Hollow-core photonic crystal fiber-based multi-wavelength light source device

Abstract

A multi-wavelength source device and associated method for generating output radiation comprising a plurality of discrete output wavelength bands. The multi-wavelength source device includes a pump radiation source arrangement configured to generate input radiation comprising at least a first frequency component and a second frequency component, and a hollow-core photonic crystal fiber configured to confine a working medium. The hollow-core photonic crystal fiber is configured to receive the input radiation and generate a plurality of discrete output wavelength bands distributed across a wavelength range of interest via a seed-assisted cascaded four-wave mixing (FWM) process in the working medium.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to European Patent Application No. 21162934.0, filed March 16, 2021, which is incorporated by reference in its entirety into this specification.

[0002] The present invention relates to hollow-core photonic crystal fiber-based multi-wavelength radiation generators, and in particular to such multi-wavelength radiation generators 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, multi-wavelength radiation sources are increasingly used for such metrology applications. It is desirable to improve the present devices for multi-wavelength radiation generation. Summary of the Invention

[0007]

[0007] In a first aspect of the present invention, there is provided a multi-wavelength light source device configured to generate output radiation comprising a plurality of discrete output wavelength bands, the multi-wavelength light source device comprising: a pump radiation source arrangement configured to generate input radiation comprising at least a first frequency component and a second frequency component; and a hollow core photonic crystal fiber configured to confine a working medium, the hollow core photonic crystal fiber configured to receive the input radiation and generate in the working medium a plurality of discrete output wavelength bands distributed across a wavelength range of interest via a seed-assisted cascaded four-wave mixing (FWM) process.

[0008]

[0008] In a second aspect of the present invention, a method is provided for generating output radiation comprising a plurality of discrete output wavelength bands, the method comprising generating input radiation comprising at least a first frequency component and a second frequency component, and exciting a confined working medium with the input radiation to generate a plurality of discrete output wavelength bands distributed across a wavelength range of interest via a seed-assisted cascaded four-wave mixing (FWM) process within the working medium.

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

[0010]

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

[0011] [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 9] 1 is an output spectrum as obtained via a seed-assisted cascaded four-wave mixing (FWM) process in a method according to an embodiment. [Figure 10] FIG. 2 is a schematic diagram of a radiation source arrangement according to an embodiment. [Figure 11] 1 is a plot of fiber length versus pulse energy showing regions of particular combinations of these parameters for obtaining desired spectral characteristics. [Figure 12] FIG. 2 shows a block diagram of a computer system for controlling a radiation source, according to an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012]

[0011] 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).

[0013]

[0012] 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.

[0014]

[0013] 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.

[0015]

[0014] In operation, the illumination system IL receives a radiation beam from a radiation source SO (e.g. via a beam delivery system BD). The illumination system IL may include various types of optical components for directing, shaping and / or controlling the radiation, 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.

[0016]

[0015] 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.

[0017]

[0016] The lithographic apparatus LA may be of a type in which at least a part of the substrate may be covered in 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.

[0018] 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.

[0019]

[0018] 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.

[0020] In operation, a radiation beam B is incident on a patterning device (e.g. a mask MA held on a mask support MT) and is patterned according to a pattern (design layout) on the patterning device MA. After traversing the mask MA, the radiation beam B passes through 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.

[0021]

[0020] 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).

[0022]

[0021] 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.

[0023]

[0022] 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).

[0024] 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.

[0025]

[0024] 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).

[0026]

[0025] 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).

[0027]

[0026] In lithographic processes, it is desirable to frequently measure the created structures (e.g. for process control and verification). Tools performing such measurements are commonly called metrology tools MT. Different types of metrology tools MT performing 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.

[0028]

[0027] 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.

[0029]

[0028] 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.

[0030]

[0029] 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.

[0031]

[0030] 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.

[0032]

[0031] Other parameters of interest can be focus and dose. Focus and dose can be determined simultaneously by scatterometry (or alternatively by scanning electron microscopy), as described in 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.

[0033]

[0032] A 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 therefrom.

[0034]

[0033] 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.

