System and method for generating supercontinuum radiation

Abstract

A system for generating supercontinuum radiation, the system comprising: a pump light source configured to generate a pump light pulse, the pump light pulse having a pump wavelength; and a hollow core photonic crystal fiber configured to receive the pump light pulse, the hollow core photonic crystal fiber containing a gas; wherein the gas and the hollow core photonic crystal fiber are selected such that the pump wavelength is within a range corresponding to a normal group velocity dispersion state of the gas; and wherein the gas has any one of: a rotational spectral line spacing greater than 0.5 THz; or no rotating spectral line exists. A method of generating supercontinuum radiation is also described.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP application 23152628.6 filed on January 20, 2023 and EP application 23153839.8 filed on January 30, 2023, which are incorporated herein by reference in their entirety. Technical Field

[0003] The present invention relates to systems and methods for generating supercontinuum radiation. Background Art

[0004] A lithographic apparatus is a machine configured to apply a desired pattern to a substrate. Lithographic apparatuses are used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus can project a pattern (often also referred to as a "design layout" or "design"), for example, at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) disposed on a substrate (e.g., a wafer).

[0005] To project patterns onto substrates, lithographic equipment uses electromagnetic radiation. The wavelength of this radiation determines the minimum size of features that can be formed on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. Compared to lithographic equipment using radiation with a wavelength of, for example, 193 nm, lithographic equipment using extreme ultraviolet (EUV) radiation with a wavelength in the 4 nm to 20 nm range (e.g., 6.7 nm or 13.5 nm) can be used to form smaller features on substrates.

[0006] Low-k1 lithography can be used to process features smaller than the classical resolution limit of the lithographic apparatus. In such processes, the resolution equation can be expressed as CD = k1 × λ / NA, where λ is the wavelength of the radiation employed, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the "critical dimension" (usually the smallest feature size printed, but in this case, the half-pitch), and k1 is the empirical resolution factor. Generally, the smaller k1, the more difficult it is to replicate on a substrate a pattern with a shape and size similar to that planned by the circuit designer to achieve specific electrical functionality and performance. To overcome these difficulties, complex fine-tuning steps can be applied to the lithographic projection apparatus and / or design layout. These include, for example, but not limited to, optimization of the NA, customized illumination schemes, the use of phase-shifting patterning devices, various optimizations of the design layout (such as optical proximity correction (OPC), sometimes also called "optical process correction") in the design layout, or other methods generally referred to as "resolution enhancement techniques" (RET). Alternatively, rigorous control loops for controlling the stability of the lithographic apparatus can be used to improve pattern reproduction at low k1.

[0007] In the field of lithography, a variety of measurement systems can be used both within and outside of a lithographic apparatus. Typically, such measurement systems utilize a radiation source to illuminate a target with radiation, and a detection system operable to measure at least one property of a portion of the incident radiation scattered from the target. Examples of measurement systems external to a lithographic apparatus include inspection equipment or metrology equipment, which can be used to determine properties of a pattern previously projected onto a substrate by the lithographic apparatus. For example, such external inspection equipment can include a scatterometer. Examples of measurement systems that can be provided within a lithographic apparatus include topography measurement systems (also known as level sensors); position measurement systems (e.g., interferometry devices) for determining the position of a reticle or wafer stage; and alignment sensors for determining the position of alignment marks. These measurement devices can perform measurements using electromagnetic radiation.

[0008] Different types of radiation can be used to capture different properties of the pattern. Some measurement systems can use a broadband radiation source. This broadband radiation source can be a supercontinuum source and can include a waveguide (e.g., an optical fiber) with a nonlinear medium through which a pulsed pump radiation beam propagates to broaden the spectrum of the radiation.

[0009] Supercontinuum radiation can be generated by pumping a gas in a hollow-core optical fiber using a pump light source (e.g., a narrowband laser). For gases in optical fibers that exhibit anomalous group velocity dispersion, spectral broadening of the input radiation provided by the pump light source can be driven by modulation instabilities, self-phase modulation, or soliton dynamics. Further details on the spectral broadening mechanism in soliton-driven broadband radiation generation are provided in WO2022122325, the entire contents of which are incorporated herein by reference. Summary of the Invention

[0010] As an alternative to using a hollow-core fiber filled with a gas in an anomalous group velocity dispersion regime, a supercontinuum radiation source can be achieved by pumping a gas (e.g., a molecular gas) in a hollow-core fiber in a normal dispersion regime using a pump light source (e.g., a narrowband laser) to generate a broad vibrational Raman frequency comb consisting of discrete spectral lines. As the radiation propagates through the fiber (e.g., via rotationally stimulated Raman scattering and / or the optical Kerr effect), the spectral lines of the frequency comb can be broadened to achieve a supercontinuum that can extend from the ultraviolet to the mid-infrared (e.g., from below 400 nm to above 2000 nm). Advantageously, this technique can provide reduced shot-to-shot noise, as well as improved spectral flatness and spectral range, compared to techniques that pump the gas in an anomalous group velocity dispersion regime.

[0011] However, the inventors discovered that when the coherent phonon waves generated by the pump-Stokes beat are identical to the coherent phonon waves annihilated by the pump-anti-Stokes beat, gain suppression can occur, thereby balancing the phonon generation and annihilation rates. Stokes lines can still be formed in higher-order modes (HOMs) via multimode coherent waves.

[0012] In the context of supercontinuum generation, gain suppression leads to a reduction in the supercontinuum spectral energy density (particularly in the normal dispersion region, typically covering the visible and ultraviolet regions) and a deviation of the beam profile from the fundamental mode. Due to gain suppression, the spectral energy density in the visible spectral region is lower. Beams traveling in higher-order modes contribute significantly less to the supercontinuum formation due to their lower peak powers.

[0013] Gain suppression occurs when the rotational offset is small enough that the propagation constants of the Stokes and anti-Stokes lines are very similar. Therefore, the gain suppression problem can be mitigated by choosing a gas with a sufficiently large rotational line spacing (e.g., greater than 0.5 THz) so that the propagation constants of the Stokes and anti-Stokes lines are sufficiently different that the generation and annihilation of phonons are unbalanced. In some examples, a gas with no rotational lines can be chosen, i.e., no rotational Raman response at all.

[0014] This document describes a system for generating supercontinuum radiation, the system comprising: a pump light source configured to generate pump light pulses having a pump wavelength; and a hollow core photonic crystal fiber (HC-PCF, also referred to herein as "optical fiber" or "optical fiber") configured to receive the pump light pulses, the HC-PCF containing a gas; wherein the gas and the HC-PCF are selected so that the pump wavelength is within a range corresponding to a normal group velocity dispersion state of the gas; and wherein the gas has any of the following: a rotational line spacing greater than 0.5 THz; or no rotational lines.

[0015] Also described herein is a method for generating supercontinuum radiation, the method comprising: providing a pump light pulse to a hollow core photonic crystal fiber, the pump light pulse having a pump wavelength, and the hollow core photonic crystal fiber containing a gas; wherein the gas and the hollow core photonic crystal fiber are selected so that the pump wavelength is within a range corresponding to a normal group velocity dispersion state of the gas; and wherein the gas has any of the following: a rotational line spacing greater than 0.5 THz; or no rotational lines.

[0016] It should be understood that "rotational lines" refer to lines observable in a rotational spectrum of a gas, such as lines observable in a rotational spectroscopy measurement of a gas (e.g., microwave spectroscopy, infrared spectroscopy, Raman spectroscopy, etc.). A gas without rotational lines is a gas without a rotational Raman response.

[0017] Preferably, the gas is selected to exhibit a vibrational Raman response. Preferably, the gas should exhibit a strong vibrational Raman response to achieve a broad vibrational Raman frequency comb.

[0018] It will also be understood that the normal group velocity dispersion state corresponds to a state of positive group velocity dispersion, where the group velocity dispersion can also be expressed as β2. This is in contrast to anomalous group velocity dispersion (i.e., negative β2). Those skilled in the art will understand that the normal group velocity dispersion state of the gas refers to the normal group velocity dispersion state of the gas in the HC-PCF.

[0019] By pumping the gas in the normal group velocity dispersion regime, the effects of modulation instabilities are minimized, and supercontinuum radiation is instead generated by the cascaded processes described herein involving vibrational and rotational Raman scattering and the Kerr effect.

[0020] The rotational line spacing and / or the normal group velocity dispersion state can depend on the gas pressure. In some examples, the gas pressure is also selected such that the pump wavelength is within a range corresponding to the normal group velocity dispersion state of the gas at the gas pressure. Selecting the gas can include selecting the gas pressure (e.g., a suitable gas pressure for a desired rotational line spacing, amount of nonlinearity, amount of dispersion, etc.).

[0021] The normal group velocity dispersion state of the gas within the HC-PCF depends on the structure of the HC-PCF. Therefore, selecting an HC-PCF may include selecting the structure of the HC-PCF (e.g., hollow core size, cladding microstructure, etc.).

[0022] Advantageously, gases with rotational line spacing greater than 0.5 THz or gases with no rotational lines (ie, no rotational Raman response) are not (or are less) affected by the gain suppression problem described above compared to gases with closer rotational line spacing.

[0023] In some examples, the rotational line spacing of the gas is greater than 0.7 THz, greater than 1 THz, greater than 2 THz, or greater than 3 THz.

[0024] The gas may comprise a mixture of gases.

[0025] The gas may include molecular gases.

[0026] Preferably, the gas exhibits good linear transmission of radiation across the supercontinuum.

