Gas mixture for hollow core fibers used in broadband radiation generation

Abstract

A hollow core fiber and light source assembly for generating broadband radiation, wherein the hollow core of the hollow core fiber is filled with a gas composition containing a working gas. The fiber is configured to receive pulsed pump radiation at the input end of the hollow core fiber, the pulsed pump radiation having a pulse power exceeding the ionization threshold of the gas composition. The fiber is configured to confine and guide the pulsed pump radiation within the fiber, thereby causing the pulsed pump radiation to interact with the working gas to generate broadband radiation through the nonlinear spreading of the pulsed pump radiation. The gas composition contains less than 1% hydrogen component of the total gas composition in the hollow core fiber.

Description

Technical Field

[0001] [Cross-reference to Related Applications] This application claims priority to European Application No. 23165303.1, filed Mar. 30, 2023, and European Application No. 23191688.3, filed Aug. 16, 2023, which are hereby incorporated by reference in their entirety.

[0002] [Technical Field] The present invention relates to a hollow-core fiber and a light source assembly for broadband generation. In particular, it relates to a hollow-core fiber configured to contain a gas composition that causes non-linear spreading.

Background Art

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

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

[0005] Low k1 lithography allows for the processing of features smaller in dimensions than the classical resolution limit of the lithography apparatus. In such processes, the resolution formula can be expressed as CD = k1 × λ / NA, where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection optics of the lithography apparatus, CD is the "limiting dimension" (usually the smallest feature size to be printed, in this case half-pitch), and k1 is the empirical resolution factor. 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 to achieve a particular electrical function and performance. To overcome these difficulties, advanced fine-tuning steps can be applied to the lithography projection apparatus and / or design layout. These include, but are not limited to, NA optimization, customized illumination schemes, the use of phase-shift patterning devices, various optimizations of the design layout such as optical proximity correction (OPC), also known as "optical and process corrections," or other methods generally defined as "resolution enhancement techniques" (RET). Alternatively, pattern reproduction at low k1 can be improved by using a strict control loop to control the stability of the lithography device. [Overview of the project] [Problems that the invention aims to solve]

[0006] In the field of lithography, many measurement systems may be used both inside and outside the lithography apparatus. Generally, such measurement systems may use a radiation source for irradiating a target and a detection system capable of measuring at least one characteristic of a portion of the incident radiation scattered from the target. Examples of measurement systems outside the lithography apparatus include inspection or measurement devices, which can be used to determine the characteristics of patterns previously projected onto a substrate by the lithography apparatus. Such external inspection devices may include, for example, a scatterometer. Examples of measurement systems that may be installed inside the lithography apparatus include topography measurement systems (also called level sensors), position measurement systems (e.g., interferometers) for determining the position of the reticle or wafer stage, and alignment sensors for determining the position of alignment marks. These measurement devices may use electromagnetic radiation to perform measurements.

[0007] Depending on the measurement system, radiation with various wavelength ranges can be used to perform one or more measurements. This is made possible, for example, by incorporating a broadband radiation source such as a supercontinuum radiation source. A supercontinuum radiation source may have a hollow core fiber from which broadband radiation is generated by the spectral broadening of the received input radiation. Here, the input radiation is often called pump radiation. The spectral broadening process may depend on nonlinear effects, such as the interaction between confined pump radiation and a gas composition / gas mixture that exhibits a substantially nonlinear response.

[0008] This specification describes assemblies, apparatus, and methods for providing broadband radiation with improved output and / or lifetime. [Means for solving the problem]

[0009] According to a first aspect of the present disclosure, a hollow core fiber for broadband generation is provided. The hollow core of the hollow core fiber is filled with a gas composition containing a working gas. The hollow core fiber is configured to receive pulsed pump radiation having a pulse power exceeding the ionization threshold of the gas composition at the input end of the hollow core fiber, to confine and guide the pulsed pump radiation within the fiber, and to interact with the working gas to generate broadband radiation by the nonlinear spreading of the pulsed pump radiation. The gas composition contains less than 1% hydrogen component of the total gas composition in the hollow core fiber. [Brief explanation of the drawing]

[0010] Embodiments of the present invention will be described only as examples with reference to the accompanying schematic diagrams. [Figure 1] This is a schematic diagram of a lithography machine. [Figure 2] This is a schematic diagram of a lithography cell. [Figure 3] This is an overall lithography schematic diagram illustrating the coordination between three key technologies for optimizing semiconductor manufacturing. [Figure 4] This is a schematic diagram of a scatometer measurement tool. [Figure 5] This is a schematic diagram of a level sensor measurement tool. [Figure 6] This is a schematic diagram of the alignment sensor measurement tool. [Figure 7] Figure 7(a) shows an example graph illustrating the change in output power over time for a light source assembly containing a high-purity gas consisting of 50% helium and 50% working gas. Figure 7(b) shows an example graph illustrating the change in output power spectral density over time after a sudden breakdown phenomenon. [Figure 8] Figure 8(a) is a schematic diagram showing a cross-section of an example of a hollow core fiber that can be used for broadband light generation. Figure 8(b) is a schematic diagram of a hollow core fiber in a radiation source assembly. [Modes for carrying out the invention]

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

[0012] As used herein, the terms “reticle,” “mask,” or “patterning device” can be broadly interpreted to refer to any common patterning device that can be used to give an incident radiation beam a patterned cross-section corresponding to a pattern to be created on a target portion of a substrate. The term “light bulb” can also be used in this context. In addition to standard masks (transmissive or reflective, binary, phase-shifted, hybrid, etc.), other examples of such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0013] Figure 1 schematically shows a lithography apparatus LA. This 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 positioning device 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 resist-coated wafer) W and connected to a second positioning device 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).

[0014] During operation, the illumination system IL receives a beam from the radiation source SO, for example, via the beam delivery system BD. The illumination system IL may include various optical elements, or any combination thereof, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical elements, for directing, shaping, and / or controlling the radiation. The illuminator IL may be used to adjust the radiation beam B to have a desired spatial and angular intensity distribution in its cross-section in the plane of the patterning device MA.

[0015] As used herein, the term “projection system” PS should be interpreted broadly to encompass any type of projection system, including, for example, refractive optical systems, reflective optical systems, reflective-refractory optical systems, anamorphic optical systems, magneto-optical systems, electromagnetic optical systems, and / or electrostatic optical systems, or any combination thereof, depending on the exposure radiation used and / or other factors such as the use of immersion liquid or vacuum. Where the term “projection lens” is used herein, it can be considered synonymous with the more general term “projection system” PS.

[0016] The lithography apparatus LA may be of a type in which at least a portion of the substrate is covered with a liquid having a relatively high refractive index (e.g., water) so as to fill the gap between the projection system PS and the substrate W. This is also known as immersion lithography. Details of the immersion technique are described in U.S. Patent No. 6,952,253, which is incorporated herein by reference.

[0017] Lithography apparatus LA may also be of the type having two or more substrate support WTs (also called “dual stages”). In such a “multistage” machine, the substrate support WTs can be used in parallel, and / or steps in preparing the substrate W for subsequent exposure can be performed on the substrate W located on one of the substrate support WTs. The substrate W on the other substrate support WT is used to expose the pattern on the other substrate W.

[0018] In addition to the substrate support WT, the lithographic apparatus LA may include a measurement stage. The measurement stage is arranged to hold a sensor and / or a cleaning device. The sensor can be arranged to measure the characteristics of the projection system PS or the radiation beam B. The measurement stage can hold a plurality of sensors. The cleaning device can be configured to clean a part of the lithographic apparatus, for example, a part of the projection system PS or a part of the system that provides the immersion liquid. When the substrate support WT is away from the projection system PS, the measurement stage can move under the projection system PS.

