Method for generating broadband radiation and associated broadband source and metrology device

Abstract

A method of generating broadband output radiation and an associated broadband radiation source are disclosed. The method comprises: generating a pulse of input radiation, the pulse having a duration in the range 50 fs to 400 fs and having a rise time of less than 60 fs; and exciting a working medium within a hollow-core fiber with the pulse of input radiation.

Description

[0001] Cross-references to related applications

[0002] This application claims priority to European Application 20189212.2, filed on August 3, 2020, and European Application 20198713.8, filed on September 28, 2020, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to a light source and a method for operating the light source, the light source being particularly a broadband light source for photolithography equipment or measurement tools. Background Technology

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

[0005] To project a pattern onto a substrate, photolithography 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 used are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. Photolithography equipment using extreme ultraviolet (EUV) radiation with wavelengths in the range of 4 nm to 20 nm (e.g., 6.7 nm or 13.5 nm) can be used to form smaller features on the substrate compared to photolithography equipment using, for example, radiation with a wavelength of 193 nm.

[0006] Low-k1 lithography can be used to process features with dimensions smaller than the classical resolution limit of lithography equipment. In this process, the resolution formula can be expressed as CD = k1 × λ / NA, where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection optics in the lithography equipment, CD is the "critical size" (typically the smallest feature size printed, but in this case, half a pitch), and k1 is an empirical resolution factor. Generally, 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 functions and performance. To overcome these difficulties, complex fine-tuning steps can be applied to the lithographic projection equipment and / or design layout. These steps include, for example, but not limited to: NA optimization, customized illumination schemes, use of phase-shifting patterning apparatus, 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 generally defined as "resolution enhancement techniques" (RET). Alternatively, a tight control loop for controlling the stability of the lithography apparatus can be used to improve the reproduction of the pattern under low k1 conditions.

[0007] The measurement equipment can be used to measure parameters of interest related to the structure on the substrate. For example, the measurement equipment can be used to measure parameters such as critical dimensions, overlap between layers on the substrate, and asymmetry of the pattern on the substrate. A beam of radiation is used to irradiate the substrate. The radiation is diffracted by the structure on the substrate. The diffracted radiation is collected by an objective lens and captured by a sensor.

[0008] The radiation measured is provided by light emitted through a light source. This light is directed onto the substrate via a beam splitter and an objective lens, which collects the diffracted radiation from the substrate.

[0009] The light source providing the radiation measurement can be a broadband light source. The broadband light source can be generated using a gas-filled optical fiber. A laser source can be coupled to the input of the optical fiber of the light source and broadened spectrally within the fiber by excitation by pulsed laser pulses.

[0010] The goal is to improve one or more spectral broadening properties in optical fibers. Summary of the Invention

[0011] According to a first aspect, a method for generating broadband output radiation by exciting a working medium within a hollow core optical fiber is provided, the method comprising:

[0012] A pulse of input radiation is generated, the pulse having a duration in the range of 50 fs to 400 fs and a rise time of less than 60 fs; and the working medium is excited by the pulse of input radiation.

[0013] According to a second aspect of the present invention, a broadband radiation source for generating broadband output radiation is provided, comprising: an input radiation source operable to generate a pulse of input radiation having a duration in the range of 50 fs to 400 fs and a rise time of less than 60 fs; and a hollow core optical fiber including a working medium and an optical input section for receiving the input radiation to excite the working medium to generate the broadband output radiation. Attached Figure Description

[0014] Embodiments of the invention will now be described by way of example only with reference to the accompanying illustrative drawings, in which:

[0015] - Figure 1A A schematic schematic diagram depicting a photolithography apparatus;

[0016] - Figure 1B A schematic schematic diagram depicting a photolithography unit;

[0017] - Figure 2 A schematic representation of overall photolithography depicting the collaboration between three key technologies used to optimize semiconductor manufacturing;

[0018] - Figure 3A Describe a schematic block diagram of the alignment sensor;

[0019] - Figure 3B A schematic block diagram depicting the horizontal sensor;

[0020] - Figure 4 It is a schematic cross-sectional view of a hollow core optical fiber that can form a portion of the radiation source according to the embodiment in a transverse plane (i.e., perpendicular to the axis of the optical fiber).

[0021] - Figure 5 A schematic representation depicting a radiation source according to an embodiment for providing broadband output radiation; and

[0022] - Figure 6 (a) and (b) schematically depict cross-sections of example hollow-core photonic crystal fiber (HC-PCF) designs for supercontinuum spectrum generation, each of which can form part of a radiation source according to an embodiment;

[0023] - Figure 7It is a graph of the input radiation intensity versus time, which illustrates the prior art pulse used to excite the working medium within the HC-PCF;

[0024] - Figure 8 This is a graph of the input radiation intensity versus time according to a first embodiment of the present invention, illustrating a pulse that can be used to excite the working medium within the HC-PCF; and

[0025] - Figure 9 This is a graph of the input radiation intensity versus time according to a second embodiment of the present invention, illustrating a pulse that can be used to excite the working medium within the HC-PCF. Detailed Implementation

[0026] In this document, the terms “radiation” and “beam” are used to cover all types of electromagnetic radiation, including ultraviolet radiation (e.g., with wavelengths of 365 nm, 248 nm, 193 nm, 157 nm, or 126 nm) and EUV (e.g., extreme ultraviolet radiation with wavelengths in the range of about 5 nm to 100 nm).

[0027] As used herein, the terms “mask,” “mask,” or “patterning apparatus” can be broadly interpreted to refer to a general patterning apparatus 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 the substrate. The term “optical valve” can also be used in this context. Examples of such patterning apparatuses, in addition to classic masks (transmissive or reflective masks, binary masks, phase-shifting masks, hybrid masks, etc.), include programmable mirror arrays and programmable LCD arrays.

