Hollow core photonic crystal fiber-based broadband radiation source device, metronome device, and broadband radiation generation method

Abstract

1. A broadband radiation source device comprising a fiber assembly including a plurality of optical fibers, each optical fiber filled with a gaseous medium, the broadband radiation source device operable such that a subset of the optical fibers are individually selectable for receiving a beam of input radiation so as to generate broadband output from only one subset of the plurality of optical fibers at any one time.

Description

[Technical Field]

[0001] Cross-reference of related applications

[0001] This application claims priority to European Patent Application No. 20194353.7 filed on 3 September 2020, European Patent Application No. 20202720.7 filed on 20 October 2020, and European Patent Application No. 21169105.0 filed on 19 April 2021, which are incorporated herein by reference in their entirety.

[0002]

[0001] The present invention relates to a hollow core photonic crystal fiber-based broadband radiation generator, and more particularly to such a broadband radiation generator related to metrologic applications in the manufacture of integrated circuits. [Background technology]

[0003]

[0002] A lithography apparatus is a machine built to apply a desired pattern to a substrate. Lithography apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithography apparatus can project a pattern (often called a "design layout" or "design") located on a patterning device (e.g., a mask) onto a radiative-sensitive material (resist) layer provided on a substrate (e.g., a wafer).

[0004]

[0003] A lithography apparatus may use electromagnetic radiation to project a pattern onto a substrate. The wavelength of this radiation determines the minimum size of the feature 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. A lithography apparatus using extreme ultraviolet (EUV) radiation with a wavelength in the range of 4 to 100 nm, for example 6.7 nm or 13.5 nm, can form smaller features on the substrate than a lithography apparatus using radiation with a wavelength of 193 nm, for example.

[0005]

[0004] Low k1 lithography can be used to process features having dimensions smaller than the classical resolution limit of a lithography apparatus. In such a process, the resolution formula can be expressed as CD = k1 × λ / NA, where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection optical system of the lithography apparatus, CD is the "critical dimension" (generally the smallest feature size to be printed, but in this case half-pitch), and k1 is the empirical resolution factor. Generally, the smaller k1, the more difficult it becomes to copy a pattern onto the substrate that resembles the shape and dimensions planned by the circuit designer to achieve a particular electrical functionality and performance. To overcome such difficulties, advanced fine-tuning steps can be applied to the lithography projection apparatus and / or design layout. Such steps include, but are not limited to, optimizing the numerical aperture (NA), customizing the illumination scheme, using phase-shift patterning equipment, various optimizations of the design layout, such as optical proximity correction (OPC, sometimes called "optical and process correction") or other methods generally defined as "resolution enhancement techniques" (RET) in the design layout. Alternatively, to improve pattern copying at low k1, a strict control loop may be used to manage the stability of the lithography equipment.

[0006]

[0005] In many aspects of IC manufacturing processes, metronome tools are used, for example, as alignment tools for proper positioning of substrates before exposure, as leveling tools for measuring the surface topology of substrates for focus control, and as scantometry-based tools for inspecting / measuring exposed and / or etched products in process control. In each case, a radiation source is required. For various reasons, including measurement robustness and accuracy, broadband or white light radiation sources are increasingly used for such metronome applications. It is desirable to improve current devices for broadband radiation generation. [Overview of the Initiative]

[0007]

[0006] In a first aspect of the present invention, a broadband radiation source device is provided comprising a fiber assembly including a plurality of optical fibers, each optical fiber being filled with a gaseous medium, wherein a subset of the optical fibers is operable to be individually selected as the target for receiving a beam of input radiation so that at any given time a broadband output is generated from only one subset of the plurality of optical fibers. For example, a suitable subset of optical fibers is individually selected to generate a broadband output from only one subset of the plurality of optical fibers.

[0008]

[0007] A second aspect of the present invention provides a method for generating broadband radiation, comprising: emitting input radiation from a pump source; receiving input radiation through a selected subset of optical fibers; and generating broadband output from the selected subset of optical fibers.

[0009]

[0008] Another aspect of the present invention includes a metrology device comprising a broadband light source device of the first aspect.

[0010]

[0009] Hereinafter, embodiments of the present invention will be described with reference to the attached schematic drawings, merely as examples. [Brief explanation of the drawing]

[0011] [Figure 1]

[0009] A general overview of the lithography apparatus is shown. [Figure 2]

[0009] A general overview of the lithography cell is shown. [Figure 3]

[0009] A schematic diagram of holistic lithography is shown, illustrating the coordination between three key technologies for optimizing semiconductor manufacturing. [Figure 4]

[0009] A schematic diagram of a scatrometry device used as a metrologic device, which may include a radiation source according to an embodiment of the present invention, is shown. [Figure 5]

[0009] Schematic diagram of a level sensor device that may include a radiation source according to an embodiment of the present invention. [Figure 6]

[0009] Schematic diagram of an alignment sensor device that may include a radiation source according to an embodiment of the present invention. [Figure 7]

[0009] Schematic cross-sectional view of a hollow-core optical fiber that may form part of a radiation source in a cross-section (i.e., a plane perpendicular to the axis of the optical fiber). [Figure 8]

[0009] Schematic diagram of a radiation source for providing broadband output radiation according to an embodiment. [Figure 9a]

[0009] Schematically shows a cross-section of an example of a hollow-core photonic crystal fiber (HC-PCF) design for supercontinuum generation. [Figure 9b]

[0009] Schematically shows a cross-section of an example of a hollow-core photonic crystal fiber (HC-PCF) design for supercontinuum generation. [Figure 10a]

[0009] Schematically shows a rectangular fiber assembly including a two-dimensional (2D) array of optical fibers (e.g., single-ring HC-PCF) according to an embodiment. [Figure 10b]

[0009] Schematically shows a circular fiber assembly including a two-dimensional (2D) array of optical fibers (e.g., single-ring HC-PCF) according to an embodiment. [Figure 11a]

[0009] Shows a 1D fiber assembly using individual single-core optical fibers according to an embodiment. [Figure 11b]

[0009] Shows a 2D fiber assembly using individual single-core optical fibers according to an embodiment. [Figure 12]

[0009] Schematically shows a broadband radiation source and a first single-beam configuration including a fiber assembly according to an embodiment. [Figure 13]

[0009] Schematically shows a broadband radiation source and a multi-beam configuration including a fiber assembly according to an embodiment. [Figure 14]

[0009] A schematic diagram shows a broadband radiation source RDS3 including a fiber assembly according to an embodiment and a second single-beam configuration. [Figure 15]

[0009] A schematic diagram of the multiple fiber integration arrangement according to the embodiment is shown. [Figure 16a]

[0009] An exemplary fiber integrating arrangement for mechanically clamping optical fibers is shown. [Figure 16b]

[0009] Another exemplary fiber integrating arrangement for mechanically clamping optical fibers is shown. [Figure 17a]

[0009] An improved fiber optic arrangement according to the embodiment is shown. [Figure 17b]

[0009] Another improved fiber optic arrangement according to an embodiment is shown. [Figure 17c]

[0009] Further improved fiber integration arrangement according to the embodiment is shown. [Figure 18]

[0009] This is a schematic diagram showing the process of creating a fiber ensemble using heat-shrinkable tubing. [Figure 19a]

[0009] This is a schematic cross-sectional view of an exemplary fiber integrating arrangement manufactured using heat-shrinkable fiber tubes. [Figure 19b]

[0009] This is a schematic cross-sectional view of an exemplary fiber integrating arrangement manufactured using heat-shrinkable fiber tubes. [Figure 19c]

[0009] This is a schematic cross-sectional view of an exemplary fiber integrating arrangement manufactured using heat-shrinkable fiber tubes. [Figure 19d]

[0009] This is a schematic cross-sectional view of an exemplary fiber integrating arrangement manufactured using heat-shrinkable fiber tubes. [Figure 19e]

[0009] This is a schematic cross-sectional view of an exemplary fiber integrating arrangement manufactured using heat-shrinkable fiber tubes. [Figure 20]

[0009] A block diagram of a computer system for controlling a broadband radiation source is shown. [Modes for carrying out the invention]

[0012]

[0010] In this document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, which include ultraviolet (e.g., having wavelengths of 365, 248, 193, 157 or 126 nm) and EUV (e.g., extreme ultraviolet having wavelengths in the range of about 5 to 100 nm).

[0013] <Reticle>

[0011] As used herein, the terms “reticle,” “mask,” or “patterning device” may be broadly interpreted to mean a general patterning device that can be used to provide an incident radiation beam with a patterned cross section corresponding to a pattern to be created on a target portion of a substrate. In this context, the term “light bulb” may also be used. In addition to classic masks (transmissive or reflective masks, binary masks, phase-shift masks, hybrid masks, etc.), other examples of such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0014]

[0012] Figure 1 schematically shows a lithography apparatus LA. The lithography apparatus LA includes an illumination system IL (also called an illuminator) configured to adjust a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation), a mask support (e.g., a mask table) MT connected to a first positioner PM built to support a patterning device (e.g., a mask) MA and configured to precisely position the patterning device MA according to specific parameters, a substrate support (e.g., a wafer table) WT connected to a second positioner PW built to hold a substrate (e.g., a resist-coated wafer) W and configured to precisely position the substrate support according to specific parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project the pattern applied to the radiation beam B by the patterning device MA onto a target portion C of the substrate W (e.g., including one or more dies).

[0015]

[0013] During operation, the illumination system IL receives a radiant beam from the radiation source SO (for example, via the beam delivery system BD). The illumination system IL may include various types of optical components for induction, shaping and / or control of the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical components or any combination thereof. The illuminator IL may be used to adjust the radiant beam B so that the radiant beam B has a desired spatial intensity distribution and angular intensity distribution in its cross-section on the surface of the patterning device MA.

[0016]

[0014] The term “projection system” PS as used herein should be interpreted broadly to encompass various types of projection systems. Such systems may include refractive, reflective, reflective-refracting, anamorphic, magnetic, electromagnetic, and / or electrostatic-optical systems or any combination thereof, depending on the requirements of the exposure radiation used and / or other factors (e.g., the use of immersion liquid or vacuum). Where the term “projection lens” is used herein, they can all be considered synonymous with the more general term “projection system” PS.

[0017]

[0015] The lithography apparatus LA may be of a type in which at least a portion of the substrate is covered with a liquid (e.g., water) with a relatively high refractive index so as to fill the space between the projection system PS and the substrate W, which is also called immersion lithography. Details of the immersion technique are given in U.S. Patent No. 6,952,253, which is incorporated herein by reference.

[0018]

[0016] The lithography apparatus LA may be of a type having two or more substrate support units WT (also called a "dual-stage"). In such a "multi-stage" machine, the substrate support units WT may be used in parallel, and / or, while a substrate W on one of the substrate support units WT is being used to expose a pattern onto that substrate W, procedures for preparing another substrate W on the other substrate support unit WT for subsequent exposure of that other substrate W may be performed.

[0019]

[0017] In addition to the substrate support WT, the lithography apparatus LA may include a measurement stage. The measurement stage is configured to hold sensors and / or a cleaning device. The sensors may be configured to measure the characteristics of the projection system PS or the characteristics of the radiation beam B. The measurement stage may hold multiple sensors. The cleaning device may be configured to clean a part of the lithography apparatus, for example, a part of the projection system PS or a part of the system that supplies the immersion fluid. The measurement stage may move under the projection system PS when the substrate support WT is away from the projection system PS.

[0020]

[0018] During operation, the radiant beam B is incident on the patterning device (e.g., a mask MA held on a mask support MT) and patterned by the pattern (design layout) on the patterning device MA. After the radiant beam B crosses the mask MA, it passes through the projection system PS, which focuses the beam onto the target portion C of the substrate W. With the assistance of the second positioner PW and the position measurement system IF, the substrate support WT can move precisely, for example, so that various target portions C are positioned to be focused and aligned along the path of the radiant beam B. Similarly, a first positioner PM and possibly another position sensor (not shown in Figure 1) may be used to precisely position the patterning device MA relative to the path of the radiant beam B. The patterning device MA and the substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. The substrate alignment marks P1 and P2 occupy dedicated target areas as shown in the figure, but may also be placed in the space between target areas. When the substrate alignment marks P1 and P2 are placed between target areas C, they are called scribe line alignment marks.

[0021]

[0019] As shown in Figure 2, the lithography apparatus LA may be part of a lithography cell LC (sometimes called a lithocell or (litho)cluster), which often includes equipment for performing pre-exposure and post-exposure processes on the substrate W. Conventionally, such equipment includes a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a cooling plate CH and a bake plate BK (these, for example, regulate the temperature of the substrate W, which is done, for example, to regulate the solvent in the resist layer). A substrate handler (i.e., robot) RO picks up the substrates W from input / output ports I / O1 and I / O2, moves the substrates W between various process equipment, and delivers the substrates W to the loading bay LB of the lithography apparatus LA. The devices within a lithocell are often collectively referred to as tracks, and are typically under the control of a track control unit (TCU). The TCU itself may be controlled by a monitoring and control system (SCS), which in turn may control the lithography apparatus (LA) (for example, via a lithography control unit (LACU)).

[0022]

[0020] In order for the substrate W to be exposed by the lithography apparatus LA to be exposed accurately and reliably, it is desirable to inspect the substrate and measure the characteristics of the patterned structure, such as the overlay error between consecutive layers, line thickness, critical dimension (CD), etc. For this purpose, an inspection tool (not shown) may be included in the lithocell LC. If an error is detected, adjustments may be made, for example, to the exposure of the next substrate or to other process steps to be performed on the substrate W, which may be done especially if the inspection is performed before other substrates W of the same batch or lot are subsequently exposed or processed.

[0023]

[0021] Inspection equipment, sometimes called metrologic equipment, is used to measure the properties of a substrate W, in particular how the properties of different substrates W vary, or how the properties associated with different layers of the same substrate W vary from layer to layer. The inspection equipment may instead be constructed to identify defects on the substrate W, and may be, for example, part of a lithocell LC, or incorporated into a lithography apparatus LA, or be a standalone device. The inspection equipment may measure properties relating to a latent image (an image in the resist layer after exposure), or properties relating to a semi-latent image (an image in the resist layer after a post-exposure bake process PEB), or properties relating to a developed resist image (where the exposed or unexposed parts of the resist have been removed), or even properties relating to an etched image (after a pattern transfer process such as etching).

[0024]

[0022] Typically, the patterning process in a lithography apparatus LA is one of the most critical steps in the process, requiring high precision in dimensionalizing and positioning structures on a substrate W. To ensure this high precision, three systems can be combined as a so-called "holistic" control environment, as schematically shown in Figure 3. One of these systems is the lithography apparatus LA, which is (virtually) connected to the metrologic tool MT (second system) and the computer system CL (third system). The key to such a "holistic" environment is to optimize the coordination between these three systems to strengthen the entire process window and realize a strict control loop, thereby ensuring that the patterning performed by the lithography apparatus LA remains within the process window. The process window defines a range of process parameters (e.g., dose, focus, overlay) within which a particular manufacturing process produces a defined result (e.g., a functional semiconductor device), and typically, the process parameters of the lithography or patterning process can vary within this range.

[0025]

[0023] The computer system CL can predict which resolution enhancement techniques should be used by using (part of) the design layout to be patterned, and can perform computer lithography simulations and calculations to determine the mask layout and lithography apparatus settings that achieve maximization of the entire process window of the patterning process (indicated by the bidirectional arrows on the first scale SC1 in Figure 3). Typically, the resolution enhancement techniques are prepared to suit the patterning capabilities of the lithography apparatus LA. The computer system CL can further predict whether defects may exist (for example, for suboptimal processing) by detecting where in the process window the lithography apparatus LA is currently operating (for example, using input from the metrology tool MT) (indicated by the arrow pointing to "0" on the second scale SC2 in Figure 3).

[0026]

[0024] The Metrology Tool MT can provide input to the computer system CL that enables accurate simulation and prediction, and can provide feedback to the lithography apparatus LA that identifies possible drifts (for example, in the calibration status of the lithography apparatus LA) (indicated by multiple arrows on the third scale SC3 in Figure 3).

[0027]

[0025] In the lithography process, it is desirable to frequently measure the created structure (for example, for process control and verification). Tools used to perform such measurements are generally called metrologic tools (MT). Various types of metrologic tools (MT) are known to perform such measurements, such as scanning electron microscopes or various forms of scattrometer metrologic tools (MT). A scattrometer is a multi-purpose instrument that enables the measurement of parameters in the lithography process, and the measurements are performed by having a sensor on the pupil or conjugate plane of the pupil of the scattrometer's objective lens (usually called a pupil-based measurement), or by having a sensor on the image plane or conjugate plane of the image plane (in this case, usually called an image-based or field-based measurement). Such scatorometers and related measurement techniques are described in detail in U.S. Patent Applications Publications 20100328655, 2011102753A1, 20120044470A, 20110249244, 20110026032 or European Patent Application Publication 1,628,164A, which are incorporated herein by reference as a whole. The aforementioned scatorometers are capable of measuring gratings using soft X-rays and light in the visible to near-infrared wavelength range.

[0028]

[0026] In the first embodiment, the scatorometer MT is an angle-resolved scatorometer. In such a scatorometer, a reconstruction method that reconstructs or calculates the properties of the grating may be applied to the measured signal. Such reconstruction may be the result of, for example, simulating the interaction between scattered radiation and a mathematical model of the target structure and comparing the simulation results with the measured results. The parameters of the mathematical model are adjusted by the simulation of the interaction until a diffraction pattern similar to the diffraction pattern observed from the actual target is produced.

[0029]

[0027] In a second embodiment, the scatorometer MT is a spectrometer MT. In such a spectrometer MT, radiation emitted from a radiation source is directed toward a target, and radiation reflected or scattered from the target is directed toward a spectrometer detector, which measures the spectrum of the spectrometer-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 that is causing the detected spectrum, which can be done, for example, by exact coupled wave theory and nonlinear regression, or by comparison with a library of simulated spectra.