[0019] During operation, the radiation beam B is incident on a patterning device, such as a mask MA, held by the mask support MT, and is patterned by the pattern (design layout) present on the patterning device MA. After passing through the mask MA, the radiation beam B passes through a projection system PS that focuses the beam onto a target portion C of the substrate W. With the aid of the second positioning device PW and the position measurement system IF, the substrate support WT can be accurately moved so that different target portions C in the path of the radiation beam B are located at focused and aligned positions. Similarly, the first positioning device PM and optionally another position sensor (not explicitly shown in FIG. 1) can be used to accurately position the patterning device MA with respect to the path of the radiation 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 they may also be arranged in the spaces between the target portions. The substrate alignment marks P1, P2 are known as scribe lane alignment marks when they are arranged between the target portions C.

[0020] As shown in Figure 2, the lithography apparatus LA may sometimes be referred to as a lithocell or (litho)cluster and may form part of a lithography sicel LC that also includes equipment for performing pre-exposure and post-exposure processes on a substrate W. Conventionally, these include a spin-coating apparatus SC for depositing a resist layer, for example to adjust the temperature of the substrate W, for example to adjust the solvent of the resist layer, a developing apparatus DE for developing the exposed resist, a cooling plate CH, and a bake plate BK. A substrate handler or robot RO takes the substrate W from input / output ports I / O1, I / O2, moves the substrate between different process apparatuses, and carries the substrate W to the loading bay LB of the lithography apparatus LA. The lithocell apparatus (often collectively referred to as a track) is usually under the control of a track control unit TCU, the TCU itself is controlled by a monitoring and control system SCS, which may control the lithography apparatus LA, for example, via a lithography control unit LACU.

[0021] To ensure accurate and consistent exposure of substrates W exposed by a lithography apparatus LA, it is desirable to inspect the substrates and measure characteristics of the pattern structure, such as subsequent interlayer overlay errors, line widths, and critical dimensions (CD). For this purpose, inspection tools (not shown) can be included in the lithocell LC. If an error is detected, for example, if the inspection is performed before other substrates W of the same batch or lot have been exposed or processed, the exposure of subsequent substrates or other processing steps performed on the substrate W can be adjusted.

[0022] Inspection equipment, sometimes called measuring devices, is used to determine the properties of a substrate W, particularly 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 equipment may be configured to identify defects on the substrate W, and may, for example, be part of a lithocell LC, or integrated into a lithography apparatus LA, or be a standalone device. The inspection equipment can measure the properties of a latent image (an image of the resist layer after exposure), a semi-latent image (an image of the resist layer after a post-exposure bake step PEB), a developed resist image (an image from which the exposed or unexposed parts of the resist have been removed), or an etched image (after a pattern transfer step such as etching).

[0023] Typically, the patterning process in a lithography apparatus LA is one of the most critical steps in a process requiring high precision in the dimensionaling and placement of structures on a substrate W. To ensure this high precision, three systems can be combined in a so-called "overall" control environment, as schematically shown in Figure 3. One of these systems is the lithography apparatus LA, which is (substantially) connected to the measurement tool MT (second system) and the computer system CL (third system). The key to such an "overall" environment is to optimize the coordination between these three systems to enhance the entire process window and provide a strict control loop that ensures the patterning performed by the lithography apparatus LA stays within the process window. The process window defines a set of process parameters (e.g., dose, focus, overlay) within which a particular manufacturing process produces a defined result (such as a functional semiconductor device), and within which the process parameters of the lithography or patterning process are usually allowed to change.

[0024] The computer system CL can use (part of) the patterned design layout to predict which resolution enhancement techniques to use and perform computer-based lithography simulations and calculations to determine which mask layout and lithography equipment settings will achieve the largest overall process window of the patterning process (indicated by the double arrow on the first scale SC1 in Figure 3). Typically, the resolution enhancement techniques are configured to match the patterning possibilities of the lithography equipment LA. The computer system CL may also be used to detect where the lithography equipment LA is currently operating within the process window (e.g., using input from the measurement tool MT) to predict, for example, whether defects exist due to suboptimal processing (indicated by the arrow pointing to "0" on the second scale SC2 in Figure 3).

[0025] The measurement tool MT can provide input to the computer system CL to enable accurate simulation and prediction, and can provide feedback to the lithography apparatus LA to identify, for example, the drift that may occur in the calibration state of the lithography apparatus LA (indicated by multiple arrows on the third scale SC3 in Figure 3). Different types of measurement tools MT for measuring one or more characteristics related to the lithography apparatus and / or the substrate being patterned are described below.

[0026] In lithography processes, it is desirable to frequently measure the created structures, for example, for process control and verification. Tools for performing such measurements are usually called measuring tools (MT). Various types of measuring tools (MT) for performing such measurements are known, including scanning electron microscopes or various forms of scatometer measuring tools (MT). A scatometer is a versatile instrument that enables the measurement of parameters in a lithography process (in this case, the measurement is usually called an image or field-based measurement) by having a sensor on the pupil or the conjugate plane of the scatometer's objective lens with the pupil (a measurement usually called a pupil-based measurement), or by having a sensor on the image plane or the conjugate plane of the image plane. Such scatorometers and related measurement techniques are further described in Japanese Patent Applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032, or EP1,628,164A, which are incorporated herein by reference in their entirety. The scatorometers described above can measure gratings using light from soft X-rays and light in the visible to near-infrared wavelength range.

[0027] In the first embodiment, the scatometer MT is an angle-resolved scatometer. In such a scatometer, a reconstruction method can be applied to the measured signal to reconstruct or calculate the properties of the diffraction grating. Such reconstruction may result, for example, from simulating the interaction between scattered radiation and a mathematical model of the target structure and comparing the simulation results with the measured results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from an actual target.

[0028] In a second embodiment, the scatorometer MT is a spectrometer MT. In such a spectrometer MT, radiation emitted from a radiation source is directed to a target, and reflected or scattered radiation from the target is directed to a spectrometer detector that measures the spectrum of specularly reflected radiation (i.e., measures the intensity as a function of wavelength). From this data, the structure or profile of the target that produces 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.

[0029] In a third embodiment, the scatometer MT is an ellipsometric scatometer. An ellipsometric scatometer allows for the determination of parameters of a lithography process by measuring the scattered radiation of each polarization state. Such a measuring device emits polarization (such as linear, circular, or elliptical) by, for example, using a suitable polarization filter in the illumination section of the measuring device. A radiation source suitable for the measuring device may also provide polarized radiation. Various embodiments of existing ellipsometric cattrometers are described in U.S. Patent Applications No. 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.

[0030] In one embodiment of a scatorometer MT, the scatorometer MT is configured to measure the overlay of two misaligned gratings or periodic structures by measuring the asymmetry of the reflectance spectrum and / or the detection configuration. The asymmetry relates to the degree of overlay. The two (typically overlapping) grating structures may be applied to two different layers (not necessarily contiguous layers) and may be formed at substantially the same location on the wafer. The scatorometer may have a symmetric detection configuration so that the asymmetry can be clearly distinguished, as described, for example, in co-owned patent application EP1,628,164A. This allows for easy measurement of diffraction grating misalignment. Further examples of measuring overlay errors between two layers containing periodic structures as targets to be measured via the asymmetry of periodic structures can be found in International Patent Application Publication No. WO2011 / 012624 or U.S. Patent Application No. US20160161863, which are incorporated herein by reference in their entirety.

