting the solvent in the resist layer). A substrate transfer device or robot RO picks up the substrate W from the input / output ports I / O1, I / O2, moves the substrate W between different process devices and transfers the substrate W to the feed table LB of the lithographic apparatus LA. Devices in the lithographic cell, which are generally collectively referred to as a track or coat develop system, are typically under the control of a track or coat develop system control unit TCU, which itself may be controlled by a management control system SCS, which may also control the lithographic apparatus LA via, for example, a lithography control unit LACU.

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

[0037] An inspection device, which can also be referred to as a metrology device, is used to determine the properties of the substrate W and, in particular, how the properties of different substrates W vary or how the properties associated with different layers of the same substrate W vary between different layers. The inspection device is alternatively configured to identify defects on the substrate W and can be, for example, part of the lithographic cell LC, or can be integrated into the lithographic apparatus LA, or can even be a separate device. The inspection device can measure the properties on a latent image (the image in the resist layer after exposure), or a semi-latent image (the image in the resist layer after the 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).

[0038] Generally, the patterning process in the lithographic apparatus LA is one of the most critical steps in the process, which requires a high degree of accuracy in the sizing and placement of the structures on the substrate W. To ensure such high accuracy, three systems can be combined in Figure 3In the so-called "integrated" control environment schematically 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 such an "integrated" environment is to optimize the collaboration 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 allowing the process parameters in the lithographic process or patterning process to vary within the defined result.

[0039] The computer system CL can use (parts of) the design layout to be patterned to predict which resolution enhancement techniques to use and perform computational lithography simulations and calculations to determine which mask layout and lithographic apparatus settings achieve the maximum overall process window for the patterning process (depicted by the double arrows in the first scale SC1 in Figure 3 . Typically, the resolution enhancement techniques are arranged to match the patterning capabilities of the lithographic apparatus LA. The computer system CL can also be used to detect where within the process window the lithographic apparatus LA is currently operating (e.g., using inputs from the metrology tool MT) to predict whether there might be defects attributable to, for example, suboptimal processing (depicted by the arrow pointing to "0" in the second scale SC2 in Figure 3 ).

[0040] The metrology tool MT can provide inputs to the computer system CL to enable accurate simulations and predictions and can provide feedback to the lithographic apparatus LA to identify possible drifts, for example, in the calibration state of the lithographic apparatus LA (depicted by the multiple arrows in the third scale SC3 in Figure 3 ).

[0041] During the lithography process, it is desirable to frequently measure the structures produced, for example, for process control and verification. Tools used to make 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 the measurement of lithography process parameters by having a sensor in the pupil or in a plane conjugate to the pupil of the scatterometer's objective lens (the measurement is commonly referred to as pupil-based measurement), or by having a sensor in the image plane or in a plane conjugate to the image plane (in this case, the measurement is commonly referred to as image- or field-based measurement). Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032, or 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.

[0042] In a first embodiment, the scatterometer MT is an angular-resolved scatterometer. In such a scatterometer, a reconstruction method can be applied to the measured signal to reconstruct or calculate the properties of the grating. Such reconstruction can be caused, for example, by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulated results with the measured results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to the diffraction pattern observed from the real target.

[0043] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, the radiation emitted by a radiation source is directed onto a target, and the reflected or scattered radiation from the target is directed onto a spectrometer detector that measures the spectrum of the radiation reflected by the mirror (i.e., the measurement of the intensity as a function of wavelength). From such data, the structure or profile of the target that produced the detected spectrum can be reconstructed, for example, by rigorous coupled-wave analysis and non-linear regression or by comparison with a simulated spectral library.

[0044] 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. Such metrology equipment emits polarized light (such as linear, circular, or elliptical) by using, for example, a suitable polarization filter in the illumination section of the metrology equipment. A source suitable for the metrology equipment can also provide polarized radiation. 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.

[0045] In one embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the overlap of two misaligned gratings or periodic structures by measuring the reflection spectrum and / or detecting an asymmetry in the detection configuration (the asymmetry being related to the overlapping range). Two (usually stacked) grating structures can be applied in two different layers (not necessarily consecutive layers), and the two grating structures can be formed substantially at the same location on the wafer. The scatterometer can have, for example, a symmetric detection configuration as described in the co-owned patent application EP1,628,164A, such that any asymmetry is clearly distinguishable. This provides a direct way to measure misalignment in the grating. Further examples for measuring the overlap error between two layers including periodic structures when measuring a target via the asymmetry of the periodic structure can be found in PCT Patent Application Publication No. WO 2011 / 012624 or U.S. Patent Application US 20160161863, which are incorporated herein by reference in their entirety.

[0046] Other parameters of interest can be focus and dose. Focus and dose can be determined simultaneously by scatterometry (or alternatively by scanning electron microscopy) as described in U.S. Patent Application US2011-0249244, which is incorporated herein by reference in its entirety. A single structure with a unique combination of critical dimension and sidewall angle measurements for each point in a focus energy matrix (FEM, also known as a focus exposure matrix) can be used. If these unique combinations of critical dimension and sidewall angle are obtainable, the focus and dose values can be uniquely determined based on these measurements.

[0047] The measurement target can be the totality of a composite grating formed mainly in a resist by a lithography process and also formed, for example, after an etching process. Generally, the pitch and linewidth of the structures in the grating largely depend on the measurement optics (especially the NA of the optics) to be able to capture the diffraction orders from the measurement target. As previously indicated, the diffraction signal can be used to determine the shift (also referred to as "overlay") between two layers, or can be used to reconstruct at least a portion of the original grating produced by the lithography process. Such reconstruction can be used to provide guidance on the quality of the lithography process and can be used to control at least a portion of the lithography process. The target can have a smaller sub-segment configured to mimic the dimensions of the functional portion of the design layout in the target. Due to such a sub-segment, the target will behave more similarly to the functional portion of the design layout, such that the overall process parameter measurement is preferably similar to the functional portion of the design layout. The target can be measured in an underfill mode or in an overfill mode. In the underfill mode, the measurement beam produces a spot smaller than the overall target. In the overfill mode, the measurement beam produces a spot larger than the overall target. In such an overfill mode, it is also possible to measure different targets simultaneously, thereby determining different process parameters simultaneously.

[0048] The overall measurement quality of the lithography parameters using a particular target is at least partially determined by the measurement selection scheme used to measure such lithography parameters. The term "substrate measurement selection scheme" can include one or more parameters of the measurement itself, one or more parameters of one or more patterns being measured, or both. For example, if the measurement used in the substrate measurement selection scheme is a diffraction-based optical measurement, one or more of the measured parameters can include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the direction of the radiation relative to the pattern on the substrate, etc. One of the criteria for selecting the measurement selection scheme can be, for example, the sensitivity of one of the measurement parameters to process variations. More examples are described in U.S. Patent Application US2016-0161863, which is incorporated herein by reference in its entirety, and the published U.S. Patent Application US 2016 / 0370717A1.

[0049] Figure 4 depicts a metrology device, such as a scatterometer. The measurement device includes a broadband (white light) radiation projector 2 that projects radiation onto a substrate 6. The reflected or scattered radiation is transmitted to a spectrometer detector 4, which measures the spectrum 10 of the radiation reflected by the mirror (i.e., the measurement of the intensity as a function of wavelength). From such data, the processing unit PU can, for example, by rigorous coupled wave analysis and non-linear regression or by Figure 3Reconstruct the structure or profile that generated the detected spectrum by comparing it with a simulated spectral library shown at the bottom. 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. Such a scatterometer can be configured as a normal incidence scatterometer or an oblique incidence scatterometer.

[0050] The overall measurement quality of lithography parameters of a metrology target measured via measurement 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 parameters measured can include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the direction 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.

[0051] Another type of metrology tool used in IC manufacturing is a topography measurement system, a leveling sensor, or a height sensor. Such a tool can be integrated into a lithography apparatus for measuring the topography of the top surface of a substrate (or wafer). A topographic map of the substrate (also referred to as a height map) can be generated from these measurements indicating the height of the substrate as a function of the position on the substrate. Such a height map can subsequently be used to correct the position of the substrate during transfer of a pattern onto the substrate in order to provide a spatial image of the patterning device at the proper focus position on the substrate. It should be understood that "height" in this context generally refers to the out-of-plane dimension to the substrate (also referred to as the Z-axis). Generally, a leveling or height sensor performs measurements at a fixed location (relative to its own optical system), and relative movement between the substrate and the optical system of the leveling or height sensor results in height measurements at locations across the substrate.

