Supercontinuum radiation source

Abstract

Disclosed is a broadband radiation source for generating output broadband radiation, comprising a plurality of supercontinuum generation stages arranged in series, each supercontinuum generation stage comprising a respective nonlinear generating element. The plurality of supercontinuum generation stages comprises at least a first supercontinuum generation stage and a second supercontinuum generation stage, the second supercontinuum generation stage following the first supercontinuum generation stage in series. A damage tolerance of a first nonlinear generating element included in the first supercontinuum generation stage is greater than a damage tolerance of at least a second nonlinear generating element included in the second supercontinuum generation stage.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to European Patent Application Publication No. 22211917.4, filed December 7, 2022, which is incorporated herein by reference in its entirety.

[0002] The present invention relates to supercontinuum or broadband radiation sources, and in particular to such broadband radiation sources associated with metrology applications in the manufacture of integrated circuits. [Background technology]

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

[0004]

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

[0005]

[0005] Low k1 lithography can be used to process features with dimensions smaller than the classical resolution limit of a lithographic apparatus. In such processes, the resolution equation can be expressed as CD = k1 × λ / NA, where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection optics of the lithographic apparatus, CD is the "critical dimension" (generally the smallest feature size to be printed, in this case the half pitch), and k1 is an empirical resolution factor. In general, the smaller k1 is, the more difficult it is to reproduce on a substrate a pattern that resembles the shape and dimensions planned by a circuit designer to achieve a particular electrical functionality and performance. To overcome such difficulties, advanced fine-tuning steps can be applied to the lithographic projection apparatus and / or the design layout. Such steps include, but are not limited to, optimization of the NA, customization of the illumination scheme, 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") in the design layout or other methods commonly defined as "resolution enhancement techniques" (RET). Alternatively, a strict control loop can be used to manage the stability of the lithographic apparatus to improve pattern replication at low k1.

[0006]

[0006] In many aspects of the IC manufacturing process, metrology tools are used, for example, as alignment tools for proper positioning of the substrate before exposure, as leveling tools for measuring the surface topology of the substrate for focus control, and as scatterometry-based tools for inspecting / measuring exposed and / or etched products in process control. In each case, a radiation source is required. For various reasons, including measurement robustness and accuracy, broadband or white light radiation sources are increasingly being used for such metrology applications. It would be desirable to improve this device for broadband radiation generation. Summary of the Invention

[0007]

[0007] In a first aspect of the present invention, there is provided a broadband radiation source for generating output broadband radiation, comprising a plurality of supercontinuum generation stages arranged in series, each supercontinuum generation stage comprising a respective nonlinear generating element, the plurality of supercontinuum generation stages comprising at least a first supercontinuum generation stage and a second supercontinuum generation stage, the second supercontinuum generation stage following the first supercontinuum generation stage in series, and the damage tolerance of the first nonlinear generating element comprised within the first supercontinuum generation stage being greater than the damage tolerance of at least a second nonlinear generating element comprised within the second supercontinuum generation stage.

[0008]

[0008] In a second aspect of the present invention, there is provided a broadband radiation source for generating output broadband radiation, comprising a plurality of supercontinuum generation stages arranged in series, each supercontinuum generation stage comprising a respective nonlinear generating element, the plurality of supercontinuum generation stages comprising at least a first supercontinuum generation stage and a second supercontinuum generation stage, the second supercontinuum generation stage following the first supercontinuum generation stage in series, and the optical nonlinearity of the first nonlinear generating element comprised within the first supercontinuum generation stage being lower than the optical nonlinearity of at least a second nonlinear generating element comprised within the second supercontinuum generation stage.

[0009] Another aspect of the invention includes a metrology device including the broadband radiation source device of the first aspect or the second aspect.

[0010]

[0010] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which: [Brief explanation of the drawings]

[0011] [Figure 1] 1 depicts a schematic overview of a lithographic apparatus; [Figure 2]1 shows a schematic overview of a lithography cell. [Figure 3] A schematic diagram of holistic lithography is shown, illustrating the collaboration between three key technologies for optimizing semiconductor manufacturing. [Figure 4] 1 depicts a schematic diagram of a scatterometry apparatus for use as a metrology device, which may include a radiation source according to an embodiment of the present invention; [Figure 5] 1 shows a schematic diagram of a level sensor device that may include a radiation source according to an embodiment of the present invention; [Figure 6] 1 shows a schematic diagram of an alignment sensor apparatus that may include a radiation source according to an embodiment of the present invention; [Figure 7] 1 is a schematic cross-sectional view of a hollow core optical fiber that may form part of a radiation source according to an embodiment, taken in a transverse plane (i.e. perpendicular to the axis of the optical fiber); [Figure 8] 1 shows a schematic diagram of a known radiation source for providing broadband output radiation; [Figure 9a] 1A and 1B show schematic cross-sectional views of examples of hollow-core photonic crystal fiber (HC-PCF) designs for supercontinuum generation. [Figure 9b] 1A and 1B show schematic cross-sectional views of examples of hollow-core photonic crystal fiber (HC-PCF) designs for supercontinuum generation. [Figure 10] 1 shows a schematic representation of a broadband radiation source according to a first embodiment; [Figure 11] 1 shows a schematic representation of a broadband radiation source according to a first embodiment; [Figure 12] 2 shows a schematic representation of a broadband radiation source according to a second embodiment; [Figure 13] 5 illustrates schematically a broadband radiation source according to a third embodiment. [Figure 14] 5 illustrates schematically a broadband radiation source according to a fourth embodiment; [Figure 15] 10 illustrates schematically a broadband radiation source according to a fifth embodiment. [Figure 16] FIG. 1 shows a block diagram of a computer system for controlling a broadband radiation source. DETAILED DESCRIPTION OF THE INVENTION

[0012]

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

[0013]

[0012] As used herein, the terms "reticle," "mask," or "patterning device" may be broadly interpreted to refer to a general patterning device that can be used to provide an incident radiation beam with a patterned cross-section that corresponds to the pattern to be created on a target portion of a substrate. The term "light valve" is sometimes also used in this context. In addition to classic masks (transmissive or reflective masks, binary masks, phase-shifting masks, hybrid masks, etc.), examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0014] 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes an illumination system IL (also called an illuminator) configured to condition a radiation beam B (e.g. UV radiation, DUV radiation or EUV radiation), a mask support (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to certain parameters, a substrate support (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support according to certain parameters, and a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0015]

[0014] In operation, the illumination system IL receives a radiation beam from a radiation source SO (e.g. via a beam delivery system BD). The illumination system IL may include various types of optical components for directing, shaping and / or controlling the radiation, for example refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical components or any combination thereof. The illuminator IL may be used to condition the radiation beam B so that it has a desired spatial and angular intensity distribution in its cross-section in the plane of the patterning device MA.

[0016]

[0015] As used herein, the term "projection system" PS should be interpreted broadly to encompass various types of projection systems. Such systems may include refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as required by the exposure radiation being used and / or other factors (e.g., the use of an immersion liquid or a vacuum). Where the term "projection lens" is used herein, it may all be considered as synonymous with the more general term "projection system" PS.

[0017]

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

[0018] The lithographic apparatus LA may be of a type having two or more substrate supports WT (also known as "dual stage"). In such a "multiple stage" machine, the substrate supports WT may be used in parallel, and / or a substrate W on one of the substrate supports WT may be used to expose a pattern thereon, while a procedure is being performed on another substrate W on the other substrate support WT in preparation for a subsequent exposure of that other substrate W.

[0019] In addition to the substrate support WT, the lithographic apparatus LA may include a measurement stage. The measurement stage is configured to hold a sensor and / or a cleaning apparatus. The sensor may be configured to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold multiple sensors. The cleaning apparatus may be configured to clean part of the lithographic apparatus, for example part of the projection system PS or part of a system for supplying immersion liquid. The measurement stage may move below the projection system PS when the substrate support WT is spaced apart from the projection system PS.

[0020] In operation, a radiation beam B is incident on a patterning device (e.g. a mask MA held on a mask support MT) and is patterned according to a pattern (design layout) on the patterning device MA. After traversing the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and the position measurement system IF, the substrate support WT can be precisely moved, for example so that different target portions C are positioned at focused and aligned positions in the path of the radiation beam B. Similarly, the first positioner PM, and possibly another position sensor (which is not explicitly shown in Figure 1), may be used to precisely position the patterning device MA with respect to the path of the radiation beam B. The patterning device MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although substrate alignment marks P1, P2 occupy dedicated target portions as illustrated, they may be located in spaces between target portions. When located between target portions C, substrate alignment marks P1, P2 are referred to as scribe-lane alignment marks.

[0021] 2, the lithography apparatus LA may be part of a lithography cell LC (sometimes called a litho-cell or (litho)-cluster), which often also includes apparatus for performing pre-exposure and post-exposure processes on the substrate W. Conventionally, such apparatus include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH, and a bake plate BK (which, for example, adjust the temperature of the substrate W, e.g., to adjust the solvent in the resist layer). A substrate handler (i.e., robot) RO picks up substrates W from input / output ports I / O1, I / O2, moves them between various process tools, and delivers them to a loading bay LB of the lithography apparatus LA. The devices within a lithocell are often collectively referred to as a track and are typically under the control of a track control unit TCU, which itself may be controlled by a supervisory control system SCS, which may also control the lithographic apparatus LA (e.g., via a lithography control unit LACU).

[0022] To ensure that a substrate W to be exposed by the lithographic apparatus LA is exposed accurately, it is desirable to inspect the substrate to measure properties of the patterned structures, such as overlay error between successive layers, line thickness, critical dimension (CD), etc. To that end, an inspection tool (not shown) may be included in the lithocell LC. If an error is detected, adjustments may be made, for example, to the exposure of subsequent substrates or other process steps to be performed on the substrate W, particularly if inspection is carried out before other substrates W of the same batch or lot are subsequently exposed or processed.

