Methods of making optical fiber intermediate products and producing photonic crystal fibers

Abstract

A method of manufacturing an optical fiber and manufacturing an optical fiber intermediate product are disclosed. The manufacture of an optical fiber intermediate product includes: an outer sheath; and a plurality of capillaries wherein the capillaries comprise an intermediate stage capillary diameter ratio of a capillary inner diameter to a capillary outer diameter greater than 0.90, and wherein each of the capillaries of the intermediate stage has a nominal wall thickness greater than 1500 nm. The intermediate product may be drawn into an optical fiber, wherein each capillary in the drawn optical fiber has a wall thickness of less than 200 nm.

Description

[0001] Cross - Reference to Related Applications

[0002] This application claims the priority of EP application 22207945.1, filed on November 17, 2022, the entire content of which is incorporated herein by reference. Technical Field

[0003] The present invention relates to methods for producing photonic crystal fibers (in particular, hollow-core photonic crystal fibers) and 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). For example, a lithographic apparatus can project a pattern (also often referred to as a "design layout" or "design") at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).

[0005] To project a pattern onto a substrate, a lithographic apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum feature size 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. Smaller features can be formed on a substrate using a lithographic apparatus that uses 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) than using a lithographic apparatus that uses radiation having a wavelength of, for example, 193 nm.

[0006] Features that are smaller than the classical resolution limit of a lithographic apparatus can be patterned using low k1 lithography. 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" (generally, 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 that is similar in shape and size to that planned by a circuit designer to achieve a particular electrical function and performance. To overcome these difficulties, complex fine-tuning steps can be applied to the lithographic projection apparatus and / or the design layout. These include, for example but not limited to, optimization of 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, tight control loops for controlling the stability of the lithographic apparatus can be used to improve pattern reproduction at low k1.

[0007] Measurement tools are used in many aspects of the IC manufacturing process. For example, they are used as alignment tools for correctly positioning a substrate before exposure, leveling tools for measuring the surface topology of a substrate (e.g., for focus control), and scatterometry tools for inspecting / measuring an exposed and / or etched product in process control. In each case, a radiation source is required. For various reasons (including measurement robustness and accuracy), broadband or white light radiation sources are increasingly used for such metrology applications. It would be desirable to improve the devices currently used for broadband radiation generation. Among other techniques for white light generation, photonic crystal fibers (PCFs) can be used to convert narrowband input radiation into broadband output radiation via one or more non-linear optical processes. The output performance of a PCF-based radiation source strongly depends on the quality and characteristics 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 typically result in a change in one or more dimensions of the internal structure of the PCF that is greater than desired. Such changes 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 undesirable characteristics. Accordingly, it is an object of the present invention 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 intermediate product, the method comprising: an outer sheath; and a plurality of capillaries, wherein the capillaries have an intermediate capillary diameter ratio of capillary inner diameter to capillary outer diameter greater than 0.90, and wherein the nominal wall thickness of each of the intermediate capillaries is greater than 1500 nm.

[0009] According to a second aspect of the present invention, there is provided a method of manufacturing an optical fiber, the method comprising: obtaining an optical fiber intermediate product according to the first aspect; and drawing an optical fiber therefrom, the drawing being such that the fiber-grade capillary wall thickness of each capillary in the drawn optical fiber is less than 200 nm.

[0010] 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 second aspect. Brief Description of the Drawings

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

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

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

[0014] Figure 3 depicts a schematic representation of holistic lithography, showing the collaboration between three key technologies for optimizing semiconductor manufacturing;

[0015] Figure 4 depicts a schematic overview of a scatterometry apparatus used as a metrology device that can include a radiation source according to an embodiment of the present invention;

[0016] Figure 5 depicts a schematic overview of a level sensor device that can include a radiation source according to an embodiment of the present invention;

[0017] Figure 6 depicts a schematic overview of an alignment sensor device that can include a radiation source according to an embodiment of the present invention;

[0018] Figure 7 is a schematic cross - sectional view of a hollow - core optical fiber that can form part of a radiation source according to an embodiment in a transverse plane (i.e., perpendicular to the axis of the optical fiber);

[0019] Figure 8 depicts a schematic representation of a radiation source according to an embodiment for providing broadband output radiation;

[0020] Figure 9 (a) and Figure 9 (b) schematically depict transverse cross - sections of examples of hollow - core photonic crystal fiber (HC - PCF) designs for supercontinuum generation;

[0021] Figure 10 is a flowchart describing a method for manufacturing HC - PCF that can form the basis of the concepts and embodiments disclosed herein;

[0022] 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;

[0023] Figure 12 is a graph of the capillary outer diameter at the fiber level as a function of the relative capillary outer diameter change at the preform level for different values of the ratio Γ (capillary inner diameter / capillary outer diameter) at the preform level, illustrating the sensitivity of the final capillary outer diameter to changes in the preform - level capillary outer diameter for different values of the capillary diameter ratio Γ;

[0024] Figure 13A graph of the capillary outer diameter at the optical fiber level as a function of the relative capillary wall thickness change at the preform level, for different values of the capillary diameter ratio Γ (capillary inner diameter / capillary outer diameter) at the preform level, illustrating the sensitivity of the final capillary outer diameter to changes in the capillary wall thickness at the preform level for different values of the capillary diameter ratio Γ;

[0025] Figure 14 A graph of the capillary inner diameter at the optical fiber level as a function of the relative pressure (compared to the nominal value) when the same preform is drawn into the same optical fiber at different tension values, illustrating the sensitivity of the final capillary inner diameter to pressure for different tension values; and

[0026] Figure 15 Depicts a lithographic system including a lithographic apparatus and a radiation source; the lithographic system is Figure 1 an alternative to the lithographic system of Detailed Description

[0027] In this document, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g., having a wavelength of 365, 248, 193, 157, or 126 nm) and EUV (e.g., extreme ultraviolet radiation having a wavelength in the range of approximately 5 nm to 100 nm).

[0028] The terms "reticle", "mask", or "patterning device" as used in this text can be broadly interpreted to refer to a general patterning device that can be used to impart a patterned cross-section to an incident radiation beam, the patterned cross-section corresponding to a pattern to be created in a target portion of a substrate. The term "light valve" can also be used in this context. Examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays in addition to classical masks (transmission or reflection, binary, phase-shifting, hybrid, etc.).

[0029] Figure 1 Schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes: an illumination system (also referred to as an illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation); a mask support (e.g., a mask table) MT configured to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to certain parameters; a substrate support (e.g., a wafer table) WT configured to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support according to certain parameters; and a projection system (e.g., a refractive projection lens system) PS configured to project the pattern imparted to the radiation beam B through the patterning device MA onto a target portion C (e.g., including one or more dies) of the substrate W.

[0030] In operation, the illumination system IL receives a radiation beam from a radiation source SO, e.g., via a beam delivery system BD. The illumination system IL may include various types of optical components for directing, shaping, and / or controlling the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof. The illuminator IL may be used to adjust the cross-section of the radiation beam B at the plane of the patterning device MA to have a desired spatial and angular intensity distribution.

[0031] As used herein, the term "projection system" PS shall be broadly construed to encompass various types of projection systems suitable for the exposure radiation in use, and / or other factors (such as the use of an 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 common term "projection system" PS.

[0032] The lithographic apparatus LA may be of the type in which at least a portion of the substrate is covered by a liquid having a relatively high refractive index (e.g., water) in order to fill the space between the projection system PS and the substrate W - this is also referred to as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0033] The lithographic apparatus LA may also be of the type having more than two substrate supports WT (also referred to as "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or the steps of preparing the substrate W for a subsequent exposure may be performed on the substrate W located on one of the substrate supports WT, while another substrate W on another substrate support WT is used for exposing the pattern on another substrate W.

[0034] In addition to the substrate support WT, the lithographic apparatus LA may include a measurement stage. The measurement stage is arranged to hold a sensor and / or a cleaning device. The sensor may be arranged to measure the characteristics of the projection system PS or the radiation beam B. The measurement stage may hold a plurality of sensors. The cleaning device may be arranged to clean parts of the lithographic apparatus, such as parts of the projection system PS or parts of the system providing the immersion liquid. The measurement stage may move under the projection system PS when the substrate support WT moves away from the projection system PS.

