Methods, components, and devices for improving control of broadband radiation generation

Abstract

A hollow-core photonic crystal fiber (HC-PCF) assembly for converting input radiation into broadband radiation, the hollow-core fiber assembly comprising: a microstructured fiber having a hollow core that extends along a length of the fiber from an input end configured to receive the input radiation to an output end configured to output the broadband radiation, wherein the hollow core of the fiber is configured to contain a medium; and a density control system configured to control a density profile of the medium along at least a portion of the length of the fiber to establish a desired zero-dispersion wavelength profile along at least a portion of the length of the fiber.

Description

[0001] Cross-reference to related applications

[0002] This application claims priority to European Application No. 20151863.6, filed on January 15, 2020, and European Application No. 20156804.5, filed on February 12, 2020, the entire contents of which are incorporated herein by reference. Technical field

[0003] The present invention relates to methods, components, and devices for converting input radiation into broadband radiation. In particular, the present invention relates to controlling the density distribution of a medium within an optical fiber. Background art

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

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

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

[0007] In the field of lithography, many different measurement systems can be used to obtain information about the lithographically patterned structures on a substrate. Metrology can be used, for example, for the inspection of the lithographically patterned structures and the analysis of the associated lithographic patterning process. The measurement system can use radiation to interrogate the structures on the substrate, such as electromagnetic radiation. Due to the smaller dimensions of the patterned features, the nature of the radiation used to inspect and measure the properties of the substrate and the structures patterned thereon can affect what information can be obtained through those measurements. Different wavelengths of radiation can be suitable for measuring different properties on the substrate. In addition, the quality of the beam can affect the quality of the measurements obtained. The radiation properties that can affect the measurement results can include, for example, the size and shape of the radiation beam, the intensity of the radiation and the wavelength of the radiation. The wavelength and / or the beam size can, for example, affect the smallest feature that can be distinguished in the measurement. Different materials and / or structures can also have a wavelength-dependent response (e.g., due to absorption, transmission, interference effects, etc.). Therefore, it is desirable to be able to obtain a radiation source that can provide the desired radiation wavelength and beam properties. Summary of the Invention

[0008] According to a first aspect of the present disclosure, there is provided a hollow-core photonic crystal fiber (HC-PCF) assembly for converting input radiation into broadband radiation. The hollow-core fiber assembly includes: a microstructured fiber having a hollow core that extends along the length of the fiber from an input end configured to receive the input radiation to an output end configured to output the broadband radiation, wherein the hollow core of the fiber is configured to contain a medium; and a density control system configured to control the density distribution of the medium along at least a portion of the length of the fiber to establish a desired zero-dispersion wavelength distribution along at least a portion of the length of the fiber.

[0009] Optionally, the desired zero-dispersion wavelength distribution may be configured to enhance optical soliton trapping.

[0010] Optionally, the desired zero-dispersion wavelength distribution may be configured to extend the wavelength range of the broadband radiation generated due to modulation instability.

[0011] Optionally, the desired zero-dispersion wavelength distribution may be configured to enhance the interaction between the optical soliton and the dispersive wave generated by the HC-PCF upon receiving the input radiation.

[0012] Optionally, the desired zero-dispersion wavelength distribution may be configured to enhance the conversion efficiency of the HC-PCF.

[0013] Optionally, the broadband radiation may have a continuous wavelength range between 350 nm and 2000 nm.

[0014] Optionally, the density distribution along at least a portion of the length of the fiber may be a negative gradient distribution.

[0015] Optionally, the density control system may include a temperature control system configured to control the temperature along at least a portion of the length of the fiber.

[0016] Optionally, the diameter of the hollow core may vary along at least a portion of the length of the fiber.

[0017] Optionally, the diameter of the hollow core may decrease along at least a portion of the length of the fiber.

[0018] Optionally, the medium may include at least one of helium, neon, argon, krypton, xenon, O2 gas, and N2 gas.

[0019] Optionally, the hollow-core fiber may be a single-ring photonic crystal fiber.

[0020] Optionally, the hollow core fiber may include a single loop of capillaries surrounding the hollow core.

[0021] Optionally, the broadband radiation may include supercontinuum radiation.

[0022] Optionally, the broadband radiation may include one or more wavelengths in the range of 350 nm to 3000 nm.

[0023] According to another aspect of the present disclosure, there is provided a radiation source for outputting broadband radiation, the radiation source including a hollow core fiber assembly as described above and a pump radiation source configured to couple radiation at a pump wavelength into the hollow core fiber for generating broadband radiation inside the fiber.

[0024] According to another aspect of the present disclosure, there is provided a method for converting input radiation into broadband radiation, the method including: providing a microstructured fiber having a hollow core that extends along a length of the fiber from an input end to an output end, wherein the hollow core of the fiber contains a medium; guiding input radiation into the hollow core fiber at the input end; controlling a density distribution of the medium along at least a portion of the length of the fiber to establish a desired zero-dispersion wavelength distribution along at least a portion of the length of the fiber; and outputting broadband radiation at the output end.

[0025] Optionally, controlling the density distribution may include setting a negative gradient distribution along at least a portion of the length of the fiber.

[0026] Optionally, controlling the density distribution may include controlling a temperature along at least a portion of the length of the fiber.

[0027] According to another aspect of the present disclosure, there is provided a measuring device including the radiation source as described above.

[0028] According to another aspect of the present disclosure, there is provided an inspection tool including the measuring device as described above.

[0029] According to another aspect of the present disclosure, there is provided a measuring tool including the measuring device as described above.

[0030] According to another aspect of the present disclosure, there is provided a lithography apparatus including the measuring device as described above.

[0031] According to another aspect of the present disclosure, there is provided a lithography cell including the measuring device as described above. Description of the Drawings

[0032] Embodiments of the present invention will now be described with reference to the accompanying schematic drawings, which are given by way of example only, in which:

[0033] - Figure 1 A schematic overview of a lithographic apparatus is depicted;

[0034] - Figure 2 A schematic overview of a lithography cell is depicted;

[0035] - Figure 3 A schematic representation of overall lithography is depicted, which represents the collaboration between three key technologies for optimizing semiconductor manufacturing;

[0036] - Figure 4 A schematic representation of a scatterometer is depicted;

[0037] - Figure 5 A schematic representation of a level sensor is depicted;

[0038] - Figure 6 A schematic representation of an alignment sensor is depicted;

[0039] - Figure 7 A schematic representation of a hollow-core fiber assembly is depicted;

[0040] - Figure 8 A graph depicting an exemplary dispersion profile of a hollow-core photonic crystal fiber is depicted. Detailed Description

[0041] In this document, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g., having a wavelength of 365 nm, 248 nm, 193 nm, 157 nm, or 126 nm) and EUV (extreme ultraviolet radiation, e.g., having a wavelength in the range of about 5 nm to 100 nm).

[0042] As used herein, the term "mask", "reticle", or "patterning device" can be broadly interpreted to mean a general patterning device that can be used to impart a patterned cross-section to an incident radiation beam, the patterned cross-section corresponding to a pattern to be created in a target portion of a substrate. In such a context, the term "light valve" can also be used. Examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays in addition to classical masks (transmission or reflection; binary, phase-shift, hybrid, etc.).

[0043] Figure 1Schematically depict a lithographic apparatus LA. The lithographic apparatus LA includes: an illumination system (also referred to as an illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation); a mask support (e.g., a mask table) T configured 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 configured to hold a substrate (e.g., a wafer coated with resist) 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., including one or more dies) of the substrate W.

[0044] In operation, the illumination system IL receives the radiation beam from a radiation source SO, for example via a beam delivery system BD. The illumination system IL may include various types of optical components for guiding, shaping, and / or controlling the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in a plane of the patterning device MA in its cross-section.

[0045] The term "projection system" PS as used herein should be broadly interpreted to cover various types of projection systems suitable for the exposure radiation used and / or for other factors such as the use of an immersion liquid or the use of a vacuum, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic, and / or electrostatic optical systems, or any combination thereof. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0046] The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index (e.g., water) to fill the space between the projection system PS and the substrate W - this is also known as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0047] The lithographic apparatus LA can also be of the type having two or more substrate supports WT (also known as "dual platforms"). In such a "multi-platform" machine, the substrate supports WT can be used in parallel, and / or steps for preparing a substrate W for a subsequent exposure of the substrate W located on one of the substrate supports WT can be carried out while another substrate W on another substrate support WT is used for exposing a pattern on said other substrate W.

