Fiber manufacturing intermediate product and method for manufacturing photonic crystal fiber

Abstract

A method for manufacturing an optical fiber and a fiber manufacturing intermediate product are disclosed, the fiber manufacturing intermediate product comprising an outer jacket and a plurality of capillaries, the capillaries comprising a mid-level capillary diameter ratio of an inner diameter of the capillary to an outer diameter of the capillary greater than 0.90, and a nominal wall thickness of each capillary at the mid-level greater than 1500 nm, which can be drawn into a fiber in which the wall thickness of each capillary in the drawn fiber is less than 200 nm.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to European Patent Application Publication No. 22207945.1, filed November 17, 2022, which is incorporated herein by reference in its entirety.

[0002] The present invention relates to methods for fabricating photonic crystal fibers, particularly hollow-core photonic crystal fibers, and their application in the fabrication of integrated circuits. [Background technology]

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus can, for example, project a pattern (often called a "design layout" or "design") in a patterning device (e.g., mask) onto a layer of radiation-sensitive material (resist) provided on the substrate (e.g., wafer).

[0004]

[0004] Lithographic apparatus may use electromagnetic radiation to project a pattern onto a substrate. The wavelength of this radiation determines the minimum size of features that can be formed on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. Lithographic apparatus using extreme ultraviolet (EUV) radiation with a wavelength in the range of 4 to 20 nm, for example 6.7 nm or 13.5 nm, can form smaller features on a substrate than lithographic apparatus using radiation with a wavelength of, for example, 193 nm.

[0005] Low k1 lithography can be used to process features with dimensions smaller than the classical resolution limit of a lithographic apparatus. In such processes, the resolution equation 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 of the lithographic apparatus, CD is the “critical dimension” (generally the smallest feature size to be printed, in this case the half pitch), and k1 is an empirical resolution factor. In general, the smaller k1 is, the more difficult it is to reproduce on a substrate a pattern resembling the shape and dimensions planned by a circuit designer to achieve a particular electrical functionality and performance. To overcome such difficulties, advanced fine-tuning steps can be applied to the lithographic projection apparatus and / or the design layout. Such steps may include, but are not limited to, optimization of the NA, customization of the illumination scheme, use of phase-shifting patterning devices, various optimizations of the design layout (e.g., optical proximity correction (OPC, sometimes also called "optical and process correction") in the design layout), or other methods commonly defined as "resolution enhancement techniques" (RET). Alternatively, strict control loops may be used to control the stability of the lithographic apparatus to improve pattern reproduction at low k1.

[0006]

[0006] In many aspects of IC manufacturing processes, metrology tools are used, for example, as alignment tools for proper positioning of substrates before exposure, leveling tools for measuring the surface topology of substrates for focus control, etc., and scatterometry tools for inspecting / measuring exposed and / or etched products in process control. In each case, a radiation source is required. Broadband or white light radiation sources are increasingly being used for such metrology applications for various reasons, including measurement robustness and accuracy. It is desirable to improve upon conventional devices 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 nonlinear optical processes. The output performance of PCF-based radiation sources strongly depends on the quality and characteristics of the PCF, which are primarily determined by the fiber manufacturing process. Currently, PCFs are drawn in optical fiber draw towers that continuously draw the fiber. Current fiber manufacturing methods often result in undesirable variations in one or more dimensions of the PCF's internal structure. Such variations can result in a non-optimal structure of the drawn PCF, and therefore a radiation source using such a PCF to generate broadband radiation will output light with undesirable characteristics. It is therefore an object of the present invention to improve the method of manufacturing PCFs. Summary of the Invention

[0007]

[0007] According to a first aspect of the present invention, there is provided a fiber manufacturing intermediate product comprising an outer jacket and a plurality of capillaries, wherein the capillaries have an intermediate level capillary diameter ratio of the capillary inner diameter to the capillary outer diameter greater than 0.90, and the nominal wall thickness of each capillary at the intermediate level is greater than 1500 nm.

[0008]

[0008] According to a second aspect of the present invention, there is provided a method for manufacturing an optical fiber, comprising obtaining a fiber manufacturing intermediate product according to the first aspect, and drawing a fiber from the fiber manufacturing intermediate product, wherein the drawing is performed such that the capillary wall thickness at fiber level of each capillary in the drawn fiber is less than 200 nm.

[0009] Another aspect of the invention includes a metrology device comprising a radiation source device comprising an optical fiber manufactured by the method of the second aspect.

[0010]

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

[0011] [Figure 1] 1 depicts a schematic diagram of a lithographic apparatus; [Figure 2] 1 shows a schematic diagram of a lithographic cell. [Figure 3] A schematic diagram of holistic lithography is shown, illustrating the collaboration between three key technologies for optimizing semiconductor manufacturing. [Figure 4] 1 depicts a schematic diagram of a scatterometry apparatus for use as a metrology device, which may comprise a radiation source according to an embodiment of the present invention; [Figure 5] 1 shows a schematic diagram of a level sensor device that may comprise a radiation source according to an embodiment of the present invention; [Figure 6] 1 shows a schematic diagram of an alignment sensor apparatus that may include a radiation source according to an embodiment of the present invention; [Figure 7] 1 is a schematic cross-sectional view of a hollow-core optical fiber that may form part of a radiation source, according to one embodiment, in a transverse plane (i.e., a transverse plane perpendicular to the axis of the optical fiber); [Figure 8] 1 shows a schematic diagram of a radiation source according to an embodiment that provides broadband output radiation; [Figure 9a]1A and 1B show schematic cross-sections of example hollow-core photonic crystal fiber (HC-PCF) designs for supercontinuum generation. [Figure 9b] 1A and 1B show schematic cross-sections of example hollow-core photonic crystal fiber (HC-PCF) designs for supercontinuum generation. [Figure 10] FIG. 1 is a flow diagram illustrating a method for manufacturing an HC-PCF that may form the basis of concepts and embodiments disclosed herein. [Figure 11] 8 is an exemplary simulation of the confinement loss of an HC-PCF (eg, the fiber shown in FIG. 7) as a function of wavelength. [Figure 12] 1 is a plot of capillary outer diameter at the fiber level as a function of relative capillary outer diameter variation at the cane level for different values ​​of the ratio Γ (capillary inner diameter / capillary outer diameter) at the cane level, showing the sensitivity of the final capillary outer diameter to capillary outer diameter variation at the cane level for different values ​​of the capillary diameter ratio Γ. [Figure 13] 1 is a plot of capillary outer diameter at the fiber level as a function of relative capillary wall thickness variation at the cane level for different values ​​of capillary diameter ratio Γ (capillary inner diameter / capillary outer diameter) at the cane level, showing the sensitivity of the final capillary outer diameter to capillary wall thickness variation at the cane level for different values ​​of capillary diameter ratio Γ. [Figure 14] 1 is a plot of capillary inner diameter at the fiber level as a function of relative pressure (compared to the nominal value) when drawing the same cane into the same fiber at different tension values, showing the sensitivity of the final capillary inner diameter to pressure for different tension values. [Figure 15] 2 depicts an alternative lithography system to that of FIG. 1, comprising a lithography apparatus and a radiation source; DETAILED DESCRIPTION OF THE INVENTION

[0012]

[0011] In this specification, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g., having wavelengths of 365, 248, 193, 157, or 126 nm) and EUV (extreme ultraviolet radiation, e.g., having wavelengths in the range of about 5 to 100 nm).

[0013] As used herein, the terms "reticle," "mask," or "patterning device" may be broadly interpreted to refer to a general patterning device that can be used to impart an incident radiation beam with a patterned cross-section that corresponds to the pattern to be created in a target portion of a substrate. The term "light valve" may also be used in this context. In addition to classic masks (such as transmissive or reflective masks, binary masks, phase-shifting masks, hybrid masks), examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0014] 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA comprises: an illumination system (also called 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 constructed to support a patterning device (e.g. a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters, a substrate support (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support in accordance with certain parameters, and a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0015]

[0014] In operation, the illumination system IL receives a radiation beam from the radiation source SO (e.g. via the beam delivery system BD). The illumination system IL may comprise various types of optical components for directing, shaping and / or controlling the radiation, for example refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical components, or any combination thereof. The illuminator IL may be used to condition the radiation beam B so that it has a desired spatial and angular intensity distribution in its cross-section in the plane of the patterning device MA.

[0016]

[0015] As used herein, the term "projection system" PS should be interpreted broadly as encompassing various types of projection systems. Such systems may be refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic, and / or electrostatic optical systems, or any combination thereof, as required by the exposure radiation being used and / or other factors (e.g., the use of an immersion liquid or a vacuum). Where the term "projection lens" is used herein, it may all be considered as synonymous with the more general term "projection system" PS.

[0017]

[0016] The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid with a relatively high refractive index (e.g. water) so as to fill a space between the projection system PS and the substrate W, which is also known as immersion lithography. Further information about immersion techniques is given in US Patent No. 6,952,253, which is incorporated herein by reference.

[0018] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also known as "dual stage"). In such a "multiple stage" machine, the substrate supports WT may be used in parallel, and / or a substrate W on one of the substrate supports WT may be being used to expose a pattern on that substrate W while a procedure is being performed on another substrate W on the other substrate support WT in preparation for a subsequent exposure of that other substrate W.

[0019] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement stage. The measurement stage is configured to hold a sensor and / or a cleaning device. The sensor may be configured to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold a plurality of sensors. The cleaning device may be configured to clean part of the lithographic apparatus, for example part of the projection system PS or part of a system for supplying immersion liquid. The measurement stage may move below the projection system PS when the substrate support WT is spaced apart from the projection system PS.

