Broadband radiation source

Abstract

A radiation device for generating broadband radiation upon receiving pump radiation, comprising: an optical waveguide having a hollow core along a longitudinal axis of the optical waveguide; wherein the optical waveguide is configured to substantially confine the pump radiation within the hollow core; a working medium comprising molecules having a permanent dipole moment included within the hollow core of the optical waveguide wherein the working medium is configured to spectrally widen the pump radiation to generate the broadband radiation; and an electric field generating device for generating an electric field across the working medium, where the electric field is configured to orient molecules of the working medium.

Description

[0001] Cross-reference to related applications

[0002] This application claims priority to European Application No. 22207704.2, filed on Nov. 16, 2022, the entire content of which is incorporated herein by reference. Technical field

[0003] The present invention relates to a broadband radiation source based on an optical hollow-core waveguide, and more particularly, to such a broadband radiation source related to metrology applications in integrated circuit manufacturing. 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 (often also referred to as a “design layout” or “design”) present on a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).

[0005] In order 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 that are 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 employed, 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 a substrate a pattern that is 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, customized 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 tight control loop for controlling the stability of the lithographic apparatus can be used to improve the reproduction of patterns at low k1.

[0007] Metrology tools are used in many aspects of the IC manufacturing process, for example as alignment tools for properly positioning a substrate before exposure, leveling tools for measuring the surface topography of the substrate, and focus control and scatterometry-based tools for inspecting / measuring the exposed and / or etched product, for example in process control. In each case, a laser source is required. For various reasons including measurement stability, i.e. robustness and accuracy, broadband or white light radiation sources are increasingly being used for such metrology applications. Among other techniques for white light generation, hollow core optical waveguides can be used to convert narrowband input radiation into broadband output radiation via a non-linear optical process. Various different types of hollow core optical waveguides (such as, for example, hollow core photonic crystal fibers (PCF)) have been developed to meet the need for generating broadband radiation. It would be desirable to improve current devices for broadband radiation generation. SUMMARY OF THE INVENTION

[0008] According to a first aspect of the invention, there is provided a radiation device for generating broadband radiation upon receipt of pump radiation, comprising: an optical waveguide having a hollow core along its longitudinal axis; wherein the optical waveguide is configured to confine the pump radiation substantially within the hollow core; a working medium comprising molecules having a permanent dipole moment, included within the hollow core of the optical waveguide; wherein the working medium is configured to spectrally broaden the pump radiation to generate the broadband radiation; and an electric field generating means for generating an electric field across the working medium; wherein the electric field is configured to orient the molecules of the working medium.

[0009] According to a second aspect of the present invention, there is provided a radiation source, comprising: a radiation device according to the first aspect; and a pump radiation source for outputting the pump radiation.

[0010] According to a third aspect of the present invention, there is provided a measuring device, comprising a radiation source according to the second aspect.

[0011] According to a fourth aspect of the present invention, there is provided a method of generating broadband radiation, comprising: providing pump radiation; providing an optical waveguide having a hollow core along a longitudinal axis of the optical waveguide; introducing a working medium including molecules having a permanent dipole moment into the hollow core of the optical waveguide; causing the pump radiation to propagate generally within the hollow core of the optical waveguide so as to spectrally broaden the pump radiation as the pump radiation traverses the optical waveguide to generate the broadband radiation; and applying an electric field across the working medium to orient the molecules of the working medium so as to control the spectral broadening of the pump radiation and thus control the characteristics of the broadband radiation. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0013] Figure 1 A schematic general view of a lithographic apparatus is depicted;

[0014] Figure 2 A schematic general view of a lithographic cell is depicted;

[0015] Figure 3 A schematic representation of overall lithography, which represents the collaboration between three key technologies for optimizing semiconductor manufacturing;

[0016] Figure 4 A schematic general view of a scatterometry apparatus serving as a measuring device that may include a radiation source according to an embodiment of the present invention is depicted;

[0017] Figure 5 A schematic general view of a level sensor device that may include a radiation source according to an embodiment of the present invention is depicted;

[0018] Figure 6 A schematic general view of an alignment sensor device that may include a radiation source according to an embodiment of the present invention is depicted;

[0019] Figure 7 is a schematic cross-sectional view of a hollow-core optical fiber that may form part of a radiation source according to an embodiment in a transverse plane (i.e., perpendicular to the axis of the optical fiber);

[0020] Figure 8Schematic representation of a radiation source for providing broadband output radiation according to an embodiment;

[0021] Figure 9 (a) and Figure 9 (b) Schematically depict a cross-section of an example of a hollow-core photonic crystal fiber (HC-PCF) design for supercontinuum generation;

[0022] Figure 10 Schematic representation of an example broadband radiation device based on a working medium comprising molecules having a permanent dipole moment and oriented randomly;

[0023] Figure 11 Schematic representation of an embodiment broadband radiation device based on a working medium comprising molecules having a permanent dipole moment and oriented in a controlled manner;

[0024] Figure 12 Block diagram of a computer system for controlling a broadband radiation source. Detailed Description

[0025] In this document, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g., having wavelengths of 365 nm, 248 nm, 193 nm, 157 nm, or 126 nm) and EUV (extreme ultraviolet radiation, e.g., having wavelengths in the range of about 5 nm to 100 nm).

[0026] The terms "reticle", "mask", or "patterning device" as used herein can be broadly interpreted to refer to a general patterning device that can be used to impart a patterned cross-section to an incident radiation beam, the patterned cross-section corresponding to a pattern to be 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.).

[0027] Figure 1Schematically depicts 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) MT 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.

[0028] In operation, the illumination system IL receives the radiation beam from a radiation source SO, e.g., via a beam delivery system BD. The illumination system IL may include various types of optical components for 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 cross-section at the plane of the patterning device MA.

[0029] The term "projection system" PS as used herein should be interpreted broadly to encompass 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.

[0030] The lithographic apparatus LA may be of a type in which at least a portion of the substrate is 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, which is also referred to as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0031] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also referred to as a "dual platform"). In such a "multi-platform" machine, the substrate supports WT may 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 may be carried out while another substrate W on another substrate support WT is used for exposing a pattern on the other substrate W.

[0032] In addition to the substrate support WT, the lithographic apparatus LA may also include a measurement stage. The measurement stage is arranged to hold sensors and / or cleaning devices. The sensors may be arranged to measure properties of the projection system PS or of the radiation beam B. The measurement stage may hold a plurality of sensors. The cleaning device may be arranged to clean parts of the lithographic apparatus, such as part of the projection system PS or part of a system providing the immersion liquid. The measurement stage may be movable below the projection system PS when the substrate support WT is moved away from the projection system PS.

[0033] In operation, the radiation beam B is incident on a patterning device (e.g., a mask) MA held on a mask support MT 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, e.g., in order to position different target portions C in the path of the radiation beam B in a focused and aligned position. 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 may be used to align the patterning device MA and the substrate W. Although the illustrated substrate alignment marks P1, P2 occupy dedicated target portions, the substrate alignment marks may 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 are referred to as scribe alignment marks.

[0034] As Figure 2As shown, the lithographic apparatus LA can form part of a lithographic cell LC (sometimes also referred to as a lithographic cell or (lithographic) cluster), which typically also includes equipment for performing pre-exposure and post-exposure processes on the substrate W. Conventionally, these devices include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH and a bake plate BK 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 devices in the lithographic cell, which are generally collectively referred to as the track or coat and develop system, are typically under the control of a track or coat and develop system control unit TCU, which may itself be controlled by a management control system SCS, which may also control the lithographic apparatus LA via, for example, a lithographic control unit LACU.

[0035] In order 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) may be included in the lithographic cell LC. If an error is detected, the exposure of subsequent substrates or other processing steps to be performed on the substrate W can be adjusted, especially if the inspection is performed before the other substrates W in the same lot or batch are still to be exposed or processed.

[0036] An inspection device, which may also be referred to as 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 is alternatively configured to identify defects on the substrate W and may, for example, be part of the lithographic cell LC, or may be integrated into the lithographic apparatus LA, or may 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 portions of the resist have been removed), or even an etched image (after a pattern transfer step such as etching).

[0037] Generally, the patterning process in the lithographic apparatus LA is one of the most critical steps in the process, which requires a high degree of accuracy in the sizing and placement of the structures on the substrate W. To ensure this high degree of accuracy, three systems can be combined in Figure 3In the so-called "integrated" control environment schematically depicted. One of these systems is a lithographic apparatus LA, which is (virtually) connected to a metrology tool MT (second system) and to a computer system CL (third system). The key to such an "integrated" environment is to optimize 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 a range of process parameters (e.g., dose, focus, overlay), within which a particular manufacturing process produces a defined result (e.g., a functional semiconductor device) - typically allowing the process parameters in the lithography process or patterning process to vary within the defined result.

[0038] The computer system CL can use (a portion 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 the lithographic apparatus LA is currently operating within the process window (e.g., using input from the metrology tool MT), to predict whether there may be defects attributable to, for example, suboptimal processing (depicted by the arrow pointing to "0" in the second scale SC2 in Figure 3 ).