[0048] In addition to the substrate support WT, the lithographic apparatus LA can also include a measurement platform. The measurement platform is arranged to hold sensors and / or cleaning devices. The sensors can be arranged to measure properties of the projection system PS or of the radiation beam B. The measurement platform can hold multiple sensors. The cleaning device can be arranged to clean parts of the lithographic apparatus, such as parts of the projection system PS or parts of the system providing the immersion liquid. The measurement platform can move under the projection system PS when the substrate support WT is moved away from the projection system PS.

[0049] In operation, the radiation beam B is incident on a patterning device (e.g., a mask) MA held on a mask support T and is patterned by a pattern (design layout) present on the patterning device MA. After traversing the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. By means of a second positioner PW and a position measurement system IF, the substrate support WT can be accurately moved, for example to position different target portions C in the path of the radiation beam B at focused and aligned positions. Similarly, a first positioner PM and possibly another position sensor (which is not explicitly depicted in Figure 1 ) can be used to accurately position the patterning device MA relative to the path of the radiation beam B. Mask alignment marks M1, M2 and substrate alignment marks P1, P2 can be used to align the patterning device MA and the substrate W. Although the substrate alignment marks P1, P2 as illustrated occupy dedicated target portions, the marks can be located in the spaces between the target portions. When the substrate alignment marks P1, P2 are located between the target portions C, these substrate alignment marks P1, P2 are referred to as scribe alignment marks.

[0050] As Figure 2As shown, the lithographic apparatus LA can form part of a lithographic cell LC (sometimes also referred to as a lithographic element or (lithographic) cluster), which often also includes equipment for performing pre-exposure processes and post-exposure processes on the substrate W. Generally, such equipment includes a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH and a bake plate BK for adjusting the temperature of the substrate W (e.g., for adjusting the solvent in the resist layer). A substrate transfer device or robot RO picks up the substrate W from the input / output ports I / O1, I / O2, moves the substrate W between different process devices and transfers the substrate W to the feed table LB of the lithographic apparatus LA. The device, often collectively referred to as a track or coat and develop system in the lithographic cell, is typically under the control of a track or coat and develop system control unit TCU, which itself may be controlled by a management control system SCS, and the management control system SCS can also control the lithographic apparatus LA, for example, via a lithography control unit LACU.

[0051] To correctly and consistently expose the substrate W exposed by the lithographic apparatus LA, it is desirable to inspect the substrate to measure the properties of the patterned structures, such as overlay errors between subsequent layers, line thickness, critical dimension (CD), etc. For this purpose, an inspection tool (not shown in the figure) can be included in the lithographic cell LC. Especially in the case where the inspection is performed before other substrates W in the same lot or batch are still to be exposed or processed, if an error is detected, the exposure of subsequent substrates or other processing steps to be performed on the substrate W can be adjusted, for example.

[0052] The inspection device, which can also be called a metrology device, is used to determine the properties of the substrate W, and in particular how the properties of different substrates W vary or how the properties associated with different layers of the same substrate W vary between different layers. The inspection device can alternatively be configured to identify defects on the substrate W and can be, for example, part of the lithographic cell LC, or can be integrated into the lithographic apparatus LA, or can even be a separate device. The inspection device can measure the properties of a latent image (the image in the resist layer after exposure), or a semi-latent image (the image in the resist layer after a post-exposure bake step PEB), or a developed resist image (where the exposed or unexposed part of the resist has been removed), or even the properties of an etched image (after a pattern transfer step such as etching).

[0053] Generally, the patterning process in the lithographic apparatus LA is one of the most critical steps in the processing, and this most critical step requires high accuracy in the dimension calibration and placement of the structures on the substrate W. To ensure such high accuracy, three systems can be combined in a so-called "integrated" control environment, as Figure 3Schematically depicted, one of these systems is a lithographic apparatus LA, which is (in practice) connected to a metrology tool MT (second system) and to a computer system CL (third system). The key to such an "integrated" environment lies in optimizing the collaboration between these three systems to enhance the overall process window and provide a tight control loop, thereby ensuring that the patterning performed by the lithographic apparatus LA remains within the process window. The process window defines the range of process parameters (e.g., dose, focus, overlay) within which a particular manufacturing process yields the defined result (e.g., a functional semiconductor device) - typically within this range of process parameters, the process parameters in the lithography process or patterning process are allowed to vary.

[0054] The computer system CL can use (parts of) the design layout to be patterned to predict which resolution enhancement techniques to use and perform computational lithography simulations and calculations to determine which mask layout and lithographic apparatus settings achieve the maximum overall process window for the patterning process (depicted by the double arrows in the first scale SC1 in Figure 3 . Typically, the resolution enhancement techniques are arranged to match the patterning capabilities of the lithographic apparatus LA. The computer system CL can also be used to detect where within the process window the lithographic apparatus LA is currently operating (e.g., using input from the metrology tool MT) to predict whether defects are likely to occur due to, for example, suboptimal processing (depicted by the arrow pointing to "0" in the second scale SC2 in Figure 3 ).

[0055] The metrology tool MT can provide input to the computer system CL to enable accurate simulations and predictions, and can provide feedback to the lithographic apparatus LA to identify possible drifts in, for example, the calibration state of the lithographic apparatus LA (depicted by the multiple arrows in the third scale SC3 in Figure 3 ).

[0056] During the lithography process, it is desirable to frequently perform measurements of the resulting structures, for example for process control and verification. Tools used to perform such measurements are commonly referred to as metrology tools MT. Different types of metrology tools MT for performing such measurements are well known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. A scatterometer is a multi-functional instrument that allows measurement of lithography process parameters by having a sensor in the pupil or in a plane conjugate to the pupil of the scatterometer's objective lens (the measurement is typically referred to as pupil-based measurement), or by having a sensor in the image plane or in a plane conjugate to the image plane, in which case the measurement is typically referred to as image- or field-based measurement. Patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP1,628,164A, which are hereby incorporated by reference in their entirety, further describe such scatterometers and associated measurement techniques. The aforementioned scatterometers can use light from the soft x-ray and visible to near-IR wavelength ranges to measure gratings.

[0057] In a first embodiment, the scatterometer MT is an angular-resolved scatterometer. In such a scatterometer, a reconstruction method can be applied to the measured signal to reconstruct or calculate the properties of the grating. This reconstruction can be caused, for example, by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulated results with the measured results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to the diffraction pattern observed from the real target.

[0058] 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 onto a target and the reflected or scattered radiation from the target is directed onto a spectrometer detector that measures the spectrum of the specularly reflected radiation (i.e., the measurement of the intensity as a function of wavelength). Based on this data, the structure or profile of the target that produced the detected spectrum can be reconstructed, for example, by rigorous coupled-wave analysis and non-linear regression or by comparison with a simulated spectral library.

[0059] In a third embodiment, the scatterometer MT is an ellipsometric scatterometer. An ellipsometric scatterometer allows for the determination of parameters of a lithography process by measuring scattered radiation for each polarization state. Such metrology equipment emits polarized light (such as linear, circular, or elliptical) by using, for example, a suitable polarization filter in the illumination section of the metrology equipment. Sources suitable for the metrology equipment can also provide polarized radiation. Various embodiments of existing ellipsometric scatterometers are described in U.S. Patent Applications 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 hereby incorporated by reference in their entirety.

[0060] Examples of known scatterometers often rely on the supply of dedicated metrology targets, such as underfilled targets (targets in the form of simple gratings or stacked gratings in different layers, which are large enough such that the measurement beam generates a spot smaller than the grating) or overfilled targets (such that the illumination spot partially or fully encompasses the target). Additionally, using metrology tools (such as an angular-resolved scatterometer that illuminates an underfilled target such as a grating) allows for the use of so-called reconstruction methods, where the properties of the grating can be calculated by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measured results. The parameters of the model are adjusted until the simulated interaction produces a diffraction pattern similar to the diffraction pattern observed from the real target.

[0061] In one embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the overlap of two misaligned gratings or periodic structures by measuring the reflection spectrum and / or detecting an asymmetry in the detection configuration (the asymmetry being related to the overlap range). Two (usually stacked) grating structures can be applied in two different layers (not necessarily consecutive layers), and the two grating structures can be formed at substantially the same location on the wafer. The scatterometer can have a symmetric detection configuration as described, for example, in co-owned Patent Application EP1,628,164A, such that any asymmetry is clearly distinguishable. This provides a direct way to measure misalignment in the grating. Additional examples of measuring the overlap error between two layers containing periodic structures as targets via the asymmetry of the periodic structures can be found in PCT Patent Application Publication No. WO 2011 / 012624 or U.S. Patent Application No. US 20160161863, which are hereby incorporated by reference in their entirety.

