Systems and methods for generating supercontinuum radiation

Abstract

SUPERCONTINUUM RADIATION SYSTEM AND METHOD FOR GENERATING SUPERCONTINUUM RADIATION SYSTEM TECHNICAL FIELD [0001] The present invention relates to 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 configured to receive the pump light pulses, the hollow-core photonic crystal fiber containing a gas, the gas and the hollow-core photonic crystal fiber being selected such that the pump wavelength is within a range corresponding to a normal group velocity dispersion region of the gas, the gas either having a rotational line spacing of greater than 0.5 THz or having no rotational lines. Methods for generating supercontinuum radiation are also described.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to European Patent Application Publication No. 23152628.6, filed January 20, 2023, and European Patent Application Publication No. 23153839.8, filed January 30, 2023, which are incorporated by reference in their entireties.

[0002] The present invention relates to systems and methods for generating supercontinuum radiation. [Background technology]

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

[0004] To project a pattern onto a substrate, a lithographic apparatus may use 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. Lithographic apparatus using extreme ultraviolet (EUV) radiation with a wavelength in the range of 4 to 20 nm, for example 6.7 nm or 13.5 nm, can be used to form smaller features on a substrate than lithographic apparatus using radiation with a wavelength of, for example, 193 nm.

[0005] Low k1 lithography can be used to process features with dimensions smaller than the conventional resolution limit of a lithographic apparatus. In such processes, the resolution equation can be expressed as CD=k1×λ / NA, where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection optics of the lithographic apparatus, CD is the “critical dimension” (generally the minimum printed feature size, but in this case the half pitch), and k1 is an empirical resolution factor. In general, the smaller k1 is, the more difficult it is to reproduce on a substrate a pattern similar to the shape and dimensions planned by a circuit designer to achieve a particular electrical function and performance. To overcome these difficulties, advanced fine-tuning steps can be applied to the lithographic projection apparatus and / or the design layout. These include, for example but not limited to, NA optimization, 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 and process correction") in the design layout or other methods commonly defined as "resolution enhancement techniques" (RET). Alternatively, tight control loops that control the stability of the lithographic apparatus can be used to improve the reproducibility of patterns at low k1.

[0006]

[0006] In the field of lithography, many measurement systems may be used both within the lithographic apparatus and external to the lithographic apparatus. Generally, such measurement systems may use a radiation source that irradiates a target with radiation and a detection system operable to measure at least one property of the incident radiation scattered from the target. An example of a measurement system that is external to the lithographic apparatus is an inspection or metrology apparatus, which may be used to determine properties of a pattern previously projected onto a substrate by the lithographic apparatus. Such an external inspection apparatus may include, for example, a scatterometer. Examples of measurement systems that may be provided within the lithographic apparatus include topography measurement systems (also known as level sensors), position measurement systems (e.g., interferometric devices) that determine the position of a reticle or wafer stage, and alignment sensors that determine the position of alignment marks. These measurement devices may perform measurements using electromagnetic radiation.

[0007]

[0007] Different types of radiation can be used to investigate the properties of different types of patterns. Some measurement systems may use a broadband radiation source. Such a broadband radiation source may be a supercontinuum source and may include a waveguide (e.g., an optical fiber) having a nonlinear medium through which a pulsed pump radiation beam propagates in order to broaden the spectrum of the radiation.

[0008]

[0008] A supercontinuum radiation source can be realized by pumping a gas into 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, the spectral broadening of the input radiation provided by the pump light source can be dominated by modulation instability, self-phase modulation, or soliton dynamics. Further details of mechanisms for achieving spectral broadening in soliton-driven broadband radiation generation are provided in WO2022122325, which is incorporated herein by reference in its entirety. Summary of the Invention

[0009]

[0009] As an alternative to the use of gas-filled hollow-core optical fibers in the anomalous group velocity dispersion regime, a supercontinuum radiation source may be realized by pumping a gas (e.g., a molecular gas) into a hollow-core optical fiber using a pump light source (e.g., a narrowband laser) in the normal dispersion regime to generate a broad vibrational Raman frequency comb containing discrete spectral lines. The spectral lines of the frequency comb may be broadened (e.g., by rotationally stimulated Raman scattering and / or the optical Kerr effect) as the radiation propagates through the fiber, potentially achieving a supercontinuum spectrum spanning from the ultraviolet to the mid-infrared (e.g., from less than 400 nm to more than 2000 nm). Advantageously, this technique may reduce shot-to-shot noise as well as improve spectral flatness and spectral reach compared to techniques in which a gas is pumped in the anomalous group velocity dispersion regime.

[0010] However, the present inventors have found that gain suppression can occur when the phonon coherence waves generated by the pump Stokes pulsations are identical to the phonon coherence waves annihilated by the pump anti-Stokes pulsations, thereby balancing the phonon generation and annihilation rates. Stokes lines can still be formed in higher-order modes (HOMs) due to the inter-modal coherence waves.

[0011]

[0011] In relation to supercontinuum generation, gain suppression can reduce the supercontinuum spectral energy density, especially in the normal dispersion region, which typically covers the visible and UV regions, and cause the beam profile to deviate from the fundamental mode. Gain suppression reduces the spectral energy density in the visible spectral region. Beams traveling in higher-order modes have significantly lower contributions to the formation of the supercontinuum due to their lower peak power.

[0012]

[0012] Gain suppression occurs when the rotational shift is small enough, so that the propagation constants of the Stokes and anti-Stokes lines are very similar. The problem of gain suppression can therefore be alleviated by selecting a gas with a rotational line spacing large enough (e.g., greater than 0.5 THz) that the propagation constants of the Stokes and anti-Stokes lines are sufficiently different so that phonon generation and annihilation are not balanced. In some examples, a gas can be selected that has no rotational lines, i.e., no rotational Raman response.

[0013]

[0013] Described herein is a system for generating supercontinuum radiation, the system including 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 an "optical fiber" or "fiber") configured to receive the pump light pulses, the hollow-core photonic crystal fiber containing a gas (HC-PCF, also referred to herein as an "optical fiber" or "fiber"), wherein the gas and HC-PCF are selected such that the pump wavelength is within a range corresponding to the normal group velocity dispersion region of the gas, and the gas either has a rotational line spacing of greater than 0.5 THz or has no rotational lines.

[0014]

[0014] Also described in this specification 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, the gas and the hollow-core photonic crystal fiber being selected such that the pump wavelength is within a range corresponding to the normal group velocity dispersion region of the gas, and the gas either has a rotational line spacing of greater than 0.5 THz or has no rotational lines.

[0015]

[0015] A "rotational line" will be understood to refer to a line observable in the rotational spectrum of a gas, for example, in rotational spectroscopy of the gas (e.g., microwave spectroscopy, infrared spectroscopy, Raman spectroscopy, etc.). A gas that has no rotational lines is one that does not have a rotational Raman response.

[0016]

[0016] The gas is preferably selected to exhibit a vibrational Raman response. Preferably, the gas should exhibit a strong vibrational Raman response in order to achieve a broad vibrational Raman frequency comb.

[0017] It will be further understood that the normal group velocity dispersion region corresponds to the positive group velocity dispersion region, and group velocity dispersion can be expressed as β2. This is in contrast to anomalous group velocity dispersion (i.e., negative β2). It will be understood by those skilled in the art that the normal group velocity dispersion region of a gas refers to the normal group velocity dispersion region of the gas in the HC-PCF.

[0018]

[0018] By pumping the gas in the normal group velocity dispersion regime, the effects of modulation instability are minimized and instead supercontinuum radiation is generated through a cascade process involving vibrational and rotational Raman scattering and the Kerr effect described herein.

[0019]

[0019] The rotational line spacing and / or normal group velocity dispersion region may depend on the pressure of the gas. In some examples, the gas pressure is also selected so that the pump wavelength is within a range corresponding to the normal group velocity dispersion region of the gas at the gas pressure. Selecting the gas may include selecting a gas pressure (e.g., an appropriate gas pressure at a desired rotational line spacing, an amount of nonlinearity, an amount of dispersion, etc.).

[0020]

[0020] The normal group velocity dispersion region of a gas in an HC-PCF depends on the structure of the HC-PCF. Selecting an HC-PCF may therefore include selecting the structure of the HC-PCF (e.g., hollow core size, cladding microstructure, etc.).

[0021]

[0021] Advantageously, gases with rotational line spacing greater than 0.5 THz or gases with no rotational lines (i.e., no rotational Raman response) are not susceptible (or are less susceptible) to the gain suppression problems described above compared to gases with closer rotational line spacings.

[0022] 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.

[0023]

[0023] The gas may include a mixture of gases.

[0024]

[0024] The gas may include a molecular gas.

[0025]

[0025] Preferably, the gas exhibits good linear transmission of radiation across the supercontinuum spectrum.

[0026] In some instances, more than one pump source or pump wavelength is used, in which case at least one of the pump wavelengths is in a range corresponding to the normal group velocity dispersion region of the gas.

[0027]

[0027] Examples of gases with rotational line spacings greater than 0.5 THz include hydrogen (H2) and deuterium (D2).