[0027] In some examples, more than one pump light source or pump wavelength is used. In this case, at least one pump wavelength is within a range corresponding to the normal group velocity dispersion state of the gas.

[0028] Examples of gases with rotational line spacing greater than 0.5 THz include hydrogen (H2) and deuterium (D2).

[0029] Examples of gases that do not have rotational lines include methane and SF6.

[0030] Thus, the gas may include one or more of H2, D2, methane and SF6.

[0031] Selection of a hollow core crystalline fiber may include selecting one or more physical characteristics of the fiber, such as length, core diameter, material composition, internal structure, jacket region thickness, wall thickness, and the like.

[0032] The gas dispersion distribution depends on the combination of the HC-PCF's physical structure, gas type, gas pressure, and pump wavelength. For a given HC-PCF and gas type, the zero-dispersion wavelength (ZDW) can be shifted to a longer wavelength by increasing the gas pressure. For pump wavelengths shorter than the ZDW, the group dispersion velocity is normal, while for pump wavelengths longer than the ZDW, the group dispersion velocity is anomalous. Accordingly, the gas type, HC-PCF structure, and pump wavelength can be selected to induce a ZDW at a specific gas pressure.

[0033] In some examples, the pump wavelength is less than 1000 nm. Thus, advantageously, a normal group velocity dispersion state for a given gas species can be achieved at lower gas pressures than when using a pump wavelength of 1000 nm or greater. In some examples, the pump wavelength is less than 800 nm. In some examples, the pump wavelength is less than 600 nm. In some examples, the pump wavelength is a visible wavelength (e.g., in the range of approximately 380 nm to approximately 800 nm). In some examples, the pump wavelength is an ultraviolet wavelength (e.g., in the range of approximately 100 nm to approximately 380 nm). In some examples, the pump wavelength is a green wavelength (e.g., in the range of approximately 500 nm to approximately 565 nm, such as approximately 515 nm).

[0034] In some examples, the ZDW, and therefore the normal group velocity dispersion state of the gas, is particularly sensitive to the core diameter of the HC-PCF. Thus, the selection of the HC-PCF can include the selection of the core diameter of the HC-PCF.

[0035] In some examples, the pump light pulses may have a pulse duration of 700 ps or less to avoid requiring excessive pulse energy. In some examples, the pulse duration may be 600 ps or less, 500 ps or less, 400 ps or less, 300 ps or less, 200 ps or less, or 100 ps or less. Preferably, the pulse duration should be selected to be long enough to avoid self-phase modulation dominating the formation of the Raman vibrational comb, i.e., 50 fs or greater, preferably 100 fs or greater. In some examples, the pulse duration of the pump light pulses may be between 50 fs and 700 ps. In some examples, the pulse duration of the pump light pulses may be between 300 fs and 700 ps. In some examples, the pulse duration of the pump light pulses may be between 300 fs and 27 ps. In some examples, the pulse duration of the pump light pulses may be between 100 fs and 100 ps.

[0036] A metrology apparatus comprising the system described herein is also described. A lithographic apparatus comprising the system described herein is further described.

[0037] Those skilled in the art will understand that, as used herein, a "supercontinuum" generally refers to a continuous spectral power distribution that exhibits substantial flatness. In some examples, a supercontinuum comprises a continuous spectral power distribution over a wavelength range of at least 100 nm. In some examples, the flatness of the supercontinuum corresponds to a peak-to-valley spectral power ratio of less than 100:1 or 20 dB. In some examples, the flatness of the supercontinuum corresponds to a peak-to-valley spectral power ratio of less than 10:1 or 10 dB. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0039] Figure 1 A schematic overview of a lithographic apparatus is depicted;

[0040] Figure 2 A lithographic system including a lithographic apparatus and a radiation source is depicted;

[0041] Figure 3 A schematic overview of a lithography unit is depicted;

[0042] Figure 4 Depicted is a schematic representation of overall lithography showing the collaboration between three key technologies used to optimize semiconductor manufacturing;

[0043] Figure 5 A schematic overview of a scatterometer measurement tool is depicted;

[0044] Figure 6 A schematic diagram of a level sensor measurement tool is depicted;

[0045] Figure 7 A schematic overview of an alignment sensor metrology tool is depicted;

[0046] Figure 8 Depicted is an apparatus for broadening the frequency range of received input radiation, the apparatus comprising a hollow core photonic crystal fiber;

[0047] Figure 9 Depicts Figure 7 A device of the type indicated for broadening the frequency range of received input radiation, the device also comprising a reservoir;

[0048] Figure 10 Depicted is a schematic representation of a radiation source for providing broadband output radiation, the radiation source comprising Figure 9 The device shown for broadening the frequency range of received input radiation;

[0049] Figure 11 is a schematic cross-sectional view of an example of a hollow core photonic crystal fiber in a transverse plane (ie, perpendicular to the axis of the fiber);

[0050] Figure 12 yes Figure 11 A schematic cross-sectional view of a hollow-core photonic crystal fiber in the illustrated example, taken in a plane including the axis of the fiber;

[0051] Figure 13 Describes the numerical simulation results of generating a broadened Raman frequency comb in a hollow core photonic crystal fiber for supercontinuum generation; and

[0052] Figure 14 A method of generating supercontinuum radiation according to the present disclosure is schematically illustrated. DETAILED DESCRIPTION

[0053] In this document, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g., having a wavelength of 365, 248, 193, 157, or 126 nm) and EUV (extreme ultraviolet radiation, e.g., having a wavelength in the range of about 5-100 nm).

[0054] <Mask>

[0055] As used herein, the terms "reticle," "mask," or "patterning device" should be broadly interpreted to refer to a general-purpose patterning device that can be used to impart an incident radiation beam with a patterned cross-section corresponding to the pattern to be produced in a target portion of the substrate. The term "light valve" may also be used in this context. In addition to classical masks (transmissive or reflective, binary, phase-shift, hybrid, etc.), examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0056] Figure 1 A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA comprises an illumination system (also called illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation), a mask support (e.g., mask table) T configured to support a patterning device (e.g., mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to specific parameters, a substrate support (e.g., wafer stage) WT configured to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support according to specific parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0057] In operation, the illumination system IL receives a radiation beam from a radiation source SO, for example, via a beam delivery system BD. The illumination system IL may include various types of optical components for directing, shaping, and / or controlling the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof. The illuminator IL may be used to condition the radiation beam B so that it has a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0058] The term "projection system" PS as used herein should be broadly interpreted as covering various types of projection systems including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, which projection system is appropriate to the exposure radiation used and / or other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0059] The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index (e.g., water) to fill the space between the projection system PS and the substrate W, which is also known as immersion lithography. More information on immersion technology is given in US6952253 (which is incorporated herein by reference).

[0060] The lithographic apparatus LA may also be of a type having two (also referred to as "dual stage") or more substrate supports WT. In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or steps for subsequent exposure preparation of a substrate W may be performed on a substrate W on one of the substrate supports WT while another substrate W on another substrate support WT is being used to expose a pattern on another substrate W.

[0061] In addition to the substrate support WT, the lithographic apparatus LA can include a measurement stage. The measurement stage is arranged to hold sensors and / or cleaning devices. The sensors can be arranged to measure properties of the projection system PS or properties of the radiation beam B. The measurement stage can hold multiple sensors. The cleaning devices can be arranged to clean part of the lithographic apparatus, such as part of the projection system PS or part of the system for providing immersion liquid. The measurement stage can be moved beneath the projection system PS when the substrate support WT is away from the projection system PS.

[0062] In operation, a radiation beam B is incident on a patterning device (e.g. a mask) MA held on a mask support MT and is patterned by a 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 which focuses the beam onto a target portion C of a substrate W. With the aid of a second positioner PW and a position measurement system IF, the substrate support WT can be accurately moved, for example, in order to position different target portions C in a focused and aligned position in the path of the radiation beam B. Similarly, a first positioner PM and possibly another position sensor (which is not in the Figure 1 The patterning device MA may be accurately positioned relative to the path of the radiation beam B using the mask alignment marks M1, M2 and the substrate alignment marks P1, P2. Although the substrate alignment marks P1, P2 are shown occupying dedicated target portions, they may be located in spaces between target portions. When the substrate alignment marks P1, P2 are located between target portions C, they are referred to as scribe lane alignment marks.

[0063] Figure 2A lithographic system is shown comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and provide the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS, and a substrate table WT configured to support a substrate W.

[0064] The illumination system IL is configured to condition the EUV radiation beam B before it is incident on the patterning device MA. Thereby, the illumination system IL may comprise a faceted field mirror arrangement 10 and a faceted pupil mirror arrangement 11. The faceted field mirror arrangement 10 and the faceted pupil mirror arrangement 11 together provide a desired cross-sectional shape and a desired intensity distribution for the EUV radiation beam B. The illumination system IL may comprise other mirrors or arrangements in addition to or instead of the faceted field mirror arrangement 10 and the faceted pupil mirror arrangement 11.

[0065] After this conditioning, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B' is produced. The projection system PS is configured to project the patterned EUV radiation beam B' onto the substrate W. To this end, the projection system PS may comprise a plurality of mirrors 13, 14 configured to project the patterned EUV radiation beam B' onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B', thereby forming an image having features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although in Figure 2 The projection system PS is shown in FIG. 2 with only two mirrors 13 , 14 , but the projection system PS may comprise a different number of mirrors (eg six or eight mirrors).

[0066] The substrate W may include a previously formed pattern. In this case, the lithographic apparatus LA aligns the image formed by the patterned EUV radiation beam B' with the pattern previously formed on the substrate W.