[0039] 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, for example, in the calibration state of the lithographic apparatus LA (depicted by the multiple arrows in the third scale SC3 in Figure 3 ).

[0040] During the lithography process, it is desirable to frequently measure the structures produced, for example, for process control and verification. Tools used to make such measurements are commonly referred to as metrology tools MT. Different types of metrology tools MT for making such measurements are known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. A scatterometer is a versatile instrument that allows for the measurement of lithography process parameters by having a sensor in the pupil or a plane conjugate to the pupil of the scatterometer's objective lens (the measurement is commonly referred to as pupil-based measurement), or by having a sensor in the image plane or a plane conjugate to the image plane (in this case, the measurement is commonly referred to as image- or field-based measurement). Such scatterometers and associated measurement techniques are further described in the patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032, or EP1,628,164A, which are incorporated herein by reference in their entirety. The aforementioned scatterometers can use radiation from soft x-rays and light visible in the near-IR wavelength range to measure gratings.

[0041] 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. Such reconstruction can be caused, for example, by simulating the interaction of 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.

[0042] 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 radiation reflected by the mirror (i.e., the measurement of intensity as a function of wavelength). Based on such 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.

[0043] 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 incorporated herein by reference in their entirety.

[0044] 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 the asymmetry in the detection configuration (the asymmetry being related to the overlapping range). Two (usually overlapping) grating structures can be applied in two different layers (not necessarily consecutive layers), and the two grating structures can be formed substantially at the same location on the wafer. The scatterometer can have, for example, a symmetric detection configuration as described in the co-owned patent application EP1,628,164A, such that any asymmetry is clearly distinguishable. This provides a direct way to measure the misalignment in the grating. Further examples for measuring the overlap error between two layers including periodic structures when measuring a target via the asymmetry of the periodic structure can be found in PCT Patent Application Publication No. WO 2011 / 012624 or U.S. Patent Application US 20160161863, which are incorporated herein by reference in their entirety.

[0045] Other parameters of interest can be focus and dose. Focus and dose can be determined simultaneously by scatterometry (or alternatively by scanning electron microscopy) as described in U.S. Patent Application 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 focus energy matrix (FEM, also known as a focus exposure matrix) can be used. If these unique combinations of critical dimension and sidewall angle are obtainable, the focus and dose values can be uniquely determined based on these measurements.

[0046] The measurement target can be the totality of a composite grating formed mainly 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 measurement target. As previously indicated, the diffraction signal can be used to determine the shift (also referred to as "overlay") between two layers, or can be used to reconstruct at least a portion of the original grating produced by the 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 a smaller sub-segment configured to mimic the dimensions of the functional portion of the design layout in the target. Due to such sub-segmentation, the target will behave more like the functional portion of the design layout, such that the overall process parameter measurement is preferably similar to the functional portion 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 produces a spot smaller than the overall target. In the overfill mode, the measurement beam produces a spot larger than the overall target. In such an overfill mode, it may also be possible to measure different targets simultaneously, thereby determining different process parameters simultaneously.

[0047] The overall measurement quality of the 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 direction of the radiation relative to the pattern on the substrate, etc. 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 the published U.S. Patent Application US 2016 / 0370717A1.

[0048] Figure 4 depicts a metrology device, such as a scatterometer. The measurement device includes a broadband (white light) radiation projector 2 that projects radiation onto a substrate 6. The reflected or scattered radiation is passed to a spectrometer detector 4 that measures the spectrum 10 of the radiation reflected by the mirror (i.e., the measurement of the intensity as a function of wavelength). From such data, a processing unit PU can, for example, by rigorous coupled-wave analysis and non-linear regression or by Figure 3Reconstruct the structure or profile that generated the detected spectrum by comparing it to a simulated spectral library shown at the bottom. Generally, for 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 data. Such a scatterometer can be configured as a normal incidence scatterometer or an oblique incidence scatterometer.

[0049] The overall measurement quality of lithography parameters of a measurement target via measurement is at least partially determined by the measurement option scheme used to measure such lithography parameters. The term "substrate measurement option 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 option scheme is a diffraction-based optical measurement, one or more of the parameters measured can include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the direction of the radiation relative to the pattern on the substrate, etc. One of the criteria for selecting a measurement option 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 and Published U.S. Patent Application US 2016 / 0370717A1, which are incorporated herein by reference in their entirety.

[0050] Another type of metrology tool used in IC manufacturing is a topography measurement system, a level sensor, or a height sensor. Such a tool can be integrated into a lithography apparatus for measuring the topography of the top surface of a substrate (or wafer). A topographic map of the substrate (also referred to as a height map) can be generated from these measurements indicating the height of the substrate as a function of position on the substrate. Such a height map can subsequently be used to correct the position of the substrate during the transfer of a pattern onto the substrate in order to provide a spatial image of the patterning device at the proper focus position on the substrate. It should be understood that "height" in this context generally refers to the out-of-plane dimension to the substrate (also referred to as the Z-axis). Generally, a 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.

[0051] Figure 5 An example of a level or height sensor LS known in the art is schematically shown in Figure 5Only the operating principle is illustrated. In this example, the level sensor includes an optical system, which includes a projection unit LSP and a detection unit LSD. The projection unit LSP includes a radiation source LSO that provides a radiation beam LSB, which is imparted by the projection grating PGR of the projection unit LSP. The radiation source LSO can be, for example, a narrow-band or broadband light source, such as a polarized or non-polarized, pulsed or continuous supercontinuum light source, such as a polarized or non-polarized laser beam. The radiation source LSO can include multiple radiation sources with different colors or wavelength ranges, such as multiple LEDs. The radiation source LSO of the level sensor LS is not limited to visible 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.

[0052] The projection grating PGR is a periodic grating that includes a periodic structure that generates a radiation beam BE1 with a periodically varying intensity. The radiation beam BE1 with a periodically varying intensity is directed towards the measurement site MLO on the substrate W, and the radiation beam BE1 has an incident angle ANG between 0 degrees and 90 degrees, typically between 70 degrees and 80 degrees, with respect to the axis (Z-axis) perpendicular to the incident substrate surface. At the measurement site MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by the arrow BE2) and directed towards the detection unit LSD.

[0053] To determine the height level at the measurement site MLO, the level sensor further includes a detection system, which includes a detection grating DGR, a detector DET, and a processing unit (not shown) for processing the output signal of the detector DET. The detection grating DGR can be identical to 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 photodetector, 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.

[0054] 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.

[0055] The projection unit LSP and / or the detection unit LSD can include additional optical elements, such as lenses and / or mirrors, along the path of the patterned radiation beam between the projection grating PGR and the detection grating DGR (not shown).

[0056] In an embodiment, the detection grating DGR may be omitted, and the detector DET may be placed at the location where the detection grating DGR was located. This configuration provides a more direct detection of the image of the projection grating PGR.

[0057] To effectively cover the surface of the substrate W, the horizontal sensor LS may be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby creating an array of measurement regions MLO or spots that cover a larger measurement range.

[0058] Various height sensors of a general type are disclosed, for example, in both 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 that uses a multi-element detector to detect and identify the position of a grating image without a detection grating is described.

[0059] Another type of metrology tool used in IC manufacturing is an alignment sensor. Thus, a key aspect of the performance of a lithographic apparatus is the ability to correctly and accurately place the pattern (by the same or a different lithographic apparatus) relative to features placed in a previous layer. For this purpose, the substrate is provided with one or more sets of marks or targets. Each mark is a structure whose position can be measured later using a position sensor, typically an optical position sensor. The position sensor may be referred to as an "alignment sensor", and the marks may be referred to as "alignment marks".

[0060] A lithographic apparatus may include one or more (e.g., a plurality of) alignment sensors that can accurately measure the position of alignment marks provided on a substrate. The alignment (or position) sensor may use optical phenomena such as diffraction and interference to obtain position information from alignment marks formed on the substrate. An example of an alignment sensor used in current lithographic apparatuses is based on the self-referencing interferometer described in US6961116. Various enhancements and modifications of the position sensor have been developed, such as those disclosed in US2015261097A1. All of these disclosures are incorporated herein by reference.

[0061] Figure 6It is a schematic block diagram of an embodiment of a known alignment sensor AS such as described in, for example, US6961116 and incorporated herein by reference. A radiation source RSO provides a radiation beam RB having one or more wavelengths, and the radiation beam RB is steered by steering optics onto a mark (such as a mark AM located on a substrate W) as an illumination spot SP. In this example, the steering optics includes a point mirror SM and an objective lens OL. Thus, the diameter of the illumination spot SP that illuminates the mark AM can be slightly smaller than the width of the mark itself.

[0062] The radiation diffracted by the alignment 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 (which may be referred to as reflection). For example, a self-referencing interferometer SRI of the type disclosed in US6961116 mentioned above causes the beam IB to interfere with itself, and then the beam is received by a photodetector PD. Additional optics (not shown) may be included to provide separate beams in the case where the radiation source RSO produces more than one wavelength. The photodetector may be a single element, or it may include multiple pixels as needed. The photodetector may include a sensor array.