[0052] Figure 5 An example of a leveling or height sensor LS known in the art is schematically shown in Figure 5Only the operating principle is illustrated. In this example, the level sensor includes an optical system, which includes a projection unit LSP and a detection unit LSD. The projection unit LSP includes a radiation source LSO that provides a radiation beam LSB, which is imparted by the projection grating PGR of the projection unit LSP. The radiation source LSO can be, for example, a narrow-band or broadband light source, such as a polarized or non-polarized, pulsed or continuous supercontinuum light source, such as a polarized or non-polarized laser beam. The radiation source LSO can include multiple radiation sources with different colors or wavelength ranges, such as multiple LEDs. The radiation source LSO of the level sensor LS is not limited to visible light radiation, but additionally or alternatively, can cover UV and / or IR radiation and any wavelength range suitable for reflection from the surface of the substrate.

[0053] The projection grating PGR is a periodic grating including a periodic structure that generates a radiation beam BE1 with a periodically varying intensity. The radiation beam BE1 with a periodically varying intensity is directed towards the measurement site MLO on the substrate W, and the radiation beam BE1 has an incident angle ANG between 0 degrees and 90 degrees, typically between 70 degrees and 80 degrees, with respect to the axis (Z-axis) perpendicular to the incident substrate surface. At the measurement site MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by the arrow BE2) and directed towards the detection unit LSD.

[0054] To determine the height level at the measurement site MLO, the level sensor further includes a detection system, which includes a detection grating DGR, a detector DET, and a processing unit (not shown) for processing the output signal of the detector DET. The detection grating DGR can be identical to 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 photodetector, or represents the spatial distribution of the received intensity, such as a camera. The detector DET can include any combination of one or more detector types.

[0055] By means of triangulation techniques, the height level at the measurement site MLO can be determined. The detected height level is generally related to the signal intensity measured by the detector DET, and the signal intensity has a periodicity that depends particularly on the design of the projection grating PGR and the (tilted) incident angle ANG.

[0056] The projection unit LSP and / or the detection unit LSD can include additional optical elements, such as lenses and / or mirrors, along the path of the patterned radiation beam between the projection grating PGR and the detection grating DGR (not shown).

[0057] In an embodiment, the detection grating DGR may be omitted, and the detector DET may be placed at the location where the detection grating DGR was located. This configuration provides a more direct detection of the image of the projection grating PGR.

[0058] To effectively cover the surface of the substrate W, the horizontal sensor LS may be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby creating an array of measurement regions MLO or spots that cover a larger measurement range.

[0059] Various height sensors of a general type are disclosed in both US7265364 and US7646471, 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 identify the position of a grating image without a detection grating is described.

[0060] 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 correctly and accurately place the pattern (by the same apparatus or a different lithographic apparatus) relative to features placed in a previous layer. For this purpose, the substrate is provided with one or more sets of marks or targets. Each mark is a structure whose position can be measured later 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".

[0061] A lithographic apparatus may include one or more (e.g., multiple) alignment sensors that can accurately measure the position of alignment marks provided on a substrate. The alignment (or position) sensor may use optical phenomena such as diffraction and interference to obtain position information from alignment marks formed on the substrate. An example of an alignment sensor used in current lithographic apparatuses is based on the self-referencing interferometer described in US6961116. Various enhancements and modifications of the position sensor have been developed, such as those disclosed in US2015261097A1. All of these disclosures are incorporated herein by reference.

[0062] Figure 6It is a schematic block diagram of an embodiment of a known alignment sensor AS such as described and incorporated by reference in, for example, US6961116. A radiation source RSO provides a radiation beam RB having one or more wavelengths, and the radiation beam RB 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 point mirror SM and an objective lens OL. Thus, the diameter of the illumination spot SP that illuminates the mark AM can be slightly smaller than the width of the mark itself.

[0063] The radiation diffracted by the alignment mark AM (in this example via the objective lens OL) is collimated into an information-bearing beam IB. The term "diffraction" is intended to include zero-order diffraction from the mark (which may be referred to as reflection). A self-referencing interferometer SRI of the type disclosed, for example, in US6961116 mentioned above causes the beam IB to interfere with itself, and then the beam is received by a photodetector 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 photodetector can be a single element, or it can include multiple pixels as needed. The photodetector can include a sensor array.

[0064] The steering optics including the point mirror SM in this example can also be used to block the zero-order radiation reflected from the mark, such that the information-bearing beam IB includes only higher-order diffracted radiation from the mark AM (this is not necessary for measurement but improves the signal-to-noise ratio).

[0065] An intensity signal SI is supplied to a processing unit PU. By combining the optical processing performed in block SRI with the computational processing performed in unit PU, values of the X position and Y position of the substrate relative to a reference frame are output.

[0066] A single measurement of the type illustrated only fixes the position of the mark within a certain range corresponding to one pitch of the mark. A coarser measurement technique is used in combination with this to identify which period of a sine wave is the period that includes the marked position. The same process can be repeated at different wavelengths at coarser and / or finer levels for improving accuracy and / or for robustly detecting the mark, regardless of the material on which the mark is made and the material on and / or under which the mark is provided. The wavelengths can be multiplexed and demultiplexed optically to process the wavelengths simultaneously, and / or the wavelengths can be multiplexed by time division or frequency division.

[0067] In this example, the alignment sensor and the spot SP remain stationary while the substrate W moves. Thus, the alignment sensor can be mounted rigidly, i.e., firmly and accurately, relative to the reference system, while effectively scanning the mark AM in a direction opposite to the direction of movement of the substrate W. In such a movement, the substrate W is controlled by a substrate positioning system that mounts the substrate W on a substrate support and controls 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 marks provided on the substrate support allows calibration of the position of the substrate support determined by the position sensor (e.g., relative to the frame to which the alignment system is connected). Measuring the position of the alignment marks provided on the substrate allows determination of the position of the substrate relative to the substrate support.

[0068] The above-mentioned metrology tool MT (such as a scatterometer, a topography measurement system, or a position measurement system) can perform measurements using radiation from a radiation source. The nature of the radiation used by the metrology tool may affect the type and quality of the measurements that can be performed. For some applications, it may be advantageous to use multiple radiation frequencies to measure the substrate, for example, broadband radiation can be used. Multiple different frequencies may be able to propagate, irradiate, and scatter off the metrology target without interfering with other frequencies or with minimal interference with other frequencies. Thus, more metrology data can be obtained simultaneously using different frequencies, for example. Different radiation frequencies may also be able to probe and discover different properties of the metrology target. Broadband radiation can be applicable in the metrology system MT, such as, for example, a level sensor, an alignment mark measurement system, a scatter measurement tool, or an inspection tool. A broadband radiation source can be a supercontinuum source.

[0069] It may be difficult to generate high-quality broadband radiation such as supercontinuum radiation, for example. One method for generating broadband radiation can be, for example, to utilize non-linear higher-order effects to broaden high-power narrow-band or single-frequency input radiation or pump radiation. The input radiation (which can be generated using a laser) can be referred to as pump radiation. Alternatively, the input radiation can be referred to as seed radiation. To obtain high-power radiation for the broadening effect, the radiation can be confined to a small area so that a largely localized high-intensity radiation is achieved. In those areas, the radiation can interact with the broadening structure and / or the material forming the non-linear medium to produce broadband output radiation. In the high-intensity radiation areas, different materials and / or structures can be used to achieve and / or improve radiation broadening by providing a suitable non-linear medium.

[0070] In some implementations, broadband output radiation is generated in a photonic crystal fiber (PCF). In a number of embodiments, such a photonic crystal fiber has a microstructure around its fiber core, which helps to confine the radiation traveling through the fiber in the core. The fiber core can be made of a solid material that has nonlinear properties and is capable of generating broadband radiation when high-intensity pump radiation is transmitted through the fiber in the core. Although it is feasible to generate broadband radiation in a solid-core photonic crystal fiber, there can be several drawbacks to using solid materials. For example, if UV radiation is generated in the solid core, such radiation may not be present in the output spectrum of the fiber because the radiation is absorbed by most solid materials.