[0028] Figure 1A A lithography apparatus LA is schematically depicted. The lithography apparatus LA includes: an irradiation system (also called an irradiator) IL configured to modulate a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation); a mask support (e.g., a mask stage) MT configured to support a patterning apparatus (e.g., a mask) MA and connected to a first positioning device PM configured to accurately position the patterning apparatus MA according to specific parameters; a substrate support (e.g., a wafer stage) WT configured to hold a substrate (e.g., a wafer coated with resist) W and connected to a second positioning device PW configured to accurately position the substrate support according to specific parameters; and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted by the patterning apparatus MA to the radiation beam B onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0029] As used herein, the term "projection system" PS should be interpreted broadly to encompass all types of projection systems, including refractive, reflective, reflective-refractive, distorting, magnetic, electromagnetic, and / or electrostatic optical systems, or any combination thereof, depending on the application of the exposure radiation being used and / or other factors such as immersion in liquids or vacuum. Any use of the term "projection lens" herein may be considered synonymous with the broader term "projection system" PS.

[0030] In addition to the substrate support WT, the lithography apparatus LA may include a measurement platform. The measurement platform is arranged to hold sensors and / or cleaning devices. The sensors may be arranged to measure properties of the projection system PS or the radiation beam B. The measurement platform may hold multiple sensors. The cleaning devices may be arranged to clean a portion of the lithography apparatus, such as a portion of the projection system PS or a portion of the system providing the immersion liquid. The measurement platform may move below the projection system PS as the substrate support WT moves away from the projection system PS.

[0031] In operation, the radiation beam B is incident on the pattern forming apparatus (e.g., mask MA) held on the mask support MT and patterned by the pattern (design layout) present on the pattern forming apparatus MA. After passing through the mask MA, the radiation beam B is passed through the projection system PS, which focuses the beam onto the target portion C of the substrate W. With the aid of the second positioning device PW and the position measurement system IF, the substrate support WT can be accurately moved, for example, to position different target portions C in a focused and aligned position within the path of the radiation beam B. Similarly, the first positioning device PM and possibly another position sensor (which in...) Figure 1A (Not explicitly depicted) can be used to accurately position the patterning apparatus MA relative to the path of the radiation beam B. Mask alignment marks M1, M2 and substrate alignment marks P1, P2 can be used to align the patterning apparatus MA and the substrate W. Although the substrate alignment marks P1, P2, as illustrated, occupy dedicated target portions, they can be located in the space between the target portions. When the substrate alignment marks P1, P2 are located between the target portions C, they are referred to as scribe alignment marks.

[0032] like Figure 1BAs shown, the lithography apparatus LA can form part of a lithography unit LC (sometimes also called a lithography cell or (lithography) cluster), which typically also includes equipment for performing pre-exposure and post-exposure processes on a substrate W. Conventionally, these devices include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chiller CH for regulating the temperature of the substrate W, and a baking plate BK (e.g., for regulating the solvent in the resist layer). A substrate transport device, or robot RO, picks up the substrate W from input port I / O1 and output port I / O2, moves these substrates W between different process devices, and transfers the substrate W to the loading stage LB of the lithography apparatus LA. The devices in the lithography cell, also commonly referred to as tracks or coating / developing systems, are typically under the control of a track or coating / developing system control unit TCU, which itself can be controlled by a management control system SCS, which can also control the lithography apparatus LA, for example, via a lithography control unit LACU.

[0033] To ensure accurate and consistent exposure of the substrate W exposed by the lithography apparatus LA, it is necessary to inspect the substrate to measure properties of the patterned structure, such as overlap error between subsequent layers, line thickness, critical dimension (CD), etc. For this purpose, an inspection tool (not shown) may be included in the lithography unit LC. If errors or mistakes are detected, adjustments can be made, for example, to the exposure of subsequent substrates or to other processing steps to be performed on the substrate W, particularly if inspection is performed on other substrates W in the same batch or group before they are exposed or processed.

[0034] Inspection equipment, also known as metrology equipment, is used to determine the properties of the substrate W, and in particular to determine how the properties of different substrates W vary, or how the properties associated with different layers of the same substrate W vary between different layers. The inspection equipment may alternatively be configured to identify defects on the substrate W, and may be, for example, part of the photolithography unit LC, or integrated into the photolithography apparatus LA, or even a separate device. The inspection equipment can measure properties on a latent image (an image in a resist layer after exposure), or a semi-latent image (an image in a resist layer after a post-exposure baking (PEB) step), or a developed resist image (in which the exposed or unexposed portions of the resist have been removed), or even on an etched image (after a pattern transfer step such as etching).

[0035] Typically, the patterning process in a photolithography (LA) apparatus is one of the most critical steps in the process, requiring high accuracy in determining the size and placement of the structure on the substrate W. To ensure this high accuracy, such as... Figure 2 The diagram schematically depicts three systems that can be combined within a so-called “holistic” control environment. One of these systems is the lithography apparatus LA, which is (in effect) connected to a metrology tool MT (the second system) and a computer system CL (the third system). The key to this “holistic” environment is optimizing the coordination between these three systems to enhance the overall process window and provide a tight, i.e., stringent control loop to ensure 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., dose, focus, overlap) within which a particular manufacturing process produces a defined result (e.g., a functional semiconductor device), typically allowing the process parameters during the lithography or patterning process to vary within this range.

[0036] The computer system CL can use the design layout (partial) to be patterned to predict which resolution enhancement techniques will be used, and perform computational lithography simulations and calculations to determine which mask layouts and lithography equipment settings achieve the maximum overall process window (in) of the patterning process. Figure 2 (Depicted by a double arrow in the first scale SC1). Typically, the resolution enhancement technique is arranged to match the patterning possibilities of the lithography apparatus LA. The computer system CL can also be used to detect where the lithography apparatus LA is currently operating within the process window (e.g., using input from the metrology tool MT) to predict whether defects exist due to, for example, suboptimal processing (in... Figure 2 (This is depicted by the arrow pointing to "0" in the second scale SC2).