[0023] Inspection apparatus, sometimes called metrology apparatus, are used to measure properties of substrates W, and in particular to measure how the properties of different substrates W vary, or how properties associated with different layers of the same substrate W vary from layer to layer. The inspection apparatus may alternatively be constructed to identify defects on substrates W and may, for example, be part of a lithocell LC, or integrated into a lithography apparatus LA, or may be a stand-alone apparatus. The inspection apparatus may measure properties related to 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 bake step PEB), or a developed resist image (from which exposed or unexposed parts of the resist have been removed), or even a property related to an etched image (after a pattern transfer step such as etching).

[0024] Typically, the patterning process in a lithography apparatus LA is one of the most critical steps in processing, requiring high accuracy in the dimensioning and placement of structures on a substrate W. To ensure this high accuracy, three systems can be combined in a so-called "holistic" control environment, as shown schematically in FIG. 3. One of these systems is a lithography apparatus LA, which is (virtually) connected to a metrology tool MT (a second system) and a computer system CL (a third system). The key to such a "holistic" environment is optimizing the coordination between these three systems to enforce the overall process window and achieve a tight control loop so that the patterning performed by the lithography apparatus LA stays within the process window. The process window defines the range of process parameters (e.g., dose, focus, overlay) within which a particular manufacturing process produces a specified result (e.g., a functioning semiconductor device), and typically within which the process parameters of a lithography process or patterning process can vary.

[0025]

[0024] The computer system CL is capable of predicting which resolution enhancement techniques should be used by using (parts of) the design layout to be patterned, and is capable of performing computational lithography simulations and calculations to determine mask layouts and lithography apparatus settings that maximize the overall process window of the patterning process (shown in FIG. 3 by the double-headed arrow at the first scale SC1). Typically, resolution enhancement techniques are tailored to the patterning capabilities of the lithography apparatus LA. The computer system CL is further capable of predicting whether defects are likely to exist (e.g., due to suboptimal processing) by detecting where in the process window the lithography apparatus LA is currently operating (e.g., using input from the metrology tool MT) (shown in FIG. 3 by the arrow pointing to "0" at the second scale SC2).

[0026]

[0025] The metrology tool MT is capable of providing input to the computer system CL that enables accurate simulation and prediction, and is capable of providing feedback to the lithography apparatus LA that identifies possible drift (e.g., in the calibration status of the lithography apparatus LA) (shown by multiple arrows at the third scale SC3 in Figure 3).

[0027]

[0026] In lithographic processes, it is desirable to frequently measure the structures produced (e.g., for process control and verification). Tools that perform such measurements are commonly called metrology tools MT. Various types of metrology tools MT that perform such measurements are known, such as scanning electron microscopes or various forms of scatterometer metrology tools MT. A scatterometer is a multipurpose instrument that allows measurements of parameters of the lithographic process, the measurements being made by having a sensor in the pupil or a conjugate plane to the pupil of the scatterometer objective lens (usually referred to as pupil-based measurements), or by having a sensor in the image plane or a conjugate plane to the image plane (in which case they are usually referred to as image-based or field-based measurements). Such scatterometers and associated measurement techniques are described in detail in U.S. Patent Application Publication Nos. 20100328655, 2011102753A1, 20120044470A, 20110249244, 20110026032, or EP 1,628,164A, which are incorporated by reference in their entireties. The scatterometers described above are capable of measuring gratings using soft x-rays and light in the visible to near-infrared wavelength range.

[0028] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, a reconstruction method may be applied to the measurement signal to reconstruct or calculate the properties of the grating. Such a reconstruction may, for example, be the result of simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measurement results. Parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from the real target.

[0029]

[0028] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, radiation emitted by a radiation source is directed towards a target, and radiation reflected or scattered from the target is directed towards a spectrometer detector, which measures the spectrum of the specularly reflected radiation (i.e., measures the intensity as a function of wavelength). From this data, it is possible to reconstruct the structure or profile of the target giving rise to the detected spectrum, for example by rigorous coupled wave theory and nonlinear regression, or by comparison with a library of simulated spectra.

[0030] In a third embodiment, the scatterometer MT is an ellipsometric scatterometer. An ellipsometric scatterometer makes it possible to determine parameters of a lithographic process by measuring scattered radiation for each polarization state. Such a metrology apparatus emits polarized light (e.g., linearly, circularly or elliptically polarized light), for example using appropriate polarizing filters in the illumination section of the metrology apparatus. A source suitable for the metrology apparatus can provide polarized radiation as well. Various embodiments of existing ellipsometric scatterometers are described in U.S. Patent Application Publication Nos. 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,449, 12 / 920,968, 12 / 922,587, 13 / 000,229, 13 / 033,135, 13 / 533,110, and 13 / 891,410, which are incorporated by reference herein in their entireties.

[0031] In one embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the overlay of two misaligned grating or periodic structures by measuring the asymmetry of the reflectance spectra and / or the detection configuration, where the asymmetry is related to the degree of overlay. The two (typically overlapping) grating structures can be applied in two different (not necessarily consecutive) layers and formed at substantially the same location on the wafer. The scatterometer can have a symmetric detection configuration, for example, as described in co-owned European Patent Application Publication No. 1628164A, so that any asymmetry can be clearly distinguished. This provides a straightforward method for measuring grating misalignment. Further examples for measuring the overlay error between two layers containing periodic structures when the target is measured through the asymmetry of the periodic structures can be obtained from PCT Patent Application Publication No. WO 2011 / 012624 or U.S. Patent Application No. 20160161863, which are incorporated herein by reference in their entirety.

[0032]

[0031] Other parameters of interest can be focus and dose. Focus and dose can be determined simultaneously by scatterometry (or alternatively by scanning electron microscopy), as described in U.S. Patent Application No. 2011-0249244, which is incorporated herein by reference in its entirety. A single structure can be used that has a unique combination of critical dimension and sidewall angle measurements for each point of the focus-energy matrix (FEM, also called focus-exposure matrix). If these unique combinations of critical dimension and sidewall angle are available, focus and dose values ​​can be uniquely determined from these measurements.

[0033]

[0032] A metrology target can be a collection of composite gratings, mostly formed by a lithography process in resist, but also formed after, for example, an etching process. Typically, the pitch and linewidth of the grating structures strongly depend on the measurement optics (specifically, the NA of the optics) so that the diffraction orders obtained from the metrology target can be captured. As previously shown, the diffraction signal can be used to determine the shift between two layers (also called "overlay") or to reconstruct at least a portion of the original grating as produced by the lithography process. This reconstruction can be used to provide guidance on the quality of the lithography process and can be used to control at least a portion of the lithography process. The target can have smaller subsegments configured to mimic the dimensions of the features of the design layout in the target. This subsegmentation causes the target to behave more similarly to the features of the design layout, so that all process parameter measurements closely resemble the features of the design layout. The target can be measured in underfill mode or overfill mode. In underfill mode, the measurement beam generates a spot that is smaller than the entire target. In overfill mode, the measurement beam generates a spot that is larger than the entire target. In such an overfill mode, it may be possible to simultaneously measure different targets, and therefore simultaneously determine different process parameters.

[0034] The overall measurement quality of a lithography parameter using a particular target depends, at least in part, on the measurement recipe used to measure that lithography parameter. The term "substrate measurement recipe" can include one or more parameters of the measurement itself, one or more parameters of the measured pattern(s), or both. For example, if the measurement used in the substrate measurement recipe is a diffraction-based optical measurement, one or more of the parameters of the measurement can include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation to the substrate, the orientation of the radiation relative to the pattern on the substrate, etc. One of the criteria for selecting a measurement recipe can be, for example, the sensitivity of one of the measurement parameters to process variations. Further examples are described in U.S. Patent Application No. 2016-0161863 and published U.S. Patent Application No. 2016 / 0370717A1, which are incorporated herein by reference in their entireties.

[0035] A metrology apparatus such as a scatterometer is shown in FIG. 4. It includes a broadband (white light) radiation projector 2 that projects radiation onto a substrate 6. Reflected or scattered radiation is sent to a spectrometer detector 4, which measures the spectrum 10 of the specularly reflected radiation (i.e., a measurement of intensity as a function of wavelength). From this data, the structure or profile giving rise to the detected spectrum can be reconstructed by a processing unit (PU), for example by rigorous coupled-wave analysis and nonlinear regression, or by comparison with a library of simulated spectra such as that shown at the bottom of FIG. 3. Typically, for reconstruction, the general form of the structure is known and some parameters are assumed from knowledge of the process by which the structure was created, thereby leaving only a few parameters of the structure to be determined from the scatterometry data. Such a scatterometer may be configured as a normal-incidence scatterometer or an oblique-incidence scatterometer.

[0036] The overall measurement quality of a lithography parameter through measurement of a metrology target depends, at least in part, on the measurement recipe used to measure the lithography parameter. The term “substrate measurement recipe” can encompass one or more parameters of the measurement itself, one or more parameters of one or more measured patterns, or both. For example, if the measurement performed in the substrate measurement recipe is a diffraction-based optical measurement, 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 patterns on the substrate, etc. One criterion for selecting a measurement recipe can be, for example, the sensitivity of any measurement parameter to process variations. Further examples are described in U.S. Patent Application No. 2016 / 0161863 and published U.S. Patent Application No. 2016 / 0370717A1, which are incorporated herein by reference in their entireties.

[0037] Another type of metrology tool used in IC manufacturing is a topography measurement system, level sensor, or height sensor. Such a tool may be incorporated into a lithography apparatus to measure the topography of the top surface of a substrate (or wafer). A map of the substrate's topography (also called a height map) can be generated from these measurements, showing the substrate's height as a function of position on the substrate. This height map can later be used to correct the position of the substrate during transfer of a pattern onto the substrate in order to provide an aerial image of the patterning device at the proper focus position on the substrate. It will be understood that "height" in this context refers broadly to the out-of-plane dimension relative to the substrate (also called the Z-axis). Typically, a level or height sensor performs measurements at a fixed position (relative to its own optics), and relative movement of the substrate and the optics of the level or height sensor results in height measurements at locations across the substrate.