[0030]

[0028] In a third embodiment, the scatometer MT is an ellipsoscalometer. The ellipsoscalometer makes it possible to determine the parameters of the lithography process by measuring the scattered radiation for each polarization state. Such a metronome emits polarized light (e.g., linearly polarized light, circularly polarized light, or ellipsoscalometer) using a suitable polarizing filter, for example, in the illumination section of the metronome. A suitable source for the metronome is also possible to provide polarized radiation. Various embodiments of existing ellipsoscalculometers are described in U.S. Patent Applications Publications 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,449, 12 / 920,968, 12 / 922,587, 13 / 000,229, 13 / 033,135, 13 / 533,110, and 13 / 891,410, which are incorporated herein by reference as a whole.

[0031]

[0029] In one embodiment of the scatrometer MT, the scatrometer MT is adapted to measure the overlay of two misaligned grids or periodic structures by measuring the asymmetry of the reflectance spectrum and / or the detection configuration, where the asymmetry relates to the degree of overlay. The two (typically overlapping) grid structures can be applied in two different layers (not necessarily continuous layers) and can be formed at substantially the same location on the wafer. The scatrometer may have a symmetric detection configuration, such as described, for example, in the jointly owned European Patent Application Publication No. 1628164A, so that any asymmetry can be clearly distinguished. This provides a straightforward method for measuring grid misalignment. Further examples for measuring overlay errors between two layers containing periodic structures, when the target is measured through the asymmetry of the periodic structure, can be found in the PCT Patent Application Publication International Publication No. 2011 / 012624 or the US Patent Application No. 20160161863, which are incorporated herein by reference in whole.

[0032]

[0030] Other parameters of the subject may be focus and dose. Focus and dose can be determined simultaneously by scantometry (or by scanning electron microscopy) as described in U.S. Patent Application No. 2011-0249244, which is incorporated herein by reference as a whole. A single structure can be used which has a unique combination of critical dimension and sidewall angle measurements for each point of the focus energy matrix (FEM, also called the focus exposure matrix). If these unique combinations of critical dimension and sidewall angle are available, the focus and dose values ​​can be uniquely determined from these measurements.

[0033]

[0031] A metrologic target can be an assembly of composite gratings, mostly formed by the lithography process in the resist, but also formed, for example, after the etching process. Typically, the pitch and linewidth of the grating structure depend heavily on the measurement optical system (specifically, the NA of the optical system) so that the diffraction order obtained from the metrologic target can be captured. As previously shown, the diffraction signal can be used to determine the shift between two layers (also called "overlay") or to reconstruct at least a portion of the original grating as produced by the lithography process. This reconstruction can be used to provide guidance on the quality of the lithography process and can be used to control at least a portion of the lithography process. The target may have smaller subsegmentations configured to mimic the dimensions of the functional parts of the design layout in the target. These subsegments cause the target to behave more like the functional parts of the design layout so that the total process parameter measurements closely resemble those of the functional parts of the design layout. The target can 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 larger than the entire target. In such overfill mode, it may be possible to measure different targets simultaneously and, therefore, to determine different processing parameters at the same time.

[0034]

[0032] The overall quality of a lithography parameter measurement using a particular target is determined, at least in part, by the measurement recipe used to measure the lithography parameter. The term “substrate measurement recipe” may include one or more parameters of the measurement itself, one or more parameters of the one or more patterns measured, or both. For example, if the measurement used in the substrate measurement recipe is a diffraction-based optical measurement, one or more parameters of this measurement may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation on the substrate, and the orientation of the radiation on the pattern on the substrate. One criterion for selecting a measurement recipe may be, for example, the sensitivity of one of the measurement parameters to process variations. Further examples are given in U.S. Patent Application No. 2016-0161863 and the published U.S. Patent Application No. 2016 / 0370717A1, which are incorporated herein by reference in whole.

[0035]

[0033] A metronome device such as a scatrometer is shown in Figure 4. It includes a broadband (white light) radiation projector 2 that projects radiation onto a substrate 6. The reflected or scattered radiation is sent to a spectrometer detector 4, which measures the spectrum 10 of the specularly reflected radiation (i.e., measures the intensity as a function of wavelength). From this data, the structure or profile that gives rise to the detected spectrum can be reconstructed by a processing unit (PU), for example, by exact coupled wave analysis and nonlinear regression, or by comparison with a library of simulated spectra, as shown at the bottom of Figure 3. Generally, for reconstruction, the general form of the structure is known, and several parameters are assumed from knowledge of the process by which the structure was made, thereby leaving only a few parameters of the structure that should be determined from the scatrometry data. Such a scatrometer may be configured as a normal incidence scatrometer or an oblique incidence scatrometer.

[0036]

[0034] The overall measurement quality of lithography parameters obtained through measurement of a metrologic target is determined, at least in part, by the measurement recipe used to measure the lithography parameters. The term “substrate measurement recipe” may encompass one or more parameters of the measurement itself, one or more parameters of one or more patterns measured, or both. For example, if the measurement performed in a substrate measurement recipe is a diffraction-based optical measurement, one or more parameters of this measurement may include the wavelength of radiation, the polarization of radiation, the angle of incidence of radiation on the substrate, the orientation of radiation on the pattern on the substrate, etc. One criterion for selecting a measurement recipe may be, for example, the sensitivity of any of the measurement parameters to process variations. Further examples are described in U.S. Patent Application No. 2016-0161863 and the published U.S. Patent Application No. 2016 / 0370717A1, which are incorporated herein by reference in their entirety.

[0037]

[0035] Another type of metrologic tool used in IC manufacturing is a topography measurement system, level sensor, or height sensor. Such tools may be incorporated into a lithography apparatus to measure the topography of the top surface of a substrate (or wafer). A topographic map of the substrate (also called a height map) can be generated from these measurements, showing the height of the substrate as a function of its position on the substrate. This height map may later be used to correct the position of the substrate during pattern transfer on the substrate in order to provide a spatial image of the patterning device at the correct focus position on the substrate. In this context, “height” will be understood to broadly refer to the out-of-plane dimension relative to the substrate (also called the Z-axis). Generally, a level or height sensor makes measurements at a fixed position (relative to its own optics), and the relative movement of the substrate and the optics of the level or height sensor results in a height measurement at a location across the substrate.

[0038]

[0036] An example of a level or height sensor LS known in the art is schematically shown in Figure 5, which shows only the operating principle. In this example, the level sensor includes an optical system, the optical system includes a projection unit LSP and a detection unit LSD. The projection unit LSP includes a radiation source LSO that provides a radiation beam LSB provided by the projection grating PGR of the projection unit LSP. The radiation source LSO may be a narrowband or broadband light source, such as a polarized or unpolarized, pulsed or continuous (polarized or unpolarized laser beam, etc.) supercontinuum light source. The radiation source LSO may include multiple radiation sources (such as multiple LEDs) having different colors or wavelength ranges. The radiation source LSO of the level sensor LS is not limited to visible radiation and may additionally or alternatively include UV and / or IR radiation, as well as any wavelength range suitable for reflection from the surface of the substrate.

[0039]

[0037] The projection grating PGR is a periodic grating that includes a periodic structure that produces a radiation beam BE1 with periodically changing intensity. The radiation beam BE1 with periodically changing intensity is directed toward a measurement location MLO on a substrate W having an incident angle ANG with respect to an axis perpendicular to the incident substrate surface (Z axis) of 0 to 90 degrees, generally 70 to 80 degrees. At the measurement location MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and directed toward the detection unit LSD.

[0040]

[0038] To determine the height level at the measurement location MLO, the level sensor further includes a detection system comprising a detection grid DGR, a detector DET, and a processing unit (not shown) for processing the 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 that indicates the received light, for example, the intensity of the received light (e.g., a photodetector), or represents the spatial distribution of the received intensity (e.g., a camera). The detector DET may include any combination of one or more types of detectors.

[0041]

[0039] Triangulation techniques can be used to determine the height level at the measurement location MLO. The detected height level is generally related to the signal intensity measured by the detector DET, which has a periodicity that depends, among other things, on the design of the projection grid PGR and the (oblique) incidence angle ANG.

[0042]

[0040] The projection unit LSP and / or detection unit LSD may include further optical elements such as lenses and / or mirrors (not shown) along the path of the patterned radiation beam between the projection grid PGR and the detection grid DGR.

[0043]

[0041] In one embodiment, the detection grid DGR may be omitted, and the detector DET may be placed in the same position as the detection grid DGR. Such a configuration provides a more direct detection of the image of the projection grid PGR.

[0044]

[0042] 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 that cover a larger measurement range.

[0045]

[0043] Various types of general height sensors 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. International Publication No. 2016102127A1, which is incorporated by reference, describes a miniature height sensor that uses a multi-element detector to detect and recognize the position of a grid image without requiring a detection grid.

[0046]

[0044] Another type of metronome tool used in IC manufacturing is the alignment sensor. Therefore, a crucial aspect of the performance of a lithography apparatus is its ability to correctly and accurately position a pattern in relation to a feature built on a previous layer (by the same apparatus or a different lithography apparatus). For this purpose, the substrate is provided with one or more sets of marks or targets. Each mark is a structure whose position can later be measured using a position sensor (generally an optical position sensor). The position sensor is sometimes called an "alignment sensor," and the marks are sometimes called "alignment marks."

[0047]

[0045] A lithography apparatus may include one or more (e.g., multiple) alignment sensors that can accurately measure the position of alignment marks provided on a substrate. Alignment (or position) sensors can acquire position information from alignment marks formed on a substrate using optical phenomena such as diffraction and interference. An example of an alignment sensor used in current lithography apparatuses is based on a self-referencing interferometer, as described in U.S. Patent No. 6,961,116. Various improvements and modifications of position sensors have been developed, for example, as disclosed in U.S. Patent Application Publication No. 2015261097A1. All of the contents of these publications are incorporated herein by reference.

[0048]

[0046] Figure 6 is a schematic block diagram of one embodiment of a known alignment sensor AS, such as described in U.S. Patent No. 6,961,116, which is referenced. The radiation source RSO provides a radiation beam RB of one or more wavelengths, and the radiation beam RB is redirected by a redirecting optical component to an illumination spot SP onto a mark (such as a mark AM located on a substrate W). In this example, the redirecting optical component includes a spot mirror SM and an objective lens OL. The illumination spot SP (by which the mark AM is illuminated) may have a diameter slightly smaller than the width of the mark itself.

[0049]

[0047] The radiation diffracted by the alignment mark AM is collimated (in this example, by the objective lens OL) into the information transmission beam IB. The term “diffracted” is intended to include zero-order diffraction from the mark (which is sometimes called reflection). For example, a self-reference interferometer SRI of the type disclosed in U.S. Patent No. 6961116 above interferes the beam IB with itself, and the beam is then received by a photodetector PD. Further optical components (not shown) may be included to provide separate beams if two or more wavelengths are produced by the radiation source RSO. The photodetector may be a single element, or it may include several pixels as needed. The photodetector may include a sensor array.

[0050]

[0048] A redirection optical component (in this example, the redirection optical component includes a spot mirror SM) may also function to block zeroth-order radiation reflected from the mark so that the information transmission beam IB contains only higher-order diffraction radiation from the mark AM (which is not essential for the measurement but improves the signal-to-noise ratio).

[0051]

[0049] The intensity signal SI is supplied to the processing unit PU. The optical processing of block SRI and the computational processing of unit PU are combined to output the X and Y position values ​​on the substrate relative to the reference frame.

[0052]

[0050] The illustrated type of single measurement fixes the position of the mark only within a specific range corresponding to one pitch of the mark. In conjunction with this, a coarser measurement technique is used to identify which period of the sine wave contains the position of the mark. For improved accuracy and / or for robust detection of the mark regardless of the material making the mark and the material on which the mark is placed above and / or below, the same process may be repeated at coarser and / or finer levels at different wavelengths. The wavelengths may be optically multiplexed and demultiplexed so that they are processed simultaneously, and / or the wavelengths may be multiplexed by time division or frequency division.

[0053]

[0051] In this example, the alignment sensor and spot SP remain stationary, while the substrate W moves. Therefore, the alignment sensor can be firmly and accurately mounted on the reference frame while effectively scanning the mark AM in the opposite direction to the movement of the substrate W. The substrate W is controlled in this movement by a substrate positioning system that controls its mounting onto 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 some embodiments, one or more (alignment) marks are provided on the substrate support. Measuring the position of the marks provided on the substrate support allows the position of the substrate support to be calibrated (e.g., relative to the frame to which the alignment system is connected) as determined by the position sensor. Measuring the position of the alignment marks provided on the substrate allows the position of the substrate relative to the substrate support to be determined.

[0054]

[0052] Metrology tools MT, such as the above-mentioned scatrometer, topography measurement system, or position measurement system, may use radiation from 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. In some applications, it may be advantageous to use multiple radiation frequencies to measure the substrate, for example, broadband radiation can be used. Multiple different frequencies may be able to propagate, irradiate, and scatter from the metrology target with little to no interference with other frequencies. Therefore, for example, different frequencies can be used to obtain more metrology data simultaneously. Different radiation frequencies may also be able to query or discover different characteristics of the metrology target. Broadband radiation may be useful in metrology systems MT, for example, level sensors, alignment mark measurement systems, scatrometry tools, or inspection tools. A broadband radiation source may be a supercontinuum light source.

[0055]

[0053] High-quality broadband radiation (e.g., supercontinuous radiation) can be difficult to generate. One method for generating broadband radiation is to broaden high-power narrowband or single-frequency input radiation or pump radiation by utilizing nonlinear and higher-order effects, for example. Input radiation (which can be generated using a laser) can be called pump radiation. Alternatively, input radiation can be called seed radiation. To obtain high-power radiation for broadening effects, radiation can be confined to small areas such that strong localized high-intensity radiation is achieved. In those areas, radiation interacts with broadening structures and / or materials that form a nonlinear medium, and broadband power radiation can be produced. Different materials and / or structures can be used in high-intensity radiation areas to enable and / or improve radiation broadening by providing a suitable nonlinear medium.

[0056]

[0054] In some implementations, broadband output radiation is generated in a photonic crystal fiber (PCF). In some embodiments, such a photonic crystal fiber has a microstructure along the circumference of its fiber core to help confine radiation moving through the fiber into the fiber core. The fiber core may 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, there may be some disadvantages to using solid materials. For example, if UV radiation is generated in a solid core, this radiation may not be seen in the fiber's output spectrum because most of the radiation is absorbed by the solid material.

[0057]

[0055] In some implementations, as will be discussed further below with reference to Figure 8, methods and apparatus for broadening input radiation can use a fiber for confining the input radiation and broadening the input radiation into output broadband radiation. The fiber may be a hollow-core fiber and may include an internal structure to achieve effective guidance and confinement of radiation within the fiber. The fiber may be a hollow-core photonic crystal fiber (HC-PCF), which is particularly suitable for strong radiation confinement mainly 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 for broadening the input radiation. Such a fiber and gas arrangement can be used to produce a supercontinuous radiation source. The radiation input to the fiber may be electromagnetic radiation, and may be, for example, one or more of the infrared, visible light, UV and extreme UV spectra. The output radiation may consist of or include broadband radiation which may be referred to herein as white light.

[0058]

[0056] Several embodiments relate to novel designs of such broadband radiation sources, including optical fibers. The optical fiber is a hollow-core photonic crystal fiber (HC-PCF). Specifically, the optical fiber may be a type of hollow-core photonic crystal fiber that includes an anti-resonant structure for radiation confinement. Such fibers including an anti-resonant structure are known in the art as anti-resonant fibers, tubular fibers, single-ring fibers, negative curvature fibers, or suppression-coupled fibers. Various different designs of such fibers are known in the art. Alternatively, the optical fiber may be a photonic bandgap fiber (HC-PBF, e.g., a kagome fiber).

[0059]

[0057] Several types of HC-PCFs can be engineered, each based on a different physical guidance mechanism. Two such HC-PCFs include hollow-core photonic bandgap fibers (HC-PBFs) and hollow-core anti-resonant reflecting fibers (HC-ARFs). Details relating to the design and manufacture of HC-PCFs can be obtained from U.S. Patent No. 2004 / 015085A1 (for HC-PBFs) and International Publication No. 2017 / 032454A1 (for hollow-core anti-resonant reflecting fibers), which are incorporated herein by reference. Figure 9(a) shows a kagome fiber including a kagome lattice structure.

[0060]

[0058] Here, with reference to Figure 7, an example of an optical fiber used as a radiation source will be described. Figure 7 is a schematic cross-sectional view of an optical fiber OF in cross-section. Further embodiments similar to the practical example of the fiber in Figure 7 are disclosed in International Publication No. 2017 / 032454A1.

[0061]

[0059] An optical fiber OF includes an elongated body, and one dimension of the fiber OF is longer than the other two dimensions. This longer dimension can be called the axial direction and can define the axis of the optical fiber OF. The other two dimensions define a plane that can be called the cross-section. Figure 7 shows a cross-section of an optical fiber OF in this cross-section (i.e., a plane perpendicular to the axis), which is labeled the xy plane. The cross-section of an optical fiber OF can be substantially constant along the fiber axis.

[0062]

[0060] It will be understood that optical fibers (OFs) have a certain degree of flexibility, and therefore the axial direction is generally not uniform along the length of the optical fiber (OF). It will be understood that terms such as optical axis, cross-section, and the like refer to the local optical axis, local cross-section, etc. Furthermore, if the component is described as cylindrical or tubular, it will be understood that these terms include shapes that would deform if the optical fiber (OF) were bent.

[0063]

[0061] It will be understood that optical fiber OF can have any length, and the length of optical fiber OF can depend on the application. Optical fiber OF can have a length of 1 cm to 10 m, for example, optical fiber OF can have a length of 10 cm to 100 cm.