[0037] The measurement tool MT can provide input to the computer system CL to enable accurate simulation and prediction, and can provide feedback to the lithography equipment LA to identify, for example, its calibration status (in...). Figure 2 Possible drift (described by multiple arrows in the third scale SC3).

[0038] During photolithography, it is desirable to frequently measure the resulting structure, for example, for process control and verification. Different types of metrology equipment (MTs) are known for performing these measurements, including scanning electron microscopes or various forms of scatterometer metrology equipment.

[0039] A scattering meter is a multifunctional instrument that allows for the measurement of parameters of a photolithography process by placing a sensor in the pupil of the objective lens of the scattering meter or in a conjugate plane conjugate to the pupil (the measurement is generally referred to as a pupil-based measurement), or by placing a sensor in an image plane or in a plane conjugate to the image plane (in this case, the measurement is generally referred to as an image- or field-based measurement). Such scattering meters 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 scattering meter can measure gratings using light from embodiments of the light sources discussed in this document.

[0040] The overall measurement quality for a particular lithography parameter is determined at least in part by the measurement scheme used to measure it. The term "substrate measurement scheme" can include one or more parameters of the measurement itself, one or more parameters of the measured pattern, or both. For example, if the measurement used in a substrate measurement scheme is a diffraction-based optical measurement, the one or more parameters of the measurement can include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, and so on. One of the criteria for selecting a measurement scheme can be, for example, the sensitivity of one of the measurement parameters to handling variations. Further examples are described in U.S. Patent Application US2016-0161863, which is incorporated herein by reference in its entirety, and published U.S. Patent Application US2016 / 0370717A1. The light source described in this document can be configured to be controllable with respect to the light source requirements of these substrate measurement schemes.

[0041] Photolithography apparatuses may include one or more alignment sensors that can accurately measure the position of alignment marks formed on a substrate. The alignment (or position) sensors can use optical phenomena such as diffraction and interference to obtain positional information from the alignment marks formed on the substrate. Examples of alignment sensors used in current photolithography apparatuses are based on self-reference interferometers as described in US6961116. Various enhancements and modifications of position sensors have been developed, for example, as disclosed in US2015261097A1. All of these disclosures are incorporated herein by reference.

[0042] Markings, or alignment marks, may comprise a series of gratings formed on or within a layer disposed on the substrate, or (directly) formed in the substrate. These gratings may be regularly spaced and act as grating lines, such that the marks can be considered as diffraction gratings with a known spatial period (pitch). Depending on the orientation of these grating lines, the marks can be designed to allow measurement of position along the X-axis or along the Y-axis (the Y-axis being substantially perpendicular to the X-axis orientation). Markings comprising gratings arranged at +45 degrees and / or -45 degrees relative to both the X and Y axes allow for combined X and Y measurements using techniques described in US2009 / 195768A, which is incorporated herein by reference.

[0043] The alignment sensor optically scans each mark using 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 thus the position of the substrate relative to the alignment sensor, which is then fixed relative to the reference frame of the photolithography apparatus. So-called coarse and fine marks associated with different (coarse and fine) mark sizes can be provided, allowing the alignment sensor to distinguish different periods of the periodic signal and the precise position (phase) within each period. Marks with different pitches can also be used for this purpose.

[0044] Measuring the location of the markings can also provide information about the deformation of a substrate on which the markings (e.g., in the form of a wafer grid) are disposed. For example, substrate deformation may occur by electrostatically clamping the substrate to a substrate stage, and / or by heating the substrate when it is exposed to radiation.

[0045] Figure 3A This is a schematic block diagram of an embodiment of a known alignment sensor AS (such as that described, for example, in US6961116, which is incorporated herein by reference). A radiation source RSO provides a radiation beam RB of one or more wavelengths, which is directed by a steering optics onto a marker (such as a marker 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 embodiments of the light source disclosed in this document. The diameter of the illumination spot SP used to illuminate the marker AM may be slightly smaller than the width of the marker itself.

[0046] The radiation diffracted by the marker AM (via the objective lens OL in this example) is collimated 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). The beam IB is interfered with by a self-reference interferometer (SRI) (e.g., the type disclosed in US6961116 mentioned above), after which the beam is received by a photodetector PD. In cases where more than one wavelength is generated by the radiation source RSO, additional optics (not shown) may be included to provide multiple separate beams. The photodetector may be a single element, or it may include multiple pixels, if desired. The photodetector may include a sensor array.

[0047] A topography measurement system, a level sensor, or a height sensor, which can be integrated into a photolithography apparatus, is arranged to measure the topography of the top surface of a substrate (or wafer). A topography map (also called a height map) of the substrate can be generated from these measurements, which indicate the height of the substrate as a function of its position. This height map can then be used to correct the position of the substrate during pattern transfer onto the substrate, so as to provide a spatial image of the pattern forming apparatus at an appropriate focal position on the substrate. It will be understood that "height" in this context refers to a dimension significantly outside a plane (also called the Z-axis) relative to the substrate. Typically, the level sensor or height sensor performs measurements at a fixed location (relative to its own optical system), and the relative motion between the substrate and the optical system of the level sensor or height sensor produces height measurements at multiple locations across the entire substrate.

[0048] Figure 3B The diagram schematically illustrates an example of a level or height sensor LS as known in the art, the example being merely an illustration of the principle of operation. 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, the radiation beam LSB being imparted by a projection grating PGR of the projection unit LSP. The radiation source LSO may include embodiments of the disclosures in this document.