[0038]

[0037] An example of a level or height sensor LS known in the art is shown schematically in Figure 5, which shows only the operating principle. In this example, the level sensor includes an optical system, which includes a projection unit LSP and a detection unit LSD. The projection unit LSP includes a radiation source LSO that provides a radiation beam LSB, which is provided by a projection grating PGR of the projection unit LSP. The radiation source LSO may be, for example, a narrowband or broadband light source, such as a polarized or unpolarized, pulsed or continuous (such as a polarized or unpolarized laser beam) supercontinuum light source. The radiation source LSO may include multiple radiation sources (such as multiple LEDs) with different colors or wavelength ranges. The radiation source LSO of the level sensor LS is not limited to visible radiation but may additionally or alternatively include UV and / or IR radiation, as well as any wavelength range suitable for reflection from the surface of the substrate.

[0039]

[0038] The projection grating PGR is a periodic grating comprising a periodic structure which results in a radiation beam BE1 with a periodically varying intensity. The radiation beam BE1 with a periodically varying intensity is directed towards a measurement location MLO on the substrate W with an angle of incidence ANG relative to an axis normal to the substrate surface of incidence (Z-axis) of between 0 and 90 degrees, typically between 70 and 80 degrees. At the measurement location MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and directed towards a detection unit LSD.

[0040]

[0039] To determine the height level at the measurement location MLO, the level sensor further includes a detection system including a detection grid DGR, a detector DET, and a processing unit (not shown) for processing an output signal of the detector DET. The detection grid DGR may be the same as the projection grid PGR. The detector DET generates a detector output signal indicative of the received light, for example indicative of the intensity of the received light (e.g., a photodetector) or representing the spatial distribution of the received intensity (e.g., a camera). The detector DET may include any combination of one or more types of detectors.

[0041]

[0040] Triangulation techniques allow the height level at the measurement location MLO to be determined. The detected height level is generally related to the signal intensity as measured by the detector DET, which has a periodicity that depends, among other things, on the design of the projection grating PGR and the (oblique) angle of incidence ANG.

[0042]

[0041] The projection unit LSP and / or the detection unit LSD may include further optical elements, such as lenses and / or mirrors, along the path of the patterned radiation beam between the projection grating PGR and the detection grating DGR (not shown).

[0043] In an embodiment, the detection grating DGR may be omitted and the detector DET may be arranged at the position where the detection grating DGR is located. Such a configuration provides a more direct detection of the image of the projection grating PGR.

[0044]

[0043] In order to effectively cover the surface of the substrate W, the level sensor LS may be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating an array of measurement areas MLO or spots that cover a larger measurement range.

[0045] Various height sensors of the general type are disclosed, for example, in U.S. Patent Nos. 7,265,364 and 7,646,471, both of which are incorporated by reference. A height sensor that uses UV radiation instead of visible or infrared radiation is disclosed in incorporated U.S. Patent Application Publication No. 2010233600A1. In incorporated WO 2016102127A1, a miniature height sensor is described that uses a multi-element detector to detect and recognize the position of a grating image without the need for a detection grating.

[0046] Another type of metrology tool used in IC manufacturing is an alignment sensor. Therefore, an important aspect of the performance of a lithographic apparatus is the ability to correctly and accurately position a pattern applied relative to features built up on a previous layer (by the same apparatus or a different lithographic apparatus). For this purpose, the substrate is provided with one or more sets of marks or targets. Each mark is a structure whose position can be subsequently measured using a position sensor (typically an optical position sensor). The position sensor is sometimes called an "alignment sensor" and the marks are sometimes called "alignment marks".

[0047] A lithographic apparatus may include one or more (e.g., multiple) alignment sensors that can accurately measure the position of alignment marks provided on a substrate. Alignment (or position) sensors may use optical phenomena such as diffraction and interference to obtain position information from alignment marks formed on the substrate. One example of an alignment sensor used in current lithographic apparatuses is based on a self-referencing interferometer, such as that described in U.S. Patent No. 6,961,116. Various improvements and modifications to position sensors have been developed, as disclosed, for example, in U.S. Patent Application Publication No. 2015261097A1. The entire contents of these publications are incorporated herein by reference.

[0048] 6 is a schematic block diagram of an embodiment of a known alignment sensor AS, for example as described in the incorporated US Pat. No. 6,961,116. A radiation source RSO provides a radiation beam RB of one or more wavelengths, which is redirected by redirecting optics as an illumination spot SP onto a mark (such as a mark AM located on a substrate W). In this example, the redirecting optics include a spot mirror SM and an objective lens OL. The illumination spot SP (by which the mark AM is illuminated) may have a diameter slightly smaller than the width of the mark itself.

[0049]

[0048] Radiation diffracted by the alignment mark AM is collimated (in this example by an objective lens OL) into an information-bearing beam IB. The term "diffracted" is intended to include zeroth order diffraction from the mark (which is sometimes called reflection). A self-referencing interferometer SRI, for example of the type disclosed in the above-mentioned U.S. Patent No. 6,961,116, causes the beam IB to interfere with itself, after which it is received by a photodetector PD. Further optical components (not shown) may be included to provide separate beams if more than one wavelength is produced by the radiation source RSO. The photodetector may be a single element, or it may include several pixels, if required. The photodetector may include a sensor array.

[0050]

[0049] The redirecting optical element (in this example, the redirecting optical element includes a spot mirror SM) can also function to block zero-order radiation reflected from the mark AM so that the information-carrying beam IB contains only higher-order diffracted radiation from the mark AM (this is not essential for the measurement, but improves the signal-to-noise ratio).

[0051]

[0050] The intensity signal SI is fed to a processing unit PU. The combination of optical processing in block SRI and computational processing in unit PU outputs values ​​of the X and Y position on the substrate relative to a reference frame.

[0052] A single measurement of the type shown fixes the position of the mark only within a certain range corresponding to one pitch of the mark. In conjunction with this, coarser measurement techniques are used to identify which period of the sine wave contains the mark position. For improved accuracy and / or for robust detection of marks regardless of the material from which they are made and the material on which they are placed and / or below, the same process can be repeated at coarser and / or finer levels with different wavelengths. Wavelengths may be optically multiplexed and demultiplexed to be processed simultaneously, and / or wavelengths may be multiplexed by time division or frequency division.

[0053] In this example, the alignment sensor and spot SP remain stationary, and it is the substrate W that moves. The alignment sensor can therefore be rigidly and accurately mounted to a reference frame, effectively scanning the mark AM in a direction opposite to the direction of movement of the substrate W. The substrate W is controlled in this movement by a substrate positioning system that controls its mounting on the substrate support and the movement of the substrate support. A substrate support position sensor (e.g., an interferometer) measures the position of the substrate support (not shown). In an embodiment, one or more (alignment) marks are provided on the substrate support. Measuring the position of the marks provided on the substrate support allows the position of the substrate support, as determined by the position sensor, to be calibrated (e.g., relative to a frame to which the alignment system is connected). Measuring the position of the alignment marks provided on the substrate allows the position of the substrate relative to the substrate support to be determined.

[0054]

[0053] Metrology tools MT, such as the scatterometers, topography measurement systems, or position measurement systems described above, may use radiation generated by a radiation source to perform measurements. The characteristics of the radiation used by the metrology tool may affect the type and quality of 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 may be used. Multiple different frequencies may be capable of propagating, irradiating, and scattering from the metrology target with no or minimal interference with other frequencies. Thus, for example, different frequencies may be used to simultaneously obtain more metrology data. Different radiation frequencies may also be capable of interrogating or discovering different characteristics of the metrology target. Broadband radiation may be useful in metrology systems MT, for example, level sensors, alignment mark measurement systems, scatterometry tools, or inspection tools. The broadband radiation source may be a supercontinuum light source.

[0055]

[0054] High-quality broadband radiation (e.g., supercontinuum radiation) can be difficult to generate. One way to generate broadband radiation can be to broaden high-power narrowband or single-frequency input radiation or pump radiation, for example, by utilizing nonlinear and higher-order effects. The input radiation (which can be generated using a laser) can be called pump radiation. Alternatively, the input radiation can be called seed radiation. To obtain high-power radiation due to the broadening effect, the radiation can be confined within a small area so that strong, localized, high-intensity radiation is achieved. In these areas, the radiation can interact with broadening structures and / or materials that form a nonlinear medium, and broadband output radiation can be produced. In high-intensity radiation areas, different materials and / or structures can be used to enable and / or improve radiation broadening by providing a suitable nonlinear medium.

[0056] In some implementations, broadband output radiation is generated in a photonic crystal fiber (PCF). In some embodiments, such photonic crystal fibers have a microstructure around the core that helps confine radiation traveling in the waveguide. 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, the use of solid materials can have some disadvantages. For example, if UV radiation is generated in the solid core, this radiation may not be visible in the output spectrum of the fiber because the radiation is absorbed by most solid materials.

[0057]

[0056] In some implementations, as further described below with reference to FIG. 8, methods and apparatus for broadening input radiation may use a fiber to confine the input radiation and to broaden the input radiation to output broadband radiation. The fiber may be a hollow-core fiber and may include an internal structure to achieve effective guiding and confinement of radiation within the fiber. The fiber may be a hollow-core photonic crystal fiber (HC-PCF), which is particularly suitable for strong radiation confinement primarily within the hollow core of the fiber, achieving high radiation intensity. The hollow core of the fiber may be filled with a gas that acts as a nonlinearly active medium to broaden the input radiation. Such a fiber and gas arrangement may be used to create a supercontinuum radiation source. The radiation input to the fiber may be electromagnetic radiation, for example, radiation in one or more of the infrared, visible, UV, and extreme UV spectrums. The output radiation may consist of or include broadband radiation, sometimes referred to herein as white light.

