Radiation source arrangement and metrology device

Abstract

Disclosed is a radiation source arrangement comprising; a radiation source operable to generate source radiation comprising source energy pulses; and at least one non-linear energy-filter being operable to filter said source radiation to obtain filtered radiation comprising filtered energy pulses. The at least one non-linear energy-filter is operable to mitigate variation in energy in said filtered radiation by reducing the energy level of said source energy pulses which have an energy level corresponding to one of both extremities of an energy distribution of said source energy pulses by a greater amount than said source energy pulses which have an energy level corresponding to a peak of said energy distribution.

Description

[Technical Field]

[0001] This invention relates to a light source, and more specifically, to a broadband light source for use in lithography equipment or measuring instruments. [Previous Technology]

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

[0003] To project a pattern onto a substrate, lithography equipment can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the feature that can be formed on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. Compared to lithography equipment using radiation with a wavelength of, for example, 193 nm, lithography equipment using extreme ultraviolet (EUV) radiation with wavelengths in the range of 4 nm to 20 nm, such as 6.7 nm or 13.5 nm, can be used to form smaller features on the substrate.

[0004] Low-k1 lithography can be used to process features smaller than the classical resolution limit of lithography equipment. In this process, the resolution 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 in the lithography equipment, CD is the "critical size" (usually the smallest printed feature size, but in this case half pitch), and k1 is an empirical resolution factor. Generally speaking, the smaller k1 is, the more difficult it becomes to reproduce patterns on the substrate that resemble the shape and size planned by the circuit designer to achieve specific electrical functionality and performance. To overcome these difficulties, complex fine-tuning steps can be applied to the lithography projection equipment and / or design layout. These steps include, for example, but not limited to, optimization of NA, customized illumination schemes, use of phase-shift patterning devices, various optimizations of the design layout such as optical proximity correction (OPC, sometimes also called "optical and process correction"), or other methods generally defined as "resolution enhancement techniques" (RET). Alternatively, a strict control loop used to control the stability of lithography equipment can be used to improve pattern reproduction at low k1.

[0005] Metrological instruments can be used to measure parameters of interest related to the structure on a substrate. For example, metrological instruments can be used to measure parameters such as critical dimensions, layer stacking on the substrate, asymmetry of the pattern on the substrate, and the position of this pattern relative to the wafer. Measurement radiation is irradiated onto the substrate. The radiation diffracts from the structure on the substrate. The diffracted radiation is collected by an objective lens and captured by a sensor.

[0006] The radiation to be measured is provided by light emitted by a light source. This light is guided to the substrate by a beam splitter and an objective lens, which collects the diffracted radiation from the substrate.

[0007] The light source providing the radiation measurement can be a broadband light source. Broadband light sources may have inter-pulse amplitude variations or radiation noise, which negatively affect measurement reproducibility (or accuracy) and therefore measurement accuracy. The light source can also be a narrowband light source, and suffers from the same inter-pulse amplitude variation problem.

[0008] This radiation noise needs to be reduced or mitigated. [Summary of the Invention]

[0009] According to a first state, a radiation source configuration is provided, comprising: a radiation source operable to generate source radiation containing source energy pulses; and at least one nonlinear energy filter operable to filter the source radiation to obtain filtered radiation containing filtered energy pulses; wherein the at least one nonlinear energy filter is operable to mitigate energy variations in the filtered radiation by reducing the energy level of the source energy pulses having an energy level corresponding to one of the two ends of the energy distribution of the source energy pulses, the reduction being greater than the reduction of the source energy pulses having an energy level corresponding to the peak of the energy distribution.

Implementation Method

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

[0012] As used herein, the terms "reducing mask," "mask," or "patterning device" can be broadly interpreted as referring to a general patterning device that can be used to impart a patterned cross-section to an incident radiation beam, the patterned cross-section corresponding to a pattern to be generated in a target portion of a substrate. In this context, the term "light valve" may also be used. Examples of such patterning devices, besides classic masks (transmitting or reflecting, binary, phase-shifting, hybrid, etc.), include programmable mirror arrays and programmable LCD arrays.

[0013] Figure 1A schematically depicts a lithography apparatus LA. The lithography apparatus LA includes: an illumination system (also called an illuminator) IL configured to modulate a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation); a photomask support (e.g., a photomask stage) MT configured to support a patterning device (e.g., a photomask) 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 stage) WT configured to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support according to certain parameters; and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern applied to the radiation beam B onto a target portion C (e.g., containing one or more dies) of the substrate W by means of the patterning device MA.

[0014] The term “projection system” PS as used herein should be broadly interpreted to encompass all types of projection systems suitable for the exposure radiation used and / or other factors suitable for use with a wetted liquid or a vacuum, including refractive, reflective, reflective-refractive, composite, magnetic, electromagnetic and / or electrostatic optical systems or any combination thereof. Any use of the term “projection lens” herein may be considered synonymous with the more general term “projection system” PS.

[0015] In addition to the substrate support WT, the lithography apparatus LA may include a measurement stage. The measurement stage is configured to hold sensors and / or cleaning devices. The sensors may be configured to measure the properties of the projection system PS or the properties of the radiated beam B. The measurement stage may hold multiple sensors. The cleaning devices may be configured to clean parts of the lithography apparatus, such as parts of the projection system PS or parts of the system providing the wetting liquid. The measurement stage may move below the projection system PS when the substrate support WT is away from the projection system PS.

[0016] In operation, a radiation beam B is incident on a patterning device MA, such as a photomask, which is held on a photomask support MT, and is patterned by a pattern (design layout) present on the patterning device MA. Having traversed the photomask MA, the radiation beam B passes through a projection system PS, which focuses the beam onto a target portion C of the substrate W. The substrate support WT can be accurately moved by means of a second locator PW and a position measurement system IF, for example, to position different target portions C in the path of the radiation beam B at a focused and aligned position. Similarly, a first locator PM and possibly another position sensor (not explicitly depicted in FIG. 1A) can be used to accurately position the patterning device MA relative to the path of the radiation beam B. The patterning device MA and the substrate W can be aligned using photomask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks P1, P2, as described, occupy dedicated target portions, they can be located in the space between the target portions. When substrate alignment marks P1 and P2 are located between target portions C, these substrate alignment marks are called cut track alignment marks.

[0017] As shown in Figure 1B, the lithography equipment LA can form part of a lithography unit LC (sometimes also called a lithocell or a cluster of lithocells), which often also includes equipment for performing pre-exposure and post-exposure processes on the substrate W. Conventionally, such equipment includes a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a cooling plate CH, and a baking plate BK, which are used, for example, to regulate the temperature of the substrate W, or to regulate the solvent in the resist layer. A substrate handler or robot RO picks up the substrate W from input / output ports I / O1 and I / O2, moves the substrate W between different process equipment, and delivers the substrate W to the loading cassette LB of the lithography equipment LA. The devices in the lithography unit, often referred to as the coating and developing system, are usually under the control of the coating and developing system control unit (TCU). The coating and developing system control unit itself can be controlled by the supervisory control system (SCS), which can also control the lithography equipment (LA) through the lithography control unit (LACU).

[0018] To ensure accurate and consistent exposure of the substrate W exposed by the lithography equipment LA, it is necessary to inspect the substrate to measure the properties of the patterned structure, such as the overlap error between subsequent layers, line thickness, and critical dimension (CD). For this purpose, inspection tools (not shown) can be included in the lithography manufacturing unit LC. If an error is detected, adjustments can be made, for example, to the exposure of subsequent substrates or other processing steps to be performed on the substrate W, especially when inspection is performed before other substrates W in the same batch or group are exposed or processed.