[0049] This disclosure relates to improving the operational lifetime of light sources, particularly broadband light sources including hollow-core photonic crystal fibers (HC-PCF). The broadband light sources of this disclosure can be used in measurement tools, such as scatterometers, alignment sensors, height or level sensors as described above.

[0050] The measurement tools (MTs) mentioned above (such as scatterometers, topography measurement systems, or position measurement systems) can perform measurements using radiation originating from a radiation source. The nature of the radiation used by the measurement tool can affect the type and quality of the measurements that can be performed. For some applications, it can be advantageous to use multiple radiation frequencies to measure the substrate, for example, broadband radiation can be used. Multiple different frequencies can propagate, illuminate, and scatter away from the measurement target without interfering with other frequencies or with minimal interference. Therefore, different frequencies can be used, for example, to obtain more measurement data simultaneously. Different radiation frequencies can also enable the querying and discovery of different properties of the measurement target. Broadband radiation can be used in measurement systems (MTs) such as, for example, level sensors, alignment mark measurement systems, scattering measurement tools, or inspection tools. The broadband radiation source can be a supercontinuum source.

[0051] High-quality broadband radiation (e.g., supercontinuum radiation) can be difficult to generate. One approach to generating broadband radiation is, for example, to broaden high-power narrowband or single-frequency input radiation using nonlinear higher-order effects. This input radiation (which can be generated using a laser) can be referred to as pump radiation. Alternatively, it can be referred to as seed radiation. To obtain high-power radiation for the broadening effect, the radiation can be confined to a small region to achieve highly localized high-intensity radiation. In those 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.

[0052] In some embodiments, the broadband output radiation (PCF) is generated within a photonic crystal fiber. In several embodiments, such a photonic crystal fiber has a microstructure around its fiber core that helps confine the radiation traveling through the fiber within the fiber core. The fiber core can be made of a solid material with nonlinear properties that can generate broadband radiation when high-intensity pump radiation is transmitted through the fiber core. While generating broadband radiation in a solid-core photonic crystal fiber is feasible, using a solid material can have several disadvantages. 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.

[0053] In some implementations, see the following references Figure 5Furthermore, methods and apparatus for broadening input radiation can utilize optical fibers used to confine input radiation and broaden it to output broadband radiation. The optical fiber may be a hollow-core fiber and may include internal structures for effectively guiding and confining 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 by confining radiation primarily within the hollow core of the fiber. The hollow core of the optical 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 arrangement can be used to generate a supercontinuum radiation source. The radiation input to the optical fiber may be electromagnetic radiation, such as radiation in one or more of the infrared, 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 precise spectrum and power density of the output radiation will be determined by multiple parameters such as fiber structure, gas mixture composition, gas pressure, energy of input radiation, pulse duration and pulse shape of input radiation.

[0054] Some embodiments relate to novel designs of such broadband radiation sources comprising optical fibers. The optical fiber is a hollow-core photonic crystal fiber (HC-PCF). Specifically, the optical fiber can be a type of hollow-core photonic crystal fiber including an anti-resonant structure for confining radiation. Such fibers including anti-resonant structures are known in the art as anti-resonant fibers, tubular fibers, single-loop fibers, negative curvature fibers, or suppression-coupled fibers. Various different designs of such fibers are known in the art. Alternatively, the optical fiber can be a photonic bandgap fiber (HC-PBF, such as Kagome fiber).

[0055] Several types of HC-PCFs can be designed, each based on a different physical guidance mechanism. Two such HC-PCFs include: Hollow Core Photonic Bandgap Fiber (HC-PBF) and Hollow Core Anti-Resonant Reflective Fiber (HC-ARF). Details regarding 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 optical fiber including a Kagome lattice structure.

[0056] Now for reference Figure 4 Describe an example of the optical fiber used in the radiation source. Figure 4This is a schematic cross-sectional view of the optical fiber (OF) in the transverse plane. It is disclosed in WO2017 / 0324541. Figure 4 Other embodiments similar to the actual examples of optical fibers.

[0057] An optical fiber (OF) includes an elongated body that is longer in one dimension than the other two. This longer dimension may be referred to as the axial direction and defines the axis of the optical fiber. The other two dimensions define planes that may be referred to as transverse planes. Figure 4 A cross-sectional view of the optical fiber OF is shown in this transverse plane (i.e., perpendicular to the axis), labeled as the xy plane. The transverse cross-section of the optical fiber OF can be substantially constant along the fiber axis.

[0058] It will be understood that optical fibers (OFs) possess a degree of flexibility or flexibility, and therefore the direction of the axis will not typically be uniform along the length of the optical fiber (OF). Terms such as optical axis, lateral cross-section, etc., will be understood to refer to local optical axis, local lateral cross-section, etc. Furthermore, when components are described as cylindrical or tubular, these terms will be understood to encompass these shapes that may deform when the optical fiber (OF) is bent.

[0059] Optical fiber OF can have any length, and it will be understood that the length of optical fiber OF can depend on the application. The length of optical fiber OF can be between 1cm and 10m or between 0.1cm and 10m, for example, the length of optical fiber OF can be between 10cm and 100cm.

[0060] An optical fiber (OF) includes: a hollow core (COR); a cladding portion surrounding the hollow core (COR); and a support portion (SP) surrounding and supporting the cladding portion. An optical fiber (OF) can be considered to include a body (including the cladding portion and the support portion SP) having a hollow core (COR). The cladding portion includes a plurality of anti-resonant elements for guiding radiation through the hollow core (COR). Specifically, the plurality of anti-resonant elements are arranged to primarily confine radiation propagating through the optical fiber (OF) within the hollow core (HC) and guide the radiation along the optical fiber (OF). The hollow core (HC) 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 (HC).

[0061] The cladding portion includes multiple anti-resonant elements for guiding radiation propagation through the optical fiber (OF). Specifically, in this embodiment, the cladding portion includes a single ring having six tubular capillary CAPs. Each tubular capillary CAP acts as an anti-resonant element.

