Hollow-core optical fiber-based radiation source

Abstract

1. A broadband emission device comprising: a pulse shaper configured to impart a temporal profile to an input pump pulse to generate a temporally modulated pump pulse, the temporally modulated pump pulse having a different temporal profile than the input pump pulse; and a hollow-core photonic crystal fiber (HC-PCF) having a hollow core for confining a working medium under pressure during use, the HC-PCF being operable to receive the temporally modulated pump pulse, wherein the temporally modulated pump pulse is configured to undergo spectral broadening by a soliton self-compression process while propagating through the hollow core of the HC-PCF to form a broadband output radiation, the temporal profile being configured to configure a spectrum of the broadband output radiation to have a target spectral characteristic.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to European Patent Application No. 22167300.7, filed April 8, 2022, and European Patent Application No. 22171192.2, filed May 2, 2022, which are incorporated by reference in their entireties into this specification.

[0002] The present invention relates to hollow-core optical fiber based broadband radiation sources, and in particular to such broadband radiation sources relevant for metrology applications in the manufacture of integrated circuits. [Background technology]

[0003]

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

[0004]

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

[0005]

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

[0006]

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

[0007]

[0007] In a first aspect of the present invention, there is provided a broadband emission device comprising a pulse shaper configured to impart a temporal profile to an input pump pulse to generate a temporally modulated pump pulse, the temporally modulated pump pulse having a different temporal profile than the input pump pulse, and a hollow-core photonic crystal fiber (HC-PCF) having a hollow core for confining a working medium under pressure during use, the HC-PCF being operable to receive the temporally modulated pump pulse, wherein the temporally modulated pump pulse is configured to undergo increased spectral broadening by a soliton self-compression process while propagating through the hollow core of the HC-PCF to form a broadband output radiation, and the temporal profile is configured to configure the spectrum of the broadband output radiation to have a target spectral characteristic.

[0008]

[0008] In a second aspect of the present invention, a method is provided for optimizing a temporal profile of a pump pulse to increase spectral broadening via a soliton self-compression process within a working medium in a hollow-core photonic crystal fiber (HC-PCF) to form broadband output radiation, the method comprising optimizing the temporal profile to configure the broadband output radiation to have target spectral characteristics.

[0009]

[0009] In a third aspect of the present invention, there is provided a method for generating broadband output radiation having target spectral characteristics, the method comprising: imparting a temporal profile to an input pump pulse to generate a temporally modulated pump pulse, the temporally modulated pump pulse having a different temporal profile than the input pump pulse, the temporal profile being configured to configure a spectrum of the broadband output radiation to have the target spectral characteristics; and exciting a working medium in a hollow-core photonic crystal fiber (HC-PCF) using the temporally modulated pump pulse such that the temporally modulated pump pulse exhibits increased spectral broadening due to a soliton self-compression process to form the broadband output radiation having the target spectral characteristics.

[0010] Another aspect of the invention includes a metrology device including the broadband radiating device of the first aspect.

[0011]

[0011] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which: [Brief description of the drawings]

[0012] [Figure 1] 1 depicts a schematic overview of a lithographic apparatus; [Diagram 2] 1 shows a schematic overview of a lithography cell. [Diagram 3] A schematic diagram of Holistic Lithography is shown, illustrating the collaboration between three key technologies to optimize semiconductor manufacturing. [Figure 4] 1 depicts a schematic diagram of a scatterometry apparatus for use as a metrology device, which may include a radiation source according to an embodiment of the present invention; [Diagram 5] 1 shows a schematic diagram of a level sensor apparatus that may include a radiation source according to an embodiment of the present invention; [Figure 6] 1 shows a schematic diagram of an alignment sensor apparatus which may include a radiation source according to an embodiment of the present invention; [Figure 7]1 is a schematic cross-sectional view in a transverse plane (i.e. perpendicular to the axis of the optical fiber) of a hollow core optical fiber that may form part of a radiation source according to an embodiment; [Figure 8] 1 shows a schematic representation of a radiation source for providing broadband output radiation, according to an embodiment. [Figure 9a] 1A and 1B show schematic cross-sections of example hollow-core photonic crystal fiber (HC-PCF) designs for supercontinuum generation. [Figure 9b] 1A and 1B show schematic cross-sections of example hollow-core photonic crystal fiber (HC-PCF) designs for supercontinuum generation. [Figure 10A] 9 shows wavelength λ power spectral density plot (or signal sig(dB)) versus position P along the fiber length for a simulation illustrating the soliton self-compression based spectral evolution of a pulse of pump radiation within the hollow-core optical fiber of the radiation source shown in FIG. [Figure 10B] Figure 1 shows power spectral density plots (or signal sig(dB)) versus time t versus position P along the fiber length for a simulation illustrating the soliton self-compression based time evolution of a pulse of pump radiation (up to the maximum compression point) in a hollow-core optical fiber of the same source. [Figure 11] 9 illustrates an exemplary simulated spectrum of broadband output radiation generated by soliton self-compression based spectral broadening of pulses of pump radiation in the hollow-core optical fiber of the radiation source illustrated in FIG. 8 . [Figure 12A] We present a forward soliton self-compression based nonlinear process for converting an input pump pulse into broadband output radiation with undesired spectral characteristics. [Figure 12B] A backward soliton self-compression based nonlinear process is presented for converting broadband output radiation with target spectral characteristics into a target pump pulse with a desired or target temporal profile. [Figure 13]2 illustrates generally a first radiation source with a pulse shaper for providing broadband output radiation with targeted spectral characteristics according to an embodiment; [Figure 14] 13 illustrates diagrammatically a second radiation source with a pulse shaper for providing broadband output radiation with targeted characteristics according to an embodiment; [Figure 15] FIG. 15 is a flow diagram of a method for optimizing the temporal profile of a pump pulse for use in the secondary radiation source shown in FIG. 14. [Figure 16] FIG. 2 is a block diagram of a computer system for controlling a broadband radiation source. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013]

[0012] In this document, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including ultraviolet (e.g., having wavelengths of 365, 248, 193, 157 or 126 nm) and EUV (e.g., extreme ultraviolet having wavelengths in the range of about 5 to 100 nm).

[0014]

[0013] As used herein, the terms "reticle", "mask" or "patterning device" may be broadly interpreted to mean a general patterning device that can be used to provide an incident radiation beam with a patterned cross-section that corresponds to a pattern to be created on a target portion of a substrate. The term "light valve" may also be used in this context. In addition to classical masks (transmissive or reflective masks, binary masks, phase-shifting masks, hybrid masks, etc.), examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0015]

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

[0016]

[0015] In operation, the illumination system IL receives a radiation beam from a radiation source SO (e.g. via a beam delivery system BD). The illumination system IL may include various types of optical components for directing, shaping and / or controlling the radiation, 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 such that it has a desired spatial and angular intensity distribution in its cross-section in the plane of the patterning device MA.

[0017]

[0016] The term "projection system" PS as used herein should be broadly interpreted to encompass various types of projection systems. Such systems may include refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems or any combination thereof, as required by the exposure radiation being used and / or other factors (e.g., the use of an immersion liquid or the use of a vacuum). Where the term "projection lens" is used herein, it may all be considered as synonymous with the more general term "projection system" PS.

[0018]

[0017] The lithographic apparatus LA may be of a type in which at least a part of the substrate may be covered with a liquid having a relatively high refractive index (e.g. water) so as to fill a space between the projection system PS and the substrate W, which is also referred to as immersion lithography. Details of immersion techniques are given in US Pat. No. 6,952,253, which is incorporated herein by reference.

[0019] The lithographic apparatus LA may be of a type having two or more substrate supports WT (also referred to as “dual stage”). In such a “multiple stage” machine, the substrate supports WT may be used in parallel and / or a substrate W on one of the substrate supports WT may be used to expose a pattern thereon while a procedure is being performed on another substrate W on the other substrate support WT in preparation for a subsequent exposure of the other substrate W.

[0020] In addition to the substrate support WT, the lithographic apparatus LA may include a measurement stage. The measurement stage may be configured to hold a sensor and / or a cleaning apparatus. The sensor may be configured to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold a number of sensors. The cleaning apparatus may be configured to clean part of the lithographic apparatus, for example part of the projection system PS or part of a system for supplying immersion liquid. The measurement stage may move below the projection system PS when the substrate support WT is spaced apart from the projection system PS.

[0021] In operation, a radiation beam B is incident on a patterning device (e.g. a mask MA held on a mask support MT) and is patterned according to a pattern (design layout) on the patterning device MA. After traversing the mask MA, the radiation beam B passes through a projection system PS, which focuses the beam onto a target portion C of a substrate W. With the aid of a second positioner PW and a position measurement system IF, the substrate support WT can be precisely moved, e.g. such that different target portions C are positioned at focused and aligned positions in the path of the radiation beam B. Similarly, a first positioner PM and possibly another position sensor (not explicitly shown in FIG. 1 ) may be used to precisely position the patterning device MA relative to the path of the radiation beam B. The patterning device MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Substrate alignment marks P1, P2 occupy dedicated target portions as illustrated, but may be located in spaces between target portions. When substrate alignment marks P1, P2 are located between target portions C, they are referred to as scribe-lane alignment marks.