[0020]

[0019] In operation, a radiation beam B is incident on a patterning device (e.g. a mask MA held on a mask support MT) and is patterned according to a pattern (design layout) on the patterning device MA. After traversing the mask MA, the radiation beam B passes through a projection system PS, which focuses the beam onto a target portion C of a substrate W. With the aid of the second positioner PW and the position measurement system IF, the substrate support WT can be precisely moved, for example so that different target portions C are positioned at focused and aligned positions in the path of the radiation beam B. Similarly, the first positioner PM, and possibly another position sensor (which is not explicitly depicted in Figure 1), can be used to precisely position the patterning device MA with respect to the path of the radiation beam B. The patterning device MA and substrate W can be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks P1, P2 as illustrated occupy dedicated target portions, they may be located in spaces between target portions. When located between target portions C, the substrate alignment marks P1, P2 are known as scribe-lane alignment marks.

[0021] 2, lithographic apparatus LA may form part of a lithographic cell LC (sometimes called a litho-cell or (litho) cluster), which often also includes apparatus for performing pre-exposure and post-exposure processes on substrates W. Conventionally, such apparatus 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 (which, for example, adjust the temperature of the substrate W, e.g., to adjust the solvent in the resist layer). A substrate handler (i.e., robot) RO picks up substrates W from input / output ports I / O1, I / O2, moves them between various process tools, and delivers them to a loading bay LB of lithographic apparatus LA. The devices in a lithocell are often collectively referred to as a track and are typically under the control of a track control unit TCU, which may itself be controlled by a supervisory control system SCS, which may also control the lithographic apparatus LA (e.g., via a lithography control unit LACU).

[0022] To ensure that a substrate W to be exposed by the lithographic apparatus LA is exposed accurately, it is desirable to inspect the substrate to measure properties of the patterned structures (such as overlay error between subsequent layers, line thickness, critical dimension (CD), etc.). For this purpose, an inspection tool (not shown) may be provided in the lithocell LC. If an error is detected, adjustments may be made, for example, to the exposure of a subsequent substrate or other process step performed on the substrate W, especially if the inspection is performed before other substrates W of the same batch or lot that have not yet been exposed or processed.

[0023] Inspection apparatus, sometimes called metrology apparatus, are used to measure properties of substrates W, and in particular to measure how the properties of different substrates W vary, or how properties associated with different layers of the same substrate W vary from layer to layer. The inspection apparatus may alternatively be constructed to identify defects on substrates W and may, for example, be part of a lithocell LC, or integrated into a lithography apparatus LA, or even be a stand-alone apparatus. The inspection apparatus may measure properties related to a latent image (an image in a resist layer after exposure), or a semi-latent image (an image in a resist layer after a post-exposure bake step PEB), or a developed resist image (from which exposed or unexposed parts of the resist have been removed), or even an etched image (after a pattern transfer step such as etching).

[0024] Typically, the patterning process in a lithography apparatus LA is one of the most critical steps in processing, requiring high accuracy in the dimensioning and placement of structures on a substrate W. To ensure this high accuracy, three systems can be combined in a so-called "holistic" control environment, as shown schematically in FIG. 3 . One of these systems is the lithography apparatus LA, which is (virtually) connected to a metrology tool MT (a second system) and a computer system CL (a third system). The key to such a "holistic" environment is to optimize the coordination between these three systems to enforce the overall process window and achieve a tight control loop, thereby ensuring that the patterning performed by the lithography apparatus LA stays within the process window. The process window defines the range of process parameters (e.g., dose, focus, overlay) within which a particular manufacturing process will produce a specified result (e.g., a functioning semiconductor device), and within which the process parameters of the lithography or patterning process are typically allowed to vary.

[0025]

[0024] By using (part of) the design layout to be patterned, the computer system CL can predict which resolution enhancement techniques should be used and can perform computational lithography simulations and calculations to determine mask layouts and lithography apparatus settings that maximize the overall process window of the patterning process (shown in FIG. 3 by the double-headed arrow at a first scale SC1). Typically, the resolution enhancement techniques are configured to match the patterning capabilities of the lithography apparatus LA. The computer system CL can also be used to predict whether defects (e.g., due to sub-optimal processing) may be present by detecting where in the process window the lithography apparatus LA is currently operating (e.g., using input from the metrology tool MT) (shown in FIG. 3 by the arrow pointing to "0" at a second scale SC2).

[0026]

[0025] The metrology tool MT can provide input to the computer system CL that enables accurate simulation and prediction, and can provide feedback to the lithographic apparatus LA that identifies possible drifts (e.g., in the calibration status of the lithographic apparatus LA) (shown by multiple arrows at a third scale SC3 in Figure 3).

[0027]

[0026] In lithographic processes, it is desirable to frequently measure the structures produced (e.g., for process control and verification). Tools that perform such measurements are commonly called metrology tools MT. Various types of metrology tools MT that perform such measurements are known, such as scanning electron microscopes or various forms of scatterometer metrology tools MT. A scatterometer is a multipurpose instrument that allows measurements of parameters of the lithographic process to be made by having a sensor in the pupil or a conjugate plane to the pupil of the scatterometer objective (usually referred to as pupil-based measurements), or by having a sensor in the image plane or a conjugate plane to the image plane (in which case these measurements are usually referred to as image-based or field-based measurements). Such scatterometers and associated measurement techniques are described in detail in U.S. Patent Application Publication Nos. 20100328655, 2011102753A1, 20120044470A, 20110249244, 20110026032, or EP 1,628,164A, the entireties of which are incorporated by reference herein. The scatterometers described above can measure gratings using soft x-rays and light in the visible to near-infrared wavelength range.

[0028] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, a reconstruction method may be applied to the measurement signal to reconstruct or calculate the properties of the grating. Such a reconstruction may, for example, be obtained as a result of 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 that observed from the real target.

[0029]

[0028] 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 radiation reflected or scattered from the target is directed to a spectrometer detector, which measures the spectrum of the specularly reflected radiation (i.e., measures the intensity as a function of wavelength). From this data, it is possible to reconstruct the structure or profile of the target giving rise to the detected spectrum, for example by rigorous coupled-wave analysis and nonlinear regression, or by comparison with a library of simulated spectra.

[0030] In a third embodiment, the scatterometer MT is an ellipsometric scatterometer. An ellipsometric scatterometer makes it possible to determine parameters of a lithographic process by measuring scattered radiation for each of the polarization states. Such a metrology apparatus emits polarized light (e.g., linearly, circularly, or elliptically polarized light), for example using appropriate polarizing filters in the illumination section of the metrology apparatus. A radiation source suitable for the metrology apparatus may provide polarized radiation as well. Various embodiments of existing ellipsometric scatterometers are described in U.S. patent application Ser. Nos. 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,449, 12 / 920,968, 12 / 922,587, 13 / 000,229, 13 / 033,135, 13 / 533,110, and 13 / 891,410, which are incorporated by reference herein in their entireties.

[0031] In one embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the overlay of two misaligned grating or periodic structures by measuring the asymmetry of the reflectance spectra and / or the detection configuration, where the asymmetry is related to the degree of overlay. The two (typically overlapping) grating structures can be applied in two different layers (not necessarily consecutive layers) and formed at substantially the same location on the wafer. The scatterometer can have a symmetric detection configuration, such as that described in co-owned European Patent Application Publication No. 1,628,164A, so that any asymmetry can be clearly distinguished. This provides a simple method for measuring grating misalignment. Further examples for measuring the overlay error between two layers containing periodic structures when the target is measured through the asymmetry of the periodic structures can be found in PCT Patent Application Publication No. WO 2011 / 012624 or U.S. Patent Application Publication No. 20160161863, the entire contents of which are incorporated herein by reference.

[0032]

[0031] Other parameters of interest can be focus and dose. Focus and dose can be determined simultaneously by scatterometry (or alternatively by scanning electron microscopy), as described in U.S. Patent Application Publication No. 2011-0249244, the entire contents of which are incorporated herein by reference. A single structure can be used that has a unique combination of critical dimension and sidewall angle measurements for each point in the focus-energy matrix (FEM, also called focus-exposure matrix). If these unique combinations of critical dimension and sidewall angle are available, focus and dose values ​​can be uniquely determined from these measurements.

[0033]

[0032] A metrology target may be a collection of composite gratings, mostly formed by a lithography process in resist, but also formed after, for example, an etching process. Typically, the pitch and linewidth of the grating structures strongly depend on the measurement optics (especially the NA of the optics) so as to capture the diffraction orders obtained from the metrology target. As previously indicated, the diffraction signal may be used to determine the shift between two layers (also called "overlay") or to reconstruct at least a portion of the original grating produced by the lithography process. This reconstruction may be used to provide guidance on the quality of the lithography process or to control at least a portion of the lithography process. The target may have smaller subsections configured to mimic the dimensions of features of the design layout on the target. This subsection causes the target to behave more similarly to the features of the design layout, so that the overall process parameter measurements more closely resemble the features of the design layout. The target may be measured in underfill mode or overfill mode. In underfill mode, the measurement beam generates a spot smaller than the entire target. In overfill mode, the measurement beam generates a spot that is larger than the entire target, and in such overfill mode, it may be possible to simultaneously measure different targets, and therefore determine different process parameters therefrom.

[0034] The overall quality of a lithography parameter measurement using a particular target depends, at least in part, on the measurement recipe used to measure this 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 measured patterns, or both. For example, if the measurement used in the substrate measurement recipe is a diffraction-based optical measurement, one or more of the parameters of the measurement may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation to the substrate, the orientation of the radiation relative to the pattern on the substrate, etc. One criterion for selecting a measurement recipe can be, for example, the sensitivity of one of the measurement parameters to process variations. Further examples are described in U.S. Patent Application Publication No. 2016-0161863 and U.S. Patent Application Publication No. 2016 / 0370717A1, which are incorporated herein by reference in their entireties.