[0062] A capillary CAP can also be referred to as a tube. The cross-section of a capillary CAP can be circular or may have other shapes. 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 against radiation propagating through the hollow core HC (and which may be incident on the wall portion WP at a grazing angle of incidence). The thickness of the wall portion WP can be adapted to ensure substantially enhanced reflections returning to the hollow core HC, while substantially suppressing transmission into the capillary cavity CC. In some embodiments, the capillary wall portion WP can have a thickness between 0.01 μm and 10.0 μm.

[0063] It will be understood that, as used herein, the term "covered portion" is intended to refer to the portion of an optical fiber (OF) used to guide radiation propagation through the OF (i.e., the capillary CAP that confines the radiation within the hollow core (COR)). The radiation can be confined in the form of lateral modes propagating along the fiber axis.

[0064] The support portion is generally tubular and supports six capillary CAPs of the covering portion. If it is an inner support portion SP, the six capillary CAPs are evenly distributed around the inner surface. The six capillary CAPs can be described as being arranged in a generally hexagonal shape.

[0065] The capillary CAPs are arranged such that each capillary does not contact any other capillary CAP. Each capillary CAP contacts the inner support portion SP and is spaced apart from adjacent capillary CAPs in the annular structure. This arrangement can be advantageous because it can increase the transmission bandwidth of the optical fiber OF (e.g., relative to an arrangement where capillaries are in contact with each other). Alternatively, in some embodiments, each capillary CAP may contact adjacent capillary CAPs in the annular structure.

[0066] The six capillary capillaries (CAPs) of the cladding portion are arranged 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, mode complexity (i.e., modal multivariability), and nonlinear properties of the hollow core optical fiber (OF).

[0067] In this embodiment, the cladding portion comprises a single-ring arrangement of capillary CAPs (which act as anti-resonant elements). Therefore, no line in any radial direction from the center of the hollow core HC to the outside of the optical fiber OF will pass through more than one capillary CAP.

[0068] It will be understood that other embodiments may have different arrangements of anti-resonant elements. These may include arrangements of multiple rings with anti-resonant elements, and arrangements of nested, i.e., nested, anti-resonant elements. Furthermore, although... Figure 4 The illustrated embodiment includes a ring of six capillaries, but in other embodiments, one or more rings may be provided in the encapsulation portion, the one or more rings including any number of anti-resonant elements (e.g., 4, 5, 6, 7, 8, 9, 10, 11 or 12 capillaries).

[0069] Figure 6 (b) illustrates a modified embodiment of the single-ring HC-PCF with multiple tubular capillaries discussed above. Figure 6 In example (b), there are two coaxial rings of the tubular capillary 21. To hold the inner and outer rings of the tubular capillary 21 together, a support tube ST may be included in the HC-PCF. The support tube may be made of silica.

[0070] Figure 4 and Figure 6 The tubular capillaries in examples (a) and (b) can have a circular cross-sectional shape. Tubular capillaries can also be other shapes, such as elliptical or polygonal cross-sections. Alternatively, Figure 4 and Figure 6 The solid material of the tubular capillary in examples (a) and (b) may include plastic materials such as PMA, glass such as silica, or soft glass.

[0071] Figure 5 A radiation source RDS for providing broadband output radiation is described. The radiation source RDS includes a pulse-pumped radiation source PRS, or any other type of source capable of generating short pulses with desired length and energy levels; an optical fiber OF having a hollow core COR (e.g., Figure 4 (as shown in the image); and the working medium WM (e.g., gas) disposed within the hollow core COR. Although in Figure 5 In this context, the radiation source RDS includes... Figure 4 The optical fiber OF shown is used, but in alternative embodiments, other types of hollow core optical fibers can be used.

[0072] A pulse-pumped radiation source (PRS) is configured to provide an input radiation IRD. The hollow core (HC) of the optical fiber (OF) is arranged to receive the input radiation IRD from the PRS and widen the input radiation IRD to provide an output radiation ORD. The operating medium (WM) is capable of widening the frequency range of the received input radiation IRD to provide a broadband output radiation ORD.

[0073] The radiation source RDS also 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 the working medium WM. The storage unit RSV may include one or more characteristics known in the art for controlling, regulating, and / or monitoring the 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 a portion 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 majority thereof) can be coupled into the optical fiber OF located within the storage unit RSV. It will be understood that optics (not shown) may be provided for coupling the input radiation IRD into the optical fiber OF.

[0074] The storage unit RSV includes a second transparent window TW2, which forms 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 positioned close to the output end OE of the optical fiber OF. The second transparent window TW2 can be transparent at least to the frequency of the broadband output radiation ORD of the device.

[0075] Alternatively, in another embodiment, the two opposing ends of the optical fiber OF can be housed within different receptacles. The optical fiber OF may include a first end segment configured to receive an input radiation IRD and a second end segment configured to output a broadband output radiation ORD. The first end segment may be housed within a first receptacle comprising a working medium WM. The second end segment may be housed within a second receptacle, which may also include the working medium WM. The receptacles may operate as described above. Figure 5The 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. Both the first and second reservoirs may also include sealable openings to allow the optical fiber OF to be partially placed inside and partially outside the reservoir, so that gas can be sealed inside the reservoir. The optical fiber OF may also include intermediate segments not contained within the reservoirs. This arrangement of two separate gas reservoirs may be particularly convenient for embodiments where the optical fiber OF is relatively long (e.g., when the length is greater than 1 m). It will be understood that, with this arrangement 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 inside the two reservoirs) can be considered as providing a device for supplying the working medium WM into the hollow core HC of the optical fiber OF.

[0076] In this context, the window may be transparent to the frequency if at least 50%, 75%, 85%, 90%, 95%, or 99% of the incident radiation of a frequency incident on the window is transmitted through the window.