[0035]

[0034] A metrology device such as a scatterometer is shown in Figure 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 Figure 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.

[0036]

[0035] 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.

[0037] Another type of metrology tool used in IC manufacturing is a topography measurement system, level sensor, or height sensor. Such a tool may be incorporated into a lithography apparatus to measure the topography of the top surface of a substrate (or wafer). A map of the 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 performs measurements at a fixed position (relative to its own optics), and relative movement of the substrate and the optics of the level or height sensor results in height measurements at locations across the substrate.

[0038]

[0037] An example of a level or height sensor LS known in the art is shown 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.

[0039]

[0038] 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.

[0040]

[0039] 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.

[0041]

[0040] 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.

[0042]

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

[0043]

[0042] 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.

[0044]

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

[0045]

[0044] 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.

[0046]

[0045] 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".

[0047]

[0046] 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.

[0048]

[0047] 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.

[0049]

[0048] 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.

[0050]

[0049] 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).

[0051]

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

[0052]

[0051] 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.

[0053]

[0052] 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.

[0054]

[0053] 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.

[0055]

[0054] High quality broadband radiation (e.g., supercontinuum radiation) can be difficult to generate. One way to generate broadband radiation can be to broaden high power narrowband or single frequency input radiation or pump radiation, for example, by utilizing nonlinear and higher order effects. The input radiation (which can be generated using a laser) can be called pump radiation. Alternatively, the input radiation can be called seed radiation. To obtain high power radiation 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.

[0056]

[0055] 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 may 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 may 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.

[0057]

[0056] 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. 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 multiple discrete wavelengths.

[0058]

[0057] 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).

[0059]

[0058] 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 manufacture 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.

[0060]

[0059] 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.

[0061]

[0060] 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.

[0062]

[0061] 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.

[0063]

[0062] It will be understood that the optical fiber OF can have any length, and the length of the optical fiber OF can depend on the application. The optical fiber OF can have a length between 1 cm and 10 cm, for example, the optical fiber OF can have a length between 1 cm and 7 cm, or between 1 cm and 5 cm.

[0064]

[0063] The optical fiber OF includes a hollow core HC, a cladding portion surrounding the hollow core HC, and a support portion SP surrounding and supporting the cladding portion. The optical fiber OF can be considered to include a body (including the cladding portion and the support portion SP) having the hollow core HC. The cladding portion includes a plurality of anti-resonant elements for guiding radiation through the hollow core HC. Specifically, the plurality of anti-resonant elements are arranged to confine radiation propagating through the optical fiber OF primarily 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.

[0065]

[0064] The cladding portion includes a plurality 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.

[0066]

[0065] 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.

[0067]

[0066] It will be understood that, as used herein, the term cladding portion 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).

[0068]

[0067] The support portion is generally tubular and supports six capillary CAPs of the cladding portion. The six capillary CAPs are evenly distributed around the 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.

[0069]

[0068] 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.

[0070]

[0069] 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.

[0071]

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

[0072] It will be appreciated that other embodiments may be provided with different arrangements of anti-resonant elements. These may include arrangements having multiple rings of anti-resonant elements, or arrangements having nested anti-resonant elements. 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).

[0073]

[0072] 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.

[0074]

[0073] 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.

[0075]

[0074] 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.

[0076]

[0075] 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.

[0077]

[0076] 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.

[0078] In this context, a window may be transparent to a frequency if at least 50%, 75%, 85%, 90%, 95% or 99% of radiation incident on the window at that frequency is transmitted through the window.

[0079]

[0078] 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.

[0080]

[0079] 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 changing 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.

[0081]

[0080] 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.

[0082]

[0081] 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.

[0083]

[0082] 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.

[0084]

[0083] 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.

[0085]

[0084] 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 be generated by a laser. Similarly, the output radiation ORD may be collimated and / or coherent.