[0019] The inspection equipment, also known as a measuring device, is used to determine the properties of the substrate W, and specifically, to determine how the properties of different substrates W change or how the properties associated with different layers of the same substrate W change between layers. The inspection equipment may alternatively be constructed to identify defects on the substrate W and may be, for example, part of a lithography unit LC, or integrated into a lithography apparatus LA, or even a standalone device. The inspection equipment can measure the properties of latent images (images in the resist layer after exposure), or half-latent images (images in the resist layer after the post-exposure baking step PEB), or properties of developed resist images (where the exposed or unexposed portions of the resist have been removed), or even properties of etched images (after a pattern transfer step such as etching).

[0020] The patterning process in a typical lithography apparatus (LA) is one of the most critical steps in the process, requiring high accuracy in the dimensional calibration and placement of the structure on the substrate W. To ensure this high accuracy, three systems can be combined in what is schematically depicted in Figure 2 as a so-called "overall" control environment. One of these systems is the lithography apparatus (LA), which is (in fact) connected to a metrology tool (MT) (the second system) and to a computer system (CL) (the third system). The key to this "overall" environment is optimizing the collaboration between these three systems to enhance the overall process window and provide a tight control loop, thereby ensuring that the patterning performed by the lithography apparatus (LA) remains within the process window. The process window defines a range of process parameters (e.g., dosage, focus, stacking) within which a specific manufacturing process yields a defined result (e.g., a functional semiconductor device)—typically, within this range, process parameters in the lithography or patterning process are allowed to vary.

[0021] The computer system CL can use the design layout (partially) to be patterned to predict which resolution enhancement technique to use and perform computational lithography simulation and calculations to determine which mask layout and lithography equipment settings achieve the maximum overall process window for the patterning process (described by the double arrows in the first scale SC1 in Figure 2). Typically, the resolution enhancement technique is configured to match the patterning possibilities of the lithography equipment LA. The computer system CL can also be used to detect where the lithography equipment LA is currently operating within the process window (e.g., using input from the metric tool MT) to predict whether there may be defects attributable to, for example, suboptimal processing (described by the arrow pointing to "0" in the second scale SC2 in Figure 2).

[0022] The measurement tool MT can provide input to the computer system CL to achieve accurate simulation and prediction, and can provide feedback to the lithography device LA to identify possible variations, such as in the calibration state of the lithography device LA (described by multiple arrows in the third scale SC3 in Figure 2).

[0023] In lithography processes, it is frequently necessary to measure the resulting structure, for example, for process control and verification. Different types of metrological instruments (MTs) are known for performing such measurements, including scanning electron microscopes or various forms of scatterometer metrological instruments (MTs).

[0024] A scatterometer is a multifunctional instrument that allows for the measurement of lithography process parameters by means of a sensor in a plane conjugate to the pupil of the scatterometer's objective lens. This measurement is generally referred to as a pupil-based measurement. Alternatively, it can be used to measure lithography process parameters by means of a sensor in an image plane or a plane conjugate to the image plane. In this case, the measurement is generally referred to as an image or field-based measurement. Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032, or EP1,628,164, which are incorporated herein by reference in their entirety. The aforementioned scatterometer can use light from embodiments of the light sources discussed in this document to measure gratings.

[0025] The overall measurement quality of a lithography parameter for a specific target is determined at least in part by the measurement formulation used to measure that lithography parameter. The term "substrate measurement formulation" may include measuring one or more parameters of itself, measuring one or more parameters of one or more patterns, or both. For example, if the measurement used in the substrate measurement formulation is a diffraction-based optical measurement, one or more of the measured parameters may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, etc. One of the criteria used to select the measurement formulation may be, for example, the sensitivity of one of the measured parameters to processing variations. Further examples are described in U.S. Patent Application US2016-0161863 and U.S. Patent Application US2016 / 0370717A1, which are incorporated herein by reference in their entirety. The light source of this document may be configured to be controllable relative to the light source requirements of these substrate measurement formulations.

[0026] A lithography apparatus may include one or more (e.g., a plurality of) alignment sensors capable of accurately measuring the position of alignment marks provided on a substrate or wafer. The alignment (or position) sensors may use optical phenomena such as diffraction and interference to obtain position information from the alignment marks formed on the substrate or wafer. Examples of alignment sensors used in current lithography apparatuses are based on self-reference interferometers as described in US6961116. Various enhancements and modifications of position sensors have been developed, such as those disclosed in US2015261097A1. All such disclosures are incorporated herein by reference.

[0027] The markings or alignment marks may comprise a series of strips formed on or in a layer provided on a substrate or (directly) formed in the substrate. These strips may be regularly spaced and act as grating lines, such that the markings can be considered as diffraction gratings with a known spatial period (spacing). Depending on the orientation of these grating lines, the markings may be designed to allow measurement of position along the X-axis or along the Y-axis (which is oriented substantially perpendicular to the X-axis). Markings comprising strips configured at +45 degrees and / or -45 degrees relative to both the X-axis and Y-axis allow for combined X and Y measurements using techniques as described in US2009 / 195768A, which is incorporated by reference.

[0028] The alignment sensor optically scans each mark using a radiating spot to obtain a periodically changing signal, such as a sine wave. The phase of this signal is analyzed to determine the position of the mark, and thus the position of the substrate relative to the alignment sensor, which is fixed relative to the reference frame of the lithography apparatus. So-called coarse and fine marks related to different (coarse and fine) mark sizes can be provided so that the alignment sensor can distinguish different cycles of the periodic signal, and the exact position (phase) within a cycle. Marks with different spacings can also be used for this purpose.

[0029] The location of the measurement marks can also provide information about the deformation of the substrate on which the marks are provided, for example, by providing the marks in the form of a wafer grid. The deformation of the substrate can occur, for example, by electrostatically clamping the substrate to a substrate stage and / or by heating the substrate when it is exposed to radiation.

[0030] Figure 3A is a schematic block diagram of an embodiment of a known alignment sensor AS, such as that described, for example, in US6961116 and incorporated herein by reference. A radiation source RSO provides a radiation beam RB having one or more wavelengths, which is directed by a steering optics to a mark, such as a mark AM located on a substrate W, as an illumination spot SP. In this example, the steering optics include a spot mirror SM and an objective lens OL. The radiation source RSO may be provided by an embodiment of the light source of the invention described herein. The diameter of the illumination spot SP illuminating the mark AM may be slightly smaller than the width of the mark itself.

[0031] Radiation diffracted by the marker AM is collimated (in this example via the objective lens OL) into an information-carrying beam IB. The term "diffraction" is intended to include zero-order diffraction from the marker (which may be referred to as reflection). For example, a self-reference interferometer SRI of the type disclosed in US6961116 mentioned above interferes with itself with a beam IB, which is then received by a photodetector PD. Additional optics (not shown) may be included to provide a separate beam in cases where more than one wavelength is generated by the radiation source RSO. The photodetector may be a single element, or may include multiple pixels as needed. The photodetector may include a sensor array.

[0032] A configuration measurement system, a level sensor, or a height sensor, which can be integrated into a lithography apparatus, is configured to measure the configuration of the top surface of a substrate (or wafer). A map of the substrate configuration, also known as a height map, can be generated from such measurements indicating the substrate height as a function of position on the substrate. This height map can then be used to correct the position of the substrate during pattern transfer onto the substrate to provide an aerial image of the patterning apparatus at the appropriate focus position on the substrate. It will be understood that "height" in this context refers to the broad dimension from the plane to the substrate (also known as the Z-axis). Typically, the level or height sensor performs measurements at a fixed position (relative to its own optical system), and the relative movement between the substrate and the optical system of the level or height sensor across the substrate produces height measurements at various positions.