[0071] In some implementations, as discussed further below Figure 8 A method and apparatus for broadening input radiation can use an optical fiber for confining the input radiation and for broadening the input radiation to output broadband radiation. The optical fiber can be a hollow-core fiber and can include internal structures for enabling efficient guiding and confinement of the radiation in the fiber. The optical fiber can be a hollow-core photonic crystal fiber (HC-PCF), which is particularly suitable for strong radiation confinement mainly inside the hollow core of the fiber, thereby enabling high radiation intensities. The hollow core of the fiber can be filled with a gas that acts as a broadening medium for broadening the input radiation. Such an optical fiber and gas arrangement can be used to generate a supercontinuum radiation source. The radiation input to the fiber can be electromagnetic radiation, such as radiation in one or more of the infrared, visible, UV, and extreme UV spectra. The output radiation can consist of or include broadband radiation, which can be referred to herein as white light.

[0072] Some embodiments relate to a new design of such a broadband radiation source that includes an optical fiber. The optical fiber is a hollow-core photonic crystal fiber (HC-PCF). In particular, the optical fiber can be of the type of hollow-core photonic crystal fiber that includes an antiresonant structure for confining radiation. Such an optical fiber including an antiresonant structure is known in the art as an antiresonant fiber, a tubular fiber, a single-ring fiber, a negative-curvature fiber, or a suppressed-coupling fiber. Various different designs of such optical fibers are known in the art. Alternatively, the optical fiber can be a photonic bandgap fiber (HC-PBF, such as a Kagome fiber).

[0073] Multiple types of HC-PCF can be designed, each based on a different physical guiding mechanism. Two such HC-PCF include: hollow-core photonic bandgap fiber (HC-PBF) and hollow-core anti-resonant reflecting fiber (HC-ARF). Details of the design and manufacture of HC-PCF can be found in U.S. Patent US2004 / 015085A1 (for HC-PBF) and International PCT Patent Application WO2017 / 032454A1 (for hollow-core anti-resonant reflecting fiber), which are incorporated herein by reference. Figure 9 FIG. (a) shows a Kagome fiber including a Kagome lattice structure.

[0074] Reference will now be made to Figure 7 describe an example of a fiber for use in a radiation source, Figure 7 which is a schematic cross-sectional view of a fiber OF in a transverse plane. Additional embodiments similar to the actual example of the fiber disclosed in WO2017 / 032454A1 are Figure 7 disclosed.

[0075] The fiber OF includes an elongated body that is longer in one dimension compared to the other two dimensions of the fiber OF. This longer dimension can be referred to as the axial direction and can define the axis of the fiber OF. The other two dimensions define a plane that can be referred to as the transverse plane. Figure 7 FIG. shows a cross-section of the fiber OF in such a transverse plane (i.e., perpendicular to the axis) labeled as the x-y plane. The transverse cross-section of the fiber OF can be substantially constant along the fiber axis.

[0076] It should be understood that the fiber OF has a certain degree of flexibility, and thus, generally, the direction of the axis will not be uniform along the length of the fiber OF. Terms such as the optical axis, transverse cross-section, and the like should be understood to mean the local optical axis, local transverse cross-section, etc. Additionally, in cases where a component is described as cylindrical or tubular, these terms should be understood to encompass such shapes that may have been deformed when the fiber OF is flexed.

[0077] The fiber OF can have any length, and it should be understood that the length of the fiber OF can depend on the application. The fiber OF can have a length between 1 cm and 10 m. For example, the fiber OF can have a length between 10 cm and 100 cm.

[0078] The optical fiber OF includes a hollow core HC, a cladding portion surrounding the hollow core HC, and a support portion SP surrounding and supporting the cladding portion. The optical fiber OF can be regarded as including a body (including the cladding portion and the support portion SP) having a hollow core HC. The cladding portion includes a plurality of anti-resonant elements or capillaries CAP for guiding radiation through the hollow core HC. In particular, the plurality of anti-resonant elements are arranged to mainly confine the radiation propagating through the optical fiber OF inside the hollow core HC and are configured to guide the radiation along the optical fiber OF. The hollow core HC of the optical fiber OF can be generally disposed in the central region of the optical fiber OF such that the axis of the optical fiber OF can also define the axis of the hollow core HC of the optical fiber OF.

[0079] The cladding portion includes a plurality of anti-resonant elements for guiding the radiation propagating through the optical fiber OF. In particular, in this embodiment, the cladding portion includes a single ring of six tubular capillaries CAP. Each of the tubular capillaries CAP serves as an anti-resonant element.

[0080] The capillary CAP can also be referred to as a tube. The cross-section of the capillary CAP can be circular or can have another shape. Each capillary CAP includes a generally cylindrical wall portion WP that at least partially defines the hollow core HC of the optical fiber OF and separates the hollow core HC from the capillary cavity CC. It should be understood that the wall portion WP can serve as an anti-reflection Fabry - Perot resonator for the radiation that propagates through the hollow core HC (and the radiation can be incident on the wall portion WP at a grazing angle of incidence). The thickness of the wall portion WP can be appropriate to ensure substantially enhanced reflection back into the hollow core HC while substantially suppressing transmission into the capillary cavity CC.

[0081] It should be understood that, as used herein, the term cladding portion is intended to mean the part of the optical fiber OF that guides the radiation propagating through the optical fiber OF (i.e., the capillaries CAP that confine the radiation within the hollow core HC). The radiation can be confined in the form of a transverse mode propagating along the optical fiber axis.

[0082] The support portion is generally tubular and supports the six capillaries CAP of the cladding portion. The six capillaries CAP are evenly distributed around the inner surface of the inner support portion SP. The six capillaries CAP can be described as being arranged in a generally hexagonal form.

[0083] The capillary CAPs are arranged such that each capillary does not contact any of the other capillary CAPs. The items in the capillary CAPs contact the inner support portion SP and are spaced apart from adjacent capillary CAPs in the annular structure. Such an arrangement can be beneficial in that it can increase the transmission bandwidth of the optical fiber OF (relative to an arrangement where, for example, the capillaries contact each other). Alternatively, in some embodiments, the items in the capillary CAPs can contact adjacent capillary CAPs in the annular structure.

[0084] Six capillary CAPs of the cladding portion are arranged in a ring structure around the hollow core HC. The inner surface of the ring structure of the capillary CAPs at least partially defines the hollow core HC of the optical fiber OF. The diameter D of the hollow core HC (which can be defined as the minimum dimension between opposing capillaries, indicated by arrow d) can be between 10 µm and 1000 µm. The diameter D of the hollow core HC can affect the mode field diameter, impact loss, dispersion, modal multiplicity i.e., mode complexity, and non - linear properties of the optical fiber OF in the hollow core HC.

[0085] Other dimensions shown are the adjacent capillary pitch Λ, the gap δ between adjacent capillaries, the inner capillary radius r c and the outer capillary radius R c and the capillary wall thickness t = R c - r c and the radius r of the hollow region j and the optical fiber radius R j .

[0086] In such an embodiment, the cladding portion includes a single - ring arrangement of capillary CAPs (which act as anti - resonant elements). Thus, a line in any radial direction from the center of the hollow core HC to the outside of the optical fiber OF passes through no more than one capillary CAP.

[0087] It should be understood that other embodiments can be provided with different arrangements of anti - resonant elements. These arrangements can include arrangements with multiple rings of anti - resonant elements and arrangements with nested anti - resonant elements. Figure 9 (a) shows an embodiment of an HC - PCF with three rings of capillary CAPs, the rings being stacked on top of each other in the radial direction. In such an embodiment, each capillary CAP contacts other capillaries both in the same ring and in different rings. Additionally, while Figure 7 the embodiment shown includes a ring of six capillaries, in other embodiments, one or more rings including any number of anti - resonant elements (such as 4, 5, 6, 7, 8, 9, 10, 11, or 12 capillaries) can be provided in the cladding portion.

[0088] Figure 9(b) shows a modified embodiment of the HC-PCF with a single loop having a tubular capillary as discussed above. In Figure 9 In the example of (b), there are two coaxial loops of the tubular capillary 21. To hold the inner and outer loops of the tubular capillary 21, a support tube ST can be included in the HC-PCF. The support tube can be made of silica.

[0089] Figure 7 and Figure 9 (a) and Figure 9 The tubular capillary in the examples of (a) and (b) can have a circular cross-sectional shape. For the tubular capillary, other shapes are also possible, such as an elliptical or polygonal cross-section. Additionally, Figure 7 and Figure 9 (a) and Figure 9 The solid material of the tubular capillary in the examples of (a) and (b) can include plastic materials (such as PMA), glasses (such as silica or soda-lime glass).