[0062] Other parameters of interest can be the focal length and the dose. The focal length and the dose can be determined simultaneously by scatterometry (or alternatively by scanning electron microscopy) as described in U.S. Patent Application US2011-0249244, which is incorporated herein by reference in its entirety. A single structure with a unique combination of critical dimension and sidewall angle measurements for each point in a focal length energy matrix (FEM - also known as a focal length exposure matrix) can be used. If these unique combinations of critical dimension and sidewall angle are available, the focal length and dose values can be uniquely determined based on these measurements.

[0063] The metrology target can be an ensemble of composite gratings formed primarily in a resist by a lithography process and also formed, for example, after an etching process. Generally, the pitch and linewidth of the structures in the grating depend largely on the measurement optics (especially the NA of the optics) to be able to capture the diffraction orders from the metrology target. As indicated earlier, the diffraction signal can be used to determine the shift (also known as "overlay") between two layers or can be used to reconstruct at least a portion of the original grating as produced by a lithography process. Such 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 sub-segments that are configured to mimic the dimensions of the functional portions of the design layout in the target. Due to such sub-segments, the target will behave more like the functional portions of the design layout, such that the overall process parameter measurements better resemble the functional portions of the design layout. The target can be measured in an underfill mode or in an overfill mode. In the underfill mode, the measurement beam creates a spot smaller than the overall target. In the overfill mode, the measurement beam creates a spot larger than the overall target. In such an overfill mode, it may also be possible to measure different targets simultaneously, thus determining different process parameters simultaneously.

[0064] The overall measurement quality of lithography parameters using a particular target is determined at least in part by the measurement selection scheme used to measure such lithography parameters. The term "substrate measurement selection scheme" can include one or more parameters of the measurement itself, one or more parameters of one or more patterns being measured, or both. For example, if the measurement used in the substrate measurement selection scheme is a diffraction-based optical measurement, one or more of the measured parameters can include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, and so on. One of the criteria for selecting the measurement selection scheme can be, for example, the sensitivity of one of the measurement parameters to process variations. More examples are described in U.S. Patent Application US2016-0161863, which is incorporated herein by reference in its entirety, and Published U.S. Patent Application US 2016 / 0370717A1.

[0065] Figure 4A metrology apparatus, such as scatterometer SM1, is depicted. The scatterometer SM1 includes a broadband (white light) radiation projector 2 that projects radiation onto a substrate 6. The reflected or scattered radiation is transmitted to a spectrometer detector 4 that measures the spectrum 10 of the specularly reflected radiation (i.e., the measurement of the intensity INT as a function of the wavelength λ). From such data, the structure or profile that caused the detected spectrum can be reconstructed by a processing unit PU, for example, by rigorous coupled-wave analysis and non-linear regression, or by comparison with a simulated spectrum library as shown at the bottom of Figure 4 . Usually, for the reconstruction, the general form of the structure is known and some parameters are assumed based on knowledge of the process used to fabricate the structure, leaving only a few parameters of the structure to be determined from the scatterometry measurement data. Such a scatterometer can be configured as a normal incidence scatterometer or an oblique incidence scatterometer.

[0066] A topography measurement system, a level sensor, or a height sensor (and the topography measurement system, level sensor, or height sensor can be integrated in a lithographic apparatus) is arranged to measure the topography of the top surface of the substrate (or wafer). A topographic map (also referred to as a height map) of the substrate can be generated from these measurements indicating the height of the substrate as a function of the position on the substrate. Such a height map can subsequently be used to correct the position of the substrate during transfer of a pattern onto the substrate in order to provide a spatial image of the patterning device at an appropriate focus position on the substrate. It should be understood that "height" in this context refers to a dimension that is substantially out of the plane of the substrate (also referred to as the Z-axis). Usually, the level or height sensor performs measurements at a fixed location (relative to its own optical system), and relative movement between the substrate and the optical system of the level or height sensor results in height measurements at locations across the substrate.

[0067] Figure 5 An example of a level or height sensor LS known in the art is schematically shown in Figure 5 ; only the operating principle is illustrated. In this example, the level sensor includes an optical system that 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 that is imparted by a projection grating PGR of the projection unit LSP. The radiation source LSO can be, for example, a narrowband or broadband radiation source, such as a supercontinuum light source, polarized or non-polarized, pulsed or continuous, such as a polarized or non-polarized laser beam. The radiation source LSO can include multiple radiation sources having different colors or wavelength ranges, such as multiple LEDs. The radiation source LSO of the level sensor LS is not limited to visible light radiation, but additionally or alternatively, can cover UV and / or IR radiation and any wavelength range suitable for reflection from the surface of the substrate.

[0068] The projection grating PGR is a periodic grating including a periodic structure that generates a radiation beam BE1 having a periodically varying intensity. The radiation beam BE1 having a periodically varying intensity is directed towards a measurement site MLO on a substrate W, and the radiation beam has an incident angle ANG between 0 degrees and 90 degrees, typically between 70 degrees and 80 degrees, with respect to an axis (Z-axis) perpendicular to the incident bottom surface of the substrate. At the measurement site MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and directed towards a detection unit LSD.

[0069] To determine the height level at the measurement site MLO, the level sensor further includes a detection system that includes a detection grating DGR, a detector DET, and a processing unit (not shown in the figure) for processing the output signal of the detector DET. The detection grating DGR can be the same as the projection grating PGR. The detector DET generates a detector output signal that indicates the received light, for example, indicates the intensity of the received light, such as a light detector, or represents the spatial distribution of the received intensity, such as a camera. The detector DET can include any combination of one or more detector types.

[0070] By means of triangulation techniques, the height level at the measurement site MLO can be determined. The detected height level is generally related to the signal intensity measured by the detector DET, and the signal intensity has a periodicity that depends particularly on the design of the projection grating PGR and the (tilted) incident angle ANG.

[0071] The projection unit LSP and / or the detection unit LSD can include other optical elements along the path of the patterned radiation beam between the projection grating PGR and the detection grating DGR, such as lenses and / or mirrors (not shown in the figure).

[0072] In an embodiment, the detection grating DGR can be omitted, and the detector DET can be placed at the position where the detection grating DGR is located. This configuration provides a more direct detection of the image of the projection grating PGR.

[0073] To effectively cover the surface of the substrate W, the level sensor LS can be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating an array of measurement regions MLO or patches that cover a larger measurement range.

[0074] For example, various height sensors of a general type are disclosed in US7265364 and US7646471, which are incorporated herein by reference. A height sensor that uses UV radiation instead of visible or infrared radiation is disclosed in US2010233600A1, which is incorporated herein by reference. In WO2016102127A1, which is incorporated herein by reference, a compact height sensor is described that uses a multi-element detector to detect and identify the position of a grating image without detecting the grating.

[0075] In the manufacture of complex devices, many lithographic patterning steps are typically performed, thereby forming functional features in successive layers on a substrate. Thus, a key aspect of the performance of a lithographic apparatus is the ability to place the applied pattern correctly and accurately (by the same apparatus or a different lithographic apparatus) relative to features laid down in a previous layer. For this purpose, the substrate is provided with one or more sets of marks. Each mark is a structure whose position can later be measured using a position sensor, typically an optical position sensor. The position sensor may be referred to as an "alignment sensor" and the mark may be referred to as an "alignment mark".

[0076] A lithographic apparatus may include one or more (e.g., multiple) alignment sensors by which the position of alignment marks provided on a substrate can be accurately measured. The alignment (or position) sensor may use optical phenomena such as diffraction and interference to obtain position information from alignment marks formed on a substrate. An example of an alignment sensor used in current lithographic apparatuses is based on a self-referencing interferometer as described in US6961116. Various enhancements and modifications of the position sensor have been developed, such as those disclosed in US2015261097A1. All of these publications are incorporated herein by reference.

[0077] The mark or alignment mark may include a series of grating bars formed on or in a layer provided on a substrate or (directly) in the substrate. The grating bars may be regularly spaced and serve as grating lines such that the mark can be regarded as a diffraction grating having a well-known spatial period (pitch). Depending on the orientation of these grating lines, the mark can be designed to allow measurement of the position along the X-axis or along the Y-axis (the Y-axis being oriented substantially perpendicular to the X-axis). Marks including grating bars arranged at +45 degrees and / or -45 degrees relative to both the X-axis and the Y-axis allow combined X and Y measurements using the techniques described in US2009 / 195768A, which is incorporated herein by reference.

