Hollow core fiber optic-based radiation source

Abstract

A broadband radiating device comprising an HC-PCF, the HC-PCF comprising a hollow core extending along the length of the HC-PCF for confining a working medium under pressure during use, an input end operable to receive pulse pump radiation, and an output end operable to emit broadband output radiation resulting from the spectral spreading of the pulse pump radiation in the working medium confined within the HC-PCF, wherein the HC-PCF is divided into a first section and a second section, the first section comprising the input end, at least a portion of the first section comprising a bend and / or one or more coils, and the second section comprising the output end, being substantially linear, and the spectral spreading being governed by a self-phase modulation process in the first section and a nonlinear optical process in the second section, respectively.

Description

[Technical Field]

[0001] Cross-reference of related applications

[0001] This application claims priority to European Patent Application Publication No. 23161377.9, filed on 13 March 2023, which is incorporated herein by reference in its entirety.

[0002]

[0001] The present invention relates to a broadband radiation source based on a hollow core optical fiber, and more particularly to such a broadband radiation source related to measurement applications in the manufacturing of integrated circuits. [Background technology]

[0003]

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

[0004]

[0003] To project a pattern onto a substrate, a lithography apparatus may use electromagnetic radiation. 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 20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on the substrate compared to a lithography apparatus using radiation with a wavelength of, for example, 193 nm.

[0005]

[0004] Low k1 lithography may be used to process features having dimensions smaller than the conventional resolution limit of a lithography apparatus. In such a process, the resolution formula may be expressed as CD = k1 × λ / NA, where λ is the wavelength of 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 printed, but in this case it is the half-pitch), and k1 is an empirical resolution coefficient. Generally, the smaller k1, the more difficult it becomes to reproduce a pattern on the substrate that resembles the shape and dimensions planned by the circuit designer in order to achieve a particular electrical function and performance. To overcome these difficulties, advanced fine-tuning steps may be applied to the lithography projection apparatus and / or design layout. These include, but are not limited to, optimization of NA, customized illumination schemes, use of phase-shift patterning devices, 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, a strict control loop may be used to control the stability of the lithography equipment in order to improve pattern reproducibility at low k1.

[0006]

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

[0007]

[0006] In a first aspect of the present invention, a broadband radiating device comprising a hollow core photonic crystal fiber HC-PCF, the hollow core photonic crystal fiber HC-PCF comprising a hollow core extending along the length of the HC-PCF for confining a working medium under pressure during use, an input end operable to receive pulse pump radiation, and an output end operable to emit broadband output radiation resulting from the spectral spreading of pulse pump radiation in the working medium confined within the HC-PCF, wherein the HC-PCF comprises a first section extending over a first portion of the length of the HC-PCF A broadband radiating device is provided, which is divided into a first section and a second section extending over a second portion of the length of the HC-PCF, wherein the first section includes an input end, and at least a portion of the first section includes a bend and / or one or more coils, and the second section includes an output end and is substantially linear, and the HC-PCF is configured such that in the first section, spectral broadening occurs mainly by a self-phase modulation process, and in the second section, spectral broadening occurs mainly by a nonlinear optical process different from the self-phase modulation process.

[0008]

[0007] A second aspect of the present invention provides a method for generating broadband power radiation, comprising receiving pulse pump radiation at an input end of a hollow core photonic crystal fiber HC-PCF having a hollow core that confines a working medium under pressure, and emitting broadband power radiation at an output end of the HC-PCF, wherein the broadband power radiation is emitted from the spectral spreading of pulse pump radiation in a working medium confined within the HC-PCF, the HC-PCF being divided into a first section extending over a first portion of the length of the HC-PCF and a second section extending over a second portion of the length of the HC-PCF, the first section including an input end, at least a portion of the first section including a bend and / or one or more coils, the second section including an output end and being substantially linear, and the HC-PCF being configured such that in the first section, spectral spreading occurs mainly by a self-phase modulation process, and in the second section, spectral spreading occurs mainly by a nonlinear optical process different from the self-phase modulation process.

[0009]

[0008] Another aspect of the present invention includes a measuring device comprising a broadband radiating device of the first aspect.

[0010]

[0009] Hereinafter, embodiments of the present invention will be described simply by reference to the attached schematic drawings. [Brief explanation of the drawing]

[0011] [Figure 1] A schematic diagram of a lithography apparatus is shown. [Figure 2] A schematic diagram of a lithography system including an EUV radiation source and an EUV lithography apparatus or scanner is shown. [Figure 3] A schematic diagram of a lithographic cell is shown. [Figure 4] This diagram illustrates the coordination between three key technologies for optimizing semiconductor manufacturing, representing holistic lithography. [Figure 5]A schematic diagram of a scatometry device used as a measuring device, which may include a radiation source according to an embodiment of the present invention, is shown. [Figure 6] A schematic diagram of a level sensor device, which may include a radiation source according to an embodiment of the present invention, is shown. [Figure 7] A schematic diagram of an alignment sensor device, which may include a radiation source according to an embodiment of the present invention, is shown. [Figure 8] This is a schematic cross-sectional view of a hollow core optical fiber that can form part of a radiation source according to one embodiment, in a cross-section (i.e., a cross-section perpendicular to the axis of the optical fiber). [Figure 9] A schematic diagram of a radiation source according to one embodiment for providing broadband output radiation is shown. [Figure 10a] A schematic cross-section of an example of a hollow core photonic crystal fiber (HC-PCF) design for supercontinium generation is shown. [Figure 10b] A schematic cross-section of an example of a hollow core photonic crystal fiber (HC-PCF) design for supercontinium generation is shown. [Figure 11a] For example, Figure 9 shows a plot of wavelength λ against a normalized position NP along the fiber length for a simulation, illustrating the spectral evolution based on the modulation instability of the input radiation pulse within the HC-PCF of the radiation source. [Figure 11b] For example, Figure 9 shows a plot of wavelength λ against a normalized position NP along the fiber length for a simulation, illustrating the spectral evolution based on soliton self-compression of the pulse of input radiation within the HC-PCF of the radiation source. [Figure 12a] For example, Figure 9 shows a plot of the bandwidth expansion / increase rate BWR against a normalized position NP along the fiber length for simulation, illustrating how the rate of increase in the spectral bandwidth of the input emission pulse changes due to soliton self-compression as the pulse propagates along the HC-PCF of the emission source. [Figure 12b]For example, as shown in FIG. 9, it shows a plot of the bandwidth expansion / increase rate BWR against the normalized position NP along the fiber length for simulation, explaining how the increase rate of the spectral bandwidth of the input radiation pulse changes due to modulation instability while the pulse propagates along the HC-PCF of the radiation source. [Figure 13a] It shows a group of different simulation plots, and each plot shows the numerically simulated bending loss BL as a function of the bending radius BR of the HC-PCF for different wavelengths (e.g., as shown in FIG. 8). [Figure 13b] It shows a group of different simulation plots, and each plot shows the numerically simulated bending loss BL as a function of the bending radius BR of the HC-PCF for different wavelengths (e.g., as shown in FIG. 8). [Figure 13c] It shows a group of different simulation plots, and each plot shows the numerically simulated bending loss BL as a function of the bending radius BR of the HC-PCF for different wavelengths (e.g., as shown in FIG. 8). [Figure 13d] It shows a group of different simulation plots, and each plot shows the numerically simulated bending loss BL as a function of the bending radius BR of the HC-PCF for different wavelengths (e.g., as shown in FIG. 8). [Figure 13e] It shows a group of different simulation plots, and each plot shows the numerically simulated bending loss BL as a function of the bending radius BR of the HC-PCF for different wavelengths (e.g., as shown in FIG. 8). [Figure 13f] It shows a group of different simulation plots, and each plot shows the numerically simulated bending loss BL as a function of the bending radius BR of the HC-PCF for different wavelengths (e.g., as shown in FIG. 8). [Figure 13g] It shows a group of different simulation plots, and each plot shows the numerically simulated bending loss BL as a function of the bending radius BR of the HC-PCF for different wavelengths (e.g., as shown in FIG. 8). [Figure 13h] A group of different simulation plots is shown, where each plot shows numerically simulated bending loss BL as a function of the bending radius BR of an HC-PCF for different wavelengths (e.g., as shown in FIG. 8). [Figure 13i] A group of different simulation plots is shown, where each plot shows numerically simulated bending loss BL as a function of the bending radius BR of an HC-PCF for different wavelengths (e.g., as shown in FIG. 8). [Figure 14a] Five embodiments of a broadband radiation source RDS are schematically shown, where at least a part of the first section or the first section of the HC-PCF is bent or coiled, and the HC-PCF is at least partially enclosed in one or more gas cells configured differently for different embodiments. [Figure 14b] Five embodiments of a broadband radiation source RDS are schematically shown, where at least a part of the first section or the first section of the HC-PCF is bent or coiled, and the HC-PCF is at least partially enclosed in one or more gas cells configured differently for different embodiments. [Figure 14c] Five embodiments of a broadband radiation source RDS are schematically shown, where at least a part of the first section or the first section of the HC-PCF is bent or coiled, and the HC-PCF is at least partially enclosed in one or more gas cells configured differently for different embodiments. [Figure 14d] Five embodiments of a broadband radiation source RDS are schematically shown, where at least a part of the first section or the first section of the HC-PCF is bent or coiled, and the HC-PCF is at least partially enclosed in one or more gas cells configured differently for different embodiments. [Figure 14e] Five embodiments of a broadband radiation source RDS are schematically shown, where at least a part of the first section or the first section of the HC-PCF is bent or coiled, and the HC-PCF is at least partially enclosed in one or more gas cells configured differently for different embodiments. [Figure 15] This shows a block diagram of a computer system for controlling a broadband radiation source. [Modes for carrying out the invention]

[0012]

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

[0013]

[0011] The terms “reticle,” “mask,” or “patterning device” as used in this document may be broadly interpreted to refer to any general patterning device that can be used to provide an incident radiation beam with a patterned cross section corresponding to a pattern formed on a target portion of a substrate. In this context, the term “light bulb” may also be used. In addition to conventional masks (transmissive or reflective masks, binary, phase-shifted, hybrid, 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 (also called an illuminator) IL 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 constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to precisely position the patterning device MA according to specific parameters, a substrate support (e.g., a wafer table) WT constructed to hold a substrate (e.g., a wafer coated with resist) W and connected to a second positioner PW 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 the 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, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components or any combination thereof, to guide, shape and / or control the radiation. The illuminator IL may be used to adjust the radiant beam B so that the radiant beam B has a desired spatial and angular intensity distribution in its cross-section on the surface of the patterning device MA.