[0090] Figure 8 Depicts a radiation source RDS for providing broadband output radiation. The radiation source RDS includes: a pulsed pump radiation source PRS or any other type of source capable of generating short pulses of a desired length and energy level; an optical fiber OF having a hollow core HC (e.g., of the type shown in Figure 7 ); and a working medium WM (such as a gas) disposed within the hollow core HC. Although in Figure 8 the radiation source RDS includes the optical fiber OF shown in Figure 7 , in alternative embodiments, other types of hollow core HC optical fibers OF can be used.

[0091] The pulsed pump radiation source PRS is configured to provide input radiation IRD. The hollow core HC of the optical fiber OF is arranged to receive the input radiation IRD from the pulsed pump radiation source PRS and broaden the input radiation IRD to provide output radiation ORD. The working medium WM is capable of broadening the frequency range of the received input radiation IRD to provide broadband output radiation ORD.

[0092] The radiation source RDS further includes a reservoir RSV. The optical fiber OF is disposed inside the reservoir RSV. The reservoir RSV may also be referred to as a housing, a container, or a gas chamber. The reservoir RSV is configured to include a working medium WM. The reservoir RSV may include one or more features known in the art for controlling, regulating, and / or monitoring the composition of the working medium WM (which may be a gas) inside the reservoir RSV. The reservoir RSV may include a first transparent window TW1. In use, the optical fiber OF is disposed inside the reservoir RSV such that the first transparent window TW1 is positioned close to the input end IE of the optical fiber OF. The first transparent window TW1 may form part of the wall of the reservoir RSV. The first transparent window TW1 may be transparent at least for the received input radiation frequency such that the received input radiation IRD (or at least most of it) may be coupled into the optical fiber OF located inside the reservoir RSV. It should be understood that optical means (not shown) may be provided for coupling the input radiation IRD into the optical fiber OF.

[0093] The reservoir RSV includes a second transparent window TW2 that forms part of the wall of the reservoir RSV. In use, when the optical fiber OF is disposed inside the reservoir RSV, the second transparent window TW2 is positioned close to the output end OE of the optical fiber OF. The second transparent window TW2 may be transparent at least for the frequency of the broadband output radiation ORD of the device 120.

[0094] Alternatively, in another embodiment, the two opposite ends of the optical fiber OF may be placed inside different reservoirs. The optical fiber OF may include a first end section configured to receive the input radiation IRD and a second end section for outputting the broadband output radiation ORD. The first end section may be placed inside a first reservoir including the working medium WM. The second end section may be placed inside a second reservoir, where the second reservoir may also include the working medium WM. The functions of the reservoirs may be as described above with respect to Figure 8As described. The first reservoir may include a first transparent window configured to be transparent to the input radiation IRD. The second reservoir may include a second transparent window configured to be transparent to the broadband output broadband radiation ORD. The first and second reservoirs may also include sealable openings to allow the optical fiber OF to be partially placed inside the reservoir and partially outside the reservoir such that gas can be sealed inside the reservoir. The optical fiber OF may also include an intermediate section not contained inside the reservoir. Such an arrangement using two separate gas reservoirs may be particularly convenient for embodiments where the optical fiber OF is relatively long (e.g., when the length exceeds 1 m). It should be understood that for such an arrangement using two separate gas reservoirs, the two reservoirs (which may include one or more features known in the art for controlling, regulating, and / or monitoring the composition of the gas inside the two reservoirs) can be considered as devices for providing the working medium WM inside the hollow core HC of the optical fiber OF.

[0095] In this context, a window may be transparent to a frequency if at least 50%, 75%, 85%, 90%, 95%, or 99% of the incident radiation at that frequency transmits through the window.

[0096] Both the first TW1 and second TW2 transparent windows may form an airtight seal within the wall of the reservoir RSV such that the working medium WM (which may be a gas) can be contained within the reservoir RSV. It should be understood that the gas WM may be contained within the reservoir RSV at a pressure different from the ambient pressure of the reservoir RSV.

[0097] The working medium WM may include: inert gases such as argon, krypton, and xenon; Raman active gases such as hydrogen, deuterium, and nitrogen; or gas mixtures such as argon / hydrogen mixtures, xenon / deuterium mixtures, krypton / nitrogen mixtures, or nitrogen / hydrogen mixtures. Depending on the type of filling gas, the nonlinear optical processes may include modulation instability (MI), optical soliton self-compression, optical soliton splitting, Kerr effect, Raman effect, and dispersive wave generation (DWG), the details of which are described in WO2018 / 127266A1 and US9160137B1 (both of which are hereby incorporated by reference). Since the dispersion of the filling gas can be tuned by changing the pressure of the working medium WM in the reservoir RSR (i.e., the gas chamber pressure), the generated broadband pulse dynamics and associated spectral broadening characteristics can be adjusted to optimize frequency conversion.

[0098] In one embodiment, the working medium WM can be disposed within the hollow core HC at least during reception of the input radiation IRD for generating the broadband output radiation ORD. It should be understood that when the optical fiber OF does not receive the input radiation IRD for generating the broadband output radiation, the gas WM can be wholly or partly absent from the hollow core HC.

[0099] To achieve frequency broadening, high-intensity radiation may be desirable. The advantage of an optical fiber OF with a hollow core HC is that it can achieve high-intensity radiation by strong spatial confinement of the radiation propagating through the optical fiber OF, thereby achieving a highly localized radiation intensity. The radiation intensity inside the optical fiber OF can be relatively high, for example, due to a high received input radiation intensity and / or due to strong spatial confinement of the radiation inside the optical fiber OF. The advantage of a hollow-core optical fiber is that it can guide radiation with a wider wavelength range than a solid-core optical fiber, and in particular, a hollow-core optical fiber can guide radiation in both the ultraviolet and infrared ranges.

[0100] The advantage of using an optical fiber OF with a hollow core HC can be that most of the radiation guided inside the optical fiber OF is confined within the hollow core HC. Therefore, most of the interaction of the radiation inside the optical fiber OF is with the working medium WM disposed inside the hollow core HC of the optical fiber OF. Thus, the broadening effect of the working medium WM on the radiation can be enhanced.

[0101] The received input radiation IRD can be electromagnetic radiation. The input radiation IRD can be received as pulsed radiation. For example, the input radiation IRD can include, for example, ultrafast pulses generated by a laser.

[0102] The input radiation IRD can be coherent radiation. The input radiation IRD can be collimated radiation, and its advantage can be to facilitate and improve the efficiency of coupling the input radiation IRD into the optical fiber OF. The input radiation IRD can include a single frequency or a narrow frequency range. The input radiation IRD can be generated by a laser. Similarly, the output radiation ORD can be collimated and / or can be coherent.

[0103] The broadband range of the output radiation ORD can be a continuous range, including a continuous radiation frequency range. The output radiation ORD can include supercontinuum radiation. The continuous radiation can be beneficial for use in a plurality of applications (such as in measurement applications). For example, the continuous frequency range can be used to interrogate a large number of properties. The continuous frequency range can be used, for example, to determine and / or eliminate the frequency dependence of the measured properties. The supercontinuum output radiation ORD can include, for example, electromagnetic radiation in the wavelength range from 100 nm to 4000 nm. The broadband output radiation ORD frequency range can be, for example, 400 nm to 900 nm, 500 nm to 900 nm, or 200 nm to 2000 nm. The supercontinuum output radiation ORD can include white light.

[0104] The input radiation IRD provided by the pulsed pump radiation source PRS can be pulsed. The input radiation IRD can include electromagnetic radiation of one or more frequencies between 200 nm and 2 µm. The input radiation IRD can include, for example, electromagnetic radiation having a wavelength of 1.03 µm. The repetition rate of the pulsed radiation IRD can be on the order of 1 kHz to 100 MHz. The pulse energy can be on the order of 0.1 µJ to 100 µJ, for example 1 to 10 µJ. The pulse duration of the input radiation IRD can be between 10 fs and 10 ps, for example 300 fs. The average power of the input radiation IRD can be between 100 mW and several hundreds of W. The average power of the input radiation IRD can be, for example, 20 W to 50 W.