[0064]

[0062] The optical fiber OF includes a hollow core HC, a cladding portion surrounding the hollow core HC, and a support portion SP surrounding and supporting the cladding portion. The optical fiber OF can be thought of as including a body (including the cladding portion and the support portion SP) having a hollow core HC. The cladding portion includes a plurality of anti-resonant elements for guiding radiation through the hollow core HC. Specifically, the plurality of anti-resonant elements are arranged to confine the 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 substantially be located in the central region of the optical fiber OF, and as a result, the axis of the optical fiber OF can also define the axis of the hollow core HC of the optical fiber OF.

[0065]

[0063] The cladding portion includes a plurality of anti-resonant elements to guide the radiation propagating through the optical fiber OF. Specifically, in this embodiment, the cladding portion includes a single ring of six capillary tubes CAP. Each of the capillary tubes CAP acts as an anti-resonant element.

[0066]

[0064] The capillary CAP may also be called a tube. The capillary CAP may have a circular cross-section or other shapes. Each capillary CAP generally includes a 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 will 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 an oblique angle of incidence). The thickness of the wall portion WP may be suitable to ensure that transmission to the capillary cavity CC is generally suppressed, while reflection back to the hollow core HC is generally enhanced. In some embodiments, the capillary wall portion WP may have a thickness of 0.01 to 10.0 μm.

[0067]

[0065] As used herein, the term cladding portion is intended to mean the portion of the optical fiber OF that guides radiation propagating through the optical fiber OF (i.e., the capillary CAP that confines the radiation within the hollow core HC). The radiation can be confined in transverse mode (propagating along the fiber axis).

[0068]

[0066] The support portion is generally tubular and supports the six capillary CAPs of the clad portion. In the case of the inner support portion SP, the six capillary CAPs are evenly distributed along the circumferential direction of the inner surface. The six capillary CAPs can be described as being arranged in a general hexagonal form.

[0069]

[0067] The capillaries are arranged such that each capillary does not come into contact with any other capillary. Each capillary is in contact with the inner support portion SP and is separated from adjacent capillary in the ring structure. Such an arrangement may be beneficial because it can increase the transmission bandwidth of the optical fiber OF (compared to, for example, an arrangement in which the capillaries are in contact with each other). Alternatively, in some embodiments, each capillary may come into contact with adjacent capillary in the ring structure.

[0070]

[0068] The six capillaries CAP in the cladding portion are arranged in a ring structure along the circumference of the hollow core HC. The inner surface of the ring structure of the capillaries CAP defines the hollow core HC of the optical fiber OF at least partially. The diameter d of the hollow core HC (which may be defined as the minimum dimension between opposing capillaries and indicated by arrow d) can be 10 to 1000 μm. The diameter d of the hollow core HC may affect the mode field diameter, collision loss, dispersion, mode multiplicity, and nonlinearity characteristics of the hollow core HC optical fiber OF.

[0071]

[0069] In this embodiment, the cladding portion includes a single ring arrangement of capillary CAPs (which act as anti-resonant elements). Therefore, 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.

[0072]

[0070] It will be understood that other embodiments can provide different arrangements of anti-resonant elements. These may include arrangements having multiple rings of anti-resonant elements or arrangements having nested anti-resonant elements. Figure 9(a) shows an embodiment of an HC-PCF having three rings of capillaries stacked radially. In this embodiment, each capillary is in contact with other capillaries, both in the same ring and in different rings. Furthermore, while the embodiment shown in Figure 7 includes one ring containing six capillaries, other embodiments can provide one or more rings in the cladding portion containing any number of anti-resonant elements (e.g., 4, 5, 6, 7, 8, 9, 10, 11, or 12 capillaries).

[0073]

[0071] Figure 9(b) shows a modified embodiment of the HC-PCF discussed above having a single ring in the capillary tube. In the example of Figure 9(b), there are two coaxial rings in the capillary tube 21. To hold the inner and outer rings of the capillary tube 21, the HC-PCF may include a support tube ST. The support tube may be made of quartz.

[0074]

[0072] The capillary tubes in the examples of Figures 7 and 9(a), (b) may have a circular cross-sectional shape. Other shapes are also possible for the capillary tubes, such as elliptical or polygonal cross-sections. Furthermore, the solid material of the capillary tubes in the examples of Figures 7 and 9(a), (b) may include plastic materials such as PMA, glass such as quartz, or soft glass.

[0075]

[0073] Figure 8 shows a radiation source RDS for providing broadband power radiation. The radiation source RDS includes a pulse 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 (e.g., the type shown in Figure 7), and a working medium WM (e.g., gas) placed within the hollow core HC. In Figure 8, the radiation source RDS includes the optical fiber OF shown in Figure 7, but in alternative embodiments, other types of hollow core HC optical fiber OFs may be used.

[0076]

[0074] The pulse pump radiation source PRS is configured to provide an input radiated IRD. The hollow core HC of the optical fiber OF receives the input radiated IRD from the pulse pump radiation source PRS and is arranged to broaden the input radiated IRD to provide an output radiated ORD. The working medium WM is configured to broaden the frequency range of the received input radiated IRD to provide a broadband output radiated ORD.

[0077]

[0075] The radiation source RDS further includes a reservoir RSV. An optical fiber OF is placed inside the reservoir RSV. The reservoir RSV may also be called a housing, container, or gas cell. The reservoir RSV is configured to include a working medium WM. The reservoir RSV may include one or more features known in the art for controlling, adjusting 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 placed inside the reservoir RSV such that the first transparent window TW1 is located very close to the input end IE of the optical fiber OF. The first transparent window TW1 may form part of the wall of the reservoir RSV. The first transparent window TW1 may be at least transparent to the received input radiation frequency, and as a result, the received input radiation IRD (or at least a large portion thereof) can be coupled to the optical fiber OF located inside the reservoir RSV. It will be understood that an optical system (not shown) can be provided for coupling the input radiation IRD to an optical fiber OF.

[0078]

[0076] The reservoir RSV includes a second transparent window TW2 which forms part of the wall of the reservoir RSV. When the optical fiber OF is placed inside the reservoir RSV during use, the second transparent window TW2 is located very close to the output end OE of the optical fiber OF. The second transparent window TW2 may be at least transparent to the frequencies of the broadband output radiation ORD of the device 120.

[0079]

[0077] Alternatively, in another embodiment, the ends of the optical fiber OF may be placed in different reservoirs. The optical fiber OF may include a first end section configured to receive an input radiated IRD and a second end section for outputting a broadband output radiated ORD. The first end section may be placed in a first reservoir containing a working medium WM. The second end section may be placed in a second reservoir, which may also contain a working medium WM. The function of the reservoirs may be as described with respect to Figure 8 above. The first reservoir may include a first transparent window configured to be transparent to the input radiated IRD. The second reservoir may include a second transparent window configured to be transparent to the broadband output broadband radiated ORD. The first and second reservoirs may also include sealable openings to allow the optical fiber OF to be placed partially inside and partially outside the reservoir, thereby allowing gas to be sealed inside the reservoir. The optical fiber OF may further include intermediate sections not contained within the reservoir. Such an arrangement using two separate gas reservoirs may be particularly convenient for embodiments where the optical fiber OF is relatively long (e.g., when the length exceeds 1 m). In the case of an arrangement using two separate gas reservoirs, it will be understood that the two reservoirs (which may include one or more features known in the art for controlling, adjusting and / or monitoring the composition of the gas in the two reservoirs) provide a device for providing a working medium WM within the hollow core HC of the optical fiber OF.

[0080]

[0078] In this context, a window may be transparent to frequency if at least 50%, 75%, 85%, 90%, 95%, or 99% of the incident radiation of that frequency into the window is transmitted through the window.

[0081]

[0079] Both the first transparent window TW1 and the second transparent window TW2 can form an airtight seal within the wall of the reservoir RSV, and as a result, the working medium WM (which may be a gas) can be contained within the reservoir RSV. It will be understood that the gas WM can be contained within the reservoir RSV at a pressure different from the ambient pressure of the reservoir RSV.

[0082]

[0080] The working medium WM may include noble gases such as argon, krypton, and xenon, Raman active gases such as hydrogen, deuterium, and nitrogen, or gas mixtures such as argon / hydrogen mixture, xenon / deuterium mixture, krypton / nitrogen mixture, or nitrogen / hydrogen mixture. Depending on the type of filling gas, the nonlinear optical process may include modulation instability (MI), soliton self-compression, soliton splitting, Kerr effect, Raman effect, and dispersion wave generation, details of which are described in International Publication No. 2018 / 127266A1 and U.S. Patent No. 9160137B1 (both 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 resulting broadband pulse dynamics and associated spectral broadening characteristics can be adjusted to optimize frequency conversion.

[0083]

[0081] In one implementation, the working medium WM can be located within the hollow core HC while it is receiving input radiation IRD to generate broadband output radiation ORD. It will be understood that when the optical fiber OF is not receiving input radiation IRD to generate broadband output radiation ORD, the gas WM does not need to be present entirely or partially within the hollow core HC.

[0084]

[0082] High-intensity radiation is desirable to achieve frequency spread. An advantage of having a hollow-core HC optical fiber OF is that high-intensity radiation can be achieved through strong spatial confinement of radiation propagating through the optical fiber OF, and localized high-intensity radiation can be achieved. The radiation intensity within the optical fiber OF can be high, for example, due to the received high-intensity input radiation and / or strong spatial confinement of radiation within the optical fiber OF. An advantage of hollow-core optical fibers is that they can derive radiation with a wider wavelength range than solid-core fibers and, more specifically, hollow-core optical fibers, which can derive radiation in both the ultraviolet and infrared ranges.

[0085]

[0083] An advantage of using a hollow core HC optical fiber OF is that most of the radiation induced within the optical fiber OF is confined within the hollow core HC. Therefore, most of the radiation interaction within the optical fiber OF occurs with the working medium WM provided within the hollow core of the optical fiber OF. As a result, the radiation spreading effect of the working medium WM can be enhanced.

[0086]

[0084] The input radiation IRD received may be electromagnetic radiation. The input radiation IRD may be received as pulsed radiation. For example, the input radiation IRD may include ultrafast pulses generated by, for example, a laser.

[0087]

[0085] The input radiation IRD may be coherent radiation. The input radiation IRD may be collimated radiation, the advantage of which may be that it promotes and improves the efficiency of coupling the input radiation IRD to the optical fiber OF. The input radiation IRD may include a single frequency or a narrow range of frequencies. The input radiation IRD can be generated by a laser. Similarly, the output radiation ORD may be collimated and / or coherent.

[0088]

[0086] The broadband range of the output emission ORD can be a continuous range, including a continuous range of emission frequencies. The output emission ORD may include supercontinuous emission. Continuous emission can be beneficial for use in many applications (e.g., metrology applications). For example, a continuous range of frequencies can be used for querying the majority of characteristics. A continuous range of frequencies can be used, for example, to determine and / or eliminate the frequency dependence of measured characteristics. The supercontinuous output emission ORD may include, for example, electromagnetic emission over a wavelength range of 100 nm to 4000 nm. The frequency range of the broadband output emission ORD may be, for example, 400 nm to 900 nm, 500 nm to 900 nm, or 200 nm to 2000 nm. The supercontinuous output emission ORD may include white light.

[0089]

[0087] The input radiation IRD provided by the pulse pump radiation source PRS may be pulsed. The input radiation IRD may include electromagnetic radiation of one or more frequencies between 200 nm and 2 μm. The input radiation IRD may include electromagnetic radiation having a wavelength of, for example, 1.03 μm. The repetition rate of the pulse radiation IRD may be as large as 1 kHz to 100 MHz. The pulse energy may be as large as 0.1 μJ to 100 μJ (e.g., 1 to 10 μJ). The pulse duration of the input radiation IRD may be 10 fs to 10 ps (e.g., 300 fs). The average power of the input radiation IRD may be 100 mW to several hundred watts. The average power of the input radiation IRD may be, for example, 20 to 50 watts.

[0090]

[0088] The pulse 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 the adjustment of (pump) laser parameters, operating component WM variations, and optical fiber OF parameters. The spatiotemporal transmission characteristics may include one or more of the output power, output mode profile, output time profile, width of the output time profile (or output pulse width), output spectral profile, and bandwidth of the output spectral profile (or output spectral bandwidth). The pulse pump radiation source PRS parameters may include one or more of the pump wavelength, pump pulse energy, pump pulse width, and pump pulse repetition rate. The optical fiber OF parameters may include one or more of the optical fiber length, size and shape of the hollow core HC, size and shape of the capillary, and thickness of the capillary wall surrounding the hollow core HC. The operating component WM (e.g., filling gas) parameters may include one or more of the gas type, gas pressure, and gas temperature.

[0091]

[0089] 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 a 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 entire wavelength band of the output radiation. The power spectral density across the entire wavelength band of the broadband output radiation may be at least 3 mW / nm.

[0092]

[0090] HC-PCF-based broadband radiation sources (RDS) tend to suffer from problems of rapid performance degradation and short lifetime. Several failure mechanisms have been identified at present. The first failure mechanism is fiber contamination, typically induced by hydrocarbon deposition at one or both ends of the fiber. Hydrocarbons can be introduced into the reservoir RSV when the radiation source is assembled. Hydrocarbons can also be generated by thermal gas release from components within the reservoir RSV, including the optical fiber OF itself.

[0093]

[0091] The second failure mechanism is overheating of the fiber. White light generation involves ionization and heat generation due to atomic collisions in gas species and recombination dynamics in the plasma. Such heat generated during operation increases the temperature of the inner cladding surface of the optical fiber OF. However, in the case of existing HC-PCFs, such as those shown in Figure 7, 9(a) or 9(b), their fiber geometry prevents access to and efficient cooling of the surface of the inner cladding waveguide structure. Without effective heat dissipation, the generated heat will accumulate within the fiber until the fiber overheats and is eventually damaged, especially when the fiber is pumped / driven with high repetition rate pulses. Overheating of the surface of the inner cladding waveguide structure (e.g., capillary tube CAP as shown in Figure 7, 9(a) or 9(b)) can also trigger unwanted chemical reactions that result in gas release and, consequently, further contamination.

[0094]

[0092] The third failure mechanism is hydrogen-induced fiber surface reduction and glassy growth. After operating an HC-PCF-based broadband radiation source RDS with a hydrogen-containing gas mixture for several hundred hours or more, silicon dioxide or SiO xNanostructures and fuzzy glass are known to grow primarily at the output ends of fibers. One of the main root causes of silicon dioxide growth is the reduction of the inner fiber surface in the presence of hydrogen plasma. The reduction and etching of silicon dioxide in the presence of hydrogen plasma is a known phenomenon. Hydrogen ions and radicals, such as atomic hydrogen, attack the inner fiber surface, causing reduction of the contact surface by converting quartz to silicon, or etching of the surface by generating volatile silicon monoxide. The growth of silicon dioxide at the fiber ends results in a stepwise cutoff and loss of output power, which ultimately leads to fiber damage and a shortened lifetime for broadband radiation sources (RDS).

[0095]

[0093] Currently, various measures are being taken individually or in combination to reduce the degradation of HC-PCF and, consequently, to extend the lifespan of HC-PCF-based radiation sources. For example, better cleaning methods are used to clean all components of the HC-PCF-based radiation source RDS (specifically, those contained in the reservoir RSV). Furthermore, components made of materials with low gas emission characteristics are preferred over homogeneous components made of materials with high gas emission characteristics. In addition, gas mixtures with higher thermal conductivity are used to help improve heat dissipation within the HC-PCF. While the above measures are effective to some extent, preventing fiber damage remains difficult.

[0096]

[0094] Due to the aforementioned fiber lifetime problem, HC-PCF-based radiation source RDS may fail to function properly after only a short period of operation. When an HC-PCF-based light source is used in a metrology tool (e.g., the above-mentioned scatrometer, topography measurement system, or position measurement system), an unexpected and / or premature failure of the HC-PCF means that the entire light source must be removed from the tool for repair or replacement. Replacing a damaged / degraded fiber requires re-optimizing the operation of coupling the pump laser beam to the new HC-PCF, which is a very time-consuming process. After the repaired or replaced radiation source RDS is incorporated into the same metrology tool, a complete optical alignment of the radiation beam emitted from the radiation source, as well as other necessary calibrations and characterizations, must be performed again. The entire process not only increases costs but also results in considerable system downtime.

[0097]

[0095] As described above, in an HC-ARF, a cladding section containing multiple anti-resonant elements, such as a capillary tube CAP, is used to guide radiation through the hollow core HC. Between fiber resonances, a low-loss transmission window of a given HC-ARF is formed. Within the low-loss transmission window, the cladding modes supported by the anti-resonant elements (e.g., the capillary tube CAP) are not phase-matched with the core modes (i.e., the core modes and cladding modes resonate), thereby ensuring that the core modes propagate mainly within the hollow core HC. In contrast, within each fiber resonance band, the core modes are phase-matched with the cladding modes (i.e., the core modes and cladding modes resonate), thereby strongly coupling the core modes to the anti-resonant elements. Consequently, the corresponding portion of the light is no longer guided within the hollow core HC. The strong coupling of the core modes to the surrounding glass structure exacerbates the overheating problem described above, and consequently shortens the fiber lifetime. Therefore, it is desirable to optimize the output spectrum so that overlap with fiber resonances is minimized in order to reduce the interaction between the core modes and the cladding structure.

[0098]

[0096] In the case of a typical HC-ARF as shown in Figure 7, 9(a) or 9(b), the spectral position of the fiber resonance is determined by the thickness of the core wall (e.g., the thickness of the capillary tube) and the geometry of the surrounding cladding structure (e.g., the arrangement of the capillary tube). Fiber resonance cannot be eliminated and must be controlled to optimize a given spectral bandwidth. The geometry and core wall thickness, as well as the HC-PCF material, cannot be changed once the fiber has been fabricated. Changing the position of the fiber resonance requires changing one or more of the above fiber parameters, and therefore requires using a different fiber. The lack of flexibility in controlling fiber resonance after fabrication imposes stringent requirements on the fiber fabrication process, inevitably leading to high manufacturing costs.