[0035] In operation, a radiation beam B is incident on a patterning device (e.g., a mask held on a mask support MT) MA, and is patterned by a pattern (design layout) present on the patterning device MA. After traversing the mask MA, the radiation beam B passes through a projection system PS which focuses the radiation beam onto a target portion C of a substrate W. With the assistance of a second positioner PW and a position measurement system IF, the substrate support WT can be accurately moved, e.g., so as to position different target portions C in the path of the radiation beam B at the focused and aligned positions. Similarly, a first positioner PM and (possibly) another position sensor ( Figure 1 not explicitly depicted in) can be used to accurately position the patterning device MA relative to the path of the radiation beam B. Mask alignment marks M1, M2 and substrate alignment marks P1, P2 can be used to align the patterning device MA and the substrate W. Although the illustrated substrate alignment marks P1, P2 occupy dedicated target portions, they can be located in the spaces between the target portions. When the substrate alignment marks P1, P2 are located between the target portions C, they are referred to as scribe alignment marks.

[0036] As Figure 2 shown, the lithographic apparatus LA can form part of a lithographic cell LC, which is sometimes also referred to as a lithocell or (lithographic) cluster and typically also includes apparatus for performing pre-exposure processes and post-exposure processes on the substrate W. Conventionally, these 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, e.g., for regulating the temperature of the substrate W, e.g., for conditioning the solvent in the resist layer. A substrate handler or robot RO picks up the substrates W from input / output ports I / O1, I / O2, moves them between the different process apparatus and delivers the substrates W to a feed table LB of the lithographic apparatus LA. The apparatus in the lithographic cell, which are often collectively referred to as a track, are typically under the control of a track control unit TCU, which itself can be controlled by an administrative control system SCS, which can also control the lithographic apparatus LA, e.g., via a lithography control unit LACU.

[0037] In order for the substrates W exposed by the lithographic apparatus LA to be correctly and consistently exposed, it is desirable to inspect the substrates to measure characteristics of the patterned structures, such as overlay errors between subsequent layers, line thicknesses, critical dimensions (CD), etc. For this purpose, an inspection tool (not shown) can be included in the lithographic cell LC. If an error is detected, then, e.g., 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 same batch or other substrates W of the same lot are still to be exposed or processed.

[0038] The characteristics of a substrate W are determined using an inspection device, which may also be referred to as a metrology device, and in particular how the characteristics of different substrates W vary or how the characteristics associated with different layers of the same substrate W vary from layer to layer. Alternatively, the inspection device may be configured to identify defects on the substrate W and may be, for example, part of a lithography cell LC, or may be integrated in a lithography apparatus LA, or may even be a stand-alone device. The inspection device may measure characteristics 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).

[0039] Generally, the patterning process in a lithography apparatus LA is one of the most critical steps in the process, which requires high accuracy in dimension determination and structure layout on the substrate W. To ensure this high accuracy, three systems can be combined in a so-called "integrated" control environment, as Figure 3 schematically depicted. One of these systems is the lithography apparatus LA, which is (virtually) connected to a metrology tool MT (the second system) and a computer system CL (the 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, thereby ensuring that the patterning performed by the lithography apparatus LA stays within the process window. This process window defines the range of process parameters (e.g., dose, focus, overlay) within which a particular manufacturing process produces a defined result (e.g., a functional semiconductor device) - typically within this range, variations in the process parameters in the lithography process or patterning process are allowed.

[0040] The computer system CL can use (parts of) the design layout to be patterned to predict which resolution enhancement techniques are to be used and perform computational lithography simulations and calculations to determine which mask layouts and lithography apparatus settings achieve the maximum overall process window for the patterning process (depicted by the double arrow in Figure 3 the first scale SC1). Generally, the resolution enhancement techniques are arranged to match the patterning capabilities of the lithography apparatus LA. The computer system CL can also be used to detect where within the process window the lithography apparatus LA is currently operating (e.g., using input from the metrology tool MT) to predict whether there may be defects due to, for example, sub-optimal processing (depicted by the arrow pointing to "0" in Figure 3 the second scale SC2).

[0041] The metrology tool MT can provide input to the computer system CL to enable accurate simulations and predictions, and for example in the calibration state of the lithography apparatus LA (in Figure 3as depicted by the multiple arrows in the third scale SC3), feedback can be provided to the lithographic apparatus LA to identify possible drifts.

[0042] In a lithographic process, it is desirable to frequently measure the structures produced, for example for process control and verification. Tools for performing such measurements are commonly referred to as metrology tools MT. Different types of metrology tools MT for performing such measurements are known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. A scatterometer is a versatile instrument that allows the measurement of parameters of a lithographic process by having a sensor in the pupil of the scatterometer objective or in a plane conjugate to the pupil (measurements are typically referred to as pupil-based measurements), or by having a sensor in the image plane or in a plane conjugate to the image plane (in which case the measurements are typically referred to as image- or field-based measurements). Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032, or EP1,628,164A, the entire contents of which are incorporated herein by reference. The aforementioned scatterometers can use light from the soft x-ray and visible to near-IR wavelength ranges to measure gratings.

[0043] 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 characteristics of the grating. Such a reconstruction can be obtained, for example, by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measurement 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.

[0044] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, radiation emitted by a radiation source is directed onto a target, and the reflected or scattered radiation from the target is directed onto a spectrometer detector that measures the spectrum of the specularly reflected radiation (i.e., the measurement of intensity as a function of wavelength). From this data, the structure or profile of the target that caused the detected spectrum can be reconstructed, for example, by rigorous coupled-wave analysis and non-linear regression or by comparison with a library of simulated spectra.

[0045] In a third embodiment, the scatterometer MT is an ellipsometric scatterometer. An ellipsometric scatterometer allows determination of parameters of a lithography process by measuring the scattered radiation for each polarization state. Such a metrology device emits polarized light (such as linear, circular or elliptical) by using, for example, a suitable polarization filter in the illumination part of the metrology device. A source suitable for the metrology device can also provide polarized radiation. Various embodiments of existing ellipsometric scatterometers are described in U.S. patent applications 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,449, 12 / 920,968, 12 / 922,587, 13 / 000,229, 13 / 033,135, 13 / 533,110 and 13 / 891,410, the entire contents of which are incorporated herein by reference.

[0046] In one embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the overlay of two misaligned gratings or periodic structures by measuring the reflected spectrum and / or detecting an asymmetry in the detection configuration, the asymmetry being related to the degree of overlay. The two (usually overlapping) grating structures can be applied in two different layers (not necessarily consecutive layers) and can be substantially formed at the same location on the wafer. The scatterometer can have a symmetry detection configuration as described, for example, in co-owned patent application EP1,628,164A, such that any asymmetry is clearly distinguishable. This provides a straightforward way to measure misalignment in the grating. Other examples for using the asymmetry of periodic structures to measure the overlap error between two layers containing periodic structures when measuring a measurement target can be found in PCT patent application publication number WO 2011 / 012624 or U.S. patent application US20160161863, the entire contents of which are incorporated herein by reference.

[0047] Other parameters of interest can be focal length and dose. As described in U.S. patent application US2011-0249244, the entire contents of which are incorporated herein by reference, the focal length and dose can be determined simultaneously by scatterometry (or alternatively, by scanning electron microscopy). A single structure can be used that has a unique combination of critical dimension and sidewall angle measurement for each point in a focus energy matrix (FEM - also known as a focus exposure matrix). If these unique combinations of critical dimension and sidewall angle are available, the focal length and dose values can be uniquely determined from these measurements.