[0058] Some embodiments relate to a novel design of such a broadband radiation source comprising an optical fiber. The optical fiber is a hollow-core photonic crystal fiber (HC-PCF). In particular, the optical fiber can be a type of hollow-core photonic crystal fiber that includes an anti-resonant structure for confining the radiation. Such fibers that include an anti-resonant structure are known in the art as anti-resonant fibers, tubular fibers, single-ring fibers, negative curvature fibers, or depressive coupling fibers. Various different designs of such fibers are known in the art. Alternatively, the optical fiber can be a photonic bandgap fiber (HC-PBF, e.g., Kagome fiber).

[0059] Many 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 antiresonant reflecting fiber (HC-ARF). Details regarding the design and fabrication of HC-PCFs can be found in U.S. Patent Application Publication No. 2004 / 015085 A1 (for HC-PBF) and International PCT Patent Application Publication No. WO 2017 / 032454 A1 (for hollow-core antiresonant reflecting fiber), both of which are incorporated herein by reference. Figure 9(a) shows a kagome fiber, including a kagome lattice structure.

[0060]

[0059] An example of an optical fiber used for a radiation source will now be described with reference to Figure 7. Figure 7 shows a schematic cross-section of an optical fiber OF in a transverse plane. Further embodiments similar to the application of the fiber of Figure 7 are disclosed in WO 2017 / 032454 A1.

[0061] The optical fiber OF includes an elongated body, with one dimension of the fiber OF being longer than the other two dimensions. This longer dimension may be referred to as the axial direction and may define the axis of the optical fiber OF. The other two dimensions define a plane, which may be referred to as the transverse plane. Figure 7 shows a cross-section of the optical fiber OF at this transverse plane (i.e., perpendicular to the axis), labeled as the xy plane. The transverse cross section of the optical fiber OF may be substantially constant along the fiber axis.

[0062] It will be appreciated that because the optical fiber OF has a degree of flexibility, the axial orientation will generally not be uniform along the length of the optical fiber OF. Terms such as optical axis, cross section, etc. will be understood to mean the local optical axis, local cross section, etc. Furthermore, when a component is described as being cylindrical or tubular, these terms will be understood to encompass the shape that the optical fiber OF would be distorted into when bent.

[0063]

[0062] It will be appreciated that the optical fiber OF can have any length, and the length of the optical fiber OF can depend on the application. The optical fiber OF can have a length between 1 cm and 10 m, for example, the optical fiber OF can have a length between 10 cm and 100 cm.

[0064]

[0063] The optical fiber OF includes a hollow core HC, a cladding portion surrounding the hollow core HC, and a support portion SP surrounding and supporting the cladding portion. The optical fiber OF can be considered to include a body (including the cladding portion and the support portion SP) having the hollow core HC. The cladding portion includes a plurality of anti-resonant elements for guiding radiation through the hollow core HC. In particular, the plurality of anti-resonant elements are arranged to confine radiation propagating through the optical fiber OF primarily within the hollow core HC to guide the radiation along the optical fiber OF. The hollow core HC of the optical fiber OF is located in a substantially central region of the optical fiber OF, such that the axis of the optical fiber OF may also define the axis of the hollow core HC of the optical fiber OF.

[0065] The cladding portion includes a plurality of anti-resonant elements for guiding radiation propagating through the optical fiber OF. In particular, 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. It should be appreciated that these anti-resonant elements may be provided with different cross sections, for example, elliptical cross sections, or nested circular cross sections with a smaller diameter circular tube within a larger diameter circular tube.

[0066]

[0065] The capillary CAP may also be referred to as a tube. The capillary CAP may be circular in cross section 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 appreciated that the wall portion WP may function as an anti-reflection Fabry-Perot resonator for radiation propagating through the hollow core HC (and that may be incident on the wall portion WP at an oblique incidence angle). The thickness of the wall portion WP may be suitable to ensure that reflection back into the hollow core HC is generally enhanced while transmission into the capillary cavity CC is generally suppressed. In some embodiments, the capillary wall portion WP may have a thickness of 0.01 to 10.0 μm.

[0067] It will be appreciated that, as used herein, the term cladding portion is intended to mean a portion of the optical fiber OF for guiding radiation propagating therethrough (i.e., the capillary CAP that confines said radiation within the hollow core HC). The radiation is confined in the form of transverse modes and can propagate along the fiber axis.

[0068]

[0067] The support portion is generally tubular and supports six capillary caps of the cladding portion. The six capillary caps are evenly distributed around the inner surface in the case of the inner support portion SP. The six capillary caps can be described as being arranged in a generally hexagonal configuration.

[0069]

[0068] The capillary CAPs are arranged so that each capillary does not contact any of the other capillary CAPs. Each of the capillary CAPs contacts the inner support portion SP and is spaced apart from adjacent capillary CAPs in a ring structure. Such an arrangement may be beneficial because it may increase the transmission bandwidth of the optical fiber OF (e.g., compared to an arrangement in which the capillaries contact each other). Alternatively, in some embodiments, each of the capillary CAPs may contact adjacent capillary CAPs in a ring structure.

[0070]

[0069] The six capillary CAPs of the cladding portion are arranged in a ring structure around the hollow core HC. The inner surfaces of the ring structure of the capillary CAPs at least partially define the hollow core HC of the optical fiber OF. The diameter d of the hollow core HC (which may be defined as the smallest dimension between opposing capillaries, indicated by arrow d) may be 10 to 1000 μm. The diameter d of the hollow core HC may affect the mode field diameter, collisional loss, dispersion, mode multiplicity, and nonlinear characteristics of the hollow-core HC optical fiber OF.

[0071]

[0070] In this embodiment, the cladding portion includes a single ring arrangement of capillary CAPs (which act as anti-resonant elements), so that any radial ray from the center of the hollow core HC to the outside of the optical fiber OF passes through only one capillary CAP.

[0072] It will be appreciated that other embodiments may be provided with different arrangements of anti-resonant elements. These may include arrangements with multiple rings of anti-resonant elements and arrangements with nested anti-resonant elements. FIG. 9(a) shows an embodiment of an HC-PCF with three rings of capillary CAPs stacked along the radial direction. In this embodiment, each capillary CAP contacts other capillaries, both in the same ring and in different rings. Furthermore, while the embodiment shown in FIG. 7 includes six rings of capillaries, in other embodiments, the cladding portion may be provided with one or more rings including any number of anti-resonant elements (e.g., 4, 5, 6, 7, 8, 9, 10, 11, or 12 capillaries).

[0073]

[0072] Figure 9(b) shows an alternative embodiment of the above-described HC-PCF with a single ring of tubular capillary. In the example of Figure 9(b), there are two concentric rings of tubular capillary 21. A support tube ST may be included in the HC-PCF to hold the inner and outer rings of tubular capillary 21. The support tube may be made of silica.

[0074]

[0073] The tubular capillaries of the examples of Figures 7 and 9(a) and (b) may have a circular cross-sectional shape. Other shapes are also possible for the tubular capillaries, such as elliptical or polygonal cross-sections. Additionally, the solid material of the tubular capillaries of the examples of Figures 7 and 9(a) and (b) may include a plastic material such as PMA, a glass such as silica, or a soft glass.

[0075]

[0074] Figure 8 shows a radiation source RDS for providing a known broadband output radiation. The radiation source RDS may include a pulsed pump radiation source PRS, a continuous wave radiation source or any type of radiation source capable of generating short pulses of a desired length and energy level, an optical fiber OF having a hollow core HC (for example of the type shown in Figure 7), and a working medium WM (e.g. a gas) disposed within the hollow core HC. In Figure 8, the radiation source RDS includes the optical fiber OF shown in Figure 7, but in alternative embodiments other types of hollow core HC optical fiber OF may be used.

[0076]

[0075] The pulsed pump radiation source PRS is configured to provide input radiation IRD. The hollow core HC of the optical fiber OF is arranged to receive the input radiation IRD from the pulsed pump radiation source PRS and to broaden the input radiation IRD to provide output radiation ORD. The working medium WM enables broadening the frequency range of the received input radiation IRD to provide broadband output radiation ORD.

[0077]

[0076] The radiation source RDS further includes a reservoir RSV. The optical fiber OF is disposed within the reservoir RSV. The reservoir RSV may also be referred to as a housing, a container, or a gas cell. The reservoir RSV is configured to contain a working medium WM. The reservoir RSV may include one or more features known in the art for controlling, regulating, and / or monitoring the composition of the working medium WM (which may be a gas or a liquid) within the reservoir RSV. The reservoir RSV may include a first transparent window TW1. In use, the optical fiber OF is disposed within the reservoir RSV such that the first transparent window TW1 is located proximate to the input end IE of the optical fiber OF. The first transparent window TW1 may form a part of a wall of the reservoir RSV. The first transparent window TW1 may be at least transparent to the received input radiation frequency, such that the received input radiation IRD (or at least a majority thereof) may be coupled to the optical fiber OF located within the reservoir RSV. It will be appreciated that optics (not shown) may be provided for coupling the input radiation IRD into the optical fiber OF.

[0078]

[0077] The reservoir RSV includes a second transparent window TW2 forming part of a wall of the reservoir RSV. In use, when the optical fiber OF is placed within the reservoir RSV, the second transparent window TW2 is located adjacent to the output end OE of the optical fiber OF. The second transparent window TW may be transparent to at least the frequencies of the broadband output radiation ORD of the device 120.

[0079] Alternatively, in another embodiment, the two opposite ends of the optical fiber OF may be disposed in 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 disposed in a first reservoir containing the working medium WM. The second end section may be disposed in a second reservoir, which may also contain the working medium WM. The reservoirs may function as described above with respect to FIG. 8. 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 disposed partially inside and partially outside the reservoirs, thereby sealing the gas within the reservoirs. The optical fiber OF may further include an intermediate section that is not housed within a reservoir. Such an arrangement using two separate gas reservoirs may be particularly convenient for embodiments in which the optical fiber OF is relatively long (e.g., greater than 1 meter in length). It will be appreciated that in such an 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 gases in the two reservoirs) may be considered to provide an apparatus for providing the working medium WM within the hollow core HC of the optical fiber OF.