[0031] Other parameters of interest may be focus and dose. Focus and dose can be determined simultaneously by scantometry (or alternatively by scanning electron microscopy), as described in U.S. Patent Application US2011-0249244, which is incorporated in its entirety by reference herein. A single structure may be used, which has a unique combination of critical dimension and sidewall angle measurements for each point of the focus energy matrix (FEM, also called the focus exposure matrix). If these unique combinations of critical dimension and sidewall angle are available, the focus and dose values ​​can be uniquely determined from these measurements.

[0032] The measurement target may be an assembly of composite gratings, often formed within the resist by the lithography process, but also formed, for example, after the etching process. Typically, the pitch and linewidth of the structures within the grating depend heavily on the measurement optical system (especially the NA of the optical system) capable of capturing the diffraction order coming from the measurement target. As previously mentioned, the diffracted signal may be used to determine the shift (also called "overlay") between two layers, 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, or to control at least a portion of the lithography process. The target may have smaller subsegments configured to mimic the dimensions of the functional parts of the design layout within the target. Due to these subsegments, the target will behave more similarly to the functional parts of the design layout, such that the measurement results of the overall process parameters closely resemble those of the functional parts of the design layout. The target may be measured in underfill mode or overfill mode. In underfill mode, the measurement beam generates a spot smaller than the entire target. In overfill mode, the measurement beam generates a spot larger than the entire target. In such overfill mode, it may be possible to measure different targets simultaneously and therefore determine different process parameters at the same time.

[0033] The overall measurement quality of lithography parameters using a specific 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, or the measured parameters. The above patterns may include one or more parameters, or both. For example, if the measurement used in a substrate measurement recipe is a diffraction-based optical measurement, 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 on the substrate, and the direction of the radiation on 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 US2016-0161863 and the published U.S. Patent Application US2016 / 0370717, which are incorporated herein by reference in their entirety.

[0034] A measuring device such as a scatromometer SM1 is shown in Figure 4. It comprises a broadband (white light) radiation projector 2 that projects radiation onto a substrate 6. The reflected or scattered radiation is passed to a spectrometer detector 4, which measures the spectrum 10 of the specularly reflected radiation (i.e., the measurement of intensity In1 as a function of wavelength λ). From this data, the structure or profile that gives rise to the detected spectrum can be reconstructed by a processing unit PU, for example, by exact coupled wave analysis and nonlinear regression, or by comparison with a library of simulated spectra as shown at the bottom of Figure 4. Generally, in the reconstruction, the general form of the structure is known, some parameters are inferred from knowledge of the process by which the structure was created, and only a few parameters of the structure remain determined from the scatromometer data. Such a scatromometer can be configured as a normal incidence scatromometer or an oblique incidence scatromometer.

[0035] In lithography processes, it is desirable to frequently measure the generated structure, for example, for process control and verification. Various tools are known for performing such measurements, including scanning electron microscopes and various forms of measuring instruments such as scatterometers. Known examples of scatterometers often rely on the provision of dedicated measurement targets, such as underfill targets (targets with the shape of a simple grating or a grating with different layers overlapping, and large enough for the measurement beam to produce a spot smaller than the grating) and overfill targets (targets in which the illumination spot partially or completely encloses the target). Furthermore, so-called reconstruction methods can be used by employing measurement tools such as angle-resolved scatterometers that illuminate underfill targets, such as gratings. In this method, the interaction between scattered light and a mathematical model of the target structure can be simulated, and the properties of the grating can be calculated by comparing the simulation results with the measurement results. The parameters of the model are adjusted until the simulated interaction produces a diffraction pattern similar to the diffraction pattern observed from the actual target.

[0036] A scatterometer is a versatile measuring instrument capable of measuring parameters of a lithography process by positioning a sensor on the pupil or conjugate plane of the scatterometer's objective lens (this measurement is typically called pupil-based measurement), or by positioning a sensor on the image plane or conjugate plane of the image plane (this measurement is typically called image-based measurement or field-of-view measurement). Such scatterometers and related measuring techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032, or EP1,628,164A, which are incorporated herein by reference in their entirety. The aforementioned scatterometers can measure multiple targets from multiple gratings in a single image using soft X-rays and light in the visible to near-infrared wavelength range.

[0037] A topography measurement system, level sensor, or height sensor, which can be integrated into a lithography apparatus, is configured to measure the topography of the top surface of a substrate (or wafer). A topographic map of the substrate, also called a height map, can be generated from these measurements, showing the height of the substrate as a function of its position on the substrate. This height map can then be used to correct the position of the substrate when transferring a pattern to the substrate, in order to provide a spatial image of the patterning device at the correct focus position on the substrate. In this context, "height" will be understood to generally refer to the out-of-plane dimension (also called the Z-axis) relative to the substrate. Typically, the level or height sensor performs measurements at a fixed location (relative to its optics), and the relative movement between the substrate and the optics of the level or height sensor results in height measurements at various locations across the substrate.

[0038] An example of a known level or height sensor LS in the art is schematically shown in Figure 5. The figure illustrates only the principle of operation. In this example, the level sensor comprises an optical system including a projection unit LSP and a detection unit LSD. The projection unit LSP includes a radiation source LSO that provides a radiation beam LSB provided by the projection grating PGR of the projection unit LSP. The radiation source LSO may be a narrowband or broadband radiation source, such as a supercontinuum light source, polarized or unpolarized, such as a polarized or unpolarized laser beam, pulsed or continuous. 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 radiation and may additionally or alternatively include UV and / or IR radiation and any range of wavelengths suitable for reflection from the surface of the substrate.

[0039] The projection grating PGR is a periodic grating with a periodic structure that produces a radiation beam BE1 with periodically changing intensity. The radiation beam BE1 with periodically changing intensity is guided toward the measurement location MLO on the substrate W with an incident angle ANG of 0 to 90 degrees, typically 70 to 80 degrees, with respect to the axis perpendicular to the incident substrate surface (Z-axis). At the measurement location MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and guided toward the detection unit LSD.

[0040] To determine the height level at the measurement location MLO, the level sensor further comprises a detection system including a detection grid DGR, a detector DET, and a processing unit (not shown) that processes 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, indicating the intensity of the received light like a photodetector, or representing the spatial distribution of the received intensity like a camera. The detector DET may comprise any combination of one or more detector types.

[0041] Triangulation techniques can be used to determine the height level at the measurement site MLO. The detected height level is typically related to the signal intensity measured by the detector DET, which has a periodicity determined, in particular, by the design of the projection grid PGR and the (oblique) incidence angle ANG.

[0042] 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).

[0043] In one embodiment, the detection grid DGR may be omitted, and the detector DET may be installed in the same location as the detection grid DGR. Such a configuration provides a more direct detection of the image of the projection grid PGR.

[0044] 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 larger measurement range.

[0045] Various types of common height sensors are disclosed, for example, in U.S. Patent No. 7,265,364 and U.S. Patent No. 7,646,471, which are incorporated by reference. A height sensor using UV radiation instead of visible or infrared radiation is disclosed in U.S. Patent Application Publication No. 2010233600A1, which is incorporated by reference. International Publication No. 2016102127A1, which is incorporated by reference, describes a small height sensor that uses a multi-element detector to detect and recognize the position of a grid image without requiring a detection grid.

[0046] The position measurement system (PMS) may include any type of sensor suitable for determining the position of the substrate support WT. The position measurement system (PMS) may also include any type of sensor suitable for determining the position of the mask support MT. The sensors may be optical sensors such as interferometers or encoders. The position measurement system (PMS) may include a system combining an interferometer and an encoder. The sensors may be other types of sensors such as magnetic sensors, capacitive sensors, or inductive sensors. The position measurement system (PMS) can determine the position relative to a reference, such as a measurement frame (MF) or projection system (PS). The position measurement system (PMS) can determine the position of the substrate table WT and / or mask support MT by measuring the position or by measuring the time derivative of the position (e.g., velocity or acceleration).