[0078] The alignment sensor optically scans each mark using a radiation spot to obtain a periodically varying signal, such as a sine wave. The phase of this signal is analyzed to determine the position of the mark and thus the position of the substrate relative to the alignment sensor, which is in turn fixed relative to the reference coordinate system of the lithographic apparatus. So-called coarse marks and fine marks can be provided, which are related to different (coarse and fine) mark sizes, such that the alignment sensor can distinguish different cycles of the periodic signal and the exact position (phase) within a cycle. Marks with different pitches can also be used for this purpose.

[0079] Measuring the position of the marks can also provide information about the deformation of the substrate provided with marks, for example in the form of a wafer grid. The deformation of the substrate can occur, for example, by electrostatic clamping of the substrate to the substrate table and / or by heating of the substrate when the substrate is exposed to radiation.

[0080] Figure 6 FIG. 7 is a schematic block diagram of an embodiment of a known alignment sensor AS, such as described in and incorporated by reference in US6961116. A radiation source RSO provides a radiation beam RB having one or more wavelengths, which radiation beam is directed as an illumination spot SP by steering optics onto a mark (such as a mark AM located on a substrate W). In this example, the steering optics include a spot mirror SM and an objective lens OL. The diameter of the illumination spot SP (by which the mark AM is illuminated) can be slightly smaller than the width of the mark itself.

[0081] The radiation diffracted by the mark AM (in this example via the objective lens OL) is collimated into an information-bearing beam IB. The term "diffraction" is intended to include zero-order diffraction from the mark (zero-order diffraction can be referred to as reflection). A self-referencing interferometer SRI, for example of the type disclosed in the above-mentioned US6961116, interferes the beam IB with itself, after which the beam is received by a photodetector PD. Additional optics (not shown in the figure) can be included to provide separate beams in the case where the radiation source RSO generates more than one wavelength. The photodetector can be a single element, or the photodetector can include multiple pixels as required. The photodetector can include a sensor array.

[0082] The steering optics, which include the spot mirror SM in this example, can also be used to block the zero-order radiation reflected from the mark, such that the information-bearing beam IB only includes higher-order diffracted radiation from the mark AM (this is not necessary for the measurement, but improves the signal-to-noise ratio).

[0083] An intensity signal SI is supplied to a processing unit PU. By a combination of optical processing in block SRI and computational processing in unit PU, values of the X position and the Y position relative to the reference coordinate system on the substrate are output.

[0084] A single measurement of the type illustrated only fixes the position of the mark within a certain range corresponding to one pitch of the mark. A coarser measurement technique is used in combination with such measurement to identify which period of the sine wave contains the period of the marked position. The same process at coarser and / or finer levels can be repeated at different wavelengths for improving accuracy and / or for robustly detecting the mark, independent of the material on which the mark is made and the material disposed above and / or below the mark. The wavelengths can be optically multiplexed and demultiplexed to process the wavelengths simultaneously, and / or the wavelengths can be multiplexed by time division or frequency division.

[0085] In this example, the alignment sensor and the spot SP remain stationary while the substrate W moves. The alignment sensor can thus be rigidly and accurately mounted to a reference coordinate system while effectively scanning the mark AM in a direction opposite to the direction of movement of the substrate W. The substrate W is controlled in such movement by being mounted on a substrate support and the substrate positioning system controlling 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 the figure). In an embodiment, one or more (alignment) marks are provided on the substrate support. Measurement of the position of the mark provided on the substrate support allows calibration of the position of the substrate support as determined by the position sensor (e.g., relative to the frame to which the alignment system is connected). Measurement of the position of the alignment mark provided on the substrate allows determination of the position of the substrate relative to the substrate support.

[0086] A metrology tool MT, such as a scatterometer, a level sensor, and an alignment sensor as described above, can use radiation to perform measurements. The radiation can be electromagnetic radiation. The radiation can be optical radiation, for example including wavelengths in one or more of the infrared portion, the visible portion, and / or the ultraviolet portion of the electromagnetic spectrum. The radiation can include wavelengths in the deep ultraviolet DUV, extreme ultraviolet EUV (e.g., 1 nm to 100 nm), and / or soft X-ray SXR (e.g., 0.1 nm to 10 nm) portions of the electromagnetic spectrum. The metrology tool MT can include or be connected to a radiation source. The type and quality of the measurements performed by the metrology tool MT can be affected by the nature of the radiation used. Different types of radiation can be provided by different types of sources. Some sources can provide radiation at a single wavelength or within a narrow wavelength range. Some sources can provide radiation at multiple wavelengths (e.g., within a broadband wavelength range). The physical effects and techniques used to generate the radiation can vary depending on one or more wavelengths and / or the width of the wavelength range. For example, a source providing broadband radiation can use spectral broadening of narrowband or single-wavelength radiation. Having a broadband radiation source can be advantageous because it can enable a wider range of measurements to be performed, thereby taking advantage of the different wavelengths available. To provide a high-quality broadband radiation source, it may be desirable to have a high degree of control over the radiation output by the source. The control can be, for example, providing radiation over the entire desired wavelength range, and / or controlling radiation beam properties, such as, for example, the intensity, size, and shape of the beam. Increased control over the radiation output by the source can lead to improved measurement results. Methods and components for providing an improved broadband radiation source are described herein.

[0087] Nonlinear processes can be used to generate broadband radiation. Nonlinear processes may require efficient excitation of high radiation intensity. This can be achieved, for example, by coupling high-intensity radiation into an optical fiber. Within the optical fiber core, radiation with a stronger local intensity can be obtained. The optical fiber can be a photonic crystal fiber (PCF), and the photonic crystal fiber can achieve, for example, stronger confinement of the radiation within the optical fiber core. This can result in providing radiation with a local high intensity. Nonlinear processes may also require a nonlinear medium in which the nonlinear process can occur. Such a nonlinear medium can be, for example, a nonlinear crystal or a nonlinear fluid, such as a nonlinear gas or a gas mixture. The nonlinear medium can be disposed within the optical fiber. The optical fiber can be a hollow-core photonic crystal fiber (HC-PCF), where the nonlinear medium (such as a nonlinear fluid) can be disposed within the hollow core. The high-intensity radiation can then be largely confined within the hollow core of the optical fiber, allowing the high-intensity radiation to interact with the nonlinear medium for generating broadband radiation. Hollow-core optical fibers, such as hollow-core photonic crystal fibers, can be used, for example, to generate supercontinuum radiation from input radiation provided at one or more pump wavelengths.

[0088] Optical fibers having uniform properties along their length, such as an optical fiber having a uniform cross-section and uniform medium properties along its length, may not provide optimal conditions for broadband wavelength generation. This can be attributed, for example, to dispersion and / or optical non-linearity, which means that different wavelengths will experience different properties as the radiation propagates along the length of the optical fiber. Specifically, the uniform properties along the propagation direction of the radiation confined within the optical fiber may affect the efficiency of generation of radiation having shorter wavelengths (e.g., UV, DUV, EUV, SXR radiation). The uniform properties along the length of the optical fiber may also negatively affect the conversion efficiency from pump generation to broadband generation, and may also result in differences in the intensities generated across the broadband spectrum. Therefore, it is proposed herein to provide control over one or more conditions along the optical fiber to provide non-uniform properties for aspects of improving the broadband radiation generation process.

[0089] Generally, a hollow-core optical fiber assembly for converting input radiation into broadband radiation is disclosed herein. Figure 7 A schematic representation of a hollow-core optical fiber assembly 100 is depicted. The hollow-core optical fiber assembly 100 includes an optical fiber 102 having a hollow core 104 that extends along the length of the optical fiber from an input end 106 of the optical fiber to an output end 108. The input end 106 is configured to receive input radiation 202, and the output end 108 is configured to output broadband radiation 204. The hollow core 104 of the optical fiber may contain a medium 110. The optical fiber 102 having the hollow core 104 may also be referred to as a hollow-core optical fiber 102. The conversion from the input radiation 202 to the broadband radiation 204 may include generating the broadband radiation 204 via an interaction of the input radiation 202 with the medium 110. The assembly further includes a density control system 112 that is configured to control the density distribution of the medium 110 along at least a portion of the length of the optical fiber 102. The density distribution may be controlled depending on a desired zero-dispersion wavelength distribution along at least a portion of the length of the optical fiber 102. The desired zero-dispersion wavelength distribution is generally associated with desired characteristics of the wavelength spectrum associated with the broadband radiation 204 and / or the conversion efficiency of the input radiation to the broadband radiation 204. Therefore, alternatively, the density distribution is controlled depending on characteristics associated with the wavelength spectrum of the broadband radiation 204 and / or the conversion efficiency of the input radiation to the broadband radiation 204. The optical fiber 102 may optionally be disposed within a reservoir 114.