[0077] Both the first transparent window TW1 and the second transparent window TW2 can form an airtight seal within the wall of the reservoir RSV, allowing the working medium WM (which can be a gas) to 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.

[0078] The working medium WM can include inert 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, or nitrogen / hydrogen mixtures). Depending on the type of fill gas, nonlinear optical processes can include modulation instabilities (MI), soliton self-compression, soliton fission, the Kerr effect, the Raman effect, and the generation of dispersed waves, details of which are described in WO2018 / 127266A1 and US9160137B1 (both incorporated herein by reference). Since the dispersion of the fill gas can be tuned by varying 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.

[0079] In one embodiment, the working medium WM can be disposed within the hollow core COR, at least during the process of receiving input radiation IRD for generating broadband output radiation ORD. It will be understood that when the optical fiber OF does not receive input radiation IRD for generating broadband output radiation, the gas WM may be completely or partially absent from the hollow core COR.

[0080] To achieve frequency broadening, high-intensity radiation may be required. The advantage of hollow-core optical fibers (OFs) lies in the ability to achieve high-intensity radiation, resulting in high localized radiation intensity, through strong spatial confinement of radiation propagating through the OF. For example, the internal radiation intensity of an OF can be high due to the high intensity of received input radiation and / or the strong spatial confinement of radiation within the OF. Hollow-core fibers also offer the advantage of guiding radiation over a wider wavelength range than solid-core fibers, and in particular, they can guide radiation in both the ultraviolet and infrared ranges.

[0081] The advantage of using hollow-core optical fiber (OF) lies in the fact that most of the guided radiation 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 disposed within the hollow core (HC) of the OF. This increases the broadening effect of the working medium (WM) on the radiation.

[0082] The received input radiation IRD can be electromagnetic radiation. The input radiation IRD can be received as pulsed radiation. For example, the input radiation IRD can include, for example, ultrafast pulses generated by a laser.

[0083] The input radiation IRD can be coherent radiation. The input radiation IRD can also be collimated radiation, which has the advantage of promoting and improving the efficiency of coupling the input radiation IRD to the optical fiber OF. The input radiation IRD can include 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 radiation and / or coherent radiation.

[0084] The broadband range of the output radiation ORD can be a continuous range, which includes a continuous range of radiation frequencies. The output radiation ORD can include supercontinuum radiation. Continuous radiation is beneficial in many applications, such as measurement applications. For example, a continuous range of frequencies can be used to query a large number of properties. For example, a continuous range of frequencies can be used to determine and / or eliminate the frequency dependence, i.e., frequency correlation, of the measured property. For example, a supercontinuum output radiation ORD can include electromagnetic radiation with wavelengths ranging from 100 nm to 4000 nm, or even up to 10 μm. For example, the frequency range of a broadband output radiation ORD can be 400 nm to 900 nm, 500 nm to 900 nm, or 200 nm to 2000 nm. A supercontinuum output radiation ORD can include white light.

[0085] The input radiation IRD provided by the pulsed pump source (PRS) can be pulsed. The PRS can be a laser. The spatiotemporal transmission characteristics (e.g., its spectral amplitude and phase) of such a laser pulse transmitted along the fiber OF can be altered and tuned by adjusting the (pump) laser parameters, the working component WM, and the fiber OF parameters. The spatiotemporal transmission characteristics can include one or more of the following: output power, output mode distribution, output time distribution, width of the output time distribution (or output pulse width), output spectral distribution, and bandwidth of the output spectral distribution (or output spectral bandwidth). The PRS parameters can include one or more of the following: pump wavelength, pump pulse energy, pump pulse width, and pump pulse repetition rate. The OF parameters can include one or more of the following: fiber length, size and shape of the hollow core 101, size and shape of the capillary, and thickness of the capillary wall surrounding the hollow core. The working component WM (e.g., filling gas) parameters can include one or more of the following: gas type, gas pressure, and gas temperature.

[0086] The broadband output radiation ORD provided by the radiation source RDS can have an average output power of at least 1 W. The average output power can be at least 5 W. The average output power can be at least 10 W. The broadband output radiation ORD can be a pulsed broadband output radiation ORD. The power spectral density of the output radiation across the entire wavelength band of the broadband output radiation ORD can be at least 0.01 mW / nm. The power spectral density of the broadband output radiation across the entire wavelength band can be at least 3 mW / nm.

[0087] In all embodiments described herein, the pulsed pump radiation may include electromagnetic radiation of one or more frequencies between 200 nm and 2 μm. The pulsed pump radiation may, for example, include electromagnetic radiation with a wavelength of 1.03 μm. The repetition rate of the pulsed radiation 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 duration (e.g., FWHM pulse width) of the input radiation IRD may be between 10 fs and 10 ps, ​​or for example, between 50 fs and 400 fs or between 100 fs and 400 fs. The average power of the pump radiation may be between 100 mW and several 100 W values. For example, the average power of the input radiation IRD may be between 20 W and 50 W.

[0088] Figure 7 This is a graph of the input radiation intensity I versus time t, illustrating a common form of laser pulse used to excite the input radiation IRD of the working medium WM within the hollow core fiber. The pulse can have a typical Gaussian shape with a pulse width (FWHM) PW that peaks at time t0. Such a pulse can have parameter values ​​within one or more ranges described in the previous paragraph. For a pulse with a pulse width (FWHM) of, for example, 250 fs to 300 fs, the rise time (which can always be limited to 10% to 90% of the peak value) is greater than 250 fs (e.g., about 300 fs).

[0089] Due to the high nonlinearity of the broadband generation process, continuous spectrum generation occurs only when the pulse intensity reaches a certain threshold. Before reaching this threshold, the pump laser radiation will not be converted but will leave the system as a slightly redshifted spike in the output spectrum, which is undesirable in many applications. When a pre-pulse is used, for example, to adjust the working medium before the main generation pulse, even more unconverted pump radiation will exist, i.e., more pump power loss.