[0067] A relative vacuum, ie, a small amount of gas (eg, hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, illumination system IL, and / or projection system PS.

[0068] The radiation source SO may be a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a free electron laser (FEL), or any other radiation source capable of generating EUV radiation.

[0069] like Figure 3As shown, the lithography apparatus LA may form part of a lithocell LC, sometimes also referred to as a litho cell or (lithography) cluster. The lithocell LC typically also includes equipment for performing pre-exposure and post-exposure processes on a substrate W. Typically, this equipment includes a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH, and a bake plate BK, for example, to adjust the temperature of the substrate W, such as for adjusting the solvent in the resist layer. A substrate handling device (or robot) RO picks up substrates W from input / output ports I / O1 and I / O2, moves them between various processing devices, and delivers them to a loading station LB of the lithography apparatus LA. The equipment within the lithocell (often collectively referred to as a track) is typically controlled by a track control unit TCU, which itself may be controlled by a supervisory control system SCS, which in turn controls the lithography apparatus LA, for example, via a litho control unit LACU.

[0070] In order to correctly and consistently expose substrates W exposed by the lithographic apparatus LA, it is desirable to inspect the substrates to measure properties of the patterned structures, such as overlay errors between subsequent layers, line thickness, critical dimensions (CDs), etc. To this end, an inspection tool (not shown) may be included in the lithography cell LC. If an error is detected, particularly if the inspection is performed before exposing or processing other substrates W from the same batch or lot, for example, adjustments may be made to the exposure of subsequent substrates and / or other processing steps to be performed on the substrates W.

[0071] Inspection equipment (which may also be referred to as metrology equipment) is used to determine properties of substrate W, in particular, to determine how properties vary between different substrates W or how properties associated with different layers of the same substrate W vary from layer to layer. Alternatively, the inspection equipment may be configured to identify defects on substrate W and may, for example, be part of a lithography cell LC, integrated into a lithography apparatus LA, or even a standalone device. The inspection equipment may measure properties on a latent image (the image in the resist layer after exposure), a semi-latent image (the image in the resist layer after a post-exposure bake step (PEB), a developed resist image (in which exposed or unexposed portions of the resist have been removed), or even an etched image (after a pattern transfer step such as etching).

[0072] Typically, the patterning process in the lithographic apparatus LA is one of the most critical steps in the process requiring high-precision sizing and placement of structures on the substrate W. To ensure this high precision, the three systems can be combined into a so-called "holistic" control environment, such as Figure 4One of these systems is a lithography apparatus LA, which is (physically) connected to a metrology tool MT (a second system) and a computer system CL (a third system). The key to this "holistic" environment is optimizing the collaboration between these three systems to enhance the overall process window, and providing a tight control loop to ensure that the patterning performed by the lithography apparatus LA remains within the process window. The process window defines the range of process parameters (e.g., dose, focus, overlay) within which a particular manufacturing process produces a defined outcome (e.g., a functional semiconductor device). Typically, process parameters in the lithography or patterning process are allowed to vary within this range.

[0073] The computer system CL can use (a portion of) the design layout to be patterned to predict which resolution enhancement technology to use and to perform computational lithography simulations and calculations to determine which mask layouts and lithographic equipment settings achieve the largest overall process window for the patterning process (in Figure 4 Typically, the resolution enhancement technique is arranged to match the patterning possibilities of the lithographic apparatus LA. The computer system CL may also be used to detect where within the process window the lithographic apparatus LA is currently operating (e.g. by using input from a metrology tool MT) to predict whether defects may be present due to, for example, a suboptimal process (e.g., in the example of FIG. Figure 4 , which is represented by an arrow pointing to “0” in the second scale SC2).

[0074] The metrology tool MT may provide input to the computer system CL to enable accurate simulations and predictions, and may provide feedback to the lithographic apparatus LA to identify, for example, possible drift in the calibration state of the lithographic apparatus LA (e.g., Figure 3 represented by multiple arrows in the third scale SC3).

[0075] During a lithographic process, it is often desirable to measure the resulting structures, for example, for process control and verification. The tool that performs such measurements is typically referred to as a metrology tool MT. Different types of metrology tools MT for performing such measurements are known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. Scatterometers are versatile instruments that allow parameters of the lithographic process to be measured by placing a sensor in the pupil of the scatterometer objective, or in a plane conjugate to the pupil, typically referred to as pupil-based measurements, or by placing the sensor in the image plane, or in a plane conjugate to the image plane, typically referred to as image-based or field-based measurements. Such scatterometers and related measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032, or EP1,628,164A, which are incorporated herein by reference in their entireties. The scatterometer described above can measure gratings using light from soft X-rays and visible to near IR wavelength ranges.

[0076] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, reconstruction methods can be applied to the measurement signals to reconstruct or calculate the properties of the grating. For example, such a reconstruction can be obtained by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measurement results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to the diffraction pattern observed from a real target.

[0077] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, radiation emitted by a radiation source is directed toward a target, and radiation reflected or scattered from the target is directed toward a spectroscopic detector, which measures the spectrum of the specularly reflected radiation (i.e., a measurement of intensity as a function of wavelength). From this data, the structure or profile of the target that produced the detected spectrum can be reconstructed, for example, by rigorous coupled wave analysis and nonlinear regression, or by comparison with a library of simulated spectra.

[0078] In a third embodiment, the scatterometer MT is an ellipsometer. Ellipsometers allow parameters of a lithography process to be determined by measuring scattered radiation for each polarization state. Such metrology equipment emits polarized light (such as linear, circular, or elliptical) by, for example, using appropriate polarization filters in the illumination portion of the metrology equipment. A source suitable for the metrology equipment can also provide polarized radiation. Various embodiments of existing ellipsometers are described in U.S. patent applications Ser. Nos. 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,449, 12 / 920,968, 12 / 922,587, 13 / 000,229, 13 / 033,135, 13 / 533,110, and 13 / 891,410, the entire contents of which are incorporated herein by reference.

[0079] In one embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the overlay of two misaligned gratings or periodic structures (e.g., gratings) by measuring the reflection spectrum and / or an asymmetry in the detection configuration, the asymmetry being related to the degree of overlap. The two (typically overlapping) grating 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 scatterometer may have a symmetrical detection configuration, such as described in commonly owned patent application EP 1,628,164A, so that any asymmetry can be clearly distinguished. This provides a straightforward method of measuring grating misalignment. Further examples of measuring overlay error between two layers comprising periodic structures when the target is measured via an asymmetry of the periodic structure may be found in PCT patent application publication number WO 2011 / 012624 or U.S. patent application US 20160161863, the entire contents of which are incorporated herein by reference.

[0080] Other parameters of interest may be focus and dose. Focus and dose can be determined simultaneously using a scatterometer (or alternatively, a scanning electron microscope), as described in U.S. Patent Application No. US2011-0249244, the entire contents of which are incorporated herein by reference. A single structure can be used that has a unique combination of critical dimension and sidewall angle measurements for each point in the focus energy matrix (FEM—also known as the focus exposure matrix). If these unique combinations of critical dimensions and sidewall angles are available, focus and dose values can be uniquely determined based on these measurements.

[0081] The metrology target can be a composite grating formed by the photolithography process, primarily in the resist, but also, for example, after an etching process. Typically, the pitch and linewidth of the structures in the grating are largely determined by the measurement optics (particularly the optical aperture) to capture the diffraction orders from the metrology target. As previously described, the diffraction signal can be used to determine the drift between two layers (also known as "overlay") or to reconstruct at least a portion of the original grating produced by the photolithography process. This reconstruction can provide guidance on the quality of the photolithography process and can be used to control at least part of the photolithography process. The target can have smaller subsegments configured to mimic the dimensions of functional portions of the design layout within the target. Due to this subsegmentation, the target's behavior will more closely resemble that of the functional portions of the design layout, resulting in better alignment of overall process parameter measurements with those of the functional portions of the design layout. The target can be measured in either an underfill mode or an overfill mode. In underfill mode, the spot size produced by the measurement beam is smaller than the entire target. In overfill mode, the spot size produced by the measurement beam is larger than the entire target. In this overfill mode, different targets can also be measured simultaneously, allowing for simultaneous determination of different process parameters.

[0082] The overall measurement quality of a lithography parameter using a particular target is determined at least in part by the measurement profile used to measure the lithography parameter. The term "substrate measurement profile" may include one or more parameters of the measurement itself, one or more parameters of one or more measured patterns, or both. For example, if the measurement used in the substrate measurement profile is a diffraction-based optical measurement, the 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 relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, and the like. For example, one of the criteria for selecting a measurement profile may be the sensitivity of one of the measurement parameters to process variations. Further examples are described in U.S. patent application US2016-0161863 and published U.S. patent application US 2016 / 0370717A1, the entire contents of which are incorporated herein by reference.

[0083] Figure 5 A measurement device, such as a scatterometer SM1, is depicted. The measurement device comprises a broadband (white light) radiation projector 2, which projects radiation onto a substrate W. The reflected or scattered radiation is passed to a spectral detector 4, which measures the spectrum 6 of the specularly reflected radiation (i.e., a measurement of the intensity I as a function of the wavelength λ). From this data, for example, as in Figure 5The processing unit PU can reconstruct the structure or profile that gave rise to the detected spectrum by rigorous coupled wave analysis and nonlinear regression, as shown at the bottom of the figure, or by comparison with a library of simulated spectra 8. Typically, for the reconstruction, the general form of the structure is known, and some parameters are assumed based on knowledge of the process that formed the structure, so only a few parameters of the structure need to be determined from the scatterometry data. Such a scatterometer can be configured as a normal-incidence scatterometer or an oblique-incidence scatterometer.