[0099]

[0097] Existing HC-PCF-based broadband radiation source RDSs are typically based on a single-fiber configuration (e.g., the example shown in Figure 8). To meet the growing need for larger bandwidths, HC-PCFs with a multi-octave spectral bandwidth (from deep ultraviolet (UV) to near-infrared (NIR)) are desirable. However, HC-PCFs with such large bandwidths are extremely difficult to fabricate and require precise control of the fiber fabrication process, which limits production yield and increases manufacturing costs.

[0100]

[0098] The embodiments described below relate to a method and apparatus that can mitigate the aforementioned short lifetime problem and / or, at the same time, enable adjustment of desired flexibility or optimization of the spectral characteristics of the output radiation (e.g., bandwidth and / or location of the output spectrum) that cannot be achieved with existing single-fiber-based radiation sources RDS. The main concept of the method is to use two or more optical fiber OFs (e.g., HC-PCFs) arranged in a desired manner such that the current optical fiber OF can be automatically and / or quickly replaced with a new optical fiber having substantially the same fiber parameters as the current optical fiber OF or having other different fiber parameters in order to obtain different output characteristics (e.g., different fiber resonance and / or spectral bandwidth). This method allows for automatic and non-invasive switching of multiple optical fiber OFs, thus eliminating the need to go through existing time-consuming fiber replacement processes (e.g., removing the current optical fiber OF from the reservoir RSV, inserting a new optical fiber OF into the reservoir, and coupling the input radiation to the new optical fiber OF).

[0101]

[0099] In all embodiments, the number of fibers in the fiber assembly may include, for example, 2 to 100 optical fibers, 2 to 50 optical fibers, 2 to 30 optical fibers, 5 to 100 optical fibers, 5 to 50 optical fibers, 5 to 30 optical fibers, 10 to 100 optical fibers, or 10 to 50 optical fibers. The arrangement may be a one-dimensional (1D) array, a two-dimensional array, an irregular 1D or 2D arrangement, a circular arrangement, or any other arrangement. The optical fibers may all be substantially similar, or at least one or more subsets may have different characteristics.

[0102] [000100] Figure 10(a) schematically shows a rectangular fiber assembly FAA1 comprising a two-dimensional (2D) array of optical fibers (e.g., single-ring HC-PCF) according to an embodiment. In the particular embodiment shown, the fiber assembly FAA1 may be a multicore microstructure fiber comprising a total of 25 separate cores of HC-PCF OF1 to OF25. The 25 separate hollow cores HC may be formed within a single support portion SP. Note that the fiber assembly FAA1 may include any number and / or any type of optical fibers OF, the number and / or type of which can be determined based on the needs and / or practicality of the application of the mounting configuration.

[0103] [000101] The spatial distance DS between the centers of two neighboring hollow cores HC may be at least greater than the inner diameter ds of the support portion SP of each HC-PCF that supports direct contact with the (six) capillary tubes. Thus, any two HC-PCFs may be spatially separated. In some embodiments, the spatial distance DS between any two HC-PCFs may be large enough so that the adjacent optical fiber OF is less affected or not affected at all by, for example, gas emission and / or glassy (quartz) growth originating from the neighboring optical fiber OF. The spatial distance DS may be, for example, up to twice, three times, four times, or five times the inner diameter ds of the support portion ds of each HC-PCF.

[0104] [000102] Figure 10(b) schematically shows a circular fiber assembly FAA2 including a two-dimensional (2D) arrangement of optical fibers (e.g., single-ring HC-PCF) according to an embodiment. In this particular example, the fiber assembly FAA2 includes a total of six HC-PCF OF1 to OF6 arranged in a single-ring geometry. It should be understood that in different embodiments, the circular fiber assembly FAA2 may include multiple concentric rings of optical fibers OF. Any number of optical fibers OF may be present in each fiber ring, the number of which can be determined based on the needs and / or practicality of the application of the implementation form. All optical fibers OF included in each fiber ring may have the same radial distance to the axis AX of the fiber assembly FAA2. Similar to the embodiment shown in Figure 10(a), the spatial distance DS between the centers of two neighboring hollow cores HC may be at least greater than the inner diameter ds of the supporting portion of each HC-PCF that supports or is in direct contact with the (six) capillary tubes. The spatial distance DS can be, for example, up to 2 times, 3 times, 4 times, or 5 times the inner diameter ds of the support portion of each HC-PCF.

[0105] [000103] Both fiber assemblies FAA1 and FAA2 shown in Figures 10(a) and 10(b) can be manufactured using standard multicore fiber fabrication techniques (e.g., the so-called stack-and-draw technique). This three (or more) step process includes assembling multiple glass tubes into a glass preform, drawing the preform onto canes with smaller lateral dimensions, and further drawing the canes onto fibers. Fiber assemblies consisting of multiple hollow core fibers can be fabricated directly, based on variations of this method, more specifically by including the additional step of assembling the canes together before fiber drawing.

[0106] [000104] Instead of using fiber assemblies based on directly fabricated multicore microstructured fibers, other types of fiber assemblies may be based on assembling multiple individual single-core fibers. Such fiber assemblies may have the advantage of allowing greater flexibility in the selection of optical fibers and eliminating some of the fabrication difficulties that arise when directly manufacturing a single multicore microstructured fiber.

[0107] [000105] Figures 11(a) and 11(b) show 1D and 2D fiber assemblies FAA3 and FAA4, respectively, using individual single-core optical fibers according to an embodiment. Fiber assemblies FAA3 and FAA4 can be obtained by assembling multiple individual single-core optical fibers OF (e.g., HC-PCF) and arranging them in a fiber array. The size of the fiber array and, consequently, the total number of optical fibers OF can be flexibly selected depending on the application.

[0108] [000106] Referring to Figure 11(a), the fiber assembly FAA3 may include a 1D fiber array having five optical fibers OF1 to OF5 (in this particular example). The five optical fibers OF1 to OF5 may be five HC-PCFs, which can optionally be obtained from a single HC-PCF and thus may have substantially the same fiber characteristics. Alternatively, the five HC-PCFs may be obtained from two or more different HC-PCFs and thus may have at least two different fiber characteristics. The five HC-PCFs can be held by a V-groove mount VG having a 1D array of V-grooves and fiber clamps CP. Each V-groove may hold a single fiber. Once the clamps CP are fixed onto the V-groove mount VG, the clamps CP can physically contact the five HC-PCFs and provide appropriate clamping force. The strength of the clamping force may be adjustable by varying the depth of the V-grooves. The clamping force can be fixed in place by pressing each HC-PCF against the wall of its respective V-groove. In some embodiments, a fiber sleeve can be used where the clamp CP contacts the fiber to reduce or minimize the stress on the fiber induced by the clamping force.

[0109] [000107] Referring to Figure 11(b), the fiber assembly FAA4 may include a 2D fiber array having 10 optical fibers OF1 to OF10 (in this particular example). The 10 optical fibers OF1 to OF10 may be 10 HC-PCFs, which can be obtained from a single HC-PCF and therefore may have substantially the same fiber characteristics. Alternatively, the 10 HC-PCFs may be obtained from two or more different HC-PCFs and therefore some fibers may have substantially the same fiber characteristics, others may have different fiber characteristics, or all may be different. For example, the five HC-PCFs OF1 to OF5 in the top row may have one fiber structure, and the five HC-PCFs OF6 to OF10 in the bottom row may have different fiber structures. Depending on the application, other combinations of different fibers may be used in the fiber assembly FAA4. The 10 HC-PCFs may be held by a V-groove mount VG' having a corresponding 2D array of V-grooves and two fiber clamps CP. Each V-groove can hold a single fiber. Two fiber clamps can be mounted on the top and bottom surfaces of the V-groove mount VG', respectively, to secure all optical fibers OF1 to OF10 in place.

[0110] [000108] Note that the 1D or 2D V-groove mounts described above are merely examples of fiber holders, and other different types of fiber holders are equally applicable. In some embodiments, a single glass substrate containing a 1D or 2D array of cylindrical bores can be used as a fiber array holder. Each of the cylindrical bores may hold a single optical fiber OF (e.g., HC-PCF). After all the fibers have been inserted into their respective cylindrical bores, they can be fixed to their respective bores, for example, by laser bonding or adhesive.

[0111] [000109] Figure 12 schematically shows a broadband radiation source RDS1 arrangement including a fiber assembly FAA and a first single-input beam configuration according to an embodiment. The fiber assembly FAA may be, for example, any one of the fiber assemblies FAA1 to FAA4 described above. During operation, one of the HC-PCFs (e.g., OF3) can be aligned with the optical axis of the radiation source RDS. Within the reservoir RSV, the optical axis of the radiation source RDS may be the beam path of the input radiation IRD. In some embodiments, at any given time, only one HC-PCF of the fiber assembly FAA may be used to generate broadband radiation. When the current HC-PCF reaches the end of its lifespan (e.g., when its performance has degraded to a certain threshold level, when damage occurs) or when different output characteristics are desired, a neighboring or selected new HC-PCF (e.g., OF4) can be inserted (e.g., automatically) into the beam path of the input radiation by translating the fiber assembly FAA parallel (perpendicular) to the beam path of the input radiation IRD (e.g., a fixed one). If fiber assembly FAA2 is used with radiation source RDS1, a neighboring or selected new HC-PCF (e.g., OF4) can be inserted (e.g., automatically) into the beampath of the input radiation IRD by rotating fiber assembly FAA2 around its axis AX. Alternatively or additionally, fiber core switching can also be achieved by shifting the beampath of the input radiation IRD relative to a fiber assembly FAA (e.g., a fixed one). In some embodiments, both the beampath of the input radiation IRD and the position or orientation of fiber assembly FAA can be adjusted together to achieve fiber core switching.

[0112] [000110] Referring to Figure 12, if the switching of fiber cores is achieved solely through the translation of the fiber assembly FAA, the fiber assembly FAA may be mounted on a stage module (not shown) which includes one or more actuators (e.g., electromagnetic actuators). The stage module may provide the fiber assembly FAA with multiple (e.g., 6) degrees of freedom of movement. In this particular embodiment in which a linear 1D fiber array is used, the stage module may include three linear motion actuators that provide precise movement in the X, Y, and Z directions, and two rotary actuators that provide precise angular tilt in the sagittal plane (tilt Rx around the X direction) and the tangential plane (tilt Rz around the Z direction). The X, Y, and Z directions are defined with respect to the coordinate system shown in Figure 12. The linear actuators may provide a spatial resolution of, for example, less than 1 micrometer, and the rotary actuators may have an angular resolution of, for example, less than 1 microradian. The range of movement and angular tilt may be large enough to cover all the fibers of the fiber assembly FAA. In some embodiments, in addition to high-precision linear actuators, the stage module may further include coarse actuators, which are less precise but responsible for moving over long distances. When it is necessary to replace a fiber assembly FAA (for example, if all fibers are damaged or degraded), coarse actuators can be used to ensure that the difference in relative position between the new fiber assembly FAA and the beam of the input radiated IRD is within the range of motion of the high-precision linear actuator.

[0113] [000111] Whenever it is time to replace the current optical fiber OF, the input radiated IRD can be at least partially blocked, for example, by a beam block. The stage module can move the fiber assembly FAA in parallel to substantially align the hollow core HC of the new optical fiber OF with the beam path of the input radiated IRD. The distance of the movement can be predetermined based on the spatial separation between any two neighboring fiber cores. Once the new optical fiber OF has been moved to the target position, the attenuated or complete input radiated IRD can enter the hollow core HC of the new fiber, and a broadband output radiated ORD can be generated. The radiation source RDS1 can then enter an input coupling optimization mode in which the coupling between the input radiated IRD and the hollow core HC of the new fiber is optimized. Input coupling optimization can be evaluated by monitoring specific optical properties of the output radiation (e.g., the spectrum and / or power of the output radiated ORD) while adjusting the position of the fiber assembly FAA.

[0114] [000112] The optimization routine may involve, for example, a sequential micro-scan of the position of the fiber assembly FAA in the X, Y, and Z directions. For example, the stage module may receive a command to first scan the position of the fiber assembly FAA in the XZ plane to optimize the lateral overlap between the input radiated IRD and the hollow core HC. The stage module may then receive a command to scan the position of the fiber assembly FAA in the Y direction to optimize the focus position of the input radiated IRD to the hollow core HC. In some embodiments, the stage module may receive a command to tilt the fiber assembly FAA in the Rx and / or Rz directions to further optimize the fiber coupling of the input radiated IRD. Once the input coupling optimization is complete, the radiation source RDS1 can enter a normal operating mode in which the position of the fiber assembly FAA is fixed. Note that input coupling optimization may be performed only once when a new fiber assembly FAA is installed. After the initial input coupling optimization, subsequent fiber core switching can be performed repeatedly without requiring further optimization. However, input coupling optimization can be repeated at any time if necessary (e.g., for fault detection purposes). For example, if the optical properties (e.g., spectrum and / or power) of the output radiation ORD generated from a new fiber differ significantly from those generated by the same fiber before its degradation / damage or from predefined values, input coupling optimization can be used to verify whether the performance loss is due to fiber coupling of the input radiation IRD.

[0115] [000113] In some embodiments, all optical fibers OF contained within a fiber assembly FAA may have substantially the same fiber characteristics, and therefore may result in substantially the same output optical characteristics. When the current optical fiber is already damaged or on the verge of damage (determined, for example, by existing performance degradation), a fiber core switching can be initiated. Since HC-PCF is typically considered a lifetime-limiting component of an HC-PCF-based broadband radiation source RS, a fiber assembly FAA that includes redundant collections of multiple copies (e.g., five copies) of a particular fiber structure can extend the overall lifetime of the radiation source RS considerably (e.g., by five times).

[0116] [000114] In different embodiments, some or all of the optical fiber OFs may have different fiber characteristics / structures resulting in different output optical characteristics. If a 2D fiber assembly (e.g., assembly FAA1 in Figure 10(a)) is used in the radiation source RDS1 in Figure 12, the optical fiber OFs in each column may have substantially the same fiber characteristics, while the optical fiber OFs in each row may have different fiber characteristics. The differences in fiber characteristics between the fibers in a row can be tailored in a customized manner to optimize the output optical characteristics from each or one of the different fiber cores and / or to complement the output optical characteristics from the other fibers.

[0117] [000115] In the embodiment, the HC-PCFs in each column may have substantially the same fiber characteristics, and they may differ from the characteristics of the fibers in different columns in order to provide radiation with different radiation characteristics. Thus, the output radiation generated from the fibers in a particular column can be optimized in a particular spectral range. For example, the output radiation ORD from the fibers in the first column (e.g., the leftmost column of the fiber assembly FAA in Figure 12) can be optimized in the spectral range of 200 nm to 800 nm, the fibers in the second column (i.e., the column next to the first column) can be optimized in the spectral range of 600 nm to 1200 nm, the fibers in the third column can be optimized in the spectral range of 1000 nm to 1600 nm, the fibers in the fourth column can be optimized in the spectral range of 1400 nm to 2000 nm, and the fibers in the fifth column can be optimized in the spectral range of 1900 nm to 2500 nm. It should be noted that the same spectral coverage (e.g., 200nm to 2500nm) can be equally achieved by using any other number of optical fibers (e.g., 2, 3, 4, or 6).

[0118] [000116] In such a customized fiber arrangement, broad spectral coverage (e.g., 200 nm to 2500 nm) can be obtained by sequentially switching between fibers in different rows, and the overall fiber lifetime can be extended by sequentially switching between fibers in different rows. In this case, the switching of fiber cores can be initiated by commands sent by a metrology tool or operator, and can be enabled, for example, by a control system of a broadband radiation source RDS1 that controls the movement of the fiber assembly FAA. This arrangement may be preferred for applications where multiple different spectral bands are desirable but not required simultaneously. This customized arrangement allows each fiber to be optimized for a specific spectral range / band so that the output radiation from each fiber does not overlap with the fiber resonant band determined by the fiber structure (e.g., core wall thickness and / or core diameter). Less or no overlap of light glass (e.g., cladding structure) means less heating within the cladding structure, and consequently, a longer fiber lifetime.

[0119] [000117] If it is desirable to provide broad spectral coverage (e.g., 200 nm to 2500 nm) simultaneously rather than sequentially from the fiber assembly FAA, the input radiation can be spatially split into multiple separate beams, each beam then focused onto a different fiber in a row. The multiple beams of the output radiation ORD having multiple different spectral bands can then be spatially and temporally recombined into a single broadband output beam. Figure 13 schematically shows a multi-beam configuration of a broadband radiation source including a fiber assembly FAA according to an embodiment. The fiber assembly FAA can be pumped simultaneously by multiple beams of the input radiation IRD. The fiber assembly FAA may be, for example, any one of the fiber assemblies FAA1 to FAA4 described above.

[0120] [000118] If the fiber assembly includes a 2D fiber assembly (e.g., assembly FAA1 in Figure 10(a)) to a radiation source such as the radiation source RDS2 in Figure 13, a 1D array of multiple (e.g., five) input beams IB1-IB5 may be generated by the input radiation IRD passing through a beam splitting arrangement BSA. The beam splitting arrangement BSA may include, for example, an array of individual beam splitters (e.g., parallel glass plates). Each beam splitter may have a suitable optical coating optimized for a desired splitting ratio of the spectral band of the input radiation IRD. The five input beams IB1-IB5 may have substantially the same optical properties and are focused by a first focus arrangement FS1 onto five hollow cores HC in a particular row of the fiber assembly FAA1 (e.g., OF1-OF5 in the upper row). The first focus arrangement FS1 may be placed between the beam splitting arrangement BSA and a first transparent window TW1. At the output end, a second focus configuration FS2 can be used to collimate the five beams of the output emission ORD, and a beam coupling configuration BCA can be used to spatially recombine the five beams of the output emission ORD into a single output beam. The beam coupling configuration BCA may include, for example, five mirrors arranged to guide each of the beams of the output emission ORD to the same output beampath. Each mirror may have a highly reflective coating optimized for the spectral band of the output beam being guided by the mirror. Additionally or optionally, an optical delay configuration may be placed between the second focus configuration FS2 and the beam coupling configuration BCA to compensate for or customize the optical delay between the different beams of output emission.