[0063] The steering optics including the point mirror SM in this example can also be used to block the zero-order radiation reflected from the mark, so that the information-bearing beam IB only includes higher-order diffracted radiation from the mark AM (this is not necessary for measurement, but improves the signal-to-noise ratio).

[0064] An intensity signal SI is supplied to a processing unit PU. By combining the optical processing performed in the block SRI and the computational processing performed in the unit PU, values of the X position and Y position of the substrate relative to a reference frame are output.

[0065] 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 this to identify which period of the sine wave is the period that includes the marked position. The same process can be repeated at different wavelengths, at coarser and / or finer levels, for improving accuracy and / or for robustly detecting the mark, regardless of the material on which the mark is made and the material on and / or under which the mark is provided. The wavelengths can be multiplexed and demultiplexed optically to process the wavelengths simultaneously, and / or the wavelengths can be multiplexed by time division or frequency division.

[0066] In this example, the alignment sensor and the fiducial mark SP remain stationary while the substrate W moves. Thus, the alignment sensor can be mounted rigidly, i.e., firmly and accurately, with respect to a reference system, while effectively scanning the mark AM in a direction opposite to the direction of movement of the substrate W. In such movement, the substrate W is controlled by a substrate positioning system that mounts the substrate W on a substrate support and controls the movement of the substrate support. A substrate support position sensor (e.g., an interferometer) measures the position of the substrate support (not shown). In an embodiment, one or more (alignment) marks are provided on the substrate support. Measuring the position of the marks provided on the substrate support allows calibration of the position of the substrate support determined by the position sensor (e.g., with respect to the frame to which the alignment system is connected). Measuring the position of the alignment marks provided on the substrate allows determination of the position of the substrate relative to the substrate support.

[0067] The above-mentioned metrology tool MT (such as, a scatterometer, a topography measurement system, or a position measurement system) can perform measurements using radiation from a radiation source. The nature of the radiation used by the metrology tool may affect the type and quality of the measurements that can be performed. For some applications, it may be advantageous to use multiple radiation frequencies to measure the substrate, for example, broadband radiation can be used. Multiple different frequencies may be able to propagate, illuminate, and scatter off the metrology target without interfering with other frequencies or with minimal interference with other frequencies. Thus, more metrology data can be obtained simultaneously using different frequencies, for example. Different radiation frequencies may also be able to probe and uncover different properties of the metrology target. Broadband radiation can be applicable in the metrology system MT, such as, for example, a level sensor, an alignment mark measurement system, a scatterometry tool, or an inspection tool. A broadband radiation source can be a supercontinuum source.

[0068] It may be difficult to generate high-quality broadband radiation such as supercontinuum radiation, for example. One method for generating broadband radiation can be, for example, to utilize non-linear higher-order effects to broaden high-power narrow-band or single-frequency input radiation or pump radiation. The input radiation (which can be generated using a laser) can be referred to as pump radiation. Alternatively, the input radiation can be referred to as seed radiation. To obtain high-power radiation for the broadening effect, the radiation can be confined to a small area such that a highly localized high-intensity radiation is achieved. In those areas, the radiation can interact with a broadening structure and / or a material forming a non-linear medium to produce broadband output radiation. In the high-intensity radiation areas, different materials and / or structures can be used to achieve and / or improve radiation broadening by providing a suitable non-linear medium.

[0069] In some implementations, broadband output radiation is generated in a photonic crystal fiber (PCF). In a number of embodiments, such a photonic crystal fiber has a microstructure around its core, which helps to confine the radiation traveling through the waveguide. The fiber core can be made of a solid material having nonlinear properties and capable of generating broadband radiation when high-intensity pump radiation is transmitted through the fiber core. Although it is feasible to generate broadband radiation in a solid-core photonic crystal fiber, there can be several drawbacks to using solid materials. For example, if UV radiation is generated in the solid core, such radiation may not be present in the output spectrum of the fiber because the radiation is absorbed by most solid materials.

[0070] In some implementations, as discussed further below with reference to Figure 8 A method and apparatus for broadening input radiation can use an optical fiber for confining the input radiation and for broadening the input radiation to output broadband radiation. The optical fiber can be a hollow-core optical fiber and can include an internal structure for effecting efficient guiding and confinement of the radiation in the optical fiber. The optical fiber can be a hollow-core photonic crystal fiber (HC-PCF), which is particularly suitable for strong radiation confinement mainly inside the hollow core of the optical fiber, thereby achieving a high radiation intensity. The hollow core of the optical fiber can be filled with a gas that acts as an active nonlinear medium for broadening the input radiation. Such an optical fiber and gas arrangement can be used to generate a supercontinuum radiation source. The radiation input to the optical fiber can be electromagnetic radiation, such as radiation in one or more of the infrared, visible, UV, and extreme UV spectra. The output radiation can consist of or include broadband radiation, which can be referred to herein as white light.

[0071] Some embodiments relate to a novel design of such a broadband radiation source including an optical fiber. The optical fiber is a hollow-core photonic crystal fiber (HC-PCF). In particular, the optical fiber can be of a type that includes an antiresonant structure for confining radiation. Such an optical fiber including an antiresonant structure is known in the art as an antiresonant fiber, a tubular fiber, a single-ring fiber, a negative-curvature fiber, or a suppressed-coupling fiber. Various different designs of such optical fibers are known in the art. Alternatively, the optical fiber can be a photonic bandgap fiber (HC-PBF, such as a Kagome fiber).

[0072] Multiple types of HC-PCF can be designed, each based on a different physical guiding mechanism. Two such HC-PCF include: a hollow-core photonic bandgap fiber (HC-PBF) and a hollow-core antiresonant reflecting fiber (HC-ARF). Details of the design and manufacture of HC-PCF can be found in U.S. Patent US2004 / 015085A1 (directed to HC-PBF) and International PCT Patent Application WO2017 / 032454A1 (directed to a hollow-core antiresonant reflecting fiber), which are incorporated herein by reference.Figure 9 (a) shows a Kagome optical fiber including a Kagome lattice structure.

[0073] Reference will now be made to Figure 7 describe an example of an optical fiber for use in a radiation source, Figure 7 which is a schematic cross-sectional view of an optical fiber OF in a transverse plane. Additional embodiments similar to the actual example of the optical fiber disclosed in WO2017 / 032454A1 are Figure 7 presented.

[0074] The optical fiber OF includes an elongated body that is longer in one dimension compared to the other two dimensions of the optical fiber OF. This longer dimension may be referred to as the axial direction and may define the axis of the optical fiber OF. The other two dimensions define a plane that may be referred to as the transverse plane. Figure 7 Shows a cross-section of the optical fiber OF in such a transverse plane (i.e., perpendicular to the axis) labeled as the x-y plane. The transverse cross-section of the optical fiber OF may be substantially constant along the optical fiber axis.

[0075] It should be understood that the optical fiber OF has a certain degree of flexibility, and thus, typically, the direction of the axis will not be uniform along the length of the optical fiber OF. Terms such as the optical axis, transverse cross-section, and the like should be understood to mean the local optical axis, local transverse cross-section, etc. In addition, in cases where a component is described as cylindrical or tubular, these terms should be understood to cover such shapes that may have been deformed when the optical fiber OF is flexed.

[0076] The optical fiber OF may have any length, and it should be understood that the length of the optical fiber OF may depend on the application. The optical fiber OF may have a length between 1 cm and 10 m. For example, the optical fiber OF may have a length between 10 cm and 100 cm.

[0077] The optical fiber OF includes a hollow core HC, a cladding portion surrounding the hollow core HC, and a support portion SP surrounding and supporting the cladding portion. The optical fiber OF may be considered to include a body (including the cladding portion and the support portion SP) having a hollow core HC. The cladding portion includes a plurality of antiresonant elements for guiding radiation through the hollow core HC. In particular, the plurality of antiresonant elements are arranged to mainly confine the radiation propagating through the optical fiber OF inside the hollow core HC and are configured to guide the radiation along the optical fiber OF. The hollow core HC of the optical fiber OF may be generally disposed in the central region of the optical fiber OF such that the axis of the optical fiber OF may also define the axis of the hollow core HC of the optical fiber OF.

[0078] The cladding portion includes a plurality of antiresonant elements for guiding the radiation propagating through the optical fiber OF. In particular, in this embodiment, the cladding portion includes a single loop of six tubular capillaries CAP. Each of the tubular capillaries CAP serves as an antiresonant element.

[0079] The capillary CAP can also be referred to as a tube. The cross-section of the capillary CAP can be circular or can have another shape. Each capillary CAP includes a generally cylindrical wall portion WP that at least partially defines a hollow core HC of the optical fiber OF and separates the hollow core HC from the capillary cavity CC. It should be understood that the wall portion WP can act as an antireflection Fabry-Perot resonator for radiation that propagates through the hollow core HC (and the radiation can be incident on the wall portion WP at a grazing angle of incidence). The thickness of the wall portion WP can be suitable to ensure a substantially enhanced reflection back into the hollow core HC while substantially suppressing transmission into the capillary cavity CC. In some embodiments, the capillary wall portion WP can have a thickness between 0.01 µm and 10.0 µm.