[0033] An example of a level or height sensor LS known in the art is schematically shown in FIG. 3B, which is only to illustrate the operating principle. In this example, the level sensor includes an optical system comprising a projection unit LSP and a detection unit LSD. The projection unit LSP includes a radiation source LSO that provides a radiation beam LSB, which is imparted by a projection grating PGR of the projection unit LSP. The radiation source LSO may include embodiments of the invention described in this document.

[0034] This invention relates to improving the operational lifetime or noise performance of a light source. The light source referred to herein may be a broadband light source comprising a hollow core photonic crystal fiber (HC-PCF). The broadband light source of this invention can be used in metrological instruments, such as scatterometers, alignment sensors, height or level sensors as described above. Generally speaking, these two aspects of a light source—lifetime and noise performance—are contradictory. Specifically, lifetime can be improved by reducing the repetition rate of the light source. However, the lower the repetition rate, the greater the observed variation between pulses. This is because detectors, such as photodiodes, measure the optical signal by integrating over a period of time. If, for example, 100 laser pulses exist during this period, the result is actually the average of these pulses. Statistically, averaging a large number of pulses produces data points with less noise; conversely, if only one pulse exists, the noise will be more apparent. The concept revealed here reduces inter-pulse variation (noise) by lowering the repetition rate while maintaining a given performance level, thereby allowing noise performance to be traded off against lifetime.

[0035] The metrological instruments (MTs) mentioned above, such as scatterometers, configuration measurement systems, or position measurement systems, can use radiation from a radiation source to perform measurements. The nature of the radiation used by the metrological instrument can affect the type and quality of the measurements that can be performed. For some applications, it may be advantageous to use multiple radiation frequencies to measure the substrate, for example, broadband radiation. Multiple different frequencies can propagate, irradiate, and scatter the metrological target without interfering with other frequencies or with minimal interference. Therefore, different frequencies can be used, for example, to obtain more metrological data simultaneously. Different radiation frequencies can also be used to query and discover different properties of the metrological target. Broadband radiation is suitable for metrological systems (MTs) such as, for example, level sensors, alignment mark measurement systems, scattering measurement instruments, or detection instruments. The broadband radiation source can be a hypercontinuum source.

[0036] For example, high-quality broadband radiation from supercontinuum radiation may be difficult to generate. One method for generating broadband radiation is, for example, to broaden high-power narrow-band or single-frequency input radiation using nonlinear, higher-order effects. The input radiation (which can be generated using a laser) may be called pump radiation. Alternatively, the input radiation may be called seed radiation. To obtain high-power radiation for the broadening effect, the radiation can be confined to a smaller region to achieve highly localized high-intensity radiation. In such regions, the radiation can interact with the broadening structure and / or the material forming the nonlinear medium to form broadband output radiation. In the high-intensity radiation region, different materials and / or structures can be used to achieve and / or improve radiation broadening by providing a suitable nonlinear medium.

[0037] In some embodiments, broadband output radiation is generated in a photonic crystal fiber (PCF). In several embodiments, such a photonic crystal fiber has a microstructure around its fiber core that helps to confine the radiation traveling through the fiber core. The fiber core may be made of a solid material that has nonlinear properties and is capable of generating broadband radiation when high-intensity pump radiation is transmitted through the fiber core. Although generating broadband radiation in a solid-core photonic crystal fiber is feasible, there are several disadvantages to using a solid material. For example, if UV radiation is generated in a solid core, this radiation may not be present in the output spectrum of the fiber because the radiation is absorbed by most solid materials.

[0038] In some embodiments, as further discussed below with reference to Figure 5, the method and apparatus for broadening input radiation may use an optical fiber for confining input radiation and for broadening the input radiation to output broadband radiation. The optical fiber may be a hollow-core fiber and may contain internal structures for effective guidance and confinement of radiation within the fiber. The optical fiber may be a hollow-core photonic crystal fiber (HC-PCF), which is particularly suitable for achieving high radiation intensity primarily within the hollow core of the fiber. The hollow core of the fiber may be filled with a gas or gas mixture, which acts as a broadening medium for broadening the input radiation. This fiber and gas mixture configuration can be used to generate a supercontinuum radiation source. The radiation input of the optical fiber may be electromagnetic radiation, such as radiation in one or more of the infrared, visible, UV, and extreme UV spectra. The output radiation may consist of or include broadband radiation, which may be referred to herein as white light. The output radiation may cover the UV, visible, and near-infrared ranges. The exact spectrum and power density of the output radiation will be determined by a number of parameters, such as fiber structure, gas mixture composition, gas pressure, energy of input radiation, pulse duration, and pulse shape of input radiation.

[0039] Some embodiments relate to novel designs of this broadband radiation source incorporating an optical fiber. The optical fiber is a hollow core photonic crystal fiber (HC-PCF). Specifically, the optical fiber may be a hollow core photonic crystal fiber containing an anti-resonant structure for confining radiation. Such fibers containing an anti-resonant structure are known in the art as anti-resonant fibers, tubular fibers, single-loop fibers, negative curvature fibers, or suppression-coupled fibers. Various designs of such fibers are known in the art. Alternatively, the optical fiber may be a photonic bandgap fiber (HC-PBF, such as Kagome fiber).

[0040] Various types of HC-PCFs can be engineered, each based on a different physical guidance mechanism. Two such HC-PCFs include: Hollow Core Photonic Bandgap Fiber (HC-PBF) and Hollow Core Anti-Resonant Reflective Fiber (HC-ARF). Details of the design and fabrication of HC-PCFs can be found in U.S. Patent US2004 / 015085A1 (for HC-PBF) and International PCT Patent Application WO2017 / 032454A1 (for Hollow Core Anti-Resonant Reflective Fiber), which are incorporated herein by reference. Figure 6(a) shows a Kagome fiber containing a Kagome lattice structure.

[0041] An example of an optical fiber used for a radiation source will now be described with reference to FIG4, which is a schematic cross-sectional view of the optical fiber OF in the transverse plane. Other embodiments similar to the actual example of the optical fiber in FIG4 are disclosed in WO2017 / 0324541.

[0042] An optical fiber (OF) comprises an elongated body, which is longer than the other two dimensions of the optical fiber in one dimension. This longer dimension may be called the axial direction and defines the axis of the optical fiber. The two other dimensions define a plane that may be called the transverse plane. Figure 4 shows a cross-section of the optical fiber in a transverse plane (i.e., perpendicular to the axis) labeled xy plane. The cross-section of the optical fiber can be substantially constant along the fiber axis.

[0043] It will be understood that optical fiber OF has a certain degree of flexibility, and therefore, the direction of the axis will generally not be consistent with the direction along the length of the optical fiber OF. Terms such as optical axis, cross section and the like should be understood to mean local optical axis, local cross section, etc. Furthermore, when the component is described as cylindrical or tubular, these terms should be understood to cover such shapes that may have been deformed when the optical fiber OF is bent.

[0044] Fiber optic OF can have any length and it will be understood that the length of fiber optic OF can depend on the application. Fiber optic OF can have lengths between 1 cm and 10 m or 0.1 cm and 10 m, for example, fiber optic OF can have lengths between 10 cm and 100 cm.