[0022]

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

[0023]

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

[0024]

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

[0025] Typically, the patterning process in the lithography apparatus LA is one of the most critical steps of the process, requiring high accuracy in the dimensioning and placement of structures on the substrate W. To ensure this high accuracy, three systems may be combined as a so-called "holistic" controlled environment, as shown diagrammatically in FIG. 3. One of these systems is the lithography apparatus LA, which is (virtually) connected with a metrology tool MT (second system) and a computer system CL (third system). The key to such a "holistic" environment is to optimize the coordination between these three systems to enforce the overall process window and achieve a tight control loop, so that the patterning performed by the lithography apparatus LA stays within the process window. The process window defines the range of process parameters (e.g., dose, focus, overlay) within which a particular manufacturing process produces a defined result (e.g., a functioning semiconductor device), and within which the process parameters of the lithography process or patterning process may typically vary.

[0026]

[0025] The computer system CL can predict which resolution enhancement technique should be used by using (part of) the design layout to be patterned, and can perform computational lithography simulations and calculations to determine the mask layout and lithography device settings that achieve the maximization of the overall process window of the patterning process (shown in FIG. 3 by the double-headed arrow at the first scale SC1). Typically, the resolution enhancement technique is prepared to match the patterning capability of the lithography device LA. The computer system CL can further predict whether defects may be present (e.g., due to suboptimal processing) by detecting (e.g., using input from the metrology tool MT) where in the process window the lithography device LA is currently operating (shown in FIG. 3 by the arrow pointing to "0" at the second scale SC2).

[0027]

[0026] The metrology tool MT is capable of providing input to the computer system CL enabling accurate simulation and prediction, and is capable of providing feedback to the lithographic apparatus LA identifying possible drift (e.g., in the calibration status of the lithographic apparatus LA) (as shown by multiple arrows at the third scale SC3 in Figure 3).

[0028]

[0027] In lithographic processes, it is desirable to frequently measure the created structures (e.g. for process control and verification). Tools that perform such measurements are commonly called metrology tools MT. Different types of metrology tools MT that perform such measurements are known, such as scanning electron microscopes or different forms of scatterometer metrology tools MT. A scatterometer is a multipurpose instrument that allows the measurement of parameters of the lithographic process, either by having a sensor in the pupil or in a conjugate plane to the pupil of the scatterometer objective (usually called pupil-based measurement) or by having a sensor in the image plane or in a conjugate plane to the image plane (usually called image-based or field-based measurement). Such scatterometers and associated measurement techniques are described in detail in U.S. Patent Application Publication Nos. 20100328655, 2011102753A1, 20120044470A, 20110249244, 20110026032, or EP 1,628,164A, which are incorporated by reference in their entireties. The scatterometers described above are capable of measuring gratings using soft x-rays and light in the visible to near infrared wavelength range.

[0029]

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

[0030]

[0029] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, radiation emitted from a radiation source is directed to a target, and radiation reflected or scattered from the target is directed to a spectrometer detector, which measures the spectrum of the specularly reflected radiation (i.e., the intensity as a function of wavelength). From this data, it is possible to reconstruct the structure or profile of the target causing the detected spectrum, for example by rigorous coupled wave theory and nonlinear regression, or by comparison with a library of simulated spectra.

[0031]

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

[0032]

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

[0033]

[0032] 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 US Patent Application No. 2011-0249244, which is incorporated herein by reference in its entirety. A single structure can be used that has a unique combination of critical dimension and sidewall angle measurements for each point of the focus-energy matrix (FEM, also called focus-exposure matrix). If these unique combinations of critical dimension and sidewall angle are available, focus and dose values ​​can be uniquely determined from these measurements.

[0034]

[0033] The metrology target may be a collection of composite gratings, mostly formed by lithography processes in resist, but also after, for example, etching processes. Typically, the pitch and line width of the structures of the gratings are strongly dependent on the measurement optics (specifically, the NA of the optics) so that the diffraction orders obtained from the metrology target can be captured. As previously shown, the diffraction signal can be used to determine the shift between the two layers (also called "overlay"), or to reconstruct at least a portion of the original grating as produced by the lithography process. This reconstruction can be used to provide guidance on the quality of the lithography process and can be used to control at least a portion of the lithography process. The target may have smaller sub-segmentations configured to mimic the dimensions of the features of the design layout in the target. This sub-segmentation makes the target behave more similarly to the features of the design layout, so that the overall process parameter measurements closely resemble the features of the design layout. The target can be measured in underfill or overfill mode. In underfill mode, the measurement beam generates a spot that is smaller than the entire target. In overfill mode, the measurement beam generates a spot that is larger than the entire target. In such an overfill mode, it may be possible to simultaneously measure different targets and therefore determine different process parameters at the same time.

[0035]

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

[0036]

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

[0037]

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

[0038] Another type of metrology tool used in IC manufacturing is a topography measurement system, level sensor, or height sensor. Such a tool may be incorporated into a lithography apparatus to measure the topography of the top surface of a substrate (or wafer). A map of the topography of the substrate (also called a height map) may be generated from these measurements showing the height of the substrate as a function of position on the substrate. This height map may later be used to correct the position of the substrate during transfer of a pattern on the substrate to provide an aerial image of the patterning device at the correct focus position on the substrate. It will be understood that "height" in this context refers broadly to the out-of-plane dimension relative to the substrate (also called the Z-axis). Typically, a level or height sensor measures at a fixed position (relative to its own optics), and relative movement of the substrate and the optics of the level or height sensor results in height measurements at locations across the substrate.

[0039]

[0038] An example of a level or height sensor LS known in the art is shown diagrammatically in Fig. 5, which shows only the principle of operation. 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, which provides a radiation beam LSB provided by a projection grating PGR of the projection unit LSP. The radiation source LSO may be, for example, a narrowband or broadband light source, such as a polarized or unpolarized, pulsed or continuous (such as a polarized or unpolarized laser beam) supercontinuum light source. The radiation source LSO may include multiple radiation sources (such as multiple LEDs) with different colors or wavelength ranges. The radiation source LSO of the level sensor LS is not limited to visible radiation, but may additionally or alternatively include UV and / or IR radiation, as well as any wavelength range suitable for reflection from the surface of the substrate.

[0040]

[0039] The projection grating PGR is a periodic grating including a periodic structure resulting in a radiation beam BE1 with a periodically varying intensity. The radiation beam BE1 with a periodically varying intensity is directed towards a measurement location MLO on the substrate W with an angle of incidence ANG relative to an axis normal to the incident substrate surface (Z-axis) between 0 degrees and 90 degrees, typically between 70 degrees and 80 degrees. At the measurement location MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and directed towards a detection unit LSD.

[0041]

[0040] To determine the height level at the measurement location MLO, the level sensor further comprises a detection system including a detection grid DGR, a detector DET and a processing unit (not shown) for processing an output signal of the detector DET. The detection grid DGR may be the same as the projection grid PGR. The detector DET generates a detector output signal indicative of the received light, for example indicative of the intensity of the received light (e.g. a photodetector) or representative of the spatial distribution of the received intensity (e.g. a camera). The detector DET may comprise any combination of one or more detector types.

[0042]

[0041] Triangulation techniques make it possible to determine the height level at the measurement location MLO. The detected height level is generally related to the signal intensity as measured by the detector DET, which has a periodicity that depends, among other things, on the design of the projection grating PGR and on the (oblique) angle of incidence ANG.

[0043]

[0042] The projection unit LSP and / or the detection unit LSD may include further optical elements, such as lenses and / or mirrors, along the path of the patterned radiation beam between the projection grating PGR and the detection grating DGR (not shown).

[0044]

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

[0045]

[0044] In order to effectively cover the surface of the substrate W, the level sensor LS may be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating an array of measurement areas MLO or spots covering a larger measurement range.

[0046]

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

[0047]

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

[0048]

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

[0049]

[0048] Figure 6 is a schematic block diagram of an embodiment of a known alignment sensor AS, for example as described in the incorporated US6961116. A radiation source RSO provides a radiation beam RB of one or more wavelengths, which is redirected by redirecting optics onto a mark (such as a mark AM located on a substrate W) as an illumination spot SP. In this example, the redirecting optics include a spot mirror SM and an objective lens OL. The illumination spot SP (by which the mark AM is illuminated) may be slightly smaller in diameter than the width of the mark itself.

[0050]

[0049] The radiation diffracted by the alignment mark AM is collimated (in this example by the objective lens OL) into an information-bearing beam IB. The term "diffracted" is intended to include zero order diffraction from the mark (which may be called reflection). A self-referencing interferometer SRI, for example of the type disclosed in the above-mentioned US Pat. No. 6,961,116, interferes the beam IB with itself, after which the beam is received by a photodetector PD. Further optical components (not shown) may be included to provide separate beams if more than one wavelength is produced by the radiation source RSO. The photodetector may be a single element, or it may include several pixels, as required. The photodetector may include a sensor array.

[0051]

[0050] The redirecting optical component (in this example, the redirecting optical component includes a spot mirror SM) may also function to block zero order radiation reflected from the mark so that the information-carrying beam IB contains only higher order diffracted radiation from the mark AM (this is not essential for the measurement, but improves the signal-to-noise ratio).

[0052]

[0051] The intensity signal SI is fed to a processing unit PU. The combination of optical processing in block SRI and computational processing in unit PU results in values ​​of the X and Y positions on the substrate relative to a reference frame.