[0086]

[0085] Typically, for metrology applications for which such sources are designed, it is desirable for the source to provide multiple (e.g., narrow) discrete wavelength bands. For example, some metrology tools may use any number of such discrete wavelength bands, from 3 or 4 to 30, 20, or 15 (e.g., per polarization state or states). One particular metrology tool (alignment sensor) uses 12 such discrete wavelength bands (and 2 polarization states, i.e., 24 illumination settings in total).

[0087]

[0086] The present method for generating a desired spectrum of multiple (e.g., narrow) discrete wavelength bands includes generating a supercontinuum output (e.g., using the supercontinuum radiation source and related techniques described) and filtering the desired wavelength bands from the supercontinuum output, e.g., using a static optical filter (having a multi-layer coating). However, such filtering is an inefficient way to obtain the desired spectrum. Typically, for example, the supercontinuum output may be less than 10 W of power (obtained using a >20 W pump source), of which only 1-2 W includes visible or usable wavelengths. After filtering, the actual usable light may be less than 100 mW.

[0088]

[0087] In addition to this inefficiency, the central wavelength of any filtered band depends on the tolerance of multi-layer coatings (complex), and therefore its construction is not easy. Also, current modulation instability (MI) based supercontinuum generation techniques require long fiber lengths (tens of cm) to generate a smooth spectrum. This makes the design quite tedious. Also, drawing a good quality fiber is very difficult.

[0089] To address these issues, it is proposed to use a seed-assisted cascaded four-wave mixing (FWM) process in a hollow-core fiber (e.g., filled with a noble gas) to directly generate the desired spectrum of multiple wavelength bands. Such an approach exploits the third-order nonlinearity χ 3 Based on.

[0090] For example, assume that a pump laser contains a first frequency component at f. A typical FWM process typically m +f k =2 * In other words, the energy of two photons of frequency f0 is converted to the energy of two photons of frequency f m and f k is converted into two new photons.

[0091] The proposed method involves providing the system with seed radiation having a slightly detuned second frequency component f1, which, together with the pump frequency component f0, m +f k =f0+f1. The frequency f1 can be expressed as the pump frequency f0 plus a small frequency delta Δf (i.e., f1=f0+Δf). A cascading effect occurs, where each newly generated frequency further interacts with the original pump frequency to generate subsequent frequencies on either side of it (the "cascade"), with Δf separating adjacent generated frequencies.

[0092]

[0091] Figure 9 is a schematic diagram of an output spectrum illustrating this cascading effect. The pump or first frequency component f0 and the seed or second frequency component f0+Δf together generate frequencies of f0-Δf and f0+2Δf (i.e., f0+(f0+Δf)=(f0-Δf)+(f0+2Δf)). The pump frequency f0 and the generated frequency f0+2Δf together generate frequencies of f0-Δf and f0+3Δf (i.e., f0+(f0+2Δf)=(f0-Δf)+(f0+3Δf)). The pump frequency f0 and the generated frequency f0-Δf together generate frequencies of f0-2Δf and f0+Δf (i.e., f0+(f0-Δf)=(f0-2Δf)+(f0+Δf)). This process cascades to generate multiple discrete frequencies. As will be described below, the number of discrete frequencies that are generated (at available power) depends on a number of parameters related to the fiber and the working medium (eg, gas pressure and fiber length).

[0093]

[0092] Figure 10 is a schematic diagram of a multi-wavelength light source device according to such an embodiment. It should be understood that the seed radiation can be generated in various ways via a pump laser, and this is just an example of an arrangement. A portion of the pump radiation PR from the pump laser PL is por is directed towards the seed generation element SG to generate a broadened radiation. In an embodiment, the seed generation element SG comprises a piece (e.g., a short one) of solid-core PCF or other suitable spectral broadening element. The detuning required in the proposed method is only small (e.g., in terms of wavelength, 10 nm to 200 nm, 50 nm to 200 nm or about 100 nm detuning), which can be achieved with a few cm of PCF. Following this, a filter F can be used to filter out the desired seed wavelength to obtain the seed radiation SR. Both the pump radiation and the seed radiation can then be used to excite the working medium in the fiber HC-PCF. Alternative methods for seed generation include frequency mixing processes such as difference frequency generation DFG, optical parametric generation OPG, or optical parametric amplification OPA or optical parametric oscillation OPO.