[0121] [000119] In some embodiments, the focus configurations FS1, FS2 may include a 1D or 2D array of individual optical lenses. In different embodiments, the focus configurations FS1, FS2 may include a 1D or 2D microlens array, which includes an array of microlenses fabricated on a single optical substrate. In other different embodiments, the focus configurations FS1, FS2 may include a spatial light modulator, such as a digital micromirror device (DMD), which includes a 1D or 2D array of individually controllable micromirrors. Each micromirror may have a suitable radius of curvature configured to focus one of the input beams onto one of the hollow cores. By using a spatial light modulator (e.g., a DMD) as the focus configurations FS1, FS2, the radiation source RDS2 in Figure 13 can enable not only sequential pumping of multiple hollow cores HC of the fiber assembly FAA with a single beam of input radiation IRD, but also simultaneous pumping of multiple hollow cores HC of the fiber assembly FAA with an array of multiple input beams. When the RDS2 radiation source is used with any of the aforementioned metrology tools, multiple illumination beams and numerical apertures with different spectral bands can be achieved to provide simultaneous illumination of different parts and / or different spectral bands of the sample.

[0122] [000120] As described above, switching of the fiber core may be achieved by translating the fiber assembly FAA relative to the beam of the input radiating IRD and / or by shifting the beam of the input radiating IRD relative to the fiber assembly FAA. Figure 14 schematically shows a broadband radiating source RDS3 arrangement including a fiber assembly FAA and a second single-input beam configuration according to an embodiment. Unlike the first configuration shown in Figure 12, in the second configuration, the beam of the input radiating IRD can be shifted relative to the fiber assembly FAA. Spatial shifting of the input beam may be achieved by a first optical beam shifter OBS1. In some embodiments, the first optical beam shifter OBS1 may include, for example, at least one glass plate with parallel planes. The glass plate with parallel planes may have an optical input surface and an optical output surface, both of which are substantially flat and substantially parallel to each other. The glass plate with parallel planes may only displace the optical beam without causing a change in the directionality of the beam of the input radiating IRD. The amount of beam displacement may depend on the input angle formed between the input beam and the optical input surface. If parallel glass plates are positioned such that the optical input surface (and optical output surface) is perpendicular to the input optical beam of the input emitting IRD (i.e., the input angle is 90°), then beam displacement of the input beam may be absent or negligible. Conversely, if the input angle moves away from 90° by rotating the glass plates relative to the input beam (e.g., around the X-axis in Figure 12), then spatial displacement of the input beam may occur (e.g., in the YZ plane). The amount of spatial displacement may be proportional to the angular difference between the new angular position and the non-displaced angular position (i.e., the input angle of 90°). The input and output surfaces of the parallel glass plates may have a coating that provides anti-reflective properties in the spectral range of the input emitting IRD.

[0123] [000121] When a 1D fiber assembly (e.g., fiber assembly FAA3 in Figure 11(a)) is used with the radiation source RDS3 in Figure 14, a 1D beam shift element, such as a single parallel glass plate, can be used to shift the beam of the input radiation IRD vertically or in a plane defined by the Y and Z directions. By changing the rotation angle of the parallel glass plate, the beam path of the input radiation IRD can be shifted, for example, from a first beam path BP1 to a second beam path BP2, and then to a third beam path BP3. The parallel glass plate can be mounted on a rotary actuator capable of providing high-precision rotational motion. The angular position of the parallel glass plate can be predetermined based on the spatial separation between two neighboring fiber cores of the fiber assembly FAA3 and the magnification of the first optical lens OL1. Whenever it is time to replace the current optical fiber OF, the input radiation IRD can be at least partially blocked, for example, by a beam block. A rotary actuator can rotate parallel glass plates to ensure that the beam of the input radiated IRD propagating along the new beampath is substantially aligned with the hollow core HC of the new optical fiber OF. For example, when changing the beampath of the input radiated IRD from the first beampath BP1 to the second beampath BP2, the new optical fiber OF3 is used rather than the previous optical fiber OF1 to generate a broadband output radiated ORD.

[0124] [000122] Once the beam of the input radiation IRD has moved to the target beam path, the attenuated or complete input radiation IRD can enter the hollow core HC of the new fiber. Subsequently, the radiation source RDS3, or radiation source RDS1, can enter an input coupling optimization mode in which the coupling between the input radiation IRD and the hollow core HC of the new fiber is optimized. Input coupling optimization can be evaluated by monitoring specific optical properties (e.g., spectrum and / or power) of the output radiation while adjusting one or more components of the radiation source RDS3. In some embodiments, input coupling optimization may involve, for example, fine-tuning scanning of the position of the input beam relative to the fiber core in the X, Y, and Z directions. Additionally or optionally, the optimization routine may also involve tilting the first optical lens OL1 (e.g., tilt Rx around the X direction, tilt Rz around the Z direction) to adjust or optimize the orientation of the input beam and / or the translation of the first optical lens OL1 (e.g., along the Y direction) to adjust or optimize the focus position of the input beam to the fiber core. Similar to the embodiment in Figure 12, input coupling optimization may be performed only once when a new fiber assembly FAA is installed. After the initial input coupling optimization, subsequent fiber core switching can be repeatedly performed without requiring further optimization. However, input coupling optimization can be restarted at any time if necessary (e.g., for fault detection purposes).

[0125] [000123] As shown in Figure 14, the radiation source RDS3 may include a second optical beam shifter OBS2, which may also include at least one glass plate with parallel planes mounted on a rotary actuator. The glass plate with parallel planes may have a coating that provides anti-reflection in the spectral range of the output radiation ORD. The rotary actuators of the first optical beam shifter OBS1 and the second optical beam shifter OBS2 can be moved in combination to their respective new angular positions, which is intended to return the beam of the output radiation to its default or to a desired beam path, and based on this, the downstream optics (e.g., the optics of a metronome tool) may be aligned. The default or desired beam path can be determined, for example, by the optical alignment of the metronome tool if the output radiation is used in a tool, and / or by the position of the output delivery fiber if the output radiation is coupled to an output delivery fiber. Once input coupling optimization is complete, the radiation source RDS3 can enter an output coupling mode in which the position of the output beam can be optimized. In some embodiments, output coupling optimization may include, for example, fine-tuning of the second optical beam shifter OBS2 by tilting and / or translating the second optical lens OL2.

[0126] [000124] Plates with parallel planes can be made of low-dispersion material (e.g., N-FK58 made of CaF2). Low dispersion means that there is little variation in refractive index with respect to wavelength, so the advantage of such material is that it can reduce spectral variations in beam shift d. The relationship is,

number

[0127] [000125] In different embodiments, one or both of the first optical beam shifter OBS1 and the second optical beam shifter OBS2 may include one or more galvanoscanners or piezo mirrors whose sagittal and tangential surfaces are controlled with submicrometer resolution. By using two or more such mirrors sequentially, perfect alignment of the optical beam with respect to a particular hollow fiber of the fiber assembly is made possible.

[0128] [000126] Referring to Figure 14, if a 2D fiber assembly (e.g., fiber assembly FAA1 in Figure 10(a)) is used in the radiation source RDS3, the first optical beam shift OBS1 may include two plane-to-plane glass plates (e.g., a first plate and a second plate). The first plate may be rotatable around a first axis, and the second plate may be rotatable around a second axis, with the first and second axes being perpendicular to each other and forming planes parallel to the input facets of the fiber assembly FAA. In a preferred embodiment, the input facets of the fiber assembly FAA may be perpendicular to the beam of the input radiation IRD. Referring to Figure 4, for example, the first glass plate may be positioned to rotate horizontally or around the Z axis, and the second glass plate may be positioned to rotate vertically or around the X axis. The first glass plate may be placed downstream or upstream of the second glass plate. The downstream glass plate can be sufficiently large in the direction in which the input beam is shifted by the upstream glass plate. Therefore, the downstream glass plate can capture the displaced input beam regardless of how large an angle the upstream glass plate is rotated by.

[0129] [000127] In other embodiments, a fiber assembly FAA (e.g., FAA1, FAA2, FAA3, or FAA4) can be configured to selectively pressurize each of the individual fiber cores with a different gaseous medium or in a vacuum. Selective pressurization of each individual fiber core can be achieved, for example, by creating one or more gas channels connecting the fiber cores to different gas supplies in the support portion of each fiber. Alternatively, such embodiments may involve the use of HC-PCFs that are not contained within a gas cell (however, the HC-PCFs are individually sealed with a gaseous medium (e.g., sealed)). Such configurations can provide greater flexibility in controlling the optical properties of the output radiation ORD.

[0130] [000128] Figure 15 shows another embodiment of a fiber assembly including a multi-fiber integration. The figure shows the number of optical fibers OF in a fiber guide FG in cross-section. In contrast to configurations in which multiple optical fibers OF are individually assembled in a 1D or 2D groove mount (such as the examples shown in Figures 11(a) and 11(b)), the fiber integration-based configuration allows more fibers to be placed in a given volume. This has two advantages: a longer gas cell life due to increased redundancy and a smaller pitch that reduces the optical beam tuning range and velocity. In the embodiment of Figure 15, fiber clamping can be achieved, for example, by mechanical clamping with two or more mechanical members forming the fiber guide FG. Alternatively or additionally, fiber clamping can be achieved by bonding, melting, or (laser) welding LW of the fiber ends together.

[0131] [000129] For example, in many applications, such as those where a clean operating environment is desirable or those involving UV radiation, the use of multi-fiber arrangements in which optical fibers are clamped or fixed with adhesive may not be preferable. This is due to the fact that adhesives tend to release gases, which can lead to contamination, and that adhesives are prone to degradation under UV exposure, which can cause fiber drift. Furthermore, adhesives can also impose undesirable and changing stresses on the optical fibers, which can result in changes in performance or operating conditions over their entire lifespan. Even if the problems of contamination from gas release and degradation due to UV are successfully avoided, clamping stacks of optical fibers using melting or welding is time-consuming, especially when the multi-fiber arrangement contains a large number of optical fibers.

[0132] [000130] Mechanical clamping can avoid contamination and UV-induced degradation problems, and at the same time, it can be implemented quickly. Through research, the inventors have found that robust and stable mechanical clamping of optical fiber stacks can be obtained using a multi-fiber ensemble or configuration that satisfies the following three criteria. The first such criterion is that each optical fiber in the multi-fiber ensemble has a defined clamp, which means that each optical fiber has two supports (or contact points). If an optical fiber has more than two supports in a multi-fiber ensemble, the clamping of such an optical fiber is overdefined. If an optical fiber has more than two supports or its clamping is overdefined, the force vector at at least one of the optical fiber supports becomes uncertain and depends on the tolerances of the optical fiber and fiber holder. In contrast, if an optical fiber has fewer than two supports, the clamping of the optical fiber is underdefined. If an optical fiber has fewer than two supports or its clamping is underdefined, such an optical fiber will not be properly clamped.

[0133] [000131] The second criterion is that the resultant force or composite force vector affecting each optical fiber intersects the line formed by connecting the two support points (or contact points) of the optical fiber.

[0134] [000132] The third criterion is that the clamping force does not exceed the maximum force that each optical fiber (e.g., the top optical fiber to which the clamping force is directly applied) can tolerate. Such a maximum force is determined by the robustness of the fiber to stress and mode matching. In other words, the maximum force may be such that, when applied to the optical fiber, the properties of the optical fiber and the optical properties of the radiation emitted from the optical fiber remain substantially unchanged. The clamping force may also be strong enough to allow each optical fiber to withstand sufficient force to firmly clamp those fibers located at the bottom of the fiber stack.

[0135] [000133] Existing methods for mechanically clamping stacks of optical fibers do not meet the aforementioned criteria and therefore often result in unstable fiber clamping. When such unstablely clamped fiber stacks are used in any of the above embodiments (e.g., the embodiments shown in Figures 12-14), it will cause suboptimal coupling of the input radiated IRD, thereby resulting in fiber damage or a decrease in the performance of the output radiated ORD. Figures 16(a) and 16(b) schematically show two exemplary mechanically clamped fiber stack arrangements. In both exemplary fiber stack arrangements FSA1 and FSA2, mechanical clamping is achieved by using fiber holders in combination with fiber clamps, which together form a hollow fiber channel. The two exemplary arrangements include different fiber holders (and thus different fiber clamps) and support different numbers of optical fibers. For the purpose of illustrating the operating principle and related issues, both exemplary arrangements are simplified and only the fiber holders are shown. The fiber clamp is represented by a clamping force F.

[0136] [000134] Referring to Figure 16(a), the fiber holder 1 FH1 of the fiber integrating arrangement FSA1 holds seven optical fibers OF1 to OF7. A clamping force F is applied directly to the optical fiber 1 OF1. The clamping force F is then transmitted from the uppermost fiber (e.g., optical fiber 1 OF1) to the lowermost fiber (e.g., optical fiber 4 OF4) via various contact points or supports (as indicated by solid points) formed between adjacent optical fibers OF1 to OF7 or between the optical fibers and the inner wall of the fiber holder 1 FH1. For example, the clamping force F is transmitted through optical fiber 1 OF1 to optical fibers 6 OF6, 7 OF7, and 2 OF2 via contact points 1 CP1, 2 CP2, and 3 CP3, respectively. Since optical fiber 1 OF1 has three supports or contact points CP1 to CP3, its clamp is overdefined and therefore unstable. This is because the force vectors at these three contact points are highly dependent on and susceptible to the manufacturing tolerances of the three contacting optical fibers OF2, OF6, and OF7. For example, in the case of HC-PCF with a nominal diameter of 100 μm to 300 μm (after removing the outer coating), the typical manufacturing tolerance of the fiber diameter is approximately ±3 μm. The manufacturing tolerance of the fiber holder FH1 is typically in the range of ±5 μm to ±10 μm (depending on cost).

[0137] [000135] For example, if optical fiber 2 OF2 is smaller than usual (e.g., has a fiber diameter smaller than its nominal diameter), optical fiber 1 OF1 is supported, mainly or entirely, by optical fiber 6 OF6 and optical fiber 7 OF7, respectively, via contact points 1 CP1 and 2 CP2. In such a case, optical fiber 2 OF2 is barely touching optical fiber 1 OF1 or completely detached from optical fiber 1 OF1. Therefore, the force vector at contact point 3 CP3 is negligible or zero. Because there is no or negligible clamping force transmitted to optical fiber 2 OF2 via contact point 3 CP3, optical fiber 2 OF2 is lightly resting on optical fiber 3 OF3 via contact point 4 CP4. Optical fiber 2 OF2 may wobble between the contact point with optical fiber 7 OF7 and the contact point with the inner wall of fiber holder 1 FH1. Therefore, the clamp on optical fiber 2 OF2 is underdefined and loose. Similarly, if optical fiber 6 OF6 or optical fiber 7 OF7 is smaller than normal, the corresponding contact points with optical fiber 1 OF1 (e.g., contact point 1 CP1, contact point 2 CP2) will have negligible or zero force vectors, resulting in a loose clamping of the optical fiber. Similar loose or unstable clamping can also occur from some optical fibers that are larger than normal. Because the clamping of optical fibers OF1-OF7 in the fiber integrating arrangement FSA1 is either overdefined or underdefined, high requirements for manufacturing tolerances of the optical fibers and fiber holders are necessary to ensure stable and robust fiber clamping.

[0138] [000136] Figure 16(b) schematically shows another fiber stack configuration FSA2. In this exemplary configuration, fiber holder 2 FH2 supports a stack of six optical fibers OF8-OF13. All optical fibers except optical fiber 11 OF11 are supported by two contact points. Optical fiber 11 OF11 is supported by only one contact point (i.e., contact point 9 CP9). To mechanically clamp this fiber stack, a clamping force F is applied directly to the uppermost optical fiber (i.e., optical fiber 8 OF8). Since optical fiber 8 OF8 is supported by contact points 5 CP5 and 6 CP6, the force vector at each of these two contact points is half the force vector that affects optical fiber 1 OF1. Optical fiber 1 OF1 is supported in a stable position. Although both optical fibers 13 OF13 and 9 OF9 are supported by two contact points, their force vectors are directed only to a single contact point (e.g., contact point 7 CP7 and contact point 11 CP11, respectively). More specifically, in the case of optical fiber 13 OF13, its force vector transmitted from optical fiber 8 is directed only to contact point 7 CP7, leaving contact point 8 CP8 undefined (or having a negligible or zero force vector). Similarly, the force vector of optical fiber 9 OF9 is directed only to contact point 11 CP11, leaving contact point 10 CP10 undefined (or having a negligible force vector). Consequently, optical fiber 11 OF11 does not experience any clamping force and is therefore not properly clamped. Clamping stability is degraded if optical fiber 11 OF11 is smaller than normal (e.g., has a fiber diameter smaller than its nominal diameter), as it may wobble between optical fiber 13 and optical fiber 9 OF9. When such fiber cluster arrangements are used in any of the above embodiments (for example, the embodiments shown in Figures 12-14), uncertainty in fiber position can lead to potential fiber damage.