[0053]

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

[0054]

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

[0055]

[0054] A metrology tool MT, such as the scatterometer, topography measurement system or position measurement system described above, may use radiation originating from a radiation source to perform measurements. The characteristics of the radiation used by the metrology tool may affect the type and quality of measurements that may be performed. For some applications, it may be advantageous to use multiple radiation frequencies to measure the substrate, for example broadband radiation may be used. Multiple different frequencies may be capable of propagating, illuminating and scattering from the metrology target with no or minimal interference with other frequencies. Thus, for example, different frequencies may be used to simultaneously obtain more metrology data. Different radiation frequencies may also be capable of interrogating or discovering different characteristics of the metrology target. Broadband radiation may be useful in metrology systems MT, such as level sensors, alignment mark measurement systems, scatterometry tools or inspection tools. The broadband radiation source may be a supercontinuum source.

[0056]

[0055] High quality broadband radiation (e.g., supercontinuum radiation) can be difficult to generate. One way to generate broadband radiation can be to broaden high power narrowband or single frequency input radiation or pump radiation, for example, by utilizing nonlinear and higher order effects. The input radiation (which can be generated using a laser) can be called pump radiation. Alternatively, the input radiation can be called seed radiation. To obtain high power radiation for broadening effects, the radiation can be confined within a small area such that strong localized high intensity radiation is achieved. In those areas, the radiation can interact with a broadening structure and / or material that forms a nonlinear medium, and broadband output radiation can be produced. In high intensity radiation areas, different materials and / or structures can be used to enable and / or improve radiation broadening by providing a suitable nonlinear medium.

[0057]

[0056] In some implementations, broadband output radiation is produced in a photonic crystal fiber (PCF). In some embodiments, such a photonic crystal fiber has a microstructure along the circumferential direction of its fiber core to help confine radiation traveling in the fiber core through the fiber. The fiber core can be made of a solid material that has nonlinear properties and can generate broadband radiation when high-intensity pump radiation is transmitted through the fiber core. Although it is possible to generate broadband radiation in a solid-core photonic crystal fiber, the use of solid materials can have some disadvantages. For example, if UV radiation is generated in a solid core, it may not be seen in the output spectrum of the fiber because most of the radiation is absorbed by the solid material.

[0058]

[0057] In some implementations, as further discussed below with reference to FIG. 8, the method and apparatus for spreading the input radiation can use a fiber for confining the input radiation and spreading the input radiation into an output broadband radiation. The fiber can be a hollow core fiber and can include an internal structure for achieving effective guiding and confinement of the radiation in the fiber. The 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 fiber and can achieve high radiation intensity. The hollow core of the fiber can be filled with a gas that acts as a spreading medium for spreading the input radiation. Such a fiber and gas arrangement can be used to create a supercontinuum radiation source. The radiation input to the fiber can be electromagnetic radiation, for example, radiation in one or more of the infrared, visible, UV and extreme UV spectrums. The output radiation can consist of or include broadband radiation, which can be referred to herein as white light.

[0059]

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

[0060]

[0059] Several types of HC-PCFs can be engineered, each based on a different physical guidance mechanism. Two such HC-PCFs include hollow-core photonic bandgap fiber (HC-PBF) and hollow-core anti-resonant reflecting fiber (HC-ARF). Details regarding the design and fabrication of HC-PCFs can be obtained from U.S. Patent No. 2004 / 015085A1 (for HC-PBF) and International PCT Patent Application Publication No. WO 2017 / 032454A1 (for hollow-core anti-resonant reflecting fiber), which are incorporated herein by reference. Figure 9(a) shows a Kagome fiber including a Kagome lattice structure.

[0061]

[0060] An example of an optical fiber for use in a radiation source will now be described with reference to Fig. 7. Fig. 7 is a schematic cross-sectional view in transverse section of an optical fiber OF. Further embodiments similar to the practical application of the fiber of Fig. 7 are disclosed in WO 2017 / 032454 A1.

[0062]

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

[0063]

[0062] It will be understood that the optical fiber OF has a degree of flexibility, and thus the axial direction will generally not be uniform along the length of the optical fiber OF. Terms such as optical axis, cross section, and the like will be understood to mean local optical axis, local cross section, etc. Furthermore, where a component is described as being cylindrical or tubular, it will be understood that these terms encompass shapes that would be distorted if the optical fiber OF were bent.

[0064]

[0063] It will be understood that the optical fiber OF may have any length, and 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. In an embodiment, the optical fiber OF may be a tapered fiber that may include a waist section. The waist section may include a down taper section where the diameter of the fiber decreases. In an embodiment, the waist section may further include a central region with a constant diameter. In an embodiment, the waist section may further include an up taper section where the diameter of the fiber increases to the original diameter. The length of the tapered waist section may extend, for example, from a few millimeters to tens of centimeters. By varying the fiber parameters during fiber drawing, it is also possible to provide a tapered waist section with a long length, for example, tens of meters.

[0065]

[0064] The optical fiber OF includes a hollow core HC, a cladding portion surrounding the hollow core HC, and a support portion SP surrounding and supporting the cladding portion. The optical fiber OF can be considered to include a body (including the cladding portion and the support portion SP) having the hollow core HC. The cladding portion includes a number of anti-resonant elements for guiding radiation through the hollow core HC. Specifically, the number of anti-resonant elements are arranged to confine radiation propagating through the optical fiber OF mainly within the hollow core HC and guide the radiation along the optical fiber OF. The hollow core HC of the optical fiber OF can be substantially located in a central region of the optical fiber OF, such that the axis of the optical fiber OF can also define the axis of the hollow core HC of the optical fiber OF.

[0066]

[0065] The cladding portion includes a number of anti-resonant elements for guiding radiation propagating through the optical fiber OF. Specifically, in this embodiment, the cladding portion includes a single ring of six capillary tubes CAP. Each of the capillary tubes CAP acts as an anti-resonant element.

[0067]

[0066] The capillary CAP may also be referred to as a tube. The capillary CAP may be circular in cross section or have another shape. Each capillary CAP includes a generally cylindrical wall portion WP, which at least partially defines the hollow core HC of the optical fiber OF and separates the hollow core HC from the capillary cavity CC. It will be appreciated that the wall portion WP may act as an anti-reflection Fabry-Perot resonator for radiation propagating through the hollow core HC (which may be incident on the wall portion WP at an oblique incidence angle). The thickness of the wall portion WP may be suitable to ensure that transmission into the capillary cavity CC is generally suppressed, while reflection back into the hollow core HC is generally enhanced. In some embodiments, the capillary wall portion WP may have a thickness of 0.01 to 10.0 μm.

[0068]

[0067] It will be understood that the term cladding portion, as used herein, is intended to mean the portion of the optical fiber OF for guiding the 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 fiber axis).

[0069]

[0068] The support portion is generally tubular and supports six capillary CAPs of the cladding portion. The six capillary CAPs are evenly distributed around the circumference of the inner surface in the case of the inner support portion SP. The six capillary CAPs may be described as being arranged in a general hexagonal configuration.

[0070]

[0069] The capillary CAPs are arranged such that each capillary does not contact any other capillary CAP. Each capillary CAP contacts the inner support portion SP and is spaced apart from adjacent capillary CAPs in a ring structure. Such an arrangement may be beneficial because it may increase the transmission bandwidth of the optical fiber OF (e.g., compared to an arrangement in which the capillaries are in contact with each other). Alternatively, in some embodiments, each capillary CAP may contact adjacent capillary CAPs in a ring structure.

[0071]

[0070] The six capillary CAPs of the cladding section are arranged in a ring structure along the circumferential direction of 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 may be defined as the smallest dimension between the opposing capillaries and is indicated by the arrow d) may be 10 to 1000 μm. The diameter d of the hollow core HC may affect the mode field diameter, collision loss, dispersion, mode plurality and nonlinearity characteristics of the hollow core HC optical fiber OF.

[0072]

[0071] In this embodiment, the cladding portion includes a single ring arrangement of capillary CAPs (which act as anti-resonant elements), so that any radial line from the center of the hollow core HC to the outside of the optical fiber OF passes through only one capillary CAP.

[0073] It will be appreciated that other embodiments may be provided with different arrangements of anti-resonant elements. These may include arrangements with multiple rings of anti-resonant elements, or arrangements with nested anti-resonant elements. FIG. 9(a) shows an embodiment of an HC-PCF with three rings of capillary CAPs stacked along the radial direction. In this embodiment, each capillary CAP is in contact with other capillaries, both in the same ring and in different rings. Furthermore, while the embodiment shown in FIG. 7 includes one ring containing six capillaries, in other embodiments, the cladding portion may be provided with one or more rings containing any number of anti-resonant elements (e.g., 4, 5, 6, 7, 8, 9, 10, 11, or 12 capillaries).

[0074]

[0073] Figure 9(b) shows a modified embodiment of the HC-PCF discussed above with a single ring of capillary tubes. In the example of Figure 9(b), there are two concentric rings of capillary tubes 21. To hold the inner and outer rings of capillary tubes 21, the HC-PCF may include a support tube ST. The support tube may be made of quartz.

[0075]

[0074] The capillary tubes in the examples of Figures 7 and 9(a), (b) may have a circular cross-sectional shape. Other shapes such as elliptical or polygonal cross-sections are also possible for the capillary tubes. Furthermore, the solid material of the capillary tubes in the examples of Figures 7 and 9(a), (b) may include plastic materials such as PMA, glass such as quartz, or soft glass.

[0076]

[0075] Figure 8 shows 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 radiation source capable of generating short pulses of a desired length and energy level, an optical fiber OF having a hollow core HC (e.g. of the type shown in Figure 7), and a working medium WM (e.g. a gas) disposed within the hollow core HC. In Figure 8, the radiation source RDS includes the optical fiber OF shown in Figure 7, but in alternative embodiments other types of hollow core HC optical fibers OF can be used.