[0084] During photolithography, it is desirable to frequently measure the resulting structures, for example, for process control and verification. Various tools are known for performing such measurements, including scanning electron microscopes or various forms of metrology equipment, such as scatterometers. Examples of known scatterometers typically rely on providing a dedicated metrology target, such as an underfilled target (a target in the form of a simple grating or overlapping gratings in different layers, which is large enough that the measurement beam produces a spot smaller than the grating) or an overfilled target (where the illumination spot partially or completely encompasses the target). Furthermore, the use of metrology tools (e.g., angle-resolved scatterometers for illuminating underfilled targets such as gratings) allows for the use of so-called reconstruction methods, in which the properties of the grating can be calculated by simulating the interaction of scattered radiation with a mathematical model of the target structure and 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 that observed from a real target.

[0085] Scatterometers are versatile instruments that allow measurement of parameters of a lithographic process by placing a sensor in the pupil of the scatterometer's objective lens, or in a plane conjugate to the pupil (often referred to as pupil-based measurement), or by placing a sensor in the image plane, or in a plane conjugate to the image plane (often referred to as image-based or field-based measurement). Such scatterometers and related measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032, or EP1,628,164A, the entire contents of which are incorporated herein by reference. These scatterometers can measure multiple targets from multiple gratings in a single image using light from soft X-rays and visible to near-IR wavelengths.

[0086] A topography measurement system, a level sensor or height sensor, which can be integrated into a lithographic apparatus, is arranged to measure the topography of the top surface of a substrate (or wafer). A topography map (also called a height map) of the substrate can be generated from these measurements indicating the height of the substrate as a function of position on the substrate. This height map can then be used to correct the position of the substrate during transfer of the pattern onto the substrate so as to provide an aerial image of the patterning device at an appropriate focus position on the substrate. It will be understood that "height" in this context refers to a dimension that is significantly out of plane (also called the Z-axis) relative to the substrate. Typically, the level sensor or height sensor performs measurements at a fixed location (relative to its own optical system), and relative motion between the substrate and the optical system of the level sensor or height sensor produces height measurements at multiple locations across the substrate.

[0087] Figure 6 An example of a level sensor or height sensor LS known in the art is shown schematically, which merely illustrates the operating principle. In this example, the level sensor comprises an optical system comprising a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO, which provides a radiation beam LSB imparted by a projection grating PGR of the projection unit LSP. The radiation source LSO can, for example, be a narrowband radiation source or a broadband radiation source, such as a supercontinuum light source, a polarized or unpolarized radiation source, a pulsed or continuous radiation source, such as a polarized or unpolarized laser beam. The radiation source LSO can comprise a plurality of radiation sources having different colors or wavelength ranges, such as a plurality of LEDs. The radiation source LSO of the level sensor LS is not limited to visible radiation, but can additionally or alternatively encompass UV and / or IR radiation and any wavelength range suitable for reflection from the surface of the substrate.

[0088] Projection grating PGR is a periodic grating comprising a periodic structure that generates a radiation beam BE1 having a periodically varying intensity. This periodically varying intensity radiation beam BE1 is directed toward a measurement position MLO on a substrate W at an angle of incidence ANG between 0 and 90 degrees, typically between 70 and 80 degrees, relative to an axis perpendicular to the substrate surface (the Z axis). At measurement position MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and directed toward a detection unit LSD.

[0089] To determine the height level at the measurement location MLO, the level sensor further includes a detection system comprising a detection grating DGR, a detector DET, and a processing unit (not shown) for processing an output signal of the detector DET. The detection grating DGR can be equivalent to the projection grating PGR. The detector DET generates a detector output signal indicative of the received light, for example, a detector output signal indicative of the intensity of the received light (e.g., a photodetector) or a detector output signal representative of the spatial distribution of the received intensity (e.g., a camera). The detector DET can include any combination of one or more detector types.

[0090] By means of triangulation techniques the height level at the measurement position MLO can be determined. The detected height level is generally related to the signal intensity measured by the detector DET, which has a periodicity that depends inter alia on the design of the projection grating PGR and the (oblique) angle of incidence ANG.

[0091] The projection unit LSP and / or the detection unit LSD may comprise further optical elements, such as lenses and / or mirrors, along the path of the patterned radiation beam between the projection grating PGR and the detection grating DGR (not shown).

[0092] In an embodiment, the detection grating DGR can be omitted and the detector DET can be placed where the detection grating DGR is located. Such a configuration provides a more direct detection of the image of the projection grating PGR.

[0093] In order to effectively cover the surface of the substrate W, the level sensor LS may be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating an array of measurement areas MLO or spots covering a larger measurement range.

[0094] Various general types of height sensors are disclosed, for example, in US7265364 and US7646471, both of which are incorporated by reference. A height sensor that uses UV radiation rather than visible or infrared radiation is disclosed in US2010233600A1, which is incorporated by reference. In WO2016102127A1, incorporated by reference, a compact height sensor is described that uses a multi-element detector to detect and identify the position of a grating image without requiring a detection grating.

[0095] The position measurement system IF may include any type of sensor suitable for determining the position of the substrate support WT. The position measurement system IF may include any type of sensor suitable for determining the position of the mask support MT. The sensor may be an optical sensor, such as an interferometer or an encoder. The position measurement system IF may include a combined interferometer and encoder system. The sensor may be another type of sensor, such as a magnetic sensor, a capacitive sensor, or an inductive sensor. The position measurement system IF may determine the position relative to a reference (e.g., the metrology frame MF or the projection system PS). The position measurement system IF may determine the position of the substrate table WT and / or the mask support MT by measuring the position or by measuring a time derivative of the position, such as a velocity or acceleration.

[0096] The position measurement system IF may include an encoder system. The encoder system may be known, for example, from U.S. patent application US2007 / 0058173A1 filed on September 7, 2006, which is incorporated herein by reference. The encoder system includes an encoder head, a grating, and a sensor. The encoder system may receive a primary radiation beam and a secondary radiation beam. The primary radiation beam and the secondary radiation beam both originate from the same radiation beam, namely the primary radiation beam. At least one of the primary radiation beam and the secondary radiation beam is produced by diffracting the primary radiation beam with a grating. If both the primary radiation beam and the secondary radiation beam are produced by diffracting the primary radiation beam with a grating, the primary radiation beam needs to have a different diffraction order than the secondary radiation beam. For example, the different diffraction orders are +1, -1, +2, and -2. The encoder system optically combines the primary radiation beam and the secondary radiation beam into 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 the phase or phase difference. The signal represents the position of the encoder head relative to the grating. One of the encoder head and the optical grating can be arranged on the substrate structure WT. The other of the encoder head and the optical grating can be arranged on the metrology frame MF or the base frame BF. For example, a plurality of encoder heads are arranged on the metrology frame MF, while the optical grating is arranged on the top surface of the substrate support WT. In another example, the optical grating is arranged on the bottom surface of the substrate support WT, while the encoder head is arranged below the substrate support WT.

[0097] The position measurement system IF may include an interferometer system. The interferometer system may be known, for example, from U.S. Pat. No. 6,020,964, filed on July 13, 1998, which is incorporated herein by reference. The interferometer system may include a beam splitter, a reflector, a reference reflector, and a sensor. The radiation beam is split into a reference beam and a measurement beam by the beam splitter. The measurement beam propagates to the reflector and is reflected back to the beam splitter by the reflector. The reference beam propagates to the reference reflector and is reflected back to the beam splitter by the reference reflector. At the beam splitter, the measurement beam and the reference beam are combined into a combined radiation beam. The combined radiation beam is incident on the sensor. The sensor determines the phase or frequency of the combined radiation beam. The sensor generates a signal based on the phase or frequency. The signal represents the displacement of the reflector. In an embodiment, the reflector is connected to the substrate support WT. The reference reflector may be connected to the measurement frame MF. In an embodiment, the measurement beam and the reference beam are combined into a combined radiation beam by additional optical components instead of a beam splitter.

[0098] In the manufacture of complex devices, numerous photolithographic patterning steps are typically performed to form functional features in successive layers on a substrate. Therefore, a key aspect of the performance of a lithographic apparatus is its ability to correctly and accurately place the applied pattern relative to features laid down in previous layers (either by the same apparatus or a different apparatus). To this end, the substrate is provided with one or more sets of marks. Each mark is a structure whose position can be measured at a later time using a position sensor (typically an optical position sensor). The position sensor may be referred to as an "alignment sensor," and the mark may be referred to as an "alignment mark." The mask may also be referred to as a metrology target.

[0099] A lithographic apparatus may include one or more (e.g., multiple) alignment sensors that accurately measure the positions of alignment marks provided on a substrate or wafer. Alignment sensors (or position sensors) may utilize optical phenomena such as diffraction and interference to obtain position information from alignment marks formed on the substrate. Examples of alignment sensors currently used in lithographic apparatus are based on self-referencing interferometers, as described in US Pat. No. 6,961,116. Various enhancements and modifications to position sensors have been developed, for example, as disclosed in US Pat. No. 2015,261,097, A1. All of these disclosures are incorporated herein by reference.

[0100] A mark or alignment mark can include a series of stripes formed on or in a layer provided on a substrate, or formed (directly) in the substrate. These stripes can be regularly spaced and act as grating lines, so that the mark can be considered a diffraction grating with a known spatial period (pitch). Depending on the orientation of these grating lines, the mark can be designed to allow position measurement along the X-axis or along the Y-axis (where the Y-axis is oriented substantially perpendicular to the X-axis). Marks that include stripes arranged at +45 degrees and / or -45 degrees relative to both the X-axis and the Y-axis allow for combined X and Y measurements using the techniques described in US 2009 / 195768A (which is incorporated herein by reference).

