Multi-pass radiation device

Abstract

A radiation device for generating broadband output radiation upon receiving pulsed input radiation, the radiation device comprising: an oscillation cavity comprising a first reflective surface and a second reflective surface; and a non-linear medium, the non-linear medium being located between the first reflective surface and the second reflective surface; wherein the oscillation chamber is configured to receive the pulsed input radiation and oscillate the pulsed input radiation therein, such that the pulsed input radiation undergoes a filamentation process, resulting in the formation of one or more filaments in the non-linear medium, and causing the pulsed input radiation to undergo multiple passes through the non-linear medium such that the pulsed input radiation is spectrally broadened by the non-linear medium to form the broadband output radiation.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP application 22195017.3 filed on September 12, 2022, and the EP application is incorporated herein by reference in its entirety. Technical Field

[0003] The present invention relates to a multi-pass broadband radiation source and, in particular, to such a broadband radiation source in connection with metrology applications in integrated circuit manufacturing. Background Art

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

[0005] To project a pattern on a substrate, a lithographic apparatus may use electromagnetic radiation. 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. A lithographic apparatus having extreme ultraviolet (EUV) radiation with a wavelength in the range of 4 nm to 20 nm (for example, 6.7 nm or 13.5 nm) may be used to form smaller features on a substrate compared to a lithographic apparatus using radiation having a wavelength of, for example, 193 nm.

[0006] Low k1 lithography can be used to process features where the dimensions are smaller than the classical resolution limit of the lithographic apparatus. In such a process, the resolution formula can be expressed as CD=k1×λ / NA, where λ is the wavelength of the radiation employed, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the "critical dimension" (usually the smallest feature size printed, but in this case the half pitch), and k1 is an empirical resolution factor. In general, the smaller the k1, the more difficult it becomes to reproduce a pattern on a substrate that is similar in shape and size to what the circuit designer planned to achieve specific electrical functionality and performance. To overcome these difficulties, complex fine-tuning steps can be applied to the lithographic projection apparatus and / or the design layout. These include, by way of example and without limitation, optimization of NA, customized illumination schemes, use of phase-shifted patterning devices, various optimizations of the design layout (e.g., optical proximity correction (OPC, sometimes also referred to as "optical and process correction") in the design layout, or other methods generally defined as "resolution enhancement techniques" (RET). Alternatively, a tight control loop for controlling the stability of the lithographic apparatus can be used to improve the reproduction of patterns at low k1.

[0007] Metrology tools are used in many aspects of the IC manufacturing process, such as alignment tools for correctly positioning a substrate prior to exposure, leveling tools for measuring the surface topology of a substrate, for example, focus control and scatterometry-based tools for checking / measuring the products of exposure and / or etching in process control. In each case, a radiation source is required. Broadband or white light radiation sources are increasingly used for such metrology applications for a variety of reasons, including measurement robustness and accuracy. It would be desirable to improve existing devices for broadband radiation generation. Summary of the invention

[0008] According to a first aspect of the present invention, there is provided a radiation device for generating broadband output radiation upon receiving pulsed input radiation, the radiation device comprising: an oscillation cavity comprising a first reflection surface and a second reflection surface; and a nonlinear medium located between the first reflection surface and the second reflection surface; wherein the oscillation cavity is configured to receive and oscillate the pulsed input radiation therein, so that the pulsed input radiation undergoes a filamentation process, thereby causing one or more filaments to be formed in the nonlinear medium, and so that the pulsed input radiation undergoes multiple passes through the nonlinear medium, so that the pulsed input radiation is spectrally broadened by the nonlinear medium to form the broadband output radiation.

[0009] According to a second aspect of the present invention, there is provided a radiation source comprising: the radiation device according to the first aspect and a pump radiation source configured to output pulsed input radiation.

[0010] According to a third aspect of the present invention, there is provided a method for constructing a radiation device according to the first aspect, comprising: providing a pulse input radiation having an input spectrum; defining a target output spectrum; determining an amount of nonlinear spectral broadening required to broaden the input spectrum to the target output spectrum via the nonlinear spectral broadening process; determining a total number of passes through the nonlinear medium to achieve the determined amount of nonlinear spectral broadening; determining an input direction of the pulse input radiation into an oscillation cavity so as to achieve the determined total number of passes through the nonlinear medium; and guiding the pulse input radiation to follow the determined input direction.