[0045] An optical fiber OF comprises: a hollow core COR; an cladding portion surrounding the hollow core COR; and a support portion SP surrounding and supporting the cladding portion. The optical fiber OF can be viewed as a body comprising a hollow core COR (including the cladding portion and the support portion SP). The cladding portion comprises a plurality of anti-resonant elements for guiding radiation through the hollow core COR. Specifically, these plurality of anti-resonant elements are configured to limit radiation propagating primarily within the hollow core COR through the optical fiber OF and are configured to guide radiation along the optical fiber OF. The hollow core COR of the optical fiber OF can be substantially disposed in the central region of the optical fiber OF such that the axis of the optical fiber OF can also define the axis of the hollow core COR.

[0046] The cladding portion includes a plurality of anti-resonant elements for guiding radiation propagating through the optical fiber (OF). Specifically, in this embodiment, the cladding portion includes a single ring of six tubular capillary CAPs. Each of the tubular capillary CAPs acts as an anti-resonant element.

[0047] The capillary CAP can also be referred to as a tube. In cross-section, the capillary CAP can be circular or may have another shape. Each capillary CAP includes a generally cylindrical wall portion WP that at least partially defines the hollow core HC of the optical fiber OF and separates the hollow core HC from the capillary cavity CC. It will be understood that the wall portion WP can act as an anti-reflection Fabry-Perot resonator for radiation propagating through the hollow core HC (and the radiation can be incident on the wall portion WP at a grazing angle of incidence). The thickness of the wall portion WP can be appropriate to ensure that reflections returning to the hollow core HC are generally enhanced, while transmission into the capillary cavity CC is generally suppressed. In some embodiments, the capillary wall portion WP may have a thickness between 0.01 µm and 10.0 µm.

[0048] It will be understood that, as used herein, the term "covered portion" is intended to refer to the portion of the optical fiber OF used to guide the radiation propagating through the optical fiber OF (i.e., the capillary CAP that confines the radiation within the hollow core COR). The radiation may be confined in a lateral mode, thereby propagating along the fiber axis.

[0049] The support portion is generally tubular and supports six capillary CAPs covering the portion. The six capillary CAPs are evenly distributed around the inner surface of the inner support portion SP. The six capillary CAPs can be described as being arranged in a generally hexagonal form.

[0050] The capillary CAPs are configured such that each capillary does not contact any of the other capillary CAPs. Each of the capillary CAPs contacts the internal support portion SP and is spaced apart from adjacent capillary CAPs in the ring structure. This configuration can be advantageous because it can increase the transmit bandwidth of the optical fiber OF (compared to, for example, a configuration in which the capillaries contact each other). Alternatively, in some embodiments, each of the capillary CAPs may contact adjacent capillary CAPs in the ring structure.

[0051] Six capillary capillaries (CAPs) of the cladding portion are housed in a ring structure surrounding the hollow core (COR). The inner surface of the ring structure of the capillary capillaries at least partially defines the hollow core (HC) of the optical fiber (OF). The diameter d of the hollow core (HC) (which can be defined as the minimum dimension between opposing capillaries, indicated by arrow d) can be between 10 µm and 1000 µm. The diameter d of the hollow core (HC) can affect the mode field diameter, impulse loss, dispersion, modal multivariability, and nonlinear properties of the hollow core optical fiber (OF).

[0052] In this embodiment, the cladding portion comprises a single-ring configuration of capillary CAPs (which act as anti-resonant elements). Therefore, a line in any radial direction from the center of the hollow core HC to the outside of the optical fiber OF passes through no more than one capillary CAP.

[0053] It will be understood that other embodiments may have different configurations of anti-resonant elements. Such configurations may include configurations with multiple rings having anti-resonant elements and configurations with nested anti-resonant elements. Furthermore, although the embodiment shown in FIG4 includes six capillary rings, in other embodiments, one or more rings containing any number of anti-resonant elements (e.g., 4, 5, 6, 7, 8, 9, 10, 11, or 12 capillaries) may be disposed in the covering portion.

[0054] Figure 6(b) shows a modified embodiment of the HC-PCF with a single ring having a tubular capillary discussed above. In the example of Figure 6(b), there are two coaxial rings of the tubular capillary 21. To hold the inner and outer rings of the tubular capillary 21, a support tube ST may be included in the HC-PCF. This support tube may be made of silicon dioxide.

[0055] The tubular capillary in the examples of Figures 4 and 6(a) and (b) may have a circular cross-sectional shape. Other shapes are also possible for the tubular capillary, such as elliptical or polygonal cross-sections. In addition, the solid material of the tubular capillary in the examples of Figures 4 and 6(a) and (b) may include plastic materials, such as PMA; and glass, such as silica or soft glass.

[0056] Figure 5 depicts a radiation source RDS for providing broadband output radiation. The radiation source RDS includes: a pulsed pump radiation source PRS or any other type of source capable of generating short pulses of desired length and energy level; an optical fiber OF having a hollow core HC (e.g., the type shown in Figure 4); and a working medium WM (e.g., gas) disposed within the hollow core HC. Although the radiation source RDS in Figure 5 includes the optical fiber OF shown in Figure 4, other types of hollow core optical fibers may be used in alternative embodiments.

[0057] The pulsed pump radiation source (PRS) is configured to provide the input radiation IRD. The hollow core HC of the optical fiber OF is configured to receive the input radiation IRD from the pulsed pump radiation source (PRS) and widen the input radiation IRD to provide the output radiation ORD. The working medium WM can widen the frequency range of the received input radiation IRD to provide a wideband output radiation ORD.

[0058] The radiation source RDS further includes a storage unit RSV. An optical fiber OF is disposed within the storage unit RSV. The storage unit RSV may also be referred to as a housing, container, or gas chamber. The storage unit RSV is configured to contain a working medium WM. The storage unit RSV may include one or more features known in the art for controlling, regulating, and / or monitoring a composition of the working medium WM (which may be a gas) inside the storage unit RSV. The storage unit RSV may include a first transparent window TW1. In use, the optical fiber OF is disposed within the storage unit RSV such that the first transparent window TW1 is positioned close to the input end IE of the optical fiber OF. The first transparent window TW1 may form part of the wall of the storage unit RSV. The first transparent window TW1 may be transparent at least to the received input radiation frequency, such that the received input radiation IRD (or at least a large portion thereof) can be coupled to the optical fiber OF located inside the storage unit RSV. It will be understood that optical components (not shown) may be provided for coupling the input radiation IRD to the fiber OF.

[0059] The storage unit RSV includes a second transparent window TW2 forming part of the wall of the storage unit RSV. In use, when the optical fiber OF is placed inside the storage unit RSV, the second transparent window TW2 is located near the output end OE of the optical fiber OF. The second transparent window TW2 is transparent at least to the frequency of the broadband output radiation ORD of the device.

[0060] Alternatively, in another embodiment, the two opposing ends of the optical fiber OF may be placed inside different reservoirs. The optical fiber OF may include a first end section configured to receive input radiation IRD and a second end section for outputting broadband output radiation ORD. The first end section may be placed inside a first reservoir containing a working medium WM. The second end section may be placed inside a second reservoir, which may also contain the working medium WM. The function of the reservoirs may be as described above with respect to FIG5. The first reservoir may include a first transparent window configured to be transparent to the input radiation IRD. The second reservoir may include a second transparent window configured to be transparent to the broadband output radiation ORD. The first and second reservoirs may also include sealable openings to allow the optical fiber OF to be partially placed inside and partially placed outside the reservoir, so that gas can be sealed inside the reservoir. The optical fiber OF may further include an intermediate section not contained within the reservoir. This configuration using two separate gas reservoirs is particularly convenient for embodiments in which the optical fiber OF is relatively long (e.g., when the length exceeds 1 m). It will be understood that for such 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 gases within the two reservoirs) can be considered as devices for providing the working medium WM within the hollow core HC of the optical fiber OF.