[0080] It should be understood that, as used herein, the term cladding portion is intended to mean a portion of the optical fiber OF that guides radiation propagating through the optical fiber OF (i.e., the capillary CAP that confines the radiation within the hollow core HC). The radiation can be confined in the form of a transverse mode propagating along the optical fiber axis.

[0081] The support portion is generally tubular and supports six capillary CAPs of the cladding portion. The six capillary CAPs are uniformly distributed around the inner surface of the inner support portion SP. The six capillary CAPs can be described as being arranged in a generally hexagonal form.

[0082] The capillary CAPs are arranged such that each capillary does not contact any of the other capillary CAPs. Each of the capillary CAPs contacts the inner support portion SP and is spaced apart from adjacent capillary CAPs in the annular structure. This arrangement can be beneficial in that it can increase the transmission bandwidth of the optical fiber OF (relative to, for example, an arrangement in which the capillaries are in contact with each other). Alternatively, in some embodiments, each of the capillary CAPs can contact adjacent capillary CAPs in the annular structure.

[0083] The six capillary CAPs of the cladding portion are arranged in a ring structure around the hollow core HC. The inner surface of the ring structure of the capillary CAPs at least partially defines the hollow core HC of the optical fiber OF. The diameter d of the hollow core HC (which can be defined as the minimum dimension between opposing capillaries, indicated by arrow d) can be between 10 µm and 1000 µm. The diameter d of the hollow core HC can affect the mode field diameter, impact loss, dispersion, modal multiplicity i.e., modal complexity, and non-linear properties of the optical fiber OF in the hollow core HC.

[0084] In this embodiment, the cladding portion includes a single loop arrangement of capillaries CAP that serve as anti-resonant elements. Thus, a line in any radial direction from the center of the hollow core HC to the outside of the optical fiber OF passes through no more than one capillary CAP.

[0085] It should be understood that other embodiments may be provided with different arrangements of anti-resonant elements. These arrangements may include arrangements having multiple loops of anti-resonant elements and arrangements having nested anti-resonant elements. Figure 9 (a) shows an embodiment of an HC-PCF having three loops of capillaries CAP, the loops being stacked on top of each other in the radial direction. In this embodiment, each capillary CAP contacts other capillaries both within the same loop and in different loops. Further, while Figure 7 the embodiment shown in includes a loop of six capillaries, in other embodiments, one or more loops including any number of anti-resonant elements (such as 4, 5, 6, 7, 8, 9, 10, 11, or 12 capillaries) may be provided in the cladding portion.

[0086] Figure 9 (b) shows a modified embodiment of the HC-PCF having a single loop of tubular capillaries as discussed above. In Figure 9 the example of (b), there are two coaxial loops of tubular capillaries 21. To hold the inner and outer loops of the tubular capillaries 21, a support tube ST may be included in the HC-PCF. The support tube may be made of silica.

[0087] Figure 7 and Figure 9 (a) and Figure 9 the tubular capillaries in the example of (b) may have a circular cross-sectional shape. For the tubular capillaries, other shapes are possible, such as an elliptical or polygonal cross-section. Additionally, Figure 7 and Figure 9 the solid material of the tubular capillaries in the examples of (a) and Figure 9 (b) may include a plastic material (such as PMA), glass (such as silica or soft glass).

[0088] Figure 8 Depicts a radiation source RDS for providing broadband output radiation. The radiation source RDS includes: a pulsed pump radiation source PRS or any other type of source capable of generating short pulses of a desired length and energy level; an optical fiber OF having a hollow core HC (such as Figure 7 the type shown in ); and a working medium WM (such as a gas) disposed within the hollow core HC. While in Figure 8 the radiation source RDS includes Figure 7 the optical fiber OF shown in, in alternative embodiments, other types of hollow core HC optical fibers OF may be used.

[0089] The pulsed pump radiation source PRS is configured to provide input radiation IRD. The hollow core HC of the optical fiber OF is arranged to receive the input radiation IRD from the pulsed pump radiation source PRS and broaden the input radiation IRD to provide output radiation ORD. The working medium WM is capable of broadening the frequency range of the received input radiation IRD to provide broadband output radiation ORD.

[0090] The radiation source RDS further includes a reservoir RSV. The optical fiber OF is disposed inside the reservoir RSV. The reservoir RSV may also be referred to as a housing, a container, or a gas chamber. The reservoir RSV is configured to include the working medium WM. The reservoir RSV may include one or more features known in the art for controlling, regulating, and / or monitoring the composition of the working medium WM (which may be a gas) inside the reservoir RSV. The reservoir RSV may include a first transparent window TW1. In use, the optical fiber OF is disposed inside the reservoir RSV such that the first transparent window TW1 is positioned close to the input end IE of the optical fiber OF. The first transparent window TW1 may form part of the wall of the reservoir RSV. The first transparent window TW1 may be transparent at least for the frequency of the received input radiation, such that the received input radiation IRD (or at least most of it) can be coupled into the optical fiber OF located inside the reservoir RSV. It should be understood that optical devices (not shown) may be provided for coupling the input radiation IRD into the optical fiber OF.

[0091] The reservoir RSV includes a second transparent window TW2 that forms part of the wall of the reservoir RSV. In use, when the optical fiber OF is disposed inside the reservoir RSV, the second transparent window TW2 is positioned close to the output end OE of the optical fiber OF. The second transparent window TW2 may be transparent at least for the frequency of the broadband output radiation ORD of the device 120.

[0092] Alternatively, in another embodiment, the two opposite ends of the optical fiber OF may be placed inside different reservoirs. The optical fiber OF may include a first end section configured to receive the input radiation IRD and a second end section for outputting the broadband output radiation ORD. The first end section may be placed inside a first reservoir including the working medium WM. The second end section may be placed inside a second reservoir, where the second reservoir may also include the working medium WM. The functions of the reservoirs may be as described above with respect to Figure 8As described. The first reservoir may include a first transparent window configured to be transparent to the input radiation IRD. The second reservoir may include a second transparent window configured to be transparent to the broadband output broadband radiation ORD. The first and second reservoirs may also include a sealable opening to allow the optical fiber OF to be partially placed inside the reservoir and partially outside the reservoir such that gas can be sealed inside the reservoir. The optical fiber OF may also include an intermediate section not contained inside the reservoir. Such an arrangement using two separate gas reservoirs may be particularly convenient for embodiments where the optical fiber OF is relatively long (e.g., when the length exceeds 1 m). It should be understood that for such an arrangement using two separate gas reservoirs, the two reservoirs (which may include one or more features known in the art for controlling, regulating, and / or monitoring the composition of the gas inside the two reservoirs) can be considered as devices for providing the working medium WM inside the hollow core HC of the optical fiber OF.

[0093] In this context, a window may be transparent to a frequency if at least 50%, 75%, 85%, 90%, 95%, or 99% of the incident radiation at that frequency transmits through the window.

[0094] Both the first TW1 and second TW2 transparent windows may form an airtight seal within the wall of the reservoir RSV such that the working medium WM (which may be a gas) can be contained within the reservoir RSV. It should be understood that the gas WM may be contained within the reservoir RSV at a pressure different from the ambient pressure of the reservoir RSV.

[0095] The working medium WM may include: inert gases such as argon, krypton, and xenon; Raman active gases such as hydrogen, deuterium, and nitrogen; or gas mixtures such as argon / hydrogen mixtures, xenon / deuterium mixtures, krypton / nitrogen mixtures, or nitrogen / hydrogen mixtures. Depending on the type of filling gas, the nonlinear optical processes may include modulation instability (MI), optical soliton self-compression, optical soliton splitting, Kerr effect, Raman effect, and dispersive wave generation (DWG), the details of which are described in WO2018 / 127266A1 and US9160137B1 (both of which are hereby incorporated by reference). Since the dispersion of the filling gas can be tuned by changing the pressure of the working medium WM in the reservoir RSR (i.e., the gas chamber pressure), the resulting broadband pulse dynamics and associated spectral broadening characteristics can be adjusted to optimize frequency conversion.

[0096] In one implementation, the working medium WM can be disposed within the hollow core HC at least during reception of input radiation IRD for generating broadband output radiation ORD. It should be understood that when the optical fiber OF does not receive input radiation IRD for generating broadband output radiation, the gas WM can be wholly or partially absent from the hollow core HC.

[0097] To achieve frequency broadening, high-intensity radiation may be desirable. The advantage of an optical fiber OF with a hollow core HC is that it can achieve high-intensity radiation due to strong spatial confinement of the radiation propagating through the optical fiber OF, thereby achieving a highly localized radiation intensity. The radiation intensity inside the optical fiber OF can be relatively high, for example, due to a high received input radiation intensity and / or due to strong spatial confinement of the radiation inside the optical fiber OF. The advantage of a hollow-core optical fiber is that it can guide radiation with a wider wavelength range than a solid-core optical fiber, and in particular, a hollow-core optical fiber can guide radiation in both the ultraviolet and infrared ranges.