[0016]

[0014] As used herein, the term “projection system” PS should be interpreted broadly to encompass a variety of projection systems, including refractive, reflective, reflective-refracting, anamorphic, magnetic, electromagnetic, and / or electrostatic-optical systems or any combination thereof, depending on the exposure radiation and / or other factors used, such as the use of immersion liquid or vacuum. As used herein, the term “projection lens” may 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 may be covered with a liquid with a relatively high refractive index, such as water, to fill the gap between the projection system PS and the substrate W, also known as immersion lithography. Further information relating to immersion technology is given in U.S. Patent No. 6,952,253, which is incorporated herein by reference.

[0018]

[0016] The lithography apparatus LA may also be of a type having two or more substrate support WTs (also called a “dual-stage”). In such a “multi-stage” machine, the substrate support WTs may be used in parallel, and / or the step of preparing for the subsequent exposure of a substrate W may be performed on a substrate W located on one substrate support WT, while in the meantime, another substrate W located on the other substrate support WT is used for exposure of a pattern onto the other substrate W.

[0019]

[0017] In addition to the substrate support WT, the lithography apparatus LA may include a measurement stage. The measurement stage is positioned to hold sensors and / or cleaning devices. Sensors may be positioned to measure the characteristics of the projection system PS or the characteristics of the radiation beam B. The measurement stage may hold multiple sensors. Cleaning devices may be positioned to clean parts of the lithography apparatus, such as parts of the projection system PS or parts 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 radiating beam B is incident on a patterning device, for example, a mask MA held on a mask support MT, and a pattern is formed by the pattern (design layout) present on the patterning device MA. After traversing the mask MA, the radiating beam B passes through a projection system PS that focuses the beam onto a target portion C of the substrate W. A second positioner PW and a position measuring system IF can be used to precisely move the substrate support WT to position different target portions C in a focused and aligned position within the path of the radiating beam B, for example. Similarly, a first positioner PM and optionally (not explicitly shown in Figure 1) another position sensor may be used to precisely position the patterning device MA relative to the path of the radiating beam B. The patterning device MA and the substrate W can be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. The illustrated substrate alignment marks P1, P2 occupy dedicated target portions but may be located in the space between target portions. When substrate alignment marks P1 and P2 are located between target portions C, they are known as scribe line alignment marks.

[0021]

[0019] Figure 2 shows a lithography system including a radiation source SO and a lithography apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithography apparatus LA. The lithography apparatus LA includes an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS, and a substrate table WT configured to support a substrate W.

[0022]

[0020] The illumination system IL is configured to adjust the EUV radiation beam B before it is incident on the patterning device MA. Furthermore, the illumination system IL may include a faceted field mirror device 10 and a faceted pupil mirror device 11. Together, the faceted field mirror device 10 and the faceted pupil mirror device 11 provide the EUV radiation beam B having a desired cross-sectional shape and a desired intensity distribution. In addition to or instead of the faceted field mirror device 10 and the faceted pupil mirror device 11, the illumination system IL may include other mirrors or devices.

[0023]

[0021] After being adjusted in this manner, the EUV radiation beam B interacts with the patterning device MA. This interaction results in the generation of a patterned EUV radiation beam B'. The projection system PS is configured to project the patterned EUV radiation beam B' onto the substrate W. For this purpose, the projection system PS may include a number of mirrors 13, 14 configured to project the patterned EUV radiation beam B' onto the substrate W held by the substrate table WT. The projection system PS applies a reduction factor to the patterned EUV radiation beam B', thereby forming an image with features smaller than the corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. In Figure 2, the projection system PS is shown to have only two mirrors 13, 14, but the projection system PS may also include a different number of mirrors (e.g., 6 or 8 mirrors).

[0024]

[0022] The substrate W may include a previously formed pattern. In this case, the lithography apparatus LA aligns the image formed by the patterned EUV radiation beam B' with the previously formed pattern on the substrate W.

[0025]

[0023] A small amount of gas (e.g., hydrogen) at a relative vacuum, i.e., a pressure far below atmospheric pressure, may be supplied to the radiation source SO, the illumination system IL, and / or the projection system PS.

[0026]

[0024] The radiation source SO may be a laser-generated plasma (LPP) source, a discharge-generated plasma (DPP) source, a free-electron laser (FEL), or any other radiation source capable of generating EUV radiation.

[0027]

[0025] As shown in Figure 3, the lithography apparatus LA may form part of a lithographic cell LC, sometimes called a lithocell or (litho)cluster, which often also includes apparatus for performing pre-exposure and post-exposure processes on the substrate W. Conventionally, these include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, and cooling plates CH and bake plates BK for adjusting the temperature of the substrate W, for example, to adjust the solvent in the resist layer. A substrate handler or robot RO picks up the substrate W from input / output ports I / O1 and I / O2 and moves it between various process apparatuses to deliver the substrate W to the loading bay LB of the lithography apparatus LA. The devices within the lithocell, often collectively called a track, are typically under the control of a track control unit TCU, which itself may be controlled by a monitoring and control system SCS, which may also control the lithography apparatus LA, for example, via a lithography control unit LACU.

[0028]

[0026] It is desirable to inspect the substrate W to be exposed by the lithography apparatus LA to measure the characteristics of the patterned structure, such as overlay errors between subsequent layers, line thickness, and critical dimension (CD), so that the substrate W is exposed accurately and consistently. For this purpose, an inspection tool (not shown) may be included in the lithocell LC. If an error is detected, especially if the inspection is performed before other substrates W of the same batch or lot are exposed or processed, adjustments may be made to the exposure of subsequent substrates or other process steps performed on the substrate W.

[0029]

[0027] The inspection device, sometimes called a measuring device, is used to determine the properties of the substrate W, in particular how the properties of different substrates W change, or how the properties related to different layers of the same substrate W change layer by layer. Alternatively, the inspection device may 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 may be a standalone device. The inspection device may measure the properties of a latent image (image in the resist layer after exposure), or a semi-latent image (image in the resist layer after a post-exposure bake step PEB), or a developed resist image (with the exposed or unexposed parts of the resist removed), or even an etched image (after a pattern transfer step such as etching).

[0030]

[0028] The patterning process in a lithography apparatus LA is typically one of the most critical steps in the process, requiring highly accurate dimensionality and placement of structures on a substrate W. To ensure this high precision, three systems can be combined in a so-called "holistic" control environment, as schematically shown in Figure 4. One of these systems is the lithography apparatus LA, (virtually) connected to a measurement tool MT (second system) and a computer system CL (third system). The essence of such a "holistic" environment is to optimize the coordination between these three systems to enhance the entire process window and establish a strict control loop to ensure that the patterning performed by the lithography apparatus LA stays within the process window. The process window defines the range of process parameters (e.g., dose, focus, overlay) that a particular manufacturing process brings to a defined result (e.g., a functional semiconductor device), and typically, within this range, changes in process parameters in the lithography or patterning process are permitted.

[0031]

[0029] The computer system CL may use (part of) the patterned design layout to predict which resolution enhancement techniques should be used, and may perform computational lithography simulations and calculations to determine which mask layout and lithography apparatus settings will achieve the largest overall process window of the patterning process (indicated by the bidirectional arrows on the first scale SC1 in Figure 4). Typically, the resolution enhancement techniques are configured to match the patterning capabilities of the lithography apparatus LA. The computer system CL may also be used to detect where the lithography apparatus LA is currently operating within the process window (for example, using input from the measurement tool MT) to predict whether defects may exist, for example, due to suboptimal processing (indicated by the arrow pointing to "0" on the second scale SC2 in Figure 4).

[0032]

[0030] The measurement tool MT can provide input to the computer system CL to enable accurate simulation and prediction, and can also provide feedback to the lithography apparatus LA to identify possible drifts in the calibration status of the lithography apparatus LA (indicated by multiple arrows on the third scale SC3 in Figure 4).

[0033]

[0031] In lithography processes, it is desirable to frequently measure the formed structures, for example, for process control and verification. Tools used to perform such measurements are typically called measuring tools (MT). Different types of measuring tools (MT) for performing such measurements are known, including scanning electron microscopes or various forms of scaltrometer measuring tools (MT). A scaltrometer is a general-purpose instrument that enables the measurement of parameters in a lithography process, and by having a sensor on the pupil or conjugate plane of the pupil of the scaltrometer's objective system, measurements are usually performed which are called pupil-based measurements, or by having a sensor on the image plane or conjugate plane of the image plane, measurements are usually performed which are called image or field-based measurements. Such scattrometers and related measurement techniques are further described in U.S. Patent Publication No. 20100328655, U.S. Patent Publication No. 2011102753A1, U.S. Patent Publication No. 20120044470A, U.S. Patent Publication No. 20110249244, U.S. Patent Publication No. 20110026032, or European Patent Publication No. 1,628,164A, which are incorporated by reference in their entirety. The aforementioned scattrometers may measure gratings using soft X-rays and light in the visible to near-infrared wavelength range.

[0034]

[0032] In the first embodiment, the scatometer MT is an angle-resolved scatometer. In such a scatometer, a reconstruction method may be applied to the measured signal in order to reconstruct or calculate the properties of the lattice. Such reconstruction may be obtained, for example, by simulating the interaction between the scattered rays 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 until the simulated interaction produces a diffraction pattern similar to that observed from the actual target.

[0035]

[0033] In a second embodiment, the scatorometer MT is a spectrometer MT. In such a spectrometer MT, radiation emitted from a radiation source is guided onto a target, and reflected or scattered radiation from the target is guided to a spectrometer detector to measure the spectrum of specular reflection (i.e., a measurement of intensity as a function of wavelength). From this data, the structure or profile of the target that yields the detected spectrum can be reconstructed, for example, by exact coupled wave analysis and nonlinear regression or by comparison with a library of simulated spectra.

[0036]

[0034] In a third embodiment, the scatometer MT is a polarization analysis scatometer. A polarization analysis scatometer makes it possible to determine the parameters of the lithography process by measuring scattered radiation for each polarization state. Such a measuring device emits polarized light (such as a straight line, circle, or ellipse) by using a suitable polarizing filter in the illumination section of the measuring device, for example. A radiation source suitable for the measuring device may also provide polarized radiation. Various embodiments of existing elliptic polarization scatromometers are described in U.S. Patent Applications Nos. 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,449, 12 / 920,968, 12 / 922,587, 13 / 000,229, 13 / 033,135, 13 / 533,110, and 13 / 891,410, which are incorporated herein by reference in their entirety.