[0061] In this context, if at least 50%, 75%, 85%, 90%, 95% or 99% of incident radiation of a certain frequency is transmitted through the window, then the window may be transparent to that frequency.

[0062] The first TW1 and the second TW2 transparent windows can form an airtight seal within the wall of the reservoir RSV, so that the working medium WM (which may be a gas) can be contained within the reservoir RSV. It will be understood that the gas WM can be contained within the reservoir RSV at a pressure different from the ambient pressure of the reservoir RSV.

[0063] The working medium WM may contain: rare gases such as argon, krypton, and xenon; Raman-active gases such as hydrogen, deuterium, and nitrogen; or gas mixtures such as argon / hydrogen mixtures, xenon / deuterium mixtures, krypton / nitrogen mixtures, krypton / helium mixtures, or nitrogen / hydrogen mixtures. Depending on the type of fill gas, nonlinear optical processes may include modulation instabilities (MI), soliton self-compression, soliton splitting, Kerr effect, Raman effect, and dispersive wave generation, detailed in WO2018 / 127266A1 and US9160137B1 (both are hereby incorporated by reference). Since the dispersion of the fill gas can be tuned by changing the pressure of the working medium WM (i.e., the gas chamber pressure) in the reservoir RSR, the resulting broadband pulse dynamics and associated spectral broadening characteristics can be adjusted to optimize frequency conversion.

[0064] In one embodiment, the working medium WM may be housed within the hollow core COR, at least during the period when it receives the input radiation IRD used to generate the broadband output radiation ORD. It will be understood that when the optical fiber OF does not receive the input radiation IRD used to generate the broadband output radiation, the gas WM may disappear wholly or partially from the hollow core COR.

[0065] To achieve frequency broadening, high-intensity radiation may be required. The advantage of hollow-core optical fiber (OF) is that it can achieve high-intensity radiation through the strong spatial confinement of radiation propagating through the OF, thereby achieving high localized radiation intensity. The higher radiation intensity inside the OF can be attributed, for example, to the high received input radiation intensity and / or to the strong spatial confinement of radiation within the OF. The advantage of hollow-core optical fiber is that it can guide radiation with a wider wavelength range than solid-core optical fiber, and specifically, it can guide radiation in both the ultraviolet and infrared ranges.

[0066] The advantage of using a hollow core optical fiber (OF) is that most of the radiation guided inside the OF is confined to the hollow core (COR). Therefore, most of the radiation interaction inside the OF is with the working medium (WM), which is located inside the hollow core (HC) of the OF. This increases the broadening effect of the working medium (WM) on the radiation.

[0067] The received input radiation IRD may be electromagnetic radiation. The input radiation IRD may be a pulsed radiation receiver. For example, the input radiation IRD may contain, for example, an ultrafast pulse generated by a laser.

[0068] The input radiation IRD can be coherent radiation. The input radiation IRD can also be collimated radiation, which has the advantage of improving the efficiency of coupling the input radiation IRD to the optical fiber OF. The input radiation IRD can contain a single frequency or a narrow frequency range. The input radiation IRD can be generated by a laser. Similarly, the output radiation ORD can be collimated and / or coherent.

[0069] The broadband range of the output radiation ORD can be a continuous range, encompassing a continuous range of radiation frequencies. The output radiation ORD may include supercontinuum radiation. Continuous radiation can be beneficial in various applications, such as in metrology applications. For example, the range of continuous frequencies can be used to look up a large number of properties. The range of continuous frequencies can, for example, be used to determine and / or eliminate the frequency dependence of the measured properties. The supercontinuum output radiation ORD may include, for example, electromagnetic radiation in the wavelength range of 100 nm to 4000 nm, or even up to 10 µm. The frequency range of the broadband output radiation ORD may be, for example, 400 nm to 900 nm, 500 nm to 900 nm, or 200 nm to 2000 nm. The supercontinuum output radiation ORD may include white light.

[0070] The input radiation IRD provided by the pulsed pump radiation source (PRS) can be pulsed. The pulsed pump radiation source (PRS) can be a laser. The spatiotemporal transmission characteristics of this laser pulse transmitted along the fiber OF can be changed and tuned by adjusting the (pump) laser parameters, the working component WM variation, and the fiber OF parameters, such as its spectral amplitude and phase. These spatiotemporal transmission characteristics may include one or more of the following: output power, output mode profile, output time profile, width of output time profile (or output pulse width), output spectral profile, and bandwidth of output spectral profile (or output spectral bandwidth). The pulsed pump radiation source (PRS) parameters may include one or more of the following: pump wavelength, pump pulse energy, pump pulse width, and pump pulse repetition rate. The fiber OF parameters may include one or more of the following: the fiber length, size, and shape of the hollow core 101; the size and shape of the capillary; and the thickness of the wall surrounding the hollow core of the capillary. These working components WM, such as filling gas, may include one or more of the following parameters: gas type, gas pressure, and gas temperature.

[0071] The broadband output radiation ORD provided by the radiation source RDS may have an average output power of at least 1 W. The average output power may be at least 5 W. The average output power may be at least 10 W. The broadband output radiation ORD may be a pulsed broadband output radiation ORD. The broadband output radiation ORD may have a power spectral density of at least 0.01 mW / nm across the entire wavelength band of the output radiation. The power spectral density of the broadband output radiation across the entire wavelength band may be at least 3 mW / nm.

[0072] Any laser source has associated inter-pulse amplitude variations, which generate laser noise. This noise has a direct impact on alignment reproducibility, which in turn directly affects the parameters of interest being measured (e.g., overlay). It has been observed that for at least some colors, such as the broadband sources schematically illustrated in Figure 5, there are unacceptably high noise levels, which can significantly affect reproducibility. While algorithms exist to reduce the impact of laser noise, no algorithm can completely eliminate laser noise, and some laser noise will always be present. This may still be too high for a given application.

[0073] Because this inter-pulse noise is fundamental to the light generation process, there is currently no inherent direct solution to this problem for the light source. A complete redesign of the source to solve this problem (even if possible) is undesirable in terms of effort, cost, and complexity. Therefore, an indirect method for reducing this noise downstream of the light source will be described, which can be implemented in conjunction with (or as an alternative to) a noise cancellation algorithm. In that case, the total noise reduction achieved will be even higher.

[0074] Figure 7 illustrates the problem of laser noise. Figure 7(a) shows a plot of the amplitude A of a hypothetical perfect source against the pulse count PC (i.e., the amplitude of each individual pulse), where there is no inter-pulse variation in amplitude. Figure 7(b) shows an equivalent plot as seen for an actual source, which shows the described inter-pulse amplitude variation.

[0075] It is proposed to redistribute pulse amplitude statistics using downstream optics. For an uncorrected source, pulse amplitude statistics may follow a Poisson distribution, as illustrated in the histogram of Figure 8(a), which shows the number of counts C of the divided amplitudes A. The proposed concept aims to reshape this distribution. Ideally, of course, the reshaping will produce a single peak where all values ​​are within a single amplitude range. Figure 8(b) shows a more achievable modified distribution that can be induced by the concept disclosed herein. This shows that the tail of the distribution in Figure 8(a) corresponding to the rarer events of the pulse amplitude minimum / maximum has been removed. This can be achieved by simply removing these rarer pulses completely (e.g., attenuating them to zero). Alternatively or additionally, these rarer pulses may be moved toward the center of the distribution, for example by attenuating only a portion of the energy of these pulses (e.g., converting only a portion of the energy of these pulses into other forms of energy). In embodiments, both methods may be used in combination. More specifically, high-energy or high-amplitude pulses can be partially attenuated to bring their energy or amplitude closer to the average pulse energy, with only excess energy lost. Low-energy pulses can be completely attenuated and effectively blocked.