[0105] The pulsed pump radiation source PRS can be a laser. The spatio-temporal transmission characteristics (e.g., its spectral amplitude and phase) of such laser pulses transmitted along the optical fiber OF can be changed and tuned by the adjustment of (pump) laser parameters, working component WM variations, and optical fiber OF parameters. The spatio-temporal transmission characteristics can include one or more of output power, output mode distribution, output time distribution, the width of the output time distribution (or output pulse width), output spectral distribution, and the bandwidth of the output spectral distribution (or output spectral bandwidth). The pulsed pump radiation source PRS parameters can include one or more of pump wavelength, pump pulse energy, pump pulse width, pump pulse repetition rate. The optical fiber OF parameters can include one or more of fiber length, the size and shape of the hollow core HC, the size and shape of the capillary, the thickness of the wall of the capillary surrounding the hollow core HC. The working component WM (e.g., filling gas) parameters can include one or more of gas type, gas pressure, and gas temperature.

[0106] The broadband output radiation ORD provided by the radiation source RDS can have an average output power of at least 1 W. The average output power can be at least 5 W. The average output power can be at least 10 W. The broadband output radiation ORD can be pulsed broadband output radiation ORD. The broadband output radiation ORD can have a power spectral density of at least 0.01 mW / nm over the entire wavelength band of the output radiation. The power spectral density over the entire wavelength band of the broadband output radiation can be at least 3 mW / nm.

[0107] Figure 10It is a flowchart of a method for manufacturing a hollow-core PCF (HC-PCF) according to the concepts disclosed herein. At step 1000, initial raw materials are obtained. These materials can include glass tubes having a length of, for example, about one meter or longer. At step 1005, at least some of these tubes can be drawn downward into capillaries. This step is illustrated in FIG. 1032, which shows a glass tube 1035 being heated to, for example, above 1000 °C, above 1500 °C, or heated to about 2000 °C by a heat source or furnace (e.g., a graphite resistance furnace) 1045 and being drawn into a capillary 1040. Each tube 1035 can have a diameter of, for example, more than 1 cm, for example, between 1 cm and 3 cm. The outer diameter of the drawn capillary 1040 can be, for example, between 100 µm and 1500 µm or between 100 µm and 1000 µm.

[0108] At step 1010, a PCF preform can be assembled. According to an example method (the so-called stacking and drawing technique), the drawn preform capillaries 1040 are stacked together with spacers to form a stacked assembly. The stacked assembly can be inserted into a large protective tube (e.g., having a diameter of several centimeters) to form a preform. The gap between the stacked assembly and the protective tube can be filled with, for example, silica rods of various outer diameters. The preform can have an outer diameter between, for example, 10 mm and 100 mm and a length between, for example, 0.3 m and 2 m (e.g., about 1 m).

[0109] Figure 10 A transverse cross-sectional image 1065 of an example preform is also shown. As shown in the figure, the example preform is formed by inserting a stacked assembly including seven (or other number) of preform capillaries CAP-1 and other elements such as spacers (not shown) into a protective tube JT. In some cases, the spacers can be in the form of rods or columns. In some cases, the spacers can be made of the same material as the preform capillaries.

[0110] According to another exemplary method, instead of inserting a stack of preformed capillaries into the jacket tube JT, the preformed capillaries CAP-1 are inserted into the jacket tube JT individually and continuously. In such an exemplary method, the jacket tube JT is held horizontally. Each of the preformed capillaries CAP-1 is inserted into the jacket tube JT via one of its ends. Once inserted, the preformed capillary CAP-1 is located at the bottom of the jacket tube JT. Then, the preformed capillary CAP-1 is fixed to the inner wall of the jacket tube JT, for example, by fusion or melting. Then, the jacket tube JT is rotated by a predefined angle (e.g., 60°) to allow the next preformed capillary CAP-1 to be inserted at the bottom of the jacket tube JT. This insertion, fixing, and rotation process is repeated until all the preformed capillaries are inserted and fixed to the inner wall of the jacket tube JT.

[0111] At step 1015, a glass rod is drawn from the preform. This step is illustrated in FIG. 1038, which shows a preform 1050 including a capillary 1040 being drawn into a glass rod 1055. The preform 1050 may be held at the top of a conventional fiber drawing tower (not shown), where the preform is fed downward into a furnace 1045 to heat a lower section of the preform. The heated zone of the preform softens and elongates, and a teardrop-shaped droplet pulls the fiber downward (also referred to as dropping). Additional pulling force and pulling force control are used to ensure that the preform is drawn into a glass rod 1055. Typically, a one-meter-long preform can be drawn into a glass rod 1055 that is 10 m to 1000 m long. Then, this long glass rod is cut into shorter pieces, each having a length, for example, between 0.5 m and 2 m (e.g., about 1 m).

[0112] Also shown is a transverse cross-sectional image of an exemplary glass rod drawn via step 1015. The exemplary glass rod includes seven capillaries CAP-2 that surround a hollow core HC-2 and are supported by a support portion SP-2. Although the exemplary glass rod looks more like the final HC-PCF (or the target HC-PCF), it is rigid and its physical dimensions (e.g., capillary dimensions, support portion diameter) are larger than those of the final HC-PCF. At the end of step 1015, the glass rod may have an outer diameter, for example, between 0.5 mm and 10 mm, and each of the glass rod capillaries CAP-2 surrounding the hollow core HC of the glass rod may have an outer diameter, for example, between 100 µm and 1500 µm or between 100 µm and 1000 µm, and a wall portion thickness, for example, between 1.5 µm and 100 µm.

[0113] The glass rod preparation step 1020 prepares a glass rod 1055 for the optical fiber drawing step 1025. Such a glass rod preparation step 1020 may include connecting a pressure tube to the glass rod capillary 1040'. In this way, the glass rod can be connected to a pressure supplier that is used to pressurize the internal structure of the glass rod during the optical fiber drawing step 1025 so that each capillary in the capillary tube expands in a desired and controlled manner.

[0114] The optical fiber drawing step 1025 is illustrated in FIG. 1042. In such a step, the glass rod 1055 is installed in another drawing tower (e.g., properly equipped to draw HC-PCF) and heated in a furnace 1045. The glass rod 1055 is drawn into a final optical fiber or HC-PCF 1060 such that the HC-PCF 1060 has a target size. Generally, a one-meter-long glass rod can be drawn into hundreds of meters of optical fiber in a continuous manner. In its final form, the outer diameter of the HC-PCF can be reduced to, for example, between 100 µm and 300 µm. By carefully tuning the relative pressure between different parts of the internal structure of the glass rod (e.g., the capillary cavity CC and the hollow core HC), the glass rod capillary CAP-2 expands, and thus its outer diameter increases relative to the inner diameter of the support part SP-2. This effect is evident by comparing two transverse cross-sectional images 1070 and 1075, where the drawn optical fiber shown in image 1075 has a higher ratio of the outer diameter of each capillary CAP-3 to the inner diameter of the support part SP-3. At the end of step 1025, the outer diameter of the capillary CAP-3 can be increased to, for example, between 10 µm and 50 µm, and correspondingly, the thickness of the capillary wall part WP can be reduced to, for example, between 0.05 µm and 0.2 µm. The positive pressure difference between each capillary cavity CC and the hollow core HC can be in the range between 1 millibar (mbar) and 1000 mbar.

[0115] Finally, step 1030 includes an optional rewinding step, for example, where the drawn optical fiber 1060 is rewound from a larger drum to a smaller spool.

[0116] As described above, multiple capillaries or anti-resonant elements of the HC-PCF (e.g., Figure 7The single - loop arrangement of the glass capillary CAP (shown in [ID]) is used to guide the radiation propagating through the optical fiber. The optical guiding effect is established by anti - resonant interference (inverse Fabry - Perot effect) at the glass - air boundary, and the optical guiding effect is strongest at the region of the capillary wall portion WP facing the core. In a hollow - core negatively - curved optical fiber (e.g., HC - PCF), the total fiber loss is affected by both modal - confinement loss and material loss. The confinement loss is the loss caused by the leaky nature of the mode and the imperfect structure of the HC - PCF, while the material loss is caused by the optical absorption in the fiber material. Importantly, the confinement loss of the HC - PCF is strongly affected by the thickness of the capillary wall portion WP. Specifically, the capillary wall thickness forms a resonance at which light leaks out of the capillary CAP. The resonance position can be estimated by the following:

[0117]

[0118] where q is a positive integer (resonance order), t is the nominal wall thickness, and n g is the refractive index of the glass (e.g., for silica glass, n g is approximately 1.45).

[0119] Figure 11 shows an example simulation of the confinement loss of the HC - PCF as a function of wavelength. As can be seen in the figure, there are sharp resonance features that peak at λ q This sharp resonance feature results in the confinement loss increasing by more than three orders of magnitude significantly relative to wavelengths, for example, adjacent to the resonance.