[0037]

[0035] 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 in the reflectance spectrum and / or the detection configuration, the asymmetry relating to the degree of overlay. The two (typically overlapping) grid structures may be applied to two different layers (not necessarily consecutive layers) and may be formed at substantially the same location on the wafer. The scatrometer may have a symmetric detection configuration such that any asymmetry is clearly distinguishable, as described, for example, in jointly owned European Patent Application Publication 1,628,164A. This provides a simple method for measuring grid misalignment. Further examples of measuring overlay errors between two layers containing periodic structures, such that the target is measured by the asymmetry of the periodic structure, can be found in PCT Patent Application International Publication 2011 / 012624 or U.S. Patent Application Publication 20160161863, which are incorporated herein by reference in their entirety.

[0038]

[0036] Other parameters of the subject may be focus and dose. Focus and dose may be determined simultaneously by scantometry (or alternatively by scanning electron microscopy), as described in U.S. Patent Application Publication 2011 / 0249244, which is incorporated herein by reference in whole. A single structure may be used that 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 ​​may be uniquely determined from these measurements.

[0039]

[0037] The measurement target may be an assembly of composite gratings, which are mostly formed within the resist by the lithography process, but also formed after, for example, the etching process. Typically, the pitch and linewidth of the grating structure depend heavily on the measurement optics (especially the NA of the optics) so that the diffraction order from the measurement target can be captured. As previously shown, the diffraction signal may be used to determine the shift between two layers (also called "overlay") or to reconstruct at least a portion of the original grating produced by the lithography process. This reconstruction may be used to provide an indication of the quality of the lithography process and may be used to control at least a portion of the lithography process. The target may have smaller subdivisions configured to mimic the dimensions of the functional parts of the design layout within the target. Due to these subdivisions, the target will behave more similarly to the functional parts of the design layout so that the measurements of the overall process parameters more closely resemble the functional parts of the design layout. The target may be measured in unfilled mode or overfilled mode. In unfilled mode, the measurement beam produces 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, thereby determining different processing parameters at the same time.

[0040]

[0038] The overall measurement quality of lithography parameters using a particular target is determined at least in part by the measurement recipe used to measure these lithography parameters. The term “substrate measurement recipe” may include 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 used in the substrate measurement recipe is an optical measurement based on diffraction, one or more parameters of the measurement may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation to the substrate, and the direction of the radiation to the pattern on the substrate. One criterion for selecting a measurement recipe may be, for example, the sensitivity of one measurement parameter to process variations. More examples are described in U.S. Patent Application Publication 2016 / 0161863 and U.S. Patent Application Publication 2016 / 0370717A1, which are incorporated herein by reference in their entirety.

[0041]

[0039] A measuring device such as a scatrometer is shown in Figure 5. The measuring device includes a broadband (white light) radiation projector 2 that projects radiation onto a substrate W. The reflected or scattered radiation is passed to a spectrometer detector 4 that measures the specular reflected radiation spectrum 6 (i.e., a measurement of intensity as a function of wavelength). From this data, the structure or profile 8 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 5. Generally, for reconstruction, the general form of the structure is known, some parameters are estimated from knowledge of the process in which the structure was fabricated, and only a few parameters of the structure determined from the scatrometry data remain. Such a scatrometer may be configured as a normal incidence scatrometer or an oblique incidence scatrometer.

[0042]

[0040] The overall measurement quality of lithography parameters obtained by measuring a measurement target is determined at least in part by the measurement recipe used to measure these lithography parameters. The term “substrate measurement recipe” may include 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 used in the substrate measurement recipe is an optical measurement based on diffraction, one or more parameters of the measurement may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation to the substrate, and the direction of the radiation to the pattern on the substrate. One criterion for selecting a measurement recipe may be, for example, the sensitivity of one measurement parameter to process variations. More examples are described in U.S. Patent Application Publication No. 2016 / 0161863 and U.S. Patent Application Publication No. 2016 / 0370717A1, which are incorporated herein by reference in their entirety.

[0043]

[0041] Another type of measuring tool used in IC manufacturing is a topography measuring 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 map of the substrate's topography, also called a height map, may be generated from these measurements, showing the height of the substrate as a function of its position on the substrate. This height map may then be used to correct the position of the substrate during the transfer of a pattern on the substrate in order to provide a spatial image of the patterning device at the appropriate focus position on the substrate. In this context, it will be understood that “height” refers to the roughly out-of-plane dimension relative to the substrate (also called the Z-axis). Typically, a level or height sensor performs measurements at a fixed position (relative to its own optics), and as a result of relative movement between the substrate and the optics of the level or height sensor, height measurements are obtained at multiple positions across the substrate.

[0044]

[0042] An example of a level or height sensor LS well known in the art is schematically shown in Figure 6, although only the operating principle is shown in Figure 6. In this example, the level sensor includes an optical system, which includes a projection unit LSP and a detection unit LSD. The projection unit LSP includes a radiation source LSO that provides a radiation beam LSB emitted 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.) supercontinium light source. The radiation source LSO may include multiple radiation sources having different color or wavelength ranges, such as multiple LEDs. The radiation source LSO of the level sensor LS is not limited to visible light radiation and may additionally or alternatively include UV and / or IR radiation and any wavelength range suitable for reflection from the surface of the substrate.

[0045]

[0043] The projection grating PGR is a periodic grating that includes a periodic structure that produces a radiation beam BE1 having a periodically changing intensity. The radiation beam BE1, having a periodically fluctuating intensity, is directed toward a measurement position MLO on the substrate W having an incident angle ANG of 0 to 90 degrees, typically 70 to 80 degrees, with respect to an axis perpendicular to the incident substrate surface (Z axis). At the measurement position MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and directed toward the detection unit LSD.

[0046]

[0044] To determine the height level at the measurement position 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 identical to 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, like a photodetector, or the spatial distribution of the received intensity, like a camera. The detector DET may include any combination of one or more types of detectors.

[0047]

[0045] The height level at the measurement position MLO can be determined by triangulation techniques. The detected height level is typically related to the signal intensity measured by the detector DET, which has a periodicity that depends particularly on the design of the projection grid PGR and the (oblique) incidence angle ANG.

[0048]

[0046] The projection unit LSP and / or detection unit LSD may include further optical elements such as lenses and / or mirrors along the path of the patterned radiation beam between the projection grid PGR and the detection grid DGR (not shown).

[0049]

[0047] In one embodiment, the detection grid DGR may be omitted, and the detector DET may be located in the position where the detection grid DGR is located. Such a configuration provides a more direct detection of the image of the projection grid PGR.

[0050]

[0048] 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 a measurement area MLO or an array of spots that cover a wider measurement range.

[0051]

[0049] Various types of 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 herein by reference. A height sensor that uses UV radiation as an alternative to visible light radiation 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 small height sensor that uses a multi-element detector to detect and recognize the position of a grid image without requiring a detection grid.

[0052]

[0050] Another type of measuring tool used in IC manufacturing is the alignment sensor. Therefore, a crucial aspect of the performance of a lithography apparatus is its ability to accurately and precisely position the pattern to be applied with respect to features defined in a preceding layer (by the same or different lithography apparatus). For this purpose, the substrate is provided with one or more sets of marks or targets. Each mark is a structure that can be measured at a later point in time using a position sensor, typically an optical position sensor. The position sensor is sometimes called an "alignment sensor," and the marks are sometimes called "alignment marks."

[0053]

[0051] A lithography apparatus may include one or more alignment sensors capable of accurately measuring the position of alignment marks provided on a substrate. Alignment (or position) sensors may acquire positional 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 apparatus is based on a self-referencing interferometer, as described in U.S. Patent No. 6,961,116. Various developments 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.

[0054]

[0052] Figure 7 is a schematic block diagram of one embodiment of a known alignment sensor AS, such as that described in U.S. Patent No. 6,961,116, which is incorporated by reference. The radiation source RSO provides a radiation beam RB of one or more wavelengths, which is bypassed by a bypass optical system, as an illumination spot SP to a mark such as a mark AM located on the substrate W. In this example, the bypass optical system includes a spot mirror SM and an objective lens OL. The illumination spot SP that illuminates the mark AM may have a diameter slightly smaller than the width of the mark itself.

[0055]

[0053] The radiation diffracted by the alignment mark AM is collimated to the information transmission beam IB (through the objective lens OL in this example). The term “diffracted” is intended to include zero-order diffraction (sometimes called reflection) from the mark. For example, a self-reference interferometer SRI of the type disclosed in U.S. Patent No. 6,961,116 above interferes with the beam IB by itself, and the beam is then received by a photodetector PD. If the radiation source RSO produces two or more wavelengths, additional optics (not shown) may be included to provide separate beams. The photodetector may be a single element or may include multiple pixels as needed. The photodetector may include a sensor array.

[0056]

[0054] In this example, the bypass optical system, including the spot mirror SM, also plays a role in blocking the zero-order radiation reflected from the mark, so that the information transmission beam IB contains only the higher-order diffracted radiation from the mark AM (which is not essential for the measurement but improves the signal-to-noise ratio).

[0057]

[0055] The intensity signal SI is supplied to the processing unit PU. By combining the optical processing in block SRI and the computational processing in unit PU, the X and Y position values ​​on the substrate relative to the reference frame are output.

[0058]

[0056] A single measurement of the type shown in the illustration merely fixes the position of the mark within a specific range corresponding to one pitch of the mark. In conjunction with this, a coarser measurement technique is used to determine whether any period of the sine wave includes the marked position. Regardless of the material on which the mark is made 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 for improved accuracy and / or robust detection of the mark. Wavelengths may be optically multiplexed and demultiplexed so as to be processed simultaneously, and / or wavelengths may be multiplexed by time division or frequency division.

[0059]

[0057] 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 to the reference frame while effectively scanning the mark AM in the opposite direction to the movement of the substrate W. During this movement, the substrate W is controlled by the mounting of the substrate W to the substrate support and by a substrate positioning system that controls the movement of the substrate support. A substrate support position sensor (e.g., an interferometer) measures the position of the substrate support (not shown). In one embodiment, 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, as determined by the position sensor, to be calibrated (e.g., with respect to the frame to which the alignment system is connected). Measuring the position of the alignment marks provided on the substrate makes it possible to determine the position of the substrate relative to the substrate support.