[0076] In one embodiment, the optical element may attenuate most or all high-energy or high-amplitude pulses with energy or amplitude greater than a first threshold energy or amplitude level, and / or attenuate most or all low-energy or low-amplitude pulses with energy or amplitude less than a second threshold energy or amplitude level.

[0077] The distribution in Figure 8(b) is purely illustrative of the principle and is not feasible in practice because most pulses (e.g., pulses closer to the peak of the distribution) do not decay and all energy just above or below the threshold is cut off. However, due to the nature of the proposed (passive) optics, some decay of all pulses will likely cause an overall shift in the output distribution (i.e., the distribution after the optics) relative to the input distribution (i.e., the distribution before the optics). Furthermore, the cutoff will be less precise, allowing some high energy / amplitude (above the first threshold) and / or low energy / amplitude (below the second threshold) to still be included in the output.

[0078] The disclosed concept may refer to only one (any one) or both distribution tails. Thus, the disclosed concept can convert pulse energy above the average value toward the average value of the output distribution and / or suppress / block pulse energy below the average value.

[0079] To achieve this, energy-absorbing optics can be used in the output (e.g., broadband) radiation path, or alternatively in the pump or seed radiation path. When in the output path of a broadband source, one or more energy-absorbing optics can be provided for each band or color channel only (e.g., only for noise channels, such as those with noise levels above the threshold), or one or more energy-absorbing optics can be used across the entire output spectrum. The latter option may be preferable for reducing cost and complexity, while other options may be preferable if greater flexibility is required.

[0080] More specifically, the proposed source configuration may use nonlinear energy filters or amplitude filters, such as optical power limiting elements (e.g., saturable absorption elements), to attenuate a first subset of the source energy pulses, i.e. higher source energy pulses, toward the average value of the filtered energy pulse distribution, and / or use nonlinear energy filters or amplitude filters, such as antisaturable absorption elements, to attenuate or block a second subset of the source energy pulses, i.e. lower energy pulses.

[0081] The saturable absorption element comprises a material with nonlinear transmittance, such that input pulses with energy / amplitude higher than the average value are attenuated more than input pulses closer to the average value. For example, input pulses with energy / amplitude higher than a first threshold may experience greater attenuation than input pulses between the first and second thresholds; this range corresponds to the peak region of the input distribution. Input energy pulses lower than the second threshold may attenuate to essentially zero (e.g., blocked), in which case it is called an anti-saturable absorption element. Thus, the output and / or individual color channels may each comprise a pair of nonlinear amplitude filters: for example, a saturable absorption element and an anti-saturable absorption element.

[0082] Figure 8(c) shows a more realistic example (compared to the example shown in Figure 8(b)) of the output amplitude (or energy) distribution OD of the filtered energy pulse superimposed on the input amplitude (or energy) distribution ID (e.g., prior art distribution) of the source energy pulse. The intensity values ​​on the x-axis are expressed in arbitrary units. The nonlinear response of the proposed filter is used to shape or filter the input distribution ID following the Lorenz distribution (in this particular example, the rate parameter is 12). It can be seen that the effect of the nonlinear response is a reduction in total power (the mean of the distribution shifts to the left) and a reduction in the distribution width (it becomes narrower and therefore less noisy). Thus, the intensity of most or all pulses is reduced; however, pulses corresponding to higher energy / intensity in the input distribution are attenuated more than pulses closer to the peak of the input distribution, resulting in a significantly narrower distribution with higher peak values. The pulses corresponding to lower energy / intensity in the input distribution are also attenuated more than the pulses corresponding to the peak of the input distribution. More specifically, these pulses are essentially suppressed or blocked so that they do not appear in the output distribution.

[0083] Examples of nonlinear amplitude filter materials that can be used as (e.g., forward) saturable absorption elements (i.e., for limiting pulses above the average amplitude) include saturable absorber glasses, such as: ● borosilicate glasses; such as borosilicate glass 3.3 (sold as Duran (RTM) glass) or BK7 glass; ● ultra-low expansion (ULE) glasses, such as titanium-doped silicate glasses (e.g., 7.5TiO2-92.5SiO2); ● Sk3 glass; or ● any other suitable glass having the same transmission characteristics as those listed above.

[0084] Other examples may include saturable absorber nanoparticle materials, such as metal oxide nanoparticle materials (nanospheres / nanoellipsoids); for example, Co3O4, V2O5, Fe2O3, Mn3O4, Cr2O3 and CuO nanospheres or nanoellipsoids.

[0085] Examples of nonlinear amplitude filter materials used for anti-saturation absorption elements include: ● Dichalcogenide or transition metal dichalcogenide (TMD or TMDC) monolayers, such as: MoS2, MoSe2, WS2, WSe2; ● Nanoparticles, such as Cu2O-Ag nano heterostructures; ● Indanthrene compounds, such as: indanthrene oxide, monochloroindanthrene, indanthrene oligomers.

[0086] Some sources, such as the supercontinuum source illustrated in Figure 5 (or similar devices using a solid core fiber), can generate pulses with insufficient pulse energy (or more relevantly, pulse intensity) for the desired nonlinear response in nonlinear amplitude filter materials or saturable absorber materials. Therefore, the embodiments disclosed herein may include using focusing optics to increase the light intensity (i.e., energy / area) at the saturable absorber element; for example, focusing light onto the saturable absorber element to ensure the desired nonlinear response. Such focusing elements are optional and only necessary if the intensity of the source output is insufficient to obtain the desired nonlinear response in the nonlinear amplitude filter material.

[0087] Figure 9(a) is a transmittance versus intensity plot illustrating a typical nonlinear response, which can be observed for a typical (forward) saturable absorber, while Figure 9(b) is an equivalent plot that can be observed for a typical anti-saturable absorber. The problem with these materials is that the nonlinear behavior only begins at an intensity threshold IT, which is typically higher than the intensity output of many sources (e.g., similar to the source in Figure 5). For example, this source may have a pump pulse energy of about 5 µJ and a pulse width of 300 fs. Assuming a relatively flat broadband output spectrum with a spectral width of about 1000 nm, the energy of the 10 nm spectrum after filtering is about 50 nJ. Further assuming the pulse duration is similar to that of a pump, the peak power will be in the range of 160 kW. Assuming a beam radius of 2.5 mm, this results in an intensity of 0.8 MW / cm², which is significantly below the intensity threshold IT.

[0088] However, by providing a focusing lens, the beam radius can be significantly reduced and thus the intensity increased, exceeding the intensity threshold IT. For example, a focusing element with NA=0.7 would reduce the radius to 10 μm and the intensity to 50 GW / cm². This is an order of magnitude higher than that required for (for example) SK3 glass, but other materials may have larger nonlinear thresholds. It should be noted that all values ​​provided in this example are purely illustrative to illustrate the principle of using a focusing element. Relevant values ​​can vary significantly depending on the type of source used.

[0089] Figure 10 is a schematic illustration of the source configuration according to the described concept. As in the configuration of Figure 5, the radiation source RDS includes a pulsed pumped radiation source PRS that generates the input radiation IRS and an optical fiber OF. The broadband radiation generated in the optical fiber OF is focused by the focusing element FL onto or around the saturable absorber SA and the anti-saturable absorber RSA, thereby generating an output radiation beam ORD with less noise.