[0077]

[0076] The pulsed pump radiation source PRS is configured to provide an input radiation IRD. The hollow core HC of the optical fiber OF is arranged to receive the input radiation IRD from the pulsed pump radiation source PRS and to spread the input radiation IRD to provide an output radiation ORD. The working medium WM enables spreading a frequency range of the received input radiation IRD to provide a broadband output radiation ORD.

[0078]

[0077] The radiation source RDS further includes a reservoir RSV. The optical fiber OF is disposed within the reservoir RSV. The reservoir RSV may also be referred to as a housing, a container, or a gas cell. The reservoir RSV is configured to contain a working medium WM. The reservoir RSV may include one or more features known in the art for controlling, modulating, and / or monitoring the composition of the working medium WM (which may be a gas) within the reservoir RSV. The reservoir RSV may include a first transparent window TW1. In use, the optical fiber OF is disposed within the reservoir RSV such that the first transparent window TW1 is located immediately adjacent to the input end IE of the optical fiber OF. The first transparent window TW1 may form a part of the wall of the reservoir RSV. The first transparent window TW1 may be at least transparent to the received input radiation frequency, such that the received input radiation IRD (or at least a majority thereof) can be coupled to the optical fiber OF located within the reservoir RSV. It will be appreciated that optics (not shown) can be provided for coupling the input radiation IRD into the optical fiber OF.

[0079]

[0078] The reservoir RSV includes a second transparent window TW2 forming part of the wall of the reservoir RSV. In use, when the optical fiber OF is placed in the reservoir RSV, the second transparent window TW2 is located in close proximity to the output end OE of the optical fiber OF. The second transparent window TW2 may be transparent at least to the frequency of the broadband output radiation ORD of the device 120.

[0080]

[0079] Alternatively, in another embodiment, both ends of the optical fiber OF can be placed in different reservoirs. The optical fiber OF can 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 can be placed in a first reservoir containing the working medium WM. The second end section can be placed in a second reservoir, which can also contain the working medium WM. The function of the reservoirs can be as described with respect to FIG. 8 above. The first reservoir can include a first transparent window configured to be transparent to the input radiation IRD. The second reservoir can include a second transparent window configured to be transparent to the broadband output broadband radiation ORD. The first and second reservoirs can also include sealable openings to allow the optical fiber OF to be placed partially inside and partially outside the reservoirs, so that the gas can be sealed in the reservoirs. The optical fiber OF may further include an intermediate section that is not contained within the reservoir. Such an arrangement using two separate gas reservoirs may be particularly convenient for embodiments in which the optical fiber OF is relatively long (e.g., when the length is greater than 1 m). In such an arrangement using two separate gas reservoirs, it will be understood that the two reservoirs (which may include one or more features known in the art for controlling, modulating and / or monitoring the composition of the gas in the two reservoirs) are considered to provide a device for providing the working medium WM within the hollow core HC of the optical fiber OF.

[0081] In this context, a window may be transparent to a frequency if at least 50%, 75%, 85%, 90%, 95% or 99% of radiation incident on that frequency on the window is transmitted through the window.

[0082]

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

[0083]

[0082] The working medium WM may include rare gases such as argon, krypton and xenon, Raman active gases such as hydrogen, deuterium and nitrogen, or gas mixtures such as argon / hydrogen mixtures, xenon / deuterium mixtures, krypton / nitrogen mixtures or nitrogen / hydrogen mixtures. Depending on the type of filling gas, nonlinear optical processes may include modulation instability (MI), soliton self-compression, soliton splitting, Kerr effect, Raman effect and dispersive wave generation (DWG), the details of which are described in WO 2018 / 127266 A1 and US 9160137 B1 (both of which are incorporated herein by reference). The dispersion of the filling gas can be adjusted by varying the pressure of the working medium WM in the reservoir RSR (i.e., the gas cell pressure), so that the resulting broadband pulse dynamics and associated spectral broadening characteristics can be adjusted to optimize frequency conversion.

[0084]

[0083] In one implementation, the working medium WM may be located in the hollow core HC at least while the optical fiber OF receives the input radiation IRD for generating the broadband output radiation ORD. It will be appreciated that the gas WM may be totally or partially absent from the hollow core HC while the optical fiber OF does not receive the input radiation IRD for generating the broadband output radiation.

[0085]

[0084] To achieve frequency spreading, high-intensity radiation is desirable. The advantage of having a hollow-core HC optical fiber OF is that it can achieve high-intensity radiation through strong spatial confinement of radiation propagating through the optical fiber OF, and can achieve localized high-intensity radiation. The radiation intensity in the optical fiber OF can be high, for example, due to the high-intensity input radiation received and / or the strong spatial confinement of radiation in the optical fiber OF. The advantage of hollow-core optical fibers is that they can guide radiation with a wider wavelength range, such that solid-core fibers, and specifically hollow-core optical fibers, can guide radiation in both the ultraviolet and infrared ranges.

[0086]

[0085] The advantage of using a hollow core HC optical fiber OF may be that most of the radiation guided in the optical fiber OF is confined to the hollow core HC. Therefore, most of the interaction of the radiation in the optical fiber OF occurs with the working medium WM provided in the hollow core of the optical fiber OF. As a result, the spreading effect of the radiation by the working medium WM can be enhanced.

[0087]

[0086] The received input radiation IRD may be electromagnetic radiation. The input radiation IRD may be received as pulsed radiation. For example, the input radiation IRD may comprise an ultrafast pulse, for example generated by a laser.

[0088]

[0087] The input radiation IRD may be coherent radiation. The input radiation IRD may be collimated radiation, the advantage of which may be to facilitate and improve the efficiency of coupling the input radiation IRD into the optical fiber OF. The input radiation IRD may include a single frequency or a narrow range of frequencies. The input radiation IRD may be generated by a laser. Similarly, the output radiation ORD may be collimated and / or coherent.

[0089]

[0088] The broadband range of the output radiation ORD may be a continuous range, including a continuous range of radiation frequencies. The output radiation ORD may include supercontinuum radiation. The continuous radiation may be beneficial for use in many applications (e.g., metrology applications). For example, a continuous range of frequencies may be used for interrogating a large number of properties. A continuous range of frequencies may be used, for example, to determine and / or eliminate frequency dependence of a measured property. The supercontinuum output radiation ORD may include electromagnetic radiation over a wavelength range of, for example, 100 nm to 4000 nm. The frequency range of the broadband output radiation ORD may be, for example, 400 nm to 900 nm, 500 nm to 900 nm, or 200 nm to 2000 nm. The supercontinuum output radiation ORD may include white light.

[0090]

[0089] The input radiation IRD provided by the pulsed pump radiation source PRS may be pulsed. The input radiation IRD may comprise electromagnetic radiation at one or more frequencies between 200 nm and 2 μm. The input radiation IRD may comprise electromagnetic radiation having a wavelength of, for example, 1.03 μm. The repetition rate of the pulsed radiation IRD may be as high as 1 kHz to 100 MHz. The pulse energy may have an amount of 0.1 μJ to 100 μJ (for example, 1 to 10 μJ). The pulse duration of the input radiation IRD may be 10 fs to 10 ps (for example, 300 fs). The average power of the input radiation IRD may be from 100 mW to several hundred W. The average power of the input radiation IRD may be, for example, 20 to 50 W.

[0091]

[0090] The pulsed pump radiation source PRS may be a laser. The spatiotemporal transmission characteristics (e.g., its spectral amplitude and phase) of such laser pulses transmitted along the optical fiber OF can be changed and adjusted through adjustment of (pump) laser parameters, working component WM variations and optical fiber OF parameters. The spatiotemporal transmission characteristics may include one or more of the following: output power, output mode profile, output temporal profile, width of the output temporal profile (or output pulse width), output spectral profile and bandwidth of the output spectral profile (or output spectral bandwidth). The pulsed pump radiation source PRS parameters may include one or more of the following: pump wavelength, pump pulse energy, pump pulse width, pump pulse repetition rate. The optical fiber OF parameters may include one or more of the following: length of the optical fiber, size and shape of the hollow core HC, size and shape of the capillary, thickness of the wall of the capillary surrounding the hollow core HC. The working component WM (e.g., filling gas) parameters may include one or more of the following: type of gas, pressure of the gas and temperature of the gas.

[0092]

[0091] The broadband output radiation ORD provided by the radiation source RDS may have an average output power of at least 1 W. The average output power may be at least 5 W. The average output power may be at least 10 W. The broadband output radiation ORD may be a pulsed broadband output radiation ORD. The broadband output radiation ORD may have a power spectral density of at least 0.01 mW / nm across the wavelength band of the output radiation. The power spectral density of the broadband output radiation across the wavelength band may be at least 3 mW / nm.

[0093]

[0092] As explained above, there are many nonlinear optical processes involved in the generation of broadband output radiation ORD (e.g., supercontinuum or white light). Which nonlinear optical process has more prominent spectral broadening effect than others depends on how the operating parameters are set. For example, by selecting the pump wavelength and / or the optical fiber OF such that the pump pulse propagates in the normal dispersion region (positive group velocity dispersion (GVD)) through the fiber, self-phase modulation becomes the dominant nonlinear optical process and is responsible for the spectral broadening of the pump pulse. However, in most cases, the spectral broadening of the input radiation IRD provided by the pulsed pump radiation source PRS is caused by soliton dynamics and requires that the pump pulse propagates in the anomalous dispersion region (negative GVD) of the optical fiber OF. The reason for this is that in the anomalous dispersion region, the effects of Kerr nonlinearity and dispersion act against each other. When the pulse parameters of a pump pulse launched into an optical fiber OF (e.g., HC-PCF) with anomalous chromatic dispersion do not exactly match those of a soliton, the pump pulse evolves into a soliton pulse with a specific soliton order and dispersive wave.