[0028]

[0028] Examples of gases that do not have rotational lines include methane and SF6.

[0029]

[0029] Thus, the gas may include one or more of H2, D2, methane and SF6.

[0030]

[0030] The selection of a hollow-core crystal 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.

[0031]

[0031] The dispersion profile of a gas depends on the combination of the physical structure of the HC-PCF, the gas species, the gas pressure, and the pump wavelength. For a given HC-PCF and gas species, increasing the gas pressure can shift the zero dispersion wavelength (ZDW) to a longer wavelength. At wavelengths shorter than the ZDW, the group dispersion velocity is normal, while at wavelengths longer than the ZDW, the group dispersion velocity is anomalous. Correspondingly, the gas species, HC-PCF structure, and pump wavelength can be selected so that the ZDW occurs at a specific gas pressure.

[0032] In some examples, the pump wavelength is less than 1000 nm. Thus, advantageously, the normal group velocity dispersion region for a given gas species may be achieved for lower gas pressures than when pump wavelengths of 1000 nm or greater are used. 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 light wavelength (e.g., in the range of about 380 nm to about 800 nm). In some examples, the pump wavelength is an ultraviolet wavelength (e.g., in the range of about 100 nm to about 380 nm). In some examples, the pump wavelength is a green light wavelength (e.g., in the range of about 500 nm to about 565 nm, e.g., about 515 nm).

[0033]

[0033] In some instances, the ZDW, and therefore the normal group velocity dispersion region of the gas, is particularly sensitive to the core diameter of the HC-PCF. Therefore, the selection of the HC-PCF may include the selection of the core diameter of the HC-PCF.

[0034]

[0034] In some examples, the pump light pulses may have a pulse duration of 700 ps or less to avoid the need for 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 the dominance of self-phase modulation over Raman vibration comb formation, i.e., 50 fs or more, preferably 100 fs or more. In some examples, the pump light pulses may have a pulse duration of 50 fs to 700 ps. In some examples, the pump light pulses may have a pulse duration of 300 fs to 700 ps. In some examples, the pump light pulses may have a pulse duration of 300 fs to 27 ps. In some examples, the pulsed light pulses may have a pulse duration of 100 fs to 100 ps.

[0035] Also described herein is a metrology apparatus that includes a system as described herein. Also described is a lithography apparatus that includes a system as described herein.

[0036] It will be understood by those skilled in the art that a "supercontinuum" according to the present disclosure 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 a supercontinuum corresponds to a peak-to-trough spectral power ratio of 100:1, i.e., less than 20 dB. In some examples, the flatness of a supercontinuum corresponds to a peak-to-trough spectral power ratio of 10:1, i.e., less than 10 dB.

[0037]

[0037] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which: [Brief explanation of the drawings]

[0038] [Figure 1]1 depicts a schematic overview of a lithographic apparatus; [Figure 2] 1 depicts a lithography system comprising a lithographic apparatus and a radiation source. [Figure 3] 1 shows a schematic overview of a lithographic cell. [Figure 4] A schematic diagram of holistic lithography is shown, depicting the collaboration between three key technologies to optimize semiconductor manufacturing. [Figure 5] 1 shows a schematic overview of a scatterometry metrology tool; [Figure 6] 1 shows a schematic overview of a level sensor measurement tool. [Figure 7] 1 shows a schematic overview of an alignment sensor metrology tool. [Figure 8] 1 shows a device including a hollow-core photonic crystal fiber for extending the frequency range of received input radiation. [Figure 9] 8 shows an apparatus for extending the frequency range of received input radiation of the type shown in FIG. 7, further including a reservoir. [Figure 10] 10 shows a schematic diagram of a radiation source for providing broadband output radiation, the radiation source including an apparatus for broadening the frequency range of received input radiation, as shown in FIG. [Figure 11] 1 is a schematic cross-sectional view of an example hollow-core photonic crystal fiber in a transverse plane (i.e., perpendicular to the axis of the fiber). [Figure 12] 12 is a schematic cross-sectional view of the exemplary hollow-core photonic crystal fiber shown in FIG. 11, taken in a plane containing the axis of the fiber. [Figure 13] We present the results of numerical simulations of the generation of an extended Raman frequency comb in a hollow-core photonic crystal fiber for supercontinuum radiation generation. [Figure 14] 1 illustrates a schematic diagram of a method for generating supercontinuum radiation according to the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0039]

[0038] In this specification, 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, e.g., having a wavelength in the range of about 5 to 100 nm).

[0040] <Reticle> As used herein, the terms "reticle," "mask," or "patterning device" may be broadly interpreted to refer to a general patterning device that can be used to provide an incident radiation beam with a patterned cross-section that corresponds to the pattern to be formed in a target portion of a substrate. The term "light valve" may also be used in this context. Besides conventional masks (transmissive or reflective, binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0041] 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA comprises: an illumination system (also called an illuminator) IL configured to condition a radiation beam B (e.g. UV radiation, DUV radiation or EUV radiation), a mask support (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to certain parameters, a substrate support (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support according to certain parameters, and a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0042]

[0041] In operation, the illumination system IL receives a radiation beam from a radiation source SO (e.g. via a beam delivery system BD). The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical components, or any combination thereof, for directing, shaping and / or controlling the radiation. 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.

[0043]

[0042] As used herein, the term "projection system" PS should be interpreted broadly as encompassing various types of projection systems, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as appropriate, depending on the exposure radiation used and / or other factors such as the use of an immersion liquid or the use of a vacuum. Where the term "projection lens" is used herein, it may be considered as synonymous with the more general term "projection system" PS.

[0044]

[0043] The lithographic apparatus LA may be configured for what is known as immersion lithography, in which at least a portion of the substrate is covered with a liquid having a relatively high refractive index, such as water, to fill a gap between the projection system PS and the substrate W. Further information about immersion techniques is given in US Patent No. 6,952,253, which is incorporated herein by reference.

[0045] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also known as "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or while steps to prepare a substrate W for subsequent exposure are being performed on a substrate W placed on one substrate support WT, another substrate W placed on another substrate support WT may be used to expose a pattern onto the other substrate W.

[0046] In addition to the substrate support WT, the lithographic apparatus LA may include a measurement stage. The measurement stage is configured to hold a sensor and / or a cleaning device. The sensor may be configured to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold multiple sensors. The cleaning device may be configured to clean part of the lithographic apparatus, for example part of the projection system PS or part of the system for supplying immersion liquid. The measurement stage may move below the projection system PS when the substrate support WT is spaced apart from the projection system PS.

[0047] In operation, the radiation beam B is incident on a patterning device, for example a mask MA, which is held on a mask support MT, and is patterned according to a pattern (design layout) present on the patterning device MA. After traversing the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. Using the second positioner PW and the position measurement system IF, the substrate support WT can be accurately moved, for example to position different target portions C to focused and aligned positions in the path of the radiation beam B. Similarly, the first positioner PM and possibly further position sensors (not explicitly shown in FIG. 1 ) can be used to accurately position the patterning device MA with respect to the path of the radiation beam B. The patterning device MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the illustrated substrate alignment marks P1, P2 occupy dedicated target portions, they may be located in spaces between the target portions. When the substrate alignment marks P1, P2 are located between target portions C, they are known as scribe-lane alignment marks.

[0048] 2 shows a lithography system including a radiation source SO and a lithography apparatus LA. The radiation source SO is configured to generate a beam of EUV radiation B and to provide the beam of EUV radiation B to the lithography apparatus LA. The lithography apparatus LA includes an illumination system IL, a support structure MT configured to support a patterning device MA (e.g. a mask), a projection system PS, and a substrate table WT configured to support a substrate W.

[0049] The illumination system IL is configured to condition the EUV radiation beam B before it is incident on the patterning device MA. Thereby, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The facetted field mirror device 10 and the facetted pupil mirror device 11 together provide the EUV radiation beam B having a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to or instead of the facetted field mirror device 10 and the facetted pupil mirror device 11.

[0050] After being conditioned in this way, the EUV radiation beam B interacts with the patterning device MA. This interaction results in a patterned EUV radiation beam B'. The projection system PS is configured to project the patterned EUV radiation beam B' onto the substrate W. To that end, the projection system PS may include a plurality of mirrors 13, 14 configured to project the patterned EUV radiation beam B' onto the substrate W held by a substrate table WT. The projection system PS may apply a demagnification factor to the patterned EUV radiation beam B' to form an image having smaller features than corresponding features on the patterning device MA. For example, a demagnification factor of 4 or 8 may be applied. Although in Figure 2 the projection system PS is shown as having only two mirrors 13, 14, the projection system PS may also include a different number of mirrors (e.g. 6 or 8 mirrors).

[0051]

[0050] The substrate W may include a previously formed pattern, in which case the lithographic apparatus LA aligns the image formed by the patterned EUV radiation beam B' with the previously formed pattern on the substrate W.

[0052] A relative vacuum, ie a small amount of gas (eg hydrogen) at a pressure well below atmospheric pressure, may be provided to the source SO, illumination system IL and / or projection system PS.