[0080] In this context, a window may be transparent to a frequency if at least 50%, 75%, 85%, 90%, 95%, or 99% of radiation incident on the window at that frequency is transmitted through the window.

[0081] Both the first and second transparent windows TW1 and TW2 may form an airtight seal within the wall of the reservoir RSV such that a working medium WM (which may be a gas) may be contained within the reservoir RSV. It will be appreciated that the gas WM may be contained within the reservoir RSV at a pressure different from the ambient pressure of the reservoir RSV.

[0082]

[0081] The working medium WM may include gas mixtures such as noble gases such as argon, krypton, and xenon, Raman-active gases such as hydrogen, deuterium, and nitrogen, argon / hydrogen mixtures, xenon / deuterium mixtures, krypton / nitrogen mixtures, mixtures of molecular gases (e.g., nitrogen / hydrogen mixtures), mixtures of atomic gases (e.g., argon / helium, krypton / helium, xenon / helium), or ternary gas mixtures such as argon / helium / hydrogen or krypton / helium / hydrogen. Depending on the thermodynamic conditions of the RSV (e.g., the type of filling gas, its pressure, and temperature), and the laser conditions (e.g., pulse duration, energy, and wavelength), nonlinear optical processes can include modulation instability (MI), soliton self-compression, soliton splitting, the Kerr effect, the Raman effect, and dispersive wave generation (DWG), as described in detail in International Publication No. 2018 / 127266A1 and U.S. Patent No. 9,160,137B1 (both of which are incorporated herein by reference). Because the above-mentioned parameters can be adjusted, the generated broadband pulse dynamics and associated spectral broadening characteristics can be tailored to optimize frequency conversion. For example, the dispersion of a gas-filled hollow-core fiber, a key factor in defining the interaction regime of the laser pulse with the WM, can be adjusted by changing the pressure of the working medium WM (i.e., the gas cell pressure) in the reservoir RSV.

[0083] In one implementation, the working medium WM may be disposed within the hollow core HC at least while the optical fiber OF is receiving input radiation IRD for generating the broadband output radiation ORD. It will be appreciated that the gas WM may be completely or partially absent from the hollow core HC while the optical fiber OF is not receiving input radiation IRD for generating the broadband output radiation.

[0084]

[0083] To achieve frequency broadening, high-intensity radiation may be desired. An advantage of having a hollow-core HC optical fiber OF is that this can be achieved through strong spatial confinement of the radiation propagating through the optical fiber OF, resulting in localized high-intensity radiation. The radiation intensity within the optical fiber OF can be high, for example, due to the received high-intensity input radiation and / or the strong spatial confinement of the radiation within the optical fiber OF. An advantage of hollow-core optical fibers is that solid-core fibers, and especially hollow-core optical fibers, can guide radiation having a wider wavelength range in the spectral region where fused silica (a typical glass used in optical fibers) absorbs, and can guide radiation in both the ultraviolet and infrared ranges.

[0085]

[0084] An advantage of using a hollow-core HC optical fiber OF may be that most of the radiation guided in the optical fiber OF is confined to the hollow core HC. Therefore, most of the interaction of the radiation in the optical fiber OF is with the working medium WM provided in the hollow core HC of the optical fiber OF. As a result, the spreading effect of the radiation by the working medium WM may be enhanced.

[0086]

[0085] The received input radiation IRD may be electromagnetic radiation. The input radiation IRD may be received as pulsed radiation. For example, the input radiation IRD may comprise ultrafast pulses, for example generated by a laser.

[0087]

[0086] The input radiation IRD may be coherent radiation. The input radiation IRD may be collimated radiation, the advantage of which may be to increase and improve the efficiency of coupling the input radiation IRD into the optical fiber OF. The input radiation IRD may comprise a single frequency or a narrow range of frequencies. The input radiation IRD may be generated by a laser. Similarly, the output radiation ORD may be collimated and / or coherent.

[0088]

[0087] The broadband range of the output radiation ORD can be a continuous range, including a continuous range of radiation frequencies. The output radiation ORD can include supercontinuum radiation. Continuum radiation can be beneficial for many applications, such as metrology applications. For example, a continuous range of frequencies can be used to probe multiple properties. A continuous range of frequencies can be used, for example, to identify and / or eliminate frequency dependence of a measured property and / or to enable fast selection and / or switching between different frequencies using a spectral filtering device. The supercontinuum output radiation ORD can include electromagnetic radiation over a wavelength range of, for example, 100 nm to 4000 nm. The broadband output radiation ORD frequency range can be, for example, 400 nm to 900 nm, 500 nm to 900 nm, or 200 nm to 2000 nm. The supercontinuum output radiation ORD can include white light.

[0089]

[0088] The input radiation IRD provided by the pulsed pump radiation source PRS (or other input radiation source) may be pulsed. The input radiation IRD may comprise electromagnetic radiation at one or more frequencies between 200 nm and 3 μm. The input radiation IRD may comprise electromagnetic radiation having a wavelength of, for example, 1.03 μm. The repetition rate of the pulsed radiation IRD may be on the order of 1 kHz to 100 MHz. The pulse energy may be on the order of 1 nJ to 100 μJ, for example, 1 to 10 μJ. The pulse duration of the input radiation IRD may be between 1 femtosecond and 10 picoseconds, for example, 300 femtoseconds. The average power of the input radiation IRD may be between 100 mW and several hundred W. The average power of the input radiation IRD may be, for example, 20 to 50 W.

[0090] The pulsed pump radiation source PRS may be a laser. The spatiotemporal transmission characteristics of such laser pulses transmitted along the optical fiber OF, such as their spectral amplitude and phase, can be varied and tuned through adjusting (pump) laser parameters, tuned thermodynamic properties of the working medium WM, and / or optical fiber OF parameters (e.g., its core diameter and / or length). The spatiotemporal transmission characteristics may include one or more of the following: output power, output mode profile, output temporal profile, width of the output temporal profile (or output pulse width), output spectral profile, and bandwidth of the output spectral profile (or output spectral bandwidth). The input radiation source IRS parameters may include one or more of the following: pump wavelength, pump pulse energy, pump pulse width, and pump pulse repetition rate. The optical fiber OF parameters may include one or more of the following: length of the optical fiber, size and shape of the hollow core HC, size and shape of the capillary, and thickness of the wall of the capillary surrounding the hollow core HC. The operating component WM (eg, fill gas) parameters may include one or more of the gas type, gas pressure and gas temperature, or gas composition and / or partial pressure if the WM is a mixture of different gases.

[0091]

[0090] The broadband output radiation ORD provided by the radiation source RDS may have an average output power of at least 100mW. The average output power may be at least 1W. The average output power may be at least 5W. The average output power may be at least 10W. The broadband output radiation ORD may be a pulsed broadband output radiation ORD. The broadband output radiation ORD may be a continuous wave CW broadband output radiation ORD. The broadband output radiation ORD may have a power spectral density within the entire wavelength band of the output radiation of at least 0.01mW / nm. The power spectral density within the entire wavelength band of the broadband output radiation may be at least 3mW / nm.

[0092] As mentioned above, there are many nonlinear optical processes involved in generating broadband output radiation ORD (e.g., supercontinuum or white light). Which nonlinear optical process has a more pronounced spectral broadening effect than others depends on how the operating parameters are set. For example, by selecting the pump wavelength and / or the optical fiber OF such that the pump pulse propagates through the fiber in the normal dispersion regime (positive group velocity dispersion (GVD)), self-phase modulation becomes the dominant nonlinear optical process and is responsible for the spectral broadening of the pump pulse. However, in most cases, the spectral broadening of the input radiation IRD provided by the pulsed pump radiation source PRS is induced by soliton dynamics, which requires the pump pulse to propagate through the optical fiber OF in the anomalous dispersion regime (negative GVD). This is because in the anomalous dispersion regime, the effects of Kerr nonlinearity and dispersion act against each other so that the pulse maintains and / or enhances its peak intensity. If the pulse parameters of a pump pulse injected into an optical fiber OF with anomalous chromatic dispersion (e.g., HC-PCF) do not exactly match those of a soliton, the pump pulse will evolve into a soliton pulse with a certain soliton order and dispersive wave.

[0093]

[0092] It is known that soliton self-compression and modulation instability are the two main mechanisms for spectral broadening in soliton-induced broadband radiation generation. The difference between these two mechanisms is that the soliton self-compression process is associated with low soliton orders, while the modulation instability process is associated with high soliton orders. The soliton order N of the pulsed input radiation IRD is a convenient parameter that can be used to distinguish between conditions in which spectral broadening is dominated by modulation instability and conditions in which spectral broadening is dominated by soliton self-compression. The soliton order N of the pulsed input radiation IRD is given by:

number

[0094]

[0093] Spectral broadening is typically dominated by modulation instability when N>>20, whereas spectral broadening is typically dominated by soliton self-compression when N<<20.

[0095]

[0094] Some known broadband radiation sources use arrangements that produce spectral broadening of the pulsed pump radiation, where the parameters of the pulsed pump radiation, the optical fiber, and the working medium are configured to allow modulation instability to produce the spectral broadening. There are several reasons why modulation instability is used to produce spectral broadening. First, modulation instability is known to generate broadband radiation with a relatively flat intensity-wavelength distribution if a sufficient number of pulses are averaged. Such broadband radiation sources are sometimes called white light radiation sources (due to the relatively flat spectral intensity distribution). Second, modulation instability can be achieved by using a relatively economical laser source as the pump radiation source.