[0047] A position measurement system (PMS) may include an encoder system. Such an encoder system is known, for example, from U.S. Patent Application US2007 / 0058173A1, filed September 7, 2006 (incorporated herein by reference). This encoder system comprises an encoder head, a grating, and a sensor. The encoder system can receive a primary radiation beam and a secondary radiation beam. Both the primary and secondary radiation beams originate from the same radiation beam, i.e., the original radiation beam. At least one of the primary and secondary radiation beams is produced by diffracting the original radiation beam through a grating. If both the primary and secondary radiation beams are produced by diffracting the original radiation beam through a grating, the primary radiation beam must have a different diffraction order than the secondary radiation beam. Different diffraction orders are, for example, +1st order, -1st order, +2nd order, and -2nd order. The encoder system optically combines the primary and secondary radiation beams to produce a combined radiation beam. A sensor in the encoder head determines the phase or phase difference of the combined radiation beam. The sensor generates a signal based on phase or phase difference. This signal represents the position of the encoder head relative to the grid. Either the encoder head or the grid can be placed on a substrate structure WT. The other of the encoder head or grid can be placed on a measurement frame MF or a base frame BF. For example, multiple encoder heads are placed on the measurement frame MF, and the grid is placed on the upper surface of the substrate support Wt. In another example, the grid is placed on the lower surface of the substrate support WT, and the encoder head is placed below the substrate support Wt.

[0048] A position measurement system (PMS) may include an interferometer system. An interferometer system is known, for example, from U.S. Patent No. 6,020,964, filed July 13, 1998, which is incorporated herein by reference. The interferometer system may include a beam splitter, mirrors, a reference mirror, and a sensor. The radiating beam is split into a reference beam and a measurement beam by the beam splitter. The measurement beam propagates to the mirror, is reflected by the mirror, and returns to the beam splitter. The reference beam propagates to the reference mirror, is reflected by the reference mirror, and returns to the beam splitter. At the beam splitter, the measurement beam and the reference beam are coupled into a combined radiating beam. The combined radiating beam is incident on a sensor. The sensor determines the phase or frequency of the combined radiating beam. The sensor generates a signal based on the phase or frequency. This signal represents the displacement of the mirror. In one embodiment, the mirror is connected to a substrate support WT. The reference mirror may be connected to a measurement frame MF. In one embodiment, the measurement beam and the reference beam are coupled into a combined radiating beam by an additional optical component instead of a beam splitter.

[0049] The manufacturing of complex devices typically involves numerous lithography pattern formation steps, thereby creating functional features on successive layers of a substrate. Therefore, a crucial aspect of a lithography apparatus's performance is its ability to correctly and accurately position the applied pattern relative to features defined in previous layers (by the same or different lithography apparatus). For this purpose, one or more sets of marks are provided on the substrate. Each mark is structured such that its position can later be measured by 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." The marks are also sometimes called measurement targets.

[0050] A lithography apparatus may include one or more (e.g., multiple) alignment sensors that can accurately measure the position of alignment marks on a substrate. Alignment (or position) sensors may acquire positional information from alignment marks formed on a substrate using optical phenomena such as diffraction or interference. An example of an alignment sensor used in current lithography apparatuses is based on the self-referencing interferometer described in U.S. Patent No. 6961115. Various extensions and modifications of position sensors have been developed, for example, disclosed in U.S. Patent Publication 2015261097A1. The contents of all these published documents are incorporated herein by reference.

[0051] A mark, or alignment mark, may consist of a series of bars formed on or within a layer provided on a substrate, or formed (directly) within the substrate. Since the bars are regularly spaced and function as grid lines, the mark can be considered a diffraction grating with a known spatial period (pitch). Depending on the orientation of these grid lines, the mark may be designed to allow measurements along the X-axis or along the Y-axis (oriented substantially perpendicular to the X-axis). A mark containing bars positioned at +45 degrees and / or -45 degrees with respect to both the X and Y axes allows for combined X and Y axis measurements using the technique described in US2009 / 195768A, incorporated by reference.

[0052] The alignment sensor optically scans each mark with a radiating spot, acquiring a periodically changing signal such as a sine wave. By analyzing the phase of this signal, the position of the marks, and therefore the position of the substrate relative to the alignment sensor, is determined. The alignment sensor is fixed to the reference frame of the lithography apparatus. So-called coarse and fine marks can be provided, associated with different (coarse) and fine mark dimensions, so that the alignment sensor can distinguish between different periods of the periodic signal and the precise position (phase) within that period. For this purpose, marks with different pitches can also be used.

[0053] By measuring the position of the marks, information can also be obtained about the deformation of the substrate, for example, in the form of a wafer grid. Substrate deformation can occur, for example, when the substrate is electrostatically clamped to a substrate table and / or when the substrate is heated when exposed to radiation.

[0054] Figure 6 is a schematic block diagram of one embodiment of a known alignment sensor AS, such as that described in U.S. Patent No. 6961116, which is incorporated herein by reference. The radiation source RSO provides a radiation beam RB of one or more wavelengths, which is guided by a diverting optical system as an illumination spot SP onto a mark such as a mark AM positioned on a substrate W. In this example, the diverting optical system includes a spot mirror SM and an objective lens OL. The illumination spot SP illuminating the mark AM can have a diameter slightly smaller than the width of the mark itself.

[0055] The radiation diffracted by the mark AM is collimated (through the objective lens OL in this example) into the information-holding beam IB. The term “diffracted” is intended to include zero-order diffraction (which can be called reflection) from the mark. For example, a self-referencing interferometer SRI of the type disclosed in U.S. Patent No. 6961116 above interferes beam IB with itself, and the beam is then received by a photodetector PD. If the radiation source RSO generates 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 many pixels if desired. The photodetector may include a sensor array.

[0056] In this example, the inductive optics, including a spot mirror SM, may function to block zero-order radiation reflected from the mark, so that the information-holding beam IB contains only higher-order diffraction radiation from the mark AM (this is not essential for the measurement, but improves the signal-to-noise ratio).

[0057] The intensity signal SI is supplied to the processing unit PU. The combination of optical processing in block SRI and computational processing in unit PU outputs the X and Y position values ​​on the substrate relative to the reference frame.

[0058] The type of single measurement shown only fixes the position of a mark within a specific range corresponding to one pitch of the marks. In relation to this, a coarser measurement technique is used. The period of the sine wave containing this mark position is then identified. Regardless of the material on which the mark is fabricated or the material provided below and / or above the mark, the same process can be repeated at various wavelengths, at coarse and / or fine levels, for improved accuracy and / or robust detection of the mark. Wavelengths can be optically multiplexed and demultiplexed, and / or multiplexed by time division or frequency division, so that they are processed simultaneously.

[0059] In this example, the alignment sensor and spot SP remain fixed, while the substrate W moves. Therefore, the alignment sensor, while firmly and accurately mounted on a reference frame, can scan the mark AM in substantially the opposite direction to the movement of the substrate W. This movement of the substrate W is controlled by mounting the substrate W onto a substrate support and by a substrate positioning system controlling 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. By measuring the positions of the marks on the substrate support, the position of the substrate support determined by the position sensor can be calibrated (e.g., relative to the frame to which the alignment system is connected). By measuring the positions of the alignment marks on the substrate, the position of the substrate can be determined relative to the substrate support.