[0053]

[0052] The source SO may be a laser-produced plasma (LPP) source, a discharge-produced plasma (DPP) source, a free-electron laser (FEL) or any other source capable of producing EUV radiation.

[0054] As shown in Figure 3, the lithographic apparatus LA may form part of a lithographic cell LC, sometimes referred to as a lithocell or (litho)cluster, which often also includes apparatus for performing pre-exposure and post-exposure processes on the substrate W. Conventionally, these include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, and chill plates CH and bake plates BK for adjusting the temperature of the substrate W, for example to adjust the solvent in the resist layer. A substrate handler or robot RO picks up the substrate W from input / output ports I / O1, I / O2, moves it between the various process tools, and delivers it to a loading bay LB of the lithographic apparatus LA. The devices in the lithocell, often collectively referred to as a track, are typically under the control of a track control unit TCU, which may itself be controlled by a supervisory control system SCS, which may also control the lithographic apparatus LA, for example via a lithography control unit LACU.

[0055] To ensure that substrates W exposed by lithographic apparatus LA are exposed accurately and consistently, it is desirable to inspect the substrates to measure properties of the patterned structures, such as overlay error between subsequent layers, line thickness, critical dimension (CD), etc. For this purpose, an inspection tool (not shown) may be included in the lithocell LC. If errors are detected, particularly if inspection is performed before other substrates W of the same batch or lot are exposed or processed, adjustments may be made, for example to the exposure of subsequent substrates or other process steps performed on the substrate W.

[0056] An inspection apparatus, which may also be called a metrology apparatus, is used to determine the properties of a substrate W, in particular to determine how the properties of different substrates W vary, or how properties associated with different layers of the same substrate W vary from layer to layer. The inspection apparatus may alternatively be constructed to identify defects on a substrate W and may, for example, be part of a lithocell LC, or integrated into a lithographic apparatus LA, or even be a stand-alone apparatus. The inspection apparatus may measure properties of a latent image (an image in a resist layer after exposure), or properties of a semi-latent image (an image in a resist layer after a post-exposure bake step PEB), or properties of a developed resist image (from which exposed or unexposed parts of the resist have been removed), or even properties of an etched image (after a pattern transfer step such as etching).

[0057] The patterning process in the lithography apparatus LA is typically one of the most critical steps in processing, requiring highly accurate sizing and placement of structures on the substrate W. To ensure this high accuracy, three systems can be combined into a so-called "holistic" control environment, as shown schematically in FIG. 4. One of these systems is the lithography apparatus LA (virtually) connected to a metrology tool MT (second system) and a computer system CL (third system). The essence of such a "holistic" environment is to optimize the cooperation between these three systems to enforce the overall process window and to provide a strict control loop to ensure that the patterning performed by the lithography apparatus LA stays within the process window. The process window defines the range of process parameters (e.g., dose, focus, overlay) within which a particular manufacturing process will produce a specified result (e.g., a functional semiconductor device), and typically allows the process parameters of the lithography or patterning process to vary.

[0058] The computer system CL may use (part of) the design layout to be patterned to predict which resolution enhancement techniques to use and which computer lithography simulations and calculations to perform to determine the mask layout and lithography apparatus settings that maximize the overall process window of the patterning process (shown at a first scale SC1 in FIG. 4 by a double-headed arrow). Typically, the resolution enhancement techniques are configured to match the patterning capabilities of the lithography apparatus LA. The computer system CL may also be used to detect where within the process window the lithography apparatus LA is currently operating (e.g., using input from the metrology tool MT) to predict whether defects are present, for example due to sub-optimal processing (shown at a second scale SC2 in FIG. 4 by an arrow pointing to "0").

[0059]

[0058] The metrology tool MT may provide input to the computer system CL to enable accurate simulations and predictions, for example providing feedback to the lithographic apparatus LA identifying possible drifts in the calibration status of the lithographic apparatus LA (shown at a third scale SC3 in Figure 4 by multiple arrows).

[0060]

[0059] In lithographic processes, it is desirable to frequently measure the structures formed, for example for process control and verification. Tools that perform such measurements are typically called metrology tools MT. Different types of metrology tools MT are known, including scanning electron microscopes or various types of scatterometer metrology tools MT that perform such measurements. A scatterometer is a versatile instrument that allows the measurement of parameters of a lithographic process, either by having a sensor in the pupil or in a plane conjugate to the pupil of the scatterometer objective, which is usually called pupil-based measurements, or by having a sensor in the image plane or in a plane conjugate to the image plane, which is in this case usually called image- or field-based measurements. Such scatterometers and associated measurement techniques are further described in U.S. Patent Application Publication No. 20100328655, U.S. Patent Application Publication No. 2011102753A1, U.S. Patent Application Publication No. 20120044470A, U.S. Patent Application Publication No. 20110249244, U.S. Patent Application Publication No. 20110026032, or European Patent Application Publication No. 1,628,164A, the entire contents of which are incorporated by reference. The scatterometers described above can measure gratings using soft x-rays and light in the visible to near-infrared wavelength range.

[0061] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, reconstruction methods can be applied to the measured signals to reconstruct or calculate the properties of the grating. Such reconstruction can, for example, 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 that observed from the real target.

[0062]

[0061] 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 to a target, and radiation reflected or scattered from the target is directed to a spectrometer detector, which measures the spectrum of the specularly reflected radiation (i.e., measures the intensity as a function of wavelength). From this data, the structure or profile of the target giving rise to the detected spectrum can be reconstructed, for example by rigorous coupled wave analysis and non-linear regression or comparison with a library of simulated spectra.

[0063] In a third embodiment, the scatterometer MT is an elliptically polarized scatterometer. An elliptically polarized scatterometer makes it possible to determine parameters of the lithographic process by measuring scattered radiation for each polarization state. Such metrology tools emit polarized light (linear, circular, elliptical, etc.), for example by using appropriate polarizing filters in the illumination section of the metrology tool. A radiation source suitable for the metrology tool can also provide polarized radiation. Various embodiments of existing ellipsometric scatterometers are described in U.S. Patent Application Publication Nos. 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,449, 12 / 920,968, 12 / 922,587, 13 / 000,229, 13 / 033,135, 13 / 533,110, and 13 / 891,410, which are incorporated by reference herein in their entireties.

[0064] In one embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the overlay of two misaligned grating or periodic structures by measuring asymmetry in the reflectance spectra and / or detection configuration, where the asymmetry is related to the degree of overlay. The two (typically overlapping) grating structures can be applied to two different (not necessarily consecutive) layers and formed at substantially the same location on the wafer. The scatterometer can have a symmetric detection configuration such that any asymmetry is clearly distinguishable, as described, for example, in commonly owned European Patent Application Publication No. 1,628,164 A. This provides a simple method for measuring grating misalignment. Further examples of measuring the overlay error between two layers containing periodic structures while the target is measured via the asymmetry of the periodic structures can be found in PCT Patent Application Publication No. WO 2011 / 012624 or U.S. Patent Application Publication No. 20160161863, the entire contents of which are incorporated herein by reference.

[0065] Other parameters of interest can be focus and dose. Focus and dose can be determined simultaneously by scatterometry (or alternatively, scanning electron microscopy), as described in U.S. Patent Application Publication No. 2011-0249244, the entire contents of which are incorporated herein by reference. A single structure can be used that has a unique combination of critical dimension and sidewall angle measurements for each point in the focus-energy matrix (FEM, also called focus-exposure matrix). When these unique combinations of critical dimension and sidewall angle are available, focus and dose values ​​can be uniquely determined from these measurements.

[0066]

[0065] The metrology target may be an ensemble of composite gratings formed mostly in the resist by the lithography process, but also, for example, after an etching process. Typically, the pitch and linewidth of the grating's structures strongly depend on the measurement optics (especially the NA of the optics) so that the diffraction orders from the metrology target can be captured. As previously indicated, the diffraction signal can be used to determine the shift between two layers (also called "overlay") or to reconstruct at least a portion of the original grating produced by the lithography process. This reconstruction can be used to provide an indication of the quality of the lithography process and can be used to control at least a portion of the lithography process. The target may have smaller subsections configured to mimic the dimensions of the features of the design layout within the target. This subsection will cause the target to behave more similarly to the features of the design layout, so that measurements of overall process parameters more closely resemble the features of the design layout. The target may be measured in an unfilled mode or an overfilled mode. In the unfilled mode, the measurement beam generates a spot smaller than the entire target. In overfill mode, the measurement beam creates a spot that is larger than the entire target. In such overfill mode, different targets can also be measured simultaneously, allowing different process parameters to be determined simultaneously.

[0067] The overall measurement quality of a lithography parameter using a particular target is determined at least in part by the measurement recipe used to measure this lithography parameter. The term "substrate measurement recipe" 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 recipe is a diffraction-based optical measurement, one or more of the parameters of the measurement may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation to the substrate, the orientation of the radiation relative to the pattern on the substrate, etc. One criterion for selecting a measurement recipe may be, for example, the sensitivity of one measurement parameter to process variations. Further examples are described in U.S. Patent Application Publication No. 2016-0161863 and U.S. Patent Application Publication No. 2016 / 0370717A1, which are incorporated herein by reference in their entireties.