[0090] Additionally, it is generally desirable to extend the wavelength towards the ultraviolet end of the spectrum. This is typically achieved by gradually tapering the hollow optical fiber (i.e., narrowing it) or switching to a different gas type. Neither of these options is direct or straightforward.

[0091] This paper proposes using input radiation pulses with sharp rising edges to excite the working medium within hollow-core optical fibers such as HC-PCF. It has been observed that such shaped pulses reduce unconverted radiation near the pump laser wavelength and broaden the output spectrum towards the ultraviolet range (e.g., wavelengths less than 400 nm). Therefore, the input radiation pulse in this context can be an input radiation pulse with a rise time shorter than 60 fs, 50 fs, 40 fs, 30 fs, 20 fs, or 10 fs.

[0092] Figure 8 The illustration depicts a shaped input radiation pulse according to an embodiment. The shaped input radiation pulse includes a very short rise time (e.g., less than 10 fs). Such a pulse can have a rise time of... Figure 7 The pulse has the same pulse energy, the same pulse width (PW), and the same peak intensity as the Gaussian pulse. In an embodiment, the falling edge of the pulse can approximate the falling edge of a log-normal function; for example, it can include the falling edge of a Lorentz function.

[0093] The benefits of shaping the input pulse to have a short rise time include: spectral broadening of the output radiation occurs much earlier and less unconverted input radiation (pump radiation). Pulses with sharp rise edges reach the intensity threshold more quickly, thus achieving earlier light conversion. Furthermore, the output spectrum can be broadened in the UV direction (e.g., with a significant contribution to wavelengths below 400 nm and / or around 300 nm). This is particularly beneficial for many metrology applications. Thus, it is not necessary to change the gas type or use tapered optical fibers to obtain wavelengths below 400 nm. Additionally, the output spectrum can be broadened in the infrared region.

[0094] In embodiments, for example, the concepts disclosed herein may include using shorter pulses, such as pulse widths / durations in the range of 50 fs to 200 fs, or 50 fs to 150 fs, or 100 fs to 150 fs. The advantage of using shorter pulses is that less pulse energy is required, and thus a higher repetition rate is allowed. It is well known that conversion efficiency decreases at excessively high repetition rates due to ionization. With less energy per pulse compared to a nominal pulse length of 267 fs, the ionization effect can be lower, thus allowing for a higher repetition rate. Thus, the repetition rate of shorter pulses (e.g., as defined herein) can be increased by the same factor as the reduction in the pulse length (e.g., relative to the nominal pulse length), i.e., the repetition rate of shorter pulses (e.g., as defined herein) can be increased by the same factor as the reduction in the pulse length (e.g., relative to the nominal pulse length). The nominal repetition rate of a nominal pulse length of 267 fs depends on the fiber and gas parameters. However, exemplary nominal repetition rates can be between 1 MHz and 10 MHz, 3 MHz and 8 MHz, 4 MHz and 6 MHz, or approximately 5 MHz. Thus, using half the energy and shorter (e.g., 134 fs) pulses, repetition rates can be up to twice as fast (e.g., between 6 MHz and 20 MHz, 6 MHz and 15 MHz, 8 MHz and 12 MHz, approximately 10 MHz).

[0095] Figure 9 This includes another input radiation pulse shape with a usable short rise time. This embodiment includes the use of a flat-top pulse. Figure 8 Compared to a conventional pulse, this flat-top pulse can be approximately symmetrical or at least relatively symmetrical. Using a flat-top pulse, the output spectrum is expanded more efficiently in both the UV and IR directions.

[0096] The method for generating such as Figure 8 The diagram illustrates a method for short rise-time pulses. This pulse shape can be viewed as having a one-sided exponential or log-normal distribution. Taking the one-sided exponential example, the Fourier transform of the time pulse will produce a Lorentz spectrum. An ideal Lorentz spectrum would have infinite extension, while in practice it will be slightly truncated. The truncated Lorentz spectrum transformed back to the time domain will produce a convolution of the one-sided exponential (e.g., the Fast Fourier Transform FFT of the ideal Lorentz) with a sinc function (the FFT of the squared truncation function).

[0097] Therefore, a log-normal pulse is used as the target, which can be approximated by a Lorentz function truncated using a square function (e.g., in the 32 THz range). An initial phase close to the arctangent phase of the input can be selected. Alternatively, the arctangent phase can be applied directly.

[0098] Other embodiments of the invention are disclosed in the following list of numbered aspects:

[0099] 1. A method for generating broadband output radiation by exciting the working medium within a hollow core optical fiber, the method comprising:

[0100] A pulse is generated to produce the input radiation, the pulse having a rise time of less than 60 fs; and

[0101] The working medium is excited by the pulse of input radiation.

[0102] 2. The method according to aspect 1, wherein the rise time is less than 40 fs.

[0103] 3. The method according to aspect 1, wherein the rise time is less than 20 fs.

[0104] 4. The method according to any of the foregoing aspects, wherein the falling edge of each pulse approximates the falling edge of a Gaussian function or a log-normal function.

[0105] 5. The method according to any of the foregoing aspects, wherein the pulse is obtained by generating a Lorentz spectrum truncated using a square function signal.

[0106] 6. The method according to any one of aspects 1 to 3, wherein each of the pulses comprises a flat-top pulse.

[0107] 7. The method according to any of the foregoing aspects, wherein the duration of each pulse is in the range of 50 fs to 400 fs.

[0108] 8. The method according to any of the foregoing aspects, wherein the duration of each pulse is in the range of 50 fs to 150 fs.

[0109] 9. The method according to aspect 8, wherein the repetition rate of the pulse is between 6 MHz and 15 MHz.