[0060]

[0058] Measurement tools MT such as the scatometer, topography measurement system, or position measurement system described above can perform measurements using radiation emitted from a radiation source. The characteristics of the radiation used by the measurement tool can affect the type and quality of measurements that can be performed. In some applications, it may be advantageous to measure a substrate using multiple radiation frequencies, for example, broadband radiation may be used. Multiple different frequencies may propagate, irradiate, and scatter from the measurement target with little to no interference with other frequencies. Therefore, different frequencies may be used, for example, to acquire more measurement data simultaneously. Different radiation frequencies may also be used to investigate and discover different characteristics of the measurement target. Broadband radiation may be useful in measurement systems MT such as level sensors, alignment mark measurement systems, scatometry tools, or inspection tools. A broadband radiation source may be a supercontinuum light source.

[0061]

[0059] High-quality broadband radiation, such as supercontinium radiation, can be difficult to produce. One method for generating broadband radiation may be 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 produced using a laser) is sometimes called pump radiation. Alternatively, input radiation may be called seed radiation. To obtain high-power radiation for broadening effects, radiation may be confined to small areas so that strong localized high-intensity radiation is achieved. In those areas, radiation may interact with broadening structures and / or materials that form a nonlinear medium to produce broadband power radiation. Different materials and / or structures may be used in high-intensity radiation areas to enable and / or improve radiation broadening by providing a suitable nonlinear medium.

[0062]

[0060] In some embodiments, broadband power radiation is generated in a photonic crystal fiber (PCF). In some embodiments, such a photonic crystal fiber has a microstructure around its fiber core to help confine the radiation moving through the fiber into the fiber core. The fiber core can be made of a solid material that has nonlinear properties and is capable of generating broadband radiation when high-intensity pump radiation is transmitted through the fiber core. While it is possible to generate broadband radiation in a solid-core photonic crystal fiber, there are some drawbacks to using solid materials. For example, if UV radiation is generated in a solid core, this radiation may not be present in the fiber's output spectrum because most of the radiation is absorbed by the solid material.

[0063]

[0061] In some embodiments, as will be further discussed below with reference to Figure 9, methods and apparatus for broadening input radiation may use a fiber for confining the input radiation and for broadening the input radiation to output broadband radiation. The fiber may be a hollow-core fiber and may include an internal structure for effectively inducing and confining radiation within the fiber. The fiber may be a hollow-core photonic crystal fiber (HC-PCF) which is particularly suitable for confining strong radiation 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 broadbanding medium for broadening the input radiation. Such a fiber and gas arrangement may be used to produce a supercontinuous radiation source. The radiation input to the fiber may be electromagnetic radiation, e.g., one or more radiations of the infrared, visible, UV and extreme UV spectra. The output radiation may consist of or include broadband radiation, which may be referred to herein as white light.

[0064]

[0062] Some 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). In particular, 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 coupling suppression 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).

[0065]

[0063] Several types of HC-PCFs can be designed, 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 reflective fibers (HC-ARFs). Details relating to the design and manufacture of HC-PCFs can be found in U.S. Patent Application Publication No. 2004 / 015085A1 (for HC-PBFs) and International PCT Patent Application Publication No. 2017 / 032454A1 (for hollow core anti-resonant reflective fibers), which are incorporated herein by reference. Figure 10(a) shows a kagome fiber including a kagome lattice structure.

[0066]

[0064] An example of an optical fiber used in a radiation source will now be described with reference to Figure 8, which is a schematic cross-sectional view of an optical fiber OF. Further embodiments similar to the example of the fiber in Figure 8 are disclosed in International Publication No. 2017 / 032454.

[0067]

[0065] An optical fiber OF comprises an elongated body in which one dimension of the fiber OF is longer than the other two dimensions. This longer dimension may be called the axial direction and may define the axis of the optical fiber OF. The other two dimensions define a plane which may be called the cross-section. Figure 8 shows a cross-section of an optical fiber OF in this cross-section (i.e., a cross-section perpendicular to the axis), which is labeled the xy plane. The cross-section of an optical fiber OF may be substantially constant along the fiber axis.

[0068]

[0066] It will be recognized 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 and cross-section refer to local optical axis, local cross-section, etc. Furthermore, when a component is described as cylindrical or tubular, it will be understood that these terms encompass shapes that can deform when the optical fiber (OF) is bent.

[0069]

[0067] It will be recognized that the optical fiber OF can have any length, and the length of the optical fiber OF can be application-dependent. The optical fiber OF can have a length of 1 cm to 10 m, for example, the optical fiber OF can have a length of 10 cm to 100 cm. In one embodiment, the optical fiber OF may be a tapered fiber that includes a waist section. The waist section may include a down-tapered section in which the diameter of the fiber decreases. In one embodiment, the waist section may further include a central region in which the diameter is constant. In one embodiment, the waist section may further include an up-tapered section in which the diameter of the fiber increases back to the original diameter. The length of the tapered waist section can extend, for example, from a few millimeters to several tens of centimeters. If the fiber parameters are changed during fiber stretching, tapered waist sections with longer lengths, for example, several tens of meters, may also be possible.

[0070]

[0068] 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. In particular, 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 to guide the radiation along the optical fiber OF. Since the hollow core HC of the optical fiber OF can be substantially located in the central region of the optical fiber OF, the axis of the optical fiber OF can also define the axis of the hollow core HC of the optical fiber OF.

[0071]

[0069] The cladding portion includes multiple anti-resonant elements for guiding radiation propagating through the optical fiber OF. In particular, in this embodiment, the cladding portion includes a single ring consisting of six tubular capillaries CAP. Each of the tubular capillaries CAP functions as an anti-resonant element.

[0072]

[0070] The capillary CAP is also sometimes called a tube. The capillary CAP may have a circular cross-section or other shapes. Each capillary CAP includes a substantially cylindrical wall portion WP, which at least partially defines the hollow core HC of the optical fiber OF and isolates the hollow core HC from the capillary cavity CC. It will be recognized that the wall portion WP can function 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 largely suppressed, while reflections returning into the hollow core HC are largely enhanced. In some embodiments, the capillary wall portion WP may have a thickness of 0.01 to 10.0 μm.

[0073]

[0071] As used herein, it will be recognized that 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 may be confined in transverse mode (propagating along the fiber axis).

[0074]

[0072] The support portion is substantially tubular and supports the six capillary CAPs of the clad portion. The six capillary CAPs are evenly distributed around the inner surface in the case of the inner support portion SP. The six capillary CAPs can be described as being arranged in a substantially hexagonal shape.

[0075]

[0073] The capillaries are arranged such that each capillary does not come into contact with any of the other capillaries. Each capillary is in contact with the inner support portion SP and is separated from adjacent capillaries 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 be in contact with an adjacent capillary in the ring structure.

[0076]

[0074] The six capillaries CAP of the cladding portion are arranged in a ring structure around the hollow core HC. The inner surface of the ring structure of the capillaries CAP defines at least partially the hollow core HC of the optical fiber OF. The diameter d of the hollow core HC (indicated by arrow d, which can be defined as the minimum dimension between opposing capillaries) can be 10 to 1000 μm. The diameter d of the hollow core HC can affect the mode field diameter, collision loss, dispersion, mode multiplicity, and nonlinear characteristics of the hollow core HC optical fiber OF.

[0077]

[0075] In this embodiment, the cladding portion includes a single ring arrangement of capillary CAPs (which function as anti-resonance 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.

[0078]

[0076] It will be recognized that other embodiments may provide different arrangements of anti-resonance elements. These may include arrangements having multiple rings of anti-resonance elements and arrangements having nested anti-resonance elements. Figure 10(a) shows one 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, the embodiment shown in Figure 8 includes one ring consisting of six capillaries, but in other embodiments, the cladding portion may be provided with one or more rings containing any number of anti-resonance elements (e.g., 4, 5, 6, 7, 8, 9, 10, 11, or 12 capillaries).

[0079]

[0077] Figure 10(b) shows a modified embodiment of the HC-PCF described above, which has a single ring made of a tubular capillary. In the example of Figure 10(b), there are two coaxial rings made of tubular capillaries 21. To hold the inner and outer rings made of tubular capillaries 21, the HC-PCF may include a support tube ST. The support tube may be made of silica.

[0080]

[0078] The tubular capillaries in the examples of Figures 8 and 10(a) and 10(b) may have a circular cross-sectional shape. Other shapes such as elliptical or polygonal cross-sections are also possible for tubular capillaries. Additionally, the solid material of the tubular capillaries in the examples of Figures 8 and 10(a) and 10(b) may include plastic materials such as PMA, or glass such as silica or soft glass.

[0081]

[0079] Figure 9 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 8), and a working medium WM (e.g., gas) placed within the hollow core HC. In Figure 9, the radiation source RDS includes the optical fiber OF shown in Figure 8, but in alternative embodiments, other types of hollow core HC optical fiber OFs may be used.

[0082]

[0080] 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 allows for broadening the frequency range of the received input radiated IRD to provide a broadband output radiated ORD.

[0083]

[0081] 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 house 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 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, so that the received input radiation IRD (or at least a large portion thereof) may be coupled to the optical fiber OF located inside the reservoir RSV. It will be recognized that an optical system (not shown) can be provided for coupling the input radiation IRD to the optical fiber OF.

[0084]

[0082] 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 in close proximity to the output end OE of the optical fiber OF. The second transparent window TW2 may be transparent to at least the frequency of the broadband output radiation ORD of the device 120.

[0085]

[0083] Alternatively, in another embodiment, the ends of the optical fiber OF may be located 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 located in a first reservoir containing a working medium WM. The second end section may be located in a second reservoir, which may also contain a working medium WM. The function of the reservoirs may be as described above with respect to Figure 9. 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 so that the optical fiber OF can be partially located inside and partially outside the reservoir, thereby sealing gas within the reservoir. The optical fiber OF may further include intermediate sections not housed 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 such an arrangement using two separate gas reservoirs, it will be recognized 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 gases in the two reservoirs) may be considered to provide an apparatus for supplying a working medium WM within the hollow core HC of the optical fiber OF.

[0086]

[0084] In this regard, a window may be transparent to frequency if it transmits at least 50%, 75%, 85%, 90%, 95%, or 99% of the incident radiation of the window's frequency through the window.