[0094]

[0093] In embodiments, to obtain the desired spectral characteristics, the combination of pulse energy and fiber length must be such that there is efficient mixing to wavelength ranges below 700 nm, below 600 nm, or below 500 nm, while avoiding excessive self-phase modulation (SPM) to the extent that the spectral bands overlap. Thus, the pulse energy and fiber length can be such that SPM is sufficiently avoided that each wavelength band is discrete and adjacent wavelength bands do not overlap (at least within the wavelength range of interest, such as the ranges of 100 nm to 4000 nm, 200 nm to 2000 nm, 400 nm to 900 nm, or 500 nm to 900 nm).

[0095]

[0094] Figure 11 shows the relationship between pulse energy and fiber length for the desired spectral characteristics. The top diagram is a plot of fiber length versus pulse energy. The dotted line represents the desired area DES in this space, and the combination of pulse energy and fiber length results in a first desired spectrum, spectrum A (first plot of PSD versus wavelength λ). This spectrum includes discrete usable bands of wavelengths below 500 nm. This spectrum also includes good spectral shapes (e.g., including a single peak) for each band. In some applications, this good spectral shape per band is desirable because it exhibits good band stability. However, in many other cases, this is less important. For example, another desired spectrum of the desired area DES results in a less clearly defined spectral shape for at least some wavelength bands (as shown by spectrum B). Such a spectrum is still fully usable, and in some cases may even be preferred over the characteristics demonstrated by spectrum A (e.g., due to its flatter profile and / or broader peaks).

[0096]

[0095] Above the desired region DES, there is excessive self-phase modulation and the wavelength bands begin to overlap. Below this region, as shown by spectrum C, the spectrum is not sufficiently flat and there are no discrete (e.g. usable) bands at the low end of the spectrum (e.g. below wavelengths of 500 nm).

[0097]

[0096] In general, the pulse width of the pump laser determines the bandwidth of each discrete output wavelength band, such that the larger the pulse width, the smaller the bandwidth (and vice versa). The band separation is determined by the difference between the seed and pump wavelengths. The extension to the UV is determined by the gas pressure. As has been explained, the pulse energy can be traded off against the fiber length (the smaller the pulse energy, the longer the fiber required). Thus, in addition to the desired spectral characteristics already explained (tunable via fiber length and pulse energy), the difference between the seed and pump wavelengths allows the selection of the number of wavelength bands within the desired wavelength range. The peak value for the wavelength bands scales with the pulse repetition rate of the input radiation and can therefore be controlled by controlling this parameter.

[0098]

[0097] Exemplary arrangements can provide 5 to 30 wavelength bands or 10 to 20 wavelength bands (e.g., 12 discrete wavelength bands) of radiation covering wavelength ranges of interest of 200 nm to 2000 nm, 400 nm to 2000 nm, 400 nm to 1200 nm, 400 nm to 1000 nm, 500 nm to 1200 nm, 400 to 900 or 500 nm to 900 nm.

[0099] The optical fiber OF or HC-PCF may have a length of 1 cm to 30 cm, 1 cm to 20 cm, 1 cm to 10 cm, 1 cm to 7 cm, or 1 cm to 5 cm, which is much shorter than current designs (with the ability to use shorter fibers resulting from the proposed seed-assisted cascaded four-wave mixing (FWM) process).

[0100] In summary, the method disclosed herein is much more energy efficient than current filtering methods that filter large amounts of radiation. The HC-PCF required for this method is only a few centimeters. This significantly reduces the required volume compared to current arrangements where the HC-PCF length is typically over 40 cm. This makes the HC-PCF easier to manufacture. It is much less noisy than the current noise-induced MI supercontinuum process. By using this process, stable multiple bands can be generated. The process is elastic, so the central wavelength of each band depends only on the detuning of the seed laser. It is also more efficient than unassisted four-wave mixing.

[0101] [000100] Figure 12 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.

[0102] [000101] 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.

[0103] [000102] 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.

[0104] [000103] 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.