[0011] According to a fourth aspect of the present invention, there is provided a method for constructing a radiation source according to the second aspect, comprising: determining a critical power for self-focusing in the nonlinear medium; constructing the pump radiation source so that the pulse input radiation has a peak power equal to or greater than the determined critical power for self-focusing; determining a focused radiation beam size of the pulse input radiation in the oscillation cavity, the focused radiation beam size having an intensity sufficient to achieve a filamentation process; determining a radius of curvature of a first reflecting surface and / or a radius of curvature of a second reflecting surface so that when reflected by any of the first reflecting surface and the second reflecting surface, the pulse input radiation is focused into a radiation beam size substantially the same as the determined focused radiation beam size at the focal plane of the oscillation cavity; and separating the first reflecting surface from the second reflecting surface along a common longitudinal axis by a distance equal to half the sum of the radius of curvature of the first reflecting surface and the radius of curvature of the second reflecting surface.

[0012] According to a fifth aspect of the present invention, there is provided a method for generating broadband output radiation, comprising: generating pulsed input radiation; repeatedly reflecting the pulsed input radiation back and forth during multiple passes through a nonlinear medium, so that the pulsed input radiation undergoes a filamentation process, thereby causing one or more filaments to be formed in the nonlinear medium, and so that after the multiple passes through the nonlinear medium, the pulsed input radiation is spectrally broadened by the nonlinear medium to form the broadband output radiation; and outputting the broadband output radiation.

[0013] Other aspects of the invention include a measurement device comprising a radiation source according to the second aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0015] - Figure 1 depicts a schematic overview of a lithography apparatus;

[0016] - Figure 2 A schematic overview of a lithography cell is depicted;

[0017] - Figure 3 depicts a schematic representation of overall lithography showing the collaboration between three key technologies to optimize semiconductor manufacturing;

[0018] - Figure 4 depicts a schematic overview of a scatterometry apparatus used as a metrology device which may include a radiation source according to an embodiment of the present invention;

[0019] - Figure 5 depicts a schematic overview of a level sensor device that may include a radiation source according to an embodiment of the present invention;

[0020] - Figure 6 depicts a schematic overview of an alignment sensor device that may include a radiation source according to an embodiment of the present invention;

[0021] - Figure 7 A schematic overview of a prior art laser driven radiation source is depicted;

[0022] - Figure 8 A schematic overview of a prior art hollow core photonic crystal fiber based broadband radiation source is depicted;

[0023] - Fig. 9 The diagram shows three filaments formed due to the filamentation process in a nonlinear medium;

[0024] - Fig.10 depicts a schematic overview of a multi-pass radiation device based on a gaseous nonlinear medium according to an embodiment;

[0025] - Fig.11 The diagram shows a method suitable for use in a multi-pass radiation device MPC (e.g. Fig.10 Mirror arrangement used in );

[0026] - Fig.12 A method for constructing a multi-pass radiation device (e.g., Fig.10 Flow chart of the method shown in );

[0027] - Fig.13 depicts a schematic overview of a solid-state nonlinear medium-based multi-pass radiation device according to an embodiment;

[0028] - Fig.14 A method for constructing a multi-pass radiation device (e.g., Fig.10 or Fig.13 Flow chart of a method for a radiation source as shown in ); and

[0029] - Fig.15 A block diagram of a computer system for controlling a broadband radiation source is depicted. DETAILED DESCRIPTION

[0030] In this document, the terms "radiation" and "radiation beam" are used to encompass all types of electromagnetic radiation, including UV radiation (e.g. having a wavelength of 365 nm, 248 nm, 193 nm, 157 nm or 126 nm) and EUV (extreme ultraviolet radiation, e.g. having a wavelength in the range of 5 nm to 100 nm).

[0031] The terms "reticle", "mask" or "patterning device" as employed herein may be broadly interpreted as referring to a general patterning device that may be used to impart an incident radiation beam with a patterned cross-section corresponding to the pattern to be produced in 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, binary, phase-shifting, hybrid, etc.), other examples of such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0032] Figure 1 A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA comprises an illumination system (also referred to as an illuminator) IL configured to condition a radiation beam B (e.g. UV radiation, DUV radiation or EUV radiation), a mask support (e.g. mask table) MT configured to support a patterning device (e.g. 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. wafer stage) WT configured to hold a substrate (e.g. 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. 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.