[0120] Therefore, in order to obtain good performance from an HC - PCF - based light source, it is important to fabricate an HC - PCF with a capillary wall thickness t that is small enough such that the first - order resonance is outside the region of the generated spectrum (e.g., outside 400 nm to 900 nm).

[0121] The nominal - size determination of some parameters at the preform level and / or the glass - rod level determines the process stability and the nominal size at the fiber level. Such dimensions include the capillary - diameter parameters (i.e., the capillary inner diameter ID = 2r c and the capillary outer diameter OD = 2R c ) and the aforementioned capillary wall thickness t. In particular, the inventors have determined that the important quantity is the capillary - diameter ratio ; that is, the ratio of the inner diameter ID to the outer diameter OD of the capillary at the preform or glass - rod level. This can be defined as:

[0122]

[0123] For the generated spectrum down to 400 nm, according to Equation [1], the wall thickness of the capillary at the fiber optic level (i.e., the final fiber) should be below 200 nm, or more specifically, below 190 nm.

[0124] This smaller wall thickness has an impact on the manufacturing yield of the manufacturing process. When using virgin capillaries with a capillary diameter ratio Γ = < 0.9, the resulting fiber structure can deviate significantly from the desired structure (such as Figure 7 the structure illustrated in). For example, adjacent capillaries can be in contact and / or one or more capillaries may be deformed (e.g., non-cylindrical cross-section, such as a hexagonal cross-section). Fibers with contacting and / or deformed capillaries have undesirable optical properties (e.g., high losses).

[0125] In fact, in many stretching attempts using glass rod level capillaries with a capillary diameter ratio Γ = < 0.9, a fiber structure with a capillary wall thickness below 200 nm has not been successfully achieved. Thus, new manufacturing strategies are disclosed herein to address this very poor or non-existent yield for fibers with a capillary wall thickness below 200 nm, while maintaining a relatively large fiber OD of > 150 µm and a core diameter of approximately D = 30 µm.

[0126] The inventors have determined that another important parameter in the manufacturing process is the tensile force applied when stretching the fiber from the glass rod. This force (or tensile stress τ) can be determined by: (i) the local velocity w, or more precisely, the local acceleration dw / dz (at the top of the stretching furnace, the local velocity is equal to the feed velocity of the glass rod entering the furnace; at the bottom of the furnace, the local velocity is equal to the applied stretching velocity of the fiber leaving the furnace); (ii) the glass temperature T (which can be estimated from the set furnace temperature), which in turn determines the glass viscosity µ(T); and (iii) the amount of glass material in the jacket tube. For example, the tensile force or tensile stress τ can be described (at least approximately) by:

[0127]

[0128] In an actual fiber drawing facility, the tensile stress τ can be measured. This can be achieved, for example, by using a force gauge on the fiber drawing system, such as at the lower end position of the drawing line, or by analyzing the resonant vibration of the fiber at positions along the drawing line. The returned values are often given in newtons or (kilo)grams. However, such units are not always the most appropriate, especially when comparing preforms with different glass qualities: even when measuring the same nominal tensile force (e.g., in newtons / kilograms), a glass rod with an overall thicker sheath still has a lower viscosity during stretching: the force is simply distributed over a larger glass area.

[0129] Therefore, it is proposed to express the tensile force not in terms of force or mass (N or kg), but in terms of the force applied per glass area. This definition of tensile force is in N / m 2 or Pascal as the unit.

[0130] However, two other major drawbacks in this manufacturing method have become apparent: 1) Unacceptable manufacturing yield, where only 1 out of 7 stretches produces an optical fiber with an internal structure meeting the specifications. In other attempts, the capillary remains too small or over-expands. 2) The outer diameter of the drawn optical fiber is too small, e.g., less than 150 µm (e.g., about 110 µm), which results in severe deterioration of optical performance (due to deformation of the microstructure caused by stress effects). Attempts to draw optical fibers with a sufficiently large outer diameter (e.g., > 150 µm, > 160 µm, or > 170 µm) and thin walls have not been successful.

[0131] Thus, the existing manufacturing process cannot produce the desired optical fibers (e.g., thin-walled, large OD) for measurement applications at any reasonable yield.

[0132] The inventors have identified two specific concepts for improving the yield, which can be implemented individually or together:

[0133] Manufacture a glass rod (or preform) with a capillary having a capillary diameter ratio Γ > 0.9, Γ > 0.93, Γ > 0.95, or Γ > 0.965.

[0134] Draw the glass rod into an optical fiber at a high tensile force value τ > 20 MPa, τ > 30 MPa, τ > 50 MPa, τ > 100 MPa, τ > 130 MPa, τ > 150 MPa, τ > 170 MPa, τ > 185 MPa, τ > 200 MPa, τ > 230 MPa, or τ > 250 MPa.

[0135] Maintain a capillary diameter ratio greater than at least 0.9

[0136] The inventors recognize that the main root cause of contact with the capillary is the high process sensitivity caused by the small structural tolerances of the capillary at the glass rod level. These tolerances (although nominally small) are large enough to make the expansion rate during the stretching process unstable.

[0137] The local internal radius r c of the capillary and / or the change in the external radius R c (z) (i.e., the expansion rate) can be calculated by solving the ordinary differential equation [4]:

[0138]

[0139] The parameter z is the axial distance along the furnace axis. The local variations are the interactions between the local draw velocity w, the glass viscosity µ (which depends on the furnace temperature), the surface tension γ, the nominal dimensions of the capillary (r and R), and the applied pressure difference ∆p between the capillary and the core region. Typically, the expansion rate is exponentially related to the pressure, and for higher desired values of the capillary diameter ratio ID / OD at the fiber level, the expansion rate is greater.

[0140] Figure 12 and Figure 13 Illustrates how the parameter variability depends on the capillary diameter ratio at the glass rod level (it can be noted that if the capillary diameter ratio at the preform level is plotted, substantially the same relationship will be seen). Figure 12 Shows for four different values , the (simulated) capillary outer diameter OD FL (in µm) varies with the capillary outer diameter or relative capillary outer diameter ROD CL (in % of the nominal) at the glass rod level. Figure 13 Shows a similar graph, but where the x-axis plots the capillary wall thickness variation or relative capillary wall thickness Rt CL (in % of the nominal) at the glass rod level. In Figure 12 and Figure 13 each, the horizontal grey bar highlights the region for acceptable capillary diameter variation at the fiber level for achieving acceptable source performance.

[0141] As can be seen from these graphs, the sensitivity to glass rod level parameter variations (specifically, capillary diameter and capillary wall thickness) improves (i.e., decreases) as the capillary diameter ratio (assuming the definition of ID / OD) increases. A capillary diameter ratio of 0.90 shows good sensitivity performance, while values of 0.95 and 0.98 still show relatively preferred performance (but the improvement between 0.95 and 0.98 is relatively small).

[0142] Thus, an intermediate product for fiber manufacturing (e.g., a glass rod) is disclosed, which includes an outer sheath and a plurality of capillaries, wherein the capillaries include an intermediate stage (e.g., glass rod level) capillary diameter ratio of the capillary inner diameter to the outer capillary diameter greater than 0.90 (optionally greater than 0.93, greater than 0.95, greater than 0.965), and wherein the nominal wall thickness of each of the capillaries at the intermediate stage is greater than 1500 nm (optionally greater than 2000 nm, greater than 3000 nm, greater than 4000 nm, greater than 5000 nm, greater than 6000 nm or greater than 7000 nm).

[0143] In addition, the nominal outer diameter of each capillary below the intermediate stage may be greater than 250 µm, greater than 260 µm, or greater than 270 µm.

[0144] A plurality of capillaries may be arranged in a loop configuration within an outer sheath, thereby defining a hollow core within the loop configuration. Note that in this context, the "loop configuration" should not be construed as limited to a circular arrangement, but should be understood to also include other shapes in which the capillaries surround the hollow core, such as hexagonal, pentagonal, or other polygonal arrangements.

[0145] Also disclosed is a method of manufacturing an optical fiber, comprising: obtaining an optical fiber manufacturing intermediate product as disclosed above; and drawing an optical fiber from the optical fiber manufacturing intermediate product, the drawing causing the fiber-grade capillary wall thickness of each capillary in the drawn optical fiber to be less than 200 nm (optionally less than 195 nm or less than 190 nm).

[0146] The drawing may cause the outer diameter of the drawn optical fiber to be greater than 150 µm (optionally greater than 160 µm, greater than 170 µm, or greater than 180 µm).