[0096]

[0095] On the other hand, in the soliton self-compression regime, the input pump pulse undergoes compression in the time domain, which is accompanied by an increase in spectral width. Following soliton self-compression, the compressed pulse undergoes soliton fission, in which the pulse splits into multiple solitons. This soliton fission results in a temporal broadening and spectral shift of the emitted pulse.

[0097] To summarize the principles behind the radiation source described above, e.g., in FIG. 8, current techniques for supercontinuum (SC) generation rely on the interaction of a driving laser radiation with a nonlinear element to increase the spectral bandwidth. In this scenario, the peak intensity of the laser radiation is high enough that it undergoes multiple optical nonlinear effects, thereby broadening the spectrum. Typically, the wavelength of the laser pulse is selected so that it falls into either the normal or anomalous dispersion regime of the nonlinear element. In the former case, broadening is dominated by self-phase modulation (SPM), while in the latter case, spectral broadening is dominated by soliton dynamics. Alternatively, the nonlinear element can be designed to provide fully normal dispersion, such as an all-normal-dispersion (ANDi) fiber. In this case, broadening is dominated by SPM and optical wave breaking.

[0098] This current approach can result in a highly non-uniform power spectral density (PSD). For example, the PSD from the pump laser can be down by 2-3 orders of magnitude. This indicates a low conversion efficiency from the driving laser pulse to the supercontinuum region of interest (which can be, for example, 100-1200 nm, 300-1000 nm, or 400-900 nm). This is also true for modulation instability (MI)-based systems, which is a result of the random nature of the high-order soliton-based supercontinuum generation dynamics and the noise-initiated amplification process.

[0099] It is known to use multicolor pumping to broaden the supercontinuum spectrum and / or increase its PSD at the short-wavelength end of the supercontinuum. A typical configuration may involve pumping a nonlinear element with the fundamental and its (e.g., second) harmonic of a pump laser. However, this approach has drawbacks. For example, there is a large spectral gap between the fundamental frequency of the pump laser and its second harmonic, which does not provide substantially intense radiation to the nonlinear element. Furthermore, the fundamental and its one or more harmonics reside in different dispersion regimes of the nonlinear medium. This results in laser radiation in normal dispersion broadening rapidly in time, thus losing peak intensity and resulting in reduced spectral broadening. Furthermore, several laser systems or harmonic generation stages may be required to implement such an approach. Therefore, for high-power applications, generating harmonics in high-power laser systems is very challenging and technically expensive.

[0100] In single-frequency or multicolor generation, the supercontinuum is the result of the interaction of a laser pulse (or multiple laser pulses) with only one nonlinear element stage. Therefore, the spectral broadening is limited to the interaction regime of only one nonlinear element and is limited by its nonlinear characteristics and propagation dynamics. For example, the shape of the spectrum in soliton self-compression (SSC) is determined by the phase-matching condition to the dispersive wave due to gas pressure, core diameter, etc. After this broadening stage, the PSD remains as provided by the nonlinear interaction.

[0101] [000100] Nonlinear media are pumped with laser pulses to achieve a sufficiently high peak intensity to initiate nonlinear optical processes such as SSC or MI. However, recent advances in supercontinuum generation and nonlinear optical media allow for the generation of supercontinuums with high peak intensities that may also be sufficient to initiate nonlinear optical processes in subsequent nonlinear elements / media. For example, high-energy, high-peak-intensity supercontinuums can be generated in gas-filled hollow-core fibers (e.g., HC-PCFs) by MI or SSC processes.

[0102] [000101] Therefore, it has been proposed to generate broadband radiation by pumping a nonlinear medium / element with a supercontinuum instead of a laser pulse. The supercontinuum must be intense and / or broad enough to induce further nonlinear optical phenomena in the optically nonlinear medium. In the context of the present disclosure, a supercontinuum includes radiation (e.g., produced by a laser) that has subsequently undergone at least one spectral broadening step by (or within) the nonlinear medium. The required intensity level may be proportional to the optical nonlinearity and optical damage of the subsequent nonlinear element stage.

[0103] [000102] The proposed radiation source comprises two or more successive supercontinuum generation stages, each supercontinuum generation stage comprising a respective nonlinear generating element, wherein the damage tolerance of a first nonlinear generating element comprised in the first supercontinuum generation stage is greater than the damage tolerance of at least a second nonlinear generating element comprised in the second supercontinuum generation stage, and wherein the second supercontinuum generation stage immediately follows the first supercontinuum generation stage.

[0104] [000103] The first supercontinuum generation stage may be arranged to generate a first supercontinuum and to provide the first supercontinuum to a second supercontinuum generation stage to generate a second supercontinuum. The second supercontinuum generation stage may be a final supercontinuum generation stage, and the second supercontinuum may be used as output broadband radiation. Alternatively, if the second supercontinuum generation stage is not the final supercontinuum generation stage, the second supercontinuum may be provided to an immediately following supercontinuum generation stage. Thus, each successive supercontinuum generation stage before the final supercontinuum generation stage may generate a supercontinuum for the next supercontinuum generation stage.

[0105] [000104] The radiation source may include a radiation source arranged to provide laser radiation to the first supercontinuum generation stage so as to generate the first supercontinuum.

[0106] [000105] In certain embodiments, when there are three or more successive supercontinuum generation stages, each successive supercontinuum generation stage may have a lower damage tolerance than the previous supercontinuum generation stage (i.e., the stages may be arranged in order of damage tolerance: from higher tolerance to lower tolerance). However, in other embodiments, the second supercontinuum generation stage may have a lower damage tolerance than the first supercontinuum generation stage, and successive supercontinuum generation stages after the second stage may have any damage tolerance, e.g., the damage tolerance may remain constant or become greater for successive supercontinuum generation stages after the second stage.

[0107] [000106] Damage tolerance may be assessed according to the damage threshold of each nonlinear generating element included within each supercontinuum generating stage.

[0108] [000107] Also disclosed is a radiation source in which the optical nonlinearity of a first nonlinear generating element included in the first supercontinuum generating stage is lower than the optical nonlinearity of at least a second nonlinear generating element included in the second supercontinuum generating stage.

[0109] [000108] In some embodiments, there may be free space coupling between at least the first and second supercontinuum generation stages. In some embodiments, there may be free space coupling between each pair of two or more consecutive supercontinuum generation stages.

[0110] [000109] In some embodiments, the first nonlinear generating element may include a hollow-core fiber (e.g., HC-PCF). Alternatively or additionally, the second nonlinear generating element may include a solid-state nonlinear generating medium. The solid-state nonlinear generating medium may include a solid-core fiber (e.g., SC-PCF) or a (synthetic) crystal (e.g., BBO crystal or PPLN crystal).

[0111] [000110] In the context of the present disclosure, the nonlinear generating element may include the nonlinear generating medium (e.g., if the nonlinear generating medium is a solid element), or the nonlinear generating element may include an element for confining the nonlinear generating medium (e.g., if the nonlinear generating medium is a gas, the nonlinear generating element may be, for example, a hollow-core fiber). In either case, it will be recognized that the nonlinear generating medium may be a liquid.

[0112] [000111] Figure 10 illustrates a concept according to a general embodiment. A drive laser DL generates drive radiation DR that is received by a first supercontinuum generation stage SCGS1. The first supercontinuum generation stage includes a first nonlinear generating element having a first damage threshold. The first supercontinuum generation stage generates a first supercontinuum SC1 that is received by the next successive supercontinuum generation stage or a second supercontinuum generation stage, which includes a second nonlinear generating element having a second damage threshold that is smaller than the first damage threshold. The second supercontinuum generation stage generates a supercontinuum output from this supercontinuum generation stage as a broadband output radiation SC out Alternatively, any number of supercontinuum generation stages (i.e., n>2) may be interposed between the first supercontinuum generation stage SCGS1 and the final supercontinuum generation stage SCGSn, each receiving a supercontinuum from the immediately preceding supercontinuum generation stage and generating a supercontinuum for the immediately succeeding supercontinuum generation stage, with the penultimate stage generating a supercontinuum SC to pump the final supercontinuum generation stage. n-1 It is conceivable that the damage threshold of the nonlinear generating element of each supercontinuum generation stage may also decrease (or at least not increase) with each successive supercontinuum generation stage after the second supercontinuum generation stage, i.e. from the input to the output of the radiation source.

[0113] [000112] Thus, each generated supercontinuum (at least before the final stage) has sufficient intensity, relative to the optical nonlinearity of the associated stage, to induce further nonlinear optical phenomena, e.g., frequency mixing and spectral broadening, in the next supercontinuum-generating stage, thus generating new enhanced supercontinuum radiation at the output of each successive supercontinuum-generating stage. "Enhanced" in this context may refer to having improved PSD flatness (e.g., a flatter PSD), and expanded spectral coverage, and / or improved PSD values, particularly in the supercontinuum region of interest.

[0114] [000113] Several examples of the proposed broadband radiation source will now be described, each example being an embodiment of the broadband radiation source shown in Figure 10.

[0115] [000114] Figure 11 shows an arrangement in which (at least) the first supercontinuum generation stage comprises a gas-filled hollow-core fiber HCF (e.g., HC-PCF). Such a hollow-core fiber HCF can generate a supercontinuum from drive radiation DR (e.g., emitted by a drive laser DL) by MI and / or SSC. In the case of MI, the generated first supercontinuum radiation SC1 has a fine temporal structure consisting of a well-defined (e.g., about 10 femtoseconds) temporal structure. The supercontinuum radiation SC1 can be received by a subsequent supercontinuum generation stage comprising a solid-core fiber SCF (or other solid-state nonlinear medium), such as an SC-PCF. The solid-core fiber SCF outputs (as shown) the output supercontinuum SC out or for further supercontinuum generation stages (as before, there can be any number of such stages). Because the damage threshold of a gas-filled hollow-core fiber is higher than that of a solid-core PCF, the peak intensity of the supercontinuum SC1 spike is high enough to induce nonlinear broadening in a second nonlinear element, such as a solid-core PCF.