[0048] The measurement target can be a set of composite gratings formed mainly in a resist by a lithography process (but also after, for example, an etching process). Generally, the pitch and line width of the structures in the grating strongly depend on the measurement optics (in particular, the NA of the optics) to be able to capture the diffraction orders originating from the measurement target. As previously indicated, the diffracted signal can be used to determine the shift (also called "overlay") between two layers, or the signal can be used to reconstruct at least part of the original grating produced by the lithography process. This reconstruction can be used to provide guidance on the quality of the lithography process and can be used to control at least part of the lithography process. These targets can have smaller sub-segments that are configured to mimic the dimensions of functional parts of the design layout in the target. Due to this sub-segmentation, these targets will behave more similarly to the functional parts of the design layout, making the overall process parameter measurement results better resemble the functional parts of the design layout. The target can be measured in an underfill mode or 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 may also be possible to measure different targets simultaneously, thereby determining different process parameters simultaneously.

[0049] The overall measurement quality of the lithography parameters using a particular target is at least partly determined by the measurement recipe used to measure that lithography parameter. The term "substrate measurement recipe" 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 recipe is a diffraction-based optical measurement, one or more of the measured parameters can include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, etc. One of the criteria for selecting a measurement recipe can be, for example, the sensitivity of one of the measurement parameters to process variations. More examples are described in U.S. Patent Application US2016-0161863, which is incorporated herein by reference in its entirety, and Published U.S. Patent Application US2016 / 0370717A1.

[0050] In Figure 4 a metrology device, such as a scatterometer, is depicted. It 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 that measures the spectrum 10 of the specularly reflected radiation (i.e., the measured value of the intensity as a function of wavelength). From this data, the processing unit PU can, for example, by rigorous coupled-wave analysis and non-linear regression or by comparison with, as Figure 3Compare with the library of simulated spectra shown at the bottom) to reconstruct the structure or profile that caused the detected spectrum. Generally, for reconstruction, the general form of the structure is known, and some parameters are assumed from the knowledge of the process by which the structure is made, leaving only a few parameters of the structure to be determined from the scatterometry measurement data. Such a scatterometer can be configured as a normal incidence scatterometer or an oblique incidence scatterometer.

[0051] The overall measurement quality of the lithography parameters measured via a measurement target is at least partially measured by the measurement recipe used to measure the lithography parameter. The term "substrate measurement recipe" 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 recipe is a diffraction-based optical measurement, one or more of the measured parameters can include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, etc. One of the criteria for selecting a measurement recipe can be, for example, the sensitivity of one of the measurement parameters to process variations. More examples are described in U.S. Patent Application US2016 / 0161863 and Published U.S. Patent Application US2016 / 0370717A1, which are incorporated herein by reference in their entirety.

[0052] Another type of metrology tool used in IC manufacturing is a topography measurement system, a level 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 called a height map, can be generated from these measurement results, which indicates the substrate height as a function of the position on the substrate. This height map can then be used to correct the position of the substrate during the transfer of the pattern onto the substrate in order to provide an aerial image of the patterning device at the appropriate focus position on the substrate. It should be understood that in this context, "height" refers to the dimension generally outside the substrate plane (also called the Z-axis). Generally, a level or height sensor performs measurements at a fixed position (relative to its own optical system), and the relative movement between the substrate and the optical system of the level or height sensor results in height measurement results at positions across the substrate.

[0053] In Figure 5FIG. schematically shows an example of a known in the art horizontal or height sensor LS, which only illustrates the operating principle. In this example, the horizontal 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, which provides a radiation beam LSB imparted by the projection grating PGR of the projection unit LSP. The radiation source LSO can be, for example, a narrowband or broadband light source (such as a supercontinuum light source), polarized or unpolarized, pulsed or continuous (such as a polarized or unpolarized laser beam). The radiation source LSO can include a plurality of radiation sources with different colors or wavelength ranges, such as a plurality of LEDs. The radiation source LSO of the horizontal sensor LS is not limited to visible radiation, but can additionally or alternatively cover UV and / or IR radiation and any wavelength range suitable for reflection from the substrate surface.

[0054] The projection grating PGR is a periodic grating including a periodic structure, resulting in a radiation beam BE1 with a periodically varying intensity. The radiation beam BE1 with a periodically varying intensity is directed towards a measurement position MLO on the substrate W, which has an incident angle ANG relative to an axis (Z-axis) perpendicular to the incident substrate surface between 0 degrees and 90 degrees (usually between 70 degrees and 80 degrees). At the measurement position MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by the arrow BE2) and directed towards the detection unit LSD.

[0055] To determine the height level at the measurement position MLO, the horizontal sensor further includes a detection system, which includes a detection grating DGR, a detector DET, and a processing unit (not shown) for processing the output signal of the detector DET. The detection grating DGR can be the same as the projection grating PGR. The detector DET generates a detector output signal indicating the received light (such as indicating the intensity of the received light) (such as a photodetector) or representing the spatial distribution of the received intensity (such as a camera). The detector DET can include any combination of one or more detector types.

[0056] By means of triangulation techniques, the height level at the measurement position MLO can be determined. The detected height level is generally related to the signal intensity measured by the detector DET, which has a periodicity that depends on the design of the projection grating PGR and the (tilt) incident angle ANG, among others.

[0057] The projection unit LSP and / or the detection unit LSD can include other optical elements (not shown) along the path of the patterned radiation beam between the projection grating PGR and the detection grating DGR, such as lenses and / or mirrors.

[0058] 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. Such a configuration provides a more direct detection of the image of the projection grating PGR.

[0059] 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.

[0060] For example, various types of height sensors of a general type are disclosed in US7265364 and US7646471 (both 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 herein. In WO2016102127A1, which is incorporated by reference, a small height sensor is described that uses a multi-element detector to detect and identify the position of a grating image without the need for a detection grating.

[0061] Another type of metrology tool used in IC manufacturing is an alignment sensor. Thus, a key aspect of the performance of a lithographic apparatus is the ability to place the applied pattern correctly and accurately with respect to features laid down in a previous layer (by the same apparatus or a different lithographic apparatus). For this purpose, a set of one or more markers or targets is provided on the substrate. Each marker is a structure whose position can be measured at a later time using a position sensor, typically an optical position sensor. The position sensor may be referred to as an "alignment sensor", and the marker may be referred to as an "alignment marker".

[0062] A lithographic apparatus may include one or more (e.g., multiple) alignment sensors by which the position of alignment markers provided on the substrate can be accurately measured. The alignment (or position) sensor may obtain position information from alignment markers formed on the substrate using optical phenomena such as diffraction and interference. Examples of alignment sensors used in current lithographic apparatuses are based on self-referencing interferometers such as those in US6961116. Various enhancements and modifications of position sensors have been developed, such as those disclosed in US2015261097A1. The content of all these publications is incorporated by reference herein.

[0063] Figure 6It is a schematic block diagram of an embodiment of a known alignment sensor AS (such as described in and incorporated by reference in US6961116 for example). A radiation source RSO provides a radiation beam RB of one or more wavelengths as an illumination spot SP, which is steered by a steering optical element onto a mark (such as a mark AM located on a substrate W). In this example, the steering optical element includes a point mirror SM and an objective lens OL. The diameter of the illumination spot SP by which the mark AM is illuminated can be slightly smaller than the width of the mark itself.

[0064] The radiation diffracted by the alignment mark AM is collimated (in this example, via the objective lens OL) into an information-bearing beam IB. The term "diffraction" is intended to include zero-order diffraction from the mark (which can be referred to as reflection). A self-referencing interferometer SRI of the type disclosed in US6961116 mentioned above, for example, causes the beam IB to interfere with itself, and thereafter the beam is received by a photodetector PD. Additional optical elements (not shown) can be included to provide separate beams in the case where the radiation source RSO generates more than one wavelength. If desired, the photodetector can be a single element, or it can include multiple pixels. The photodetector can include a sensor array.

[0065] The steering optical element 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 the higher-order diffracted radiation from the mark AM (this is not necessary for measurement but improves the signal-to-noise ratio).

[0066] An intensity signal SI is supplied to a processing unit PU. By a combination of optical processing in block SRI and computational processing in unit PU, values of the X position and Y position on the substrate relative to a reference frame are output.

[0067] 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. Coarser measurement techniques are used in combination with this to identify which period of a sine wave is the period containing the marked position. The same process at coarser and / or finer levels can be repeated at different wavelengths to improve accuracy and / or robustly detect the mark, regardless of the material used to make the mark and the material provided above and / or below it. The wavelengths can be optically multiplexed and demultiplexed to be processed simultaneously, and / or they can be time-division or frequency-division multiplexed.