[0110] 10. The method according to aspect 8, wherein the repetition rate of the pulse is between 8 MHz and 12 MHz.

[0111] 11. The method according to any of the foregoing aspects, wherein the output radiation comprises a wavelength of less than 400 nm.

[0112] 12. The method according to any of the foregoing aspects, wherein the output radiation comprises a wavelength of less than 350 nm.

[0113] 13. The method according to any of the foregoing aspects, wherein the hollow core optical fiber comprises a hollow core photonic crystal fiber.

[0114] 14. A broadband radiation source for generating broadband output radiation, comprising:

[0115] An input radiation source, operable to generate pulses of input radiation having a rise time of less than 60 fs; and

[0116] A hollow core optical fiber, comprising a working medium and an optical input section, wherein the optical input section is used to receive the input radiation in order to excite the working medium to generate the broadband output radiation.

[0117] 15. The broadband radiation source according to aspect 14, wherein the rise time is less than 40 fs.

[0118] 16. The broadband radiation source according to aspect 14, wherein the rise time is less than 20 fs.

[0119] 17. A broadband radiation source according to any one of aspects 14 to 16, wherein the input radiation source is operable to generate a pulse with a falling edge having a falling edge approximating a Gaussian function or a log-normal function.

[0120] 18. A broadband radiation source according to any one of aspects 14 to 17, wherein the input radiation source is operable to generate the pulse by producing a Lorentz spectrum truncated using a square function signal.

[0121] 19. A broadband radiation source according to any one of aspects 14 to 16, wherein the input radiation source is operable to generate each pulse as a flat-top pulse.

[0122] 20. The broadband radiation source according to any one of aspects 14 to 19, wherein the duration of each pulse is in the range of 50 fs to 400 fs.

[0123] 21. The broadband radiation source according to any one of aspects 14 to 20, wherein the duration of each pulse is in the range of 50 fs to 150 fs.

[0124] 22. The broadband radiation source according to aspect 21, wherein the input radiation source is operable to generate the pulse having a repetition rate between 6 MHz and 15 MHz.

[0125] 23. The broadband radiation source according to aspect 21, wherein the input radiation source is operable to generate the pulse having a repetition rate between 8 MHz and 12 MHz.

[0126] 24. The broadband radiation source according to any one of aspects 14 to 23, wherein the output radiation comprises a wavelength of less than 400 nm.

[0127] 25. The broadband radiation source according to any one of aspects 14 to 24, wherein the output radiation comprises a wavelength of less than 350 nm.

[0128] 26. The broadband radiation source according to any one of aspects 14 to 25, wherein the hollow core optical fiber includes a hollow core photonic crystal fiber.

[0129] 27. A measuring apparatus comprising a broadband radiation source according to aspect 14, wherein the radiation source is configured to generate radiation for projection onto a substrate.

[0130] 28. The measuring device according to aspect 27, wherein the measuring device is one of a scattering instrument, an alignment sensor, or a leveling sensor.

[0131] While the use of lithography equipment in IC manufacturing may be specifically mentioned herein, 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, and so on. 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, track or coating development systems (typically tools that apply a resist layer to the substrate and develop the exposed resist), metrology tools, and / or inspection tools. Where applicable, the disclosure herein may be applied to these and other substrate processing tools. Furthermore, the substrate may be processed more than once, for example, to form a multilayer IC, such that the term “substrate” as used herein may also refer to a substrate that already contains one or more processed layers.

[0132] While specific embodiments of the invention have been described above, it will be understood that the invention may be practiced in other ways than those described.

[0133] 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 following appended claims.

Claims

1. A method for generating broadband output radiation by exciting the working medium within a hollow core photonic crystal fiber, the method comprising: The input radiation includes a pulse with a duration of FWHM in the range of 50 fs to 400 fs and a rise time of less than 60 fs, the rise time being from 10% to 90% of the peak pulse intensity. and The working medium is excited by the input radiation.

2. The method according to claim 1, wherein, The rise time is less than 40 fs.

3. The method according to claim 1, wherein, The rise time is less than 20 fs.

4. The method according to claim 1, wherein, The falling edge of each pulse approximates the falling edge of a Gaussian function or a log-normal function.

5. The method according to claim 1, wherein, The pulse is obtained by generating a Lorentz spectrum that is truncated using a square function signal.

6. The method according to any one of claims 1 to 3, wherein, Each of the pulses comprises a flat-topped pulse.

7. The method according to claim 1, wherein, The duration of the FWHM for each pulse is in the range of 50 fs to 150 fs.

8. The method according to claim 7, wherein, The repetition rate of the pulses is between 6 MHz and 15 MHz.

9. The method according to claim 7, wherein, The repetition rate of the pulses is between 8 MHz and 12 MHz.

10. The method according to claim 1, wherein, The output radiation includes wavelengths less than 400 nm.

11. The method according to claim 1, wherein, The output radiation includes wavelengths less than 350 nm.

12. A broadband radiation source for generating broadband output radiation, comprising: An input radiation source, operable to generate input radiation comprising a pulse, the pulse having a duration of FWHM in the range of 50 fs to 400 fs and a rise time of less than 60 fs, the rise time being from 10% to 90% of the peak pulse intensity. and A hollow-core photonic crystal fiber, comprising a working medium and an optical input section, wherein the optical input section is used to receive the input radiation in order to excite the working medium to generate the broadband output radiation.

13. The broadband radiation source according to claim 12, wherein, The input radiation source is operable to generate pulses with a falling edge that approximates a Gaussian function or a log-normal function.

14. A measuring device comprising the broadband radiation source according to claim 12, wherein, The radiation source is configured to generate radiation for projection onto the substrate.

15. The measuring device according to claim 14, wherein, The measuring device is one of a scattering instrument, an alignment sensor, or a leveling sensor.

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