[0090] Advantages of providing a hollow core fiber assembly with a density control system are as follows: The density of the medium can vary along the length of the fiber to provide non-uniform or inhomogeneous properties along at least a portion of the fiber. The density profile of the fiber can be set to affect the generated broadband radiation. Characteristics associated with the wavelength spectrum can be considered properties or parameters capable of tuning the radiation spectrum. The characteristics can be associated with the dispersion profile (typically the zero-dispersion wavelength profile) of the hollow core fiber assembly, as the dispersion can be used to tune the radiation spectrum. Based on the dispersion profile of the assembly, other properties such as the zero-dispersion wavelength profile and the group velocity can be derived. Examples of characteristics that may be affected include, for example, the wavelength range of the broadband radiation, the intensity generated for different wavelengths over the entire broadband wavelength range. For some or all of the broadband radiation wavelength range, the controlled density profile can also affect the conversion efficiency from input radiation to broadband output radiation. The density profile can also be controlled to reduce noise in the output broadband radiation.

[0091] Also refer to: Song Z et al.: "Femtosecond pulse propagation in temperature controlled gas-filled Hollowfiner", Optics Communications, Elsevier, Amsterdam, NL, Vol. 281, Nos. 15-16, August 1, 2008, pp. 4109-4113, ISSN: 0030-4018, DOI: 10.1016 / J.OPTCOM.2008.04.037, hereinafter referred to as "Song" for short. Song describes a temperature-based control method in Section 3 that applies a density gradient along the length of the tube. Compared with the present invention disclosed in this document, the density gradient achieved by Song's density control system has no association with any behavior of the zero-dispersion wavelength within the fiber, as can be inferred from the fact that no zero-dispersion mechanism applies to Song's fiber configuration. The latter can be derived from Equation 8 of Song, demonstrating that for any wavelength, the refractive index exceeds 1 (and thus zero dispersion will not occur anywhere along the fiber). In addition, Song's fiber is not microstructured (compared with the fiber according to the present invention), and (therefore) broadband radiation generation is not based on any of the nonlinear mechanisms mentioned in this document (such as modulation instability), as can also be inferred from the very high pulse energy used by Song (>> μJ range, associated with nonlinear processes initiated within microstructured fibers).

[0092] The conversion of input radiation to broadband output radiation can include supercontinuum generation of radiation inside the hollow core of an optical fiber. Supercontinuum generation can include a process initiated by modulation instability. In an exemplary embodiment, modulation instability can be recognized as occurring in two separate stages. In the first stage, input radiation can be converted into a soliton burst in an anomalous dispersion regime, i.e., a dispersion regime where β2 < 0, where β2 is proportional to the second derivative of the refractive index n as a function of wavelength. This can occur in a first portion along the length of the central optical fiber, e.g., in the first 10 cm to 15 cm of the hollow core optical fiber. A soliton is a solution to the nonlinear wave equation. The interaction between linear dispersion and the design of the optical fiber and the nonlinear effects within the optical fiber can affect the dispersion experienced by the optical solitons within the optical fiber. Some of the energy associated with the optical solitons within the optical fiber can extend into the normal dispersion regime (β2 > 0, refractive index n decreases as wavelength increases). In the second stage, the energy in the normal dispersion regime can excite a dispersive wave (i.e., a non-soliton wave packet). The dispersive wave can be excited at a wavelength different from the pump wavelength. The excited wave can include waves having wavelengths that are generally shorter than the soliton wavelength.

[0093] In an optical fiber having a uniform property along its length, it can be challenging to meet the conditions required to implement and / or optimize the process of supercontinuum radiation generation described above. Some of the conditions that can be challenging to meet can include obtaining conditions where the zero-dispersion wavelength for the hollow core optical fiber falls within a range suitable for soliton continuum formation. To efficiently form a soliton continuum initiated by modulation instability, it may be desirable for the zero-dispersion wavelength of the hollow core optical fiber to be at or near the wavelength of the input radiation. The zero-dispersion wavelength should further fall within the anomalous dispersion regime such that a soliton continuum can be formed. It is also desirable for the dispersion profile for the hollow core optical fiber to be relatively flat around the zero-dispersion wavelength because when the dispersion profile is flatter, the phase matching conditions can be met or approximated for a wider range of wavelengths. This closer approximation for a wider range of wavelengths can lead to more efficient supercontinuum generation for a wider range of wavelengths.

[0094] Another condition for supercontinuum generation that can be challenging or may be challenging to meet is achieving group matching between the optical soliton to be generated and the dispersive wave. In order for the optical soliton continuum to transfer energy efficiently to the dispersive wave, the group velocities of the optical soliton and the dispersive wave need to be matched. The group velocity matching condition is affected by the dispersion of the optical fiber. The dispersion in a hollow-core fiber can be affected by the design of the fiber itself, as well as the material of the fiber and the medium inside the hollow core. However, the dispersion properties are relatively constrained, meaning that design changes can have a limited impact on the resulting dispersion. When the wavelength difference between the input wavelength and the generated dispersive wavelength is large, it can also be difficult to achieve group velocity matching. The input radiation can have one or more wavelengths that are longer than the generated radiation of interest. The input radiation can include, for example, one or more wavelengths in the infrared part of the spectrum (e.g., at 1550 nm). As a result, it may be more difficult to meet and / or approximate the group velocity matching for the generated shorter wavelengths.

[0095] In addition, for shorter wavelengths, such as DUV, EUV, and / or SXR wavelengths of 250 nm or less than 250 nm, the dispersion curve and slope can become dominated by the dispersion properties of the medium, and the dispersion curve and slope may deviate significantly from the preferred flat dispersion curve configuration. For shorter wavelengths, the dispersion curve can become steeper. As a result, short wavelengths can experience stronger dispersion inside a hollow-core fiber that has uniform properties along its length. In addition to the increased dispersion in the hollow-core fiber, the steeper dispersion curve also reduces the wavelength range along which group velocity matching can be met and / or approximated. To compensate for the dispersion, a non-uniform design can be provided along at least a portion of the length of the fiber. For example, to achieve group velocity matching along the length of the fiber, a smaller core diameter can be provided at the output end of the fiber compared to the input end of the fiber to compensate for the dispersion. Providing non-uniformity along at least a portion of the length of the hollow-core fiber can address the dispersion present for shorter wavelengths. This can make supercontinuum generation more efficient for shorter wavelengths. As a result, the generated wavelength range can be shifted or improved for wavelengths in the UV, DUV, EUV, and / or SRX parts of the spectrum.

[0096] Another condition for supercontinuum radiation that can be challenging or challenging to meet can be how to achieve dispersion wave trapping for increased efficiency. Under appropriate conditions, the dispersion wave generated by an optical soliton can be trapped by the optical soliton. The trapped wave and the optical soliton can propagate together along the optical fiber. This can increase the amount of interaction between the optical soliton and the dispersion wave, which means that the energy transfer from the optical soliton to the wave can be significantly increased. For dispersion trapping to occur, the group velocity of the optical soliton needs to decrease as the optical soliton propagates along the optical fiber. As the optical soliton propagates along the optical fiber, the optical soliton can self-shift to a longer wavelength. The associated dispersion wave generated from the optical soliton can conversely shift to a shorter wavelength. In a solid-core optical fiber, this decrease in group velocity can be achieved due to the Raman self-frequency shift effect, where the energy in the optical soliton spectrum transfers from a shorter wavelength to a longer wavelength. In some hollow-core optical fibers (e.g., inert gas-filled hollow-core optical fibers), this Raman effect may not be present. When radiation travels along a uniform hollow-core optical fiber, a shift to a longer wavelength and an associated decrease in group velocity can still occur. However, the amount of group velocity decrease may be limited, resulting in limited support for dispersion wave trapping. To enable dispersion wave trapping, a non-uniform design of at least a portion of the optical fiber can be given to facilitate a decrease in the group velocity of the radiation propagating through the optical fiber. This can be achieved, for example, by tapering the optical fiber down, such as by decreasing the diameter of the core of the optical fiber between the input end and the output end of the optical fiber.

[0097] Another condition for supercontinuum radiation that can be challenging or challenging to meet can be optical soliton energy matching. As the optical soliton propagates along the length of the optical fiber, the optical soliton can lose energy to the dispersion wave it is generating and / or to losses during propagation within the optical fiber. To increase the supercontinuum generation of radiation by the optical soliton, it may be desirable to maintain the soliton order. For example, it may be desirable not to perturb the optical soliton and to reduce losses for the optical soliton that are not related to dispersion wave generation. One example way to maintain the soliton order can be to increase the non-linearity experienced by the optical soliton as it propagates along the optical fiber without changing the dispersion properties of the optical fiber. This can be achieved, for example, by having a non-uniform distribution of the medium density and / or the optical fiber structure / design along the length of the optical fiber.