[0068] A metrology apparatus such as a scatterometer SM1 is shown in Figure 5. It includes a broadband (white light) radiation projector 2 which projects radiation onto a substrate W. Reflected or scattered radiation is passed to a spectrometer detector 4 which measures the spectrum 6 of the specularly reflected radiation (i.e. a measurement of the intensity I as a function of wavelength). From this data, the structure or profile 8 giving rise to the detected spectrum can be reconstructed by a processing unit PU, for example by rigorous coupled-wave analysis and non-linear regression or by comparison with a library of simulated spectra as shown at the bottom of Figure 5. Typically, for reconstruction, the general morphology of the structure is known and some parameters are assumed from knowledge of the process by which the structure was created, leaving only a few parameters of the structure to be determined from the scatterometry data. Such a scatterometer may be configured as a normal-incidence scatterometer or an oblique-incidence scatterometer.

[0069]

[0068] In lithography processes, it is desirable to frequently measure the formed structures, for example, for process control and verification. Various tools are known for performing such measurements, including various types of metrology devices such as scanning electron microscopes or scatterometers. Examples of known scatterometers often rely on providing dedicated metrology targets, such as unfilled targets (targets in the form of simple or overlapping gratings in different layers that are large enough that the measurement beam generates a spot smaller than the grating) or overfilled targets (whereby the illumination spot partially or completely covers the target). Furthermore, the use of metrology tools, such as angle-resolved scatterometers that illuminate unfilled targets such as gratings, allows the use of so-called reconstruction methods, which simulate the interaction of scattered radiation with a mathematical model of the target structure and compare the simulation results with measured results, thereby allowing the properties of the grating to be calculated. The parameters of the model are adjusted until the simulated interaction generates a diffraction pattern similar to that observed from the actual target.

[0070]

[0069] Scatterometers are versatile instruments that enable the measurement of parameters of lithography processes, either with a sensor at the pupil or a conjugate plane of the scatterometer objective, typically referred to as pupil-based measurements, or with a sensor at the image plane or a conjugate plane of the image plane, typically referred to as image- or field-based measurements. Such scatterometers and related measurement techniques are further described in U.S. Patent Application Publication Nos. 20100328655, 2011102753A1, 20120044470A, 20110249244, 20110026032, or EP 1,628,164A, the entire contents of which are incorporated herein by reference. The scatterometers described above can measure multiple targets from multiple gratings in a single image using soft x-rays and light in the visible to near-infrared wavelength range.

[0071] A topography measurement system, level sensor or height sensor, which may be integrated with a lithographic apparatus, is arranged to measure the topography of the upper surface of a substrate (or wafer). A map of the topography of the substrate, also called a height map, can be generated from these measurements, showing 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 a pattern on the substrate, in order to generate an aerial image of the patterning device at a properly focused position on the substrate. In this context, it will be understood that "height" refers generally to the dimension (also called the Z-axis) away from the substrate. Typically, a level or height sensor performs measurements at a fixed position (relative to its own optics), and relative movement between the substrate and the optics of the level or height sensor results in height measurements at multiple positions across the substrate.

[0072] An example of a level or height sensor LS known in the art is shown schematically in FIG. 6, illustrating only the principle of operation. In this example, the level sensor includes an optical system, which includes a projection unit LSP and a detection unit LSD. The projection unit LSP includes a radiation source LSO that provides a radiation beam LSB emitted by the projection grating PGR of the projection unit LSP. The radiation source LSO can be, for example, a polarized or unpolarized light source, such as a polarized or unpolarized laser beam, a narrowband or broadband light source, such as a pulsed or continuous supercontinuum light source. The radiation source LSO can include multiple radiation sources with different colors or wavelength ranges, such as multiple LEDs. The radiation source LSO of the level sensor LS is not limited to visible radiation but can additionally or alternatively include UV and / or IR radiation and any wavelength range suitable for being reflected from the surface of the substrate.

[0073] The projection grating PGR is a periodic grating comprising a periodic structure which results in a radiation beam BE1 having a periodically varying intensity. The radiation beam BE1 having a periodically varying intensity is directed towards a measurement location MLO on the substrate W with an angle of incidence ANG of between 0 and 90 degrees, typically between 70 and 80 degrees, relative to an axis normal to the incident substrate surface (the Z axis). At the measurement location MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and directed towards a detection unit LSD.

[0074]

[0073] To determine the height level at the measurement position MLO, the level sensor further comprises a detection system including a detection grid DGR, a detector DET and a processing unit (not shown) for processing the output signal of the detector DET. The detection grid DGR may be identical to the projection grid PGR. The detector DET generates a detector output signal indicative of the received light, for example a photodetector, indicating the intensity of the received light, or representing the spatial distribution of the received intensity, for example a camera. The detector DET may comprise any combination of one or more types of detectors.

[0075]

[0074] By means of triangulation techniques, the height level at the measurement location MLO can be determined. The detected height level is typically related to the signal intensity measured by the detector DET, which has a periodicity that depends, inter alia, on the design of the projection grating PGR and the (oblique) angle of incidence ANG.

[0076]

[0075] The projection unit LSP and / or the detection unit LSD may include further optical elements such as lenses and / or mirrors along the path of the patterned radiation beam between the projection grating PGR and the detection grating DGR (not shown).

[0077] In one embodiment, the detection grid DGR may be omitted and the detector DET may be placed at the position where the detection grid DGR is placed. Such a configuration provides a more direct detection of the image of the projection grid PGR.

[0078]

[0077] In order to effectively cover the surface of the substrate W, the level sensor LS may be configured to generate an array of measurement areas MLO or spots covering a wider measurement range by projecting an array of measurement beams BE1 onto the surface of the substrate W.

[0079]

[0078] Various height sensors of the general type are disclosed, for example, in U.S. Patent No. 7,265,364 and U.S. Patent No. 7,646,471, both of which are incorporated herein by reference. A height sensor that uses UV radiation as an alternative to visible or infrared radiation is disclosed in U.S. Patent Application Publication No. 2010233600A1, which is incorporated by reference. WO 2016102127A1, which is incorporated by reference, describes a compact height sensor that uses a multi-element detector to detect and recognize the position of a grid image without the need for a detection grid.

[0080] 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, for example the measurement 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 a time derivative of the position, such as the velocity or acceleration.

[0081]

[0080] The position measurement system may include an encoder system. Encoder systems are known, for example, from U.S. Patent Application Publication No. 2007 / 0058173 A1, filed September 7, 2006, which is incorporated herein by reference. The encoder system includes an encoder head, a grating, and a sensor. The encoder system can receive a primary radiation beam and a secondary radiation beam. Both the primary radiation beam and the secondary radiation beam originate from the same radiation beam, i.e., the original radiation beam. At least one of the primary radiation beam and the secondary radiation beam is generated by diffracting the original radiation beam with a grating. If both the primary radiation beam and the secondary radiation beam are generated by diffracting the original radiation beam with a grating, the diffraction order of the primary radiation beam must be different from that of the secondary radiation beam. The different diffraction orders are, for example, +1st order, -1st order, +2nd order, and -2nd order. 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 grating may be disposed on the substrate structure WT. The other of the encoder head and the grating may be disposed on the measurement frame MF or the base frame BF. For example, the encoder heads are disposed on the measurement frame MF, while the grating is disposed on the top surface of the substrate support WT. In another example, the grating is disposed on the bottom surface of the substrate support WT, and the encoder head is disposed below the substrate support WT.

[0082] The position measurement system may include an interferometer system. Interferometer systems are known, for example, from U.S. Pat. No. 6,020,964, filed July 13, 1998, which is incorporated herein by reference. The interferometer system may include a beam splitter, a mirror, a reference mirror, and a sensor. A radiation beam is split into a reference beam and a measurement beam by the beam splitter. The measurement beam propagates to the mirror and is reflected by the mirror back to the beam splitter. The reference beam propagates to the reference mirror and is reflected by the reference mirror back to the beam splitter. 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 a 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 mirror. In one embodiment, the mirror is connected to the substrate support WT. The reference mirror may be connected to the measurement frame MF. In one embodiment, the measurement and reference beams are combined into a combined radiation beam by additional optical components rather than by a beam splitter.

[0083]

[0082] In the manufacture of complex devices, typically many lithographic patterning steps are performed, thereby forming functional features in successive layers on a substrate. An important aspect of the performance of a lithographic apparatus is therefore the ability to correctly and accurately align an applied pattern with respect to features deposited in a previous layer (by the same apparatus or a different lithographic apparatus). For this purpose, the substrate is provided with a set of one or more marks. Each mark is a structure whose position can be measured at a later point in time using a position sensor, typically an optical position sensor. The position sensor may be called an "alignment sensor" and the marks may be called "alignment marks". The marks may also be called metrology targets.