[0098] The advantage of using an optical fiber OF with a hollow core HC can be that most of the radiation guided inside the optical fiber OF is confined within the hollow core HC. Therefore, most of the interaction of the radiation inside the optical fiber OF is with the working medium WM disposed inside the hollow core HC of the optical fiber OF. Therefore, the broadening effect of the working medium WM on the radiation can be enhanced.

[0099] The received input radiation IRD can be electromagnetic radiation. The input radiation IRD can be received as pulsed radiation. For example, the input radiation IRD can include, for example, ultrafast pulses generated by a laser.

[0100] The input radiation IRD can be coherent radiation. The input radiation IRD can be collimated radiation, and its advantage can be to facilitate and improve the efficiency of coupling the input radiation IRD into the optical fiber OF. The input radiation IRD can include a single frequency or a narrow frequency range. The input radiation IRD can be generated by a laser. Similarly, the output radiation ORD can be collimated and / or can be coherent.

[0101] The broadband range of the output radiation ORD can be a continuous range, including a continuous radiation frequency range. The output radiation ORD can include supercontinuum radiation. Continuous radiation can be beneficial for use in multiple applications (such as in measurement applications). For example, a continuous frequency range can be used to interrogate a large number of properties. A continuous frequency range can be used, for example, to determine and / or eliminate the frequency dependence of the measured properties. The supercontinuum output radiation ORD can include, for example, electromagnetic radiation in the wavelength range of 100 nm to 4000 nm. The broadband output radiation ORD frequency range can be, for example, 400 nm to 900 nm, 500 nm to 900 nm, or 200 nm to 2000 nm. The supercontinuum output radiation ORD can include white light.

[0102] The input radiation IRD provided by the pulsed pump radiation source PRS can be pulsed. The input radiation IRD can include electromagnetic radiation at one or more frequencies between 200 nm and 2 µm. The input radiation IRD can include, for example, electromagnetic radiation having a wavelength of 1.03 µm. The repetition rate of the pulsed radiation IRD can be on the order of 1 kHz to 100 MHz. The pulse energy can be on the order of 0.1 µJ to 100 µJ, for example 1 to 10 µJ. The pulse duration of the input radiation IRD can be between 10 fs and 10 ps, for example 300 fs. The average power of the input radiation IRD can be between 100 mW and several hundreds of W. The average power of the input radiation IRD can be, for example, 20 W to 50 W.

[0103] The pulsed pump radiation source PRS can be a laser. The spatio-temporal transmission characteristics (e.g., its spectral amplitude and phase) of such laser pulses transmitted along the optical fiber OF can be changed and tuned by adjusting (pump) laser parameters, working component WM variations, and optical fiber OF parameters. The spatio-temporal transmission characteristics can include one or more of output power, output mode distribution, output time distribution, the width of the output time distribution (or output pulse width), output spectral distribution, and the bandwidth of the output spectral distribution (or output spectral bandwidth). The pulsed pump radiation source PRS parameters can include one or more of pump wavelength, pump pulse energy, pump pulse width, pump pulse repetition rate. The optical fiber OF parameters can include one or more of fiber length, the size and shape of the hollow core HC, the size and shape of the capillary, the thickness of the wall of the capillary surrounding the hollow core HC. The working component WM (e.g., filling gas) parameters can include one or more of gas type, gas pressure, and gas temperature.

[0104] The broadband output radiation ORD provided by the radiation source RDS can have an average output power of at least 1 W. The average output power can be at least 5 W. The average output power can be at least 10 W. The broadband output radiation ORD can be pulsed broadband output radiation ORD. The broadband output radiation ORD can have a power spectral density of at least 0.01 mW / nm over the entire wavelength band of the output radiation. The power spectral density over the entire wavelength band of the broadband output radiation can be at least 3 mW / nm.

[0105] As is well known, the optical Kerr effect is one of some basic nonlinear optical effects for generating broadband radiation. The optical Kerr effect can occur when light propagates in an optical medium and can be described as a change in the refractive index of the optical material in response to the illumination of the light.

[0106] The optical Kerr effect results in an intensity-dependent refractive index n, which is expressed as:

[0107] ,[1]

[0108] ,[2]

[0109] where n0 is the linear refractive index, n2 is the quadratic nonlinear refractive index, I0 is the intensity of the light, and is the third-order optical susceptibility of the material. The quadratic nonlinear refractive index n2 is also known as the nonlinear refractive index or Kerr nonlinearity. Since the Kerr nonlinearity n2 is typically on the order of , the optical Kerr effect becomes significant only when the intensity (or irradiance) of the light is high enough (e.g., above (gigawatts per square centimeter)). The optical Kerr effect itself manifests as self-phase modulation (SPM) in the time domain and self-focusing in the spatial domain.

[0110] SPM is a well-known nonlinear effect that is caused by the nonlinear change imparted to the phase of a laser pulse by the time-varying refractive index, as represented in Equation [1]. The nonlinear phase change results in a frequency change of the time-varying instantaneous frequency and thus leads to spectral broadening of the laser pulse. The time-varying instantaneous frequency is expressed as (assuming a Gaussian temporal distribution of the laser pulse):

[0111] ,[3]

[0112] where is the pulse duration of the Gaussian pulse, is the carrier frequency, z is the propagation distance, and t is time. According to Equation [3], the frequency change is proportional to the Kerr nonlinearity n2 and the intensity I0 of the laser pulse, and inversely proportional to the pulse duration . Thus, the spectral broadening effect caused by SPM can be enhanced by increasing the Kerr nonlinearity, and / or increasing the intensity of the laser pulse, and / or decreasing the pulse duration. A stronger spectral broadening effect will result in a higher white light generation efficiency, thereby producing a broader spectrum with a higher power spectral density.

[0113] The latter two methods (i.e., increasing the laser intensity and decreasing the pulse duration) have been employed in a variety of broadband radiation sources (e.g., such as Figure 8The radiation source shown in [reference] is used to improve the efficiency of white light generation. However, using laser pulses with very high intensity and (optionally) very short pulse widths to generate broadband radiation often causes lifetime problems, such as optical damage or optical degradation of certain key components of the radiation source. In addition, for a given pulse width and a given pulse repetition frequency, the higher the pulse intensity, the greater the pulse energy will be. Using very large pulse energies to generate broadband radiation often causes thermal problems in the radiation source, which can degrade the output performance of the radiation source. Therefore, it is desirable to adopt a first method, namely, increasing the Kerr nonlinearity of the nonlinear medium, in order to further improve the efficiency of white light generation.

[0114] Figure 10 A schematic representation depicting an exemplary radiation device. Such a radiation device RDD can be similar to Figure 8 the corresponding part of the radiation source shown in [reference]. In Figure 10 the exemplary radiation device RDD of [reference], a hollow-core HC fiber OF (e.g., Figure 7 the HC-PCF shown in [reference]) can be filled with a working medium WM that can be in gaseous or liquid form. The working medium WM can include molecules having a permanent dipole moment µ0 and such molecules can be oriented randomly. The (electric) dipole moment of a molecule is a measure of the separation of positive and negative charges within the molecule. Input (or pump) radiation IRD is coupled to one end of the hollow core HC of the fiber OF and is spectrally broadened via interaction with the working medium WM as it propagates along the fiber OF. This results in the generation of broadband output radiation ORD, which then exits from the other (or opposite) end of the fiber OF.

[0115] It is well known that the dipole moment of a molecule can be oriented by an electric field. The broadband output radiation generated from a set of oriented molecules will have a greater amplitude and / or a broader spectral width than the broadband output radiation generated from the same set of molecules with randomly oriented dipoles, e.g., in Figure 10is shown. Such an effect has been confirmed by previous studies, such as "Transient birefringence of liquids induced by terahertz electric-field torque on permanent molecular dipoles" by Mohsen Sajadi, Martin Wolf, and Tobias Kampfrath, which is incorporated herein by reference, Nature Communications, DOI: 10.1038 / ncomms14963 (2017), which shows that the Kerr nonlinearity of a molecular system is enhanced in oriented molecules due to the torque exerted on the molecules by an external time-dependent electric field (e.g., a terahertz (THz) or optical pulse). As mentioned above, the Kerr nonlinearity contributes to the SPM process and is crucial in determining the characteristics of the broadband output radiation. In addition, optical Raman scattering (another nonlinear optical process contributing to broadband radiation generation) obtains a larger amplitude in this ordered system. Therefore, it is proposed to preferably control (or optimize) the characteristics of the broadband output radiation ORD by controlling the direction of the dipole moments of the molecules of the working medium.

[0116] According to a first aspect of the present invention, there is provided a radiation device for generating broadband radiation upon receipt of pump radiation, comprising: an optical waveguide having a hollow core along its longitudinal axis; wherein the optical waveguide is configured to confine the pump radiation substantially within the hollow core; a working medium including molecules having permanent dipole moments, included within the hollow core of the optical waveguide; wherein the working medium is configured to spectrally broaden the pump radiation to generate the broadband radiation; and an electric field generating device for generating an electric field across the working medium; wherein the electric field is configured to orient the molecules of the working medium.