[0068] In this example, the alignment sensor and the light spot SP remain stationary, while the substrate W moves. Therefore, the alignment sensor can be rigidly and accurately mounted on the reference frame, while effectively scanning the mark AM in a direction opposite to the moving direction of the substrate W. This movement of the substrate is controlled by mounting the substrate W on a substrate support and a substrate positioning system that controls the movement of the substrate support. A substrate support position sensor (such as 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.

[0069] A metrology tool MT (such as the scatterometer, the topography measurement system, or the position measurement system mentioned above) can perform measurements using radiation from a radiation source. The characteristics of the radiation used by the metrology tool can affect the type and quality of the measurements that can be performed. For some applications, it can be advantageous to use multiple radiation frequencies to measure the substrate, for example, broadband radiation can be used. Multiple different frequencies can be capable of propagating, illuminating, and scattering out of the metrology target with no or minimal interference to other frequencies. Therefore, for example, different frequencies can be used to obtain more metrology data simultaneously. Different radiation frequencies can also be capable of interrogating and revealing different characteristics of the metrology target. Broadband radiation can be available in a metrology system MT (such as, for example, a level sensor, an alignment mark measurement system, a scatterometry tool, or an inspection tool). A broadband radiation source can be a supercontinuum source.

[0070] Generating high-quality broadband radiation (such as supercontinuum radiation) can be difficult. One method of generating broadband radiation can be, for example, to utilize nonlinear, higher-order effects to broaden high-power narrowband 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 in a small area so that strongly localized high-intensity radiation is achieved. In those areas, the radiation can interact with the structure and / or material of the nonlinear medium for broadening to generate 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 nonlinear medium.

[0071] In some implementations, broadband output radiation is generated in a photonic crystal fiber (PCF). In some embodiments, such a photonic crystal fiber has a microstructure around its fiber core, which helps to confine the radiation traveling through the fiber within the fiber core. The fiber core can be made of a solid material that has nonlinear properties and is capable of generating broadband radiation when high-intensity pump radiation is transmitted through the fiber core. Although it is feasible to generate broadband radiation in a solid-core photonic crystal fiber, there can be several drawbacks to using a solid material. For example, if UV radiation is generated in the solid core, the radiation may not be present in the output spectrum of the fiber because the radiation is absorbed by most solid materials.

[0072] In some implementations, as further discussed below with reference to Figure 8 a method and apparatus for broadening input radiation can use an optical fiber to confine the input radiation and broaden the input radiation to output broadband radiation. The optical fiber can be a hollow-core optical fiber and can include an internal structure to enable 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 achieving a high radiation intensity. The hollow core of the fiber can be filled with a gas, which 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.

[0073] Some embodiments relate to a new design of such a broadband radiation source including 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 including an antiresonant structure for radiation confinement. 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).

[0074] Multiple types of HC-PCF can be designed and manufactured, each based on a different physical guiding mechanism. Two such HC-PCF include: hollow-core photonic bandgap fiber (HC-PBF) and hollow-core antiresonant reflecting fiber (HC-ARF). Details regarding the design and manufacture of HC-PCF can be found in U.S. Patent US2004 / 015085A1 (directed to HC-PBF) and International PCT Patent Application (WO2017 / 032454A1) (directed to hollow-core antiresonant reflecting fiber), which are incorporated herein by reference. Figure 9(a) shows a Kagome optical fiber including a Kagome lattice structure.

[0075] Now, referring to Figure 7 an example of an optical fiber used in a radiation source is described. The figure is a schematic cross-sectional view of the optical fiber OF in a transverse plane. Other embodiments similar to the actual example of the optical fiber in WO2017 / 032454A1 are disclosed. Figure 7 the actual example of the optical fiber.

[0076] The optical fiber OF includes an elongated body that is longer in one dimension of the optical fiber OF than in the other two dimensions. This longer dimension can be referred to as the axial direction and can define the axis of the optical fiber OF. The other two dimensions define a plane that can be referred to as the transverse plane. Figure 7 A cross-section of the optical fiber OF in this transverse plane (i.e., perpendicular to the axis) is shown. The transverse plane is labeled as the x-y plane. The transverse cross-section of the optical fiber OF can be substantially constant along the optical fiber axis.

[0077] It should be understood that the optical fiber OF has a certain degree of flexibility, so the direction of the axis will generally not be uniform along the length of the optical fiber OF. Terms (such as the optical axis, transverse cross-section, etc.) will be understood to mean the local optical axis, local transverse cross-section, etc. In addition, in cases where a component is described as cylindrical or tubular, these terms will be understood to cover such shapes that may deform when the optical fiber OF is flexed.

[0078] The optical fiber OF can have any length, and it should be understood that the length of the optical fiber OF can depend on the application. The optical fiber OF can have a length between 1 centimeter (cm) and 10 meters (m). For example, the optical fiber OF can have a length between 10 cm and 100 cm.

[0079] The optical fiber OF includes: a hollow core HC; a cladding portion that surrounds the hollow core HC; and a support portion SP that surrounds and supports 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 guide the radiation along the optical fiber OF. The hollow core HC of the optical fiber OF can be substantially 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.

[0080] 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 acts as an anti-resonant element.

[0081] The capillary CAP can also be referred to as a tube. The cross-section of the capillary CAP can be circular, or the capillary can have another shape. Each capillary CAP includes a generally cylindrical wall portion WP that at least partially defines a 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 act as an anti-reflection Fabry-Perot resonator for the radiation propagating through the hollow core HC (and that can be incident on the wall portion WP at a grazing incidence angle). The thickness of the wall portion WP can be appropriate to ensure that the reflections back into the hollow core HC are substantially enhanced while the transmission into the capillary cavity CC is substantially suppressed.

[0082] It should be understood that, as used herein, the term cladding portion is intended to mean a portion of the optical fiber OF that guides the radiation propagating through the optical fiber OF (i.e., the capillary CAP that confines the radiation within the hollow core HC). The radiation can be confined in the form of a transverse mode and propagate along the optical fiber axis.

[0083] The support portion is generally tubular and supports six capillary CAPs of the cladding portion. The six capillary CAPs are evenly distributed around the inner surface of the inner support portion SP. The six capillary CAPs can be described as being arranged in a generally hexagonal form.

[0084] The capillary CAPs are arranged such that each capillary does not contact any of the other capillary CAPs. Each of the capillary CAPs contacts the inner support portion SP and is spaced apart from the adjacent capillary CAPs in the ring structure. Such an arrangement can be beneficial because it can increase the transmission bandwidth of the optical fiber OF (relative to, for example, an arrangement in which the capillaries are in contact with each other). Alternatively, in some embodiments, each of the capillary CAPs can contact the adjacent capillary CAPs in the ring structure.

[0085] 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 the opposing capillaries, indicated by the 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 multiplexing, and nonlinear characteristics of the hollow core HC optical fiber OF.

[0086] Other dimensions shown are the pitch Λ of adjacent capillaries, the gap δ between adjacent capillaries, the inner capillary radius r c , the outer capillary radius R c and the capillary wall thickness t = R c - rc , the radius r of the hollow region j and the radius R of the optical fiber j .

[0087] In this embodiment, the cladding portion includes a single - loop arrangement of capillaries CAP (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.

[0088] It should be understood that other embodiments may be provided with different arrangements of anti - resonant elements. These may include arrangements having multiple loops of anti - resonant elements and arrangements having nested anti - resonant elements. Figure 9 (a) shows an embodiment of an HC - PCF having three loops of capillaries CAP, which are stacked on top of each other along the radial direction. In this embodiment, each capillary CAP contacts both other capillaries in the same loop and other capillaries in different loops. Additionally, although Figure 7 the embodiment shown includes loops of six capillaries, in other embodiments, one or more loops including any number of anti - resonant elements (e.g., 4, 5, 6, 7, 8, 9, 10, 11, or 12 capillaries) may be provided in the cladding portion.