[0090] This example is primarily described based on the use of the proposed nonlinear amplitude filter on the output radiation pulses (e.g., per channel or otherwise). However, direct filtering of the seed laser can be advantageous. Seed lasers are typically orders of magnitude larger than the output lasers and thus can reach the nonlinear region of the material's optical response without the need for a focusing element. On the other hand, while current sources based on hollow core fibers may not currently be able to generate sufficiently high energy pulses for output radiation without a focusing optics, this may change in the future. Therefore, the concepts disclosed herein can be included in filtering seed laser radiation with or without a focusing optics (which is likely not required) or filtering output radiation with or without a focusing optics.

[0091] The concepts described herein can be used for noise reduction in other types of radiation sources besides those described. For example, other such radiation sources may include those used to generate EUV, hard X-ray, or soft X-ray radiation based on higher harmonic generation (HHG) techniques; for example, techniques described in PCT application WO2017186491A1 (incorporated herein by reference). Such HHG sources may include a gas delivery system for delivering the HHG generating medium (gas) and a pump radiation source operable to emit pump radiation to excite the HHG generating medium. It is proposed that the concepts described herein can be applied to pump pulses, and thus reduce noise in such pulses before the (compressed) pump pulses interact with the gaseous medium. Other illumination sources to which this concept is also applicable may include, for example, liquid metal jet sources, inverse Compton scattering (ICS) sources, plasma channel sources, magnetic corrugated machine sources, or free electron laser (FEL) sources. The radiation source may be a narrow-band source that provides a narrow-band output, rather than a wide-band source as described. Finally, the concepts in this document apply to any radiation source that has problems with laser noise (inter-pulse amplitude or energy variation).

[0092] Depending on the materials used, laser noise can be reduced by an order of magnitude using the concepts described herein. This operation can be achieved without incurring significant costs due to the simplicity of the required materials and components. Furthermore, there are no power or size limitations because the components are expected to occupy a small volume.

[0093] The concepts described herein may be applied to only one or a subset of color channels or wavelength bands (where the source has a wideband output). For example, if the output includes frequency bands where only a subset has an intensity above the intensity threshold, it is possible to provide a nonlinear amplitude filter only to the frequency band of this subset, or to apply a filter without a focusing element to this subset while applying a filter with a focusing element to other frequency bands.

[0094] Although the concepts described herein refer to transmission energy absorbing optical elements or saturated absorption elements, the concepts described may also be achieved using reflective nonlinear amplitude filters (e.g., reflective energy absorbing optical elements or saturated absorption elements) and reflective focusing elements (which may include metamaterials with appropriate engineering design).

[0095] Although the radiation sources described herein are disclosed in the context of providing measurement radiation in lithography or IC manufacturing settings, they are not limited thereto and can be used to provide illumination for measurement purposes or for illumination for purposes other than lithography or IC manufacturing settings. The sources disclosed herein can be used in any situation where source noise is a problem.

[0096] Further embodiments are disclosed in the following list of numbered entries: 1. A radiation source configuration comprising: a radiation source operable to generate source radiation containing source energy pulses; and at least one nonlinear energy filter operable to filter the source radiation to obtain filtered radiation containing filtered energy pulses; wherein the at least one nonlinear energy filter is operable to mitigate energy variations in the filtered radiation by reducing the energy level of the source energy pulses having an energy level corresponding to one of the two ends of an energy distribution of the source energy pulses by an amount greater than that of the source energy pulses having an energy level corresponding to a peak of the energy distribution. 2. The radiation source configuration of clause 1, wherein the at least one nonlinear energy filter comprises at least one nonlinear amplitude filter operable to mitigate amplitude variations in the filtered radiation by reducing the amplitude level of the source energy pulses having an amplitude level corresponding to one of the two ends of an amplitude distribution of the source energy pulses by a greater amount than that of the source energy pulses having an amplitude level corresponding to a peak of the amplitude distribution. 3. The radiation source configuration of clause 2, wherein the at least one nonlinear amplitude filter comprises a nonlinear characteristic such that for at least some of a first subset of the source energy pulses, including those source energy pulses having an amplitude level higher than a peak amplitude in the amplitude distribution: the greater the amplitude level of the source energy pulse, the greater the attenuation of the amplitude level. 4. The radiation source configuration of clause 3, wherein the first subset of the source energy pulses includes source energy pulses having an amplitude level higher than a first threshold amplitude level. 5. The radiation source configuration of clause 3 or 4, wherein the at least one nonlinear amplitude filter is operable to reduce the amplitude level of the first subset of the source energy pulses to an amplitude level closer to the average value of an amplitude distribution of the filtered energy pulses. 6. The radiation source configuration of clauses 3, 4, or 5, wherein the at least one nonlinear amplitude filter comprises at least one saturable absorber element. 7. The radiation source configuration of clause 6, wherein the at least one saturable absorber element comprises a saturable absorber glass or a saturable absorber nanoparticle material. 8. The radiation source configuration of clause 7, wherein the saturable absorber glass comprises: a borosilicate glass, an ultra-low expansion glass, or a Sk3 glass. 9. The radiation source configuration of clause 7, wherein the saturable absorber nanoparticle material comprises a metal oxide nanosphere or nanoellipsoidal material. 10. The radiation source configuration of any one of items 2 to 9, wherein the at least one nonlinear amplitude filter includes a nonlinear characteristic such that for at least some or most of a second subset of the source energy pulses, the source energy pulses containing an amplitude level having an amplitude level below a second threshold value are suppressed.11. The radiation source configuration of clause 10, wherein the suppression attenuates at least some or most of the second subset of the energy pulses from the sources to substantially zero or the nominal amplitude level. 12. The radiation source configuration of clauses 10 or 11, wherein the at least one nonlinear amplitude filter comprises an antisaturable absorption element. 13. The radiation source configuration of clause 12, wherein the antisaturable absorption element comprises a dichalcogenide or transition metal dichalcogenide monolayer, a Cu5O-Ag nanostructure, or an indanthrene compound. 14. The radiation source configuration of any of the preceding clauses, wherein the at least one nonlinear energy filter is located at one output of the radiation source. 15. The radiation source configuration of clause 14, comprising a plurality of the at least one nonlinear energy filter, such that at least one separate nonlinear energy filter is provided for different wavelength bands of the output radiation. 16. The radiation source configuration of clauses 14 or 15, comprising at least one focusing element to reduce the beam region of the output radiation on the at least one nonlinear energy filter. 17. A radiation source configuration as described in any of clauses 1 to 13, wherein the radiation source comprises a pump or seed laser for generating input radiation and a medium for exciting by the input radiation, and the at least one nonlinear energy filter is located in the path of the input radiation. 18. A radiation source configuration as described in any of the preceding clauses, wherein the output radiation comprises broadband output radiation. 19. A radiation source configuration as described in clause 18, wherein the output radiation comprises a wavelength less than 400 nm. 20. A radiation source configuration as described in clauses 18 or 19, wherein the radiation source comprises a hollow core fiber, the hollow core fiber comprising a working medium and an optical input for receiving input radiation to excite the working medium to generate the broadband output radiation. 21. A radiation source configuration as described in clause 20, wherein the hollow core fiber comprises a hollow core photonic crystal fiber. 22. A measuring device comprising a radiation source configuration as described in any of the preceding clauses, wherein the radiation source configuration is configured to generate radiation for projection onto a substrate. 23. A measuring device as described in clause 22, wherein the measuring device is one of: a scatterometer, an alignment sensor, or a leveling sensor. a. A radiation source configuration comprising: a radiation source operable to generate source radiation containing source energy pulses; and at least one nonlinear energy filter operable to filter the source radiation to obtain filtered radiation containing filtered energy pulses; wherein the at least one nonlinear energy filter is operable to mitigate energy variations in the filtered radiation by reducing the energy level of the source energy pulses having an energy level corresponding to one or both ends of an energy distribution of the source energy pulses, the reduction being greater than the reduction of the source energy pulses having an energy level corresponding to a peak of the energy distribution.b. The radiation source configuration as described in clause a, wherein the at least one nonlinear energy filter comprises at least one nonlinear amplitude filter operable to mitigate amplitude variations in the filtered radiation by reducing the amplitude level of the source energy pulse having an amplitude level corresponding to one or both of the two ends of an amplitude distribution of the source energy pulses, by a reduction amount greater than the reduction amount of the source energy pulse having an amplitude level corresponding to a peak of the amplitude distribution.