[0084] A lithographic apparatus may include one or more (e.g., a plurality) alignment sensors capable of accurately measuring the positions of alignment marks provided on a substrate. Alignment (or position) sensors may use optical phenomena such as diffraction and interference to obtain position information from alignment marks formed on the substrate. An example of an alignment sensor used in conventional lithographic apparatus is based on a self-referencing interferometer, such as that described in U.S. Pat. No. 6,961,116. Various developments and improvements of position sensors have been developed, for example, as disclosed in U.S. Patent Application Publication No. 2015261097A1. The entire contents of these publications are incorporated herein by reference.

[0085] The mark or alignment mark may comprise a series of bars formed on or within a layer provided on the substrate or formed (directly) within the substrate. The bars may be spaced at regular intervals and function as grating lines such 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 may be designed to allow measurement of position along the X axis or along the Y axis (substantially perpendicular to the X axis). Marks containing bars positioned at +45 degrees and / or -45 degrees relative to both the X and Y axes allow combined X and Y measurements to be made using the techniques described in U.S. Patent Application Publication No. 2009 / 195768A, which is incorporated by reference.

[0086]

[0085] 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 then fixed with respect to the reference system of the lithographic apparatus. So-called coarse and fine marks, which relate to different (coarse and fine) mark dimensions, can be provided so that the alignment sensor can distinguish the exact position (phase) between and within different cycles of the periodic signal. Marks of different pitches can also be used for this purpose.

[0087]

[0086] Measuring the position of the marks can also provide information, for example in the form of a wafer grid, about deformation of the substrate on which the marks are provided, which may be caused, for example, by electrostatic clamping of the substrate to the substrate table and / or heating of the substrate when exposed to radiation.

[0088]

[0087] Figure 7 is a schematic block diagram of an embodiment of a known alignment sensor AS, for example as described in US Patent No. 6,961,116, which is incorporated by reference. The alignment sensor may be incorporated into a lithography system, for example as shown in Figure 2 and described herein. A radiation source RSO provides a radiation beam RB of one or more wavelengths that is diverted by diverter optics as an illumination spot SP onto a mark, such as a mark AM, disposed on a substrate W. In this example, the diverter optics includes a spot mirror SM and an objective lens OL. The illumination spot SP that illuminates the mark AM may have a diameter that is slightly smaller than the width of the mark itself.

[0089]

[0088] Radiation diffracted by the alignment mark AM is collimated (in this example via an objective lens OL) into an information-bearing beam IB. The term "diffracted" is intended to include zero-order diffraction (which may be called reflection) from the mark. For example, a self-referencing interferometer SRI of the type disclosed in the above-mentioned U.S. Pat. No. 6,961,116 interferes with the beam IB with itself, which is then received by a photodetector PD. If more than one wavelength is produced by the radiation source RSO, additional optics (not shown) may be included to provide separate beams. The photodetector may be a single element or may include multiple pixels as required. The photodetector may include a sensor array.

[0090]

[0089] The bypass optical system, which in this example includes a spot mirror SM, can also function to block zero-order radiation reflected from the mark, so that the information-carrying beam IB contains only higher-order diffracted radiation from the mark AM (this is not essential for the measurement, but improves the signal-to-noise ratio).

[0091]

[0090] The intensity signal SI is fed to a processing unit PU. The combination of optical processing in block SRI and computational processing in unit PU outputs values ​​for the X and Y position on the substrate relative to a reference frame.

[0092] A single measurement of the type shown merely fixes the position of the mark within a certain range corresponding to one pitch of the mark. Coarser measurement techniques are used in conjunction with this to identify which period of the sine wave contains the marked position. The same process can be repeated at coarser and / or finer levels with different wavelengths for improved accuracy and / or robust detection of the mark, regardless of the material from which the mark is made and the material on which it is provided and / or beneath. Wavelengths can be optically multiplexed and demultiplexed to be processed simultaneously, and / or wavelengths can be multiplexed by time or frequency division.

[0093] In this example, the alignment sensor and spot SP remain stationary, while the substrate W moves. The alignment sensor can therefore be reliably and accurately mounted to a reference frame, effectively scanning the mark AM in a direction opposite to the direction of movement of the substrate W. In this movement, the substrate W is controlled by a substrate positioning system, which controls its mounting to the substrate support and the movement of the substrate support. A substrate support position sensor (e.g., an interferometer) measures the position of the substrate support (not shown). In one embodiment, one or more (alignment) marks are provided on the substrate support. Measuring the positions of the marks provided on the substrate support makes it possible to calibrate the position of the substrate support determined by the position sensor (e.g., relative to a frame to which the alignment system is connected). Measuring the positions of the alignment marks provided on the substrate makes it possible to determine the position of the substrate relative to the substrate support.

[0094]

[0093] Metrology tools MT, such as the scatterometers, topography measurement systems, or position measurement systems described above, can perform measurements using radiation emitted from a radiation source. The characteristics of the radiation used by the metrology tool can affect the type and quality of measurements that can be made. In some applications, it can be advantageous to measure the substrate using multiple radiation frequencies, for example, broadband radiation can be used. Multiple different frequencies can be capable of propagating, irradiating, and scattering from the metrology target with no or minimal interference with other frequencies. Thus, for example, different frequencies can be used to simultaneously obtain more metrology data. Different radiation frequencies can also be capable of investigating and discovering different characteristics of the metrology target. Broadband radiation can be useful in metrology systems MT, such as level sensors, alignment mark measurement systems, scatterometry tools, or inspection tools. The broadband radiation source can be a supercontinuum light source.

[0095]

[0094] High-quality broadband radiation, such as supercontinuum radiation, can be difficult to generate. One way to generate broadband radiation can be to magnify high-power narrowband or single-frequency input radiation, for example, by utilizing nonlinear higher-order effects. The input radiation (which can be generated using a laser) can be called pump radiation. To obtain high-power radiation for the magnification effect, the radiation can be confined within a small region such that strong, localized, high-intensity radiation is achieved. In those regions, the radiation can interact with magnifying structures and / or materials that form a nonlinear medium to generate broadband output radiation. In high-intensity radiation regions, different materials and / or structures can be used to enable and / or enhance radiation magnification by providing a suitable nonlinear medium.

[0096]

[0095] In some implementations, as further described below with reference to Figures 8-10, methods and apparatus for expanding input radiation may use a fiber to confine the input radiation and expand it to output broadband radiation. The fiber may be a hollow-core fiber and may include an internal structure for effectively guiding and confining radiation within the fiber. The fiber may be a hollow-core photonic crystal fiber (HC-PCF), which is particularly suitable for confining intense radiation primarily within the hollow core of the fiber and can achieve high radiation intensity. The hollow core of the fiber may be filled with a gas that serves as a expanding medium for expanding the input radiation. Such a fiber and gas configuration can be used to generate a supercontinuum radiation source. The radiation input to the fiber may be electromagnetic radiation, such as one or more radiation in the infrared, visible, UV, and extreme UV spectrums. The output radiation may consist of or include broadband radiation, which may be referred to herein as white light.

[0097] 8 shows a schematic diagram of a general configuration of an apparatus 120 for receiving input radiation 122 and expanding the frequency range of the input radiation 122 to provide broadband output radiation 124. The apparatus 120 includes an optical fiber 100 having a hollow core 102 (i.e., an HC-PCF) for guiding radiation propagating through the optical fiber 100. The apparatus 120 further includes a gas 126 disposed within the hollow core 102, the gas including an active component that enables expanding the frequency range of the received input radiation 126 to provide broadband output radiation 124.

[0098]

[0097] The active component of gas 126 may include a molecular gas (e.g., N2, O2, CH4, SF6). In some examples, the active component of gas 126 may include a noble gas (e.g., one or more of argon, krypton, neon, helium, and xenon).

[0099] In one implementation, gas 126 may be present within hollow core 102 at least while apparatus 120 is receiving input radiation 122 for generating broadband output radiation 124. It will be understood that gas 126 may be entirely or partially absent within hollow core 102 while apparatus 120 is not receiving input radiation 122 for generating broadband output radiation. Generally, apparatus 120 includes an apparatus for providing gas 126 within hollow core 102 of optical fiber 100. The apparatus for providing gas 126 within hollow core 102 of optical fiber 100 may include a reservoir, as described herein with reference to FIG. 9 .

[0100] FIG. 9 illustrates the apparatus 120 shown in FIG. 8 , further including 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 a container. The reservoir 128 is configured to contain the 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. During 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 a portion of a wall of the reservoir 128. The first transparent window 130 may be transparent to at least the received input radiation frequency such that the received input radiation 122 (or at least a majority thereof) can be coupled into the optical fiber 100 disposed within the reservoir 128. The reservoir 128 may include a second transparent window 132 that forms part of a wall of the reservoir 128. In use, when the optical fiber 100 is disposed within 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.