[0117] Figure 11 A schematic representation depicting a broadband radiation device according to an embodiment. The same reference numerals are used for those components that are also included in the Figure 10 example device shown in. The main difference between the two radiation devices lies in that Figure 11 the radiation device RDD' of the embodiment can additionally include an E-field (electric field) generating device EG for generating an electric field E across the working medium WM. The electric field E can be configured to orient the dipole moments μ of the molecules of the working medium WM so as to control the spectral broadening of the pump radiation IRD and thus control the characteristics of the broadband radiation ORD'. In the case where the electric field E is linearly polarized and the molecules have cylindrical symmetry, the electric field E can induce an instantaneous dipole moment μ by polarizing the electronic distribution of the moleculesins Therefore, the dipole moment µ can be the vector sum of a permanent dipole moment µ0 and an instantaneous dipole moment µ ins (see DOI: 10.1038 / ncomms14963).

[0118] Continuing to refer to Figure 11 , the E-field generating device, i.e., the electric field generating device EG, can include, for example, a pair of conductive plates P1, P2 placed on two opposite sides of the optical fiber OF. In an embodiment, the two conductive plates P1, P2 can be configured to be parallel to each other. In an embodiment, the electric field E can be along the transverse direction of the optical fiber (e.g., along Figure 11 the Y-axis of the coordinate system shown in Figure 11 ), and the transverse direction is perpendicular to the longitudinal direction of the optical fiber (e.g.,

[0119] the Z-axis of the coordinate system shown in Figure 11 ). In an embodiment, each of the two conductive plates P1, P2 can be made of copper, aluminum, or different metals. In an embodiment, the two conductive plates P1, P2 can have similar dimensions (e.g., width W, length L, and thickness T). In an embodiment, the dimensions of the two conductive plates P1, P2 can be appropriately adjusted such that the electric field E extends along both the transverse axis (e.g., Figure 11 the X-axis of the coordinate system shown in

[0120] Continuing to refer to Figure 11 , the E-field generating device EG can further include a voltage supply operable to provide a voltage V across the two conductive plates P1, P2 such that the first plate of the pair of plates (e.g., P1) carries a positive charge and the second plate of the pair of plates (e.g., P2) carries a negative charge. The intensity or magnitude of the electric field E between the two conductive plates is controlled by the following formula:

[0121] . [4]

[0122] In the case where the working medium WM is a gas, the intensity of the electric field E can be high enough to overcome the collisions between molecules, which are proportional to the molecular kinetic energy, in order to obtain a relatively long stationary orientation of the molecular dipoles towards the electric field E. In the case where the working medium WM is a dipolar liquid, the intermolecular interactions act as a resistance to the electric field E for obtaining sufficient molecular orientation. Therefore, a liquid working medium may require a stronger electric field than a gaseous working medium. If the electric field E is tuned to the rotational transition frequency of the liquid molecules, it may be possible to obtain a resolvable orientation of the molecules of the liquid working medium.

[0123] In an embodiment, the electric field E can be an electrostatic field. In an embodiment, the E-field generating device EG can include a constant voltage power supply that is operable to provide a constant voltage V across the conductive plates P1, P2 in order to generate static charges on the plates P1, P2 and thus generate an electrostatic field E between the two plates P1, P2. The intensity of the electric field E can depend on the type of the working medium and the operating conditions. For molecules in the gas phase, the intensity of the electric field E can be, for example, at least 10 kilovolts (KV) / cm, at least 20 KV / cm, at least 50 KV / cm, at least 100 KV / cm, at least 200 KV / cm, or at least 500 KV / cm. For molecules in the liquid phase, the intensity of the electric field E can be, for example, at least 100 KV / cm, at least 200 KV / cm, at least 300 KV / cm, at least 400 KV / cm, at least 500 KV / cm, or at least 600 KV / cm.

[0124] In an embodiment, the electric field E can be an alternating electric field having an oscillation frequency. In an embodiment, the oscillation frequency of the oscillation amplitude of the electric field E can match the rotational resonance of the molecules of the working medium. For small molecules, the rotational transition frequency typically occurs in a frequency range, for example, from several gigahertz (GHz) to several terahertz (THz). The oscillation frequency of the oscillation amplitude of the electric field E can be, for example, at least 1 kHz, at least 10 kHz, at least 100 kHz, at least 500 kHz or, for example, at least 1 THz. Thus, such an oscillation frequency can be between 1 kHz (kilohertz) and 10 THz, between 100 kHz and 10 THz, between 100 kHz and 5 THz, between 500 kHz and 5 THz, between 1 GHz and 5 THz, between 1 GHz and 1 THz, between 10 GHz and 1 THz, or between 100 GHz and 1 THz.

[0125] For molecules in the gas phase and in the case where the electric field E is in resonance with the molecular rotational transition (i.e., the oscillation frequency matches the rotation of the molecule), the intensity of the electric field E can be, for example, at least 10 KV / cm, at least 20 KV / cm, at least 50 KV / cm, at least 100 KV / cm, at least 200 KV / cm, or at least 500 KV / cm. For molecules in the liquid phase and in the case where the electric field E is in resonance with the molecular rotational transition, the intensity of the electric field E can be, for example, at least 100 KV / cm, at least 500 KV / cm, at least 1000 KV / cm, at least 2000 KV / cm, at least 3000 KV / cm, or at least 5000 KV / cm.

[0126] In an embodiment, the E-field generating device EG can include a radio frequency (RF) power supply that is operable to provide an alternating voltage V to the conductive plates P1, P2.

[0127] Continuing reference Figure 11 , in an embodiment, the E-field generating device EG is operable to apply the electric field at least during the period when the pulse of the pump radiation traverses the optical fiber OF. In an embodiment, the E-field generating device EG is operable to start applying the electric field upon receiving a trigger signal in the timing of the pulse from the pump radiation IRD. In an embodiment, the E-field generating device EG can be operable to stop applying the electric field after a predetermined period has elapsed. The predetermined period for applying the electric field can be, for example, less than 1 nanosecond (ns), less than 5 ns, less than 10 ns, less than 50 ns, or less than 100 ns. In an embodiment, the E-field generating device may further include a control device configured to start and / or stop enabling the electric field in an automated manner.

[0128] Continuing reference Figure 11, the working medium WM can be a gas or a gas mixture, or a liquid. In any case, the working medium can include molecules having a permanent dipole moment µ0. The permanent dipole moment µ0 of the molecules of the working medium can be, for example, at least 1 Debye, at least 2 Debye, at least 5 Debye, at least 10 Debye. Previous studies have also found that optical Kerr nonlinearity can be enhanced by using molecules with higher polarizability. Molecular polarizability is the response of the electron distribution to an electric field and is defined as the ratio of its induced dipole moment to the electric field. More details about such findings can be found in the following publication: "Multioctave supercontinuum generation and frequency conversion based on rotational nonlinearity" by J.E. Beetar, M. Nrisimhamurty, T.C. Truong, G.C. Nagar, Y.Y. Liu, J. Nesper, O. Suarez, F. Rivas, Y. Wu, B. Shim, and M. Chini, Science Advance, 6: eabb5375 (2020), which is incorporated herein by reference. Therefore, it can be preferred that the molecules of the working medium have a linear molecular structure and a high polarizability, which can be, for example, at least 1 C·m 2 ·V -1 , at least 5 C·m 2 ·V -1 , at least 10 C·m 2 ·V -1 , at least 50 C·m 2 ·V -1 or at least 100 C·m 2 ·V -1 .

[0129] In some embodiments, the working medium WM can be one of the following: dimethyl ether, hydrogen iodide, methylamine, ketene, nitrosyl bromide, carbonyl borane, methyl bromide, hydrogen bromide, ethylamine, nitrosyl bromide, and trifluoromethane. In some embodiments, the working medium WM can be one of the following: acetonitrile, acetone, or water.

[0130] The optical fiber OF can be any kind of optical waveguide including a hollow core. In some embodiments, the optical fiber OF can be a hollow-core photonic crystal fiber (HC-PCF). In some embodiments, the optical fiber OF can be an HC-PCF with a single loop of tubular capillary (e.g., Figure 7 as shown in).

[0131] According to a second aspect of the present invention, there is provided a radiation source comprising: a radiation device according to any of the above embodiments (e.g., Figure 11 as shown); and a pump radiation source for outputting the pump radiation. In an embodiment, the radiation source may include a reservoir configured to at least partially enclose the radiation device, for example, in a manner similar to the exemplary radiation source of Figure 8 . In an embodiment, the pump radiation is linearly polarized pulsed radiation. In an embodiment, the pump radiation includes electromagnetic radiation having one or more frequencies between 100 nm and 4000 nm, between 200 nm and 2000 nm, between 200 nm and 1600 nm, or between 400 nm and 900 nm. In an embodiment, the pump radiation has a repetition frequency between 1 kHz and 100 MHz.