[0033] In operation, the illumination system IL receives a radiation beam B from a radiation source SO, for example via a radiation beam delivery system BD. The illumination system IL may include various types of optical components for directing, shaping and / or controlling the radiation, for example, refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical components, or any combination thereof. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0034] The term "projection system" PS as used herein should be broadly interpreted as encompassing various types of projection systems, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation used and / or for other factors (e.g., the use of an immersion liquid or the use of a vacuum). Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0035] The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index (e.g. water) so as to fill the space between the projection system PS and the substrate W (this is also known as immersion lithography). More information on immersion techniques is given in US6952253, incorporated herein by reference.

[0036] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also referred to as a "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or steps in preparation for subsequent exposure of a substrate W may be performed on a substrate W on one of the substrate supports WT while another substrate W on another substrate support WT is being used to expose a pattern on another substrate W.

[0037] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measuring stage. The measuring stage is arranged to hold sensors and / or cleaning means. The sensors may be arranged to measure properties of the projection system PS or properties of the radiation beam B. The measuring stage may hold a plurality of sensors. The cleaning means may be arranged to clean a part of the lithographic apparatus, for example a part of the projection system PS or a part of the system for providing immersion liquid. The measuring stage may be moved under the projection system PS when the substrate support WT is away from the projection system PS.

[0038] 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 by a pattern (design layout) present on the patterning device MA. After traversing the mask MA, the radiation beam B passes through a projection system PS which focuses the radiation beam onto a target portion C of the substrate W. With the help of a second positioner PW and a position measurement system IF, the substrate support WT can be accurately moved, for example, in order to position different target portions C in the path of the radiation beam B in a focused and aligned position. Similarly, a first positioner PM and possibly a further position sensor (which is located in the substrate support WT) are provided. Figure 1The patterning device MA (not explicitly depicted in the figure) can be used to accurately position the patterning device MA relative to the path of the radiation beam B. The patterning device MA and the substrate W can be aligned using the mask alignment marks M1, M2 and the substrate alignment marks P1, P2. Although the substrate alignment marks P1, P2 occupy dedicated target portions as shown, they can be located in the space between target portions. When the substrate alignment marks P1, P2 are located between the target portions C, the substrate alignment marks P1, P2 are called scribe-lane alignment marks.

[0039] like Figure 2 As shown in , the lithographic apparatus LA may form part of a lithographic cell LC, sometimes also referred to as a lithocell or a (lithographic) cluster, which typically also includes equipment for performing pre-exposure and post-exposure processes on a substrate W. Conventionally, these 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 example, for regulating the temperature of the substrate W (e.g., for regulating the solvent in the resist layer). A substrate handler or robot RO picks up the substrate W from the input / output ports I / O1, I / O2, moves the substrate W between the different processing equipment, and delivers the substrate W to a loading station LB of the lithographic apparatus LA. The devices in the lithographic cell (the devices are also generally referred to as tracks) are typically under the control of a track control unit TCU, which itself may be controlled by a supervisory control system SCS, which may also control the lithographic apparatus LA, for example, via a lithographic control unit LACU.

[0040] In order for substrates W exposed by the lithographic apparatus LA to be correctly and consistently exposed, it may be desirable to inspect the substrates to measure properties of the patterned structures, such as overlay errors between subsequent layers, line thickness, critical dimensions (CD), etc. For this purpose, an inspection tool (not shown) may be included in the lithography cell LC. If errors are detected, particularly if the inspection is completed before other substrates W of the same batch or lot are yet to be exposed or processed, adjustments may be made to the exposure of subsequent substrates or to other processing steps to be performed on the substrates W, for example.

[0041] An inspection apparatus, which may also be referred to as a metrology apparatus, is used to determine properties of the substrate W, and in particular, to determine how properties of different substrates W vary or how properties associated with different layers of the same substrate W vary from layer to layer. Alternatively, the inspection apparatus may be configured to identify defects on the substrate W, and may for example be part of a lithography cell LC, or may be integrated into a lithography apparatus LA, or may even be a stand-alone device. The inspection apparatus may measure properties on a latent image (the image in the resist layer after exposure), or on a semi-latent image (the image in the resist layer after a post-exposure bake step PEB), or on a developed resist image (wherein exposed or unexposed parts of the resist have been removed), or even on an etched image (after a pattern transfer step such as etching).