[0060] The measurement and / or inspection tools (also called measuring tools) described above often use radiation to acquire measurement data. Depending on the object being measured and the measurement characteristics, different types of radiation may be used. One of the different characteristics of radiation is the wavelength used to acquire the measurement results, as different wavelengths can provide different information about the object being measured. Some measuring tools may use broadband radiation, such as supercontinuum radiation, to measure using broadband radiation or to adjust and select the measurement wavelength to be used. Depending on the range of output wavelengths and the characteristics of the broadband light source, different methods may be used to acquire broadband radiation. In some embodiments for generating broadband radiation, nonlinear effects may be used to broaden input radiation (also called pump radiation) over a narrow wavelength range. A variety of known configurations and methods exist for achieving nonlinear broadening. These methods often rely on the confinement of pump radiation to achieve the high intensity required to experience a significant nonlinear effect.

[0061] Known methods for confining radiation due to nonlinear propagation include confining laser pump radiation within an optical fiber to generate broadband radiation. The laser may be an ultrashort pulse laser (e.g., picosecond to femtosecond pulses). The nonlinear propagation dynamics of this radiation within the fiber can lead to the generation of broadband radiation as a result of soliton self-compression and / or modulation instability. This can be used, for example, to generate supercontinuum radiation across the wavelength range from IR to UV.

[0062] The above method can be used to generate broadband radiation by utilizing the nonlinear spreading of laser pulses (e.g., femtosecond laser pulses) propagating along a gas-filled hollow core photonic crystal fiber. The gas mixture (also called the gas composition) filling the fiber may contain one or more components, one of which is the working gas. The working gas may be a gas that exhibits significant nonlinear effects when interacting with high-intensity radiation. The performance and lifetime of the light source may depend on the composition of the gas mixture filling the fiber. The first problem that may affect the fiber lifetime is damage / degradation caused by impurities in the gas. To address this, high purity of the gas composition is desirable to prevent the gradual progression of cloaking of the fiber. Cloaking refers to damage to the fiber due to contamination and accumulation of impurities, the gradual progression of radiation blocking, and / or scattering of radiation. Impurities include, for example, oxygen (O2).

[0063] Another issue that can affect the lifespan and performance of fibers is thermal damage. In the presence of high-intensity radiation, localized high temperatures can occur as the radiation interacts with the gas, potentially heating the fiber material. To address the risk of thermal damage, the thermal conductivity of the gas composition can be increased by adding a gas with high thermal conductivity. Good thermal conductivity can be understood as a substantially higher thermal conductivity than the thermal conductivity of other components in the gas mixture, resulting in an increased average thermal composition of the gas mixture. In some cases, good thermal conductivity may mean achieving a gas mixture thermal conductivity such that thermal damage does not occur during the normal operation of a broadband fiber light source, by adding a gas with good thermal conductivity. Gases added to improve thermal conductivity include light gases (gases with low particle weight), such as helium and neon. Light gases can be added in amounts ranging from 10% to 50% of the gas mixture. The addition of light gases can ensure good thermal conductivity and cooling in the fiber system, and can also ensure the structural integrity of the hollow core fiber.

[0064] In the above, two features that extend the lifetime of the mixed gas were described: a pure gas (low impurity level) and the addition of a lightweight gas to improve thermal conductivity. The advantages of these features mean that it is desirable to implement both in a broadband radiant fiber configuration. The inventors found that the implementation of a lightweight gas and the high purity of the mixed gas lead to a sudden and unpredictable breakdown of the broadband radiant process, as shown in Figures 7(a) and 7(b), and a significant reduction in broadband radiation emission from the radiant source. Graph 700 in Figure 7(a) shows the change in output power over time of an exemplary broadband radiant source using a high-purity mixed gas containing 50% helium and 50% working gas (krypton). In the initial startup phase, it is shown that the output power of the broadband radiant source gradually increases 702. Once the output power rises to full operation, a steady-state output condition is achieved 704, where a constant output power is supplied from the light source for a constant input pump power. At 706, a sudden and unexpected breakdown of the light source operation occurs, resulting in a decrease in output power for the same input pump power. This may be due to fiber damage. This effect is illustrated in detail in Figure 7(b), which shows an example graph of the output power spectral density as a function of wavelength. Arrow 708 indicates the time-dependent decay of the output spectral power after the sudden breakdown phenomenon 706. The spectral changes during decay may continue until almost all broadband radiated power is lost. In Figure 7(a), the breakdown is shown to occur during the steady-state operation of the light source 704, but it may also occur during the ramp-up phase 702.

[0065] This sudden breakdown is unexpected, and its cause is unknown. In this disclosure, the inventors explain the reason for this phenomenon and provide solutions to address it.

[0066] This specification explains the reason for sudden breakdowns occurring in the broadband light source device described herein. During their investigation, the inventors discovered that a stable plasma was forming inside the hollow core fiber. Due to the high intensity of the pump input pulses inside the hollow core fiber, each pulse can ionize a small amount of the gas mixture, potentially forming a plasma. This ionization can occur as a result of tunnel ionization. The plasma can be ionized while the laser pulse is present. The presence of a stable plasma depends on the ionization rate of the working gas by the pulses propagating through it. If the ionization rate is sufficiently high, the amount of ionized gas becomes large enough to withstand the period between subsequent pulses during which no ionization of the working gas occurs. In the configurations investigated by the inventors, the ionization rate can become sufficiently high when the laser pulse energy exceeds 2 μJ. The configurations investigated used a hollow core fiber with a diameter of approximately 30 μm and a laser pulse with a duration of 300 fs and a wavelength of approximately 1 μm. In this case, the corresponding peak power was 80 W, and the corresponding peak power density inside the hollow core fiber was approximately 2.5 TW / cm². 2 This is the case. With other configurations (other hollow core fiber configurations, different pump laser repetition frequencies, wavelengths, pulse durations, and gas mixture characteristics), the possibility of ionization occurring at considerably lower values ​​of the corresponding peak power density cannot be ruled out. For example, if the peak power density is 1 TW / cm² 2 Even in this case, significant ionization of the gas mixture may occur, for example, corresponding to a pulse energy of approximately 1 μJ and a pulse peak power of 30 W.

[0067] Between laser pulses, the plasma can decay / recombine. If the plasma collective decays / recombines quickly enough between subsequent pulses, plasma accumulation does not occur. However, if the plasma decay / recombination is too slow, plasma may remain when the next laser pulse arrives. The remaining free electrons can be accelerated by the next (and subsequent) laser pulses, further ionizing neutral atoms. As a result, the plasma density inside the fiber can increase exponentially. This plasma formation can ultimately lead to strong absorption of the pump laser light and the generated broadband light. The inventors concluded that plasma accumulation may be the root cause of sudden breakdown in broadband light generation using hollow core fibers containing a high-purity gas mixture.

[0068] The rate of plasma decay can depend significantly on the composition and purity of the gas mixture present within the fiber. Specifically, gas mixtures containing lighter gases (e.g., helium or neon added to improve thermal conductivity) have been found to be more susceptible to slower plasma decay, resulting in a higher likelihood of plasma breakdown. Gas mixtures containing helium, as described herein, may be particularly prone to plasma breakdown. Furthermore, certain contaminants, such as molecular gases, may accelerate plasma decay, potentially reducing the likelihood of plasma breakdown. A rapid plasma decay rate is called plasma quenching and can prevent the accumulation of critically high free electron densities.

[0069] The above observations regarding the effect of gas composition on plasma formation may explain why sudden breakdowns are less likely to occur in low-purity gas mixtures and / or gas mixtures that do not contain lighter gases. Below, we describe hollow core fibers and light source assemblies that may have improved lifetimes when operating as part of a broadband radiation source.