[0139] [000137] In order to satisfy the three criteria above simultaneously, a multi-fiber ensemble can be configured such that the center-to-center distance between any two adjacent optical fibers decreases in a bottom-up manner (i.e., from the bottom to the top of each row). Figure 17(a) schematically shows an improved fiber ensemble according to an embodiment. The improved fiber ensemble I-FSA may include a fiber holder I-FH and a fiber clamp (not shown). For simplicity, the effect of the fiber clamp is represented by the clamping force F. Note that the figure is a cross-sectional view of the fiber ensemble. In some embodiments, the fiber holder I-FH and fiber clamp may extend along the axial axis of the optical fibers OF31-OF38 and cover the entire length of the fiber. In different embodiments, the fiber holder I-FH and fiber clamp may not cover the entire length of the fiber, but instead may be applied only to each of the two end sections of the fiber. In other different embodiments, the fiber holder I-FH and fiber clamp may be applied to only one end section (e.g., the input end section of the fiber to which the input radiated IRD is coupled). For example, in such embodiments, the length of the fiber holder I-FH and fiber clamp may have a maximum length of 10 mm to 50 mm and / or a minimum length of 2 mm to 5 mm, for example, this length may be in the range of 5 mm to 50 mm, 5 mm to 20 mm, or 5 mm to 10 mm.

[0140] [000138] To avoid stress concentration due to the edge effect, clamping of the end end (fiber tip) of the fiber can be avoided, and the fiber holder I-FH and the fiber clamp are applied at a distance from the fiber tip. This distance may be such that fiber tip sections of 0.1mm to 5mm, 0.2mm to 5mm, 0.5mm to 5mm, 0.5mm to 2mm, or 0.5mm to 2mm are not clamped. Therefore, when it is stated that the clamp is applied to an end section, it should be understood in this context that such an “end section” may exclude the fiber tip section at the end (i.e., that end section or each end section that does not extend to the fiber tip). Similarly, a clamp covering the entire length of the fiber can also be understood as not covering these fiber tip sections at both ends.

[0141] [000139] In this embodiment, the fiber holder may include a multi-level (e.g., two-level) structure, where the width of the upper (or second) level is wider than the width of the lower (or first) level (e.g., D2 > D1). In this context, width may be defined as the distance between two opposing inner walls of the same level, for example, the width of D1 is the distance between IW1 and IW2 of the first level. At each level, the inner walls (e.g., inner wall 1 IW1, inner wall 2 IW2) may be substantially perpendicular to the base (e.g., B1). Such a fiber holder may also be considered to include a multi-level (e.g., two-level) rectangular groove. The resulting fiber stack may include four levels of integration, with optical fibers OF34 and OF35 located at the first level of integration (or the lowest level), optical fibers OF33, OF38, and OF36 located at the second level of integration, optical fibers OF37 and OF32 located at the third level of integration, and optical fiber 31 and OF31 located at the fourth level of integration (or the highest level). The width D1 of the first level of the improved fiber holder I-FH may be greater than the nominal fiber diameter d by a first factor (e.g., 2.7), and the width of the second level of the improved fiber holder I-FH may be greater than the nominal fiber diameter d by a second factor (e.g., 3.7) between 3.5 and 3.9, 3.6 and 3.8, 3.65 and 3.75, and 3.69 and 3.71. Such a relationship between the dimensions of the fiber holder I-FH and the dimensions of the nominal fiber diameter can ensure that the center-to-center distance between two adjacent optical fibers of the same level is reduced in a bottom-up manner. For example, the center-to-center distance d1 between optical fiber 34 OF 34 and optical fiber 35 OF 35 may be greater than the center-to-center distance d2 between optical fiber 36 OF 36 and optical fiber 38 OF 38 or between optical fiber 38 OF 38 and optical fiber 33 OF 33, and consequently, the center-to-center distance d2 may be greater than the center-to-center distance d3 between optical fiber 37 OF 37 and optical fiber 32 OF 32. In the embodiment of Figure 17(a), the center-to-center distance d1 between optical fiber 34 OF 34 and optical fiber 35 OF 35 may be greater than the nominal fiber diameter d by a third factor (e.g., 1.7).

[0142] [000140] In the embodiment, each of the optical fibers OF31 to OF38 can be supported by two contact points, and the resultant force or composite force vector of each optical fiber can intersect the line formed by connecting its two contact points. For example, each of the optical fibers OF31, OF32, OF33, OF36, and OF37 can be affected by a single force vector transmitted from an optical fiber above it (for example, optical fiber 36 OF36 can be affected by a force vector transmitted from optical fiber 37 OF37). In these cases, the single force vector (e.g., F31) of each optical fiber (e.g., OF31) directly intersects the dashed line formed by connecting two contact points (e.g., CP31 and CP32) of the same fiber (e.g., OF31).

[0143] [000141] In contrast, each of the optical fibers OF34, OF35, and OF38 may be in contact with two upper optical fibers and therefore be affected by two force vectors, each propagating in a different direction. A resultant force or composite force vector can be determined for each of these three optical fibers OF34, OF35, and OF38. As an example, optical fiber 34 may be in contact with two upper optical fibers OF33 and OF38. Consequently, optical fiber OF34 may be affected by a first force vector F34-1 transmitted from optical fiber 33 OF33 and a second force vector F34-2 transmitted from optical fiber 38 OF38. The resultant force or composite force vector F34 acting on optical fiber 34 OF34 (as shown by the thick arrow in Figure 17(a)) may intersect the dashed line formed by connecting two contact points CP34 and CP35. Similarly, the resultant or combined force vectors of optical fiber 35 OF35 and optical fiber 38 OF38 can also intersect the dashed line formed by connecting their two contact points.

[0144] [000142] Accordingly, the embodiment of Figure 17(a) simultaneously satisfies the first two of the aforementioned criteria, namely, that each optical fiber is supported by two defined support or contact points, and that the resultant force vector of the optical fiber intersects the line formed connecting the two support points. Fiber stack arrangements configured in such a manner are less susceptible to manufacturing tolerances of the optical fibers and fiber holders, and therefore the stability of the mechanical clamp can be greatly improved. Since the clamping force is reduced in a top-down manner, the clamping force F can be strong enough to allow the lower optical fibers OF34, OF35 to be affected by a sufficiently strong force vector, and therefore can be clamped firmly in place. On the other hand, the clamping force F can be kept so as not to exceed the maximum allowable force of each optical fiber (specifically, the uppermost optical fiber OF31 to which the clamping force is directly applied). This is to ensure that a stable and robust mechanical clamp of the fiber stack is achieved without any degradation of optical performance due to the application of the clamping force. Clamping force can induce stress in the fiber, which can lead to, for example, polarization angle shift and reduced mode matching, resulting in significant performance loss. The acceptable level of induced stress may depend on the thickness of the fiber cladding. The maximum clamping force may range from 10 to 50 Newtons (N) (e.g., 10N, 20N, 30N, or 40N).

[0145] [000143] It should be noted that the embodiments described above are merely examples. Other different embodiments that simultaneously satisfy the three criteria mentioned above are equally applicable. For example, in some embodiments, the improved fiber holder I-FH may include two or more levels (e.g., 1, 3, 4, 5, or 6 levels) and thus be able to clamp eight or more optical fibers (e.g., 3, 15, 24, 35, or 48 optical fibers) in a stable and robust manner.

[0146] [000144] Figure 17(b) schematically shows an improved different fiber integrating arrangement according to an embodiment. In this fiber integrating arrangement I-FSA', the fiber holder I-FH' may include a single level. The distance D1 between two opposing inner walls IW1', IW2' which are substantially perpendicular to the base B1' may be greater than the nominal fiber diameter d by a first factor (e.g., 2.7) of 2.5–2.9, 2.6–2.8, 2.65–2.75, and 2.69–2.71. Thus, the center-to-center distance d1 between optical fiber 34 OF34 and optical fiber 35 OF35 may be greater than the nominal fiber diameter d by a third factor (e.g., 1.7) of 1.5–1.9, 1.6–1.8, 1.65–1.75, and 1.69–1.71. Similar to the embodiment in Figure 17(a), each of the three optical fibers OF34, OF35, and OF38 is stably supported by two contact points and is affected by force vectors that directly intersect the line formed by connecting these two contact points. Thus, such a fiber integrating arrangement I-FSA' also enables stable and robust clamping of the fiber stack.

[0147] [000145] It should be noted that the total number of optical fibers that can be mechanically clamped in a fiber integrating arrangement can be increased in proportion to the number of levels of fiber holder I-FH. However, the total number will not increase further when any two adjacent optical fibers at the same level are in contact with each other. In such cases, the center-to-center distance between these two adjacent fibers is minimal and equal to the fiber diameter d.

[0148] [000146] In the embodiment, the support angle (relative to the fiber center) defined between any two adjacent contact points of the fiber is located in the range of 60° to 120°. Considering the example in Figure 17(b), the support angle of the first row is 120°, the support angle of the second row is 90°, and the support angle of the third row is 60°. When the total number of rows is four, the minimum angle of the force vector relative to the support is 30°.

[0149] [000147] Other integration configurations can be used, for example, there may be more than two optical fibers at the lowest level of the fiber stack. In such a configuration, the widths of different levels of the fiber holder I-FH may have different relationships (e.g., different magnifications) with respect to the fiber diameter so that the center-to-center distance between any two adjacent optical fibers is reduced in a bottom-up manner. In different embodiments, it is preferable to remove the outer coating of each optical fiber before integrating and clamping them together.

[0150] [000148] Typically, fiber holders I-FH, I-FH' may be made of a different material (e.g., glass, metal) than the material of the optical fiber, and therefore may have different coefficients of thermal expansion. The mismatch in the coefficients of thermal expansion between the fiber holder and the optical fiber can cause relative movement during temperature fluctuations. As an example, many optical fibers made of fused silica can be mechanically clamped using a stainless steel fiber holder. Since the coefficient of thermal expansion of stainless steel is much higher than that of fused silica, when the ambient temperature rises, the fiber holder expands more than the optical fiber, and as a result the optical fiber is loosely clamped (the fiber holder is too large to firmly clamp the optical fiber). When the elevated ambient temperature drops and returns to the original temperature at which the optical fiber was firmly clamped in the fiber holder, the fiber holder and the optical fiber contract and return to their original sizes. However, friction between any two contact surfaces (e.g., between any two optical fibers, or between an optical fiber and the inner wall / base of the fiber holder) can prevent some or all of the optical fibers from returning to their aligned positions relative to the fiber holder, resulting in the inability to self-recover or substantially self-recover the rigid mechanical clamp to its original state (e.g., the fiber clamp is either overdefined or underdefined). To withstand the effects of temperature cycling / variation, stricter design criteria can be implemented in addition to the above design requirements. In some embodiments, additional design criteria may include, for example, criteria regarding the support angle of each optical fiber, criteria regarding any force vector angle, and criteria regarding the coefficient of friction of any two contact surfaces.

[0151] [000149] Figure 17(c) schematically shows a further improved fiber integrating arrangement according to an embodiment. In this embodiment, the further improved fiber integrating arrangement I-FSA” can firmly clamp seven optical fibers OF41-OF47. Two fiber clamps applying substantially the same clamping force may be required to clamp the optical fibers OF41-OF47 robustly and stably. This is in contrast to the embodiments shown in Figure 17(a) or 17(b) in which a single fiber clamp or a single clamping force F is used. Again, for simplicity, the effect of each fiber clamp is expressed by clamping forces F1, F2 (e.g., F1=F2). Fiber holder I-FH” may have the same structure as fiber holder I-FH shown in Figure 17(a) (e.g., two-level rectangular groove). The dimensions of fiber holder I-FH” (for example, the distance between two opposing inner walls IW1”, IW2” at each level of fiber holder I-FH”) may vary due to the implementation of stricter design standards.

[0152] [000150] As shown in Figure 17(c), each optical fiber OF41 to OF47 can be supported by two contact points. For each optical fiber OF41 to OF47, the support angle can be formed by two lines connecting the center of the optical fiber to the two contact points, respectively. For example, optical fibers OF41, OF43, and OF44 have support angles α1, α3, and α4, respectively. If an optical fiber is supported by two other optical fibers, the support angle is equal to the sum of the two force vector angles, each of which is formed between the direction of the corresponding force vector and the z-direction with respect to the local coordinate system shown in Figure 17(c). For example, in the case of optical fiber OF41, the force vector angle β 13 This is the force vector F applied to optical fiber OF43 by optical fiber OF41. 13 A force vector angle β is formed between the direction and the perpendicular direction. 14 This is the force vector F applied to optical fiber OF44 by optical fiber OF41. 14is formed between the direction of and the perpendicular direction. The two force vector angles β 13 , β 14 may have the same or different values depending on the relative position of the optical fiber OF43 and the optical fiber OF44 (for example, the relative central position along the z direction). In the case of the optical fiber OF44, it has four contact points formed with each of the optical fibers OF41, OF42, OF46, and OF47. However, the optical fiber OF44 is still supported by two contact points (with the optical fibers OF46 and OF47), so its support is clearly defined. Similar to the optical fiber OF41, the support angle α4 is equal to the sum of the force vector angle β 41 and the force vector angle β 42 .

[0153] [000151] As described above, there is a frictional force at each contact point that can prevent the optical fiber from returning to its original position. Therefore, in order to overcome such frictional force, the tangential component of the resultant force vector of any optical fiber can be greater than the frictional force in the tangential direction, and the normal component of the resultant force vector of the same optical fiber can be greater than the frictional force in the normal direction (if applicable). It should be noted that the resultant force vector of the optical fiber is the overall force vector obtained by synthesizing all the force vectors applied to the optical fiber.

[0154] [000152] As an example, the optical fiber OF47 may have a resultant force vector F 47 applied only by the optical fiber OF44. The resultant force vector F 47 can be decomposed into a tangential component F 47h and a normal component F 47v . The optical fiber OF47 is supported by two contact points, one formed between it and the inner wall IW2”, and the other formed between it and the base B1”. At the contact point on the surface of the inner wall IW2”, there is a frictional force F fv whose magnitude is the tangential component F 47 of the resultant force vector F 47hIt depends on the size and the coefficient of friction at the contact point. At the contact point formed by the optical fiber OF47 and base B1'', the frictional force F fh There exists a resultant force vector F, whose magnitude is... 47 The normal component F 47v It depends on the size and the coefficient of friction at the contact point. The coefficient of friction at any contact point depends on the material of the contact surface. If the fiber holder I-FH" is made of metal (e.g., aluminum, stainless steel), the coefficient of friction between the optical fiber (e.g., optical fiber OF47) and the inner surface of the fiber holder (e.g., inner wall IW2" or base B1") may be 0.7 or less. For example, if the contact point is formed between a fused silica optical fiber and a stainless steel fiber holder, it may be in the range of 0.19 to 0.41. For comparison, the coefficient of friction between any two fused silica optical fibers may be in the range of 0.22 to 0.36.

[0155] [000153] In order to overcome surface friction, the resultant force vector of any optical fiber may satisfy the following conditions. F h ≥F fh ; [1] F v ≥F fv [2] During the ceremony, F h =F r ×Sin(β); ​​[3] F v =F r ×Cos(β); [4] F fh =F v ×cof; [5] F fv =F h ×cof; [6] In the formula, F h is the resultant force F r The tangential component of F is shown. v is the resultant force F r The vertical component of F is shown. fh F represents the frictional force in the tangential direction (e.g., horizontal or x-direction), and fvθ represents the frictional force in the normal direction (e.g., vertical or z-direction), cof represents the coefficient of friction at the point of contact, and β represents the angle formed between the resultant force vector and the vertical or z-direction.

[0156] [000154] When the optical fiber is directly supported by the fiber holder I-FH” (inner wall and base), the design conditions expressed by equations [1] and [2] can be converted to the following angular conditions: arctan(cof)≦β≦arccot(cof) [7] Applying the coefficient of friction to equation [7] yields a preferred range of resultant force vector angles β for any optical fiber that can overcome the frictional force. For example, if the coefficient of friction is 0.7 or less, the resultant force vector angle β is in the range of 35° ≤ β ≤ 55°. In the fiber integrating arrangement I-FSA” shown in Figure 17(c), the support angle is equal to or substantially close to 90°, as defined by the angle between the two support surfaces (e.g., inner wall IW2” and base B1”).

[0157] [000155] When an optical fiber is supported by two other optical fibers, the frictional force exists only in the tangential direction. The design conditions expressed by equation [1] can be translated into the following angular conditions: β≧arctan(cof) [8] In the case of fused silica optical fibers and with a coefficient of friction of 0.38, the preferred range for the resultant force vector angle β is calculated to be β ≥ 21°. Such a range can ensure that the resultant force vector of any optical fiber supported by two other optical fibers is stronger than the frictional force at the fiber-to-fiber contact point or interface. However, while the fiber stack arrangement is cooling, due to the mismatch in thermal expansion coefficients between the optical fiber and the fiber holder, the fiber holder may exert a (horizontal) contraction force on the fiber stack. This contraction force can push up the optical fibers supported by two other optical fibers by acting against the frictional force at the fiber-to-fiber interface. To ensure that the contraction force is stronger than the frictional force at any fiber-to-fiber interface, the resultant force vector of the optical fibers may further satisfy the following criteria: Sin(β) * Cos(β)≧cof [9] Applying the same coefficient of friction of 0.38 to equation [9] and assuming that the effect of the clamping force is negligible, the desired range of the resultant force vector angle β is calculated to be 25° ≤ β ≤ 65°. Such a narrower angle requirement (compared to the above range β ≥ 21°) allows the optical fiber to slide up and down at any interface between fibers during temperature fluctuations. In a preferred embodiment, the angle of the resultant force vector may be designed to be 45° ± 15°.

[0158] [000156] Referring to Figure 17(c), in order to ensure that all optical fibers in the fiber integrating arrangement are clearly defined (i.e., neither overdefined nor underdefined), optical fibers OF43, OF45 having two contact points with the fiber holder I-FH” (e.g., one with the inner wall and the other with the base) are not supported by other optical fibers. In some embodiments, minimum distances G1 and G2 may exist between the outer surface of optical fiber OF45 and the outer surface of optical fiber OF47 and between the outer surface of optical fiber OF43 and the outer surface of optical fiber OF46. The minimum distances G1 and G2 may be, for example, 1 μm or more. Furthermore, in order to ensure reliable base support and to facilitate improved manufacturability, the width of the base of the second level B2” may be wider than the radius of the optical fibers OF43, OF45. In some embodiments, the minimum difference G3 between the width of the base B2” of the second level and the radius of the optical fibers OF43, OF45 may be, for example, 15 μm or more.