[0105] [000104] 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.

[0106] [000105] 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.

[0107] [000106] 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.

[0108] [000107] Computer system 1600 can send messages and receive data, including program code, through the network(s), network link 1620 and communication interface 1618. In the Internet example, a server 1630 might send a requested code for an application program through the Internet 1628, ISP 1626, local network 1622 and communication interface 1618. One such downloaded application may, for example, comprise one or more of the techniques described herein. The received code may be executed by processor 1604 upon receipt and / or stored in storage device 1610 or other non-volatile storage for later execution. In this manner, computer system 1600 can obtain application code in the form of a carrier wave.

[0109] [000108] Further embodiments are disclosed in the following list of numbered clauses: 1. A multi-wavelength light source device configured to generate output radiation comprising a plurality of discrete output wavelength bands, comprising: a pump radiation source arrangement configured to generate input radiation comprising at least a first frequency component and a second frequency component; a hollow-core photonic crystal fiber configured to confine a working medium, the hollow-core photonic crystal fiber configured to receive input radiation and generate in the working medium via a seed-assisted cascaded four-wave mixing (FWM) process a plurality of discrete output wavelength bands distributed across a wavelength range of interest; A multi-wavelength light source device comprising: 2. A multi-wavelength light source device as defined in clause 1, wherein the number of the plurality of discrete output wavelength bands is between 5 and 30 wavelength bands. 3. A multi-wavelength light source device as defined in clause 1, wherein the number of the plurality of discrete output wavelength bands is between 10 and 20 wavelength bands. 4. A multi-wavelength light source device as defined in any one of the preceding clauses, wherein the hollow-core photonic crystal fiber comprises a length between 1 cm and 30 cm. 5. A multi-wavelength light source device as defined in any one of the preceding clauses, wherein the hollow-core photonic crystal fiber comprises a length between 1 cm and 10 cm. 6. A multi-wavelength light source device as defined in any one of the preceding clauses, wherein the combination of pulse energy of the input radiation and length of the hollow-core photonic crystal fiber is such that at least one of the plurality of discrete output wavelength bands is centered at or below 700 nm. 7. A multi-wavelength light source device as defined in any one of clauses 1 to 5, wherein the combination of pulse energy of the input radiation and length of the hollow-core photonic crystal fiber is such that at least one of the plurality of discrete output wavelength bands is centered at 600 nm or less. 8. A multi-wavelength light source device as defined in any one of clauses 1 to 5, wherein the combination of pulse energy of the input radiation and length of the hollow-core photonic crystal fiber is such that at least one of the plurality of discrete output wavelength bands is centered at 500 nm or less. 9. A multi-wavelength source device as defined in any one of the preceding clauses, wherein the combination of pulse energy of the input radiation and length of the hollow-core photonic crystal fiber is such that self-phase modulation is avoided. 10. A multi-wavelength light source device as defined in Clause 9, wherein the avoidance of self-phase modulation is such that the discrete wavelength bands within the wavelength range of interest do not overlap. 11. A pump radiation source comprising: a pump radiation source operable to output pump radiation comprising a first frequency component; A multi-wavelength light source device as defined in any one of the preceding clauses, wherein the seed generation element is operable to generate a second frequency component from the pump radiation. 12. The seed generating element is a spectral broadening element operable to broaden the spectrum of a portion of the pump radiation; a filter operable to separate a second frequency component from the broadened radiation; and 13. A multi-wavelength light source device as defined in Clause 11, comprising: 13. A multi-wavelength light source device as defined in clause 12, wherein the spectral broadening element comprises a solid-core photonic crystal fiber. 14. A multi-wavelength source device as defined in clause 11, wherein the seed generation element comprises one of a difference frequency generator, an optical parametric generator, an optical parametric amplifier, or an optical parametric oscillator. 