[0098] Based on the challenges associated with achieving and / or improving the supercontinuum generation described above, providing a non-uniform density distribution along at least a portion of a hollow-core fiber can be used to improve the efficiency of the supercontinuum generation process and / or control the supercontinuum generation process. Characteristics associated with the wavelength spectrum of the broadband radiation can thus include one or more of the zero-dispersion wavelength (profile), the group velocity distribution along the fiber, and the profile of the dispersion curve for the radiation propagating along the fiber. Tuning the dispersion (zero-dispersion wavelength) profile and the nonlinearity of the hollow-core fiber assembly can in turn affect the wavelength range of the generated radiation. The conversion efficiency from the input radiation to the broadband radiation can be improved for some or all of the wavelengths within the broadband radiation range.

[0099] A density control system for a hollow-core fiber assembly can provide non-uniform conditions along at least a portion of the fiber. Having a varying medium density along at least a portion of the length of the hollow core of the fiber can modify the nonlinearity and / or dispersion properties along the length of the fiber. Altering the density of the medium by controlling the medium itself can introduce a non-uniform distribution of the nonlinearity and dispersion properties (such as the zero-dispersion wavelength profile) along at least multiple portions of the length of the fiber, without having to change the cross-section along that length. However, in addition to or as an alternative to controlling the medium to introduce non-uniform nonlinearity and / or dispersion properties within the fiber, it may also be possible to vary the design of the cross-section of the fiber (e.g., a hollow-core PCF).

[0100] The negative density gradient of the medium can be expressed in terms of pressure. The negative pressure gradient can have a pressure change from an initial pressure value at a first location along the hollow-core fiber assembly to a lower pressure value at a second location along the hollow-core fiber assembly. The second location is further along the propagation direction of the fiber than the first location, such that the propagating radiation experiences the negative density gradient. The initial pressure value can be, for example, in the range of 5 bar to 60 bar, such as 5 bar, 10 bar, 20 bar, 30 bar, 40 bar, 50 bar, 60 bar. The lower pressure value is lower than the associated initial pressure value and can be, for example, in the range of 0 bar to 50 bar, such as 0 bar, 5 bar, 10 bar, 20 bar, 30 bar, 40 bar, 50 bar, where the lower pressure value is lower than the associated initial pressure value. An exemplary pressure gradient curve between the first location and the second location can include a straight line, a square root, a parabola, or any user-defined distribution or profile. The distribution can be controlled by the density control system described in more detail below.

[0101] It is proposed herein to introduce a negative density gradient distribution along at least a portion of the length of the optical fiber. The present inventors have found that a negative gradient density distribution can be used to extend the wavelength range of broadband radiation generation controlled by modulation instability to shorter wavelengths. A negative gradient density distribution can also be used to support the trapping of dispersive waves by optical solitons. Generally, by controlling the density gradient distribution along at least a portion of the optical fiber, the dispersion (zero-dispersion wavelength distribution) and non-linearity within the optical fiber, which may affect the supercontinuum generation process, can be tuned. Figure 8 A graph is shown depicting an exemplary dispersion profile for a hollow-core photonic crystal fiber filled with argon at different pressures. As can be seen on the graph, for a lower density of the medium 110, the zero-dispersion wavelength inside the hollow-core fiber 102 can be lower. Introducing a negative gradient density distribution along the length of the optical fiber can thus support the transfer of energy to shorter wavelengths. Optical soliton trapping, also supported by the negative gradient density distribution, can also contribute to a higher conversion efficiency to broadband radiation at shorter wavelengths (e.g., in the UV, DUV, EUV, and / or SXR portions of the spectrum).

[0102] As described above, including tapering in the properties of the optical fiber 102, for example by providing a negative density gradient distribution and / or by changing the design of the cross-section of the optical fiber (e.g., a non-uniform distribution of the diameter of the hollow core 104), can lead to an increased conversion efficiency for shorter wavelengths. This, in turn, may lead to a more even intensity distribution (i.e., an increased flatness of the generated broadband spectrum) across the entire generated wavelength range compared to components without the tapering properties of the optical fiber 102. The increased conversion efficiency in the shorter wavelength portion of the generated spectrum can also lead to an increased overall conversion efficiency.

[0103] The medium 110 can include a gas or a gas mixture. The medium 110 can enable and / or cause spectral broadening when radiation interacts with the medium within the optical fiber. The medium 110 can include hydrogen gas (H2). The medium 110 can include noble gases (e.g., argon Ar, helium He, neon Ne, krypton Kr, xenon Xe). The medium can include molecular gases (e.g., nitrogen (N2), oxygen (O2)).

[0104] The optical fiber 102 can be disposed inside a resealable reservoir 114. In some embodiments, the entire optical fiber can be disposed inside the reservoir 114, such as Figure 7As depicted, the reservoir can be configured to contain a medium. The reservoir 114 can include a medium control system for controlling the properties of the medium (e.g., the composition of the medium, the pressure of the medium). In other embodiments, the input end 106 and the output end 108 of the optical fiber 102 can be disposed inside separate sections of the reservoir 114 or in separate reservoirs. This can allow for the control of the properties of the medium at the input end 106 and the output end 108 of the optical fiber 102, respectively. For example, the pressure of the medium 110 disposed at the input end 106 can be different from the pressure of the medium 110 at the output end 108 to provide non-uniform medium properties along the length of the optical fiber 102.

[0105] The density control system 112 can be disposed inside or outside the reservoir 114. The density control system 112 can provide a pressure gradient of the medium 110 inside the hollow core 104 of the optical fiber 102 along at least a portion of the length of the optical fiber 102. This can be achieved, for example, by providing the medium 110 with different pressures at the input end 106 and the output end 108 of the optical fiber 102. This can be achieved, for example, by using one or more external sources to supply one or more components that form the medium 110. These sources can be used to control the pressure of the medium 110 inside the reservoir 114 at the input and output ends of the optical fiber. However, using external sources can introduce challenges, such as component downtime if the external sources need to be replenished / replaced.

[0106] The density control system 112 can include a temperature control system configured to control the temperature along at least a portion of the length of the optical fiber. The temperature control system can introduce a temperature profile along the length of the optical fiber, which in turn can affect the density of the medium 110. The temperature control system can include a plurality of temperature setting devices 116 configured to locally supply heat at a plurality of locations along the length of the optical fiber 102. The temperature control system 112 can include or be connected to one or more processors for setting the temperature control properties at each of the temperature setting devices 116.

[0107] The temperature setting devices 116 can be disposed outside the optical fiber 102, in the vicinity of the optical fiber 102, for locally heating the medium 110 inside the hollow core 104 of the optical fiber 102. The temperature setting devices 116 can indirectly heat the medium 110 inside the hollow core 104 by locally heating the optical fiber 102. Alternatively or additionally, the temperature setting devices 116 can directly supply heat to the medium 110, for example, using an alternating electric field or an electromagnetic field without heating the optical fiber 102 itself. Exemplary temperature setting devices include, for example, resistive heating elements, dielectric heating elements, Peltier elements, fan coolers, and liquid coolers. An exemplary temperature control system is described in NL2023533, which is incorporated herein by reference.

[0108] The optical fiber 102 can be a photonic crystal fiber (PCF), such as a hollow-core photonic crystal fiber (HC-PCF). An advantage of using the photonic crystal fiber 102 can be the ability to achieve strong confinement of the radiation inside the core. This can in turn achieve a high local radiation intensity to increase the efficiency of the nonlinear processes that contribute to the spectral broadening of the radiation used to form the broadband radiation. The photonic crystal fiber can include a plurality of microstructures that form a photonic crystal for achieving strong confinement inside the core 104 of the optical fiber 102. The microstructures can form a single ring of a structure surrounding the hollow core 104. The microstructures can include a plurality of capillaries surrounding the hollow core 104. In an exemplary embodiment, a single ring of capillaries surrounds the hollow core 104 of the optical fiber 102.

[0109] In some embodiments where the diameter of the hollow core of the optical fiber is non-uniform along at least a portion of the length of the optical fiber, the design of the photonic crystal microstructure can also vary (e.g., the diameter and / or wall thickness of the capillaries change along the length). In other embodiments, the diameter of the hollow core can vary while the size of the microstructures remains substantially the same (e.g., the diameter and wall thickness of the capillaries remain substantially constant). In the latter case, the relative positions of the microstructures with respect to each other can change along the length of the optical fiber to accommodate the changing diameter of the hollow core 104.