[0089] Figure 9 (b) shows an improved embodiment of the above - discussed HC - PCF having a single loop of tubular capillaries. In the Figure 9 example of (b), there are two co - axial loops of tubular capillaries 21. To hold the inner and outer loops of the tubular capillaries 21, a support tube ST may be included in the HC - PCF. The support tube may be made of silica.

[0090] Figure 7 and Figure 9 the tubular capillaries of the examples of (a) and (b) may have a circular cross - sectional shape. Other shapes of the tubular capillaries are possible, such as an elliptical or polygonal cross - section. Additionally, Figure 7 and Figure 9 the solid material of the tubular capillaries of the examples of (a) and (b) may include plastic materials (such as PMA), glass (such as silica), or soft glass.

[0091] 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 (such as the type shown in Figure 7 ); and a working medium WM (such as a gas) disposed within the hollow core HC. Although in Figure 8The mid-infrared radiation source RDS includes Figure 7 the optical fiber OF shown in Figure 7 , but in alternative embodiments, other types of hollow-core HC optical fibers OF may be used.

[0092] 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 it to provide output radiation ORD. The working medium WM enables broadening the frequency range of the received input radiation IRD so as to provide broadband output radiation ORD.

[0093] 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 cell. The reservoir RSV is configured to contain the 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 proximal 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 frequency of the received input radiation, such that the received input radiation IRD (or at least most of it) can be coupled into the optical fiber OF located inside the reservoir RSV. It should be understood that optical elements (not shown) may be provided for coupling the input radiation IRD into the optical fiber OF.

[0094] The reservoir RSV includes a second transparent window TW2, which 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 proximal 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.

[0095] 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 portion configured to receive the input radiation IRD and a second end portion for outputting the broadband output radiation ORD. The first end portion may be placed inside a first reservoir containing the working medium WM. The second end portion may be placed inside a second reservoir, which may also include the working medium WM. The function of the reservoir may be as described above for 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 a sealable opening to permit a portion of the optical fiber OF to be placed inside the reservoir and a portion outside thereof such that gas can be sealed inside the reservoir. The optical fiber may also include an intermediate portion not contained inside the reservoir. Such an arrangement using two separate gas reservoirs can be particularly convenient for embodiments where the optical fiber OF is relatively long (e.g., when the length is greater than 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 gas composition inside the two reservoirs) can be considered as a device for providing a working medium WM inside the hollow core HC of the optical fiber OF.

[0096] In this context, a window may be considered transparent to a frequency if at least 50%, 75%, 85%, 90%, 95%, or 99% of the incident radiation at that frequency on the window is transmitted through the window.

[0097] Both the first transparent window TW1 and the second transparent window TW2 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.

[0098] 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 incorporated herein by reference). Since the dispersion of the filling gas can be tuned by varying the pressure of the working medium WM (i.e., the gas cell pressure) in the storage layer RSR, the generated broadband pulse dynamics and associated spectral broadening characteristics can be adjusted to optimize frequency conversion.

[0099] In one implementation, the working medium WM can be disposed within the hollow core HC, at least during receipt of the input radiation IRD for generating the broadband output radiation ORD. It should be understood that the gaseous WM can be completely or partially absent from the hollow core HC while the optical fiber OF is not receiving the input radiation IRD for generating the broadband output radiation.

[0100] 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 through strong spatial confinement of the radiation propagating through the optical fiber OF, thereby achieving a highly localized radiation intensity. For example, due to the high input radiation intensity received and / or due to the strong spatial confinement of the radiation inside the optical fiber OF, the radiation intensity inside the optical fiber OF can be high. The advantage of hollow-core optical fibers is that they can guide radiation with a wider wavelength range than solid-core optical fibers, and in particular, hollow-core optical fibers can guide radiation in both the ultraviolet and infrared ranges.

[0101] An advantage of using a hollow-core HC optical fiber OF can be that most of the radiation guided inside the optical fiber is confined to the hollow core HC. Thus, most of the radiation interactions inside the optical fiber OF are with the working medium WM, which is disposed inside the hollow core HC of the optical fiber OF. As a result, the broadening effect of the working medium WM on the radiation can be increased.

[0102] 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.

[0103] The input radiation IRD can be coherent radiation. The input radiation IRD can be collimated radiation, the advantage of which 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 range of frequencies. The input radiation IRD can be generated by a laser. Similarly, the output radiation ORD can be collimated and / or can be coherent.

[0104] The broadband range of the output radiation ORD can be a continuous range, including a continuous range of radiation frequencies. The output radiation ORD can include supercontinuum radiation. Continuous radiation can be beneficially used in a number of applications (e.g., in metrology applications). For example, a continuous range of frequencies can be used to interrogate a large number of properties. The continuous range of frequencies 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.

[0105] The input radiation IRD provided by the pulsed pump radiation source PRS can be pulsed. The input radiation IRD can include electromagnetic radiation at one or more frequencies between 200 nm and 2 μm. The input radiation IRD can include, for example, electromagnetic radiation with 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 (e.g., 1 μJ 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 hundred W. The average power of the input radiation IRD can be, for example, 20 W to 50 W.

[0106] 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 varied and tuned by adjusting (pumping) laser parameters, working component WM variations, and optical fiber OF parameters. The spatio-temporal transmission characteristics can include one or more of the following: output power, output mode distribution, output time distribution, bandwidth of the output time distribution (or output pulse bandwidth), output spectral distribution, and bandwidth of the output spectral distribution (or output spectral bandwidth). The pulsed pump radiation source PRS parameters can include one or more of the following: pump wavelength, pump pulse energy, pump pulse width, pump pulse repetition rate. The optical fiber OF parameters can include one or more of the following: fiber length, size and shape of the hollow core HC, size and shape of the capillary, thickness of the wall of the capillary surrounding the hollow core HC. The working component WM (e.g., filling gas) parameters can include one or more of the following: gas type, gas pressure, and gas temperature.

[0107] 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 across the entire wavelength band of the output radiation. The power spectral density across the entire wavelength band of the broadband output radiation can be at least 3 mW / nm.

[0108] Figure 10is a flow chart 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 can include glass tubes having a length of, for example, approximately one meter or longer. At step 1005, at least some of these tubes can be pulled down to form capillaries. This step is illustrated in FIG. 1032, which shows a glass tube 1035 being heated to, for example, above 1000 degrees Celsius, above 1500 degrees Celsius, or about 2000 degrees Celsius by a heat source or furnace (e.g., a graphite resistance furnace) 1045 and being pulled into a capillary 1040. Each tube 1035 can have a diameter of, for example, greater than 1 cm; for example, a diameter between 1 cm and 3 cm. The pulled capillary 1040 can have a diameter of an outer diameter, for example, between 100 μm and 1500 μm or between 100 μm and 1000 μm.

[0109] At step 1010, a PCF preform can be assembled. According to an example method (the so-called stacking and pulling technique), the pulled preform capillaries 1040 are stacked 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, for example, a length of about 1 m.

[0110] Figure 10 A transverse cross-sectional image 1065 of an example preform is also shown. As shown in this figure, an example preform is formed by inserting a stacked assembly including seven (or other number) 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.

[0111] According to another example method, instead of inserting a stack of preform capillaries into the jacket tube JT, the preform capillaries CAP-1 are inserted into the jacket tube JT independently and sequentially. In this example method, the jacket tube JT is held horizontally. Each of the preform capillaries CAP-1 is inserted into the jacket tube JT via one of its ends. Once inserted, the preform capillary CAP-1 is positioned at the bottom of the jacket tube JT. Then, the preform 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 preform 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 preform capillaries are inserted and fixed to the inner wall of the jacket tube JT.

[0112] At step 1015, a blank rod is drawn from the preform. This step is illustrated in FIG. 1038, which shows a preform 1050 that includes a capillary 1040 and is drawn into a blank rod 1055. The preform 1050 can be held at the top of a conventional optical fiber drawing tower (not shown), where the preform is fed downward into a furnace 1045 to heat the lower part of the preform. The heated area of the preform softens and elongates, with teardrop-shaped drips, thereby pulling the optical fiber downward (also known as dropping). Additional pulling and tension control are used to ensure that the preform is drawn into the blank rod 1055. Generally, a one-meter-long preform can be drawn into a blank rod that is 10 m to 1000 m long. Then, such a long blank rod is cut into shorter pieces, each piece having a length, for example, between 0.5 m and 2 m, e.g., about 1 m.