[0042] Typically, the patterning process in the lithographic apparatus LA is one of the most critical steps in the process requiring high accuracy in sizing and placement of structures on the substrate W. To ensure this high accuracy, three systems can be combined in a so-called “holistic” control environment, such as Figure 3 Schematically depicted in . One of these systems is a lithography apparatus LA, which is (virtually) connected to a metrology tool MT (a second system) and a computer system CL (a third system). The key to such an "integrated" environment is to optimize the cooperation between these three systems to enhance the overall process window, and to provide a tight control loop to ensure 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 functional semiconductor device) - typically within which process parameter variations in the lithography process or patterning process are allowed.

[0043] The computer system CL can use (a portion of) the design layout to be patterned to predict which resolution enhancement technique to use and to perform computational lithography simulations and calculations to determine which mask layout and lithography equipment settings achieve the maximum overall process window for the patterning process (in Figure 3 Typically, the resolution enhancement technique is arranged to match the patterning possibilities of the lithographic apparatus LA. The computer system CL may also be used to detect where within the process window the lithographic apparatus LA is currently operating (e.g. using input from a metrology tool MT) to predict whether defects may be present (e.g. due to suboptimal processing). Figure 3 ) is depicted by an arrow pointing to “0” in the second scale SC2.

[0044] The metrology tool MT may provide input to the computer system CL to enable accurate simulations and predictions, and may provide feedback to the lithographic apparatus LA to, for example, identify possible drift in the calibration state of the lithographic apparatus LA. Figure 3 ) as depicted by multiple arrows in a third scale SC3.

[0045] In the lithographic process, it is desirable to frequently make measurements of the produced structure, for example for process control and verification. The tool that makes such measurements is generally referred to as a measurement tool MT. Different types of measurement tools MT for making such measurements are known, including scanning electron microscopes or various forms of scatterometer measurement tools MT. Scatterometers are multi-purpose instruments that allow the measurement of parameters of the lithographic process by having a sensor in a pupil or a plane conjugated with the pupil of the objective lens of the scatterometer, the measurement being generally referred to as pupil-based measurement, or by having a sensor in an image plane or a plane conjugated with the image plane to measure parameters of the lithographic process, in which case the measurement is generally referred to as image-based or field-based measurement. Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP1,628,164A, which are incorporated herein by reference in their entirety. The aforementioned scatterometer can measure the grating using light from soft X-rays and visible to the near IR wavelength range.

[0046] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, reconstruction methods can be applied to the measured signals to reconstruct or calculate the properties of the grating. Such a reconstruction can, for example, be produced by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulated results with the measured results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to the pattern observed from a real target.

[0047] In a second embodiment, the scatterometer MT is a spectral scatterometer MT. In such a spectral scatterometer MT, radiation emitted by a radiation source is directed onto a target, and reflected or scattered radiation from the target is directed to a spectrometer detector which measures the spectrum of the specularly reflected radiation (i.e. a measure of intensity as a function of wavelength). From this data, the structure or profile of the target that produced the detected spectrum can be reconstructed, for example by rigorous coupled wave analysis and nonlinear regression or by comparison with a library of simulated spectra.

[0048] In a third embodiment, the scatterometer MT is an ellipsometer. An ellipsometer allows the parameters of a lithography process to be determined by measuring the scattered radiation of each polarization state. Such a measurement device emits polarized light (e.g., linear, circular, or elliptical) by using, for example, an appropriate polarization filter in the illumination portion of the measurement device. A source suitable for the measurement device may also provide polarized radiation. Various embodiments of existing ellipsometers are described in U.S. patent applications 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,449, 12 / 920,968, 12 / 922,587, 13 / 000,229, 13 / 033,135, 13 / 533,110, and 13 / 891,410, which are incorporated herein by reference in their entirety.

[0049] In one embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the overlap of two misaligned gratings or periodic structures by measuring the reflected spectrum and / or asymmetry in the detection construction, the asymmetry being related to the degree of overlap. The two (usually overlapping) grating structures may be applied in two different layers (not necessarily continuous layers) and may be formed substantially at the same position on the wafer. The scatterometer may have a symmetrical detection construction as described, for example, in the co-owned patent application EP1,628,164A, so that any asymmetry is clearly distinguishable. This provides a direct way to measure misalignment in the grating. Other examples of measuring the overlap error between two layers containing a periodic structure when measuring a target by the asymmetry of the periodic structure may be found in PCT patent application publication No. WO 2011 / 012624 or in U.S. patent application US 20160161863, which are incorporated herein by reference in their entirety.