[0101] Alternatively, in another embodiment, both ends of the optical fiber 100 may be disposed in different reservoirs. The optical fiber 100 may include a first terminal section configured to receive the input radiation 122 and a second terminal section for outputting the broadband output radiation 124. The first terminal section may be disposed in a first reservoir containing the gas 126. The second terminal section may be disposed in a second reservoir, which may also contain the gas 126. The function of the reservoirs may be as described above with respect to FIG. 9. The first reservoir may include a first transparent window configured to be transparent to the input radiation 122. The second reservoir may include a second transparent window configured to be transparent to the broadband output broadband radiation 124. The first and second reservoirs may also include sealable openings that allow the optical fiber 100 to be disposed partially inside and partially outside the reservoirs, such that the gas is sealed within the reservoirs. The optical fiber 100 may further include an intermediate section that is not contained within a reservoir. Such a configuration using two separate gas reservoirs may be particularly advantageous for embodiments in which the optical fiber 100 is relatively long (e.g., greater than 1 meter in length). It will be understood that in a configuration using two separate gas reservoirs, the two reservoirs (which may include one or more features known in the art for controlling, regulating, and / or monitoring the composition of the gas 126 in the two reservoirs) are considered to provide a means for supplying the gas 126 within the hollow core 102 of the optical fiber 100.

[0102] [000101] In this regard, a window may be transparent to a frequency if at least 50%, 75%, 85%, 90%, 95% or 99% of incident radiation at the frequency of the window is transmitted through the window.

[0103] [000102] Both the first transparent window 130 and the second transparent window 132 may form an airtight seal within the walls of the reservoir 128 such that the gas 126 may be contained within the reservoir 128. It will be understood that the gas 126 may be contained within the reservoir 128 at a pressure different from the ambient pressure of the reservoir 128.

[0104] [000103] 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 may be achieved through strong spatial confinement of radiation propagating through the optical fiber 100, thereby achieving high local radiation intensities. In addition, hollow-core designs may result in higher quality transmission modes (e.g., having a greater proportion of single-mode transmission) (e.g., compared to solid-core designs). The radiation intensity within the optical fiber 100 may be high due to, for example, the received high-intensity input radiation and / or the strong spatial confinement of the radiation within the optical fiber 100.

[0105] [000104] An advantage of using a hollow core optical fiber 100 may be that most of the radiation guided in the optical fiber 100 is confined to the hollow core 102. Therefore, most of the interactions of the radiation in the optical fiber 100 occur with the gas 126 provided in the hollow core 102 of the optical fiber 100. As a result, the magnifying effect of the gas 126 on the radiation of the active component may be enhanced.

[0106] [000105] 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. Various mechanisms are possible for broadening the spectrum when the radiation interacts with the gas 126, such as four-wave mixing, modulation instability, ionization of the active gas, the Raman effect, Kerr nonlinearity, soliton formation, or soliton splitting. The present disclosure particularly relates to generating broadened (i.e., supercontinuum) radiation via the generation of Raman combs described herein. The input radiation 122 may be coherent radiation. The input radiation 122 may be collimated radiation, which may have the advantage of facilitating and improving 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 range of frequencies. The input radiation 122 may be generated by a laser. Similarly, the output radiation 124 may be collimated and / or coherent.

[0107] [000106] The broadband range of the output radiation 124 can be a continuous range, including a continuous range of radiation frequencies. The output radiation 124 can include supercontinuum radiation. Continuum radiation can be beneficial for use in some applications, such as metrology applications. For example, a continuous range of frequencies can be used to investigate multiple properties. A continuous range of frequencies can be used, for example, to determine and / or eliminate frequency dependence of a measured property. The supercontinuum output radiation 124 can include electromagnetic radiation over a wavelength range of, for example, 100 nm to 4000 nm. The frequency range of the broadband output radiation 124 can be, for example, 400 nm to 900 nm, 500 nm to 900 nm, or 200 nm to 2000 nm. The supercontinuum output radiation 124 can include white light.

[0108] [000107] Figure 10 shows a radiation source 134 that provides broadband output radiation. The radiation source 134 includes the apparatus 120 described above with reference to Figure 9. The radiation source 134 further includes an input radiation source 136 configured to provide input radiation 122 to the apparatus 120. The apparatus 120 may receive input radiation 122 from the input radiation source 136 and magnify it to provide output radiation 124.

[0109] [000108] The input radiation 122 provided by the input radiation source 136 may be pulsed. The input radiation 122 may include electromagnetic radiation having one or more frequencies between 200 nm and 2 μm. The input radiation 122 may include, for example, electromagnetic radiation having a wavelength of 1.03 μm, 515 μm, or 343 μm. The repetition rate of the pulsed radiation 122 may be on the order of 1 kHz to 100 MHz. The pulse energy may be on the order of 0.1 μJ to 100 μJ, for example, 1 to 10 μJ. The pulse length of the input radiation 122 may be on the order of 10 fs to 100 ps, ​​or 10 fs to 10 ps, ​​for example, 300 fs. The average power of the input radiation 122 may be from 100 mW to several hundred W. The average power of the input radiation 122 may be, for example, 20 to 50 W.

[0110] [000109] The broadband output radiation 124 provided by the radiation source 134 may have an average output power of at least 1 W. The average output power may be at least 5 W. The average output power may be at least 10 W. The broadband output radiation 124 may be pulsed broadband output radiation 124. The broadband output radiation 124 may have a power spectral density of at least 0.01 mW / nm across the wavelength band of the output radiation. The power spectral density of the broadband output radiation across the wavelength band may be at least 3 mW / nm.

[0111] [000110] The radiation source 134 described above may be provided as part of a metrology configuration for determining a parameter of interest of a structure on a substrate. The structure on the substrate may be, for example, a lithography pattern applied to the substrate. The metrology configuration may further include an illumination subsystem for illuminating the structure on the substrate. The metrology configuration may further include a detection subsystem for detecting a portion of the radiation scattered and / or reflected by the structure. The detection subsystem may further determine the parameter of interest of the structure from the portion of the radiation scattered and / or reflected by the structure. The parameter may be, for example, overlay, alignment or leveling data of the structure on the substrate.

[0112] [000111] A system for generating supercontinuum radiation according to the present disclosure may include the apparatus 120 of FIGS. 8 and 9 and / or the radiation source 134 of FIG.

[0113] [000112] Figures 11 and 12 show examples of optical fibers (ie, HC-PCFs) that may be used in the systems and / or methods of the present disclosure.

[0114] [000113] The optical fiber 100 includes an elongated body in which one dimension of the fiber 100 is longer than the other two dimensions. This 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, referred to as the transverse plane, define a plane. Figure 11 shows a cross-section of the optical fiber 100 in the transverse plane (i.e., perpendicular to the axis 101), which is labeled as the xy plane. Figure 12 shows a cross-section of the optical fiber 100 in a plane that includes the axis 101, specifically the xz plane. The cross-section of the optical fiber 100 may be substantially constant along the fiber axis 101.

[0115] [000114] It will be understood that the optical fiber 100 has some flexibility, and therefore the orientation of the axis 101 will not generally be uniform along the length of the optical fiber 100. It will be understood that terms such as optical axis 101, cross section, etc. refer to the local optical axis 101, local cross section, etc. Furthermore, when a component is described as cylindrical or tubular, it will be understood that these terms encompass shapes that the optical fiber 100 may distort when bent.

[0116] [000115] It will be understood that the optical fiber 100 may have any length, and the length of the optical fiber 100 may depend on the application (e.g., the amount of spectral broadening desired for application in a supercontinuum radiation source). The length of the optical fiber 100 may be between 1 cm and 10 m, for example, the length of the optical fiber 100 may be between 10 cm and 100 cm.

[0117] [000116] 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 anti-resonant elements for guiding radiation through the hollow core 102. In particular, the plurality of anti-resonant elements are positioned to confine radiation propagating through the optical fiber 100 primarily within the hollow core 102 and guide the radiation along the optical fiber 100. The hollow core 102 of the optical fiber 100 may be located substantially in a central region of the optical fiber 100, such that the axis 101 of the optical fiber 100 may also define the axis of the hollow core 102 of the optical fiber 100.

[0118] [000117] The inner cladding region includes a plurality of capillaries 104, e.g., tubular capillaries, surrounding the hollow core 102. In particular, in the example shown in Figures 11 and 12, the inner cladding region includes a single ring of six tubular capillaries 104.

[0119] [000118] The capillaries 104 may also be referred to as tubes. The capillaries 104 may be circular or of another shape in cross section. Each capillary 104 includes a substantially 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 a cavity 106. Each of the capillary wall portions 105 facing the hollow core functions as an anti-resonant element for guiding radiation propagating through the optical fiber 100. It will be understood that the wall portion 105 may function as an anti-reflection Fabry-Perot cavity for radiation propagating through the hollow core 102 (and that may be incident on the wall portion 105 at a grazing incidence angle). The thickness 160 of the wall portion 105 may be suitable to ensure that transmission into the cavity 106 is generally suppressed, while reflection back into the hollow core 102 is generally enhanced. 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.

[0120] [000119] It will be understood that the term "cladding region" as used herein is intended to mean the region of the optical fiber 100 for guiding 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 transverse modes that propagate along the fiber axis 101.

[0121] [000120] The jacket region 110 is generally tubular and supports the inner cladding region capillaries 104. The capillaries 104 are evenly distributed across the interior surface of the jacket region 110. Six capillaries 104 may be described as surrounding the hollow core 102 in a symmetrical arrangement. In an embodiment including six capillaries 104, the capillaries 104 may be described as arranged in a generally hexagonal format.