[0094]

[0093] Soliton self-compression and modulation instability are known to be the two main mechanisms for spectral broadening in soliton-based broadband radiation generation. The distinction between the two mechanisms is that the soliton self-compression process is associated with low soliton orders, whereas the modulation instability process is associated with high soliton orders. The soliton order N of the pulsed input radiation IRD is a convenient parameter that can be used to distinguish between situations where modulation instability dominates the spectral broadening and situations where soliton self-compression dominates the spectral broadening. The soliton order N of the pulsed input radiation IRD is given by:

number

[0095]

[0094] Spectral broadening is typically dominated by modulation instability when N>>20, whereas spectral broadening is typically dominated by soliton self-compression when N<<20.

[0096]

[0095] Some known broadband radiation sources use an arrangement that produces a spectral broadening of the pulsed pump radiation, but the parameters of the pulsed pump radiation, the optical fiber and the working medium are configured to allow the spectral broadening to be produced by modulation instability. There are several reasons for using modulation instability to produce spectral broadening. First, modulation instability is known to produce broadband radiation with a relatively flat intensity-wavelength distribution, provided that a sufficient number of pulses are averaged. Such a broadband radiation source can be called a white light radiation source (due to the relatively flat spectral intensity distribution). Second, modulation instability can be achieved using a relatively economical laser source as the pump radiation source.

[0097]

[0096] On the other hand, in the soliton self-compression regime, the input pump pulse undergoes compression in the time domain, which is accompanied by an increase in the spectral width. Following soliton self-compression, the compressed pulse undergoes soliton fission, in which the pulse splits into multiple solitons. This soliton fission results in a temporal broadening and spectral shift of the radiation pulse.

[0098] In contrast to noise-seeded modulationally unstable systems, the broadband radiation generated by such soliton self-compression is substantially free of shot-to-shot variations. This is because in the soliton self-compression process, moderate nonlinearities and anomalous dispersion cooperate to gradually and smoothly compress the duration of the input pump pulse along the fiber, whereas in the MI process, the strong amplification of noise breaks down the input pump pulse into a sequence of random sub-pulses ("soliton shower"). As a result, advantageously, in soliton self-compression systems, each single compressed pulse is characterized by having essentially the same broadband spectrum. In contrast, modulationally unstable systems require integration over many pulses to obtain a broadband spectrum and reduce its inherent shot-to-shot variations. The intensity noise of the broadband output radiation ORD generated from a soliton self-compression system can be several orders of magnitude (e.g., two orders of magnitude) lower than that of the broadband output radiation ORD generated from a modulationally unstable system, and is typically limited by the noise of the pump laser.

[0099]

[0098] As can be seen from equation (1), the soliton order of the input pulsed pump radiation is proportional to the pulse duration τ of the pulsed input radiation IRD. Therefore, in the prior art arrangements where soliton self-compression is generally dominant, the pulse duration τ of the input pulsed input radiation IRD is typically reduced to about 30 fs or less. To realize such an arrangement, a compressed high-power femtosecond fiber laser (e.g., a fiber-based chirped pulse amplification system (FCPA)) or a Ti:sapphire amplifier is typically used as the pulsed pump radiation source PRS. Such laser systems are relatively bulky (the head of a femtosecond fiber laser has, for example, dimensions of 60 x 40 x 20 cm) and in most cases require an external controller and water cooler. Furthermore, such laser systems are relatively costly.

[0100]

[0099] Instead, the inventors have determined that the pulse energy E p (E p =P pWe have realized that the soliton order of a pulsed input radiation IRD can be reduced by reducing the pulse energy E p By reducing by a factor of α, the same soliton order can be achieved using an increased pulse duration by a factor of α. This approach relaxes the requirement of short pulse durations (e.g., 30 fs or less) for soliton self-compression-based spectral broadening. As a result, it is possible to use significantly longer pulse durations (e.g., 100 fs or more) and therefore more economical laser sources for soliton self-compression.

[0101] [000100] In the soliton self-compression regime (relatively low soliton number), the pulse of input radiation IRD undergoes significant time compression, which is accompanied by spectral broadening. Eventually, the time compression reaches a maximum level (corresponding to the minimum time extent of the pulse radiation), followed by a time broadening of the radiation (soliton splitting). As the soliton (higher order) propagates along the hollow core optical fiber, it oscillates between periods of time compression and time broadening. Following the time broadening, other effects can lead to a shift in the spectrum of the radiation. For example, self-steepening (which may accompany or assist soliton self-compression) can lead to optical impulses that can seed dispersive wave emission. By adjusting the parameters of the system, a specific desired wavelength can be generated. For example, the wavelength can be selected to be suitable for interaction with a specific molecule and used in research experiments to study that molecule. Soliton self-compression is thus a known regime for generating an output radiation beam with a second shifted wavelength from an input pump laser beam with a first wavelength.

[0102] [000101] Figure 10A shows a wavelength λ power spectral density plot (or signal sig(dB)) versus position P along the fiber length for a simulation to illustrate the soliton self-compression based spectral evolution of a pulse of radiation in the hollow-core HC optical fiber OF of the radiation source RDS shown in Figure 8. The hollow-core HC optical fiber OF (e.g., the single-ring HC-ARF shown in Figure 7) has a length of 150 cm and a core diameter of 32.5 μm. The hollow core HC can be filled with a working medium WM of krypton gas at a pressure of 10 bar. In this example, the pulsed input radiation IRD has a pump pulse duration τ of 150 fs and a pulse energy E p The pulse energy E is 0.4 μJ and the wavelength λ is 1030 nm. p is about an order of magnitude lower than that currently used in broadband sources due to modulation instability. This configuration allows the 2 / cm). The soliton order of N=17 allows soliton self-compression of the pulsed input radiation IRD to modify the spectrum of the pulsed pump radiation to form a broadband output radiation ORD.

[0103] [000102] Referring to Figure 10A, in a first portion of the hollow-core HC optical fiber OF (e.g., between the input end IE of the fiber OF and a position about 100 cm from the input end IE), the input radiation IRD undergoes self-phase modulation 140 that continuously broadens the spectrum of the input radiation IRD. This is followed by soliton self-compression 142, as shown in Figure 10B, where the time range of the input radiation IRD is minimized at a distance of about 110 cm from the input end IE of the hollow-core HC optical fiber OF. The soliton self-compression is accompanied by a significant broadening 144 of the spectrum of the radiation. That is, the width of the spectrum of the input radiation IRD is maximum while the time range of the input radiation IRD is minimum.

[0104] [000103] In general, soliton self-compression may be followed by a reduction in the spectral width of the pulsed input radiation IRD and / or the appearance of spectral gaps (e.g., with soliton evolution and with emission of dispersive waves). Looking at Figure 10A, it can be seen that following soliton self-compression and associated spectral broadening 144, the spectrum of the radiation undergoes several changes. For example, dispersive waves 152 are emitted and the radiation oscillates between periods of spectral compression and spectral broadening.

[0105] [000104] One drawback of soliton self-compression based spectral broadening is the low conversion efficiency from pulsed input radiation IRD to the visible wavelength range (e.g., 400 nm to 700 nm). This is because the broadband spectrum generated by ideal solitons is typically entirely separate. 2 This is because it has a spectral profile with most of the energy concentrated near the pump wavelength (eg, 1030 nm) and a tail reaching into the visible wavelength range.

[0106] [000105] Figure 11 shows an exemplary simulated spectrum of broadband output radiation ORD generated by soliton self-compression based spectral broadening of pulses of pump radiation in the hollow-core optical fiber of the radiation source shown in Figure 8. The x-axis is the wavelength λ of the input radiation IRD spectrally broadened in the self-compression regime discussed above. The y-axis represents the amount of power per nanometer or power spectral density (PSD) of the spectrally broadened radiation normalized to its maximum value (e.g., 1090 nm, above the wavelength of the input radiation IRD). As shown in Figure 11, the PSD of the exemplary spectrum decays exponentially away from the pump wavelength (e.g., 1030 nm), which strongly limits the conversion efficiency into the visible wavelength range. The exemplary spectrum is not flat, since it has a relatively high peak PK in the wavelength region of the generated dispersive wave (e.g., dispersive wave 152 shown in Figure 10A) and a relatively low valley VR immediately after the peak PK. In this particular case, the peak PK is centered near 390 nm, and the valley VR has a dip near 470 nm. The normalized PSD of the peak PK (e.g., about 0.06) is about 11 times higher than the PSD of the trough of the valley VR (e.g., about 0.0057). Furthermore, the spectrum from 470 nm to 700 nm contains less than 2% of the integrated complete PSD, which translates to an average absolute PSD of less than 0.5 mW / nm for a typical pump laser system delivering 5 W of average power, which is too low for use in many metrology tools (e.g., the scatterometry-based metrology tools mentioned above).

[0107] [000106] Therefore, the present disclosure proposes a method and apparatus for further improving the spectral flatness of the broadband output radiation ORD generated in the soliton self-compression regime as well as further increasing the PSD of the spectrum in the visible wavelength range. Such improvement can be achieved by using a method for optimizing the temporal profile of pump pulses that increase the spectral broadening via the soliton self-compression process in the working medium in the HC-PCF to form the broadband output radiation, including optimizing the temporal profile to configure the broadband output radiation to have target spectral characteristics.