[0147] The fiber-grade core diameter of the drawn optical fiber may be greater than 15 µm, greater than 20 µm, greater than 25 µm, or greater than 28 µm. For any of these minimum values, the fiber-grade core diameter of the drawn optical fiber may be less than 45 µm, less than 40 µm, less than 35 µm, or less than 32 µm.

[0148] By way of specific example, capillaries having a capillary diameter ratio of 0.95 (e.g., between 0.94 and 0.96) in a glass rod or an optical fiber manufacturing intermediate product may have a nominal glass rod-grade capillary outer diameter of 280 µm (e.g., between 250 µm and 300 µm) and a nominal glass rod-grade wall thickness of 7400 nm (e.g., between 5000 nm and 9000 nm). Capillaries having a capillary diameter ratio of 0.98 (e.g., between 0.97 and 0.99) in a glass rod or an optical fiber manufacturing intermediate product may have a nominal glass rod-grade capillary outer diameter of 332 µm (e.g., between 300 µm and 360 µm) and a nominal glass rod-grade wall thickness of 3300 nm (e.g., between 2000 nm and 4000 nm).

[0149] Drawing the glass rod at a high tensile value

[0150] Figure 14It is a graph of the capillary inner diameter at the fiber optic level versus the relative capillary pressure (compared to the nominal) or the change in capillary pressure when different tensile forces (285 MPa, 215 MPa, 140 MPa, and 127 MPa) are applied at the glass rod. It can be seen that the capillary size increases with the increase in pressure. The curve shows that for higher nominal tensile forces, the sensitivity is smaller, indicating that the process robustness, i.e., the process stability, is increased.

[0151] Applying a high tensile force would seem to be relatively straightforward: by reducing the set furnace temperature, the viscosity of the glass increases, which results in a larger tensile force (see Equation [3]). However, the inventors have observed that the incidence of fiber breakage increases accordingly.

[0152] In practice, finding the appropriate tensile force requires a careful balance between a tensile force large enough (to have a reasonably high process stability) and a tensile force low enough (to minimize fiber breakage).

[0153] Thus, a method of manufacturing an optical fiber is disclosed, the optical fiber comprising an outer sheath and a plurality of capillaries, the method comprising obtaining an optical fiber manufacturing intermediate product and stretching the optical fiber from the optical fiber manufacturing intermediate product, the stretching causing the fiber optic capillary wall thickness of each capillary in the stretched optical fiber to be less than 200 nm (optionally less than 195 nm or less than 190 nm); wherein the tensile force applied during the stretching is greater than 20 MPa (optionally greater than 30 MPa, greater than 50 MPa, greater than 100 MP, greater than 130 MPa, greater than 150 MPa, greater than 170 MPa, greater than 185 MPa, greater than 200 MPa, greater than 230 MPa, or greater than 250 MPa).

[0154] The stretching can cause the outer diameter of the stretched optical fiber to be greater than 150 µm (optionally greater than 160 µm, greater than 170 µm, or greater than 180 µm).

[0155] The outer sheath and the plurality of capillaries can comprise materials of nominally the same composition (i.e., glass of nominally the same purity such that any differences in the materials are unintentional). This can also help prevent fiber breakage. The outer sheath can comprise a first sheath fused to the capillaries, and a second sheath into which the first sheath and the capillaries are inserted before being stretched into an optical fiber. All of these components can comprise the materials of nominally the same composition. Optionally, all of these components are as clean as possible, and the first sheath is sleeved with the second sheath in an environment of high cleanliness to prevent particles from adhering to one of the inner surface / outer surface of the first sheath and / or the second sheath.

[0156] The method may further include reducing particles in the furnace used in the stretching before performing the stretching. This may include performing the following steps before stretching: disassembling the furnace, cleaning the disassembled parts of the furnace, and reassembling the furnace. Optionally, cleaning is performed using hydrofluoric acid. The method may further include operating the furnace at a high temperature (e.g., higher than 1500 °C or higher than 2000 °C) between the reassembly and the stretching to sinter loose particles that may still be present on the surfaces inside the furnace. Such steps may help prevent fiber breakage.

[0157] The measures for reducing fiber breakage discussed above are successful. It can be seen that before implementing the measures, at the moment of fiber breakage, for many different applied tensile strength values randomly distributed in a certain way, there were multiple fiber breaks per stretch on average. Breaks also occurred at relatively low tensile strengths. After implementing the measures discussed above, the fiber breakage was reduced to about 1 per stretch, and this was also the case at relatively high tensile strengths, and such values of relatively high tensile strengths had a consistent spacing.

[0158] The optical fiber manufacturing intermediate product can be any of the optical fiber manufacturing intermediate products described in the first embodiment.

[0159] Figure 15 A lithography system including a radiation source SO and a lithography apparatus LA is shown. The radiation source SO is configured to generate an EUV radiation beam B and supply the EUV radiation beam B to the lithography apparatus LA. The lithography apparatus LA includes an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS, and a substrate table WT configured to support a substrate W.

[0160] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident on the patterning device MA. Additionally, the illumination system IL may include a faceted field mirror device 10 and a faceted pupil mirror device 11. The faceted field mirror device 10 and the faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. In addition to or instead of the faceted field mirror device 10 and the faceted pupil mirror device 11, the illumination system IL may include other mirrors or devices.

[0161] After such adjustment, the EUV radiation beam B interacts with the patterning device MA. Due to this interaction, a patterned EUV radiation beam B’ is generated. The projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For this purpose, the projection system PS may include a plurality of mirrors 13, 14 configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’, thus forming an image having features smaller than the corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated in Figure 1 as having only two mirrors 13, 14, the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[0162] The substrate W may include a previously formed pattern. In such a case, the lithographic apparatus LA aligns the image formed by the patterned EUV radiation beam B’ with the pattern previously formed on the substrate W.

[0163] A relative vacuum may be provided in the radiation source SO, in the illumination system IL, and / or in the projection system PS, i.e., a small amount of gas (e.g., hydrogen) at a pressure well below atmospheric pressure.

[0164] The radiation source SO may be a laser-produced plasma (LPP) source, a discharge-produced plasma (DPP) source, a free electron laser (FEL), or any other radiation source capable of generating EUV radiation.

[0165] Further embodiments are disclosed in the subsequent numbered list:

[0166] 1. A method of manufacturing an optical fiber, the optical fiber comprising an outer sheath and a plurality of capillaries defining a hollow core, the method comprising:

[0167] obtaining an optical fiber manufacturing intermediate product; and

[0168] drawing the optical fiber from the optical fiber manufacturing intermediate product,

[0169] wherein the drawing causes the fiber-grade capillary wall thickness of each capillary in the drawn optical fiber to be less than 200 nm; and

[0170] wherein the tensile force applied during the drawing is greater than 20 MPa.

[0171] 2. The method according to aspect 1, wherein the tensile force applied to the optical fiber manufacturing intermediate product during the drawing is greater than 30 MPa.

[0172] 3. The method according to aspect 1, wherein the tensile force applied to the intermediate product for optical fiber manufacturing during the stretching is greater than 50 MPa.

[0173] 4. The method according to aspect 1, wherein the tensile force applied to the intermediate product for optical fiber manufacturing during the stretching is greater than 100 MPa.

[0174] 5. The method according to aspect 1, wherein the tensile force applied to the intermediate product for optical fiber manufacturing during the stretching is greater than 130 MPa.

[0175] 6. The method according to aspect 1, wherein the tensile force applied to the intermediate product for optical fiber manufacturing during the stretching is greater than 150 MPa.

[0176] 7. The method according to aspect 1, wherein the tensile force applied to the intermediate product for optical fiber manufacturing during the stretching is greater than 170 MPa.

[0177] 8. The method according to aspect 1, wherein the tensile force applied to the intermediate product for optical fiber manufacturing during the stretching is greater than 185 MPa.

[0178] 9. The method according to aspect 1, wherein the tensile force applied to the intermediate product for optical fiber manufacturing during the stretching is greater than 200 MPa.

[0179] 10. The method according to aspect 1, wherein the tensile force applied to the intermediate product for optical fiber manufacturing during the stretching is greater than 230 MPa.

[0180] 11. The method according to aspect 1, wherein the tensile force applied to the intermediate product for optical fiber manufacturing during the stretching is greater than 250 MPa.

[0181] 12. The method according to any one of the preceding aspects, wherein the stretching causes the optical fiber grade capillary wall thickness of each capillary in the stretched optical fiber to be less than 195 nm.