[0113] A transverse cross-sectional image of an example blank rod drawn via step 1015 is also shown. The example blank rod includes seven capillaries CAP-2 that surround a hollow core HC-2 and are supported by a support portion SP-2. Although the example blank rod looks more like the final HC-PCF (or 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 blank rod can have an outer diameter, for example, between 0.5 mm and 10 mm, and the blank rod capillaries CAP-2 surrounding the hollow core HC of the blank rod can each 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.

[0114] The preform preparation step 1020 prepares the preform 1055 for the optical fiber drawing step 1025. This preform preparation step 1020 may include connecting a pressure tube to the preform capillary 1040'. In this way, the preform can be connected to a pressure source that is used to pressurize the internal structure of the preform during the optical fiber drawing step 1025 so that each of the capillaries expands in a desired and controlled manner.

[0115] The optical fiber drawing step 1025 is illustrated in FIG. 1042. In this step, the preform 1055 is installed in another drawing tower (e.g., suitably equipped to draw HC-PCF) and heated within a furnace 1045. The preform 1055 is drawn into the final optical fiber or HC-PCF 1060 such that the HC-PCF 1060 has a target size. Typically, a one-meter-long preform can be drawn into 100 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 preform (e.g., the capillary cavity CC and the hollow core HC), the preform capillary CAP-2 expands such that its outer diameter increases relative to the inner diameter of the support portion 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 portion SP-3. At the end of step 1025, the outer diameter of the capillary CAP-3 can increase to, for example, between 10 μm and 50 μm, and correspondingly, the thickness of the capillary wall portion 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.

[0116] Finally, step 1030 includes an optional rewinding step, e.g., in which the drawn optical fiber 1060 is rewound from a larger spool to a smaller bobbin.

[0117] As described above, the multiple capillaries or anti-resonant elements of the HC-PCF (e.g., as 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 is strongest at the core - facing region of the capillary wall portion WP. In a hollow - core negative - curvature 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 λ q can be estimated by:

[0118]

[0119] 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 ~1.45).

[0120] Figure 11 shows an example simulation of the confinement loss of the HC - PCF as a function of wavelength. As can be seen from this figure, there are sharp resonance features that peak at λ q . Such sharp resonance features result in the confinement loss increasing by more than three orders of magnitude significantly relative to wavelengths adjacent to the resonance, for example.

[0121] Therefore, for obtaining good performance from an HC - PCF - based light source, it is important to fabricate an HC - PCF with a capillary wall thickness t small enough such that the first - order resonance lies outside the region of the generated spectrum (e.g., outside 400 nm to 900 nm).

[0122] The nominal dimensions of some parameters at the preform level and / or the boule level determine the process robustness at the fiber level and the nominal dimensions. 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 above - mentioned capillary wall thickness t. In particular, the inventors have determined that an important quantity is the capillary diameter ratio Γ; that is, the ratio of the inner diameter ID to the outer diameter OD of the capillary at the preform or boule level. This can be defined as:

[0123]

[0124] For a generated spectrum down to 400 nm, according to Equation [1], the wall thickness of the capillary of the fiber grade (i.e., the final fiber) should be below 200 nm, or more specifically below 190 nm.

[0125] This small wall thickness has an impact on the manufacturing yield of the manufacturing process. When using a raw capillary with a capillary diameter ratio Γ = <0.9, the resulting fiber structure can strongly deviate from the desired structure, such as Figure 7 the structure illustrated in. For example, adjacent capillaries can touch and / or one or more capillaries can be deformed (e.g., non-cylindrical cross-section, such as a hexagonal cross-section). Fibers with touching and / or deformed capillaries have undesired optical properties (e.g., high loss).

[0126] In fact, in many drawing attempts using a preform-grade capillary 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 the very poor or non-existent yield of 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.

[0127] The inventors have found that another important parameter in the manufacturing process is the drawing force applied when drawing the fiber from the preform. This force (or tension τ) can be determined by (i) the local velocity w, or more precisely, the local acceleration dw / dz (at the top of the drawing furnace, the local velocity is equal to the feed velocity of the preform into the furnace; at the bottom of the furnace, the local velocity is equal to the applied drawing 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 drawing force or tension τ can be (at least approximately) described by:

[0128]

[0129] In an actual fiber drawing facility, the tension τ can be measured. This can be achieved, for example, by using a dynamometer on a fiber traction system located, for example, 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 values returned are usually 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 the same nominal drawing force (e.g., in newtons / grams) is measured, a preform with an overall thicker jacket has a lower viscosity during drawing: the force is simply distributed over a larger glass area.

[0130] Accordingly, it is proposed to express the tension not in units of force or mass (N or kg), but in terms of the force applied per glass area. The unit of this definition of tension is N / m 2 or Pascal.

[0131] However, two other major drawbacks in this manufacturing method have become apparent: 1) unacceptable manufacturing yield, where only 1 out of 7 draws results in an optical fiber with an internal structure conforming to the specifications. In other attempts, the capillary was either too small or over-expanded. 2) The outer diameter of the drawn optical fiber is too small, e.g., less than 150 μm (e.g., in the region of 110 μm), which leads to a strong deterioration of the optical properties (due to stress effects causing microstructural deformation). 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.

[0132] Thus, the existing manufacturing processes are unable to produce the desired optical fibers for measurement applications (e.g., thin-walled, large OD) with any reasonable yield.

[0133] The inventors have identified two specific concepts for improving the yield that can be implemented independently or together:

[0134] · Fabricating a perform (or preform) with a capillary having a capillary diameter ratio Γ > 0.9, Γ > 0.93, Γ > 0.95, or Γ > 0.965.

[0135] · Drawing the perform into an optical fiber at a high tension 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.

[0136] Maintaining the capillary diameter ratio greater than at least 0.9

[0137] The main underlying reason for the inventors' realization of touching the capillary is the high process sensitivity resulting from the small structural tolerances of the capillary at the perform level. These tolerances (although small in nominal value) are large enough to make the expansion rate during the drawing process unstable.

[0138] The local capillary inner radius r c (z) and / or the outer radius R c (z) variations (i.e., the expansion rate) can be calculated by solving the ordinary differential equation [4].

[0139]

[0140] The parameter z is the axial distance along the furnace axis. This local variation is the interaction between the local drawing speed 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 pressure difference p applied between the capillary and the core region Δp. Generally, the expansion rate depends exponentially on the pressure, and for higher desired values of the capillary diameter ratio ID / OD at the fiber level, the expansion rate is greater.

[0141] Figure 12 and Figure 13 illustrates how the parameter variability depends on the capillary diameter ratio at the preform 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 of Γ ∈ [0.85, 0.90, 0.95, 0.98], the (simulated) capillary outer diameter OD at the fiber level as a function of the capillary outer diameter variation or relative capillary outer diameter ROD CL (in % relative to the nominal value) at the preform level. FL (in μm). 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 % relative to the nominal value) at the preform level. In Figure 12 and Figure 13 each, the horizontal grey bar highlights the region of acceptable capillary diameter variation at the fiber level for acceptable source performance.

[0142] From these graphs, it can be seen that as the capillary diameter ratio Γ (assuming the definition of ID / OD) increases, the sensitivity to preform - level parameter variations (especially capillary diameter and capillary wall thickness) improves (i.e., decreases). The capillary diameter ratio Γ = 0.90 exhibits good sensitivity performance, while the values 0.95 and 0.98 still exhibit better performance (although the improvement between 0.95 and 0.98 is relatively small).

[0143] Thus, a method of manufacturing an optical fiber intermediate product (e.g., a preform) is disclosed, the product comprising an outer sheath and a plurality of capillaries, wherein the capillaries comprise an intermediate - level (e.g., preform - level) capillary diameter ratio of capillary inner diameter to capillary outer 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 level 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).