[0087]

[0085] Both the first transparent window TW1 and the second transparent window TW2 can form an airtight seal within the wall of the reservoir RSV so that the working medium WM (which may be a gas) can be contained within the reservoir RSV. It will be recognized that the gas WM can be contained within the reservoir RSV at a pressure different from the ambient pressure of the reservoir RSV.

[0088]

[0086] 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 dispersed wave generation (DWG), 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 spreading characteristics can be adjusted to optimize frequency conversion.

[0089]

[0087] In one embodiment, the working medium WM may be located within the hollow core HC while it is receiving input radiation IRD to generate broadband output radiation ORD. It will be recognized that while the optical fiber OF is not receiving input radiation IRD to generate broadband output radiation, the gas WM may not be present in the hollow core HC, either entirely or partially.

[0090]

[0088] High-intensity radiation is sometimes 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 strong spatial confinement of the received high-intensity input radiation and / or radiation within the optical fiber OF. An advantage of hollow-core optical fibers is that they can guide radiation with a wider wavelength range than solid-core fibers, and in particular, hollow-core optical fibers can guide radiation in both the ultraviolet and infrared ranges.

[0091]

[0089] 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.

[0092]

[0090] The received input radiation IRD may be electromagnetic radiation. The input radiation IRD may be received as pulsed radiation. For example, the input radiation IRD may include ultrafast pulses generated, for example, by a laser.

[0093]

[0091] The input radiation IRD may be coherent radiation. The input radiation IRD may be collimated radiation, the advantage of which may be that it facilitates 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 may be generated by a laser. Similarly, the output radiation ORD may be collimated and / or coherent.

[0094]

[0092] The broadband range of the output emission ORD may be a continuous range, including a continuous range of emission frequencies. The output emission ORD may include supercontinuous emission. Continuous emission may be useful for several applications, such as measurement applications. For example, a continuous range of frequencies may be used to investigate a number of characteristics. A continuous range of frequencies may 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.

[0095]

[0093] 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 in the range of 1 kHz to 100 MHz. The pulse energy may be in the range of 0.1 μJ to 100 μJ, for example, 1 to 10 μJ. The pulse length of the input radiation IRD may be 10 fs to 10 ps, ​​for example, 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.

[0096]

[0094] 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 the (pump) laser parameters, the operating component WM variation, and the optical fiber OF parameters. The spatiotemporal transmission characteristics may include one or more of the following: 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 following: 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 length of the optical fiber, the size and shape of the hollow core HC, the size and shape of the capillary, and the 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 type of gas, the pressure of the gas, and the temperature of the gas.

[0097]

[0095] 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.

[0098]

[0096] As described above, there are many nonlinear optical processes involved in the generation of broadband output radiation ORD (e.g., supercontinium or white light). Which nonlinear optical process has a more pronounced spectral broadening effect than others depends on how the operating parameters are set. For example, by selecting the pump wavelength and / or optical fiber OF so that the pump pulse propagates through the fiber in the normal dispersion region (positive group velocity dispersion (GVD)), self-phase modulation becomes the dominant nonlinear optical process, contributing to the spectral broadening of the pump pulse. However, in most cases, the spectral broadening of the input radiation IRD provided by a pulsed pump radiation source PRS is caused by soliton dynamics and requires the pump pulse to propagate in the anomalous dispersion region (negative GVD) of the optical fiber OF. This is because in the anomalous dispersion region, the effects of Kerr nonlinearity and dispersion act in opposite directions. If the pulse parameters of the pump pulse emitted into the optical fiber OF (e.g., HC-PCF) in anomalous chromatic dispersion do not exactly match the pulse parameters of the soliton, the pump pulse transforms into a soliton pulse with a specific soliton order and dispersion wave.

[0099]

[0097] Soliton self-compression and modulation instability are known to be two major mechanisms for spectral broadening in broadband radiation generation by solitons. The difference between the two mechanisms is that the soliton self-compression process is associated with lower soliton orders, while the modulation instability process is associated with higher soliton orders. The soliton order N of the pulsed input radiation IRD is a useful parameter that can be used to distinguish between situations in which spectral broadening is dominated by modulation instability and situations in which spectral broadening is dominated by soliton self-compression. The soliton order N of the pulsed input radiation IRD is,

number

[0100]

[0098] Typically, when N > 20, the spectral broadening is dominated by modulation instability (MI), whereas when N < 20, the spectral broadening is typically dominated by soliton self-compression (SSC).

[0101]

[0099] Some known broadband radiation sources use arrangements that produce spectral broadening of the pulsed pump radiation IRD, but the parameters of the pulsed input radiation IRD, the optical fiber, and the working medium are configured such that MI enables spectral broadening. There are several reasons why MI is used to produce spectral broadening. First, the MI process is known to produce broadband radiation with a relatively flat intensity-wavelength distribution, provided that a sufficient number of pulses are averaged. Such broadband radiation sources are sometimes called white light radiation sources (due to the relatively flat spectral intensity distribution). Second, the MI process can be achieved using a relatively economical laser source as the pump radiation source PRS.

[0102] [000100] On the other hand, in the SSC regime, the input pump pulse undergoes compression in the time domain, which is accompanied by an increase in the spectral width. Following SSC, the compressed pulse with soliton order N0 undergoes soliton splitting, where the pulse splits into multiple solitons with order N1 < N0. This soliton splitting results in a temporal broadening of the radiation pulse and a strong modulation of the spectrum by wavelength due to the interference of the multiple solitons.

[0103] [000101] In contrast to noise seed MI systems, the broadband radiation produced by such SSCs has virtually no inter-shot variation. This is because, in the SSC process, moderate nonlinearity and anomalous dispersion work together to gradually and smoothly compress the duration of the input pump pulse along the fiber, whereas in the MI process, strong amplification of noise causes the input pump pulse to decompose into a sequence of random subpulses ("soliton showers"). As a result, advantageously, in an SSC system, each of the single compressed pulses features essentially the same broadband spectrum. In contrast, in a modulation instability system, it is necessary to integrate across many pulses to obtain a broadband spectrum and reduce its inherent inter-shot variation. The intensity noise of broadband output radiated ORDs generated from an SSC system (e.g., P. Uebel, S. Bauerschmidt, Y. Ni, European Patent Application Publication No. 3796080A1 (2019), incorporated herein by reference) can be several orders of magnitude (e.g., two orders of magnitude) lower than the intensity noise of broadband output radiated ORDs generated from an MI system, and is typically limited by the noise of the pump radiation source PRS.

[0104] [000102] As can be seen from equation (1), the soliton order of the pulsed input radiated IRD is proportional to the pulse duration τ of the pulsed input radiated IRD. Therefore, in typical prior art configurations where soliton self-compression prevails, the pulse duration τ of the input pulsed input radiated IRD is typically reduced to about 30 fs or less. To achieve such configurations, typically, a pre-compressed high-power femtosecond fiber laser (e.g., a fiber-based chirp pulse amplification system (FCPA) typically provides a pulse duration τ > 100 fs for the input pulsed input radiated IRD) or a Ti:sapphire amplifier (which provides inherently short pulses that do not require pre-compression) is used as a pulse-pump radiated source (PRS). On the other hand, in the case of fiber-based FCPAs, pre-compression requires additional elements for nonlinear spectral broadening and (one or more) elements that provide negative group velocity dispersion (GVD) for time compression down to durations of 30 fs or less. Because their interfaces to subsequent spectral broadening steps are typically in free space, additional means of beam stabilization are required, which can lead to bulkier, more cost-intensive configurations. On the other hand, the thermal constraints of the crystalline medium limit the average power of Ti:sapphire amplifiers to typically less than 5W and the repetition rate to undesirably low levels (at the kHz level) in applications of optical measurement tools (MT).

[0105] [000103] As described above, for both MI and SSC processes, spectral broadening is caused by soliton dynamics and requires an anomalous dispersion environment. Figures 11(a) and 11(b) show two exemplary simulations illustrating the spectral evolution of pulses of input radiated IRDs in an HC-PCF of a radiated source RDS (e.g., as shown in Figure 9) based on MI and SSC, respectively. As can be seen in Figures 11(a) and 11(b), in both MI and SSC processes, spectral broadening within the first section of the fiber is relatively weak as it is mainly caused by SPM. The length of such a first fiber section may vary depending on the different spectral broadening process.

[0106] [000104] Spectral bandwidth Δf in the SPM-dominant section SPM (z) increases or decreases linearly with position z along the fiber, [A. Zheltikov, “Analytical insights into self-phase modulation: beyond the basic theory,” Opt. Express 26, 17571 (2018)]:

number

[0107] [000105] As shown in Figure 12(a), in the case of the SSC process, the bandwidth increase rate per fiber length rises from a relatively flat level of about 50 THz / m to over 100 THz / m at a normalization position of 0.69. Therefore, in this particular exemplary simulation, the first fiber section has a relative length of about 69% of the total fiber length. By this definition, the typical length of an SPM-dominant fiber section in the SSC process can be 64% (for low soliton orders such as N=7.5) to 77% (for high soliton orders such as N=13.0) of the total fiber length. For a given soliton order, this range may vary slightly depending on the different input pulse durations.

[0108] [000106] As shown in Figure 12(b), the bandwidth increase rate within the first fiber section is relatively flat (e.g., up to the normalization point of 0.48), so it is clear that the above definition regarding the criteria for a section where SPM is dominant also applies to the MI process. In addition, the further spread thereafter is strongly affected by noise amplification, fiber length L MI It is also possible to define the absolute length of the SPM-dominant section using the following: This length is determined using the nonlinear parameter γ and peak power Pp, [JJ.M.Dudley, et al., “Supercontinuum generation in photonic crystal fiber,” Rev.Mod.Phys.78,1135(2006)]:

number

[0109] [000107] As an example, referring to Figures 11(a) and 11(b), the first section in which SPM is dominant can be considered to start at the input end of the fiber and terminate at a position along the fiber where a spectral range of less than 400 nm is generated around a centroid pump wavelength of 1030 nm. Thus, the length of the first fiber section in which SPM is dominant is determined to be about 48% of the total fiber length in the case of an MI-dominant process (as shown in Figure 11(a)) and about 70% of the total fiber length in the case of an SSC-dominant process (as shown in Figure 11(b)).