[0122] [000121] The capillaries 104 are arranged such that each capillary does not contact any other capillary 104. Each of the capillaries 104 contacts the jacket region 110 and is spaced apart from adjacent capillaries 104 in a ring configuration. Such an arrangement may be beneficial for increasing the transmission bandwidth of the optical fiber 100 (e.g., compared to an arrangement in which the capillaries contact each other). Alternatively, in some embodiments, each of the capillaries 104 may contact adjacent capillaries 104 in a ring configuration.

[0123] [000122] The six capillaries 104 of the cladding region are arranged in a ring structure around the hollow core 102. The inner surfaces of the ring structure 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 may be defined as the smallest dimension between opposing capillaries, indicated by arrows 114) may be between 5 and 100 μm. In some embodiments, the diameter 114 of the hollow core 102 may be between 5 μm and 50 μm. In some embodiments, the diameter 114 of the hollow core 102 may be between 30 μm and 40 μm. The diameter 114 of the hollow core 102 may affect the mode field parameters, collisional loss, dispersion, mode plurality, and nonlinearity properties of the hollow-core optical fiber 100.

[0124] 11 and 12, the inner cladding region comprises a single ring arrangement of capillaries 104 (with the hollow core-facing wall portion 105 acting as the anti-resonant element). Thus, any radial line from the center of the hollow core 102 to the exterior of the optical fiber 100 passes through no more than one capillary 104.

[0125] [000124] It will be appreciated that other examples may be provided with different configurations of anti-resonant elements. These may include configurations with multiple rings of anti-resonant elements and configurations with nested anti-resonant elements. Furthermore, while the embodiment shown in Figures 11 and 12 includes six rings of capillaries 104 with wall portions 105, in other embodiments, one or more rings including any number of anti-resonant elements (e.g., 4, 5, 6, 7, 8, 9, 10, 11, or 12 capillaries) may be provided in the inner cladding region.

[0126] 11 and 12, the inner cladding region includes a circular cross-section. However, it will be understood that other embodiments may be provided with inner cladding regions whose cross-sections are shapes other than circular. For example, in one embodiment of the present invention, the cross-section of the inner cladding region may be hexagonal. A hexagonal cross-section advantageously makes it easier to arrange the capillaries 104 in a symmetrical configuration. For example, six capillaries 104 may be positioned at vertices of a hexagonal cross-section to provide an arrangement of capillaries 104 with hexagonal symmetry.

[0127] [000126] The optical fiber 100 may be referred to as a hollow-core photonic crystal fiber (HC-PCF). Typically, such hollow-core photonic crystal fibers include an inner cladding region (which may, for example, include anti-resonant elements) and a jacket region for guiding radiation within the fiber. The jacket region is typically a jacket or tube of material that supports the inner cladding region.

[0128] [000127] According to the present disclosure, inelastic scattering of pump light (e.g., narrowband light provided by a laser source) by molecules in a gas 126 disposed within the HC-PCF 120 can generate a set of broad, discrete spectral lines (called a Raman frequency comb).

[0129] [000128] 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 light source) via a large vibrational frequency shift. As the propagation distance through the fiber increases, the comb is subsequently broadened to a smooth supercontinuum due to the effects of either rotationally stimulated Raman scattering (SRS), the optical Kerr effect (instantaneous nonlinear refractive index), or a combination of both. This broadening is illustrated in Figure 13, which shows the results of a numerical simulation of a 26 μm diameter HC-PCF filled with nitrogen at 15 bar pressure and pumped with a pump light pulse of 20 ps duration, 80 μJ energy, and 532 nm wavelength. The parameter SPD denotes the relative spectral power density. Further details regarding the simulation shown in FIG. 13 can be found in the non-patent document "From Raman Frequency Combs to Supercontinuum Generation in Nitrogen-Filled Hollow-Core Anti-Resonant Fiber" by S.-F. Gao, Y.-Y. Wang, F. Belli, C. Brahms, P. Wang, and J.C. Travers, Laser & Photonics Reviews, vol. 16, no. 4, p. 2100426, 2022, the contents of which are incorporated herein by reference.

[0130] [000129] Therefore, the above-described process can be used to generate supercontinuum radiation. However, in experiments conducted using nitrogen as the gas under conditions similar to those in the simulation of FIG. 13, the inventors found that gain suppression occurs when the rotational line spacing corresponding to rotational SRS is sufficiently small so that the propagation constants of the Stokes and anti-Stokes lines are very similar. The coherence wave generated by the pump-Stokes pulsation is therefore nearly identical to the coherence wave annihilated by the pump-anti-Stokes pulsation, and the rates of phonon generation and annihilation are balanced. As a result, any signal growth above the noise level is negligible. However, the propagation constants are sufficiently different so that the Stokes lines can be formed into higher-order modes by inter-modal coherence waves.

[0131] [000130] In view of the gain suppression effects observed when using nitrogen as a gas, the inventors instead propose using a gas with sufficient rotational line spacing that the propagation constants of the Stokes and anti-Stokes lines are sufficiently different that the phonon creation and annihilation rates are not balanced, and thus the effects of gain suppression are mitigated.

[0132] [000131] Considering data from the non-patent literature J. Bendtsen, 'The rotational and rotation-vibrational Raman spectra of 14N2, 14N15N and 15N2', J. Raman Spectrosc., vol. 2, no. 2, pp. 133-145, 1974, the contents of which are incorporated herein by reference, the rotational line spacing of nitrogen can be determined to be between 145 THz and 0.36 THz.

[0133] [000132] It is therefore proposed that the gas should have a rotational line spacing of more than 0.5 THz, which should be sufficient to avoid the gain suppression problems mentioned above. Preferably, the gas should have a rotational line spacing of a few THz, for example more than 1 THz, more than 2 THz or more than 3 THz.

[0134] [000133] Examples of gases with appropriately spaced rotational lines include hydrogen (H2) and deuterium (D2).

[0135] [000134] Alternatively, gases that do not have a rotational Raman response (i.e., no rotational lines) can be used to prevent the gain suppression problem. Examples of such gases include methane (CH4) and SF6.

[0136] [000135] The gas should still have a strong vibrational Raman response to generate the broad frequency comb required for the generation of supercontinuum radiation.

[0137] [000136] It will be appreciated that the gas should also have good linear transmission in the wavelength region of interest.

[0138] [000137] The HC-PCF should be selected to ensure good guidance across the spectral region of interest (e.g., ultraviolet to near-infrared). This involves selecting an appropriate core diameter and jacket thickness 150. As shown in Figures 11 and 12, the core diameter 114 can be defined as the smallest dimension between opposing capillaries. Alternatively, for HC-PCFs without capillaries, the core diameter can be defined as the distance across the hollow core of the HC-PCF. As an example, the core diameter is less than 70 μm and the jacket thickness is less than 1 μm. The length of the HC-PCF is sufficient to fully establish the supercontinuum, but should not be so long that guidance losses result in degradation of the supercontinuum spectrum. As an example, a suitable fiber length can be between 1 cm and 10 m (e.g., 1.5 m). In some examples, the fiber length can be between 10 cm and 100 cm.

[0139] [000138] The pump wavelength is selected to be in the normal (positive) group velocity dispersion region of the gas in the HC-PCF. The group velocity dispersion at a particular wavelength depends on the gas species in the mixture, the gas pressure, the hollow core size, and the dimensions of the microstructure in the HC-PCF cladding. The group velocity dispersion can be obtained experimentally, through numerical modeling such as the finite element method, or by an appropriate empirical or analytical model. The pump wavelength should also be within the guided band of the HC-PCF.

[0140] [000139] To minimize the effects of modulation instability and thus ensure that supercontinuum radiation is generated by processes described herein, including 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 consider both gas dispersion, non-ideal scaling of gas density, and full hollow fiber waveguide dispersion, including the effects of resonance.

[0141] [000140] At pump wavelengths shorter than the zero dispersion wavelength (ZDW), the group dispersion velocity is normal, while at pump wavelengths longer than the ZDW, the group dispersion velocity is anomalous. Correspondingly, the gas species, HC-PCF structure, and pump wavelength can be selected to generate the ZDW at a specific gas pressure. Therefore, the pump wavelength should preferably be toward the short-wavelength end of the HC-PCF guiding band, since shorter pump wavelengths allow for the use of lower gas pressures. For example, to efficiently drive supercontinuum formation, the pump wavelength can be less than 1000 nm, visible wavelengths, or ultraviolet wavelengths.

[0142] [000141] When pumping with more than one pump wavelength (eg, dual pumping), at least one of the pump wavelengths should correspond to the normal dispersion region.

[0143] [000142] The pump light pulses should have a pulse duration and pulse energy such that the pulse peak power is sufficient to drive strong nonlinear effects in the gas-filled HC-PCF. Preferably, the pulse duration should be short enough (e.g., less than 100 ps) to avoid excessive pulse energy requirements. Furthermore, the pulse duration should preferably be selected to be long enough to avoid the dominance of self-phase modulation over Raman vibration comb formation. This is typically the case for pulses longer than 50 fs, preferably longer than 100 fs.