[0070] This specification provides a hollow core fiber for broadband radiation generation. Figure 8(a) shows a cross-section of an example of a hollow core fiber 800 that can be used for broadband radiation generation. Figure 8(b) shows the hollow core fiber 800 in a radiation source assembly 850. The hollow core fiber 800 has a hollow core 802 filled with a gas composition containing a working gas. The hollow core fiber 800 is configured to receive pulsed pump radiation 810 at its input end 812. The pulsed pump power has a pulse power exceeding the ionization threshold of the gas composition. The hollow core fiber 800 is further configured to generate broadband radiation by confining and guiding the pulsed pump radiation within the fiber and interacting with the working gas, thereby causing the pulsed pump radiation to spread nonlinearly. The spread light is supplied as broadband output radiation 820 at the output end 814 of the hollow core fiber 800. During propagation, the radiation is confined within the hollow core 802 of the hollow core fiber 800. The gas composition includes less than 1% hydrogen in the total gas composition within the hollow core fiber 800.

[0071] The advantage of the hollow-core fiber 800 described above is that it can maintain the fiber's lifetime during broadband radiation generation. This is thought to be because the hydrogen component in the gas composition suppresses plasma formation, thereby preventing the formation of a stable plasma. As a result, potential damage caused by plasma inside the hollow core of the fiber can be avoided. The fiber 800 may comprise a cladding 804 surrounding the hollow core. The cladding may comprise an anti-resonant element 806 configured to confine radiation within the hollow core 802. If a stable plasma forms inside the fiber, the cladding 804, and in particular the anti-resonant element, are at risk of thermal damage. The presence of the gas composition provided in this disclosure may prevent the formation of this stable plasma.

[0072] The hydrogen component can be provided in a range of 0.001% to 1% of the gas composition. Even with a low hydrogen content in the gas composition, the plasma quenching effect can be achieved. If the amount of hydrogen component in the gas mixture is too high, the gas mixture may become flammable, potentially raising safety concerns. A high amount of hydrogen component in the gas mixture also increases the risk of contamination by impurities; therefore, it is desirable to keep the hydrogen component content below an upper threshold.

[0073] The hydrogen component may contain hydrogen gas. The hydrogen component may contain at least one isotope of hydrogen gas. The isotope of hydrogen gas may contain at least one of deuterium and tritium.

[0074] The gas composition may further contain a cooling gas. The cooling gas may be a lightweight gas configured to increase the thermal conductivity of the gas composition. The cooling gas may contain at least one of helium (He) or neon (Ne). In a specific example, the cooling gas may be helium gas. The cooling gas may account for 20% to 50% of the gas composition.

[0075] The overall impurity concentration of the gas composition may be less than 0.001% (1000 ppm). The impurities may include one or more of oxygen and H2O. Low impurity concentrations can be achieved by providing the gas composition in an environment where its purity can be maintained. For example, the walls of the environment may have low permeability to impurities. The permeability may be low enough to achieve the desired impurity concentration. While the definition of permeability of a material includes properties specific to a particular configuration (e.g., surface area), this is not specified for the fiber and source assemblies described herein. Therefore, a specific definition of permeability is not provided in this context.

[0076] The working gas may include at least one of argon (Ar), krypton (Kr), and xenon (Xe). Depending on the type of working gas in the gas composition, nonlinear optical processes may include modulation instability (MI), soliton self-compression, soliton splitting, Kerr effect, Raman effect, and dispersed wave generation. These details are described in WO2018 / 127266A1 and US9160137B1 (both incorporated herein by reference). Other properties, such as the pressure of the gas mixture within the fiber, may also affect the nonlinear spreading effect.

[0077] The elements of the light source assembly 850 shown in Figure 8(b) will be described in more detail. The radiation source assembly 850 may include a pulse pump input assembly 830 for supplying pump radiation to the radiation source assembly 850. The pump input assembly 830 may be configured to supply input radiation 810 (also called pump radiation) to the hollow core fiber 800. The hollow core 802 of the hollow core fiber 800 may be arranged to receive the input radiation 810 from the pulse pump radiation source and amplify it to supply output radiation 820. A gas mixture may be used to amplify the frequency range of the received input radiation 810 to supply broadband output radiation 820. The pump input assembly 830 may be configured to receive radiation from an external radiation source, or it may include a pump radiation source such as a laser or any other type of radiation source capable of generating short radiation pulses of a desired length and energy level.

[0078] The hollow core 802 of the hollow core fiber 800 may be filled with a gas mixture. In some embodiments, the hollow core fiber 800 may be supplied filled with the gas mixture. In other embodiments, the hollow core fiber 800 may be supplied within a gas cell 840 (also called a housing, container, or reservoir). The gas cell 840 may be configured to supply the gas mixture to the hollow core fiber 800 when the light source assembly 850 is in use. An advantage of filling the hollow core fiber is that setup can be simplified because no setup is required to supply the gas mixture to the fiber. An advantage of providing a gas cell 840 configuration is that the composition of the gas mixture can be more easily adjusted / modified. Figure 8(b) shows a radiation source assembly 850 including a gas cell, but alternative implementations including a filled fiber that does not require a gas cell may also be considered. In Figure 8(b), the radiation source assembly 850 includes the optical fiber 800 shown in Figure 8(a), but in alternative embodiments, other types of hollow core optical fibers may be used.

[0079] In some embodiments, the radiation source assembly 850 may include a gas cell 840 that supplies a gas mixture into a hollow core fiber 800. The hollow core fiber 800 may be located within a reservoir in the gas cell 840. The gas cell 840 may be configured to supply and contain the gas mixture. The gas cell may have one or more functions known in the art for controlling, adjusting, and / or monitoring the composition of the gas mixture. In use, the hollow core fiber 800 is located within the gas cell 840 such that a first transparent window is close to the input end 812 of the fiber 800. The first transparent window is transparent at least to the received input radiation frequency, so that the received input radiation 810 (or at least a large portion thereof) is coupled to the hollow core 802 of the fiber 800 located within the gas cell 840. It will be understood that an optical system (not shown) may be provided for coupling the input radiation 810 to the optical fiber 800. In use, the output end 814 of the hollow core fiber 800 may be close to a second transparent window. The second transparent window may be transparent to at least the frequencies of the broadband output radiation 820 of the radiation source 850. Alternatively, in another embodiment, the two opposing ends 812, 814 of the hollow core fiber 800 may be placed in different reservoirs of the gas cell 840. Such an arrangement with two separate gas reservoirs is particularly convenient in embodiments where the hollow core fiber 800 is relatively long (e.g., when the length exceeds 1 m). In this context, the window may be transparent to frequencies to which at least 50%, 75%, 85%, 90%, 95%, or 99% of the radiation of that frequency incident on the window passes through the window.

[0080] In embodiments where a hollow core fiber 800 is provided, in which the hollow core 802 is already filled with a gas mixture, a gas cell 840 is not required. However, the elements described above in relation to the gas cell may still be present in the radiation source assembly (e.g., transparent window, coupling optical system). The filled hollow core fiber 800 can be sealed within the hollow core by closing both ends of the hollow core 802. The material closing the hollow core of the fiber may be the same material as the cladding of the fiber 800. The closed ends of the fiber may be transparent to the frequencies of radiation input to / output from the fiber 800.

[0081] High-intensity radiation is sometimes desirable to achieve frequency spread. The advantage of having a hollow core fiber is that high-intensity radiation can be achieved by strongly spatially confining the radiation propagating through the fiber 800, thereby achieving locally high radiation intensity. Radiation intensity within an optical fiber may be high, for example, due to high received input radiation intensity and / or by strongly spatially confining the radiation within the hollow core fiber. Hollow core fibers can confine and guide most of the radiation within the hollow core 802 of the fiber. Hollow core fibers 800 can guide radiation over a wider wavelength range than solid core fibers, and in particular, hollow core optical fibers can guide radiation in both ultraviolet and infrared wavelength ranges.