[0101] The alignment sensor optically scans each mark with a radiation spot to obtain a periodically varying signal, such as a sine wave. The phase of this signal is analyzed to determine the position of the mark and, therefore, the position of the substrate relative to the alignment sensor, which is fixed relative to the reference frame of the lithographic apparatus. So-called coarse and fine marks, associated with different mark sizes (coarse and fine), can be provided, allowing the alignment sensor to distinguish between different periods of the periodic signal and the exact position (phase) within a period. Marks of varying pitch can also be used for this purpose.

[0102] Measuring the position of the marks can also provide information about deformation of the substrate on which the marks are provided (e.g., in the form of a wafer grid). Deformation of the substrate may occur, for example, due to electrostatic clamping of the substrate to the substrate table and / or heating of the substrate when exposed to radiation.

[0103] Figure 7 is a schematic block diagram of an embodiment of a known alignment sensor AS, such as for example the alignment sensor described in US6961116, incorporated by reference. For example, the alignment sensor AS may be incorporated into Figure 2 The lithography system shown in FIG and described herein is shown in FIG. A radiation source RSO provides a radiation beam RB of one or more wavelengths, which is steered by steering optics onto a mark (such as a mark AM on a substrate W) as an illumination spot SP. In this example, the steering optics include a spot mirror SM and an objective lens OL. The diameter of the illumination spot SP used to illuminate the mark AM may be slightly smaller than the width of the mark itself.

[0104] Radiation diffracted by the mark AM is collimated (in this example via an objective lens OL) into an information-bearing beam IB. The term "diffraction" is intended to include zero-order diffraction from the mark (which may be referred to as reflection). A self-referencing interferometer SRI (e.g., of the type disclosed in the aforementioned US Pat. No. 6,961,116) causes beam IB to interfere with itself, after which it is received by a photodetector PD. In the event that more than one wavelength is generated by the radiation source RSO, additional optics (not shown) may be included to provide separate beams. If desired, the photodetector may be a single element, or it may comprise multiple pixels. The photodetector may comprise a sensor array.

[0105] The steering optics, which in this example comprise a spot mirror SM, can also be used to block zero-order radiation reflected from the mark so that the information-carrying beam IB comprises only higher-order diffracted radiation from the mark AM (which is not necessary for the measurement but improves the signal-to-noise ratio).

[0106] The intensity signal SI is provided to the processing unit PU. Through a combination of optical processing in block SRI and computational processing in unit PU, the values of the X position and Y position on the substrate relative to the reference frame are output.

[0107] A single measurement of the type shown only fixes the position of the mark within a specific range corresponding to one pitch of the mark. In conjunction with this, a coarse measurement technique is used to identify which period of the sine wave includes the position of the mark. The same process can be repeated at coarser or finer levels with different wavelengths to improve accuracy and / or robustly detect the mark, regardless of the material the mark is made of and what materials are disposed above and / or below it. The wavelengths can be optically multiplexed and demultiplexed for simultaneous processing, and / or they can be multiplexed using time or frequency division.

[0108] In this example, the alignment sensor and spot SP remain stationary, while the substrate W moves. Thus, the alignment sensor can be fixedly and accurately mounted to a reference frame while effectively scanning the mark AM in a direction opposite to the direction of movement of the substrate W. The substrate W is mounted on a substrate support and controlled during this movement 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 calibration of the position of the substrate support determined by the position sensor (e.g., relative to a frame to which the alignment system is attached). Measuring the position of the alignment marks provided on the substrate allows determination of the position of the substrate relative to the substrate support.

[0109] A measurement tool MT (such as a scatterometer, a topography measurement system, or a position measurement system as described above) can use radiation originating from a radiation source to perform measurements. The nature of the radiation used by the measurement tool may affect the type and quality of the measurements that may be performed. For some applications, it may be advantageous to use multiple radiation frequencies to measure the substrate, for example broadband radiation may be used. Multiple different frequencies may be able to propagate, illuminate, and scatter the measurement target with no or minimal interference from other frequencies. Thus, for example, more measurement data may be obtained simultaneously using different frequencies. Different radiation frequencies may also be able to query and discover different properties of the measurement target. Broadband radiation may be used for a measurement system MT, such as, for example, a level sensor, an alignment mark measurement system, a scatterometry tool, or an inspection tool. The broadband radiation source may be a supercontinuum source.

[0110] It can be difficult to generate high-quality broadband radiation (e.g., supercontinuum radiation). One method of generating broadband radiation can be, for example, to broaden high-power narrowband or single-frequency input radiation by utilizing nonlinear, higher-order effects. The input radiation (which can be generated using a laser) can be referred to as pump radiation. To obtain high-power radiation for the broadening effect, the radiation can be confined to a small region, thereby achieving locally enhanced high-intensity radiation. In these regions, the radiation can interact with the broadening structures and / or materials that form the nonlinear medium, thereby generating broadband output radiation. In the high-intensity radiation region, by providing a suitable nonlinear medium, different materials and / or structures can be used to achieve and / or improve radiation broadening.

[0111] In some embodiments, as described below with reference to Figures 8 to 10 As further discussed, methods and apparatus for stretching input radiation may use an optical fiber to confine the input radiation and stretch the input radiation to output broadband radiation. The optical fiber may be a hollow core optical fiber and may include internal structures for effectively guiding and confining radiation within the optical fiber. The optical fiber may be a hollow core photonic crystal fiber (HC-PCF), which is particularly suitable for strong radiation confinement primarily within the hollow core of the optical fiber, thereby achieving high radiation intensity. The hollow core of the optical fiber may be filled with a gas that serves as a stretching medium for stretching the input radiation. This optical fiber and gas arrangement may be used to generate a supercontinuum radiation source. The radiation input to the optical fiber may be electromagnetic radiation, such as radiation in one or more of the infrared spectrum, the visible spectrum, the UV spectrum, and the extreme UV spectrum. The output radiation may consist of or include broadband radiation, which may be referred to herein as white light.

[0112] Figure 8The general setup of an apparatus 120 for receiving input radiation 122 and stretching the frequency range of the input radiation 122 to provide broadband output radiation 124 is schematically shown. The apparatus 120 includes an optical fiber 100 having a hollow core 102 (i.e., an HC-PCF) for guiding radiation propagation through the optical fiber 100. The apparatus 120 also includes a gas 126 disposed within the hollow core 102, wherein the gas includes an operating component capable of stretching the frequency range of the received input radiation 126 to provide broadband output radiation 124.

[0113] The working composition of gas 126 may include a molecular gas (eg, N2, O2, CH4, SF6). In some examples, the working composition of gas 126 may include an inert gas (eg, one or more of argon, krypton, neon, helium, and xenon).

[0114] In one embodiment, the gas 126 may be located within the hollow core 102 at least during the time the input radiation 122 is received to generate the broadband output radiation 124. It should be understood that the gas 126 may be completely or partially absent from the hollow core 102 when the apparatus 120 is not receiving the input radiation 122 for generating the broadband output radiation. Typically, the apparatus 120 includes a device for providing the gas 126 within the hollow core 102 of the optical fiber 100. Such a device for providing the gas 126 within the hollow core 102 of the optical fiber 100 may include a reservoir, as now referred to. Figure 9 discussed.

[0115] Figure 9 Shown as Figure 8 The illustrated apparatus 120 also includes a reservoir 128. The optical fiber 100 is disposed within the reservoir 128. The reservoir 128 may also be referred to as a housing or container. The reservoir 128 is configured to contain a gas 126. The reservoir 128 may include one or more features known in the art for controlling, regulating, and / or monitoring the composition of the gas 126 within the reservoir 128. The reservoir may include a first transparent window 130. In use, the optical fiber 100 is disposed within the reservoir 128 such that the first transparent window 130 is located near the input end of the optical fiber 100. The first transparent window 130 may form part of a wall of the reservoir 128. The first transparent window 130 is transparent to at least the frequency of the received input radiation, allowing the received input radiation 122 (or at least a substantial portion thereof) to couple into the optical fiber 100 within the reservoir 128. The reservoir 128 may include a second transparent window 132 forming part of a wall of the reservoir 128. In use, when the optical fiber 100 is disposed inside the reservoir 128, the second transparent window 132 is located near the output end of the optical fiber 100. The second transparent window 132 may be transparent to at least the frequencies of the broadband output radiation 124 of the device 120.

[0116] Alternatively, in another embodiment, the two opposite ends of the optical fiber 100 can be placed inside different reservoirs. The optical fiber 100 can include a first end portion configured to receive input radiation 122 and a second end portion for outputting broadband output radiation 124. The first end portion can be placed in a first reservoir including a gas 126. The second end portion can be placed in a second reservoir, where the second reservoir can also include a gas 126. The function of the reservoir can be as described above with respect to Figure 9 As described above, the first reservoir may include a first transparent window configured to be transparent to input radiation 122. The second reservoir may include a second transparent window configured to be transparent to broadband output broadband radiation 124. The first and second reservoirs may also include sealable openings to allow the optical fiber 100 to be placed partially inside the reservoir and partially outside the reservoir, thereby sealing the gas within the reservoir. The optical fiber 100 may also include an intermediate portion not contained within the reservoir. This arrangement using two separate gas reservoirs may be particularly convenient for embodiments in which the optical fiber 100 is relatively long (e.g., when the length is greater than 1 meter). It will be understood that with this arrangement using two separate gas reservoirs, both reservoirs (which may include one or more features known in the art for controlling, regulating, and / or monitoring the composition of the gas 126 within both reservoirs) can be considered to be provided with a device for providing gas 126 to the hollow core 102 of the optical fiber 100.