[0144] Alternatively, the nominal outer diameter of each capillary of the intermediate stage can be greater than 250 μm, greater than 260 μm, or greater than 270 μm.

[0145] The plurality of capillaries can be arranged in an annular configuration within the outer sheath, thereby defining a hollow core within the annular configuration. It should be noted that in this context, the "annular configuration" should not be construed as limited to a circular arrangement, but also includes other shapes of the capillaries around the hollow core; for example, hexagonal, pentagonal, or other polygonal arrangements.

[0146] A method of manufacturing an optical fiber is also disclosed, the method comprising obtaining an optical fiber intermediate product as disclosed above and drawing an optical fiber therefrom, the drawing being such that the fiber-grade capillary wall thickness of each capillary in the drawn optical fiber is less than 200 nm (optionally less than 195 nm or less than 190 nm).

[0147] The drawing can be such that the outer diameter of the drawn optical fiber is greater than 150 μm (optionally greater than 160 μm, greater than 170 μm, or greater than 180 μm).

[0148] The fiber-grade core diameter of the drawn optical fiber can 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 can be less than 45 μm, less than 40 μm, less than 35 μm, or less than 32 μm.

[0149] By way of specific example, a preform or an optical fiber intermediate product including capillaries having a capillary diameter ratio of 0.95 (e.g., between 0.94 and 0.96) can have a nominal preform-grade capillary outer diameter of 280 μm (e.g., between 250 μm and 300 μm) and a nominal preform-grade wall thickness of 7400 nm (e.g., between 5000 nm and 9000 nm). A preform or an optical fiber intermediate product including capillaries having a capillary diameter ratio of 0.98 (e.g., between 0.97 and 0.99) can have a nominal preform-grade capillary outer diameter of 332 μm (e.g., between 300 μm and 360 μm) and a nominal preform-grade wall thickness of 3300 nm (e.g., between 2000 nm and 4000 nm).

[0150] Drawing the preform at high tension values

[0151] Figure 14 is a graph of the fiber-grade capillary inner diameter while applying different tensions (285 MPa, 215 MPa, 140 MPa, and 127 MPa) at the preform in contrast to the relative capillary pressure (compared to the nominal value) or the change in capillary pressure. It can be seen that the capillary size increases with the increase in pressure. The graph shows that the higher the nominal tension, the smaller the sensitivity, indicating an increase in process robustness.

[0152] Applying a high tension would seem relatively straightforward: by reducing the set furnace temperature, the viscosity of the glass increases, which results in a greater tension (see Equation [3]). However, the inventors have observed that the result is an increase in the occurrence of fiber breakage.

[0153] In practice, finding the correct tension requires a careful balance between a large enough tension that makes the process reasonably robust and a low enough tension that minimizes fiber breakage.

[0154] 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 intermediate product for manufacturing the optical fiber and drawing the optical fiber therefrom, the drawing being such that the fiber-grade capillary wall thickness of each capillary in the drawn optical fiber is less than 200 nm (optionally less than 195 nm or less than 190 nm); wherein the tension applied during the drawing is greater than 20 MPa (optionally greater than 30 MPa, greater than 50 MPa, greater than 100 MPa, 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).

[0155] The drawing may be such that the outer diameter of the drawn optical fiber is greater than 150 μm (optionally greater than 160 μm, greater than 170 μm or greater than 180 μm).

[0156] The outer sheath and the plurality of capillaries may 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 also helps to prevent fiber breakage. The outer sheath may comprise a first sheath and a second sheath, the capillaries being fused to the first sheath, and the first sheath and the capillaries being inserted into the second sheath before being drawn into the optical fiber. All of these components may comprise the material of nominally the same composition.

[0157] The method may further comprise reducing the particles in the furnace used in the drawing step before performing the drawing. This may include performing the following steps before drawing: disassembling the furnace, cleaning the disassembled parts of the furnace and reassembling the furnace. The method may further comprise operating the furnace at a high temperature (e.g., above 1500 degrees Celsius or above 2000 degrees Celsius) between the reassembly and the drawing. Such steps may help to prevent fiber breakage.

[0158] The intermediate product for manufacturing the optical fiber may be any of those described in the first embodiment.

[0159] Figure 15A 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 an EUV radiation beam B having a desired cross-sectional shape and a desired intensity distribution. As a supplement or alternative to 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 being conditioned in this way, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B' is produced. 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', thereby forming an image with 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 Figure 1 illustrated as having only two mirrors 13, 14 in, 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 this case, the lithography 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, the illumination system IL, and / or the projection system PS, i.e., a small amount of gas (e.g., hydrogen) at a pressure far 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] Other embodiments are disclosed in the following numbered clause list:

[0166] 1. A method of manufacturing an optical fiber intermediate product, comprising:

[0167] an outer sheath; and

[0168] a plurality of capillary tubes,

[0169] wherein the capillary tubes have an intermediate capillary diameter ratio of capillary inner diameter to capillary outer diameter greater than 0.90, and wherein the nominal wall thickness of each of the capillary tubes in the intermediate stage is greater than 1500 nm.

[0170] 2. The method of manufacturing an optical fiber intermediate product as defined in clause 1, wherein the capillary diameter ratio is greater than 0.93.

[0171] 3. The method of manufacturing an optical fiber intermediate product as defined in clause 1, wherein the capillary diameter ratio is greater than 0.95.

[0172] 4. The method of manufacturing an optical fiber intermediate product as defined in clause 1, wherein the capillary diameter ratio is greater than 0.965.

[0173] 5. The method of manufacturing an optical fiber intermediate product as defined in any of the preceding clauses, wherein the nominal outer diameter of each of the capillary tubes in the intermediate stage is greater than 250 μm.

[0174] 6. The method of manufacturing an optical fiber intermediate product as defined in any of the preceding clauses, wherein the nominal outer diameter of each of the capillary tubes in the intermediate stage is greater than 260 μm.

[0175] 7. The method of manufacturing an optical fiber intermediate product as defined in any of the preceding clauses, wherein the nominal outer diameter of each of the capillary tubes in the intermediate stage is greater than 270 μm.

[0176] 8. The method of manufacturing an optical fiber intermediate product as defined in clause 1, wherein the capillary diameter ratio is between 0.94 and 0.96, and the nominal outer diameter of each of the capillary tubes in the intermediate stage is between 250 μm and 300 μm.

[0177] 9. The method of manufacturing an optical fiber intermediate product as defined in any of the preceding clauses, wherein the nominal wall thickness of each of the capillary tubes in the intermediate stage is greater than 2000 nm.

[0178] 10. The method of manufacturing an optical fiber intermediate product as defined in any of the preceding clauses, wherein the nominal wall thickness of each of the capillary tubes in the intermediate stage is greater than 3000 nm.

[0179] 11. The method of manufacturing an optical fiber intermediate product as defined in any of the preceding clauses, wherein the nominal wall thickness of each of the capillary tubes in the intermediate stage is greater than 5000 nm.

[0180] 12. Manufacturing an optical fiber intermediate product as defined in any of the preceding clauses, wherein the nominal wall thickness of each of the capillaries at the intermediate stage is greater than 6000 nm.

[0181] 13. Manufacturing an optical fiber intermediate product as defined in any of the preceding clauses, wherein the nominal wall thickness of each of the capillaries at the intermediate stage is greater than 7000 nm.

[0182] 14. Manufacturing an optical fiber intermediate product as defined in any of the preceding clauses, wherein the plurality of capillaries are arranged in an annular configuration within an outer sheath, thereby defining a hollow core within the annular configuration.

[0183] 15. Manufacturing an optical fiber intermediate product as defined in any of the preceding clauses, wherein the components of the outer sheath and the plurality of capillaries both include materials of nominally the same composition, the outer sheath includes a first sheath and a second sheath, the capillaries are fused to the first sheath, and the first sheath and the capillaries are inserted into the second sheath.

[0184] 16. A method of manufacturing an optical fiber, comprising:

[0185] Obtaining an optical fiber intermediate product as defined in any of the preceding clauses; and

[0186] Drawing an optical fiber therefrom, the drawing being such that the fiber-grade capillary wall thickness of each of the capillaries in the drawn optical fiber is less than 200 nm.