[0082] An example of a hollow core fiber 800, shown in Figures 8(a) and 8(b), will be described in more detail below. The fiber 800 may have an elongated body that defines the length of the fiber. The length of the fiber is the longer dimension compared to the other two dimensions of the fiber. This longer dimension is called the axial direction and can define the axis of the hollow core fiber. As shown in Figure 8(a), the other two dimensions of the fiber define a plane called the cross-section. Figure 8(a) shows a cross-section of the hollow core fiber 800 in this cross-section (i.e., the plane perpendicular to the axis). The cross-section of the hollow core fiber 800 may be substantially constant along the fiber axis.

[0083] Because the hollow core fiber 800 has a certain degree of flexibility, it will be understood that its axial direction is generally not uniform along the length of the fiber 800. Terms such as optical axis and cross-section will be understood to refer to the local optical axis, local cross-section, etc. Furthermore, if the components are described as cylindrical or tubular, these terms will be understood to include shapes that the hollow core fiber 800 may deform when bent.

[0084] It will be understood that hollow core fibers can have any length, and the length of fiber 800 can be determined according to the application. The length of the fiber can be, for example, from 1 cm to 10 m. The length of hollow core fiber 800 can be, for example, from 10 cm to 100 cm.

[0085] A hollow core fiber may comprise a hollow core 802 surrounded by a cladding portion 804. A support portion may be provided to surround and support the cladding portion 804. A hollow core fiber 800 is considered to comprise a body (including the cladding portion and support portion SP) having a hollow core 802. A hollow core fiber 800 may comprise a plurality of anti-resonant elements 806 for inducing radiation through the hollow core 802. In some embodiments, the plurality of anti-resonant elements 806 may be arranged to confine the radiation propagating through the fiber 800 mainly within the hollow core 802. The anti-resonant elements may guide the radiation along the fiber 800. The hollow core 802 may be located in a region approximately in the center of the fiber 800. This allows the axis of the fiber 800 to also define the axis of the hollow core 802.

[0086] In some embodiments, the anti-resonant element 806 may include multiple capillaries. The capillaries may be arranged in a single capillary ring surrounding the hollow core 802. In certain examples, the cladding 804 may comprise a single ring of six tubular capillaries 806 surrounding the hollow core, with each tubular capillary acting as an anti-resonant element.

[0087] In one embodiment, a hollow core fiber containing a gas mixture may be provided, as described herein, wherein the gas mixture has a purity such that the impurity concentration is less than 0.001% (100 ppm). The gas mixture may also contain a working gas for nonlinear spreading, a light gas to improve thermal conductivity, and a hydrogen component to suppress stable plasma generation. The purity of the gas composition, the presence of the light gas component, the working gas component, and the hydrogen component may all contribute to extending the lifespan of the hollow core fiber used for broadband radiation generation.

[0088] Broadband radiation can include supercontinuum radiation. Supercontinuum radiation can include radiation in the range from ultraviolet (UV) to infrared (IR). This may be, for example, in the range of 100 nm to 2000 nm, 200 nm to 2000 nm, or 200 nm to 1600 nm.

[0089] Pump radiation may be pulsed pump radiation. Pump radiation may be a single (pulsed) radiation beam supplied to the optical input of the PIC. The radiation may be in the range of 400 nm to 2000 nm, or in the range of 800 nm to 1600 nm. Pulsed radiation may include radiation of one or more specific wavelengths, such as 400 nm, 515 nm, 800 nm, 1030 nm, 1550 nm, and / or 2000 nm.

[0090] Further embodiments of the present invention are disclosed in the following numbered list. 1. A hollow core fiber for broadband generation, wherein the hollow core of the hollow core fiber is filled with a gas composition containing a working gas, and the hollow core fiber is The input end of the hollow core fiber is configured to receive pulse pump radiation having a pulse power exceeding the ionization threshold of the gas composition, The system is configured to generate broadband radiation by confining and guiding the pulse pump radiation within the hollow core fiber and allowing it to interact with the working gas, thereby causing the pulse pump radiation to spread nonlinearly. The gas composition comprises a hollow core fiber containing less than 1% hydrogen in the total gas composition within the hollow core fiber. 2. The hollow core fiber according to item 1, wherein the hydrogen component is in the range of 0.001% to 1% or 0.01% to 1% of the gas composition. 3. The hydrogen component comprises hydrogen gas, as described in item 1 or 2, in the hollow core fiber. 4. The hollow core fiber according to any one of claims 1 to 3, wherein the hydrogen component comprises at least one isotope of hydrogen gas. 5. The hollow core fiber according to item 4, wherein at least one isotope of the hydrogen gas comprises at least one of deuterium and tritium. 6. The hollow core fiber according to any one of items 1 to 1 to 5, wherein the working gas comprises at least one of argon, krypton, and xenon. 7. The hollow core fiber according to any one of items 1 to 6, wherein the gas composition includes a cooling gas. 8. The hollow core fiber according to item 7, wherein the cooling gas comprises at least one of helium and neon. 9. The cooling gas is in the range of 20% to 50% of the gas composition, the hollow core fiber as described in item 7 or 8. 10. The hollow core fiber according to any one of items 1 to 9, wherein the impurity concentration of the gas composition is less than 0.001%. 11. The hollow core fiber according to any one of items 1 to 10, wherein the impurities include one or more of oxygen and H2O. 12. The broadband radiation includes supercontinuum radiation, as described in any one of items 1 to 11, in a hollow core fiber. 13. The hollow core fiber according to item 12, wherein the broadband radiation includes wavelengths in the range of 200 nm to 2000 nm. 14. The hollow core fiber is a hollow core photonic crystal fiber, as described in any one of items 1 to 13. 15. The hollow core photonic crystal fiber according to item 14, wherein the hollow core photonic crystal fiber comprises a single ring of capillaries around the hollow core. 16. The broadband radiation is output from the output end of the hollow core nonlinear fiber according to any one of items 1 to 15. 17. The hollow core fiber according to any one of items 1 to 16, wherein the broadband radiation is generated by the modulation instability of the working gas interacting with the pulse pump radiation. 18. The hollow core fiber according to any one of items 1 to 17, wherein the pump radiation includes radiation having one or more wavelengths in the range of 800 nm to 2000 nm or in the range of 400 nm to 550 nm. 19. The hollow core fiber according to any one of claims 1 to 18, wherein the hollow core fiber is a filled fiber containing the gas composition. 20. The pulse power is 1 TW / cm in the hollow core fiber. 2 A hollow core fiber according to any one of items 1 to 19, corresponding to a peak power density exceeding that of the above. 21. The pulse power is 2.5 TW / cm² within the hollow core fiber. 2 A hollow core fiber according to any one of items 1 to 20, corresponding to a peak power density exceeding a certain level. 22. The pulse power corresponds to a pulse peak power exceeding 30 W, as described in any of items 1 to 21, of the hollow core fiber. 23. The pulse power corresponds to a pulse peak power exceeding 80W, and is a hollow core fiber as described in any of items 1 to 21. 24. The pulse power corresponds to a pulse energy greater than 1 μJ, as described in any of items 1 to 23, for the hollow core fiber. 25. The pulse power corresponds to a pulse energy exceeding 2 μJ, as described in any of items 1 to 24, for the hollow core fiber. 26. A light source assembly for broadband radiation generation, A hollow core fiber as described in any of items 1 to 25, A light source assembly comprising a pump input assembly configured to supply pulsed pump radiation to the hollow core fiber. 27. The light source assembly according to item 26, wherein the hollow core fiber is a filled fiber containing the gas composition. 28. The light source assembly according to any one of claims 26 to 27, further comprising a gas cell configured to supply the gas composition to the hollow core fiber. 29. The light source assembly according to item 28, wherein the gas cell has an impurity transmittance such that the impurity concentration of the gas composition is less than 0.001%. 30. The light source assembly according to any one of items 26 to 29, wherein the pump input assembly comprises a pulsed pump laser. 31. The light source assembly according to item 30, wherein the pulse pump laser is configured to supply pulses having a pulse energy greater than 1 μJ. 32. The light source assembly according to item 30, wherein the pulse pump laser is configured to supply pulses having a pulse energy greater than 2 μJ. 33. The light source assembly according to paragraph 30, wherein the pulse pump laser is configured to supply pulses having a pulse peak power exceeding 30 W. 34. The light source assembly according to paragraph 30, wherein the pulse pump laser is configured to supply pulses having a pulse peak power exceeding 80 W. 35. A broadband radiation source comprising a hollow core fiber as described in any of sections 1 to 25. 36. A broadband radiation source comprising a light source assembly as described in any of sections 26 to 34. 37. A measuring device comprising a hollow core fiber as described in any of sections 1 to 25. 38. An inspection apparatus comprising a hollow core fiber as described in any of sections 1 to 25. Lithography apparatus comprising a hollow core fiber as described in any of sections 39.1 to 25. 40. A lysocell comprising the apparatus described in any one of sections 37 to 39. 41. A light source assembly for broadband radiation generation, A light source assembly comprising: a hollow core fiber (HCF) filled with a gas composition containing a working gas and a hydrogen component; and a radiation source configured to provide pulsed pump radiation to the input end of the HCF, wherein the pulsed pump radiation is configured to interact with the working gas to produce broadband radiation and has a pulse power exceeding the ionization threshold of the working gas, characterized in that the hydrogen component accounts for less than 1% of the gas composition in mole percentage. 42. A hollow core fiber (HCF) filled with a gas composition containing a hydrogen component and a working gas for generating broadband radiation by nonlinear spreading upon receiving pulse pump radiation, characterized in that the hydrogen component accounts for less than 1% of the gas composition in mole percentage. 43. A method for generating broadband radiation, To provide a hollow core fiber (HCF) filled with a gas composition containing a working gas and a trace amount of hydrogen component accounting for less than 1% of the gas composition in mole percentage, A method comprising directing pulse pump radiation towards the working gas in the HCF to generate the broadband radiation. 44. A light source assembly for broadband radiation generation, A hollow core fiber (HCF) filled with a gas composition containing a working gas, A light source assembly comprising: a radiation source configured to supply pulsed pump radiation to the input terminal of the HCF, wherein the pulsed pump radiation is configured to interact with the working gas to produce the broadband radiation, and the gas composition further comprises a trace amount of gas for neutralizing the ions formed by the pulsed pump radiation on a timescale smaller than the period between consecutive pulses of the pump radiation. 45. A hollow core fiber (HCF) filled with a gas composition comprising a working gas for generating broadband radiation by nonlinear spreading upon pulse pump radiation, and a trace amount of gas for neutralizing ions formed by the pulse pump radiation on a timescale smaller than the period between consecutive pulses of the pulse pump radiation. 46. ​​A method for generating broadband radiation, comprising the steps of: preparing a hollow core fiber (HCF) filled with a gas composition comprising a working gas and a trace amount of gas for neutralizing ions of the working gas formed during the generation of the broadband radiation; and directing pulsed pump radiation to the working gas in the HCF to initiate a nonlinear optical process. 47. The light source assembly according to any one of claims 26 to 34, wherein the gas composition further comprises helium for providing cooling of the gas composition. 48. The light source assembly according to any one of items 26 to 34 or 47, wherein the working gas comprises argon. 49. The HCF according to item 45, wherein the gas composition further comprises helium for providing cooling of the gas composition. 50. A hollow core fiber according to any one of items 1 to 25, 45, or 49, wherein the working gas comprises argon.