[0117] In this context, a window may be transparent to a frequency if at least 50%, 75%, 85%, 90%, 95% or 99% of incident radiation incident on the window at that frequency is transmitted through the window.

[0118] First transparent window 130 and second transparent window 132 may both form an airtight seal within the walls of reservoir 128 such that gas 126 may be contained within reservoir 128. It will be appreciated that gas 126 may be contained within reservoir 128 at a pressure different than the ambient pressure of reservoir 128.

[0119] To achieve frequency broadening, high-intensity radiation may be desirable. An advantage of having a hollow-core optical fiber 100 is that high-intensity radiation can be achieved by providing strong spatial confinement of radiation propagating through the optical fiber 100, thereby achieving locally high radiation intensity. Furthermore, a hollow-core design (e.g., compared to a solid-core design) can produce higher-quality transmission modes (e.g., with a greater proportion of single-mode transmission). For example, due to the high intensity of the received input radiation and / or due to the strong spatial confinement of the radiation within the optical fiber 100, the radiation intensity within the optical fiber 100 can be high.

[0120] An advantage of using a hollow-core optical fiber 100 may be that most of the radiation guided within the interior of the optical fiber 100 is confined to the hollow core 102. Therefore, the primary interaction of the radiation within the optical fiber 100 is with the gas 126 disposed within the hollow core 102 of the optical fiber 100. Consequently, the broadening effect of the working component of the gas 126 on the radiation may be increased.

[0121] The received input radiation 122 may be electromagnetic radiation. The input radiation 122 may be received as pulsed radiation (i.e., pump light pulses). For example, the input radiation 122 may include ultrafast pulses. When the radiation interacts with the gas 126, various mechanisms of spectral broadening are possible, such as four-wave mixing, modulation instability, ionization of the working gas, the Raman effect, Kerr nonlinearity, soliton formation, or soliton fission. The present disclosure particularly relates to generating broadened (i.e., supercontinuum) radiation by generating Raman combs, as described herein. The input radiation 122 may be coherent radiation. The input radiation 122 may be collimated radiation, which advantageously facilitates and improves the efficiency of coupling the input radiation 122 into the optical fiber 100. The input radiation 122 may include a single frequency or a narrow frequency range. The input radiation 122 may be generated by a laser. Similarly, the output radiation 124 may be collimated and / or coherent.

[0122] The broadband range of output radiation 124 can be a continuous range, including a continuous range of radiation frequencies. Output radiation 124 can include supercontinuum radiation. Using continuous radiation can be beneficial in many applications, such as metrology applications. For example, a continuous range of frequencies can be used to interrogate a large number of properties. For example, a continuous range of frequencies can be used to determine and / or eliminate the frequency dependence of a measured property. For example, supercontinuum output radiation 124 can include electromagnetic radiation having a wavelength range of 100 nm to 4000 nm. For example, the frequency range of broadband output radiation 124 can be 400 nm to 900 nm, 500 nm to 900 nm, or 200 nm to 2000 nm. Supercontinuum output radiation 124 can include white light.

[0123] Figure 10 A radiation source 134 is depicted for providing broadband output radiation. The radiation source 134 comprises a Figure 9 The device 120 includes a radiation source 134 that further includes an input radiation source 136 configured to provide input radiation 122 to the device 120. The device 120 may receive the input radiation 122 from the input radiation source 136 and stretch the input radiation to provide the output radiation 124.

[0124] The input radiation 122 provided by the input radiation source 136 can be pulsed. The input radiation 122 can include electromagnetic radiation at one or more frequencies between 200 nm and 2 µm. The input radiation 122 can, for example, include electromagnetic radiation at a wavelength of 1.03 µm, 515 nm, or 343 nm. The repetition rate of the pulsed radiation 122 can be on the order of 1 kHz to 100 MHz. The pulse energy can be on the order of 0.1 µJ to 100 µJ, for example, 1 µJ to 10 µJ. The pulse duration of the input radiation 122 can be between 10 fs and 100 ps, or between 10 fs and 10 ps, for example, 300 fs. The average power of the input radiation 122 can be between 100 mW and several hundred W. The average power of the input radiation 122 can be, for example, 20 W to 50 W.

[0125] The broadband output radiation 124 provided by the radiation source 134 can have an average output power of at least 1 W. The average output power can be at least 5 W. The average output power can be at least 10 W. The broadband output radiation 124 can be pulsed broadband output radiation 124. The power spectral density of the broadband output radiation 124 across the entire wavelength band of the output radiation can be at least 0.01 mW / nm. The power spectral density of the broadband output radiation across the entire wavelength band can be at least 3 mW / nm.

[0126] The aforementioned radiation source 134 can be provided as part of a metrology arrangement for determining a parameter of interest of a structure on a substrate. The structure on the substrate can be, for example, a photolithographic pattern applied to the substrate. The metrology arrangement can further include an illumination subsystem for illuminating the structure on the substrate. The metrology arrangement can further include a detection subsystem for detecting a portion of the radiation scattered and / or reflected by the structure. The detection subsystem can also determine a parameter of interest of the structure based on the portion of the radiation scattered and / or reflected by the structure. For example, the parameter can be overlay, alignment, or leveling data of the structure on the substrate.

[0127] A system for generating supercontinuum radiation according to the present disclosure may include Figure 8 and Figure 9 Device 120 and / or Figure 10 Radiation source 134.

[0128] Figure 11 and Figure 12 An example of an optical fiber (ie, HC-PCF) that may be used with the systems and / or methods of the present disclosure is shown.

[0129] The optical fiber 100 includes an elongated body having a length in one dimension that is greater than the lengths of the other two dimensions of the optical fiber 100. The longer dimension may be referred to as the axial direction and may define the axis 101 of the optical fiber 100. The other two dimensions define a plane that may be referred to as a transverse plane. Figure 11 A cross section of the optical fiber 100 is shown in a transverse plane (ie, perpendicular to the axis 101 ), which is labeled the xy plane. Figure 12 A cross section of the optical fiber 100 is shown in a plane containing the axis 101 , in particular the xz plane. The transverse cross section of the optical fiber 100 may be substantially constant along the optical fiber axis 101 .

[0130] It will be understood that the optical fiber 100 has a certain degree of flexibility, and therefore the direction of the axis 101 is generally non-uniform along the length of the optical fiber 100. Terms such as the optical axis 101, a transverse cross section, etc. will be understood to refer to a local optical axis 101, a local transverse cross section, etc. Furthermore, where components are described as cylindrical or tubular, these terms will be interpreted to encompass shapes that may have deformed when the optical fiber 100 is bent.

[0131] The optical fiber 100 can have any length, and it will be appreciated that the length of the optical fiber 100 can depend on the application (e.g., the amount of spectral broadening desired in applications within a supercontinuum radiation source). The length of the optical fiber 100 can be between 1 cm and 10 m, for example, the length of the optical fiber 100 can be between 10 cm and 100 cm.

[0132] The optical fiber 100 includes a hollow core 102, an inner cladding region surrounding the hollow core 102, and a jacket region 110 surrounding and supporting the inner cladding region. The inner cladding region includes a plurality of antiresonant elements for guiding radiation through the hollow core 102. In particular, the plurality of antiresonant elements are arranged to confine the majority of radiation propagating through the optical fiber 100 within the hollow core 102 and guide the radiation along the optical fiber 100. The hollow core 102 of the optical fiber 100 can be disposed substantially in a central region of the optical fiber 100, such that the axis 101 of the optical fiber 100 can also define the axis of the hollow core 102 of the optical fiber 100.

[0133] The inner cladding region includes a plurality of capillaries 104, such as tubular capillaries, surrounding the hollow core 102. In particular, Figure 11 and 12 In the example shown, the inner cladding region comprises a single ring of six tubular capillaries 104 .

[0134] Capillaries 104 may also be referred to as tubes. The cross-section of capillaries 104 may be circular or have other shapes. Each capillary 104 includes a generally cylindrical wall portion 105 that at least partially defines the hollow core 102 of the optical fiber 100 and separates the hollow core 102 from the cavity 106. Each capillary wall portion 105 facing the hollow core serves as an antiresonant element for guiding radiation propagating through the optical fiber 100. It should be understood that the wall portions 105 can act as antireflection Fabry-Perot resonators for radiation propagating through the hollow core 102 (and which may be incident on the wall portions 105 at grazing incidence angles). The thickness 160 of the wall portions 105 can be selected to ensure that reflection back into the hollow core 102 is generally enhanced, while transmission into the cavity 106 is generally suppressed. In some examples, the thickness 160 of the capillary wall portion 105 can be less than 400 nm; less than 300 nm; or less than 150 nm.

[0135] It should be understood that, as used herein, the term inner cladding region is intended to refer to the region of the optical fiber 100 that is used to guide radiation propagating through the optical fiber 100 (i.e., the capillary 104 that confines the radiation within the hollow core 102). The radiation may be confined in the form of a transverse mode, thereby propagating along the optical fiber axis 101.

[0136] The sheath region 110 is generally tubular and supports the capillaries 104 of the inner cladding region. The capillaries 104 are evenly distributed around the inner surface of the sheath region 110. Six capillaries 104 can be described as being arranged in a symmetrical arrangement around the hollow core 102. In embodiments including six capillaries 104, the capillaries 104 can be described as being arranged in a generally hexagonal formation.