[0187] 17. The method as defined in clause 16, wherein the drawing is such that the fiber-grade capillary wall thickness of each of the capillaries in the drawn optical fiber is less than 195 nm.

[0188] 18. The method as defined in clause 16, wherein the drawing is such that the fiber-grade capillary wall thickness of each of the capillaries in the drawn optical fiber is less than 190 nm.

[0189] 19. The method as defined in any one of clauses 16 to 18, wherein the drawing is such that the fiber-grade outer diameter of the drawn optical fiber is greater than 150 μm.

[0190] 20. The method as defined in any one of clauses 16 to 18, wherein the drawing is such that the fiber-grade outer diameter of the drawn optical fiber is greater than 160 μm.

[0191] 21. The method as defined in any one of clauses 16 to 18, wherein the drawing is such that the fiber-grade outer diameter of the drawn optical fiber is greater than 170 μm.

[0192] 22. The method as defined in any one of clauses 16 to 21, wherein the drawing is such that the fiber-grade inner core diameter of the drawn optical fiber is greater than 20 μm.

[0193] 23. According to the method defined in any one of clauses 16 to 21, wherein the drawing is such that the fiber-grade core diameter of the drawn optical fiber is greater than 25 μm.

[0194] 24. According to the method defined in any one of clauses 16 to 21, wherein the drawing is such that the fiber-grade core diameter of the drawn optical fiber is greater than 28 μm.

[0195] 25. According to the method defined in any one of clauses 16 to 24, wherein the drawing is such that the fiber-grade core diameter of the drawn optical fiber is less than 40 μm.

[0196] 26. According to the method defined in any one of clauses 16 to 24, wherein the drawing is such that the fiber-grade core diameter of the drawn optical fiber is less than 35 μm.

[0197] 27. According to the method defined in any one of clauses 16 to 24, wherein the drawing is such that the fiber-grade core diameter of the drawn optical fiber is less than 32 μm.

[0198] 28. According to the method defined in any one of clauses 16 to 27, wherein the tension applied to the intermediate product of the optical fiber being manufactured during the drawing is greater than 20 MPa.

[0199] 29. According to the method defined in any one of clauses 16 to 27, wherein the tension applied to the intermediate product of the optical fiber being manufactured during the drawing is greater than 50 MPa.

[0200] 30. According to the method defined in any one of clauses 16 to 27, wherein the tension applied to the intermediate product of the optical fiber being manufactured during the drawing is greater than 100 MPa.

[0201] 31. According to the method defined in any one of clauses 16 to 27, wherein the tension applied to the intermediate product of the optical fiber being manufactured during the drawing is greater than 130 MPa.

[0202] 32. According to the method defined in any one of clauses 16 to 27, wherein the tension applied to the intermediate product of the optical fiber being manufactured during the drawing is greater than 150 MPa.

[0203] 33. According to the method defined in any one of clauses 16 to 27, wherein the tension applied to the intermediate product of the optical fiber being manufactured during the drawing is greater than 170 MPa.

[0204] 34. According to the method defined in any one of clauses 16 to 27, wherein the tension applied to the intermediate product of the optical fiber being manufactured during the drawing is greater than 200 MPa.

[0205] 35. A method as defined in any one of clauses 16 to 34, wherein the method further comprises reducing particles in a furnace used in a drawing step before performing the drawing.

[0206] 36. A method as defined in clause 35, wherein the reducing of particles comprises performing the following steps before drawing:

[0207] Disassembling the furnace;

[0208] Cleaning the disassembled parts of the furnace; and

[0209] Reassembling the furnace.

[0210] 37. A method as defined in clause 36, wherein the method further comprises operating the furnace at a temperature above 2000 degrees Celsius between the reassembling and the drawing.

[0211] 38. An optical fiber obtained by performing the method of any one of clauses 16 to 37.

[0212] 39. An optical fiber as defined in clause 38, wherein the optical fiber comprises a hollow-core photonic crystal fiber or a microstructure fiber.

[0213] 40. A radiation source comprising an optical fiber according to clause 38 or 39.

[0214] 41. A measuring device comprising a radiation source according to clause 40.

[0215] 42. A measuring device as defined in clause 41, wherein the measuring device comprises one of a scattering measurement tool, a leveling tool, or an alignment tool.

[0216] Note that, as used herein, the term "photonic crystal fiber" includes and encompasses in particular any hollow-core fiber or microstructure fiber, including a fiber having only a single loop of capillary in the cladding.

[0217] Although in this text specific reference may be made to the use of a lithographic apparatus in IC manufacturing, it should be understood that the lithographic apparatus described herein may have other applications. Other possible applications include the manufacture of integrated optical systems, the guiding and detecting patterns of magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

[0218] Although embodiments of the invention may be specifically referred to in the context of a lithographic apparatus in this text, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus for measuring or processing an object such as a wafer (or other substrate) or a mask (or other patterning device). Such apparatus may generally be referred to as lithographic tools. Such lithographic tools may operate under vacuum conditions or ambient (non-vacuum) conditions.

[0219] Although the use of embodiments of the invention has been specifically referred to above in the context of optical lithography, it should be understood that the invention is not limited to optical lithography where the context allows, and may be used in other applications such as imprint lithography.

[0220] Although specific embodiments of the invention have been described above, it should be understood that the invention may be practiced in a different manner than that described. The above description is intended to be illustrative and not restrictive. Thus, it will be apparent to those skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims listed below.

Claims

1. A method of manufacturing an optical fiber, comprising: Obtaining an intermediate product for manufacturing an optical fiber, the intermediate product for manufacturing an optical fiber comprising i) an outer sheath; And ii) a plurality of capillaries, wherein the capillaries have an intermediate-stage capillary diameter ratio of capillary inner diameter to capillary outer diameter greater than 0.90, and wherein the nominal wall thickness of each of the capillaries at the intermediate stage is greater than 1500 nm; and Drawing an optical fiber from the intermediate product for manufacturing an optical fiber, the drawing causing the optical fiber-stage capillary wall thickness of each capillary in the drawn optical fiber to be less than 200 nm.

2. The method according to claim 1, wherein the drawing causes the optical fiber-stage outer diameter of the drawn optical fiber to be greater than 150 μm.

3. The method according to any one of claims 1 and 2, wherein the drawing causes the optical fiber-stage core diameter of the drawn optical fiber to be greater than 20 μm.

4. The method according to any one of claims 1 to 3, wherein the drawing causes the optical fiber-stage core diameter of the drawn optical fiber to be less than 40 μm.

5. The method according to any one of claims 1 to 4, wherein the tension applied to the intermediate product for manufacturing an optical fiber during the drawing is greater than 20 MPa.

6. The method according to any one of claims 1 to 5, wherein the method further comprises reducing particles in a furnace used in the drawing step before performing the drawing.

7. The method according to claim 6, wherein the reducing of particles comprises performing the following steps before the drawing: Disassembling the furnace; Cleaning the disassembled parts of the furnace; and Reassembling the furnace.

8. The method according to claim 7, wherein the method further comprises operating the furnace at a temperature higher than 2000 degrees Celsius between the reassembly and the drawing.

9. An optical fiber obtained by performing the method according to any one of claims 1 to 8.

10. A radiation source comprising the optical fiber according to claim 9.

11. A measuring device comprising the radiation source according to claim 10.

12. An intermediate product for manufacturing an optical fiber, comprising: An outer sheath; And A plurality of capillaries, Wherein the capillaries have an intermediate-stage capillary diameter ratio of capillary inner diameter to capillary outer diameter greater than 0.90, and wherein the nominal wall thickness of each of the capillaries at the intermediate stage is greater than 1500 nm.

13. The intermediate product for manufacturing an optical fiber according to claim 12, wherein the capillary diameter ratio is greater than 0.

93.

14. The intermediate product for manufacturing an optical fiber according to claim 12 or 13, wherein the nominal outer diameter of each of the capillaries at the intermediate stage is greater than 250 μm.

15. The intermediate product for manufacturing an optical fiber according to claim 12, 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 stage is between 250 μm and 300 μm.

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