[0035] A metrology apparatus, such as a scatterometer, is shown in FIG. 4. The metrology apparatus comprises a broadband (white light) radiation projector 2 which projects radiation onto a substrate W. The reflected or scattered radiation is passed to a spectrometer detector 4, which measures the spectrum 10 of the specularly reflected radiation (i.e., a measurement of intensity as a function of wavelength). From this data, the structure or profile giving rise to the detected spectrum can be reconstructed by a processing unit (PU), for example by rigorous coupled-wave analysis and nonlinear regression, or by comparison with a library of simulated spectra as shown in the bottom part of FIG. 4. Typically, for reconstruction, the general form of the structure is known and some parameters are assumed from knowledge of the process by which the structure was created, thereby leaving only a few parameters of the structure to be determined from the scatterometry data. Such a scatterometer may be configured as a normal-incidence scatterometer or an oblique-incidence scatterometer.

[0036] The overall measurement quality of a lithography parameter via measurement of a metrology target depends, at least in part, on the measurement recipe used to measure this 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 measured patterns, or both. For example, if the measurement used in the substrate measurement recipe is a diffraction-based optical measurement, one or more of the parameters of the measurement may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation to the substrate, the orientation of the radiation relative to the pattern on the substrate, etc. One criterion for selecting a measurement recipe can be, for example, the sensitivity of one of the measurement parameters to process variations. Further examples are described in U.S. Patent Application Publication No. 2016 / 0161863 and U.S. Patent Application Publication No. 2016 / 0370717A1, which are incorporated herein by reference in their entireties.

[0037] Another type of metrology tool used in IC manufacturing is a topography measurement system, level sensor, or height sensor. Such a tool may be integrated into a lithography apparatus to measure the topography of the top surface of a substrate (or wafer). A map of the substrate's topography, also called a height map, may be generated from these measurements to show the height of the substrate as a function of position on the substrate. This height map may then be used to correct the position of the substrate during transfer of a pattern onto the substrate to provide an aerial image of the patterning device at the correct focus position on the substrate. In this context, it should be understood that "height" refers to the dimension that is generally out-of-plane with respect to the substrate (also called the Z-axis). Typically, a level or height sensor makes measurements at a fixed location (relative to its own optics), and relative movement between the substrate and the optics of the level or height sensor results in height measurements at each location on the substrate.

[0038] An example of a level or height sensor LS known in the art is shown schematically in FIG. 5, which only illustrates the principle of operation. In this example, the level sensor comprises an optical system, which comprises a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO, which provides a radiation beam LSB, which is provided by a projection grating PGR of the projection unit LSP. The radiation source LSO may be, for example, a narrowband or broadband light source, such as a polarized or unpolarized, pulsed or continuous (such as a polarized or unpolarized laser beam) supercontinuum light source. The radiation source LSO may include multiple radiation sources (such as multiple LEDs) with different colors or wavelength ranges. The radiation source LSO of the level sensor LS is not limited to visible radiation, but may additionally or alternatively encompass UV and / or IR radiation, as well as any wavelength range suitable for reflection from the surface of the substrate.

[0039]

[0038] The projection grating PGR is a periodic grating with a periodic structure that results in a radiation beam BE1 having a periodically varying intensity. The radiation beam BE1 having a periodically varying intensity is directed towards a measurement location MLO on the substrate W with an angle of incidence ANG relative to an axis normal to the substrate surface of incidence (Z-axis) of between 0 and 90 degrees, typically between 70 and 80 degrees. At the measurement location MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and directed towards a detection unit LSD.

[0040]

[0039] To determine the height level at the measurement location MLO, the level sensor further comprises a detection system comprising a detection grid DGR, a detector DET and a processing unit (not shown) for processing an output signal of the detector DET. The detection grid DGR may be the same as the projection grid PGR. The detector DET generates a detector output signal indicative of the received light, for example indicative of the intensity of the received light (e.g. a photodetector) or representing the spatial distribution of the received intensity (e.g. a camera). The detector DET may comprise any combination of one or more detector types.

[0041]

[0040] By means of triangulation techniques, the height level at the measurement location MLO can be determined. The detected height level is typically related to the signal intensity measured by the detector DET, which has a periodicity that depends, inter alia, on the design of the projection grating PGR and the (oblique) angle of incidence ANG.

[0042]

[0041] The projection unit LSP and / or the detection unit LSD may include further optical elements, such as lenses and / or mirrors, along the path of the patterned radiation beam between the projection grating PGR and the detection grating DGR (not shown).

[0043] In one embodiment, the detection grid DGR may be omitted and the detector DET may be placed in the position where the detection grid DGR was located, such a configuration providing a more direct detection of the image of the projection grid PGR.

[0044]

[0043] In order to effectively cover the surface of the substrate W, the level sensor LS may be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating an array of measurement areas MLO or spots covering a wider measurement range.

[0045]

[0044] Various height sensors of the general type are disclosed, for example, in U.S. Patent No. 7,265,364 and U.S. Patent No. 7,646,471, both of which are incorporated by reference. A height sensor that uses UV radiation instead of visible or infrared radiation is disclosed in U.S. Patent Application Publication No. 2010233600A1, which is incorporated by reference. WO 2016102127A1, which is incorporated by reference, describes a compact height sensor that uses a multi-element detector to detect and recognize the position of a grid image without the need for a detection grid.

[0046] Another type of metrology tool used in IC manufacturing is an alignment sensor. Therefore, an important aspect of the performance of a lithographic apparatus is its ability to correctly and accurately align an applied pattern with respect to features deposited on a previous layer (by the same or a different lithographic apparatus). For this purpose, the substrate is provided with one or more sets of marks or targets. Each mark is a structure whose position can be subsequently measured using a position sensor, typically an optical position sensor. The position sensor may be called an "alignment sensor" and the marks may be called "alignment marks".

[0047] A lithographic apparatus may include one or more (e.g., multiple) alignment sensors capable of accurately measuring the position of alignment marks provided on a substrate. Alignment (or position) sensors may use optical phenomena such as diffraction and interference to obtain position information from alignment marks formed on the substrate. An example of an alignment sensor used in conventional lithographic apparatus is based on a self-referencing interferometer, such as that described in U.S. Patent No. 6,961,116. Various enhancements and modifications of position sensors have been developed, for example, as disclosed in U.S. Patent Application Publication No. 2015261097A1. The entire contents of these publications are incorporated herein by reference.

[0048] 6 is a schematic block diagram of an embodiment of a known alignment sensor AS, for example as described in US Pat. No. 6,961,116, which is incorporated by reference. A radiation source RSO provides a radiation beam RB of one or more wavelengths, which is redirected by redirecting optics as an illumination spot SP onto a mark, such as a mark AM arranged on a substrate W. In this example, the redirecting optics comprises a spot mirror SM and an objective lens OL. The illumination spot SP that illuminates the mark AM may have a diameter that is slightly smaller than the width of the mark itself.

[0049]

[0048] Radiation diffracted by the alignment mark AM is collimated (in this example via an objective lens OL) into an information-bearing beam IB. The term "diffracted" is intended to include zeroth order diffraction (which may be called reflection) from the mark. For example, a self-referencing interferometer SRI of the type disclosed in the above-mentioned U.S. Pat. No. 6,961,116 causes the beam IB to interfere with itself, after which the beam is received by a photodetector PD. Additional optics (not shown) may be included to provide separate beams if more than one wavelength is generated by the radiation source RSO. The photodetector may be a single element or may include multiple pixels, as required. The photodetector may include a sensor array.

[0050]

[0049] The redirecting optical system, which in this example includes a spot mirror SM, can also function to block zero-order radiation reflected from the mark, so that the information-carrying beam IB contains only higher-order diffracted radiation from the mark AM (this is not essential for the measurement, but improves the signal-to-noise ratio).

[0051]

[0050] The intensity signal SI is fed to a processing unit PU. The combination of optical processing in block SRI and computational processing in unit PU outputs values ​​for the X and Y position on the substrate relative to a reference frame.

[0052]

[0051] A single measurement of the type shown only fixes the position of the mark within a certain range corresponding to one pitch of the mark. In conjunction with this, coarser measurement techniques are used to identify which period of the sine wave contains the marked position. For improved accuracy and / or for robust detection of marks regardless of the material they are made of and the material on which they are provided above and / or below, the same process can be repeated at coarser and / or finer levels with different wavelengths. Wavelengths may be optically multiplexed and demultiplexed to be processed simultaneously, and / or wavelengths may be multiplexed by time division or frequency division.

[0053] In this example, the alignment sensor and spot SP remain stationary, and it is the substrate W that moves. The alignment sensor can therefore be rigidly and accurately mounted to a reference frame, effectively scanning the mark AM in a direction opposite to the direction of movement of the substrate W. The substrate W is controlled in this movement by a substrate positioning system that controls its mounting on the substrate support and the movement of the substrate support. A substrate support position sensor (e.g., an interferometer) measures the position of the substrate support (not shown). In one embodiment, one or more (alignment) marks are provided on the substrate support. Measuring the positions of the marks provided on the substrate support allows the position of the substrate support, as determined by the position sensor, to be calibrated (e.g., relative to a frame to which the alignment system is connected). Measuring the positions of the alignment marks provided on the substrate allows the position of the substrate relative to the substrate support to be determined.