[0144] [000143] Figure 14 shows an example of a method 1400 for generating supercontinuum radiation according to the present disclosure. The method may be implemented using one or more systems or apparatus described herein, such as the apparatus 120 shown in Figures 8 and 9 and / or the radiation source 134 shown in Figure 10. The method 1400 includes, in step S1402, 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. The gas and hollow-core photonic crystal fiber are selected such that the pump wavelength is in a range corresponding to the normal group velocity dispersion region of the gas. The gas either has a rotational line spacing of greater than 0.5 THz or has no rotational lines. The hollow-core photonic crystal fiber may be any type of fiber 100 shown in Figures 8-12 and described herein. The pump light pulse may be provided from a radiation source such as the radiation source 136 shown in Figure 10 and described herein.

[0145] [000144] Further embodiments are disclosed in the following list of numbered clauses: 1. A system for generating supercontinuum radiation, comprising: a pump light source configured to generate pump light pulses having a pump wavelength; a hollow-core photonic crystal fiber configured to receive pump light pulses, the hollow-core photonic crystal fiber containing a gas; Including, The gas and hollow-core photonic crystal fiber are selected such that the pump wavelength is in a range corresponding to the normal group velocity dispersion region of the gas; Gas is have a rotational line spacing of greater than 0.5 THz, or A system that either has no rotation line or has no rotation line. 2. The system of clause 1, wherein the gas has a rotational line spacing of greater than 1 THz. 3. The system of clause 1 or 2, wherein the gas comprises a molecular gas. 4. The system of any one of clauses 1-3, wherein the gas is selected to exhibit a vibrational Raman response. 5. The system of any one of clauses 1 to 4, wherein the gas comprises H2. 6. The system of any one of clauses 1 to 5, wherein the gas includes D2. 7. The system of any one of clauses 1 to 6, wherein the gas comprises methane. 8. A system according to any one of clauses 1 to 7, wherein the gas comprises SF6. 9. The system of any one of clauses 1 to 8, wherein the pump wavelength is less than 1000 nm. 10. The system of any one of clauses 1-9, wherein the pump wavelength is less than 800 nm. 11. The system of any one of clauses 1 to 10, wherein the pump wavelength is less than 600 nm. 12. The system of any one of clauses 1 to 11, wherein the pump wavelength is a visible light wavelength. 13. The system of clause 12, wherein the pump wavelength is a green light wavelength. 14. The system of any one of clauses 1-11, wherein the pump wavelength is an ultraviolet wavelength. 15. The system of any one of clauses 1 to 14, wherein selecting the hollow-core photonic crystal fiber includes selecting the core diameter of the hollow-core photonic crystal fiber. 16. The system of any one of clauses 1 to 15, wherein the pump light pulse has a pulse duration of 50 fs to 700 ps. 17. The system of any one of clauses 1 to 15, wherein the pump light pulse has a pulse duration of 300 fs to 700 ps. 18. The system of any one of clauses 1 to 15, wherein the pump light pulse has a pulse duration of 100 fs to 100 ps. 19. A measuring device comprising a system according to any one of clauses 1 to 18. 20. A lithographic apparatus comprising a system according to any one of clauses 1 to 18. 21. A method for generating supercontinuum radiation, comprising: providing a pump light pulse to the hollow-core photonic crystal fiber, the pump light pulse having a pump wavelength and the hollow-core photonic crystal fiber containing a gas; Including, The gas and hollow-core photonic crystal fiber are selected such that the pump wavelength is in a range corresponding to the normal group velocity dispersion region of the gas; Gas is have a rotational line spacing of greater than 0.5 THz, or The method either has no lines of rotation. 22. The method of clause 21, wherein the gas has a rotational line spacing of greater than 1 THz. 23. The method of clause 21 or 22, wherein the gas comprises a molecular gas. 24. The method of any one of clauses 21 to 23, wherein the gas is selected to exhibit a vibrational Raman response. 25. The method of any one of clauses 21 to 24, wherein the gas comprises H2. 26. The method of any one of clauses 21 to 25, wherein the gas comprises D2. 27. The method of any one of clauses 21 to 26, wherein the gas comprises methane. 28. The method of any one of clauses 21 to 27, wherein the gas comprises SF6. 29. The method of any one of clauses 21 to 28, wherein the pump wavelength is less than 1000 nm. 30. The method of any one of clauses 21 to 29, wherein the pump wavelength is less than 800 nm. 31. The method of any one of clauses 21 to 30, wherein the pump wavelength is less than 600 nm. 32. The method of any one of clauses 21 to 31, wherein the pump wavelength is a visible light wavelength. 33. The method of clause 32, wherein the pump wavelength is a green light wavelength. 34. A system according to any one of clauses 21 to 31, wherein the pump wavelength is an ultraviolet wavelength. 35. The method of any one of clauses 21 to 34, wherein selecting a hollow-core photonic crystal fiber includes selecting a core diameter of the hollow-core photonic crystal fiber. 36. The method of any one of clauses 21 to 35, wherein the pump light pulse has a pulse duration of 50 fs to 700 ps. 37. The method of any one of clauses 21 to 35, wherein the pump light pulse has a pulse duration of 300 fs to 700 ps. 38. The method of any one of clauses 21 to 35, wherein the pump light pulse has a pulse duration of 100 fs to 100 ps.

[0146] [000145] The above-mentioned metrology arrangement may form part of a metrology apparatus MT. The above-mentioned metrology arrangement may form part of an inspection apparatus. The above-mentioned metrology arrangement may be included within a lithographic apparatus LA.

[0147] [000146] Although specific reference may be made herein to the use of lithographic apparatus in the manufacture of ICs, it should be appreciated that the lithographic apparatus described herein may have other applications, such as in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0148] [000147] Although specific reference may be made herein to embodiments of the invention in relation to lithographic apparatus, embodiments of the invention may also be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes objects such as wafers (or other substrates) or masks (or other patterning devices). These apparatus may be generally referred to as lithography tools. Such lithography tools may use vacuum or ambient (non-vacuum) conditions.

[0149] [000148] While specific reference may be made above to the use of embodiments of the invention in relation to optical lithography, it will be understood that the invention is not limited to optical lithography and may be used in other applications, for example imprint lithography, where the context allows.

[0150] [000149] Although specific reference is made to a "metrology apparatus / tool / system" or an "inspection apparatus / tool / system," these terms may refer to the same or similar types of tools, apparatus, or systems. For example, an inspection or metrology apparatus incorporating embodiments of the present invention may be used to determine characteristics of structures on a substrate or wafer. For example, an inspection or metrology apparatus incorporating embodiments of the present invention may be used to detect defects in the substrate or in structures on the substrate or wafer. In such embodiments, the characteristic of interest of the structure on the substrate may relate to a defect within the structure, the absence of a particular portion of the structure, or the presence of an unwanted structure on the substrate or wafer.

[0151] [000150] While specific embodiments of the present invention have been described above, it will be understood that the invention may be practiced otherwise than as described. The above description is intended to be illustrative and not limiting. Thus, it will be apparent to those skilled in the art that modifications to the invention as described above may be made without departing from the scope of the claims set forth below.

Claims

1. 1. A system for generating supercontinuum radiation, comprising: a pump light source generating pump light pulses having a pump wavelength; a hollow-core photonic crystal fiber containing a gas, the hollow-core photonic crystal fiber receiving the pump light pulse; Including, the gas and the hollow-core photonic crystal fiber are selected such that the pump wavelength is in a range corresponding to a normal group velocity dispersion region of the gas; The gas is have a rotational line spacing of greater than 0.5 THz, or A system that either has no rotation line or has no rotation line.

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

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

4. The system of any one of claims 1 to 3, wherein the gas is selected to exhibit a vibrational Raman response.

5. The gas is H ２ , D ２ , methane, SF ６ The system according to any one of claims 1 to 4, comprising one of:

6. The system of any one of claims 1 to 5, wherein the pump wavelength is less than 1000 nm.

7. The system of any one of claims 1 to 6, wherein the pump wavelength is a visible light wavelength.

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

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

10. The system according to any one of claims 1 to 9, wherein the selection of the hollow-core photonic crystal fiber includes selection of a core diameter of the hollow-core photonic crystal fiber.

11. The system of any one of claims 1 to 10, wherein the pump light pulses have a pulse duration of 50 fs to 700 ps.

12. A metrology device comprising a system according to any one of claims 1 to 11.

13. A lithographic apparatus comprising a system according to any one of claims 1 to 11.

14. 1. A method for generating supercontinuum radiation, 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; Including, the gas and the hollow-core photonic crystal fiber are selected such that the pump wavelength is in a range corresponding to a normal group velocity dispersion region of the gas; The gas is have a rotational line spacing of greater than 0.5 THz, or The method either has no lines of rotation.

15. 15. The method of claim 14, wherein the gas has a rotational line spacing of greater than 1 THz.

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