[0116] [000115] In an embodiment, the first supercontinuum radiation SC1 may be coupled to the solid-core fiber SCF by free-space optics. Alternatively, a hollow-core fiber HCF may be spliced ​​to the solid-core fiber SCF, making the free-space optics redundant.

[0117] [000116] In another embodiment, the MI or SSC-based supercontinuum can induce further nonlinear effects in another gas. The next stage benefits from the fine temporal structure of the MI-based supercontinuum or the short pulses of the SSC supercontinuum to induce further nonlinear processes.

[0118] [000117] It has been observed that there is a large drop in the PSD between the dispersive wave and the SPM broadened spectrum of the first supercontinuum radiation SC1. This is because the output radiation SC out This can make the supercontinuum unsuitable for some metrology applications. To address this, it has been proposed to reduce the pressure of the gas (nonlinear medium) in the hollow-core fiber (HCF) to less than 100 bar. This step is optional and shifts the dispersive wave to shorter wavelengths. The first supercontinuum radiation is then coupled into a solid-core fiber (SCF). In this case, the spectrum of the supercontinuum SC1 includes both the normal and anomalous dispersion regimes of the solid-core fiber, but this is not a requirement. As the pulse propagates along the solid-core fiber, the dispersive wave is red-shifted, filling the dip in the PSD. Furthermore, the remaining portion of the SPM-broadened pump undergoes four-wave mixing (FWM) to convert energy to other wavelengths on both the shorter and longer wavelength sides, resulting in further spectral flattening. These combined effects result in the output radiation SC1. out The PSD of the image is improved.

[0119] 12 and 13 each show an arrangement in which the first supercontinuum SC1 is generated by filamentation in a transparent solid or filamentation element FE. Supercontinuum generation in a solid by filamentation is a good way to fabricate compact, alignment-free, robust solid-state broadband light sources at low cost. As already explained, the output supercontinuum SC out It is desirable to improve the spectral flatness and to increase the spectral coverage of the proposed bulk medium (e.g., first nonlinear medium) used in the filamentation element FE, e.g., sapphire, fused silica, or laser crystals such as KGW or YAG, have a fairly high damage threshold, e.g., up to 100 J / cm for femtosecond pulses. 2 fluence levels up to 1000 Hz. Therefore, these media can be used for (at least) the first supercontinuum generation stage, followed by one or more additional supercontinuum generation stages containing respective nonlinear elements with lower damage thresholds. For example, these nonlinear elements can include one or more of a solid-core fiber SCF (as shown in FIG. 12) or a frequency-mixing stage FMS (as shown in FIG. 13) to increase the bandwidth of the supercontinuum and smooth its PSD.

[0120] [000119] The frequency mixing stage FMS may include a synthetic crystal such as periodically poled lithium niobate (PPLN). The frequency mixing stage may support broadband phase matching. For example, it may use quasi-phase-matched broadband second-harmonic generation. Other techniques for broadband phase matching, such as angle-dispersive supercontinuum, may also be used.

[0121] [000120] Figure 14 illustrates another embodiment in which supercontinuum generation occurs by stage-by-stage expansion in a series of two or more (e.g., solid-core) fibers having core diameters that decrease in successive stages. In the illustrated example, three such stages are provided, including a first solid-core fiber SCF1 having a first diameter, a second solid-core fiber SCF2 having a second diameter smaller than the first diameter, and a third solid-core fiber SCF3 having a third diameter smaller than the second diameter. The laser-induced damage threshold (LIDT) of a fiber having a core diameter φ is 2 (i.e., LIDT ∝ φ 2 ), the damage threshold decreases with each successive nonlinear element or solid-core fiber SCF1, SCF2, SCF3. Such an arrangement makes it possible to take advantage of the different dispersion profiles provided by different fibers having different core diameters. Optionally, the materials comprising the solid-core fibers may be different for one or more of the fibers, thus also taking advantage of the different dispersion profiles provided by the different core materials.

[0122] [000121] The coupling between different fibers may be all free-space optics, all spliced, or a combination of the two (i.e., a first pair SCF1, SCF2 free-space coupled and a second pair SCF2, SCF3 spliced, or vice versa). If spliced, adapters may be provided between the fibers to mode-fill the diameter of each successive fiber.

[0123] 15 illustrates a further embodiment in which at least one supercontinuum generation stage after the first supercontinuum stage SCGS1 includes a crystal, such as a barium borate (BBO) crystal, as a nonlinear medium. For example, the BBO crystal has a cut angle θ c= 22°. Such a BBO crystal improves the output broadband radiation characteristics in the visible frequency range. In particular, a single BBO crystal can be used in this manner to convert most of the IR region of the output broadband radiation into visible light. In such an embodiment, the first supercontinuum stage SCGS1 can include, for example, a hollow-core fiber (e.g., HC-PCF).

[0124] [000123] It will be appreciated that Figures 11-15 illustrate a non-exhaustive list of different arrangements possible within the context of the disclosure herein and the general arrangement of Figure 10. Other arrangements are possible within the scope of this disclosure.

[0125] [000124] In an embodiment, a feedback loop may be configured to control (e.g., adjust) the input radiation (e.g., in terms of power) based on a performance metric, e.g., a measured PSD, of each supercontinuum stage; this may be done, for example, to prevent optical damage.

[0126] 16 is a block diagram illustrating a computer system 1600 that may assist in implementing the methods and flows disclosed herein. The computer system 1600 includes a bus 1602 or other communication mechanism for communicating information and a processor 1604 (or multiple processors 1604, 1605) coupled with the bus 1602 for processing information. The computer system 1600 also includes a main memory 1606 (such as a random access memory (RAM) or other dynamic storage device) coupled to the bus 1602 for storing information and instructions for execution by the processor 1604. The main memory 1606 may also be used for storing temporary variables or other intermediate information during execution of instructions by the processor 1604. The computer system 1600 further includes a read-only memory (ROM) 1608 or other static storage device coupled to the bus 1602 for storing static information and instructions for the processor 1604. A storage device 1610 (such as a magnetic disk or optical disk) is provided and coupled to the bus 1602 for storing information and instructions.

[0127] [000126] Computer system 1600 can be coupled via bus 1602 to a display 1612 (such as a cathode ray tube (CRT) or flat-panel or touch-panel display) for displaying information to a computer user. Input devices 1614 (including alphanumeric and other keys) are coupled to bus 1602 for communicating information and command selections to processor 1604. Another type of user input device is a cursor control 1616 (such as a mouse, trackball, or cursor direction keys) for communicating directional information and command selections to processor 1604 and for controlling cursor movement on display 1612. This input device typically has two degrees of freedom in two axes, i.e., a first axis (e.g., x) and a second axis (e.g., y), allowing the device to specify a position in a plane. Touch-panel (screen) displays can also be used as input devices.

[0128] [000127] One or more of the methods described herein may be performed by computer system 1600 in response to processor 1604 executing one or more sequences of one or more instructions contained in main memory 1606. Such instructions may be read into main memory 1606 from another computer-readable medium, such as storage device 1610. Execution of the sequences of instructions contained in main memory 1606 causes processor 1604 to perform the process steps described herein. One or more processors in a multi-processing arrangement may also be used to execute the sequences of instructions contained in main memory 1606. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions. Thus, the description herein is not limited to any specific combination of hardware circuitry and software.

[0129] The term "computer-readable medium," as used herein, refers to any medium that participates in providing instructions to processor 1604 for execution. Such media may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media include, for example, optical or magnetic disks, such as storage device 1610. Volatile media include dynamic memory, such as main memory 1606. Transmission media include coaxial cables, copper wire and fiber optics, including the wires that comprise bus 1602. Transmission media may also take the form of acoustic or light waves, such as those generated during radio frequency (RF) and infrared (IR) data communications. Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, a hard disk, magnetic tape, any other magnetic medium, a CD-ROM, a DVD, any other optical medium, punch cards, paper tape, any other physical medium with a pattern of holes, a RAM, a PROM, an EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described below, or any other medium from which a computer can read.

[0130] Various forms of computer-readable media may be involved in carrying one or more sequences of one or more instructions to processor 1604 for execution. For example, the instructions may initially reside on a magnetic disk of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to computer system 1600 can receive the data on the telephone line and use an infrared transmitter to convert the data to an infrared signal. An infrared detector coupled to the bus can receive the data carried in the infrared signal and place the data on bus 1602. Bus 1602 carries the data to main memory 1606, from which processor 1604 retrieves and executes the instructions. The instructions received by main memory 1606 may optionally be stored on storage device 1610 either before or after execution by processor 1604.

[0131] Computer system 1600 also preferably includes a communication interface 1618 coupled to bus 1602. The communication interface 1618 provides a two-way data communication coupling to a network link 1620 that is connected to a local network 1622. For example, communication interface 1618 may be an Integrated Services Digital Network (ISDN) card or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, communication interface 1618 may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In any such implementation, communication interface 1618 sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information.

[0132] [000131] Network link 1620 typically provides data communication through one or more networks to other data devices. For example, network link 1620 may provide a connection through local network 1622 to a host computer 1624 or to data equipment operated by an Internet Service Provider (ISP) 1626. ISP 1626, in turn, provides data communication services through the world wide packet data communication network (now commonly referred to as the "Internet" 1628). Local network 1622 and the Internet 1628 both use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on network link 1620 and through communication interface 1618 (which carry the digital data to and from computer system 1600) are exemplary forms of carrier waves transporting the information.

[0133] [000132] Computer system 1600 can send messages and receive data, including program code, through the network(s), network link 1620 and communication interface 1618. In the Internet example, a server 1630 might send a requested code for an application program through the Internet 1628, ISP 1626, local network 1622 and communication interface 1618. One such downloaded application may, for example, comprise one or more of the techniques described herein. The received code may be executed by processor 1604 upon receipt and / or stored in storage device 1610 or other non-volatile storage for later execution. In this manner, computer system 1600 can obtain application code in the form of a carrier wave.