15. A multi-wavelength light source device as defined in any one of the preceding clauses, wherein the difference in frequency between the first frequency component and the second frequency component is between 10 nm and 200 nm. 16. A multi-wavelength light source device as defined in any one of the preceding clauses, wherein the wavelength range of interest includes wavelengths of at least 400 nm to 2000 nm. 17. A multi-wavelength light source device as defined in any one of clauses 1 to 15, wherein the wavelength range of interest includes wavelengths of at least 400 nm to 1200 nm. 18. A multi-wavelength light source device as defined in any one of clauses 1 to 15, wherein the wavelength range of interest includes wavelengths of at least 500 nm to 1000 nm. 19. A method for generating output radiation comprising a plurality of discrete output wavelength bands, comprising: generating input radiation comprising at least a first frequency component and a second frequency component; exciting the confined working medium with input radiation to generate a plurality of discrete output wavelength bands distributed across a wavelength range of interest via a seed-assisted cascaded four-wave mixing (FWM) process in the working medium; A method comprising: 20. A method as defined in clause 19, wherein the number of the plurality of discrete output wavelength bands is between 5 and 30 wavelength bands. 21. The method as defined in clause 19, wherein the number of the plurality of discrete output wavelength bands is between 10 and 20 wavelength bands. 22. A method as defined in any one of clauses 19 to 21, wherein the working medium is confined within a hollow-core photonic crystal fiber having a length between 1 cm and 30 cm. 23. A method as defined in any one of clauses 19 to 21, wherein the working medium is confined within a hollow-core photonic crystal fiber having a length between 1 cm and 10 cm. 24. A method as defined in clause 22 or 23, comprising optimizing a combination of pulse energy of the input radiation and length of the hollow-core photonic crystal fiber such that at least one of the plurality of discrete output wavelength bands is centred at or below 700 nm. 25. A method as defined in clause 22 or 23, comprising optimizing a combination of pulse energy of the input radiation and length of the hollow-core photonic crystal fiber such that at least one of the plurality of discrete output wavelength bands is centred at or below 600 nm. 26. A method as defined in clause 22 or 23, comprising optimizing a combination of pulse energy of the input radiation and length of the hollow-core photonic crystal fiber such that at least one of the plurality of discrete output wavelength bands is centred at or below 500 nm. 27. A method as defined in any one of clauses 23 to 26, comprising optimizing a combination of pulse energy of the input radiation and length of the hollow-core photonic crystal fiber such that self-phase modulation is avoided. 28. A method as defined in clause 27, wherein the avoidance of self-phase modulation is such that discrete wavelength bands within the wavelength range of interest do not overlap. 29. A method as defined in any one of clauses 19 to 28 comprising setting the bandwidth of each discrete output wavelength band by appropriately setting the pulse width of the input radiation. 30. A method as defined in any one of clauses 19 to 29, setting a band separation between adjacent discrete output wavelength bands by setting a difference in frequency between the first frequency component and the second frequency component. 31. A method as defined in clause 30, wherein the frequency difference between the first frequency component and the second frequency component is set to between 10 nm and 200 nm. 32. A method as defined in any one of clauses 19 to 31, comprising generating the first frequency component and the second frequency component from a single pump radiation source operable to output pump radiation at only the first frequency. 33. Generating a second frequency component from a pump radiation source by increasing the spectral broadening of a portion of the pump radiation; Separating a second frequency component from the broadened radiation; as defined in clause 32. 34. A method as defined in clause 32, comprising generating a second frequency component from a pump radiation source by one of difference frequency generation, optical parametric generation, optical parametric amplification or optical parametric oscillation. 35. A method as defined in any one of clauses 19 to 34, wherein the wavelength range of interest includes wavelengths of at least 400 nm to 2000 nm. 36. A method as defined in any one of clauses 19 to 34, wherein the wavelength range of interest includes at least wavelengths between 400 nm and 1200 nm. 37. A method as defined in any one of clauses 19 to 34, wherein the wavelength range of interest includes wavelengths of at least 500 nm to 1000 nm. 38. A substrate support portion for supporting a substrate; A multi-wavelength light source device according to any one of clauses 1 to 18, at least one optical system operable to direct output radiation from the multi-wavelength light source device to a substrate and capture radiation scattered from the substrate; metrology devices, including 39. A metrology device as defined in Clause 38 that is operable as a scatterometer metrology tool. 40. A metrology device as defined in clause 38 operable as a level sensor or an alignment sensor. 41. A lithographic apparatus comprising at least one metrology device as defined in clause 40 for performing alignment and / or levelling metrology. 42. A lithography cell comprising a lithography apparatus as defined in clause 41 and a metrology device as defined in clause 39.