[0054]

[0053] Metrology tools MT, such as the scatterometers, topography measurement systems, or position measurement systems described above, may use radiation generated by a radiation source to perform measurements. The characteristics of the radiation used by the metrology tool may affect the type and quality of measurements that can be performed. For some applications, it may be advantageous to use multiple radiation frequencies to measure the substrate, for example, broadband radiation may be used. Multiple different frequencies may be capable of propagating, irradiating, and scattering from the metrology target with no or minimal interference with other frequencies. Thus, for example, different frequencies may be used to simultaneously obtain more metrology data. Different radiation frequencies may also be capable of interrogating or discovering different characteristics of the metrology target. Broadband radiation may be useful in metrology systems MT, for example, level sensors, alignment mark measurement systems, scatterometry tools, or inspection tools. The broadband radiation source may be a supercontinuum light source.

[0055]

[0054] High-quality broadband radiation (e.g., supercontinuum radiation) can be difficult to generate. One way to generate broadband radiation can be to broaden high-power narrowband or single-frequency input radiation or pump radiation, for example, by utilizing nonlinear and higher-order effects. 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 due to the broadening effect, the radiation can be confined within a small area so that strong, localized, high-intensity radiation is achieved. In those areas, the radiation can interact with broadening structures and / or materials that form a nonlinear medium to produce broadband output radiation. In high-intensity radiation areas, different materials and / or structures can be used to enable and / or improve radiation broadening by providing a suitable nonlinear medium.

[0056] In some embodiments, broadband output radiation is produced in a photonic crystal fiber (PCF). In some embodiments, such a photonic crystal fiber has a microstructure around its fiber core to assist in confining radiation traveling within the fiber core through the fiber. 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. While it is possible to generate broadband radiation in a solid-core photonic crystal fiber, the use of solid-state materials can have some drawbacks. For example, if UV radiation is generated in the solid core, this radiation may not be present in the output spectrum of the fiber because most of the radiation is absorbed by the solid material.

[0057] In some embodiments, as discussed further below with reference to FIG. 8 , methods and apparatus for broadening input radiation may use a fiber to confine the input radiation and to broaden the input radiation to output broadband radiation. The fiber may be a hollow-core fiber and may include an internal structure to achieve effective guiding and confinement of the radiation within the fiber. The fiber may be a hollow-core photonic crystal fiber (HC-PCF), which is particularly suitable for strong radiation confinement primarily within the hollow core of the fiber and can achieve high radiation intensity. The hollow core of the fiber may be filled with a gas that acts as a broadening medium to broaden the input radiation. Such a fiber and gas configuration may be used to create a supercontinuum radiation source. The radiation input to the fiber may be electromagnetic radiation, for example, radiation in one or more of the infrared, visible, UV, and extreme UV spectrums. The output radiation may consist of or include broadband radiation, which may be referred to herein as white light.

[0058] Some embodiments relate to a new design of such a broadband radiation source comprising an optical fiber. The optical fiber is a hollow-core photonic crystal fiber (HC-PCF). In particular, the optical fiber can be a type of hollow-core photonic crystal fiber with an anti-resonant structure for confining the radiation. Such fibers with an anti-resonant structure are known in the art as anti-resonant fibers, tubular fibers, single-ring fibers, negative curvature fibers, or inhibited coupling fibers. Various different designs of such fibers are known in the art. Alternatively, the optical fiber can be a photonic bandgap fiber (HC-PBF, e.g., Kagome fiber).

[0059]

[0058] Several types of HC-PCFs can be designed, each based on a different physical light-guiding mechanism. Two such HC-PCFs include hollow-core photonic bandgap fiber (HC-PBF) and hollow-core antiresonant reflecting fiber (HC-ARF). Details regarding the design and fabrication of HC-PCFs can be obtained from U.S. Patent Application Publication No. 2004 / 015085 A1 (for HC-PBF) and International PCT Patent Application Publication No. WO 2017 / 032454 A1 (for hollow-core antiresonant reflecting fiber), both of which are incorporated herein by reference. Figure 9(a) shows a kagome fiber with a kagome lattice structure.

[0060]

[0059] An example of an optical fiber for use in a radiation source will now be described with reference to Figure 7, which shows a schematic cross-section of an optical fiber OF in transverse cross-section. Further embodiments similar to the example fiber of Figure 7 are disclosed in WO 2017 / 032454 A1.

[0061] The optical fiber OF comprises an elongated body in which one dimension of the fiber OF is longer than the other two dimensions. This longer dimension may be referred to as the axial direction and may define the axis of the optical fiber OF. The other two dimensions define a plane that may be referred to as the transverse plane. Figure 7 shows a cross-section of the optical fiber OF in this transverse plane (i.e., a transverse plane perpendicular to the axis), which is labeled the xy plane. The transverse plane of the optical fiber OF may be substantially constant along the fiber axis.

[0062] It should be understood that the optical fiber OF has a degree of flexibility, and therefore the axial orientation will generally not be uniform along the length of the optical fiber OF. Terms such as optical axis, cross section, etc. should be understood to mean the local optical axis, local cross section, etc. Furthermore, where a component is described as being cylindrical or tubular, it should be understood that these terms encompass shapes that may be distorted when the optical fiber OF is bent.

[0063]

[0062] It should be understood that the optical fiber OF can have any length, and the length of the optical fiber OF can depend on the application. The optical fiber OF can have a length of 1 centimeter (cm) to 10 meters (m), for example, the optical fiber OF can have a length of 10 cm to 100 cm.

[0064]

[0063] The optical fiber OF comprises a hollow core HC, a cladding portion surrounding the hollow core HC, and a support portion SP surrounding and supporting the cladding portion. The optical fiber OF can be considered to comprise a body (comprising the cladding portion and the support portion SP) having the hollow core HC. The cladding portion includes several anti-resonant elements or capillaries CAP for guiding radiation through the hollow core HC. In particular, the multiple anti-resonant elements are arranged to confine radiation propagating through the optical fiber OF mainly within the hollow core HC and guide the radiation along the optical fiber OF. The hollow core HC of the optical fiber OF can be located substantially in a 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.

[0065]

[0064] The cladding portion comprises a plurality of anti-resonant elements for guiding radiation propagating through the optical fiber OF. In particular, in this embodiment, the cladding portion comprises a single ring of six tubular capillary caps, each of which acts as an anti-resonant element.

[0066]

[0065] The capillary CAP may also be referred to as a tube. The capillary CAP may have a circular cross section or another shape. Each capillary CAP comprises a substantially cylindrical wall portion WP, which at least partially defines the hollow core HC of the optical fiber OF and separates the hollow core HC from the capillary cavity CC. It should be understood that the wall portion WP may act as an anti-reflection Fabry-Perot resonator for radiation propagating through the hollow core HC (which may be incident on the wall portion WP at a grazing incidence angle). The thickness of the wall portion WP may be suitable to ensure that transmission into the capillary cavity CC is generally suppressed, while reflection back into the hollow core HC is generally enhanced.

[0067] It should be understood that, as used herein, the term cladding portion is intended to mean the portion of the optical fiber OF for guiding the radiation propagating through the optical fiber OF (i.e., the capillary CAP that confines the radiation within the hollow core HC). The radiation may be confined in the form of a transverse mode (propagating along the fiber axis).

[0068] 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 in the case of the inner support portion SP. The six capillary caps can be described as being arranged in a generally hexagonal configuration.

[0069]

[0068] The capillary CAPs are arranged so 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 adjacent capillary CAPs in a ring structure. Such an arrangement may be beneficial because it may increase the transmission bandwidth of the optical fiber OF (e.g., compared to an arrangement in which the capillaries are in contact with each other). Alternatively, in some embodiments, each of the capillary CAPs may contact adjacent capillary CAPs in a ring structure.

[0070]

[0069] The six capillary CAPs of the cladding portion are arranged in a ring structure around the hollow core HC. The inner surfaces of the ring structure of the capillary CAPs at least partially define the hollow core HC of the optical fiber OF. The diameter D of the hollow core HC (which may be defined as the smallest dimension between opposing capillaries and is indicated by arrow D) may be 10 to 1000 μm. The diameter D of the hollow core HC may affect the mode field diameter, collisional loss, dispersion, mode multiplicity, and nonlinearity characteristics of the hollow-core HC optical fiber OF.

[0071] Other dimensions shown are the pitch Λ of adjacent capillaries, the gap δ between adjacent capillaries, and the capillary inner radius r c , outer radius of capillary R c , capillary wall thickness t=R c -r c , the radius of the hollow region r j , and the radius of the fiber R j is.

[0072]

[0071] In this embodiment, the cladding portion comprises a single ring configuration of capillary CAPs (which act as anti-resonant elements), so that any radial line from the center of the hollow core HC to the outside of the optical fiber OF passes through only one capillary CAP.

[0073] It should be understood that other embodiments may be provided with different configurations of anti-resonant elements. These may include configurations with multiple rings of anti-resonant elements or configurations with nested anti-resonant elements. FIG. 9(a) shows one embodiment of an HC-PCF with three rings of capillary CAPs stacked radially. In this embodiment, each capillary CAP contacts other capillaries, both in the same ring and in different rings. Furthermore, while the embodiment shown in FIG. 7 includes one ring of six capillaries, in other embodiments, the cladding portion may be provided with one or more rings containing any number of anti-resonant elements (e.g., 4, 5, 6, 7, 8, 9, 10, 11, or 12 capillaries).

[0074]

[0073] Figure 9(b) shows a modified embodiment of the HC-PCF discussed above with a single ring of tubular capillaries. In the example of Figure 9(b), there are two concentric rings of tubular capillaries 21. To hold the inner and outer rings of tubular capillaries 21, the HC-PCF may be provided with a support tube ST. The support tube may be made of silica.