[0182] 13. The method according to any one of the preceding aspects, wherein the stretching causes the optical fiber grade capillary wall thickness of each capillary in the stretched optical fiber to be less than 190 nm.

[0183] 14. The method according to any one of the preceding aspects, wherein the stretching causes the optical fiber grade outer diameter of the stretched optical fiber to be greater than 150 µm.

[0184] 15. The method according to any one of the preceding aspects, wherein the stretching causes the optical fiber grade outer diameter of the stretched optical fiber to be greater than 160 µm.

[0185] 16. The method according to any of the foregoing aspects, wherein the drawing causes the fiber-grade outer diameter of the drawn optical fiber to be greater than 170 µm.

[0186] 17. The method according to any of the foregoing aspects, wherein the drawing causes the fiber-grade core diameter of the drawn optical fiber to be greater than 20 µm.

[0187] 18. The method according to any of the foregoing aspects, wherein the drawing causes the fiber-grade core diameter of the drawn optical fiber to be greater than 25 µm.

[0188] 19. The method according to any of the foregoing aspects, wherein the drawing causes the fiber-grade core diameter of the drawn optical fiber to be greater than 28 µm.

[0189] 20. The method according to any of the foregoing aspects, wherein the drawing causes the fiber-grade core diameter of the drawn optical fiber to be less than 40 µm.

[0190] 21. The method according to any of the foregoing aspects, wherein the drawing causes the fiber-grade core diameter of the drawn optical fiber to be less than 35 µm.

[0191] 22. The method according to any of the foregoing aspects, wherein the drawing causes the fiber-grade core diameter of the drawn optical fiber to be less than 32 µm.

[0192] 23. The method according to any of the foregoing aspects, wherein the method further comprises reducing particles in the furnace used in the drawing step before performing the drawing.

[0193] 24. The method according to aspect 23, wherein the reducing particles comprises performing the following steps before the drawing:

[0194] Disassembling the furnace;

[0195] Cleaning the disassembled parts of the furnace; and

[0196] Reassembling the furnace.

[0197] 25. The method according to aspect 24, wherein the method further comprises operating the furnace at a temperature higher than 2000 °C between the reassembly and the drawing.

[0198] 26. The method according to any of the foregoing aspects, wherein the optical fiber manufacturing intermediate product comprises:

[0199] An outer sheath; and

[0200] A plurality of capillaries, each capillary comprising an intermediate-stage capillary diameter ratio greater than 0.90, the intermediate-stage capillary diameter ratio being the ratio of the inner capillary diameter to the outer capillary diameter.

[0201] 27. The method according to aspect 26, wherein the capillary diameter ratio is greater than 0.93.

[0202] 28. The method according to aspect 26, wherein the capillary diameter ratio is greater than 0.95.

[0203] 29. The method according to aspect 26, wherein the capillary diameter ratio is greater than 0.965.

[0204] 30. The method according to any one of aspects 26 to 29, wherein the nominal outer diameter of each of the capillaries under the intermediate stage is greater than 300 µm.

[0205] 31. The method according to any one of aspects 26 to 29, wherein the nominal outer diameter of each of the capillaries under the intermediate stage is greater than 260 µm.

[0206] 32. The method according to any one of aspects 26 to 29, wherein the nominal outer diameter of each of the capillaries under the intermediate stage is greater than 270 µm.

[0207] 33. The method according to aspect 26, wherein the capillary diameter ratio is between 0.94 and 0.96, and the nominal outer diameter of each of the capillaries under the intermediate stage is between 250 µm and 300 µm.

[0208] 34. The method according to any one of aspects 26 to 33, wherein the nominal wall thickness of each of the capillaries under the intermediate stage is greater than 2000 nm.

[0209] 35. The method according to any one of aspects 26 to 33, wherein the nominal wall thickness of each of the capillaries under the intermediate stage is greater than 3000 nm.

[0210] 36. The method according to any one of aspects 26 to 33, wherein the nominal wall thickness of each of the capillaries under the intermediate stage is greater than 5000 nm.

[0211] 37. The method according to any one of aspects 26 to 33, wherein the nominal wall thickness of each of the capillaries under the intermediate stage is greater than 6000 nm.

[0212] 38. The method according to any one of aspects 26 to 33, wherein the nominal wall thickness of each of the capillaries under the intermediate stage is greater than 7000 nm.

[0213] 39. The method according to any one of the foregoing aspects, wherein the outer sheath and each of the plurality of capillary tubes are composed of materials having nominally the same composition, the outer sheath includes a first sheath and a second sheath, the capillary tubes are welded to the first sheath, and the second sheath and the capillary tubes are sleeved in the second sheath.

[0214] 40. An optical fiber obtained by performing the method according to any one of the foregoing aspects.

[0215] 41. The optical fiber according to aspect 40, wherein the optical fiber includes a hollow-core photonic crystal fiber or a hollow-core microstructured fiber.

[0216] 42. A radiation source including the optical fiber according to aspect 40 or 41.

[0217] 43. A measuring device including the radiation source according to aspect 42.

[0218] 44. The measuring device according to aspect 43, wherein the measuring device includes one of a scattering measurement tool, a leveling tool, or an alignment tool.

[0219] Note that the term "photonic crystal fiber" as used herein particularly includes and encompasses any hollow fiber or microstructured fiber, including those having only a single ring of capillary tubes in the cladding.

[0220] Although specific reference may be made herein to the use of a lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, the guidance and detection for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

[0221] Although specific reference may be made herein to embodiments of the present invention in the context of a lithographic apparatus, embodiments of the present invention may be used in other apparatuses. Embodiments of the present invention may form part of a mask inspection apparatus, a measuring 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 generally be referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0222] Although the use of embodiments of the present invention may have been specifically referred to above in the context of optical lithography, it should be understood that, where the context permits, the present invention is not limited to optical lithography and may be used in other applications (such as imprint lithography).

[0223] Although specific embodiments of the present invention have been described above, it should be understood that the present invention may be practiced in other ways different from the described manner. The above description is intended to be illustrative, not restrictive. Thus, it will be apparent to those skilled in the art that the described present invention may be modified without departing from the scope of the claims set forth below.

Claims

1. A method of manufacturing an optical fiber, the optical fiber comprising an outer sheath and a plurality of capillaries defining a hollow core, the method comprising: Obtaining an intermediate product for optical fiber manufacturing; And Stretching the optical fiber from the intermediate product for optical fiber manufacturing, wherein the stretching results in an optical fiber-grade capillary wall thickness of each capillary in the stretched optical fiber being less than 200 nm; and wherein a tensile force applied during the stretching is greater than 20 MPa.

2. The method according to claim 1, wherein The tensile force applied to the intermediate product for optical fiber manufacturing during the stretching is greater than 30 MPa.

3. The method according to any one of the preceding claims, wherein, The stretching results in an optical fiber-grade capillary wall thickness of each capillary in the stretched optical fiber being less than 195 nm.

4. The method according to any one of the preceding claims, wherein, The stretching results in an optical fiber-grade outer diameter of the stretched optical fiber being greater than 150 µm.

5. The method according to any one of the preceding claims, wherein The stretching results in an optical fiber-grade inner core diameter of the stretched optical fiber being greater than 20 µm.

6. The method according to any one of the preceding claims, wherein, The stretching results in an optical fiber-grade inner core diameter of the stretched optical fiber being less than 40 µm.

7. The method according to any one of the preceding claims, wherein, The intermediate product for optical fiber manufacturing comprises: An outer sheath; and A plurality of capillaries, each capillary having an intermediate-grade capillary diameter ratio greater than 0.90, the intermediate-grade capillary diameter ratio being the ratio of the inner capillary diameter to the outer capillary diameter.

8. The method according to claim 7, wherein, The nominal outer diameter of each of the capillaries at the intermediate grade is greater than 300 µm.

9. The method according to claim 7, wherein The capillary diameter ratio is between 0.94 and 0.96, and the nominal outer diameter of each of the capillaries at the intermediate grade is between 250 µm and 300 µm.

10. The method according to any one of claims 7 to 9, wherein, The nominal wall thickness of each of the capillaries at the intermediate grade is greater than 2000 nm.

11. An optical fiber, the optical fiber being obtained by performing the method according to any one of the preceding claims.

12. A radiation source, comprising the optical fiber according to claim 11.

13. A measuring device, comprising the radiation source according to claim 12.

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