[0132] According to a third aspect of the present invention, there is provided a method of generating broadband radiation, comprising: providing pump radiation; providing an optical waveguide having a hollow core along a longitudinal axis of the optical waveguide; introducing a working medium including molecules having a permanent dipole moment into the hollow core of the optical waveguide; propagating the pump radiation substantially within the hollow core of the optical waveguide so as to spectrally broaden the pump radiation as the pump radiation traverses the optical waveguide to generate broadband radiation; and applying an electric field across the working medium to orient the molecules of the working medium so as to control the spectral broadening of the pump radiation and thus the characteristics of the broadband radiation.

[0133] Further embodiments of the present invention are listed in the following numbered list of aspects:

[0134] 1. A radiation device for generating broadband radiation upon receipt of pump radiation, comprising: an optical waveguide having a hollow core along its longitudinal axis; wherein the optical waveguide is configured to substantially confine the pump radiation within the hollow core; a working medium including molecules having a permanent dipole moment, included within the hollow core of the optical waveguide; wherein the working medium is configured to spectrally broaden the pump radiation to generate the broadband radiation; and an electric field generating device for generating an electric field across the working medium; wherein the electric field is configured to orient the molecules of the working medium.

[0135] 2. The radiation device according to aspect 1, wherein the electric field generating device is configured such that the electric field extends in a transverse direction of the optical waveguide, the transverse direction being perpendicular to the longitudinal direction of the optical waveguide.

[0136] 3. The radiation device according to aspect 1 or 2, wherein the electric field generating device is configured such that at least a majority of the optical waveguide is included within the electric field.

[0137] 4. The radiation device according to aspect 3, wherein the electric field generating device is configured such that the entire optical waveguide is included within the electric field.

[0138] 5. The radiation device according to any one of the preceding aspects, wherein the electric field includes an electrostatic field.

[0139] 6. The radiation device according to any one of aspects 1 to 4, wherein the electric field includes an alternating electric field having an oscillation frequency.

[0140] 7. The radiation device according to aspect 6, wherein the oscillation frequency of the alternating electric field matches the rotational resonance of the molecules of the working medium.

[0141] 8. The radiation device according to aspect 6 or 7, wherein the oscillation frequency of the oscillation amplitude of the electric field is between 1 kHz and 10 THz.

[0142] 9. The radiation device according to any one of aspects 6 to 8, wherein the oscillation frequency of the oscillation amplitude of the electric field is between 5 GHz and 5 THz.

[0143] 10. The radiation device according to any one of the preceding aspects, wherein the electric field has an intensity of at least 10 kV / cm.

[0144] 11. The radiation device according to any one of the preceding aspects, wherein the electric field has an intensity of at least 100 kV / cm.

[0145] 12. The radiation device according to any one of the preceding aspects, wherein the electric field generating device is operable to generate the electric field at least during the period when the pump radiation traverses the optical waveguide.

[0146] 13. The radiation device according to any one of the preceding aspects, the electric field generating device is operable to start generating the electric field upon receiving a trigger signal in the timing derived from the pump radiation.

[0147] 14. The radiation device according to any one of the preceding aspects, the electric field generating device is operable to stop generating the electric field after a predetermined period has elapsed.

[0148] 15. The radiation device according to aspect 14, wherein the predetermined period is at least 1 nanosecond.

[0149] 16. The radiation device according to aspect 14 or 15, wherein the predetermined period is at least 10 nanoseconds.

[0150] 17. The radiation device according to any one of the preceding aspects, wherein the electric field generating device comprises: a pair of conductive plates placed on two opposite sides of the optical waveguide; and a power supply configured to apply a voltage across the pair of conductive plates to generate the electric field across the optical waveguide.

[0151] 18. The radiation device according to aspect 17, wherein the conductive plates are parallel to each other.

[0152] 19. The radiation device according to aspect 17 or 18, wherein the optical waveguide is included between the conductive plates.

[0153] 20. The radiation device according to any one of aspects 17 to 19, wherein the power supply of the electric field generating device comprises a radio frequency signal generator configured to provide an alternating voltage.

[0154] 21. The radiation device according to any one of aspects 17 to 19, wherein the power supply of the electric field generating device comprises a constant voltage power supply configured to provide a static voltage.

[0155] 22. The radiation device according to any one of aspects 17 to 21, wherein the electric field generating device further comprises a controller configured to automatically start and / or stop generating the electric field.

[0156] 23. The radiation device according to aspect 22, wherein the controller is configured to receive a control signal synchronized with a pump radiation generation signal for generating a pulse of the pump radiation.

[0157] 24. The radiation device according to any one of the preceding aspects, wherein the working medium is a liquid medium.

[0158] 25. The radiation device according to aspect 24, wherein the liquid medium is one of acetonitrile, acetone, and water.

[0159] 26. The radiation device according to any one of aspects 1 to 23, wherein the working medium is a gaseous medium.

[0160] 27. The radiation device according to aspect 26, wherein the gaseous medium is one of dimethyl ether, hydrogen iodide, methylamine, ketene, nitrosyl bromide, carbonyl borane, methyl bromide, hydrogen bromide, ethylamine, nitrosyl bromide, and trifluoromethane.

[0161] 28. The radiation device according to any one of the preceding aspects, wherein the permanent dipole moment of the molecules of the working medium is at least 1 Debye.

[0162] 29. The radiation device according to any of the foregoing aspects, wherein the molecules of the working medium have a polarizability of at least 1 C·m 2 ·V -1 .

[0163] 30. The radiation device according to any of the foregoing aspects, wherein the optical waveguide is a hollow-core photonic crystal fiber (PCF).

[0164] 31. A radiation source, comprising: the radiation device according to any of the foregoing aspects; and

[0165] a pump radiation source configured to output the pump radiation.

[0166] 32. The radiation source according to aspect 31, wherein the pump radiation is linearly polarized pulsed radiation.

[0167] 33. The radiation device according to aspect 32, wherein the electric field generating device is configured such that the electric field extends in a direction substantially parallel to the polarization direction of the pump radiation.

[0168] 34. The radiation source according to any one of aspects 31 to 33, wherein the pump radiation comprises electromagnetic radiation of one or more frequencies between 200 nm and 2000 nm.

[0169] 35. The radiation source according to any one of aspects 31 to 34, wherein the pump radiation has a repetition frequency between 1 kHz and 100 MHz.

[0170] 36. The radiation source according to any one of aspects 31 to 35, wherein the characteristics of the broadband radiation include a spectrum that at least partially overlaps with a range between 100 nm and 4000 nm.

[0171] 37. A measurement device, comprising the radiation source according to any one of aspects 31 to 36.

[0172] 38. The measurement device according to aspect 37, comprising a scatterometer measurement device, a level sensor, or an alignment sensor.

[0173] 39. A method of generating broadband radiation, comprising: providing pump radiation; providing an optical waveguide having a hollow core along a longitudinal axis of the optical waveguide; introducing a working medium including molecules having a permanent dipole moment into the hollow core of the optical waveguide; causing the pump radiation to propagate generally within the hollow core of the optical waveguide so as to spectrally broaden the pump radiation as the pump radiation traverses the optical waveguide to generate the broadband radiation; and applying an electric field across the working medium to orient the molecules of the working medium so as to control the spectral broadening of the pump radiation and thus control the characteristics of the broadband radiation.

[0174] 40. The method according to aspect 39, wherein the pump radiation is linearly polarized.

[0175] 41. The method according to aspect 40, wherein the electric field extends in a direction generally parallel to the polarization direction of the pump radiation.

[0176] 42. The method according to any one of aspects 39 to 41, wherein the electric field extends in a transverse direction of the optical waveguide, the transverse direction being perpendicular to the longitudinal direction of the optical waveguide.

[0177] 43. The method according to any one of aspects 39 to 42, wherein at least a majority of the optical waveguide is included within the electric field.

[0178] 44. The method according to any one of aspects 39 to 43, wherein the electric field is an electrostatic field.

[0179] 45. The method according to any one of aspects 39 to 44, wherein the electric field is an alternating electric field having an oscillation frequency.

[0180] 46. The method according to aspect 45, comprising matching the oscillation frequency of the alternating electric field with the rotational resonance of the molecules of the working medium.

[0181] 47. The method according to any one of aspects 39 to 46, wherein the electric field is applied at least during a period when the pump radiation traverses the optical waveguide.

[0182] 48. The radiation device according to aspect 47, comprising starting the application of the electric field upon receipt of a trigger signal in a timing derived from the pump radiation.

[0183] 49. The radiation device according to aspect 48, comprising terminating the application of the electric field after a predetermined period has elapsed.

[0184] 50. The method according to any one of aspects 39 to 49, wherein the working medium is a liquid medium.

[0185] 51. The radiation device according to aspect 50, wherein the liquid medium is one of acetonitrile, acetone, and water.

[0186] 52. The radiation device according to any one of aspects 39 to 49, wherein the working medium is a gaseous medium.

[0187] 53. The radiation device according to aspect 52, wherein the gaseous medium is one of dimethyl ether, hydrogen iodide, methylamine, ketene, nitrosyl bromide, carbonyl borane, methyl bromide, hydrogen bromide, ethylamine, nitrosyl bromide, and trifluoromethane.