[0134] [000133] Further embodiments of the present invention are set forth in the following list of numbered clauses: 1. A broadband radiation source for generating output broadband radiation, comprising a plurality of supercontinuum generation stages arranged in series, each supercontinuum generation stage comprising a respective nonlinear generating element, the plurality of supercontinuum generation stages comprising at least a first supercontinuum generation stage and a second supercontinuum generation stage, the second supercontinuum generation stage following the first supercontinuum generation stage in series, and a damage tolerance of a first nonlinear generating element comprised within the first supercontinuum generation stage greater than a damage tolerance of at least a second nonlinear generating element comprised within the second supercontinuum generation stage. 2. A broadband radiation source as described in clause 1, wherein the first supercontinuum generation stage is arranged to generate a first supercontinuum and to provide the first supercontinuum to the second supercontinuum generation stage to generate a second supercontinuum. 3. A broadband radiation source according to clause 2, further comprising a laser radiation source arranged to provide laser radiation to the first supercontinuum generation stage to generate the first supercontinuum. 4. A broadband radiation source according to any one of the preceding clauses, wherein the second supercontinuum generation stage is a final supercontinuum generation stage, such that the output broadband radiation comprises a second supercontinuum. 5. A broadband radiation source as described in clause 1, 2 or 3, comprising one or more further supercontinuum generation stages, wherein the second supercontinuum is provided to a next successive supercontinuum generation stage of the one or more further supercontinuum generation stages such that each successive supercontinuum generation stage before the final supercontinuum generation stage generates a supercontinuum for the next supercontinuum generation stage in the series, and wherein the output broadband radiation comprises the supercontinuum from the final supercontinuum generation stage. 6. A broadband radiation source as described in clause 5, wherein each successive supercontinuum generation stage after the second supercontinuum generation stage includes a respective nonlinear generating element having a damage tolerance equal to or less than the nonlinear generating element of the immediately preceding supercontinuum generation stage. 7. A broadband radiation source as described in clause 5, wherein each successive supercontinuum generation stage includes a respective nonlinear generating element having a lower damage tolerance than the nonlinear generating element of the immediately preceding supercontinuum generation stage. 8. A broadband radiation source according to any one of the preceding clauses, comprising free space coupling between at least the first supercontinuum generation stage and the second supercontinuum generation stage. 9. A broadband radiation source according to any one of the preceding clauses, including free space coupling between all of the plurality of supercontinuum generation stages. 10. A broadband radiation source according to any one of clauses 1 to 8, comprising fibre-based coupling between all of the plurality of supercontinuum generation stages. 11. A broadband radiation source according to any one of clauses 1 to 8, wherein the first nonlinear element and the second nonlinear element are spliced ​​together. 12. A broadband radiation source according to clause 11, wherein all of the respective nonlinear generating elements are spliced. 13. A broadband radiation source according to any one of the preceding clauses, wherein at least the first nonlinear generating element comprises a hollow-core fiber. 14. The broadband radiation source of clause 13, wherein the hollow-core fiber comprises a hollow-core photonic crystal fiber. 15. A broadband radiation source according to any one of clauses 1 to 12, wherein at least the first nonlinear generating element comprises a solid core fibre. 16. A broadband radiation source according to any one of clauses 1 to 12, wherein at least the first nonlinear generating element comprises a filamentation element operable to generate a supercontinuum by filamentation. 17. A broadband radiation source according to any one of the preceding clauses, wherein at least the second nonlinear generating element comprises a solid state nonlinear generating element. 18. A broadband radiation source according to clause 17, wherein the solid state nonlinear generating element comprises a solid core fibre. 19. A broadband radiation source according to clause 17, wherein the solid state nonlinear generating element comprises a crystal. 20. A broadband radiation source according to clause 19, wherein the solid state nonlinear generating element comprises a BBO crystal. 21. A broadband radiation source according to any one of clauses 1 to 17, wherein at least the second supercontinuum generation stage comprises a frequency mixing stage. 22. A broadband radiation source according to clause 21, wherein the second supercontinuum generation stage supports broadband phase matching. 23. A broadband radiation source according to clause 21 or 22, wherein the second nonlinear generating element comprises periodically poled lithium niobate. 24. A broadband radiation source according to any one of clauses 1 to 12, wherein each of the plurality of supercontinuum generation stages comprises a respective optical fiber, each optical fiber having a diameter that decreases for each successive supercontinuum generation stage. 25. A broadband radiation source according to any one of the preceding clauses, wherein the output broadband radiation comprises wavelengths of at least 100 nm to 1200 nm. 26. A broadband radiation source according to any one of the preceding clauses, wherein the output broadband radiation comprises wavelengths in the range of at least 400 nm to 900 nm. 27. A broadband radiation source for generating output broadband radiation, comprising a plurality of supercontinuum generation stages arranged in series, each supercontinuum generation stage comprising a respective nonlinear generating element, the plurality of supercontinuum generation stages comprising at least a first supercontinuum generation stage and a second supercontinuum generation stage, the second supercontinuum generation stage following the first supercontinuum generation stage in series, and wherein an optical nonlinearity of a first nonlinear generating element comprised within the first supercontinuum generation stage is lower than an optical nonlinearity of at least a second nonlinear generating element comprised within the second supercontinuum generation stage. 28. A metrology device comprising a radiation source according to any one of the preceding clauses. 29. A metrology device according to clause 28, comprising a scatterometer metrology apparatus, a level sensor or an alignment sensor.

[0135] [000134] Although specific reference is made in this specification to uses of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optics, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0136] [000135] Although embodiments of the invention may be specifically referenced herein in connection with lithography apparatus, they may be used in other apparatus. They may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes objects such as wafers (or other substrates) or masks (or other patterning devices). These apparatus may be collectively referred to as lithography tools. Such lithography tools may use vacuum or ambient (non-vacuum) conditions.

[0137] [000136] Although specific reference has been made above to the use of embodiments of the invention in connection with optical lithography, it will be understood that the invention is not limited to optical lithography and may be used in other applications, for example in imprint lithography, where the context allows.

[0138] [000137] While specific embodiments of the present invention have been described above, it will be understood that the invention may be practiced otherwise than as described. The foregoing description is intended to be illustrative and not limiting. Accordingly, it will be apparent to those skilled in the art that modifications can be made to the invention as described without departing from the scope of the claims set forth below.

Claims

1. 1. A broadband radiation source for generating output broadband radiation, comprising: a plurality of supercontinuum generation stages arranged in series, each supercontinuum generation stage comprising a respective nonlinear generating element; the plurality of supercontinuum generation stages include at least a first supercontinuum generation stage and a second supercontinuum generation stage, the second supercontinuum generation stage following the first supercontinuum generation stage in the series; A broadband radiation source, wherein a damage tolerance of a first nonlinear generating element included in the first supercontinuum generating stage is greater than a damage tolerance of at least a second nonlinear generating element included in the second supercontinuum generating stage.

2. 2. The broadband radiation source of claim 1, wherein the first supercontinuum generation stage is arranged to generate a first supercontinuum and to provide the first supercontinuum to the second supercontinuum generation stage for generating a second supercontinuum.

3. 3. The broadband radiation source of claim 2, further comprising a laser radiation source arranged to provide laser radiation to the first supercontinuum generation stage to generate the first supercontinuum.

4. 2. The broadband radiation source of claim 1, wherein the second supercontinuum generation stage is a final supercontinuum generation stage, such that the output broadband radiation comprises the second supercontinuum.

5. 2. The broadband radiation source of claim 1, comprising one or more further supercontinuum generation stages, wherein the second supercontinuum is provided to a next successive supercontinuum generation stage after the one or more further supercontinuum generation stages such that each successive supercontinuum generation stage before a final supercontinuum generation stage generates a supercontinuum for a next supercontinuum generation stage in the series, and wherein the output broadband radiation comprises the supercontinuum from the final supercontinuum generation stage.

6. 6. The broadband radiation source of claim 5, wherein each successive supercontinuum generation stage after the second supercontinuum generation stage includes a respective nonlinear generating element having a damage tolerance equal to or less than the nonlinear generating element of the immediately preceding supercontinuum generation stage.

7. 2. The broadband radiation source of claim 1, comprising a free space coupling between at least the first supercontinuum generation stage and the second supercontinuum generation stage.

8. 2. The broadband radiation source of claim 1, comprising a fiber-based coupling between the first supercontinuum generation stage and the second supercontinuum generation stage.

9. 10. The broadband radiation source of claim 1, wherein at least the first nonlinear generating element comprises a hollow-core fiber.

10. 10. The broadband radiation source of claim 9, wherein the hollow-core fiber is a hollow-core photonic crystal fiber.

11. 2. The broadband radiation source of claim 1, wherein at least the first nonlinear generating element comprises a filamentation element operable to generate a supercontinuum by filamentation.

12. 10. The broadband radiation source of claim 1, wherein at least the second nonlinear generating element comprises a solid-state nonlinear generating element.

13. 13. The broadband radiation source of claim 12, wherein the solid-state nonlinear generating element comprises a solid-core fiber.

14. The broadband radiation source of claim 12 , wherein the solid-state nonlinear generating element comprises a crystal.

15. The broadband radiation source of claim 1 , wherein at least the second supercontinuum generation stage comprises a frequency mixing stage.

16. 2. The broadband radiation source of claim 1, wherein each of the plurality of supercontinuum generation stages comprises a respective optical fiber, each optical fiber having a diameter that decreases with each successive supercontinuum generation stage.

17. A metrology device comprising the radiation source of claim 1.

18. 20. The metrology device of claim 17, comprising a scatterometer metrology tool, a level sensor, or an alignment sensor.

19. An inspection apparatus comprising the radiation source of claim 1.

20. The inspection system of claim 19 , wherein the inspection system is configured to identify defects on a substrate.

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