[0137] The capillaries 104 are arranged such that each capillary does not contact any other capillary 104. Each capillary 104 contacts the jacket region 110 and is spaced apart from adjacent capillaries 104 in the annular configuration. This arrangement may be beneficial because it may increase the transmission bandwidth of the optical fiber 100 (e.g., relative to an arrangement in which the capillaries are in contact with each other). Alternatively, in some embodiments, each capillary 104 may contact adjacent capillaries 104 in the annular configuration.

[0138] The six capillaries 104 in the inner cladding region are arranged in an annular configuration around the hollow core 102. The inner surfaces of the annular configuration of capillaries 104 at least partially define the hollow core 102 of the optical fiber 100. In some embodiments, the diameter of the hollow core 102 (which can be defined as the smallest dimension between opposing capillaries, as indicated by arrow 114) can be between 5µm and 100µm. In some embodiments, the diameter 114 of the hollow core 102 can be between 5µm and 50µm. In some embodiments, the diameter 114 of the hollow core 102 can be between 30µm and 40µm. The diameter 114 of the hollow core 102 can affect the mode field parameters, impact loss, chromatic dispersion, modal complexity, and nonlinear properties of the hollow core optical fiber 100.

[0139] exist Figure 11 and Figure 12 In the embodiment shown, the inner cladding region comprises a single ring arrangement of capillaries 104 (the wall portion 105 facing the hollow core acting as an anti-resonance element). Thus, a line in any radial direction from the center of the hollow core 102 toward the exterior of the optical fiber 100 passes through no more than one capillary 104.

[0140] It will be appreciated that other examples may be provided with different arrangements of anti-resonant elements. These may include arrangements with multiple anti-resonant element rings and arrangements with nested anti-resonant elements. Figure 11 and Figure 12 The illustrated embodiment includes a ring of six capillaries 104 having a wall portion 105, but in other embodiments one or more rings including any number of antiresonant elements (e.g., 4, 5, 6, 7, 8, 9, 10, 11, or 12 capillaries) may be provided in the inner cladding region.

[0141] exist Figure 11 and 12 In the illustrated example, the inner cladding region comprises a circular cross-section. However, it should be understood that other embodiments may provide an inner cladding region having a non-circular cross-sectional shape. For example, in an embodiment of the present invention, the inner cladding region may have a hexagonal cross-section. The hexagonal cross-section may advantageously facilitate the placement of the capillaries 104 in a symmetrical arrangement. For example, six capillaries 104 may each be placed at a vertex of the hexagonal cross-section, thereby providing an arrangement of the capillaries 104 having hexagonal symmetry.

[0142] Fiber 100 may be referred to as a hollow-core photonic crystal fiber (HC-PCF). Typically, such a hollow-core photonic crystal fiber includes an inner cladding region (e.g., which may include antiresonant elements) for guiding radiation within the fiber and a jacket region. The jacket region is typically a sheath or tube of material that supports the inner cladding region.

[0143] According to the present disclosure, inelastic scattering of pump light (eg, narrowband light provided by a laser source) by molecules in the gas 126 disposed in the HC-PCF 120 can produce a broad set of discrete spectral lines, referred to as a Raman frequency comb.

[0144] Without wishing to be bound by theory, a broad frequency comb is generated at the beginning of the fiber (i.e., closest to the pump source) by a large vibrational frequency shift. As the propagation distance through the fiber increases, the comb is subsequently broadened to a smooth supercontinuum under the influence of rotational stimulated Raman scattering (SRS), the optical Kerr effect (transient nonlinear refractive index), or a combination of both. This broadening is Figure 13 The figure shows the numerical simulation results of a 26µm diameter HC-PCF filled with nitrogen at a pressure of 15 bar and pumped with a pump light pulse of 20ps duration, 80µJ energy and a wavelength of 532nm. The parameter SPD stands for relative spectral power density. Figure 13 More details of the simulations shown can be found in the non-patent publication by S.-F. Gao, Y.-Y. Wang, F. Belli, C. Brahms, P. Wang and J.C. Travers, “From Raman Frequency Combs to Supercontinuum Generation in Nitrogen-Filled Hollow-Core Anti-Resonant Fiber,” Laser & Photonics Reviews, Vol. 16, No. 4, p. 2100426, 2022, the contents of which are incorporated herein by reference.

[0145] Therefore, the above process can be used to generate supercontinuum radiation. Figure 13 In experiments conducted under similar conditions to those simulated in the previous study using nitrogen as the gas, the inventors discovered that gain suppression occurs when the spacing of the rotational spectral lines corresponding to the rotating SRS is small enough that the propagation constants of the Stokes and anti-Stokes lines are very similar. Consequently, the coherent wave generated by the pump-Stokes beat is nearly identical to the coherent wave annihilated by the pump-anti-Stokes beat, leading to an equilibrium between the phonon creation and annihilation rates. This results in negligible signal growth above the noise level. However, Stokes lines can form in higher-order modes due to the large difference in propagation constants, which is caused by multimode coherent waves.

[0146] In view of the gain suppression effect observed when using nitrogen as the gas, the inventors propose to use a gas instead in which the rotational line spacing is sufficient to make the propagation constants of the Stokes lines and the anti-Stokes lines different enough so that the phonon creation and annihilation rates are unbalanced, thereby alleviating the gain suppression effect.

[0147] Taking into account the data from the non-patent literature by J. Bendtsen, “The rotational and rotation-vibrational Raman spectra of 14N2, 14N15N and 15N2”, J. Raman Spectroscopy, Vol. 2, No. 2, pp. 133-145, 1974 (the content of which is incorporated herein by reference), the rotational line spacing of nitrogen can be determined to be between 0.24 THz and 0.36 THz.

[0148] Therefore, it is proposed that the rotational line spacing of the gas should be greater than 0.5 THz, which should be sufficient to avoid the above-mentioned gain suppression problem. Preferably, the rotational line spacing of the gas should be several THz, such as greater than 1 THz, greater than 2 THz or greater than 3 THz.

[0149] Examples of gases with appropriately spaced rotational lines include hydrogen (H2) and deuterium (D2).

[0150] Alternatively, gases that have no rotational Raman response (i.e., no rotational lines) can be used to prevent gain suppression issues. Examples of such gases include methane (CH4) and SF6.

[0151] The gas should still have a strong vibrational Raman response to produce the broad frequency comb required for supercontinuum radiation.

[0152] It will be appreciated that the gas should also have good linear transmission in the wavelength region of interest.

[0153] The HC-PCF should be selected to ensure good guidance across the spectral region of interest (e.g., from the UV to the NIR). This involves choosing an appropriate core diameter and sheath thickness 150. Figure 11 and 12 As shown, the core diameter 114 can be defined as the minimum dimension between opposing capillaries. Alternatively, for an HC-PCF without capillaries, the core diameter can be defined as the distance across the hollow core of the HC-PCF. For example, the core diameter can be less than 70µm and the jacket thickness can be less than 1µm. The length of the HC-PCF should be sufficient to fully establish the supercontinuum, but not so long that guidance losses degrade the supercontinuum. By way of example, a suitable optical fiber can be between 1 cm and 10 m (e.g., 1.5 m). In some examples, the optical fiber can be between 10 cm and 100 cm long.

[0154] The pump wavelength is chosen to achieve normal (positive) group velocity dispersion for the gas in the HC-PCF. The group velocity dispersion at a specific wavelength depends on the gas composition, gas pressure, hollow core dimensions, and the dimensions of the microstructures in the HC-PCF cladding. Group velocity dispersion can be determined experimentally, through numerical modeling (such as the finite element method), or through appropriate empirical or analytical models. The pump wavelength should also be within the HC-PCF's guide band.

[0155] To minimize the effects of modulation instabilities and thus ensure that the supercontinuum radiation is generated by the processes described herein involving vibrational and rotational Raman scattering and the Kerr effect, the gas pressure should also be chosen so that the group velocity dispersion is normal at the pump wavelength. This analysis should account for the dispersion of the gas, nonideal scaling of the gas density, and the dispersion of the fully hollow fiber waveguide, including the effects of resonances.

[0156] For pump wavelengths shorter than the zero-dispersion wavelength (ZDW), the group dispersion velocity is normal, while for pump wavelengths longer than the ZDW, the group dispersion velocity is anomalous. Accordingly, the gas species, HC-PCF structure, and pump wavelength can be selected to induce the ZDW at a specific gas pressure. Thus, shorter pump wavelengths allow for the use of lower gas pressures, and the pump wavelength should preferably be toward the short-wavelength end of the HC-PCF guide band. For example, the pump wavelength can be less than 1000 nm, can be a visible wavelength, or can be a UV wavelength to effectively drive supercontinuum formation.

[0157] For the case of pumping with more than one pump wavelength (eg dual pumping), at least one pump wavelength should correspond to the normal dispersion state.

[0158] The pump light pulses should have a pulse duration and pulse energy such that the peak pulse power is sufficient to drive strong nonlinear effects within the gas-filled HC-PCF. Preferably, the pulse duration should be short enough to avoid requiring excessive pulse energy (e.g., less than 100 ps). Furthermore, the pulse duration should ideally be long enough to prevent self-phase modulation from dominating the formation of Raman vibrational combs. This is typically the case for pulses longer than 50 fs, preferably longer than 100 fs.