[0075]

[0074] The tubular capillaries of the examples of Figures 7 and 9(a), (b) may have a circular cross-sectional shape. Other shapes, such as oval or polygonal cross-sections, are also possible for the tubular capillaries. In addition, the solid material of the tubular capillaries of the examples of Figures 7 and 9(a), (b) may include a plastic material such as PMA, a glass such as silica, or soft glass.

[0076]

[0075] Figure 8 shows a radiation source RDS for providing broadband output radiation. The radiation source RDS comprises a pulsed pump radiation source PRS, or any other type of radiation source capable of generating short pulses of a desired length and energy level, an optical fiber OF having a hollow core HC (for example of the type shown in Figure 7), and a working medium WM (for example a gas) disposed within the hollow core HC. In Figure 8, the radiation source RDS comprises the optical fiber OF shown in Figure 7, but in alternative embodiments other types of hollow core HC optical fiber OF may be used.

[0077]

[0076] 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 to broaden the input radiation IRD to provide output radiation ORD. The working medium WM enables broadening the frequency range of the received input radiation IRD to provide broadband output radiation ORD.

[0078]

[0077] The radiation source RDS further includes a reservoir RSV. The optical fiber OF is disposed within 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) within the reservoir RSV. The reservoir RSV may include a first transparent window TW1. During use, the optical fiber OF is disposed within the reservoir RSV such that the first transparent window TW1 is located near the input end IE of the optical fiber OF. The first transparent window TW1 may form part of a wall of the reservoir RSV. The first transparent window TW1 may be at least transparent to the received input radiation frequency such that the received input radiation IRD (or at least a majority thereof) can be coupled to the optical fiber OF disposed within the reservoir RSV. It will be appreciated that optics (not shown) may be provided for coupling the input radiation IRD into the optical fiber OF.

[0079]

[0078] The reservoir RSV includes a second transparent window TW2 forming part of the wall of the reservoir RSV. In use, when the optical fiber OF is disposed within the reservoir RSV, the second transparent window TW2 is located near the output end OE of the optical fiber OF. The second transparent window TW2 may be transparent at least to the frequency of the broadband output radiation ORD of the device 120.

[0080] Alternatively, in another embodiment, both ends of the optical fiber OF may be disposed in different reservoirs. The optical fiber OF may comprise a first end section configured to receive input radiation IRD and a second end section for outputting broadband output radiation ORD. The first end section may be disposed in a first reservoir containing the working medium WM. The second end section may be disposed in a second reservoir, which may also contain the working medium WM. The reservoirs may function as described above with respect to FIG. 8. The first reservoir may comprise a first transparent window configured to be transparent to the input radiation IRD. The second reservoir may comprise a second transparent window configured to be transparent to the broadband output broadband radiation ORD. The first and second reservoirs may also comprise sealable openings for disposing the optical fiber OF partially inside and partially outside the reservoirs, thereby sealing the gas within the reservoirs. The optical fiber OF may further comprise a mid-section that is not housed within a reservoir. Such a configuration using two separate gas reservoirs may be particularly convenient for embodiments in which the optical fiber OF is relatively long (e.g., when the length is greater than 1 meter). It should be understood that in such a configuration, the two reservoirs (which may include one or more features known in the art for controlling, regulating, and / or monitoring the composition of the gases in the two reservoirs) are considered to provide a device for providing the working medium WM within the hollow core HC of the optical fiber OF.

[0081] In this regard, a window may be transparent to a frequency if at least 50%, 75%, 85%, 90%, 95%, or 99% of radiation incident on the window at that frequency is transmitted through the window.

[0082] Both the first and second transparent windows TW1 and TW2 may form an airtight seal within the wall of the reservoir RSV to contain the working medium WM (which may be a gas) 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.

[0083] The working medium WM may contain noble 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, nonlinear optical processes may include modulation instability (MI), soliton self-compression, soliton splitting, the Kerr effect, the Raman effect, and dispersive wave generation (DWG), details of which are described in International Publication No. 2018 / 127266A1 and U.S. Patent No. 9,160,137B1 (both of which are incorporated herein by reference). The dispersion of the filling gas can be adjusted by changing the pressure of the working medium WM in the reservoir RSR (i.e., the gas cell pressure), so that the generated broadband pulse dynamics and associated spectral broadening characteristics can be adjusted to optimize frequency conversion.

[0084] In one embodiment, the working medium WM may be disposed within the hollow core HC at least while the optical fiber OF is receiving input radiation IRD for generating the broadband output radiation ORD. It should be understood that the gas WM may be entirely or partially absent from the hollow core HC while the optical fiber OF is not receiving input radiation IRD for generating the broadband output radiation.

[0085]

[0084] To achieve frequency spreading, high-intensity radiation may be desired. An advantage of having a hollow-core HC optical fiber OF is that it allows high-intensity radiation to be achieved through strong spatial confinement of the radiation propagating through the optical fiber OF, allowing localized high-intensity radiation to be achieved. The radiation intensity within the optical fiber OF may be high, for example, due to the received high-intensity input radiation and / or the strong spatial confinement of the radiation within the optical fiber OF. An advantage of hollow-core optical fibers is that they guide radiation having a wider wavelength range than solid-core fibers, and in particular, hollow-core optical fibers can guide radiation in both the ultraviolet and infrared ranges.

[0086]

[0085] The advantage of using a hollow-core HC optical fiber OF may be that most of the radiation guided in the optical fiber OF is confined to the hollow core HC. Therefore, most of the interaction of the radiation in the optical fiber OF occurs with the working medium WM provided in the hollow core of the optical fiber OF. As a result, the spreading effect of the radiation by the working medium WM can be enhanced.

[0087]

[0086] The received input radiation IRD may be electromagnetic radiation. The input radiation IRD may be received as pulsed radiation. For example, the input radiation IRD may comprise an ultrafast pulse (e.g. generated by a laser).

[0088]

[0087] The input radiation IRD may be coherent radiation. The input radiation IRD may be collimated radiation, the advantage of which may be to facilitate and improve the efficiency of coupling the input radiation IRD into the optical fiber OF. The input radiation IRD may comprise a single frequency or a narrow range of frequencies. The input radiation IRD may be generated by a laser. Similarly, the output radiation ORD may be collimated and / or coherent.

[0089]

[0088] The broadband range of the output radiation ORD may be a continuous range, including a continuous range of radiation frequencies. The output radiation ORD may include supercontinuum radiation. Continuum radiation may be beneficial for use in some applications (e.g., metrology applications). For example, a continuous range of frequencies may be used to interrogate multiple properties. A continuous range of frequencies may be used, for example, to determine and / or eliminate frequency dependence of a measured property. The supercontinuum output radiation ORD may include electromagnetic radiation over a wavelength range of, for example, 100 nm to 4000 nm. The frequency range of the broadband output radiation ORD may be, for example, 400 nm to 900 nm, 500 nm to 900 nm, or 200 nm to 2000 nm. The supercontinuum output radiation ORD may include white light.

[0090]

[0089] The input radiation IRD provided by the pulsed pump radiation source PRS may be pulsed. The input radiation IRD may comprise electromagnetic radiation at one or more frequencies between 200 nm and 2 μm. The input radiation IRD may, for example, comprise electromagnetic radiation having a wavelength of 1.03 μm. The repetition rate of the pulsed radiation IRD may be on the order of magnitude of 1 kHz to 100 MHz. The pulse energy may have an order of magnitude of 0.1 μJ to 100 μJ (e.g., 1 to 10 μJ). The pulse length of the input radiation IRD may be on the order of magnitude of 10 fs to 10 ps (e.g., 300 fs). The average power of the input radiation IRD may be from 100 mW to several hundred W. The average power of the input radiation IRD may, for example, be 20 to 50 W.

[0091] The pulsed pump radiation source PRS may be a laser. The spatiotemporal transmission characteristics (e.g., its spectral amplitude and phase) of such laser pulses transmitted along the optical fiber OF can be varied and tuned through adjustment of (pump) laser parameters, operating component WM variations, and optical fiber OF parameters. The spatiotemporal transmission characteristics may include one or more of: output power, output mode profile, output temporal profile, width of the output temporal profile (or output pulse width), output spectral profile, and bandwidth of the output spectral profile (or output spectral bandwidth). The pulsed pump radiation source PRS parameters may include one or more of: pump wavelength, pump pulse energy, pump pulse width, and pump pulse repetition rate. The optical fiber OF parameters may include one or more of: length of the optical fiber, size and shape of the hollow core HC, size and shape of the capillary, and thickness of the wall of the capillary surrounding the hollow core HC. The operating component WM (e.g., filling gas) parameters may include one or more of: type of gas, pressure of the gas, and temperature of the gas.

[0092] The broadband output radiation ORD provided by the radiation source RDS may have an average output power of at least 1 W. The average output power may be at least 5 W. The average output power may be at least 10 W. The broadband output radiation ORD may be pulsed broadband output radiation ORD. The broadband output radiation ORD may have a power spectral density of at least 0.01 mW / nm across the wavelength band of the output radiation. The power spectral density of the broadband output radiation across the wavelength band may be at least 3 mW / nm.

[0093] FIG. 10 is a flow diagram of a method for manufacturing hollow-core PCFs (HC-PCFs) according to the concepts disclosed herein. In step 1000, initial raw materials are obtained. These may include, for example, glass tubes having lengths on the order of one meter or more. In step 1005, at least some of these tubes may be drawn into capillaries. This step is illustrated by diagram 1032, which shows glass tubes 1035 being heated by a heat source or furnace (e.g., a graphite resistance furnace) 1045, for example, to temperatures above 1000° C., above 1500° C., or about 2000° C., and drawn into capillaries 1040. Each tube 1035 may have a diameter, for example, greater than 1 cm (e.g., between 1 cm and 3 cm). The drawn capillaries 1040 may have diameters, for example, between 100 μm and 1500 μm, or between 100 μm and 1000 μm outer diameter.