[0108] [000107] FIG. 12A shows a forward soliton self-compression process for converting an input pump pulse IP (e.g., having a Gaussian time profile) into a broadband output radiation ORD with undesired spectral characteristics US (e.g., a non-flat spectral profile as shown in FIG. 11). The forward soliton self-compression process is a standard process on which soliton self-compression systems are based. The input pump pulse IP may be a part of the pulsed pump radiation emitted from the pulsed pump radiation source PRS. In the forward process, the input pump pulse IP is injected into the input end of the HC-PCF filled with the working medium, and can undergo a soliton self-compression process that converts the input pump pulse IP into a broadband output radiation ORD with undesired spectral characteristics US. Finally, the broadband output radiation ORD can exit from the output end of the same fiber.

[0109] [000108] The inventors have found that one of the differences in characteristics between the soliton self-compression process and the MI process is that the physical process of soliton self-compression is deterministic. This means that the pulse evolution can be numerically time-reversed. In other words, it is possible to reconstruct the initial input pulse (e.g., the input pulse IP shown in FIG. 12A) by numerically backpropagating the broadband output radiation ORD (e.g., the broadband output radiation ORD having the undesirable spectral characteristics US shown in FIG. 12A) generated through the soliton self-compression process through the same fiber under the same conditions. The inventors have found that this deterministic behavior of the soliton self-compression process may allow the characteristics of the broadband output radiation ORD to be tailored to specific requirements (e.g., the requirements of a specific metrology tool). For example, in a scatterometry-based metrology tool, it may be desirable to use a broadband spectrum with a flat spectral profile (e.g., maximum fluctuation <3 dB) and high average PSD (e.g., >5 mW / nm) in a given wavelength range (e.g., 400 nm to 900 nm, or at least 500 nm to 800 nm). Such a desired broadband spectrum can be used to perform a numerical calculation of the temporal profile of the input pump pulse that increases the spectral broadening via a forward soliton self-compression process (e.g., as shown in FIG. 12A) in the working medium in the HC-PCF to form a broadband output radiation ORD.

[0110] [000109] FIG. 12B conceptually illustrates a backward soliton self-compression process for numerically transforming a broadband output radiation ORD having a target spectral characteristic TS (e.g., the spectral profile shown by the dashed line) into a target pump pulse TP having a desired or target temporal profile. It should be noted that the backward process is based on a numerical model simulating the forward soliton self-compression process shown in FIG. 12A. In the backward process, the broadband output radiation ORD having the target spectral characteristic TS may be fed into the output end of the HC-PCF (simulated in the numerical model) and may undergo a soliton self-compression process that numerically transforms the broadband output radiation ORD into a target pump pulse TP. Finally, the target pump pulse MP may be obtained at the input end of the same fiber (simulated in the numerical model). The target pump pulse TP numerically determined in the backward process may then be used to set a reference or target temporal profile to which the input pump pulse is shaped.

[0111] [000110] Figure 13 illustrates a schematic of a first radiation source RDS-M1 with a pulse shaper PS for providing a broadband output radiation ORD with target spectral characteristics according to an embodiment. The first radiation source RDS-M1 may differ from the radiation source RDS shown in Figure 8 due to the addition of the pulse shaper PS. The pulse shaper PS of Figure 13 may be configured to impart a temporal profile to a pulse of the input radiation IRD to generate a temporally modulated pump pulse IRD-M, the temporally modulated pump pulse IRD-M having a different temporal profile than the input pump pulse IRD. The pulse shaper PS may be configured to be able to modify the phase of each spectral bin of the laser pulse. In an embodiment, the pulse shaper PS may include an optical grating and a spatial light modulator (SLM). The optical grating may be configured to spatially disperse the spectrum of the laser pulse. The dispersed spectrum may then be projected (e.g. by one or more optical mirrors) onto the SLM such that the phase of each constituent wavelength (or each color) can be independently modified. The modified, spatially dispersed laser pulse can be fed back to the same optical grating to recombine the constituent wavelengths to obtain a temporally modulated laser pulse. In an embodiment, the pulse shaper PS can include an additional optical grating configured to recombine the modified constituent wavelengths to obtain a temporally modulated laser pulse. In an embodiment, one or more optical gratings of the pulse shaper PS can be replaced with one or more optical prisms configured to perform the same function as the optical gratings (e.g., dispersing and recombining the laser pulse).

[0112] [000111] In an embodiment, the temporally modulated pump pulse IRD-M may include a temporal profile with two or more peaks (e.g., the temporal profile of the target pump pulse shown in FIG. 12B). In an embodiment, the temporally modulated pump pulse IRD-M may include a temporal profile with a single peak and an asymmetric shape (e.g., a non-Gaussian shape).

[0113] [000112] The HC-PCF may be operable to receive a temporally modulated pump pulse IRD-M, which may be configured to be spectrally broadened by a soliton self-compression process while propagating through the hollow core of the HC-PCF to form a broadband output radiation ORD, the temporal profile being configured to configure the spectrum of the broadband output radiation ORD to have target spectral characteristics. The target spectral characteristics may include, for example, a spectral flatness of 3 dB or less, or optionally 6 dB or less, or optionally 9 dB or less, and an average PSD of at least 3 mW / nm, or optionally at least 5 mW / nm, or optionally at least 7 mW / nm. One or both of the spectral flatness and the average PSD may be in a wavelength range of 400 nm to 900 nm, or optionally 300 nm to 900 nm, or optionally 300 nm to 1000 nm.

[0114] [000113] In an embodiment, the temporal profile may include a temporal profile resulting from propagation of broadband output radiation ORD having the target spectral characteristics in a backward direction through the HC-PCF, the backward direction being opposite to the direction in which the broadband output radiation is formed from the temporally modulated pump pulses. This temporal profile may be numerically determined using an appropriate model to simulate this backward propagation, as described above.

[0115] [000114] In an embodiment, the first radiation source RDS-M1 may include a processor unit PU for controlling the pulse shaper PS. The processor unit may be a computer or a computer system, as described below with reference to Fig. 16. The processor unit PU may be communicatively coupled to the pulse shaper PS. In an embodiment, the processor unit PU may be a standalone unit separate from the first radiation source RDS-M1. In an embodiment, the processor unit PU may be included within the first radiation source RDS-M1, for example as part of a control unit of the first radiation source RDS-M1, the control unit being configured to control the operation of the first radiation source RDS-M1 and / or included within the pulse shaper PS. In an embodiment, the processor unit PU may be further operable to determine a temporal profile to be imparted by the pulse shaper PS. In an embodiment, the processor unit PU may be operable to determine a temporal profile by performing numerical propagation of a broadband output radiation ORD having target spectral characteristics in a reverse direction through the HC-PCF (e.g., the reverse soliton self-compression process shown in FIG. 12B), the reverse direction being opposite to the direction in which the broadband output radiation ORD is formed from the temporally modulated pump pulse.

[0116] [000115] In an embodiment, the pulse shaper PS may be included within the pulsed pump radiation source PRS. In an embodiment, the processor unit PU may be communicatively coupled to the pulse shaper PS via a control unit of the pulsed pump radiation source PRS, the control unit being configured to control operation of the pulsed pump radiation source PRS.

[0117] [000116] Figure 14 shows a schematic diagram of a second radiation source RDS-M2 with a pulse shaper for providing a broadband output radiation ORD with target spectral characteristics according to an embodiment. Compared to the first radiation source RDS-M1, the second radiation source RDS-M2 may further include a spectrometer configured to measure the spectrum of the broadband output radiation ORD. The spectrometer SP may be communicatively coupled to a processor unit PU, which is communicatively coupled to the pulse shaper PS. While the second radiation source RDS-M2 is operating, the spectrometer SP may acquire (periodically or intermittently) the spectrum of the broadband output radiation ORD and then transmit the acquired spectral data to the processor unit PU. The processor unit PU may be operable to perform an analysis on the spectral data received from the spectrometer SP and to transmit a feedback or command signal to the pulse shaper PS for the pulse shaper PS to impart a corresponding time profile to the input radiation IRD. Thus, the spectrometer SP, the processor unit PU and the pulse shaper PS may form a feedback control mechanism allowing adaptive pulse shaping of the input radiation IRD according to the measured spectrum.

[0118] [000117] In an embodiment, the processor unit PU may be operable to optimize a temporal profile of the pump pulse that increases the spectral broadening via a soliton self-compression process in the working medium in the HC-PCF to form a broadband output radiation ORD. In an embodiment, the optimization of the temporal profile may be achieved by minimizing the difference and / or maximizing the similarity between a measured spectrum of the broadband output radiation ORD and a spectrum defined by the target spectral characteristic. In an embodiment, a first or initial temporal profile of the optimization process may be a temporal profile of the target pump pulse TP determined by numerically backpropagating the broadband output radiation ORD having the target spectral characteristic through the HC-PCF (e.g., a backpropagation process as shown in FIG. 12B). In an embodiment, the first or initial temporal profile may be one of a number of candidate temporal profiles generated by the processor unit PU.

[0119] [000118] Referring to Figure 15, the processor unit PU may be operable to execute an optimization method 1500 comprising four main steps:

[0120] [000119] Step 1510: Generating a number of candidate temporal profiles, each candidate temporal profile being distinct from the others.