[0159] Figure 14 An example of a method 1400 for generating supercontinuum radiation according to the present disclosure is shown. The method can be performed using one or more systems or devices described herein, for example Figure 8 and Figure 9 The device 120 shown and / or Figure 10The radiation source 134 is shown. The method 1400 includes providing a pump light pulse to the hollow core photonic crystal fiber in step S1402, the pump light pulse having a pump wavelength, and the hollow core photonic crystal fiber contains a gas. The gas and the hollow core photonic crystal fiber are selected so that the pump wavelength is within a range corresponding to a normal group velocity dispersion state of the gas. The gas has any of the following: a rotational line spacing greater than 0.5 THz; or no rotational line. The hollow core photonic crystal fiber can be Figures 8 to 12 The pump light pulses may be provided by a radiation source such as Figure 10 Radiation source 136 is shown and described herein.

[0160] Other embodiments are disclosed in the subsequent numbered item lists:

[0161] 1. A system for generating supercontinuum radiation, the system comprising:

[0162] a pump light source configured to generate a pump light pulse having a pump wavelength; and

[0163] a hollow core photonic crystal fiber, wherein the hollow core photonic crystal fiber is configured to receive the pump light pulse, and the hollow core photonic crystal fiber contains a gas;

[0164] wherein the gas and the hollow core photonic crystal fiber are selected so that the pump wavelength is within a range corresponding to a normal group velocity dispersion state of the gas; and

[0165] The gas has any of the following:

[0166] A rotational line spacing greater than 0.5 THz; or

[0167] There is no rotation of the spectrum.

[0168] 2. A system according to clause 1, wherein the rotational line spacing of the gas is greater than 1 THz.

[0169] 3. A system according to clause 1 or 2, wherein the gas comprises a molecular gas.

[0170] 4. The system of any preceding clause, wherein the gas is selected to exhibit a vibrational Raman response.

[0171] 5. The system of any preceding clause, wherein the gas comprises H2.

[0172] 6. The system of any preceding clause, wherein the gas comprises D2.

[0173] 7. The system of any preceding clause, wherein the gas comprises methane.

[0174] 8. The system of any preceding clause, wherein the gas comprises SF6.

[0175] 9. The system of any preceding clause, wherein the pump wavelength is less than 1000 nm.

[0176] 10. The system of any preceding clause, wherein the pump wavelength is less than 800 nm.

[0177] 11. The system of any preceding clause, wherein the pump wavelength is less than 600 nm.

[0178] 12. The system of any preceding clause, wherein the pump wavelength is a visible light wavelength.

[0179] 13. The system of clause 12, wherein the pump wavelength is a green wavelength.

[0180] 14. The system of any one of clauses 1 to 11, wherein the pump wavelength is an ultraviolet wavelength.

[0181] 15. The system of any preceding clause, wherein selecting the hollow core photonic crystal fiber comprises selecting a core diameter of the hollow core photonic crystal fiber.

[0182] 16. The system of any preceding clause, wherein the pump light pulses have a pulse duration between 50 fs and 700 ps.

[0183] 17. The system of any of clauses 1 to 15, wherein the pump light pulses have a pulse duration between 300 fs and 700 ps.

[0184] 18. The system of any one of clauses 1 to 15, wherein the pulse duration of the pump light pulses is between 100 fs and 100 ps.

[0185] 19. A metrology arrangement comprising a system according to any of the preceding clauses.

[0186] 20. A lithographic apparatus comprising a system according to any of the preceding clauses.

[0187] 21. A method of generating supercontinuum radiation, the method comprising:

[0188] providing a pump light pulse to a hollow core photonic crystal fiber, wherein the pump light pulse has a pump wavelength, and the hollow core photonic crystal fiber contains a gas;

[0189] wherein the gas and the hollow core photonic crystal fiber are selected so that the pump wavelength is within a range corresponding to a normal group velocity dispersion state of the gas; and

[0190] The gas has any of the following:

[0191] A rotational line spacing greater than 0.5 THz; or

[0192] There is no rotation of the spectrum.

[0193] 22. A method according to item 21, wherein the rotational line spacing of the gas is greater than 1 THz.

[0194] 23. A method according to clause 21 or 22, wherein the gas comprises a molecular gas.

[0195] 24. A method according to any one of clauses 21 to 23, wherein the gas is selected to exhibit a vibrational Raman response.

[0196] 25. A method according to any one of clauses 21 to 24, wherein the gas comprises H2.

[0197] 26. A method according to any one of clauses 21 to 25, wherein the gas comprises D2.

[0198] 27. A method according to any one of clauses 21 to 26, wherein the gas comprises methane.

[0199] 28. A method according to any one of clauses 21 to 27, wherein the gas comprises SF6.

[0200] 29. A method according to any one of clauses 21 to 28, wherein the pump wavelength is less than 1000 nm.

[0201] 30. A method according to any one of clauses 21 to 29, wherein the pump wavelength is less than 800 nm.

[0202] 31. A method according to any one of clauses 21 to 30, wherein the pump wavelength is less than 600 nm.

[0203] 32. A method according to any one of clauses 21 to 31, wherein the pump wavelength is a visible light wavelength.

[0204] 33. The method of clause 32, wherein the pump wavelength is a green wavelength.

[0205] 34. A system according to any of clauses 21 to 31, wherein the pump wavelength is an ultraviolet wavelength.

[0206] 35. The method of any preceding clause, wherein selecting the hollow core photonic crystal fiber comprises selecting a core diameter of the hollow core photonic crystal fiber.

[0207] 36. A method according to any of the preceding clauses, wherein the pulse duration of the pump light pulses is between 50 fs and 700 ps.

[0208] 37. A method according to any one of clauses 21 to 35, wherein the pulse duration of the pump light pulses is between 300 fs and 700 ps.

[0209] 38. A method according to any one of clauses 21 to 35, wherein the pulse duration of the pump light pulse is between 100 fs and 100 ps.

[0210] The lithographic arrangement described above may form part of a metrology apparatus MT.The lithographic arrangement described above may form part of an inspection apparatus.The lithographic arrangement described above may be comprised within a lithographic apparatus LA.

[0211] Although specific reference may be made herein to the use of lithographic apparatus in IC manufacturing, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guide and detection patterns for magnetic domain storage, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

[0212] Although embodiments of the present invention may be specifically referenced herein in the context of lithographic equipment, embodiments of the present invention may be employed in other equipment. Embodiments of the present invention may form mask inspection equipment, metrology equipment, or any equipment that measures or processes objects such as wafers (or other substrates) or masks (or other patterning devices). These equipment may generally be referred to as lithographic tools. Such lithographic tools may utilize vacuum conditions or ambient (non-vacuum) conditions.

[0213] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention is not limited to optical lithography and may be used in other applications such as imprint lithography where the context permits.

[0214] Although specific reference is made to a "measurement device / tool / system" or an "inspection device / tool / system," these terms may refer to the same or similar types of tools, devices, or systems. For example, an inspection or measurement device including embodiments of the present invention may be used to determine characteristics of structures on a substrate or wafer. For example, an inspection device or measurement 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 characteristic of interest of a structure on a substrate may relate to a defect in the structure, the absence of a particular portion of the structure, or the presence of an undesirable structure on the substrate or wafer.

[0215] Although specific embodiments of the present invention have been described above, it should be understood that the present invention may be practiced in other ways than as described. The above description is intended to be illustrative rather than restrictive. Therefore, it will be clear to those skilled in the art that modifications may be made to the described invention without departing from the scope of the claims set forth below.

Claims

1. A system for generating supercontinuum radiation, the system comprising: a pump light source configured to generate a pump light pulse having a pump wavelength; and a hollow core photonic crystal fiber, wherein the hollow core photonic crystal fiber is configured to receive the pump light pulse, and the hollow core photonic crystal fiber contains a gas; wherein the gas and the hollow core photonic crystal fiber are selected such that the pump wavelength is within a range corresponding to a normal group velocity dispersion state of the gas; and The gas has any of the following: A rotational line spacing greater than 0.5 THz; or There is no rotation of the spectrum.

2. The system of claim 1, wherein the rotational line spacing of the gas is greater than 1 THz.

3. The system of claim 1 or 2, wherein the gas comprises a molecular gas.

4. A system according to any preceding claim, wherein the gas is selected to exhibit a vibrational Raman response.

5. The system according to any one of the preceding claims, wherein the gas comprises one of H2, D2, methane, SF6.

6. The system of any preceding claim, wherein the pump wavelength is less than 1000 nm.

7. The system of any preceding claim, wherein the pump wavelength is a visible light wavelength.

8. The system of claim 7, wherein the pump wavelength is a green wavelength.

9. The system of any one of claims 1 to 6, wherein the pump wavelength is an ultraviolet wavelength.

10. The system of any preceding claim, wherein selecting the hollow core photonic crystal fiber comprises: The core diameter of the hollow core photonic crystal fiber is selected.

11. The system according to any of the preceding claims, wherein the pulse duration of the pump light pulses is between 50 fs and 700 ps.

12. A metrology arrangement comprising a system according to any one of the preceding claims.

13. A lithographic apparatus comprising a system according to any preceding claim.

14. A method for generating supercontinuum radiation, the method comprising: providing a pump light pulse to a hollow core photonic crystal fiber, wherein the pump light pulse has a pump wavelength, and the hollow core photonic crystal fiber contains a gas; wherein the gas and the hollow core photonic crystal fiber are selected so that the pump wavelength is within a range corresponding to a normal group velocity dispersion state of the gas; and The gas has any of the following: A rotational line spacing greater than 0.5 THz; or There is no rotation of the spectrum. The method according to claim 14 , wherein the rotational line spacing of the gas is greater than 1 THz.

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