[0094] In step 1010, a PCF preform can be assembled. According to an exemplary method (the so-called stack-and-draw method), drawn preform capillaries 1040 are stacked together with spacers to form a stack assembly. The stack assembly can be inserted into a larger jacket tube (e.g., having a diameter of several centimeters) to form a preform. The gap between the stack assembly and the jacket tube can be filled, for example, with silica rods of various outer diameters. The preform can have, for example, an outer diameter of 10 mm to 100 mm and a length of, for example, 0.3 m to 2 m (e.g., about 1 m).

[0095] 10 also shows a cross-sectional image 1065 of an exemplary preform. As shown, the exemplary preform is formed by inserting a stack assembly including seven (or other number) preform capillaries CAP-1 and other elements, such as spacers (not shown), into a jacket tube JT. In some cases, the spacers may be in the form of rods or pillars. In some cases, the spacers may be made of the same material as the preform capillaries.

[0096] According to another exemplary method, instead of inserting a stack of preform capillaries into the jacket tube JT, the preform capillaries CAP-1 are individually and sequentially inserted into the jacket tube JT. In this exemplary 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 seats at the bottom of the jacket tube JT. The preform capillary CAP-1 is then fixed to the inner wall of the jacket tube JT, for example, by fusing or melting. The jacket tube JT is then rotated by a predetermined 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 preform capillaries have been inserted into the jacket tube JT and fixed to its inner wall.

[0097] In step 1015, a cane is drawn from the preform. This step is illustrated by diagram 1038, which shows a preform 1050, including a capillary 1040, being drawn into a cane 1055. The preform 1050 may be held at the top of a conventional optical fiber draw tower (not shown), where the preform is fed downward into a furnace 1045 to heat the lower section of the preform. The heated region of the preform softens and stretches, creating a teardrop-like drop (also known as a drop-off) that pulls the fiber downward. Additional pulling and tension control are used to ensure the preform is drawn into a cane 1055. Typically, a 1-meter-long preform can be drawn into a 10-m to 1000-m-long cane. The long cane is then cut into shorter segments, each having a length of, for example, 0.5 m to 2 m (e.g., about 1 m).

[0098] Also shown is a cross-sectional view of an exemplary cane drawn via step 1015. The exemplary cane includes seven capillaries CAP-2 surrounding a hollow core 2 and supported by support portions SP-2. The exemplary cane appears more like the final HC-PCF (or target HC-PCF), but is rigid, and its physical dimensions (e.g., capillary dimensions, support portion diameter) are all larger than those of the final HC-PCF. At the end of step 1015, the cane may have an outer diameter of, for example, 0.5 mm to 10 mm, and the cane capillaries CAP-2 surrounding the cane's hollow core HC may each have an outer diameter of, for example, 100 μm to 1500 μm or 100 μm to 1000 μm, and a wall thickness of, for example, 1.5 μm to 100 μm.

[0099] A cane preparation step 1020 prepares the cane 1055 for the fiber drawing step 1025. This cane preparation step 1020 may include connecting pressure tubing to the cane capillaries 1040'. In this manner, the cane may be connected to a pressure supply that is used to pressurize the internal structure of the cane during the fiber drawing step 1025 to expand each of the capillaries in a desired and controlled manner.

[0100] The fiber drawing step 1025 is shown in FIG. 1042. In this step, the cane 1055 is placed in another draw tower (e.g., one appropriately equipped for drawing HC-PCF) and heated in a furnace 1045. The cane 1055 is drawn into the final fiber or HC-PCF 1060 so that the HC-PCF 1060 contains the target dimensions. Typically, a one-meter-long cane can be continuously drawn into hundreds of meters of fiber. In its final form, the outer diameter of the HC-PCF can be reduced, for example, to 100 μm to 300 μm. By carefully adjusting the relative pressures between different parts of the cane's internal structure (e.g., the capillary cavity CC and the hollow core HC), the cane capillaries CAP-2 expand, thus increasing their outer diameter relative to the inner diameter of the support portion SP-2. This effect is evident by comparing the two cross-sectional images 1070 and 1075, where the drawn 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 may be increased, for example, to 10 μm to 50 μm, and the thickness of the capillary wall portion WP may be correspondingly reduced, for example, to 0.05 μm to 0.2 μm. The positive pressure difference between each capillary cavity CC and the hollow core HC may be in the range of 1 millibar (mbar) to 1000 mbar.

[0101] [000100] Finally, step 1030 includes an optional respooling step, for example, where the drawn fiber 1060 is respooled from a larger drum onto a smaller spool.

[0102] [000101] As mentioned above, multiple capillaries or anti-resonant elements of an HC-PCF (e.g., a single ring configuration consisting of a glass capillary cap CAP as shown in Figure 7) are used to guide radiation propagating through the fiber. The light-guiding effect is established by anti-resonant interference (inverse Fabry-Perot effect) at the glass-air boundary and is strongest in the region of the capillary wall portion WP facing the core. In hollow-core negative-curvature fibers (e.g., HC-PCFs), the total fiber loss is affected by both modal confinement loss and material loss. Confinement loss is the loss resulting from the leakiness of the HC-PCF's modes and imperfect structure, while material loss results from the absorption of light in the fiber material. Importantly, the confinement loss of an HC-PCF is strongly affected by the thickness of the capillary wall portion WP. Specifically, the capillary wall thickness creates a resonance, at which light leaks out of the capillary cap. The resonance position λ is determined by the following equation: q can be estimated.

number

[0103] [000102] Figure 11 shows an example simulation of the confinement loss of an HC-PCF as a function of wavelength. As can be seen, λ q Such a sharp resonance feature results in a significant increase in confinement loss, for example, by more than three orders of magnitude, for wavelengths adjacent to the resonance.

[0104] [000103] Therefore, to obtain good performance from an HC-PCF-based light source, it is important to fabricate the HC-PCF with a capillary wall thickness t that is small enough so that the first-order resonance is outside the region of the spectrum where it is generated (e.g., outside 400-900 nm).

[0105] [000104] The nominal dimensions of several parameters at the preform level and / or cane level determine the process robustness and the nominal dimensions at the fiber level. Such dimensions include the capillary diameter parameter (i.e., capillary inner diameter ID = 2r c and capillary outer diameter OD=2R c ) and the capillary wall thickness t as described above. In particular, the inventors have found that an important quantity is the capillary diameter ratio Γ, i.e., the ratio of the inner diameter ID to the outer diameter OD of the capillary at the preform or cane level. The capillary diameter ratio Γ can be defined as follows:

number

[0106] [000105] For a generated spectrum up to 400 nm, the capillary wall thickness at the fiber level (i.e., the final fiber) according to equation [1] should be less than 200 nm, or more specifically less than 190 nm.

[0107] [000106] This thin wall thickness impacts the fabrication yield of the manufacturing process. When using raw capillaries with a capillary diameter ratio Γ≦0.9, the resulting fiber structure can deviate significantly from the desired structure, such as that shown in FIG. 7. For example, adjacent capillaries may touch, and / or one or more capillaries may have a distorted shape (e.g., a non-cylindrical cross-section, such as a hexagonal cross-section). Fibers with touching and / or distorted capillaries have undesirable optical properties (e.g., high loss).

[0108] [000107] Indeed, in many drawing attempts using capillaries at the cane level with a capillary diameter ratio Γ≦0.9, fiber structures with capillary wall thicknesses less than 200 nm have not been successfully realized. Therefore, a new manufacturing strategy is disclosed herein to address this very low or non-existent yield of fibers with capillary wall thicknesses less than 200 nm while maintaining a relatively large fiber OD of greater than 150 μm and a core diameter of D=about 30 μm.

[0109] [000108] Another important parameter in the manufacturing process that the inventors have identified is the draw force applied when drawing fiber from the cane. 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 draw furnace, where the local velocity is equal to the feed rate of the cane into the furnace; at the bottom of the furnace, the local velocity is equal to the applied draw rate of the fiber exiting 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 draw force or tension τ can be described (at least approximately) by:

number

[0110] [000109] In an actual fiber draw facility, the tension τ can be measured. This can be accomplished, for example, by using a force meter on the fiber tensioning system, located at the bottom of the draw line, or by analyzing the resonant vibration of the fiber at positions along the draw line. The returned value is often given in Newtons or (kilograms). However, such units are not always the most appropriate, especially when comparing preforms with different glass masses. A cane with an overall thicker jacket will have a lower viscosity during draw, even though the same nominal draw force (e.g., Newtons / gram) is measured; the force is simply distributed over a larger glass area.

[0111] [000110] It is therefore proposed to express tension not as force or mass (N or kg) but as force applied per glass area. This definition of tension is N / m 2 or has units of pascals.

[0112] [000111] However, two additional major drawbacks of this manufacturing method have become apparent: 1) Unacceptable production yields, with only one draw in seven resulting in a fiber with an internal structure within specifications. In the remaining attempts, the capillary remained either too small or was over-expanded. 2) The outer diameter of the drawn fiber was too small, e.g., less than 150 μm (e.g., in the 110 μm region), resulting in significantly reduced optical performance (due to the effects of stresses that deform the microstructure). Attempts to draw fibers with sufficiently large outer diameters (e.g., greater than 150 μm, 160 μm, or 170 μm) and thin walls were unsuccessful.

[0113] [000112] Therefore, existing fabrication processes are unable to produce the desired fibers (eg, thin wall, large OD) for metrology applications in any reasonable yield.