[0188] Figure 12 is a block diagram of a computer system 1400 that can assist in implementing the methods and processes disclosed herein. Computer system 1400 includes a bus 1402 or other communication mechanism for communicating information, and a processor 1404 (or processors 1404 and 1405) coupled to bus 1402 for processing information. Computer system 1400 also includes a main memory 1406 coupled to bus 1402 for storing information and instructions to be executed by processor 1404, such as random access memory (RAM) or other dynamic storage. Main memory 1406 can also be used to store transient variables or other intermediate information during the execution of instructions to be executed by processor 1404. Computer system 1400 further includes a read only memory (ROM) 1408 or other static storage device coupled to bus 1402 for storing static information and instructions for processor 1404. A storage device 1410, such as a magnetic disk or optical disk, is provided and coupled to bus 1402 for storing information and instructions.

[0189] Computer system 1400 can be coupled by bus 1402 to a display 1412 for displaying information to a computer user, such as a cathode ray tube (CRT) or a flat panel or touch panel display. An input device 1414 including alphanumeric and other keys is coupled to bus 1402 for communicating information and command selections to processor 1404. Another type of user input device is a cursor controller 1416 for communicating direction information and command selections to processor 1404 and for controlling the movement of a cursor on display 1412, such as a mouse, trackball, or cursor direction keys. Such an input device typically has two degrees of freedom in two axes (a first axis (e.g., x) and a second axis (e.g., y)), which allows the device to specify a position in a plane. A touch panel (screen) display can also be used as an input device.

[0190] One or more of the methods described herein can be performed by a computer system 1400 in response to one or more sequences of one or more instructions included in main memory 1406 being executed by a processor 1404. These instructions can be read into main memory 1406 from another computer-readable medium, such as a storage device 1410. Execution of the sequences of instructions included in main memory 1406 causes the processor 1404 to perform the process steps described herein. One or more processors in a multiprocessing arrangement can also be used to execute the sequences of instructions included in main memory 1406. In alternative embodiments, hardwired circuitry can be used in place of or in combination with software instructions. Thus, the description herein is not limited to any specific combination of hardware circuitry and software.

[0191] The term "computer-readable medium" as used herein refers to any medium that participates in providing instructions to a processor 1404 for execution. Such a medium can take many forms, including but not limited to: non-volatile media, volatile media, and transmission media. Non-volatile media includes, for example, optical or magnetic disks, such as storage device 1410. Volatile media includes volatile memory, such as main memory 1406. Transmission media includes coaxial cables, copper wire, and fiber optics, including the wires that comprise bus 1402. Transmission media can also take the form of acoustic or light waves, such as those generated during radio frequency (RF) and infrared (IR) data communications. Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, a hard disk, a magnetic tape, any other magnetic medium, a CD-ROM, a DVD, any other optical medium, punch cards, paper tape, any other physical medium with patterns of holes, a RAM, a PROM, and an EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described hereinafter, or any other medium from which a computer can read.

[0192] Various forms of computer-readable media can be involved in carrying one or more sequences of one or more instructions to a processor 1404 for execution. For example, the instructions can initially be carried on a magnetic disk of a remote computer. The remote computer can load the instructions into its volatile memory and send the instructions using a modem via a telephone line. A modem local to computer system 1400 can receive the data on the telephone line and convert the data into an infrared signal using an infrared transmitter. An infrared detector coupled to bus 1402 can receive the data carried in the infrared signal and place the data on bus 1402. Bus 1402 carries the data to main memory 1406, from which the processor 1404 retrieves and executes the instructions. The instructions received by main memory 1406 can optionally be stored on storage device 1410 either before or after being executed by processor 1404.

[0193] The computer system 1400 also preferably includes a communication interface 1418 coupled to the bus 1402. The communication interface 1418 provides a two-way data communication coupling to a network link 1420 that is connected to a local area network 1422. For example, the communication interface 1418 can be an Integrated Services Digital Network (ISDN) card or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, the communication interface 1418 can be a local area network (LAN) card to provide a data communication connection to a compatible LAN. A wireless link can also be implemented. In any such implementation, the communication interface 1418 transmits and receives electrical, electromagnetic, or optical signals that carry digital data streams representing various types of information.

[0194] The network link 1420 generally provides data communication to other data devices via one or more networks. For example, the network link 1420 can provide a connection from the local area network 1422 to a main computer 1424 or to a data device operated by an Internet Service Provider (ISP) 1426. The ISP 1426 in turn provides data communication services via the global packet data communication network (now commonly referred to as the "Internet") 1428. Both the local area network 1422 and the Internet 1428 use electrical, electromagnetic, or optical signals that carry digital data streams. Signals via the various networks and signals on the network link 1420 and via the communication interface 1418, which carry digital data to and from the computer system 1400, are exemplary forms of carriers that convey information.

[0195] The computer system 1400 can send messages and receive data, including program code, from the (a) network, the network link 1420, and the communication interface 1418. In the Internet example, a server 1430 can transmit requested program code for an application via the Internet 1428, the ISP 1426, the local area network 1422, and the communication interface 1418. For example, one such downloaded application can provide one or more of the techniques described herein. The received code can be executed by the processor 1404 when it is received, and / or stored in the storage device 1410 or other non-volatile storage for later execution. In this manner, the computer system 1400 can obtain application code in the form of a carrier wave.

[0196] Although the use of a lithographic apparatus in IC manufacture may be specifically referred to herein, it should be understood that the lithographic apparatus 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.

[0197] Although embodiments of the invention may be specifically described herein with reference to the context of a lithographic apparatus, 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). Such apparatuses may generally be referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0198] Although the above may specifically refer to the use of embodiments of the invention in the context of optical lithography, it should be understood that the invention is not limited to optical lithography and may be used in other applications, such as imprint lithography, where the context allows.

[0199] 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. Accordingly, 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.

Claims

1. A radiation device for generating broadband radiation upon receipt of pump radiation, comprising: An optical waveguide having a hollow core along its longitudinal axis; wherein the optical waveguide is configured to confine the pump radiation substantially within the hollow core; A working medium including molecules having a permanent dipole moment, the working medium being included within the hollow core of the optical waveguide; wherein the working medium is configured to spectrally broaden the pump radiation to generate the broadband radiation; and An electric field generating device for generating an alternating electric field having an oscillation frequency across the working medium, wherein the electric field is configured to orient the molecules of the working medium and the oscillation frequency of the alternating electric field matches the rotational resonance of the molecules of the working medium.

2. The radiation device according to claim 1, wherein the electric field generating device is configured such that the electric field extends in a transverse direction of the optical waveguide, the transverse direction being perpendicular to the longitudinal direction of the optical waveguide.

3. The radiation device according to claim 1 or 2, wherein the electric field generating device is configured such that at least a majority of the optical waveguide is included within the electric field.

4. The radiation device according to claim 3, wherein the electric field generating device is configured such that the entire optical waveguide is included within the electric field.

5. The radiation device according to claim 1, wherein the oscillation frequency of the oscillation amplitude of the electric field is between 1 kHz and 10 THz.

6. The radiation device according to claim 1, wherein the oscillation frequency of the oscillation amplitude of the electric field is between 5 GHz and 5 THz.

7. The radiation device according to claim 1, wherein the electric field has an intensity of at least 10 KV / cm.

8. The radiation device according to claim 1, wherein the electric field has an intensity of at least 100 KV / cm.

9. The radiation device according to claim 1, wherein the electric field generating device is operable to generate the electric field at least during a period when the pump radiation traverses the optical waveguide.

10. The radiation device according to claim 1, the electric field generating device is operable to start generating the electric field upon receipt of a trigger signal derived from the timing of the pump radiation.

11. The radiation device according to claim 1, wherein the working medium includes one or more of the following: dimethyl ether, hydrogen iodide, methylamine, ketene, nitrosyl bromide, carbonyl borane, methyl bromide, hydrogen bromide, ethylamine, nitrosyl bromide, and trifluoromethane.

12. The radiation device according to claim 1, wherein the permanent dipole moment of the molecules of the working medium is at least 1 Debye.

13. The radiation device according to claim 1, wherein the molecules of the working medium have a polarizability of at least 1 C·m 2 ·V -1 of the polarizability.

14. The radiation device according to claim 1, wherein the optical waveguide is a hollow-core photonic crystal fiber (PCF).

15. A method for generating broadband radiation, comprising: providing pump radiation; providing an optical waveguide having a hollow core along a longitudinal axis of the optical waveguide; introducing a working medium including molecules having a permanent dipole moment into the hollow core of the optical waveguide; propagating the pump radiation substantially within the hollow core of the optical waveguide so as to spectrally broaden the pump radiation as the pump radiation traverses the optical waveguide to generate the broadband radiation; and applying an alternating electric field having an oscillation frequency matching the rotation resonance of the molecules across the working medium to orient the molecules of the working medium so as to control the spectral broadening of the pump radiation and thus control the characteristics of the broadband radiation.

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