[0121] [000120] Step 1520: Command the pulse shaper PS to sequentially impart each of the multiple candidate temporal profiles to a pulse of the input radiation IRD. Thus, multiple temporally modulated pump pulses, each having one of the multiple candidate temporal profiles, may be generated in sequence. Each such temporally modulated pump pulse generates an output spectrum that is measured by the spectrometer SP.

[0122] [000121] Step 1530: Analyzing a number of measured spectra received from the spectrometer, each measured spectrum resulting from each of a number of candidate time profiles. In an embodiment, the processor unit PU may be operable to compare each of the number of measured spectra with a spectrum defined by the target spectral characteristic. For example, the processor unit PU may be operable to calculate and then compare the power within a certain wavelength range for the two spectra to be compared. Alternatively, the processor unit PU may be operable to calculate and compare the conversion efficiency for the two spectra to be compared.

[0123] [000122] Step 1540: Selecting one of a number of candidate temporal profiles that maximizes the similarity between the measured spectrum of the broadband output radiation and the spectrum defined by the target spectral characteristic. In an embodiment, the selected candidate temporal profile may be sent to a pulse shaper PS and applied by the pulse shaper to the input radiation IRD. In an embodiment, the selected candidate temporal profile may be stored in a memory of the processor unit PU.

[0124] [000123] In an embodiment, the processor unit PU may be operable to repeat the generate, command, analyze and select steps (i.e. steps 1510-1540) in order to further optimize the temporal profile (in this case only one candidate temporal profile needs to be generated and analyzed per iteration and each step may be associated with one or more candidate temporal profiles).

[0125] [000124] In an embodiment, the multiple candidate temporal profiles may be generated in a predefined manner. For example, if the first temporal profile is numerically calculated according to the above-mentioned backward soliton self-compression process (e.g., as shown in FIG. 12B), the multiple candidate temporal profiles may be generated by gradually changing the value of a parameter of interest associated with one of the spectral characteristics (e.g., the PSD at one or more specific wavelengths, or the PSD averaged over a specific wavelength range). Such an approach can fine-tune the first temporal profile to maximize the similarity (or minimize the difference) between the measured spectrum and the spectrum defined by the target spectral characteristics.

[0126] [000125] In an embodiment, a number of candidate time profiles may be generated randomly and / or independently. For example, if all time profiles are generated randomly by the processor unit PU, the optimal time profile may be determined through a search process using a sequential genetic algorithm (or other evolutionary algorithm). In particular, the processor unit PU may be operable to generate a first set (or population) of candidate time profiles from which a set of measured spectra is obtained. The processor unit PU may then be operable to compare each of the resulting set of measured spectra with a spectrum defined by the target spectral characteristic. The processor unit PU may be operable to identify and store one or more candidate time profiles that maximize the similarity (or minimize the difference) between the measured spectrum and the spectrum defined by the target spectral characteristic. The processor unit PU may then be operable to randomly generate a new or second set of candidate time profiles. Similar to how the first set of time profiles was processed, the processor unit PU may be operable to identify and store one or more candidate time profiles from the second set of time profiles. This search process can continue until an optimal temporal profile is found that produces broadband output radiation having the target spectral characteristics.

[0127] [000126] Figure 16 is a block diagram illustrating a computer system 1600 that may assist in implementing the methods and flows disclosed herein. The computer system 1600 includes a bus 1602 or other communication mechanism for communicating information and a processor 1604 (or multiple processors 1604, 1605) coupled with the bus 1602 for processing information. The computer system 1600 also includes a main memory 1606 (such as a random access memory (RAM) or other dynamic storage device) coupled to the bus 1602 for storing information and instructions for execution by the processor 1604. The main memory 1606 may also be used to store temporary variables or other intermediate information during execution of instructions for execution by the processor 1604. The computer system 1600 further includes a read only memory (ROM) 1608 or other static storage device coupled to the bus 1602 for storing static information and instructions for the processor 1604. A storage device 1610 (magnetic or optical disk) is provided and coupled to the bus 1602 for storing information and instructions.

[0128] [000127] The computer system 1600 can be coupled via a bus 1602 to a display 1612 (such as a cathode ray tube (CRT) or flat panel or touch panel display) for displaying information to a computer user. An input device 1614 (including alphanumeric and other keys) is coupled to the bus 1602 for communicating information and command selections to the processor 1604. Another type of user input device is a cursor control 1616 (such as a mouse, trackball, or cursor direction keys) for communicating directional information and command selections to the processor 1604 and for controlling cursor movement on the display 1612. This input device typically has two degrees of freedom in two axes, a first axis (e.g., x) and a second axis (e.g., y), allowing the device to specify a position in a plane. A touch panel (screen) display can also be used as an input device.

[0129] [000128] One or more of the methods described herein may be performed by computer system 1600 in response to processor 1604 executing one or more sequences of one or more instructions contained in main memory 1606. Such instructions may be read into main memory 1606 from another computer-readable medium, such as storage device 1610. Execution of the sequences of instructions contained in main memory 1606 causes processor 1604 to perform the process steps described herein. One or more processors of a multi-processing arrangement may also be used to execute the sequences of instructions contained in main memory 1606. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions. Thus, the description herein is not limited to any specific combination of hardware circuitry and software.

[0130] [000129] The term "computer-readable medium" as used herein refers to any medium that participates in providing instructions to the processor 1604 for execution. Such media may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media include, for example, optical or magnetic disks, such as storage device 1610. Volatile media include dynamic memory, such as main memory 1606. Transmission media include coaxial cables, copper wire and fiber optics, including the wires that comprise bus 1602. Transmission media may also take the form of acoustic or light waves, such as those generated during radio frequency (RF) and infrared (IR) data communications. Common forms of computer readable media include, for example, a floppy disk, a flexible disk, a hard disk, magnetic tape, any other magnetic medium, a CD-ROM, a DVD, any other optical medium, punch cards, paper tape, any other physical medium with a pattern of holes, a RAM, a PROM, an EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described below, or any other medium from which a computer can read.

[0131] [000130] Various forms of computer readable media may be involved in carrying one or more sequences of one or more instructions to the processor 1604 for execution. For example, the instructions may initially reside on a magnetic disk of a remote computer. The remote computer may load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to the computer system 1600 may receive the data on the telephone line and use an infrared transmitter to convert the data to an infrared signal. An infrared detector coupled to the bus may receive the data carried in the infrared signal and place the data on the bus 1602. The bus 1602 carries the data to the main memory 1606, from which the processor 1604 retrieves and executes the instructions. The instructions received by the main memory 1606 may optionally be stored on storage device 1610 either before or after execution by the processor 1604.

[0132] [000131] The computer system 1600 also preferably includes a communication interface 1618 coupled to the bus 1602. The communication interface 1618 provides a two-way data communication coupling to a network link 1620 that is connected to a local network 1622. For example, the communication interface 1618 may be an Integrated Services Digital Network (ISDN) card or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, the communication interface 1618 may be a Local Area Network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In any such implementation, the communication interface 1618 sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information.

[0133] [000132] The network link 1620 typically provides data communication through one or more networks to other data devices. For example, the network link 1620 may provide a connection through a local network 1622 to a host computer 1624 or to data equipment operated by an Internet Service Provider (ISP) 1626. The ISP 1626 in turn provides data communication services through the world wide packet data communications network (now commonly referred to as the "Internet" 1628). Both the local network 1622 and the Internet 1628 use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on the network link 1620 and through the communication interface 1618 (which carry the digital data to and from the computer system 1600) are exemplary forms of carrier waves transporting the information.

[0134] [000133] Computer system 1600 can send messages and receive data, including program code, through the network(s), network link 1620 and communication interface 1618. In the Internet example, a server 1630 might send a requested code for an application program through the Internet 1628, ISP 1626, local network 1622 and communication interface 1618. One such downloaded application may comprise, for example, one or more of the techniques described herein. The received code may be executed by processor 1604 upon receipt and / or stored in storage device 1610 or other non-volatile storage for later execution. In this manner, computer system 1600 can obtain application code in the form of a carrier wave.