[0159] [000157] Mechanical clamping of multiple optical fibers can provide a robust and stable fiber stack for applications such as fiber switching in HC-PCF-based broadband radiation sources, but alternative methods can be used to achieve similar or the same effect. One alternative method is to clamp multiple optical fibers using a heat-shrinkable fiber tube. The idea behind such an alternative method is that when heated by a heat source (e.g., an arc heat source), the heat-shrinkable fiber tube begins to shrink, bundling the optical fibers together into a fiber bundle or fiber stack. Such a method eliminates the need for the stringent design criteria of the fiber stack shown in Figure 17(c) and allows the multiple optical fibers to self-align into a robust and stable fiber stack. Furthermore, this method also makes the fiber stack easier to install.

[0160] [000158] Figure 18 is a schematic diagram illustrating the process of creating a fiber ensemble using heat-shrinkable tubing. In the exemplary configuration shown in Figure 18, two separate, substantially identical heat-shrinkable fiber tubes or sleeves FT1, FT2 can be applied to two end sections of multiple optical fibers OF, respectively. In such cases, each fiber tube FT1, FT2 may have a maximum length of 10 mm to 50 mm and / or a minimum length of 2 mm to 5 mm, for example, this length may be in the range of 5 mm to 50 mm, 5 mm to 20 mm, or 5 mm to 10 mm. In other exemplary configurations, a single heat-shrinkable fiber tube has a tube length that is substantially the same as or slightly longer than the total length of the optical fiber OF.

[0161] [000159] A manufacturing process for producing a fiber ensemble using heat-shrinkable fiber tubes may include the following three main steps: Step 1: Insert each end of the multiple optical fiber OFs into the heat-shrinkable fiber tubes FT1 and FT2. Before insertion into the fiber tubes FT1 and FT2, the optical fiber OFs may be pre-assembled, preferably into a desired assembly arrangement, for example, to match the internal shape of the fiber tubes FT1 and FT2. Step 2: To shrink the two tubes and thus tightly bundle the multiple optical fibers OF, the two heat-shrinkable fiber tubes FT1, FT2 are heated sequentially or in parallel using one or more heat sources HS1, HS2. The degree of shrinkage may depend, for example, the desired clamping force, the material of the fiber tubes FT1, FT2, the internal shape of the fiber tubes and / or how the optical fibers OF are assembled. Preferably, the heat (as indicated by the large gray arrow in Figure 18) is applied substantially uniformly around the circumferential area of ​​one or both fiber tubes, depending on whether the two fiber tubes are heated sequentially or in parallel. Substantially uniform heating can be achieved, for example, by rotating the entire assembly (optical fibers with heat-shrinkable tubes) around the central axis of the fiber tubes, or by simultaneously using multiple heat sources equally spaced around the circumference of the fiber tubes FT1, FT2. To minimize the thermal effects on the optical fiber, the fiber tubes FT1 and FT2 can be made from materials having a melting temperature lower than the melting temperature of the optical fiber (for example, lower than the melting temperature of fused silica). In some embodiments, the material of the fiber tubes FT1 and FT2 may be a certain type of soft glass (for example, a borosilicate). Step 3: Cleave the two fiber optic tubes sequentially or in parallel using one or more cleavers CL (e.g., fiber cleavers) so that the end face of each fiber optic tube is substantially coplanar with the end of the optical fiber OF. In many cases, the fiber optic tubes can be cleaved so that the end face of each fiber optic tube is located a short distance from the fiber tip. This distance may be such that fiber optic tube sections of 0.1mm–5mm, 0.2mm–5mm, 0.5mm–5mm, 0.5mm–2mm, or 0.5mm–2mm do not come into contact with any optical fiber OF. As shown in Figure 18, the cleavage position of fiber optic tube FT1 is indicated by small black arrows.

[0162] [000160] Figures 19(a) to 19(e) are schematic cross-sectional views of five exemplary fiber optic assemblies, all manufactured using heat-shrinkable fiber optic tubes (or heat-shrinkable fiber optic tubes at the time of assembly). For simplicity, only one fiber optic tube is shown and described for each exemplary fiber optic assemblies. However, it should be understood that in practice, substantially identical pairs of heat-shrinkable fiber optic tubes, each covering an end section of an optical fiber OF, or a single heat-shrinkable fiber optic tube having substantially the same length as the entire length of the optical fiber OF, or slightly longer, can be used.

[0163] [000161] In the first example shown in Figure 19(a), the fiber integrating arrangement F-FSA-a can be manufactured using two separate, but substantially identical, heat-shrinkable fiber tubes FT-a and FT-a' (used at the other end of the optical fiber and not shown in the figure), which clamp the two end sections of the optical fiber OF, respectively. The heat-shrinkable fiber tube FT-a may include an internal channel IC which may have a circular or substantially nearly circular shape and diameter ID. It should be understood that after shrinkage, the shape of the internal channel IC may deviate slightly from its pre-shrinkage (or original) shape. For example, after shrinkage, the original circular shape of the internal channel IC of fiber tube FT-a may eventually settle into a shape that is no longer circular but lies between a polygonal and circular shape. The fiber integrating arrangement may include three optical fibers OF, each having three contact points (one contact point between the fiber and the fiber tube, and two contact points between the fibers). In contrast to the above embodiment in which a clamping force is applied to the optical fiber located at the top of the fiber stack, the clamping force CF in this embodiment may be the thermal shrinkage force applied after the heat-shrinkable tube is heated. Furthermore, the clamping force in this embodiment may be applied to all optical fibers OF, and each optical fiber OF receives a clamping force CF of substantially the same magnitude. In addition to the clamping force applied by the fiber tube, each optical fiber may receive two additional forces applied by the other two optical fibers through the contact points between the two fibers. In such a way, the force at each of the three contact points of the optical fiber can be supported by the other two contact points. Thus, the clamp of each optical fiber is clearly defined (according to the design requirements above).

[0164] [000162] In the fiber integrating arrangement F-FSA-b shown in Figure 19(b), the heat-shrinkable fiber tube FT-b can be a variation of the fiber tube FT-a shown in Figure 19(a), the difference mainly lies in the larger diameter ID' of the internal channel IC used to clamp more optical fibers OF (e.g., more than three optical fibers OF). As shown in Figure 19(b), one central fiber OF-C and six surrounding fiber OF-O are bundled together or clamped by the heat-shrinkable fiber tube FT-b to form the fiber integrating arrangement F-FSA-b. Each of the surrounding optical fibers OF-O (surrounding the central fiber) can be subjected to substantially the same clamping force CF. In contrast to the example shown in Figure 19(a), the clamping of the optical fibers in this fiber integrating arrangement F-FSA-b is not clearly defined in that any of the surrounding optical fibers can be supported by three contact points (each contact point associated with a different optical fiber). In this exemplary configuration, the central fiber OF-C may have substantially the same diameter as any of the six surrounding optical fibers. In a different exemplary configuration, the central fiber may have a different diameter than the six surrounding optical fibers. In another exemplary configuration, a central structure (e.g., a glass rod) can be used instead of the central fiber OF-O.

[0165] [000163] To provide robust and stable clamping of more optical fibers (e.g., more than three optical fibers) using a heat-shrinkable fiber tube, it is desirable to use a heat-shrinkable fiber tube that can ensure a clearly defined clamped state for each optical fiber during shrinkage. This may require that each optical fiber in direct contact with the heat-shrinkable fiber tube have only three contact points. Fiber integrating arrangements F-FSA-c and F-FSA-d shown in Figures 19(c) and 19(d) are examples that can satisfy such requirements. A commonality between these two exemplary fiber integrating arrangements may be that a central optical fiber or central structure is surrounded by three or more optical fibers (in direct contact with the fiber tube), and each of the surrounding optical fibers has two contact points with the fiber tube and one contact point with the central fiber or central structure.

[0166] [000164] In the fiber integrating arrangement F-FSA-c shown in Figure 19(c), seven optical fibers are clamped by a heat-shrinkable tube FT-c, and its internal channel IC may have a hexagonal or substantially hexagonal shape. Each of the surrounding optical fibers OF-O is placed at each of the six corners of the hexagonal internal channel IC, and thus may have two contact points (with two sides of the corner) with the fiber tubes FT-c and FT-c'. Furthermore, each of the surrounding optical fibers OF-O may have a third contact point with the central fiber OF-C. Thus, each of the surrounding optical fibers may have a total of three contact points, resulting in a clearly defined clamped state. In a similar manner, the internal channel IC of the heat-shrinkable fiber tube FT-d of the fiber integrating arrangement F-FSA-d may have a triangular or substantially triangular shape. Each of the surrounding optical fibers OF-O may be placed at each of the three corners of the internal channel IC.

[0167] [000165] After shrinkage, the shape of the internal channel IC may deviate from its original shape before shrinkage (e.g., hexagonal or triangular), and may settle into a shape that lies between a polygonal shape (e.g., hexagonal or triangular) and a circular shape. Only if each fiber has only three contact points can a clearly defined clamping state still be maintained after shrinkage of the fiber tube. Note that heat-shrinkable fiber tubes with internal channels of different shapes or structures are equally suitable for creating robust and stable fiber ensemble arrangements. The internal channels of a heat-shrinkable fiber tube may have other different polygonal shapes, such as square, pentagonal, heptagonal, octagonal, or nonagonal shapes. Instead, the shape of the internal channel may be any shape, such as a regular or irregular polygon, or an irregular shape containing both circular regions and corners, provided that the internal channel contains multiple corners, each of which provides support to the optical fiber via two contact points, and the central fiber or central structure provides further support to the optical fiber via a third contact point. In some different exemplary arrangements, the central fiber or central structure may have a different diameter from the surrounding optical fiber.

[0168] [000166] In the fiber integrating arrangement F-FSA-e shown in Figure 19(e), the internal channel IC of the heat-shrinkable fiber tube FT-e may include a central structure (e.g., a glass rod) and three internal fixing elements FE arranged to fix and connect the central structure CS to three sides of the triangle of the internal channel IC. The internal fixing elements FE may be, for example, three thin walls, each extending radially and axially. The internal fixing elements FE may separate the internal channel into three subvolumes, each containing one surrounding optical fiber OF-O. If the internal channel IC has a different polygonal shape, the number of internal fixing elements FE may be the same as the number of sides of the polygon, and each internal fixing element may be positioned to connect the central structure to one of the sides of the polygon. Thus, each subvolume formed by sandwiching any two adjacent fixing elements and the central structure may contain only one optical fiber.

[0169] [000167] A broadband radiation source configured to extend the entire lifetime of the radiation source as disclosed herein includes a fiber assembly which may include, for example, any of the configurations shown in Figures 10(a), 10(b), 11(a), 11(b), 15, 17(a), or 17(b).

[0170] [000168] A broadband radiation source configured to extend the entire lifetime of the radiation source, as disclosed herein, includes a fiber assembly and may include any of the configurations shown in Figures 12, 13, or 14.

[0171] [000169] Figure 20 is a block diagram of a computer system 2000 that may assist in the implementation of the methods and flows disclosed herein. The computer system 2000 includes a bus 2002 or other communication mechanism for transmitting information and a processor 2004 (or a plurality of processors 2004, 2005) coupled to the bus 2002 for processing information. The computer system 2000 also includes main memory 2006 (such as random access memory (RAM) or other dynamic storage device) coupled to the bus 2002 for storing information and instructions for execution by the processor 2004. The main memory 2006 may also be used to store temporary variables or other intermediate information while the processor 2004 is executing instructions. The computer system 2000 further includes read-only memory (ROM) 2008 or other static storage device coupled to the bus 2002 for storing static information and instructions for the processor 2004. A storage device 2010 (magnetic disk or optical disk) is provided and coupled to the bus 2002 for storing information and instructions.

[0172] [000170] The computer system 2000 can be coupled via bus 2002 to a display 2012 (such as a cathode ray tube (CRT) or a flat panel or touch panel display) for displaying information to the computer user. An input device 2014 (including alphanumeric and other keys) is coupled to bus 2002 to transmit information and command selections to the processor 2004. Another type of user input device is a cursor control 2016 (such as a mouse, trackball, or cursor directional keys) for transmitting directional information and command selections to the processor 2004 and for controlling the movement of a cursor on the display 2012. This input device typically has two degrees of freedom on two axes, namely a first axis (e.g., x) and a second axis (e.g., y), so that the device can specify a position in a plane. Alternatively, a touch panel (screen) display can be used as an input device.

[0173] [000171] One or more of the methods described herein can be executed by a computer system 2000 in response to a processor 2004 executing one or more sequences of one or more instructions contained in main memory 2006. Such instructions can be read into main memory 2006 from another computer-readable medium (such as a storage device 2010). By executing the sequence of instructions contained in main memory 2006, the processor 2004 performs the process steps described herein. Alternatively, one or more processors in a multiprocessing arrangement may be used to execute the sequence of instructions contained in main memory 2006. In alternative embodiments, wiring circuits can be used instead of or in combination with software instructions. Thus, the description herein is not limited to any particular combination of hardware circuits and software.

[0174] [000172] The term “computer-readable medium” as used herein means any medium involved in providing instructions for execution to the processor 2004. Such mediums can take many forms, but are not limited to, non-volatile mediums, volatile mediums and transmission mediums. Non-volatile mediums include, for example, optical or magnetic disks such as the storage device 2010. Volatile mediums include dynamic memory such as the main memory 2006. Transmission mediums include coaxial cables, copper wires and optical fibers, including wires including the bus 2002. Transmission mediums can also take the form of acoustic or optical waves, such as those generated between radio frequency (RF) and infrared (IR) data communications. Common forms of computer-readable media include, for example, floppy disks, flexible disks, hard disks, magnetic tapes, any other magnetic media, CD-ROMs, DVDs, any other optical media, punch cards, paper tapes, any other physical media having a pattern of holes, RAM, PROMs, EPROMs, FLASH-EPROMs, any other memory chips or cartridges, the carriers described below, or any other media that a computer can read.

[0175] [000173] Various forms of computer-readable media may be involved in transporting one or more sequences of one or more instructions for execution to the processor 2004. For example, the instructions may initially be on a magnetic disk of a remote computer. The remote computer may load the instructions into its dynamic memory and transmit the instructions over a telephone line using a modem. A modem locally connected to computer system 2000 may receive the data over the telephone line and convert the data into an infrared signal using an infrared transmitter. An infrared detector coupled to the bus may receive the data being transported by the infrared signal and load the data onto the bus 2002. The bus 2002 transports the data to main memory 2006, and the processor 2004 retrieves the instructions from main memory 2006 and executes them. The instructions received by main memory 2006 may optionally be stored in storage device 2010 before or after execution by the processor 2004.

[0176] [000174] The computer system 2000 also preferably includes a communication interface 2018 coupled to the bus 2002. The communication interface 2018 provides bidirectional data communication coupled to a network link 2020 connected to a local network 2022. For example, the communication interface 2018 may be an Integrated Services Digital Network (ISDN) card or modem for providing data communication connectivity to a corresponding type of telephone line. As another example, the communication interface 2018 may be a Local Area Network (LAN) card for providing data communication connectivity to a compatible LAN. Wireless links may also be implemented. In any such implementation, the communication interface 2018 transmits and receives electrical, electromagnetic or optical signals carrying digital data streams representing various types of information.

[0177] [000175] Network link 2020 typically provides data communication with other data devices through one or more networks. For example, network link 2020 may provide a connection to a host computer 2024 or to data equipment operated by an Internet service provider (ISP) 2026 through a local network 2022. In turn, ISP 2026 provides data communication services through the World Wide Packet Data Communications Network (now commonly referred to as the “Internet” 2028). Both the local network 2022 and the Internet 2028 use electrical, electromagnetic, or optical signals to carry digital data streams. Signals through various networks, as well as signals on network link 2020 and through communication interface 2018 (carrying digital data to and from computer system 2000), are exemplary forms of carrier waves that carry information.

[0178] [000176] The computer system 2000 can send messages and receive data, including program code, through a network, network link 2020, and communication interface 2018. In the example of the Internet, server 2030 can send request code for an application program through the Internet 2028, ISP 2026, local network 2022, and communication interface 2018. One such application to be downloaded may, for example, be provided in one or more of the techniques described herein. The received code may be stored in memory 2010 or other non-volatile memory for execution by processor 2004 upon receipt and / or for later execution. In this way, the computer system 2000 can obtain application code in carrier form.