[0114] [000113] The inventors have identified two specific concepts for improving yield, which can be implemented individually or together. · Fabrication of canes (or preforms) having capillaries with capillary diameter ratios of Γ>0.9, Γ>0.93, Γ>0.95, or Γ>0.965. Drawing cane into fiber at high tension values ​​of τ>20MPa, τ>30MPa, τ>50MPa, τ>100MPa, τ>130MPa, τ>150MPa, τ>170MPa, τ>185MPa, τ>200MPa, τ>230MPa, or τ>250MPa.

[0115] Maintain a capillary diameter ratio of at least 0.9 [000114] The inventors have discovered that the primary root cause of capillary contact is high process sensitivity resulting from small structural tolerances of the capillaries at the cane level. These tolerances (although nominally small) are large enough to cause instability in the expansion rate during the drawing process.

[0116] [000115] Local capillary inner radius r c (z) and / or outer radius R c The change in (z) (i.e., the expansion rate) can be calculated by solving the ordinary differential equation [4].

number

[0117] [000116] The parameter z is the axial distance along the furnace axis. The local variation is an interaction between the local draw speed w, the glass viscosity μ (which depends on the furnace temperature), the surface tension γ, the nominal capillary dimensions (r and R), and the applied pressure difference Δp between the capillary and the core region. Typically, the expansion rate is exponentially dependent on the pressure and is larger the higher the desired value of the capillary diameter ratio ID / OD at the fiber level.

[0118] [000117] Figures 12 and 13 show how the parameter variations depend on the capillary diameter ratio at the cane level (note that essentially the same relationship is seen when plotting the capillary diameter ratio at the preform level). Figure 12 shows the capillary OD variation or relative capillary OD (%ROD from nominal) at the cane level for four different values ​​of Γ∈[0.85, 0.90, 0.95, 0.98]. CL (simulated) capillary outer diameter OD at fiber level as a function of FL Figure 13 shows a similar plot, but on the x-axis is the capillary wall thickness variation at cane level or relative capillary wall thickness (% Rt from nominal value). CLIn each of Figures 12 and 13, the horizontal grey band highlights the region of allowable capillary diameter variation at the fiber level for acceptable source performance.

[0119] [000118] These plots show that as the capillary diameter ratio Γ increases (assuming the ID / OD definition), the sensitivity to cane-level parameter variations (specifically, capillary diameter and capillary wall thickness) improves (i.e., decreases). A capillary diameter ratio Γ of 0.90 shows good sensitivity performance, while values ​​of 0.95 and 0.98 still show good performance (although the improvement from 0.95 to 0.98 is relatively small).

[0120] [000119] Thus, disclosed is a fiber manufacturing intermediate (e.g., a cane) comprising an outer jacket and a plurality of capillaries, the capillaries having a mid-level (e.g., cane level) capillary diameter ratio of inner capillary diameter to outer capillary diameter greater than 0.90 (optionally greater than 0.93, greater than 0.95, greater than 0.965), and a nominal wall thickness of each capillary at the mid-level 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).

[0121] [000120] Additionally, the nominal outer diameter of each capillary at the intermediate level may be greater than 250 μm, greater than 260 μm, or greater than 270 μm.

[0122] [000121] The plurality of capillaries may be arranged in a ring configuration within the outer jacket, thereby defining a hollow core within the ring configuration. In this regard, it should be noted that "ring configuration" should not be understood to be limited solely to circular configurations, but also includes other shapes in which the capillaries surround the hollow core, such as hexagonal, pentagonal, or other polygonal configurations.

[0123] [000122] Also disclosed is a method for manufacturing an optical fiber, the method including obtaining the fiber manufacturing intermediate disclosed above, and drawing a fiber from the fiber manufacturing intermediate, wherein the drawing is done such that a fiber-level capillary wall thickness of each capillary in the drawn fiber is less than 200 nm (optionally less than 195 nm or less than 190 nm).

[0124] [000123] The draw can be done so that the outer diameter of the drawn fiber is greater than 150 μm (optionally greater than 160 μm, greater than 170 μm, or greater than 180 μm).

[0125] [000124] The inner core diameter at fiber level of the drawn 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 inner core diameter at fiber level of the drawn fiber can be less than 45 μm, less than 40 μm, less than 35 μm, or less than 32 μm.

[0126] [000125] As a specific example, a capillary in a cane or fiber manufacturing intermediate having a capillary diameter ratio of 0.95 (e.g., 0.94-0.96) may have a nominal cane-level capillary outer diameter of 280 μm (e.g., 250 μm-300 μm) and a nominal cane-level wall thickness of 7400 nm (e.g., 5000 nm-9000 nm). A capillary in a cane or fiber manufacturing intermediate having a capillary diameter ratio of 0.98 (e.g., 0.97-0.99) may have a nominal cane-level capillary outer diameter of 332 μm (e.g., 300 μm-360 μm) and a nominal cane-level wall thickness of 3300 nm (e.g., 2000 nm-4000 nm).

[0127] Drawing cane at high tensile strength [000126] Figure 14 is a plot of the capillary inner diameter at the fiber level for different tensions (285 MPa, 215 MPa, 140 MPa, and 127 MPa) applied to the cane versus relative capillary pressure (compared to the nominal value) or capillary pressure variation. It can be seen that increasing pressure increases the capillary size. The graph shows that for higher nominal tensions, there is less sensitivity, indicating increased process robustness.

[0128] [000127] Applying high tensions appears relatively simple: by lowering the furnace temperature setting, the viscosity of the glass increases, resulting in higher tensions (see equation [3]). However, the inventors have found that this results in an increased incidence of fiber breakage.

[0129] [000128] In practice, finding the right tension requires a careful balance between a tension high enough to have reasonably high process robustness, and a tension low enough to minimize fiber breakage.

[0130] [000129] Thus, disclosed is a method for manufacturing an optical fiber, the optical fiber comprising an outer jacket and a plurality of capillaries, the method including obtaining a fiber manufacturing intermediate and drawing a fiber from the fiber manufacturing intermediate, wherein the drawing is performed such that a fiber level capillary wall thickness of each capillary in the drawn fiber is less than 200 nm (optionally less than 195 nm or less than 190 nm), and wherein a tension applied during 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).

[0131] [000130] The draw can be done so that the outer diameter of the drawn fiber is greater than 150 μm (optionally greater than 160 μm, greater than 170 μm, or greater than 180 μm).

[0132] [000131] The outer jacket and the capillaries may comprise materials of nominally the same composition (i.e., glass of nominally the same purity such that any differences in material are not intended). This may also help prevent fiber breakage. The outer jacket may comprise a first jacket to which the capillaries are fused and a second jacket into which the first jacket and capillaries are sleeved before being drawn into fiber. All of these components may comprise materials of nominally the same composition.

[0133] [000132] The method may further include reducing particles in a furnace used in the drawing step before performing the draw. This may include performing the steps of disassembling the furnace, cleaning the disassembled parts of the furnace, and reassembling the furnace before drawing. The method may further include operating the furnace at an elevated temperature, for example, greater than 1500°C or greater than 2000°C, between reassembly and drawing. Such steps may help prevent fiber breakage.

[0134] [000133] The fiber manufacturing intermediate may be any of those described in the first embodiment.

[0135] 15 shows a lithography system comprising a radiation source SO and a lithographic apparatus LA. The source SO is configured to generate a beam of EUV radiation B and to provide the beam of EUV radiation B to the lithographic apparatus LA. The lithographic apparatus LA comprises 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.

[0136] [000135] The illumination system IL is configured to condition the EUV radiation beam B before it is incident on the patterning device MA. Thereby, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The facetted field mirror device 10 and the facetted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to or instead of the facetted field mirror device 10 and the facetted pupil mirror device 11.

[0137] [000136] Having been so conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B' is generated. The projection system PS is configured to project the patterned EUV radiation beam B' onto the substrate W. To that end, the projection system PS may include a number of mirrors 13, 14 configured to project the patterned EUV radiation beam B' onto the substrate W held by a substrate table WT. The projection system PS may apply a demagnification factor to the patterned EUV radiation beam B' to form an image having smaller features than corresponding features on the patterning device MA. For example, a demagnification factor of 4 or 8 may be applied. Although in Figure 1 the projection system PS is shown as having only two mirrors 13, 14, the projection system PS may also include a different number of mirrors (for example 6 or 8 mirrors).

[0138] [000137] The substrate W may include a previously formed pattern. If the substrate W includes a previously formed pattern, the lithographic apparatus LA aligns the image formed by the patterned EUV radiation beam B' with the previously formed pattern on the substrate W.

[0139] [000138] The source SO in the illumination system IL and / or projection system PS may be supplied with a small amount of gas (eg hydrogen) at a relative vacuum, that is to say at a pressure well below atmospheric pressure.

[0140] [000139] The source SO may be a laser-produced plasma (LPP) source, a discharge-produced plasma (DPP) source, a free-electron laser (FEL) or any other source capable of generating EUV radiation.