[0110] [000108] In all current configurations of hollow-core HC optical fiber OF (e.g., HC-PCF) based radiation source RDS, the gas cell RSV is configured to allow the fiber to be mounted as straight as possible. By keeping the fiber straight, the bending loss of the fiber, which increases with decreasing bending radius and becomes larger for shorter optical wavelengths, is avoided or minimized. The geometric definition and qualitative dependence of bending loss on optical wavelength can be used as an analytically calculable lower limit of the bending radius, the critical bending radius R cr This can be understood from the perspective of the critical bending radius R. cr This is incorporated herein by reference: MHFrosz, et al., “Analyical formulation for the bend loss in single-ring hollow-core photonic crystal fibers,” Photonics Res. 5, 88-91 (2017):

number

number

[0111] [000109] However, mounting hollow-core HC optical fibers in a completely linear configuration is not always desirable because it results in a large footprint for the RDS radiation source, reducing the freedom of routing the optical path within industrial products (e.g., the measurement tool MT as described above). This is particularly relevant to SSC-based radiation sources that require fiber lengths of about 1m, which are several times longer than the typical fiber lengths required by MI-based radiation sources.

[0112] [000110] The inventors have found that in both the SSC and MI processes, the spectrum in the SPM-dominant region (as shown in Figures 11(a) and (b), for example, is sufficiently narrow, and as a result, the effect of fiber bending loss on output performance is minimal. For example, Figures 13(a) to 13(i) show a group of different simulation plots, each plot showing the numerically simulated bending loss BL as a function of the bending radius BR of the HC-PCF (as shown in Figure 8) for different wavelengths. In this exemplary set of simulation data, the centroid wavelength of the input radiation IRD varies from 500 nm to 1300 nm in 100 nm increments. Here, bending loss is defined as excess power loss, which is the difference between the power loss of optical transmission in a bent fiber and the power loss of optical transmission in the same but straight fiber. As can be seen in Figures 13(a) to 13(i), a general trend common to all wavelengths is that the bending loss of the fiber increases as the bending radius decreases. In addition, for a given (constant) bending radius, the bending loss of the fiber increases as the wavelength decreases. The spikes that appear as the wavelength of light shortens at larger bending radii are due to the coupling of hollow core modes with modes located in the capillary, generating further loss peaks.

[0113] [000111] Typically, bending losses of ≤0.1 dB / m are considered negligible. This results in a 2% transmission loss along the entire length of the fiber compared to when the fiber is held in a straight line. Because the transmission loss induced by such fiber bending is small, it does not significantly adversely affect the performance of HC-PCF-based radiation source RDS. Therefore, it is proposed herein to bend or coil at least a portion of the SPM-dominant section of the HC-PCF in order to reduce the footprint of the radiation source RDS while substantially maintaining the desired characteristics of the broadband output radiation ORD (e.g., spectral width, short-wavelength cutoff, spectral flatness, etc.).

[0114] [000112] According to a first aspect of the present disclosure, a broadband radiating device is provided, comprising a hollow core photonic crystal fiber HC-PCF, the hollow core photonic crystal fiber HC-PCF comprising a hollow core extending along the length of the HC-PCF for confining a working medium under pressure during use, an input end operable to receive pulse pump radiation, and an output end operable to emit broadband output radiation resulting from the spectral spreading of pulse pump radiation in the working medium confined within the HC-PCF. The HC-PCF may be divided into a first section extending over a first portion of the length of the HC-PCF and a second section extending over a second portion of the length of the HC-PCF, the first section comprising the input end, at least a portion of the first section comprising a bend and / or one or more coils, and the second section comprising the output end and substantially linear. An HC-PCF may be configured such that, within the first section, the spectral broadening is primarily caused by the SPM process, and within the second section, the spectral broadening is primarily caused by the self-phase modulation process and a different nonlinear optical process. The terms “primarily” or “dominated by” may suggest that the spectral broadening induced by SPM accounts for more than 50%, 60%, 70%, or 80% of the overall spectral broadening occurring within the first section of the HC-PCF.

[0115] [000113] In one embodiment, the pulsed pump radiation IRD can substantially maintain a narrow spectral bandwidth in a first section where SPM-dominant spectral spreading occurs. That is, the SPM-dominant spectral spreading in the first section can be significantly weaker than the spectral spreading caused by a different nonlinear optical process in the second section. Preferably, the length of the fiber section where SPM is dominant may be, for example, 60% to 80%, 60% to 77%, 64% to 77%, 64% to 74%, 68% to 74%, or 68% to 70% of the total fiber length.

[0116] [000114] In one embodiment, different nonlinear optical processes in which the pulsed input emission IRD is spectrally spread within a second section of the HC-PCF may be dominated by MI. The pulsed input emission IRD may be configured to change into soliton pulses having soliton orders of N>20, for example, N=30, N=35, N=40, or N=45.

[0117] [000115] In one embodiment, different nonlinear optical processes in which the pulsed input emission IRD is spectrally spread within a second section of the HC-PCF may be dominated by the SSC. The pulsed input emission IRD may be configured to change into soliton pulses having soliton orders of N < 20, for example, N ≤ 7, N ≤ 8, N ≤ 9, or N ≤ 10.

[0118] [000116] In one embodiment, the HC-PCF may be a single-ring HC-PCF, as shown in Figure 8, for example. The critical bending radius may be determined from the diameter D of the hollow core, the diameter d of the capillary, the azimuthal angle of the fiber cross-section, and the (centroid) wavelength of the pulsed input radiation using equation (4) above.

[0119] [000117] In one embodiment, the HC-PCF may have a length, i.e., the length from the input end to the output end, which is appropriately determined to generate a broadband output radiated ORD by an MI or SSC process. The fiber length may be, for example, 5 cm to 1000 cm, 10 cm to 800 cm, 20 cm to 500 cm, 50 cm to 200 cm, or about 500 cm in an MI process, and about 1000 cm in an SSC process. In one embodiment, the first section of the HC-PCF may have a length similar to that of the second section of the HC-PCF. This may be the case when the MI process dominates the spectral spread of the pulsed input radiated IRD. In one embodiment, the first section of the HC-PCF may be longer than the rest of the HC-PCF. This may be the case when the SSC process dominates the spectral spread of the pulsed input radiated IRD.

[0120] [000118] How the first section of the HC-PCF is coiled and / or bent may depend on the dimensional requirements of the HC-PCF-based broadband radiation source RDS. That is, the first section of the HC-PCF may be flexibly bent or coiled to meet certain dimensional requirements, as long as the resulting bending radius or coil radius does not fall below a critical bending radius. Alternatively, there may be a trade-off between radiation loss and dimensional requirements, for example, if the radiation loss is not too high for a particular application and compactness is considered more important, it may be permissible to exceed the critical bending radius. The critical bending radius may be, for example, such that the radiation loss in the first section does not exceed 50%, 30%, 10%, 5%, 1%, 0.5%, or 0.1%.

[0121] [000119] In some embodiments, the HC-PCF may be bent by a specific angle to follow a circular, partial circular, elliptical, or partial elliptical path. Here, the bending angle may be defined as the angle between two fiber portions on each of the two sides of the bend. The bending angle may be, for example, at least 20 degrees, at least 45 degrees, at least 60 degrees, at least 90 degrees, at least 120 degrees, or at least 135 degrees. In some embodiments, the HC-PCF may be coiled into one or more circles or ellipses. In all such embodiments, the bending radius or coil radius may be kept greater than the critical bending radius determined by inputting the relevant parameters of the HC-PCF and pulse-input radiated IRD into equation (4) above.

[0122] [000120] In one embodiment, the broadband radiation device may further include a working medium WM confined within a hollow core and a pulse pump radiation source PRS arranged to generate pulse pump radiation IRD. In one embodiment, the working medium WM is configured to produce anomalous dispersion.

[0123] [000121] In one embodiment, the input pulse width of pulse pump radiation may be shorter than 5000 fs, shorter than 1000 fs, shorter than 500 fs, shorter than 300 fs, or shorter than 100 fs.

[0124] [000122] In one embodiment, the broadband output emission ORD may include spectra that partially overlap with the ranges of 200 nm to 2000 nm, 400 nm to 1600 nm, or 500 nm to 900 nm. In one embodiment, the output broadband emission may include spectra having a full width at half maximum (FWHM) of at least 500 nm, at least 400 nm, at least 300 nm, or at least 200 nm. In one embodiment, the broadband output emission may include spectra spanning from 500 nm to 900 nm.

[0125] [000123] In one embodiment, the broadband radiating device may further include a second gas cell for housing a working medium. The first gas cell may be configured to enclose at least partially the HC-PCF. In one embodiment, the broadband radiating device may further include a second gas cell for housing a working medium. In such an embodiment, the first gas cell may be configured to enclose a first portion including the input end of the HC-PCF and operate at a first pressure, and the second gas cell may be configured to enclose a second portion including the output end of the HC-PCF and operate at a second pressure. In one embodiment, the first pressure in the first gas cell may be different from (e.g., lower than) the second gas pressure in the second gas cell.

[0126] [000124] In one embodiment, the first gas cell may be configured to enclose a first portion of the HC-PCF including the input end and / or a second portion of the HC-PCF including the output end. In one embodiment, the first gas cell may be configured to enclose only the first section (or the SPM-dominant section) of the HC-PCF. In one embodiment, the first gas cell may be configured to enclose the entire HC-PCF. In one embodiment, the first gas cell may be configured to enclose only the second section of the HC-PCF. In one embodiment, at least a portion of the first gas cell may be bent or coiled in a manner similar to that of the bent or coiled portion of the HC-PCF.

[0127] [000125] Figures 14(a) to 14(e) schematically show five embodiments of a broadband radiation source RDS, in which at least a portion of the first section of the HC-PCF is bent or coiled, and the HC-PCF is at least partially enclosed in one or more gas cells configured differently for different embodiments.

[0128] [000126] Referring to Figure 14(a), in one embodiment, at least a portion of the first section of the HC-PCF may be coiled into a substantially circular loop. It will be noted that the fiber may also be coiled into other shapes (e.g., elliptical). The HC-PCF may be completely enclosed in a first gas cell RSV1, a coiled connecting tube CCT, and a second gas cell RSV2, which may be filled with the same working medium WM. The first gas cell RSV1 may be configured to enclose a first end portion including the input end of the HC-PCF. The second gas cell RSV2 may be configured to enclose a second end portion including the output end of the HC-PCF. The second end may include part or all of the second section of the HC-PCF. Although not required, both the input and output end portions may be substantially parallel to the propagation direction of the pulse pump radiation IRD (or the Z direction, referring to the adjacent local coordinate system in Figure 14(d)). The coiled connecting tube CCT may have substantially the same coil shape (e.g., circular or elliptical) as the coiled portion of the HC-PCF and may be configured to enclose at least the coiled portion of the HC-PCF.