[0110] [000109] 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.

[0111] [000110] 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). Collectively, these apparatus may be referred to as lithography tools. Such lithography tools may use vacuum or ambient (non-vacuum) conditions.

[0112] [000111] 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.

[0113] [000112] 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. 1. A multi-wavelength light source device configured to generate output radiation comprising a plurality of discrete output wavelength bands, comprising: a pump radiation source arrangement configured to generate input radiation comprising at least a first frequency component and a second frequency component; a hollow-core photonic crystal fiber configured to confine a working medium, the hollow-core photonic crystal fiber configured to receive the input radiation and generate in the working medium the plurality of discrete output wavelength bands distributed across a wavelength range of interest via a seed-assisted cascaded four-wave mixing (FWM) process; A multi-wavelength light source device comprising:

2. 2. The multi-wavelength light source device of claim 1, wherein the number of the plurality of discrete output wavelength bands is between 5 and 30 wavelength bands.

3. The multi-wavelength light source device according to claim 1 or 2, wherein the hollow-core photonic crystal fiber comprises a length of 1 cm to 30 cm.

4. 4. A multi-wavelength light source device as claimed in any one of claims 1 to 3, wherein a combination of the pulse energy of the input radiation and the length of the hollow-core photonic crystal fibre is such that at least one of the plurality of discrete output wavelength bands has a centre less than or equal to 700 nm.

5. 5. A multi-wavelength light source device according to claim 1, wherein a combination of the pulse energy of the input radiation and the length of the hollow-core photonic crystal fibre is such that self-phase modulation is avoided.

6. 6. The multi-wavelength light source device of claim 5, wherein the avoidance of self-phase modulation is such that discrete wavelength bands within the wavelength range of interest do not overlap.

7. 7. A multi-wavelength light source device according to claim 1 , wherein the pump radiation source comprises a pump radiation source operable to output pump radiation comprising the first frequency component, and a seed generating element operable to generate the second frequency component from the pump radiation.

8. The seed generating element comprises: a spectral broadening element operable to broaden the spectrum of a portion of the pump radiation; a filter operable to separate the second frequency component from the broadened radiation; and The multi-wavelength light source device of claim 7 , comprising:

9. The multi-wavelength source device of claim 8 , wherein the spectral broadening element comprises a solid-core photonic crystal fiber.

10. The multi-wavelength light source device of claim 7 , wherein the seed generation element comprises one of a difference frequency generator, an optical parametric generator, an optical parametric amplifier, or an optical parametric oscillator.

11. 11. The multi-wavelength light source device according to claim 1, wherein a difference in frequency between the first frequency component and the second frequency component is 10 nm to 200 nm.

12. The multi-wavelength light source device according to any one of claims 1 to 11, wherein the wavelength range of interest includes at least wavelengths from 400 nm to 2000 nm.

13. 1. A method for generating output radiation comprising a plurality of discrete output wavelength bands, comprising: generating input radiation comprising at least a first frequency component and a second frequency component; exciting the confined working medium with the input radiation to generate the plurality of discrete output wavelength bands distributed across a wavelength range of interest via a seed-assisted cascaded four-wave mixing (FWM) process within the working medium; A method comprising:

14. 14. The method of claim 13, comprising generating the first and second frequency components from a single pump radiation source operable to output pump radiation at only the first frequency.

15. a substrate support portion for supporting a substrate; A multi-wavelength light source device according to any one of claims 1 to 12, at least one optical system operable to direct the output radiation from the multi-wavelength light source device to the substrate and to capture radiation scattered from the substrate; A metrology device comprising: Optionally, the metrology device is operable as a scatterometer metrology apparatus, a level sensor or an alignment sensor.

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