[0097] Although reference may be specifically made herein to the use of lithography equipment in IC manufacturing, it should be understood that the lithography equipment described herein may have other applications, such as manufacturing integrated optical systems, guiding and detecting patterns for magnetic domain memory, flat panel displays, liquid crystal displays (LCDs), thin-film magnetic heads, etc. Those skilled in the art will understand that, in the context of these alternative applications, any use of the terms "wafer" or "die" herein may be considered synonymous with the more general terms "substrate" or "target portion," respectively. The substrates mentioned herein may be processed before or after exposure in, for example, in coating and developing systems (tools that typically apply a resist layer to the substrate and develop the exposed resist), measuring tools, and / or inspection tools. Where applicable, the inventions herein may be applied to these and other substrate processing tools. Furthermore, the substrate may be processed more than once, for example, to produce multilayer ICs, such that the term "substrate" as used herein may also refer to a substrate that already contains one or more processed layers.

[0098] Although specific embodiments of the invention have been described above, it will be understood that the invention may be practiced in other ways different from those described.

[0099] The above description is intended to be illustrative and not restrictive. Therefore, it will be apparent to those skilled in the art that modifications can be made to the described invention without departing from the scope of the claims set forth below. [Simplified Explanation of the Diagram]

[0010] Embodiments of the invention will now be described by way of example only with reference to the accompanying schematic drawings, in which: - Figure 1A depicts a schematic overview of a lithography apparatus; - Figure 1B depicts a schematic overview of a lithography unit; - Figure 2 depicts a schematic representation of a bulk lithography, illustrating the collaboration between three key technologies for optimizing semiconductor manufacturing; - Figure 3A depicts a schematic block diagram of an alignment sensor; - Figure 3B depicts a schematic block diagram of a level sensor; - Figure 4 is a schematic cross-sectional view of a hollow core fiber that can form a portion of the radiation source according to the embodiment in a lateral plane (i.e., perpendicular to the axis of the fiber); - Figure 5 depicts a schematic representation of a radiation source according to the embodiment for providing broadband output radiation; and - Figures 6(a) and (b) schematically depict cross-sections of examples of designs for supercontinuous generation hollow core photonic crystal fiber (HC-PCF), each design forming a portion of the radiation source according to the embodiment; Figure 7 is a plot of amplitude versus pulse count for the following: (a) a nominally perfect source unaffected by noise; and (b) a real source illustrating the effect of radiated noise; - Figure 8 includes: (a) a histogram illustrating the amplitude distribution of a prior art radiating source; (b) a histogram illustrating the amplitude distribution of a source configuration according to an embodiment of the present invention; and (c) a histogram illustrating the input and output amplitude distribution of a stack of another example of a source configuration according to an embodiment of the present invention; - Figure 9 includes a plot of transmittance versus input intensity for the following: (a) a saturable absorber and (b) an anti-saturable absorber; and - Figure 10 is a schematic illustration of a source configuration according to an embodiment of the present invention.

Claims

1. A radiation source configuration comprising: a radiation source operable to generate source radiation containing source energy pulses; and at least one nonlinear energy filter operable to filter the source radiation to obtain filtered radiation containing filtered energy pulses; wherein the at least one nonlinear energy filter is operable to mitigate energy variations in the filtered radiation by reducing the energy level of the source energy pulses having an energy level corresponding to one of the two ends of an energy distribution of the source energy pulses by an amount greater than the amount of reduction of the source energy pulses having an energy level corresponding to a peak of the energy distribution.

2. The radiation source configuration of claim 1, wherein the at least one nonlinear energy filter includes at least one nonlinear amplitude filter operable to mitigate amplitude variations in the filtered radiation by reducing the amplitude level of the source energy pulse having an amplitude level corresponding to one of the two ends of an amplitude distribution of the source energy pulses by an amount greater than the amount of reduction of the source energy pulse having an amplitude level corresponding to a peak of the amplitude distribution.

3. The radiation source configuration as claimed in claim 2, wherein the at least one nonlinear amplitude filter includes a nonlinear characteristic such that for at least some of a first subset of the source energy pulses, which includes the source energy pulses having an amplitude level higher than a peak amplitude in the amplitude distribution: the greater the amplitude level of the source energy pulse, the greater the attenuation of the amplitude level.

4. The radiation source configuration as claimed in claim 3, wherein the first subset of the source energy pulses includes source energy pulses having an amplitude level higher than a first threshold amplitude level.

5. The radiation source configuration as claimed in claim 3 or 4, wherein the at least one nonlinear amplitude filter is operable to reduce the amplitude level of the first subset of the source energy pulses to an amplitude level closer to the average value of one amplitude distribution of the filtered energy pulses.

6. The radiation source configuration as requested in item 3 or 4, wherein the at least one nonlinear amplitude filter comprises at least one saturable absorption element.

7. The radiation source configuration as claimed in claim 6, wherein the at least one saturable absorber element comprises a saturable absorber glass or a saturable absorber nanoparticle material.

8. The radiation source configuration of claim 7, wherein at least one of the following is present: the saturable absorber glass comprises: a borosilicate glass, an ultra-low expansion glass or a Sk3 glass, and the saturable absorber nanoparticle material comprises a metal oxide nanosphere or nanoellipsoid material.

9. The radiation source configuration of any one of claims 2 to 4, wherein the at least one nonlinear amplitude filter includes a nonlinear characteristic such that for at least some or most of a second subset of the source energy pulses, which includes other source energy pulses having an amplitude level below a second threshold amplitude level, the amplitude level is suppressed, and where appropriate, the suppression causes at least some or most of the second subset of the source energy pulses to decay to substantially zero or the nominal amplitude level.

10. The radiation source configuration as claimed in claim 9, wherein the at least one nonlinear amplitude filter includes an anti-saturation absorption element.

11. The radiation source configuration as claimed in claim 10, wherein the anti-saturation absorption element comprises a dichalcogenide or transition metal dichalcogenide monolayer, a Cu5O-Ag nanoheterostructure, or an indanthrene compound.

12. A radiation source configuration as claimed in any of claims 1 to 4, wherein the at least one nonlinear energy filter is located at one of the outputs of the radiation source, and, where appropriate, the radiation source configuration includes a plurality of the at least one nonlinear energy filter, such that at least one separate nonlinear energy filter is provided for different wavelength bands of the output radiation.

13. The radiation source configuration of claim 12, which includes at least one focusing element to reduce one beam region of the output radiation on the at least one nonlinear energy filter.

14. The radiation source configuration of any one of claims 1 to 4, wherein the radiation source comprises a hollow core fiber, the hollow core fiber comprising a working medium and an optical input for receiving input radiation to excite the working medium to generate the broadband output radiation, and, where appropriate, the hollow core fiber comprises a hollow core photonic crystal fiber.

15. A measuring device comprising a radiation source configuration as described in any one of claims 1 to 14, wherein the radiation source configuration is configured to generate radiation to be projected onto a substrate.

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