[0129] [000127] The first gas cell RSV1 may have a first diameter, the second gas cell RSV2 may have a second diameter, and the coiled connecting tube CCT may have a third diameter, all of which are aligned in a transverse direction perpendicular to the fiber axis of the HC-PCF. In one embodiment, the first diameter of the first gas cell RSV1 may be the same as the second diameter of the second gas cell RSV2. The difference between the first diameter and the second diameter may be, for example, within ±10%, ±15%, ±20%, ±25%, or ±30%. In one embodiment, the third diameter of the coiled connecting tube CCT may be smaller than both the first diameter of the first gas cell RSV1 and the second diameter of the second gas cell RSV2. The third diameter may be, for example, 50% or less, 40% or less, 30% or less, or 20% or less of both the first and second diameters. In one embodiment, the radius of curvature (or coil radius) of the coiled connecting tube may be, for example, 1 cm to 500 cm, 1 cm to 400 cm, 1 cm to 300 cm, 1 cm to 200 cm, or 2 cm to 100 cm.

[0130] [000128] The first gas cell RSV1 and the second gas cell RSV2 may each include an input end and an output end. In embodiments, the first gas cell RSV1 may further include a gas connection through which the working medium WM may be fed into or discharged from the first gas cell RSV1. The two ends of the coiled connecting tube CCT may be sealedly connected to the output end of the first gas cell RSV1 and the input end of the second gas cell RSV2, respectively. Such an arrangement may allow the working medium WM to flow, for example, from the first gas cell RSV1 through the coiled connecting tube CCT to the second gas cell RSV2, so that the working medium WM may be under substantially the same pressure P1 in the first gas cell RSV1, the coiled connecting tube, and the second gas cell RSV1.

[0131] [000129] The first gas cell RSV1 may include a first transparent window TW1 located at the output end of the first gas cell RSV1, configured to substantially transmit pulse pump radiation IRD. The second gas cell RSV2 may include a second transparent window TW2 located at the output end of the second gas cell RSV2, configured to substantially transmit broadband power radiation ORD. The pulse pump radiation IRD is emitted from a pulse pump radiation source PRS in free space and can be focused through a lens into the hollow core of the HC-PCF. After passing through the first transparent window TW1 of the first gas cell RSV1, the pulse pump radiation IRD can enter the hollow core of the HC-PCF through the input end of the fiber and then spectrally spread out to become broadband power radiation ORD. The broadband power radiation ORD can exit the HC-PCF through the output end of the fiber and finally exit the second gas cell RSV2 through the second transparent window TW2.

[0132] [000130] Referring to Figure 14(b), in one embodiment, at least a portion of the first section of the HC-PCF may be coiled in a manner similar to the coiled fiber shown in Figure 14(a). The HC-PCF may be partially enclosed in a first gas cell RSV1' and a second gas cell RSV2', which may be filled with the same working medium WM. The working medium WM in the first gas cell RSV1' may be different from the working medium WM in the second gas cell RSV2'. Similar to the embodiment in Figure 14(a), the first gas cell RSV1' may be configured to enclose a first end portion including the input end of the HC-PCF, and the second gas cell RSV2' may be configured to enclose a second end portion including the output end of the HC-PCF. The second end may include part or all of the second section of the HC-PCF. However, the main difference between the two embodiments is that the embodiment in Figure 14(b) may not include a coiled connecting tube CCT, and as a result, the working medium WM may not flow between the first gas cell RSV1' and the second gas cell RSV2'. In this embodiment, an airtight seal may exist between the HC-PCF and each of the first gas cell RSV1' and the second gas cell RSV2'. For example, the first gas cell RSV1' may include a first airtight opening GTO1 at the output end of cell RSV1', and the HC-PCF can be sealed into the first gas cell RSV1' through the first airtight opening GTO1. Similarly, the second gas cell RSV2' may include a second airtight opening GTO2 at the input end of cell RSV2', and the HC-PCF can be sealed into the second gas cell RSV2' through the second airtight opening GTO2.

[0133] [000131] Since there is no gas flow between the first gas cell RSV1' and the second gas cell RSV2', the second gas cell RSV2' may include a gas connection through which the same or different working medium WM can be fed into or discharged from the second gas cell RSV2'. This configuration may allow the pressure and / or type of the working medium WM in the first gas cell RSV1' and the second gas cell RSV2' to be controlled individually. In one embodiment, the first gas cell RSV1' and the second gas cell RSV2' may be filled with the same working medium WM, and the two gas cells RSV1', RSV2' may be set to two different pressure levels, namely a first pressure P1' and a second pressure P2'. Thus, because the gas flow through the HC-PCF is negligible compared to the piping connection between gas cells (for example, the embodiment shown in Figure 14(a)), it is possible to maintain a pressure gradient between the first pressure P1' in the first gas cell RSV1' and the second pressure P2' in the second gas cell RSV2'. Such a pressure gradient can be useful in reducing incoupling fluctuations (for example, by keeping the input end of the HC-PCF fiber at a lower pressure P1'), which may allow for optimization of the spectral range of the broadband output radiation ORD generated by the SSC process.

[0134] [000132] Referring to Figure 14(c), in one embodiment, at least a portion of the first section of the HC-PCF may be coiled in a manner similar to that of the coiled fiber shown in Figure 14(a) or Figure 14(b). The HC-PCF may be completely sealed in a coiled connecting tube CCT' and a gas cell RSV2'', both of which may be filled with the same working medium WM. The coiled connecting tube CCT' may be configured to seal at least the entire first section of the HC-PCF. The gas cell RSV2'' may be configured to seal the end portion of the HC-PCF, including the output end. The end portion of the HC-PCF may include part or all of the second section of the HC-PCF.

[0135] [000133] Similar to the embodiment in Figure 14(a), the output end of the coiled connecting tube CCT' may be sealably connected to the input end of the gas cell RSV2'' so that the working medium WM can flow between the coiled connecting tube CCT' and the gas cell RSV2''. The pressure P2'' of the working medium WM in both the coiled connecting tube CCT' and the gas cell RSV2'' may be controlled by the gas connection of the gas cell RSV2''. Instead of sealably connecting to another gas cell (e.g., the first gas cell RSV1 shown in Figure 14(a)), the input end of the coiled connecting tube CCT' may be sealably connected to a fiber optic interface FSI. In one embodiment, the transverse diameter of the coiled connecting tube CCT' may be smaller than the transverse diameter of the gas cell RSV2''. The transverse diameter of the coiled connecting tube CCT' may be, for example, 50% or less, 40% or less, 30% or less, or 20% or less of the transverse diameter of the gas cell RSV2''.

[0136] [000134] The input end of the HC-PCF may be joined to a transmission fiber TF configured to transmit pulse pump radiation IRD from the pulse pump radiation source PRS to the HC-PCF. In one embodiment, the transmission fiber TF may be part of the pulse pump radiation source PRS. The fiber junction interface FSI may be configured to hold the joined fiber ends of the transmission fiber TF and the HC-PCF while providing an airtight seal to the input end of the coiled connecting tube CCT'.

[0137] [000135] While the embodiments described above require at least two separate gas containment components for partially or completely encapsulating the HC-PCF, embodiments shown in Figures 14(d) and 14(e) may use a single-piece gas cell to completely encapsulate the HC-PCF. Referring to Figures 14(d) and 14(e), the single-piece gas cell RSV3, RSV3' may have a substantially constant transverse diameter along the length of the gas cell RSV3, RSV3'. The gas cell RSV3, RSV3' may include a first straight section SS1, SS1', a coiled section CS (e.g., shown in Figure 14(d)) or a bent section BS (e.g., shown in Figure 14(e)), and a second straight section SS2, SS2'. Similar to the embodiments described above, the gas cell RSV3, RSV3' may include a first transparent window TW1 configured to transmit pulse pump radiation IRD and a second transparent window TW2 configured to transmit broadband power radiation ORD. The gas cells RSV3 and RSV3' may include a gas connection, through which the working medium WM can be supplied into or discharged from the gas cells RSV3 and RSV3', and the pressure P3 of the working medium can be controlled.

[0138] [000136] In the case of Figure 14(d), the first section of the HC-PCF may be at least partially enclosed within the coiled section CS of the single-piece gas cell RSV3. In the case of Figure 14(e), the first section of the HC-PCF may be at least partially enclosed within the bent section BS of the single-piece gas cell RSV3. In either case, the second section of the HC-PCF, in which the spectral broadening of the pulse pump emission IRD is dominated by the MI or SSC process, may be completely enclosed within the second linear sections SS2, SS2' of the gas cell RSV3, RSV3'. In one embodiment, the second linear sections SS2, SS2' may also enclose a portion of the first section of the HC-PCF.

[0139] [000137] Figure 15 is a block diagram of a computer system 1600 that may assist in the implementation of the methods and flows disclosed herein. The computer system 1600 includes a bus 1602 or other communication mechanism for communicating information and a processor 1604 (or a plurality of processors 1604 and 1605) coupled to the bus 1602 for processing information. The computer system 1600 also includes a main memory 1606, such as random access memory (RAM) or other dynamic storage device, coupled to the bus 1602 for storing information and instructions executed by the processor 1604. The main memory 1606 may also be used to store temporary variables or other intermediate information during the execution of instructions executed by the processor 1604. The computer system 1600 further includes a read-only memory (ROM) 1608 or other static storage device coupled to the bus 1602 for storing static information and instructions for the processor 1604. A storage device 1610, such as a magnetic disk or optical disk, is provided and coupled to a bus 1602 for storing information and instructions.

[0140] [000138] The computer system 1600 may be coupled via a bus 1602 to a display 1612, 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 1614, including alphanumeric and other keys, is coupled to the bus 1602 for communicating information and command selections to the processor 1604. Another type of user input device is a cursor control 1616, such as a mouse, trackball, or cursor directional keys, for communicating directional information and command selections to the processor 1604 and for controlling the movement of a cursor on the display 1612. 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), allowing the device to specify a position in a plane. A touch panel (screen) display may also be used as an input device.