[0179] [000177] Further embodiments are disclosed in the following numbered list of clauses. 1. A fiber assembly comprising multiple optical fibers, wherein each optical fiber is filled with a gaseous medium. A broadband radiation source device including, A broadband radiation source device in which a subset of optical fibers can be operated to be individually selected as the target for receiving the beam of input radiation, so that at any given time, a broadband output is generated from only one subset of multiple optical fibers. 2. A broadband radiation source device as defined in Clause 1, wherein any one of multiple optical fibers is operable to be independently selectable as the target for receiving the beam of input radiation. 3. A broadband radiation source device as defined in Clause 1 or 2, in which each optical fiber includes a hollow-core photonic crystal fiber. 4. A beam splitting configuration that can operate to spatially divide the input radiation beam into multiple input beams, Focus configuration for focusing each of multiple input beams onto each optical fiber in a subset of optical fibers, and A broadband radiation source device, including, as defined in clauses 1, 2, or 3. 5. A broadband radiation source device as defined in Clauses 1, 2, or 3, which is capable of operating so that broadband output is generated from only one of multiple optical fibers at any given time. 6. A broadband radiation source device as defined in any one of the preceding clauses, comprising multiple optical fibers, including two or more fibers having substantially the same fiber characteristics. 7. A broadband radiation source device as defined in any one of the preceding clauses, comprising multiple optical fibers, including two or more fibers having different fiber characteristics. 8. A broadband radiation source device as defined in Clause 7, wherein some or each of two or more fibers having different fiber characteristics are configured to generate broadband outputs in different spectral ranges. 9. A broadband radiation source device as defined in Clause 8, wherein each of the different spectral ranges includes different subranges in the range of 200 nm to 2500 nm. 10. A broadband radiation source device as defined in any of the preceding clauses, wherein multiple optical fibers are integrated together and mechanically clamped, melted, or welded together. 11. A broadband radiation source device as defined in any of the preceding clauses, wherein the fiber assembly further includes multiple optical fibers and a gas cell enclosing a gaseous medium. 12. A broadband radiation source device as defined in any one of the preceding clauses, wherein the fiber assembly is configured to be movable relative to the beam of input radiation so that different optical fibers of multiple optical fibers can be selected. 13. A broadband radiation source device as defined in any one of the preceding clauses, configured such that the beam of input radiation is movable relative to a fiber assembly so that different optical fibers of multiple optical fibers can be selected. 14. A broadband radiation source device as defined in Clause 12 or 13, wherein the fiber assembly or the beam of input radiation is movable in one or both directions of a plane perpendicular to the propagation direction of the beam of input radiation. 15. A broadband radiation source device as defined in any one of the preceding clauses, configured to switch to different optical fibers of multiple optical fibers in order to generate broadband output when the broadband output power drops below a threshold level. 16. A broadband radiation source device configured to switch between different optical fibers of multiple optical fibers to generate a broadband output when different optical characteristics of a broadband output are desired, wherein the different optical characteristics include different spectral ranges as defined in any of the preceding clauses. 17. A broadband radiation source device as defined in any of the preceding clauses, further comprising a first beamshift assembly configured to spatially shift input radiation relative to a fiber assembly. 18. The first beam shift assembly, At least one glass plate comprising one or both of a first plate and a second plate, wherein the first plate is rotatable around a first axis and the second plate is rotatable around a second axis, and the first and second axes are perpendicular to each other and form a plane parallel to the input facets of the fiber assembly, and optionally, at least one glass plate composed of a low-dispersion material (e.g., N-FK58 made of CaF2), At least one piezo mirror, and at least one galvanoscanner A broadband radiation source device as defined in Clause 17, including one or more of the following: 19. A broadband source device as defined in Clause 17 or 18, further comprising a second beamshift assembly configured to spatially shift a broadband output so that the broadband output substantially travels along a predetermined beampath. 20. A broadband radiation source device as defined in Clause 19, wherein the beam path is determined according to either or both the optical alignment of a metrology tool configured to use broadband output and the position of an output delivery fiber configured to deliver broadband output. 21. A broadband radiation source device as defined in any one of the preceding clauses, further comprising a beam block configured to at least partially block input radiation when switching optical fibers. 22. A broadband radiation source device as defined in any of the preceding clauses, further comprising an input optical lens arrangement configured to couple the input radiation to one of a plurality of optical fibers of a fiber assembly. 23. A broadband radiation source device as defined in Clause 22, wherein the position and / or orientation of the input optical lens arrangement is operable to be adjustable to optimize the fiber coupling of the input radiation. 24. A broadband radiation source device as defined in any of the preceding clauses, further comprising an output optical lens arrangement configured to collimate a broadband output. 25. A broadband radiation source device as defined in Clause 24, wherein the position and / or orientation of the output optical lens arrangement is operable to be adjustable to optimize the position and / or orientation of the broadband output. 26. A broadband radiation source device as defined in any one of the preceding clauses, wherein the fiber assembly includes multicore microstructured fibers, and each of the optical fibers includes a microstructured fiber core. 27. A broadband radiation source device as defined in any one of the preceding clauses, wherein the fiber assembly includes a support portion for supporting multiple optical fibers. 28. A broadband radiation source device as defined in any one of the preceding clauses, wherein multiple optical fibers are arranged in a one-dimensional or two-dimensional linear array. 29. A broadband radiation source device as defined in any one of clauses 1 to 27, wherein multiple optical fibers are arranged in a ring configuration including a single ring or multiple concentric rings. 30. A broadband radiation source device as defined in any of the preceding clauses, further comprising a pump radiation source configured to provide a beam of input radiation. 31. The fiber assembly A fiber holder configured to hold multiple optical fibers that are grouped together, A fiber clamp configured to apply a clamping force to at least one optical fiber so that multiple optical fibers are clamped in a stable manner, and Includes, A broadband radiation source device as defined in any one of clauses 1 to 25, wherein each of a plurality of optical fibers is supported by two contact points and is affected by a force vector that intersects the line formed by connecting the two contact points supporting the fiber. 32. A broadband radiation source device as defined in Clause 31, wherein the force vectors affecting each optical fiber are obtained directly from at least one clamping force or from one or more force vectors transmitted through one or more contacting optical fibers. 33. A broadband radiation source device as defined in Clause 31 or 32, wherein the fiber holder includes a multi-level structure, the distance between two opposing inner walls of a lower level is smaller than that of a higher level, and the two opposing inner walls of each level are substantially perpendicular to the base of the same level. 34. A broadband radiation source device as defined in Clause 33, wherein the multilevel structure of the fiber holder results in a multilevel fiber stack, and furthermore, the intercenter distance between any two adjacent optical fibers of the same integration level decreases with each fiber level from the lowest fiber level to the highest fiber level. 35. A broadband radiation source device as defined in Clause 34, wherein the lowest level of the fiber holder holds two optical fibers, and the center-to-center distance between the two optical fibers is 1.5 to 1.9 times, 1.6 to 1.8 times, 1.65 to 1.75 times, or 1.69 to 1.71 times greater than the diameter of the multiple optical fibers. 36. A broadband radiation source device as defined in Clause 35, wherein the multilevel structure includes a first level and a second level, the first level being lower than the second level, and further, the distance between two opposing inner walls of the first level being 2.5 to 2.9 times, 2.6 to 2.8 times, 2.65 to 2.75 times, or 2.69 to 2.71 times greater than the fiber diameter, and the distance between two opposing inner walls of the second level being 3.5 to 3.9 times, 3.6 to 3.8 times, 3.65 to 3.75 times, or 3.69 to 3.71 times greater than the fiber diameter. 37. A broadband radiation source device as defined in any one of clauses 31 to 36, wherein the diameters of the multiple optical fibers are in the range of 10 μm to 1000 μm. 38. A broadband radiation source device as defined in any one of clauses 31 to 37, wherein at least one clamping force is strong enough to firmly secure each of the optical fibers in place, and does not exceed the maximum allowable force by any of the optical fibers. 39. A broadband radiation source device as defined in Clause 38, wherein the maximum force is the force that alters at least one of the properties of any of the multiple optical fibers. 40. A broadband radiation source device as defined in any one of clauses 31 to 39, wherein the support angle defined between the two contact points of each optical fiber lies in the range of 60° to 120°. 41. A broadband radiation source device as defined in Clause 33, wherein the multilevel structure of the fiber holder results in a multilevel fiber stack, and further includes two clamping forces, each applied to two optical fibers located at the top of the multilevel fiber stack, with at least one clamping force applied to each of the two optical fibers located at the top of the multilevel fiber stack. 42. Each of the multiple optical fibers meets the following conditions, namely: For each optical fiber supported by the inner wall and base of the fiber holder, the angle formed between the force vector affected by the optical fiber and the direction substantially perpendicular to the base of the fiber holder is in the range of 35° to 55°. For each optical fiber supported by two other optical fibers, the angle formed between the force vector affected by the optical fiber and the direction substantially perpendicular to the base of the fiber holder is in the range of 25° to 65°. A broadband radiation source device, as defined in Clause 41, arranged to satisfy the following conditions. 43. A broadband radiation source device as defined in Clause 42, wherein each of the multiple optical fibers is arranged to satisfy the further condition that any two adjacent optical fibers, each supported by both the inner wall and base of the fiber holder, are physically separated. 44. A broadband radiation source device as defined in clause 43, in which any two optical fibers are separated by a distance of 1 μm or more. 45. A broadband radiation source device as defined in any one of clauses 42-44, wherein each of the multiple optical fibers is arranged to satisfy the further condition that any higher level base has a width greater than the radius of the multiple optical fibers. 46. ​​A broadband radiation source device as defined in Clause 45, wherein the difference between the base width of any higher level and the radius of multiple optical fibers is 15 μm or greater. 47. A broadband radiation source device as defined in any one of clauses 41 to 46, wherein two clamping forces are substantially the same in magnitude. 48. A broadband radiation source device as defined in any one of clauses 1 to 25, wherein the fiber assembly includes at least one heat-shrinkable fiber tube surrounding at least first and second end sections of a plurality of optical fibers, and the at least one heat-shrinkable fiber tube is configured to apply a clamping force to at least first and second end sections of the plurality of optical fibers. 49. A broadband radiation source device as defined in Clause 48, wherein at least one heat-shrinkable fiber tube comprises two separate heat-shrinkable fiber tubes, each configured to clamp a first end section and a second end section of a plurality of optical fibers, respectively. 50. A broadband radiation source device as defined in Clause 48, comprising at least one heat-shrinkable fiber tube, a single heat-shrinkable fiber tube configured to cover the entire length of multiple optical fibers. 51. A broadband radiation source device as defined in any one of the clauses 48 to 50, wherein at least one heat-shrinkable fiber tube comprises a material having a melting point lower than the melting points of multiple optical fibers. 52. A broadband radiation source device as defined in Clause 51, wherein the material is a borosilicate. 53. A broadband radiation source device as defined in any one of clauses 48 to 52, wherein each of at least one heat-shrinkable fiber tubes includes an internal channel in direct contact with at least some of a plurality of optical fibers. 54. A broadband radiation source device as defined in Clause 53, wherein each internal channel of at least one heat-shrinkable fiber tube includes a circular cross-section. 55. A broadband radiation source device as defined in Clause 53, wherein each internal channel of at least one heat-shrinkable fiber tube includes multiple angles in cross-section, and each supports one of multiple optical fibers via two contact points. 56. A broadband radiation source device as defined in Clause 55, wherein each internal channel of at least one heat-shrinkable fiber tube includes a polygonal cross-section. 57. A broadband radiation source device as defined in Clause 55, wherein each internal channel of at least one heat-shrinkable fiber tube has a regular or irregular polygonal shape, or an irregular shape including both circular regions and corners. 58. A broadband radiation source device as defined in any one of clauses 53 to 57, wherein each of at least one heat-shrinkable fiber tubes further includes a central structure into which multiple optical fibers are clamped. 59. A broadband radiation source device as defined in Clause 58, wherein a central structure is fixed and connected to an internal channel of at least one heat-shrinkable fiber tube by a plurality of internal fixing elements, each of which connects the central structure to the edge of the internal channel. 60. Metrology devices including broadband radiation source devices as defined in any of the preceding clauses. 61. Metrology devices as defined in Clause 60, including scatrometer metrology equipment, level sensors, or alignment sensors. 62. A method for generating broadband radiation, Emitting input radiation from the pump source, Receiving input radiation through a selected subset of multiple optical fibers, To generate broadband output from a selected subset of multiple optical fibers and Methods that include... 63. A method as defined in Clause 62, in which each optical fiber includes a hollow-core photonic crystal fiber. 64. A subset including a single optical fiber of multiple optical fibers, as defined in clause 62 or 63. 65. A method as defined in Clause 64, which includes switching to another optical fiber of multiple optical fibers for the receiving step and the generating step. 66. A switching step, such as that defined in Clause 65, which includes switching to a similar optical fiber when the optical fiber used to generate the signal is deemed to require replacement. 67. A method, as defined in Clause 65 or 66, in which the optical fiber used to generate the signal is deemed to require replacement when the broadband output power drops below a threshold level. 68. A method as defined in any one of clauses 65 to 67, wherein the switching step includes switching to an optical fiber having at least one different fiber characteristics in order to generate a broadband output having at least one different radiation characteristic. 69. A method as defined in Clause 67, wherein at least one different radiative characteristic includes a different spectral range. 70. The steps to switch are Moving the fiber assembly relative to the input radiation, and, Transferring input radiation to a fiber assembly A method that includes one or both of the above, as defined in any one of the clauses 62 to 69. 71. A method for mechanically clamping multiple optical fibers, Holding multiple optical fibers to form a fiber stack, To ensure that multiple optical fibers are clamped in a stable state, at least one clamping force is applied to multiple optical fibers. Includes, A method in which each of multiple optical fibers is supported by two contact points and is affected by a force vector that intersects the line formed by connecting the two contact points supporting the fiber. 72. The distance between the centers of any two adjacent optical fibers at the same stacking level of the fiber stack decreases by fiber level from the lowest fiber level to the highest fiber level, as defined in Clause 71. 73. The following conditions, namely, For each optical fiber supported by the inner wall and base of the fiber holder, the angle formed between the force vector affected by the optical fiber and the direction substantially perpendicular to the base of the fiber holder is in the range of 35° to 55°. For each optical fiber supported by two other optical fibers, the angle formed between the force vector affected by the optical fiber and the direction substantially perpendicular to the base of the fiber holder is in the range of 25° to 65°. A method as defined in Clause 60 or 61, further comprising arranging each of the multiple optical fibers to satisfy the condition. 74. A method for clamping multiple optical fibers by at least one heat-shrinkable fiber tube, Inserting both end sections of multiple optical fibers into at least one heat-shrinkable fiber tube, To shrink at least one shrinkable fiber tube, heat at least one shrinkable fiber tube, Cleave at least one retractable fiber tube such that each end face of at least one retractable fiber tube is substantially coplanar with the end faces of multiple optical fibers. Methods that include...

[0180] While this specification specifically refers to the use of lithography equipment in the manufacture of ICs, it should be understood that the lithography equipment described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memory, flat panel displays, liquid crystal displays (LCDs), thin-film magnetic heads, and the like.

[0181] [000178] While embodiments of the present invention may be specifically referenced in relation to lithography apparatus, embodiments of the present invention may be used in other apparatuses. Embodiments of the present invention may form part of any apparatus for measuring or processing objects such as mask inspection apparatuses, metrology apparatuses, or wafers (or other substrates) or masks (or other patterning devices). These apparatuses may be collectively referred to as lithography tools. Such lithography tools may operate under vacuum conditions or ambient (non-vacuum) conditions.

[0182] [000179] While embodiments of the present invention have been specifically referenced to be used in relation to photolithography, it should be understood that the present invention is not limited to photolithography and may be used in other applications, such as imprint lithography, as the context allows.

[0183] [000180] Although specific embodiments of the present invention have been described so far, it goes without saying that the present invention can be carried out in ways other than those described. The above description is illustrative and not restrictive. Accordingly, as will be apparent to those skilled in the art, modifications to the described invention can be made without departing from the scope of the claims set forth below.

Claims

1. A fiber assembly comprising multiple hollow core fibers, wherein each hollow core fiber is filled with a gaseous medium. A broadband radiation source device including, The broadband radiation source device is operable such that a subset of the hollow core fibers can be individually selected as the target for receiving the beam of input radiation, so that at any given time, a broadband output is generated from only one subset of the plurality of hollow core fibers. A broadband radiation source device in which, at any given time, only one subset of the plurality of hollow core fibers receives the beam of the input radiation.

2. The broadband radiation source device according to claim 1, wherein any one of the plurality of hollow core fibers is operable to be independently selectable as the target for receiving the beam of input radiation.

3. The broadband radiation source device according to claim 1 or 2, wherein each of the hollow core fibers includes a hollow core photonic crystal fiber.

4. A beam splitting arrangement that can operate to spatially divide the beam of input radiation into multiple input beams, A focus configuration for focusing each of the plurality of input beams onto each of the hollow core fibers of the subset of hollow core fibers, A broadband radiation source device according to claim 1, 2, or 3, including the above.

5. The broadband radiation source device according to claim 1, 2, or 3, wherein the broadband output is operable to originate from only one of the plurality of hollow core fibers at any given time.

6. The broadband radiation source device according to any one of claims 1 to 5, wherein the plurality of hollow core fibers include two or more fibers having substantially the same fiber characteristics.

7. The broadband radiation source device according to any one of claims 1 to 6, wherein the plurality of hollow core fibers include two or more fibers having different fiber characteristics.

8. The broadband radiation source device according to any one of claims 1 to 7, wherein the plurality of hollow core fibers are grouped together and mechanically clamped, melted, or welded together.

9. The broadband radiation source device according to any one of claims 1 to 8, wherein the fiber assembly further comprises a gas cell enclosing the plurality of hollow core fibers and a gaseous medium.

10. The broadband radiation source device according to any one of claims 1 to 9, wherein the fiber assembly is configured to be movable relative to the beam of input radiation so that different hollow core fibers of the plurality of hollow core fibers can be selected.

11. A broadband radiation source device according to any one of claims 1 to 10, wherein the beam of input radiation is configured to be movable relative to the fiber assembly so that a different hollow core fiber of the plurality of hollow core fibers can be selected.

12. A broadband radiation source device according to any one of claims 1 to 11, further comprising an output optical lens arrangement configured to collimate the broadband output.

13. The broadband radiation source device according to any one of claims 1 to 12, wherein the plurality of hollow core fibers are arranged in a one-dimensional or two-dimensional linear array, or the plurality of hollow core fibers are arranged in a ring arrangement including a single ring or a plurality of concentric rings.

14. A metrology device comprising a broadband radiation source device according to any one of claims 1 to 13.

15. A method for generating broadband radiation, Emitting input radiation from the pump source, The input radiation is received by a selected subset of multiple hollow core fibers, To generate broadband output from the selected subset of the plurality of hollow core fibers. Includes, A method wherein, at any given time, only one subset of the plurality of hollow core fibers receives the input radiation, and the broadband output is generated from only that one subset.

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