[0141] [000140] Further embodiments are disclosed in the following list of numbered clauses: Article 1. An outer jacket and Multiple capillaries and A fiber manufacturing intermediate product comprising: A fiber manufacturing intermediate product, wherein the capillaries comprise a mid-level capillary diameter ratio of capillary inner diameter to capillary outer diameter greater than 0.90, and wherein the nominal wall thickness of each capillary at the mid-level is greater than 1500 nm. Clause 2. The fiber manufacturing intermediate of clause 1, wherein the capillary diameter ratio is greater than 0.93. Clause 3. The fiber manufacturing intermediate of clause 1, wherein the capillary diameter ratio is greater than 0.95. Clause 4. The fiber manufacturing intermediate of clause 1, wherein the capillary diameter ratio is greater than 0.965. Clause 5. A fiber manufacturing intermediate product according to any one of the preceding clauses, wherein the nominal outer diameter of each capillary at the intermediate level is greater than 250 μm. Clause 6. A fiber manufacturing intermediate product according to any one of the preceding clauses, wherein the nominal outer diameter of each capillary at the intermediate level is greater than 260 μm. Clause 7. A fiber manufacturing intermediate product according to any one of the preceding clauses, wherein the nominal outer diameter of each capillary at the intermediate level is greater than 270 μm. Clause 8. The fiber manufacturing intermediate product according to clause 1, wherein the capillary diameter ratio is 0.94 to 0.96, and the nominal outer diameter of each capillary at the intermediate level is 250 μm to 300 μm. Clause 9. A fiber manufacturing intermediate product according to any one of the preceding clauses, wherein the nominal wall thickness of each capillary at the intermediate level is greater than 2000 nm. Clause 10. A fiber manufacturing intermediate product according to any one of the preceding clauses, wherein the nominal wall thickness of each capillary at the intermediate level is greater than 3000 nm. Clause 11. A fiber manufacturing intermediate product according to any one of the preceding clauses, wherein the nominal wall thickness of each capillary at the intermediate level is greater than 5000 nm. Clause 12. A fiber manufacturing intermediate product according to any one of the preceding clauses, wherein the nominal wall thickness of each capillary at the intermediate level is greater than 6000 nm. Clause 13. A fiber manufacturing intermediate product according to any one of the preceding clauses, wherein the nominal wall thickness of each capillary at the intermediate level is greater than 7000 nm. Clause 14. A fiber manufacturing intermediate according to any one of the preceding clauses, wherein a plurality of capillaries are arranged in a ring configuration within the outer jacket, thereby defining a hollow core within the ring configuration. Clause 15. A fiber manufacturing intermediate product described in any one of the preceding clauses, wherein the compositions of the outer jacket and the plurality of capillaries each comprise materials of nominally the same composition, and the outer jacket includes a first jacket into which the capillaries are fused and a second jacket into which the first jacket and the capillaries are sleeved. Article 16. Obtaining a fiber manufacturing intermediate product according to any one of the preceding clauses; drawing a fiber from the fiber manufacturing intermediate product, wherein the drawing is performed such that the capillary wall thickness at fiber level of each capillary in the drawn fiber is less than 200 nm; A method for manufacturing an optical fiber, comprising: Clause 17. The method of clause 16, wherein the drawing is performed such that the capillary wall thickness at fiber level of each capillary in the drawn fiber is less than 195 nm. Clause 18. The method of clause 16, wherein the drawing is performed such that the capillary wall thickness at fiber level of each capillary in the drawn fiber is less than 190 nm. Clause 19. The method of any one of clauses 16 to 18, wherein the drawing is performed such that the outer diameter of the drawn fiber at fiber level is greater than 150 μm. Clause 20. The method of any one of clauses 16 to 18, wherein the drawing is performed such that the outer diameter of the drawn fiber at fiber level is greater than 160 μm. Clause 21. The method of any one of clauses 16 to 18, wherein the drawing is performed such that the outer diameter of the drawn fiber at fiber level is greater than 170 μm. Clause 22. The method of any one of clauses 16 to 21, wherein the drawing is performed such that the inner core diameter at fiber level of the drawn fiber is greater than 20 μm. Clause 23. The method of any one of clauses 16 to 21, wherein the drawing is performed such that the inner core diameter at fiber level of the drawn fiber is greater than 25 μm. Clause 24. The method of any one of clauses 16 to 21, wherein the drawing is performed such that the inner core diameter at fiber level of the drawn fiber is greater than 28 μm. Clause 25. The method of any one of clauses 16 to 24, wherein the drawing is performed such that the inner core diameter at fiber level of the drawn fiber is less than 40 μm. Clause 26. The method of any one of clauses 16 to 24, wherein the drawing is performed such that the inner core diameter at fiber level of the drawn fiber is less than 35 μm. Clause 27. The method of any one of clauses 16 to 24, wherein the drawing is performed such that the inner core diameter at fiber level of the drawn fiber is less than 32 μm. Clause 28. The method of any one of clauses 16 to 27, wherein the tension applied to the fiber manufacturing intermediate product during drawing is greater than 20 MPa. Clause 29. The method of any one of clauses 16 to 27, wherein the tension applied to the fiber manufacturing intermediate product during drawing is greater than 50 MPa. Clause 30. The method of any one of clauses 16 to 27, wherein the tension applied to the fiber manufacturing intermediate product during drawing is greater than 100 MPa. Clause 31. The method of any one of clauses 16 to 27, wherein the tension applied to the fiber manufacturing intermediate product during drawing is greater than 130 MPa. Clause 32. The method of any one of clauses 16 to 27, wherein the tension applied to the fiber manufacturing intermediate product during drawing is greater than 150 MPa. Clause 33. The method of any one of clauses 16 to 27, wherein the tension applied to the fiber manufacturing intermediate product during drawing is greater than 170 MPa. Clause 34. The method of any one of clauses 16 to 27, wherein the tension applied to the fiber manufacturing intermediate product during drawing is greater than 200 MPa. Clause 35. The method of any one of clauses 16-34, wherein the method further comprises reducing particles in a furnace used in the drawing step before performing the drawing. Article 36. The reduction of particles is carried out before the drawing of the line. disassembling the furnace; cleaning the disassembled parts of the furnace; Steps to reassemble the furnace 36. The method of claim 35, comprising: Clause 37. The method of clause 36, wherein the method further comprises operating the furnace at a temperature greater than 2000°C between reassembly and drawing. Clause 38. An optical fiber obtained by carrying out the method according to any one of clauses 16 to 37. Clause 39. The optical fiber of clause 38, wherein the optical fiber comprises a hollow core photonic crystal fiber or a microstructured optical fiber. Clause 40. A radiation source comprising an optical fiber according to clause 38 or 39. Clause 41. A metrology device comprising a radiation source according to clause 40. Clause 42. The metrology device of clause 41, wherein the metrology device comprises one of a scatterometry tool, a leveling tool, or an alignment tool.

[0142] [000141] It should be noted that the term "photonic crystal fiber" as used herein includes and encompasses, among other things, any hollow-core fiber or microstructured optical fiber, including those having only a single ring of capillary within the cladding.

[0143] [000142] Although specific reference may have been made in this text to the use of lithographic apparatus in the manufacture of ICs, it will be appreciated that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optics, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0144] [000143] While specific reference may have been made herein to embodiments of the invention in relation to lithography apparatus, embodiments of the invention may also be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes objects such as wafers (or other substrates) or masks (or other patterning devices). These apparatus may collectively be referred to as lithography tools. Such lithography tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0145] [000144] While specific reference may have been made up to this point to the use of embodiments of the invention in connection with optical lithography, it should be understood that the invention is not limited to optical lithography and may be used in other applications, for example in imprint lithography, where the context permits.

[0146] [000145] While specific embodiments of the invention have been described above, it will be understood that the invention may be practiced otherwise than as described. The foregoing description is intended to be illustrative and not limiting. Thus, it will be apparent to those skilled in the art that modifications can be made to the invention as described without departing from the scope of the claims set forth below.

Claims

1. Obtaining a fiber manufacturing intermediate product comprising: i) an outer jacket; and ii) a plurality of capillaries, the capillaries comprising a mid-level capillary diameter ratio of an inner diameter of a capillary to an outer diameter of a capillary of greater than 0.90, and a nominal wall thickness of each of the capillaries at the mid-level being greater than 1500 nm; drawing a fiber from the fiber manufacturing intermediate product, the drawing being performed such that a fiber-level capillary wall thickness of each capillary in the drawn fiber is less than 200 nm; A method for manufacturing an optical fiber, comprising:

2. The method of claim 1 , wherein the drawing is performed such that the outer diameter of the drawn fiber at fiber level is greater than 150 μm.

3. 3. The method of claim 1 or 2, wherein the drawing is performed such that the drawn fiber has an inner core diameter at fiber level of greater than 20 μm.

4. 4. The method of claim 1, wherein the drawing is performed such that the drawn fiber has an inner core diameter at fiber level of less than 40 μm.

5. The method according to any one of claims 1 to 4, wherein the tension applied to the fiber manufacturing intermediate product 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. said reducing particles before said drawing; disassembling the furnace; cleaning the disassembled parts of the furnace; reassembling the furnace; The method of claim 6, comprising performing:

8. 8. The method of claim 7, wherein the method further comprises operating the furnace at a temperature greater than 2000°C between the reassembly and the drawing.

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

10. A radiation source comprising the optical fiber of claim 9.

11. A metrology device comprising the radiation source of claim 10.

12. An outer jacket and Multiple capillaries and A fiber manufacturing intermediate product comprising: A fiber manufacturing intermediate, wherein the capillaries comprise a mid-level capillary diameter ratio of capillary inner diameter to capillary outer diameter greater than 0.90, and a nominal wall thickness of each of the capillaries at the mid-level greater than 1500 nm.

13. The fiber manufacturing intermediate of claim 12 , wherein the capillary diameter ratio is greater than 0.

93.

14. 14. A fiber manufacturing intermediate product according to claim 12 or 13, wherein the nominal outer diameter of each of the capillaries at the intermediate level is greater than 250 μm.

15. 13. The fiber manufacturing intermediate product of claim 12, wherein the capillary diameter ratio is between 0.94 and 0.96, and the nominal outer diameter of each capillary at the intermediate level is between 250 μm and 300 μm.

Images