[0135] [000134] Further embodiments are disclosed in the following numbered clause listing: 1. A pulse shaper configured to impart a temporal profile to an input pump pulse to generate a temporally modulated pump pulse, the temporally modulated pump pulse having a different temporal profile than the input pump pulse; a hollow-core photonic crystal fiber (HC-PCF) having a hollow core for confining a working medium under pressure in use, the HC-PCF being operable to receive a temporally modulated pump pulse; A broadband emitting device comprising: A broadband emitting device, wherein a temporally modulated pump pulse is configured to undergo increased spectral broadening by a soliton self-compression process while propagating through the hollow core of the HC-PCF to form a broadband output radiation, the temporal profile being configured to configure the spectrum of the broadband output radiation to have a target spectral characteristic. 2. The broadband emitting device of clause 1, wherein the temporal profile includes a temporal profile resulting from numerical propagation of broadband output radiation having target spectral characteristics in a reverse direction through the HC-PCF, the reverse direction being opposite to the direction in which the broadband output radiation is formed from the temporally modulated pump pulses. 3. A broadband radiating device as described in clause 1 or 2, comprising a processor unit for controlling a pulse shaper, the processor unit being further operable to determine the temporal profile imparted by the pulse shaper. 4. The broadband emission device of clause 3, wherein the processor unit is operable to determine a temporal profile by performing numerical propagation of the broadband output radiation having the target spectral characteristics in a reverse direction through the HC-PCF, the reverse direction being opposite to the direction in which the broadband output radiation is formed from the temporally modulated pump pulses. 5. A broadband emitting device according to clause 3 or 4, comprising a spectrometer for measuring the spectrum of the broadband output radiation, A broadband emitting device, wherein the processor unit is operable to optimize the temporal profile to minimize differences and / or maximize similarity between a measured spectrum of the broadband output radiation and a spectrum defined by a target spectral characteristic. 6. A processor unit: generating a number of candidate temporal profiles; commanding a pulse shaper PS to impart each of a number of candidate temporal profiles to a respective pump pulse to obtain a number of temporally modulated pump pulses; analyzing a number of measured spectra received from the spectrometer, each measured spectrum resulting from a respective one of a number of candidate time profiles; selecting, based on the analysis, one candidate time profile among the plurality of candidate time profiles that maximizes a similarity between the measured spectrum of the broadband output radiation and the spectrum defined by the target spectral characteristic; 6. The broadband radiating device of claim 5, operable to: 7. A broadband radiating device as described in clause 6, operable to repeat the generating, commanding, analysing and selecting steps to further optimise the temporal profile. 8. A wideband radiating device as described in any one of clauses 1 to 6, operable to determine the temporal profile through a search process using a sequential genetic or evolutionary algorithm. 9. A broadband emitting device according to any one of the preceding clauses, wherein the temporally modulated pump pulse comprises a temporal profile having two or more peaks. 10. A broadband emitting device as described in any one of the preceding clauses, wherein the target spectral characteristics include an average power spectral density (PSD) of at least 5 mW / nm in the wavelength range of 400 nm to 900 nm. 11. A broadband radiating device as described in any one of the preceding clauses, wherein the target spectral characteristics include a spectral profile having a maximum variation of 3 dB in the wavelength range of 400 nm to 900 nm. 12. A broadband emitting device according to any one of the preceding clauses, further comprising a pump laser configured to output pump pulses. 13. A broadband emitting device as claimed in claim 12, wherein the pulse shaper is included in the pump laser. 14. A metrology device comprising a broadband radiating device according to any one of the preceding clauses. 15. A metrology device according to clause 14, comprising a scatterometer metrology tool, a level sensor or an alignment sensor. 16. A method for optimizing a pump pulse temporal profile to increase spectral broadening via a soliton self-compression process in a working medium in a hollow-core photonic crystal fiber (HC-PCF) to form a broadband output radiation, comprising: Optimizing the temporal profile to configure the broadband output radiation to have target spectral characteristics. A method comprising: 17. The method of claim 16, wherein the step of optimizing the temporal profile includes performing numerical propagation of broadband output radiation having target spectral characteristics in a reverse direction through the HC-PCF, the reverse direction being opposite to the direction in which the broadband output radiation is formed from the temporally modulated pump pulses. 18. The method of clause 16 or 17, wherein optimizing the temporal profile comprises minimizing the difference and / or maximizing the similarity between the measured spectrum of the broadband output radiation and the spectrum defined by the target spectral characteristic. 19. Generating a number of candidate temporal profiles; commanding a pulse shaper PS to impart each of a number of candidate temporal profiles to a respective pump pulse to obtain a number of temporally modulated pump pulses; analyzing a number of measured spectra received from the spectrometer, each measured spectrum resulting from a respective one of a number of candidate time profiles; selecting, based on the analysis, one candidate time profile among the plurality of candidate time profiles that maximizes similarity between the measured spectrum of the broadband output radiation and the spectrum defined by the target spectral characteristic; 19. The method according to any one of clauses 16 to 18, comprising: 20. The method of claim 19, comprising repeating the generating, commanding, analyzing and selecting steps to further optimize the time profile. 21. The method of any one of clauses 16 to 19, wherein the time profile is determined through a search process using a sequential genetic or evolutionary algorithm. 22. A method of generating broadband output radiation having target spectral characteristics, comprising: imparting a temporal profile to an input pump pulse to generate a temporally modulated pump pulse, the temporally modulated pump pulse having a different temporal profile than the input pump pulse, the temporal profile configured to configure a spectrum of the broadband output radiation to have the target spectral characteristics; and exciting a working medium in a hollow-core photonic crystal fiber (HC-PCF) using a time-modulated pump pulse such that the time-modulated pump pulse exhibits high spectral broadening due to a soliton self-compression process to form a broadband output radiation having target spectral characteristics. A method comprising: 23. The method of claim 22, wherein the time profile comprises a time profile as optimized in any one of clauses 16 to 21.

[0136] [000135] Although specific reference is made in this specification to the use of the lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optics, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0137] [000136] Although embodiments of the invention may be specifically referenced herein in connection with lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus or any apparatus that measures or processes an object such as a wafer (or other substrate) or a mask (or other patterning device). These apparatus may be collectively referred to as a lithography tool. Such lithography tools may use vacuum or ambient (non-vacuum) conditions.

[0138] [000137] Although specific reference has been made above to the use of embodiments of the invention in connection with optical lithography, it will be appreciated that the invention, where the context permits, is not limited to optical lithography and may be used in other applications, for example in imprint lithography.

[0139] [000138] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The above description is intended to be illustrative rather than limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

1. a pulse shaper configured to impart a temporal profile to an input pump pulse to generate a temporally modulated pump pulse, the temporally modulated pump pulse having a different temporal profile than the input pump pulse; a hollow-core photonic crystal fiber (HC-PCF) having a hollow core for confining a working medium under pressure in use, the hollow-core photonic crystal fiber (HC-PCF) being operable to receive the temporally modulated pump pulses; A broadband emitting device comprising: A broadband emitting device, wherein the temporally modulated pump pulse is configured to undergo spectral broadening by a soliton self-compression process while propagating through the hollow core of the HC-PCF to form a broadband output radiation, and the temporal profile is configured to configure the spectrum of the broadband output radiation to have a target spectral characteristic.

2. 2. The broadband emitting device of claim 1, wherein the temporal profile comprises a temporal profile resulting from numerical propagation of the broadband output radiation having the target spectral characteristics in a reverse direction through the HC-PCF, the reverse direction being opposite to the direction in which the broadband output radiation is formed from the temporally modulated pump pulses.

3. 3. A broadband radiating device according to claim 1 or 2, comprising a processor unit for controlling the pulse shaper, the processor unit being further operable to determine the temporal profile imparted by the pulse shaper.

4. 4. The broadband emitting device of claim 3, wherein the processor unit is operable to determine the temporal profile by performing a numerical propagation of the broadband output radiation having the target spectral characteristics in a reverse direction through the HC-PCF, the reverse direction being opposite to the direction in which the broadband output radiation is formed from the temporally modulated pump pulses.

5. 5. A broadband emitting device according to claim 3 or 4, comprising a spectrometer for measuring the spectrum of the broadband output radiation, A broadband emitting device, wherein the processor unit is operable to optimize the temporal profile to minimize differences and / or maximize similarity between the measured spectrum of the broadband output radiation and a spectrum defined by the target spectral characteristic.

6. The processor unit comprises: generating a number of candidate temporal profiles; commanding said pulse shaper PS to impart each of said multiple candidate temporal profiles to a respective pump pulse to obtain a multiple temporally modulated pump pulse; analyzing a number of measured spectra received from the spectrometer, each measured spectrum resulting from a respective one of the number of candidate time profiles; selecting, based on the analysis, one candidate temporal profile from the plurality of candidate temporal profiles that maximizes the similarity between the measured spectrum of the broadband output radiation and a spectrum defined by the target spectral characteristic; 6. The broadband emitting device of claim 5, operable to:

7. 7. The broadband emitting device of claim 6, operable to repeat the generating, commanding, analysing and selecting steps to further optimise the temporal profile.

8. A broadband radiating device according to any one of the preceding claims, operable to determine said temporal profile through a search process using a sequential genetic or evolutionary algorithm.

9. The broadband emitting device according to any one of the preceding claims, wherein the temporally modulated pump pulse comprises a temporal profile having two or more peaks.

10. 10. The broadband emitting device according to any one of the preceding claims, wherein the target spectral characteristics comprise an average power spectral density (PSD) of at least 5mW / nm in the wavelength range of 400nm to 900nm.

11. A broadband emitting device according to any one of the preceding claims, wherein the target spectral characteristics comprise a spectral profile having a maximum variation of 3 dB in the wavelength range of 400 nm to 900 nm.

12. A metrology device comprising a broadband radiating device according to any one of the preceding claims.

13. A method for optimizing a pump pulse temporal profile to increase spectral broadening via a soliton self-compression process in a working medium in a hollow-core photonic crystal fiber (HC-PCF) to form a broadband output radiation, comprising: optimizing the temporal profile to configure the broadband output radiation to have target spectral characteristics. A method comprising:

14. 14. The method of claim 13, wherein the step of optimizing the temporal profile comprises performing numerical propagation of broadband output radiation having the target spectral characteristics in a reverse direction through the HC-PCF, the reverse direction being opposite to the direction in which the broadband output radiation is formed from a temporally modulated pump pulse.

15. generating a number of candidate temporal profiles; commanding a pulse shaper PS to impart each of said multiple candidate temporal profiles to a respective pump pulse to obtain a multiple temporally modulated pump pulse; analyzing a number of measured spectra received from a spectrometer, each measured spectrum resulting from a respective one of the number of candidate time profiles; selecting, based on the analysis, one candidate temporal profile from the plurality of candidate temporal profiles that maximizes a similarity between the measured spectrum of the broadband output radiation and a spectrum defined by the target spectral characteristic; The method of claim 13 or 14, comprising:

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