[0110] A non-uniform diameter distribution along at least a portion of the length of the optical fiber can also be used to control the properties associated with the wavelength spectrum and / or the conversion efficiency of the input to the broadband radiation. In an assembly where a non-uniform diameter is provided to control the wavelength and / or conversion efficiency within the optical fiber, an optical fiber with a solid core, rather than an optical fiber with a hollow core 104, can be provided as part of the assembly.

[0111] An optical fiber having a non-uniform diameter along its length can have a negative taper, also known as a downward tapering profile, i.e., the diameter of the core can decrease along the length of the optical fiber (from the input end to the output end) in the direction of propagation of the radiation. The core diameter can have an initial value and taper to a final value. The initial core diameter value can be in the range of 20 μm to 60 μm, such as 60 μm, 50 μm, 40 μm, 30 μm, 20 μm. The final core diameter value can be in the range of 10 μm to 50 μm, such as 50 μm, 40 μm, 30 μm, 20 μm, 10 μm. In the case of a negative taper, the initial diameter value is higher than the associated final diameter value. The taper can include, for example, a linear profile or a polynomial-based profile. The core diameter taper can be achieved over all or a portion of the length of the optical fiber. The taper profile can start at 0 to 80% of the total length of the optical fiber. The taper can end at 20% to 100% of the total length of the optical fiber, where the start of the taper is closer to the input end of the optical fiber compared to the end of the taper. The optical fiber can also be provided with an upward tapering profile, where the diameter increases in the direction of propagation of the radiation within the optical fiber.

[0112] The length of the optical fiber can be selected such that the optical fiber is long enough for broadband generation to occur within the wavelength range of interest. The length of the optical fiber 102 can be, for example, in the range of 10 cm to 2 m. The length of the optical fiber 102 can be in the range of 10 cm to 40 cm. Optical fibers 102 having lengths longer than 40 cm or having lengths longer than 2 m can also be used, but it can be advantageous to provide an optical fiber having a shorter length that enables the generation of radiation in the desired broadband range.

[0113] Input radiation coupled to the hollow-core optical fiber assembly 100 can be provided at one or more appropriate wavelengths. The input radiation can be provided, for example, at the wavelengths of radiation sources that are readily available. Exemplary input radiation wavelengths include, for example, 1030 nm, 1550 nm, and / or wavelengths in the range of 700 nm to 800 nm.

[0114] The broadband radiation generated within the hollow-core optical fiber assembly can include supercontinuum radiation. The supercontinuum radiation can include radiation in a continuous wavelength range. The broadband radiation can include radiation having wavelengths in the range of 350 nm to 3000 nm. The broadband radiation can include radiation having wavelengths in the range of 350 nm to 2000 nm. The supercontinuum radiation can include a continuous wavelength range at least in the range of 350 nm to 3000 nm (e.g., in the range of 350 nm to 2000 nm).

[0115] Other embodiments are disclosed in the list in the following numbered aspects:

[0116] 1. A hollow-core photonic crystal fiber (HC-PCF) assembly for converting input radiation into broadband radiation, the hollow-core fiber assembly comprising:

[0117] A microstructured fiber having a hollow core that extends along the length of the fiber from an input end configured to receive the input radiation to an output end configured to output the broadband radiation, wherein the hollow core of the fiber is configured to contain a medium; and

[0118] A density control system configured to control the density distribution of the medium along at least a portion of the length of the fiber to establish a desired zero-dispersion wavelength distribution along at least a portion of the length of the fiber.

[0119] 2. The hollow-core fiber assembly according to aspect 1, wherein the desired zero-dispersion wavelength distribution is configured to enhance optical soliton trapping.

[0120] 3. The hollow-core fiber assembly according to aspect 1 or 2, wherein the desired zero-dispersion wavelength distribution is configured to extend through the wavelength range of the broadband radiation generated by modulation instability.

[0121] 4. The hollow-core fiber assembly according to aspect 1, 2 or 3, wherein the desired zero-dispersion wavelength distribution is configured to enhance the interaction between the optical soliton and the dispersive wave generated by the HC-PCF upon receiving the input radiation.

[0122] 5. The hollow-core fiber assembly according to any one of the preceding aspects, wherein the desired zero-dispersion wavelength distribution is configured to enhance the conversion efficiency of the HCPCF.

[0123] 6. The hollow-core fiber assembly according to any one of the preceding aspects, wherein the broadband radiation has a continuous wavelength range between 350 nm and 2000 nm.

[0124] 7. The hollow-core fiber assembly according to any one of the preceding aspects, wherein the density distribution along at least a portion of the length of the fiber is a negative gradient distribution.

[0125] 8. The hollow-core fiber assembly according to any one of the preceding aspects, wherein the density control system includes a temperature control system configured to control the temperature along at least a portion of the length of the fiber.

[0126] 9. The hollow core fiber optic assembly according to any one of the foregoing aspects, wherein the density control system includes a pressure control system configured to control the pressure along at least a portion of the length of the fiber.

[0127] 10. The hollow core fiber optic assembly according to any one of the foregoing aspects, wherein the diameter of the hollow core varies along at least a portion of the length of the fiber.

[0128] 11. The hollow core fiber optic assembly according to aspect 10, wherein the diameter of the hollow core decreases along at least a portion of the length of the fiber.

[0129] 12. The hollow core fiber optic assembly according to any one of the foregoing aspects, wherein the medium includes at least one of helium, neon, argon, krypton, xenon, O2 gas, and N2 gas.

[0130] 13. The hollow core fiber optic assembly according to any one of the foregoing aspects, wherein the hollow core fiber is a single-ring photonic crystal fiber.

[0131] 14. The hollow core fiber optic assembly according to aspect 13, wherein the hollow core fiber includes a single ring of capillaries surrounding the hollow core.

[0132] 15. The hollow core fiber optic assembly according to any one of the foregoing aspects, wherein the broadband radiation includes supercontinuum radiation.

[0133] 16. A radiation source for outputting broadband radiation, the radiation source including the hollow core fiber optic assembly according to any one of aspects 1 to 15, and a pump radiation source configured to couple radiation at a pump wavelength into the hollow core fiber to generate broadband radiation within the fiber.

[0134] 17. A method for converting input radiation into broadband radiation, the method comprising:

[0135] providing a microstructured fiber having a hollow core that extends from an input end to an output end along the length of the fiber, wherein the hollow core of the fiber contains a medium;

[0136] guiding input radiation into the hollow core fiber at the input end;

[0137] controlling the density distribution of the medium along at least a portion of the length of the fiber to establish a desired zero-dispersion wavelength distribution along at least a portion of the length of the fiber; and

[0138] Output broadband radiation at the output end.

[0139] 18. The method according to aspect 17, wherein the desired zero-dispersion wavelength distribution is configured to enhance optical soliton trapping.

[0140] 19. The method according to aspect 17 or 18, wherein the desired zero-dispersion wavelength distribution is configured to extend the wavelength range of the broadband radiation generated due to modulation instability.

[0141] 20. The method according to aspect 17, 18 or 19, wherein the desired zero-dispersion wavelength distribution is configured to enhance the interaction between the optical soliton and the dispersion wave generated by the HC-PCF when receiving the input radiation.

[0142] 21. The method according to any one of aspects 17 to 20, wherein the desired zero-dispersion wavelength distribution is configured to enhance the conversion efficiency of the HCPCF.

[0143] 22. The method according to any one of aspects 17 to 21, wherein the broadband radiation has a continuous wavelength range between 350 nm and 2000 nm.

[0144] 23. The method according to any one of aspects 17 to 22, wherein the density distribution along at least a part of the length of the optical fiber is a negative gradient distribution.

[0145] 24. The method according to any one of aspects 17 to 23, wherein the density distribution is controlled by a temperature control system configured to control the temperature along at least a part of the length of the optical fiber.

[0146] 25. The method according to any one of aspects 17 to 23, wherein the density distribution is controlled by a pressure control system configured to control the pressure along at least a part of the length of the optical fiber.

[0147] 26. The method according to any one of aspects 17 to 25, wherein the diameter of the hollow core varies along at least a part of the length of the optical fiber.

[0148] 27. The method according to aspect 26, wherein the diameter of the hollow core decreases along at least a part of the length of the optical fiber.

[0149] 28. The method according to any one of aspects 17 to 27, wherein the medium includes at least one of helium, neon, argon, krypton, xenon, O2 gas, and N2 gas.