[0091] While this specification 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 have other applications. Other possible applications include the manufacture of guidance and detection patterns for integrated optical systems, magnetic domain memory, flat panel displays, liquid crystal displays (LCDs), thin-film magnetic heads, and more.

[0092] While embodiments of the present invention may be specifically referenced in relation to lithography apparatus, embodiments of the present invention can be used in other apparatuses. Embodiments of the present invention can 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 commonly referred to as lithography tools. Such lithography tools can be used under vacuum conditions or ambient (non-vacuum) conditions.

[0093] While the above has specifically referred to the use of embodiments of the present invention in the context of optical lithography, the present invention is not limited to optical lithography and can be used in other applications, such as imprint lithography, where the context permits.

[0094] The specific terms “measuring device / tool / system” or “inspection device / tool / system” are used, but these terms may refer to the same or similar types of tools, devices, or systems. For example, an inspection device or measuring device including one embodiment of the present invention can be used to determine the characteristics of a structure on a substrate or wafer. For example, an inspection device or measuring device including one embodiment of the present invention can be used to detect defects in a substrate or defects in a structure on a substrate or wafer. In such embodiments, the characteristic of interest 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 an undesirable structure on the substrate or wafer.

[0095] While specific embodiments of the present invention have been described above, it will be understood that the invention may be carried out in ways other than those described. The above description is intended to be illustrative, not limiting. Accordingly, it will be apparent to those skilled in the art that modifications to the described invention can be made without departing from the claims set forth below.

Claims

1. A hollow core fiber for broadband generation, wherein the hollow core of the hollow core fiber is filled with a gas composition containing a working gas, and the hollow core fiber is The input end of the hollow core fiber is configured to receive pulse pump radiation having a pulse power exceeding the ionization threshold of the gas composition, The system is configured to generate broadband radiation by confining and guiding the pulse pump radiation within the hollow core fiber and allowing it to interact with the working gas, thereby causing the pulse pump radiation to spread nonlinearly. The gas composition comprises a hollow core fiber containing less than 1% hydrogen in the total gas composition within the hollow core fiber.

2. The hollow core fiber according to claim 1, wherein the hydrogen component is in the range of 0.01% to 1% of the gas composition.

3. The hollow core fiber according to claim 1, wherein the working gas comprises at least one of argon, krypton, and xenon.

4. The hollow core fiber according to claim 1, wherein the gas composition comprises a cooling gas containing at least one of helium and neon.

5. The hollow core fiber according to claim 4, wherein the cooling gas is in the range of 20% to 50% of the gas composition.

6. The hollow core fiber according to claim 1, wherein the broadband radiation includes wavelengths in the range of 200 nm to 2000 nm.

7. The hollow core fiber according to claim 1, wherein the hollow core fiber is a hollow core photonic crystal fiber.

8. The pulse power is 1 TW / cm in the hollow core fiber. ２ A hollow core fiber according to claim 1, which corresponds to a peak power density exceeding that of the above.

9. The hollow core fiber according to claim 1, wherein the pulse power corresponds to a pulse peak power exceeding 30 W.

10. The hollow core fiber according to claim 1, wherein the pulse power corresponds to a pulse energy of more than 1 μJ.

11. A light source assembly for broadband radiation generation, A hollow core fiber according to claim 1, A light source assembly comprising a pump input assembly configured to supply pulsed pump radiation to the hollow core fiber.

12. The light source assembly according to claim 11, wherein the pulse pump laser is configured to supply pulses having a pulse energy greater than 1 μJ.

13. A broadband radiation source comprising the light source assembly according to claim 11.

14. A measuring device comprising a hollow core fiber as described in claim 1.

15. An inspection apparatus comprising a hollow core fiber as described in claim 1.

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