[0141] [000139] One or more of the methods described herein may be executed by the computer system 1600 in response to the processor 1604 executing one or more sequences of one or more instructions contained in the main memory 1606. Such instructions may be read into the main memory 1606 from another computer-readable medium, such as a storage device 1610. The execution of the instruction sequence contained in the main memory 1606 causes the processor 1604 to perform the process steps described herein. One or more processors in a multiprocessing configuration may also be used to execute the instruction sequence contained in the main memory 1606. In alternative embodiments, hardwired circuits may 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.

[0142] [000140] As used herein, the term “computer-readable medium” refers to any medium involved in providing instructions to the processor 1604 for execution. Such mediums may take many forms, including but 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 1610. Volatile mediums include dynamic memory such as the main memory 1606. Transmission mediums include coaxial cables, copper wires, and optical fibers, including the wires that make up the bus 1602. Transmission mediums may also take the form of acoustic waves or optical waves, such as those generated during 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 and EPROMs, FLASH-EPROMs, any other memory chips or cartridges, carriers as described below, or any other media from which a computer can read.

[0143] [000141] Various forms of computer-readable media may be involved in transporting one or more sequences of one or more instructions to the processor 1604 for execution. For example, the instructions may initially reside on a magnetic disk of a remote computer. The remote computer may load the instructions into dynamic memory and transmit them over a telephone line using a modem. A local modem of computer system 1600 may receive data over the telephone line and convert the data into an infrared signal using an infrared transmitter. An infrared detector coupled to bus 1602 may receive the data transported by the infrared signal and place the data on bus 1602. Bus 1602 transports the data to main memory 1606, and the processor 1604 retrieves the instructions from main memory 1606 and executes them. Instructions received by main memory 1606 may optionally be stored in storage device 1610 either before or after execution by processor 1604.

[0144] [000142] The computer system 1600 may also preferably include a communication interface 1618 coupled to the bus 1602. The communication interface 1618 provides bidirectional data communication coupling to a network link 1620 connected to a local network 1622. For example, the communication interface 1618 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 1618 may be a Local Area Network (LAN) card for providing data communication connectivity to a compatible LAN. A wireless link may also be implemented. In any such embodiment, the communication interface 1618 transmits and receives electrical, electromagnetic, or optical signals carrying digital data streams representing various types of information.

[0145] [000143] Network link 1620 typically provides data communication to other data devices over one or more networks. For example, network link 1620 may provide connection to a host computer 1624 via local network 1622 or to data equipment operated by an Internet service provider (ISP) 1626. The ISP 1626 then provides data communication services over a worldwide packet data communication network, now commonly referred to as the “Internet” 1628. Both local network 1622 and the Internet 1628 use electrical, electromagnetic, or optical signals to carry digital data streams. Signals across various networks that carry digital data to and from computer system 1600, as well as signals on network link 1620 and signals through communication interface 1618, are exemplary forms of carrier waves that transmit information.

[0146] [000144] The computer system 1600 may send messages and receive data including program code via a network, network link 1620, and communication interface 1618. In the example of the Internet, server 1630 may send requested code for an application program via the Internet 1628, ISP 1626, local network 1622, and communication interface 1618. One such downloaded application may provide, for example, one or more of the techniques described herein. The received code may be executed by processor 1604 upon receipt and / or stored in storage device 1610 or other non-volatile storage device for later execution. In this way, the computer system 1600 may obtain application code in carrier form.

[0147] [000145] Further embodiments are disclosed in the following numbered clauses. 1. A broadband radiating device comprising a hollow core photonic crystal fiber (HC-PCF), A hollow core extending along the length of the HC-PCF to confine the working medium under pressure during use, An input terminal capable of receiving pulse pump radiation, Spectrum of pulse pump radiation in the working fluid confined within an HC-PCF An output terminal capable of emitting broadband output radiation resulting from its spread, Includes, The HC-PCF is divided into a first section extending over a first portion of the length of the HC-PCF and a second section extending over a second portion of the length of the HC-PCF, wherein the first section includes an input terminal, and at least a portion of the first section includes a bend and / or one or more coils, and the second section includes an output terminal and is substantially linear. HC-PCF is a method configured such that, in the first section, spectral broadening occurs mainly by a self-phase modulation process, and in the second section, spectral broadening occurs mainly by a nonlinear optical process different from the self-phase modulation process. 2. A broadband radiating device as described in Clause 1, where different nonlinear optical processes are governed by modulation instability MI or soliton self-compression SSC. 3. The HC-PCF is a single-ring HC-PCF, which is a broadband radiating device as described in Clause 1 or 2. 4. At least a portion of the first section of the HC-PCF is such that the resulting coil radius and / or bending radius is

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[0148] [000146] While this document may make specific references to the use of lithography equipment in the manufacture of ICs, it should be understood that the lithography equipment described herein may also have other applications. Other possible applications include the manufacture of integrated optical systems, induction and detection patterns for magnetic domain memory, flat panel displays, liquid crystal displays (LCDs), thin-film magnetic heads, and the like.

[0149] [000147] While this document may provide specific references to embodiments of the present invention in relation to lithography apparatus, embodiments of the present invention may also be used in other apparatuses. Embodiments of the present invention may form part of a mask inspection apparatus, a measuring apparatus, or any apparatus for measuring or processing objects such as wafers (or other substrates) or masks (or other patterning devices). These apparatuses are sometimes generally referred to as lithography tools. Such lithography tools may operate under vacuum or ambient (non-vacuum) conditions.

[0150] [000148] While specific references to the use of embodiments of the present invention in relation to photolithography may have been made above, it will be recognized that the present invention is not limited to photolithography and may be used in other applications, such as imprint lithography, as the context permits.

[0151] [000149] Specific references may be made to “measuring devices / tools / systems” or “inspection devices / tools / systems,” but these terms may refer to the same or similar types of tools, devices or systems. For example, an inspection or measuring device including embodiments of the present invention may be used to determine the characteristics of a structure on a substrate or wafer. An inspection or measuring device including embodiments of the present invention may be used to detect defects in a substrate or defects in a structure on a substrate or wafer. In such embodiments, the characteristics of the structure on the substrate may relate to defects in the structure, the absence of a particular part of the structure or the presence of unwanted structures on the substrate or wafer. While specific embodiments of the present invention have been described above, it will be understood that the present invention can be implemented in ways other than those described above. The above description is illustrative and not limiting. Therefore, it will be apparent to those skilled in the art that modifications to the present invention described above can be made without departing from the claims set forth below.

Claims

1. A broadband radiating device including a hollow core photonic crystal fiber (HC-PCF), A hollow core extending along the length of the HC-PCF for containing the working medium under pressure during use, An input terminal capable of receiving pulse pump radiation, An output terminal capable of emitting broadband output radiation resulting from the spectral broadening of the pulse pump radiation in the working medium confined within the HC-PCF, Includes, The HC-PCF is divided into a first section extending over a first portion of the length of the HC-PCF and a second section extending over a second portion of the length of the HC-PCF, wherein the first section includes the input terminal, and at least a portion of the first section includes a bend and / or one or more coils, and the second section includes the output terminal and is substantially linear. The HC-PCF is a broadband emitting device configured such that, within the first section, the spectral broadening occurs mainly by a self-phase modulation process, and within the second section, the spectral broadening occurs mainly by a nonlinear optical process different from the self-phase modulation process.

2. The broadband radiating device according to claim 1, wherein the different nonlinear optical processes are governed by modulation instability MI or soliton self-compression SSC.

3. The first section of the HC-PCF, at least a portion thereof, the resulting coil radius and / or bending radius is [Math 1] The material is coiled and / or bent in such a manner that it does not become smaller than the critical bending radius determined by the formula, where D is the diameter of the hollow core of the HC-PCF, d is the diameter of the capillary of the HC-PCF, λ is the wavelength of the pulse pump radiation, and u ０１ This is the Bessel function J ０ The broadband radiating device according to claim 1 or 2, wherein the first zero of θ is indicated, and θ is the azimuth angle between the cross section of the HC-PCF and the bending plane.

4. The broadband radiating device according to any one of claims 1 to 3, wherein the HC-PCF is bent such that the angle between the two fiber portions on each of the two sides of the bent portion is at least 45 degrees, or optionally the bent portion is bent at least 90 degrees.

5. A broadband radiating device according to any one of claims 1 to 4, comprising a first gas cell for housing the working medium, wherein the first gas cell is configured to at least partially enclose the HC-PCF.

6. The broadband radiating device according to claim 5, wherein the first gas cell is configured to enclose a first portion of the HC-PCF including the input terminal and / or a second portion of the HC-PCF including the output terminal.

7. The broadband radiating device according to claim 5 or 6, wherein the first gas cell is configured to enclose the entire HC-PCF.

8. The broadband radiating device according to any one of claims 5 to 7, wherein at least a portion of the first gas cell is bent or coiled to enclose the bent portion or coiled portion of the HC-PCF.

9. The broadband radiating device according to any one of claims 1 to 8, wherein the length between the input terminal and the output terminal of the HC-PCF is 5 cm to 1000 cm.

10. The working medium confined within the hollow core, A pulse pump radiation source arranged to generate the aforementioned pulse pump radiation, A broadband radiating device according to any one of claims 1 to 9, further comprising:

11. The broadband radiating device according to claim 10, wherein the input pulse width of the pulse pump radiation is operable to be shorter than 1000 fs or, optionally, shorter than 500 fs.

12. The broadband radiating device according to any one of claims 1 to 11, wherein the first portion is 60% to 80% of the length of the HC-PCF from the input terminal to the output terminal.

13. The first part is, [Math 2] The length is determined by the formula, where γ is a material-dependent nonlinear parameter, and P ｐ The broadband radiating device according to any one of claims 1 to 12, wherein is the peak power of pulse pump radiation.

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

15. A method for generating broadband output radiation, Receiving pulse pump radiation at the input end of a hollow core photonic crystal fiber (HC-PCF) having a hollow core that confines the working medium under pressure, The emission of broadband power radiation at the output terminal of the HC-PCF, wherein the broadband power radiation arises from the spectral broadening of the pulse pump radiation in the working medium confined within the HC-PCF. Includes, A method wherein the HC-PCF is divided into a first section extending over a first portion of the length of the HC-PCF and a second section extending over a second portion of the length of the HC-PCF, the first section including the input terminal, at least a portion of the first section including a bend and / or one or more coils, the second section including the output terminal and being substantially linear, and the HC-PCF is configured such that the spectral broadening occurs mainly by a self-phase modulation process within the first section and mainly by a nonlinear optical process different from the self-phase modulation process within the second section.

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