[0150] 29. The method according to aspect 17, wherein controlling the density distribution includes setting a negative gradient distribution along at least a portion of the length of the optical fiber.

[0151] 30. A measuring device, the measuring device including a radiation source according to aspect 16.

[0152] 31. A hollow-core optical fiber assembly for converting input radiation into broadband radiation, the hollow-core optical fiber assembly including:

[0153] An optical fiber having a hollow core that extends along the length of the optical fiber from an input end configured to receive input radiation to an output end configured to output broadband radiation, wherein the hollow core of the optical fiber is configured to contain a medium; and

[0154] A density control system configured to control a density distribution of the medium along at least a portion of the length of the optical fiber depending on a characteristic associated with a wavelength spectrum of the broadband radiation and / or a conversion efficiency of the input radiation to the broadband radiation.

[0155] 32. The hollow-core optical fiber assembly according to aspect 31, wherein the density distribution along at least a portion of the length of the optical fiber is a negative gradient distribution.

[0156] 33. The hollow-core optical fiber assembly according to aspect 31 or 32, wherein the density control system includes a temperature control system configured to control a temperature along at least a portion of the length of the optical fiber.

[0157] 34. The hollow-core optical fiber assembly according to any one of aspects 31 to 33, wherein a diameter of the hollow core varies along at least a portion of the length of the optical fiber.

[0158] 35. The hollow-core optical fiber assembly according to aspect 34, wherein the diameter of the hollow core decreases along at least a portion of the length of the optical fiber.

[0159] 36. The hollow-core optical fiber assembly according to any one of aspects 31 to 35, wherein the medium includes at least one of helium, neon, argon, krypton, xenon, O2 gas, and N2 gas.

[0160] 37. The hollow-core optical fiber assembly according to any one of aspects 31 to 36, wherein the hollow-core optical fiber is a single-ring photonic crystal fiber.

[0161] 38. The hollow core fiber assembly according to aspect 37, wherein the hollow core fiber comprises a single loop of capillary tubes surrounding the hollow core.

[0162] 39. The hollow core fiber assembly according to any one of aspects 31 to 38, wherein the broadband radiation comprises supercontinuum radiation.

[0163] 40. The hollow core fiber assembly according to any one of aspects 31 to 39, wherein the broadband radiation comprises one or more wavelengths in the range of 350 nm to 3000 nm.

[0164] 41. A radiation source for outputting broadband radiation, the radiation source comprising a hollow core fiber assembly according to any one of aspects 31 to 40, and a pump radiation source configured to couple radiation at a pump wavelength into the hollow core fiber for generating broadband radiation inside the fiber.

[0165] 42. A method for converting input radiation into broadband radiation, the method comprising:

[0166] Providing an optical fiber having a hollow core extending along a length of the optical fiber from an input end to an output end, wherein the hollow core of the optical fiber contains a medium;

[0167] Guiding input radiation into the hollow core fiber at the input end;

[0168] Depending on a property associated with a wavelength spectrum of the broadband radiation and / or a conversion efficiency of the input radiation into the broadband radiation, to control a density distribution of at least a portion of the medium along the length of the optical fiber; and

[0169] Outputting broadband radiation at the output end.

[0170] 43. The method according to aspect 42, wherein controlling the density distribution comprises setting a negative gradient distribution along at least a portion of the length of the optical fiber.

[0171] 44. The method according to any one of aspects 42 to 43, wherein controlling the density distribution comprises controlling a temperature of at least a portion of the length of the optical fiber.

[0172] 45. A measuring device, the measuring device comprising a radiation source according to aspect 41.

[0173] Although reference may be made specifically herein to the use of lithographic equipment in IC manufacture, it should be understood that the lithographic equipment described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

[0174] Although embodiments of the invention may be specifically referred to herein in the context of a lithographic apparatus, the embodiments of the invention may be used in other apparatuses. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus for measuring or processing an object such as a wafer (or other substrate) or a mask (or other patterning device). These apparatuses may generally be referred to as lithographic tools. Such lithographic tools may operate under vacuum conditions or ambient (non-vacuum) conditions.

[0175] Although reference may be made specifically above to the use of embodiments of the invention in the context of optical lithography, it should be appreciated that the invention is not limited to optical lithography and may be used in other applications (e.g., imprint lithography) where the context allows.

[0176] Although specific embodiments of the invention have been described above, it should be understood that the invention may be practiced in other ways different from those described. The foregoing description is intended to be illustrative, not limiting. Thus, those skilled in the art will appreciate that the described invention may be modified without departing from the scope of the claims set forth below.

[0177] Although reference is specifically made to a "metrology device / instrument / system" or an "inspection device / instrument / system", these terms may refer to the same or similar types of instruments, devices, or systems. For example, an inspection or metrology device including embodiments of the invention may be used to determine the characteristics of a structure on a substrate or a wafer. For example, an inspection device or a metrology device including embodiments of the invention may be used to detect defects in a substrate or in a structure on a substrate or a wafer. In such embodiments, the characteristics of interest of a structure on a substrate may relate to defects in the structure, the absence of a particular portion of the structure, or the presence of an unwanted structure on the substrate or the wafer.

Claims

1. A hollow-core photonic crystal fiber (HC-PCF) assembly (100) for converting input radiation (202) into broadband radiation (204), the hollow-core photonic crystal fiber assembly (100) comprising: A microstructured fiber (102) having a hollow core (104) that extends along the length of the fiber (102) from an input end (106) configured to receive the input radiation to an output end (108) configured to output the broadband radiation, wherein the hollow core (104) of the fiber is configured to contain a medium (110); and A density control system (112) configured to control the density distribution of the medium along at least a portion of the length of the fiber to establish a desired zero-dispersion wavelength distribution along at least a portion of the length of the fiber, wherein the density distribution along at least a portion of the length of the fiber is non-uniform.

2. The hollow-core photonic crystal fiber assembly according to claim 1, wherein, The desired zero-dispersion wavelength distribution is configured to enhance optical soliton trapping.

3. The hollow-core photonic crystal fiber component according to claim 1, wherein, The desired zero-dispersion wavelength distribution is configured to extend the wavelength range of the broadband radiation generated due to modulation instability.

4. The hollow-core photonic crystal fiber assembly according to claim 1, wherein, The desired zero-dispersion wavelength distribution is configured to enhance the interaction between the optical soliton and the dispersive wave generated by the HC-PCF upon receiving the input radiation.

5. The hollow-core photonic crystal fiber assembly according to claim 1, wherein, The desired zero-dispersion wavelength distribution is configured to enhance the conversion efficiency of the HC-PCF.

6. The hollow-core photonic crystal fiber assembly according to claim 1, wherein, The broadband radiation has a continuous wavelength range between 350 nm and 2000 nm.

7. The hollow-core photonic crystal fiber component according to claim 1, wherein, The density distribution along at least a portion of the length of the fiber is a negative gradient distribution.

8. The hollow-core photonic crystal fiber component according to claim 1, wherein, The density control system includes a temperature control system configured to control the temperature along at least a portion of the length of the fiber.

9. The hollow-core photonic crystal fiber assembly according to claim 1, wherein, The diameter of the hollow core varies along at least a portion of the length of the fiber.

10. The hollow-core photonic crystal fiber assembly according to claim 9, wherein, The diameter of the hollow core decreases along at least a portion of the length of the fiber.

11. The hollow-core photonic crystal fiber component according to claim 1, wherein, The medium includes at least one of helium, neon, argon, krypton, xenon, O2 gas, and N2 gas.

12. The hollow-core photonic crystal fiber assembly according to claim 1, wherein, The hollow-core fiber is a single-ring photonic crystal fiber.

13. A radiation source for outputting broadband radiation, the radiation source comprising the hollow-core photonic crystal fiber assembly according to any one of the preceding claims, and a pump radiation source configured to couple radiation at a pump wavelength into the hollow-core fiber for generating broadband radiation within the fiber.

14. A method for converting input radiation (202) into broadband radiation (204), the method comprising: Providing a microstructured fiber (102) having a hollow core (104) that extends along the length of the fiber (102) from an input end (106) to an output end (108), wherein the hollow core (104) of the fiber contains a medium (110); Direct an input radiation (202) into the hollow core optical fiber (102) at the input end (106); Control a density distribution of the medium along at least a portion of the length of the optical fiber (102) to establish a desired zero-dispersion wavelength distribution along at least a portion of the length of the optical fiber (102); and Output a broadband radiation (204) at the output end, wherein the density distribution along at least a portion of the length of the optical fiber is non-uniform.

15. A measuring device, the measuring device comprising a radiation source according to claim 13.

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