[0050] Other parameters of interest may be focus and dose. Focus and dose may be determined simultaneously by scatterometry (or alternatively, by scanning electron microscopy), as described in U.S. Patent Application US2011-0249244, which is incorporated herein by reference in its entirety. A single structure may be used that has a unique combination of critical dimension and sidewall angle measurements for each point in the focus energy matrix (FEM - also referred to as the focus exposure matrix). If these unique combinations of critical dimension and sidewall angle are available, focus and dose values ​​may be uniquely determined from these measurements.

[0051] The metrology target may be a collection of composite gratings formed by a lithography process (for example, mainly in resist), but also after an etching process. Typically, the pitch and line width of the structures in the gratings strongly depend on the measurement optics (specifically, the NA of the optics) to be able to capture the diffraction orders from the metrology target. As indicated earlier, the diffracted signal can be used to determine the shift between two layers (also known as 'overlap'), or can be used 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-segments that are constructed to mimic the size of a functional portion of the design layout in the target. Due to the sub-segmentation, the target will behave more similar to the functional portion of the design layout, so that the overall process parameter measurement better resembles the functional portion of the design layout. The target may be measured in an underfill mode or an overfill mode. In the underfill mode, the measurement radiation beam produces a radiation spot that is smaller than the overall target. In the overfill mode, the measurement radiation beam produces a radiation spot that is larger than the overall target. In such an overfill mode it is also possible to measure different targets simultaneously and thus to determine different process parameters simultaneously.

[0052] The overall measurement quality of the lithography parameters using a specific target is at least partially determined by the measurement scheme used to measure the lithography parameters. The term "substrate measurement scheme" can include one or more parameters of the measurement itself, one or more parameters of the one or more patterns measured, or both. For example, if the measurement used in the substrate measurement scheme is an optical measurement based on diffraction, one or more of the measured parameters may include the wavelength of the radiation, the polarization of the radiation, the incident angle of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, etc. One of the criteria for selecting the measurement scheme can be, for example, the sensitivity of one of the measurement parameters to process changes. More examples are described in U.S. patent application US2016-0161863 and published U.S. patent application US2016 / 0370717A1, which are incorporated herein by reference in their entirety.

[0053] exist Figure 4 A measurement device, e.g. a scatterometer, is depicted in FIG. It comprises a broadband (white light) radiation projector 2 which projects radiation onto a substrate 6. The reflected or scattered radiation is passed to a spectrometer detector 4 which measures the spectrum 10 of the specularly reflected radiation (i.e. a measure of the intensity as a function of wavelength). From this data, the processing unit PU may perform a multi-analysis of the specularly reflected radiation, e.g. by rigorous coupled wave analysis and nonlinear regression or by comparing the specularly reflected radiation to the spectrometer detector 4. Figure 3The structure or profile that produced the detected spectrum is reconstructed by comparing it to a library of simulated spectra shown at the bottom of the figure. Typically, for the 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, leaving only a few parameters of the structure to be determined from the scatterometry data. Such a scatterometer can be configured as a normal incidence scatterometer or an oblique incidence scatterometer.

[0054] The overall measurement quality of the lithographic parameters measured via the measurement of the measurement target is at least partially determined by the measurement scheme used to measure the lithographic parameters. The term "substrate measurement scheme" may include one or more parameters of the measurement itself, one or more parameters of the one or more patterns measured, or both. For example, if the measurement used in the substrate measurement scheme is an optical measurement based on diffraction, one or more of the measured parameters may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, etc. One of the criteria for selecting the measurement scheme may be, for example, the sensitivity of one of the measurement parameters to process variations. More examples are described in U.S. patent application US2016 / 0161863, which is incorporated herein by reference in its entirety, and in the disclosed U.S. patent application US2016 / 0370717A1.

[0055] Another type of measurement tool used in IC manufacturing is a topography measurement system, a level sensor or a height sensor. Such a tool can be integrated in a lithography device to measure the topography of the top surface of a substrate (or wafer). A topographic map (also referred to as a height map) of the substrate can be generated based on these measurement results, which indicate the height of the substrate as a function of the position on the substrate. The height map can then be used to correct the position of the substrate during the transfer of the pattern onto the substrate so as to provide an aerial image of the patterning device in an appropriate focus position on the substrate. It will be understood that "height" in this context refers to a dimension (also referred to as the Z axis) that is roughly outside the plane of the substrate. Typically, a level or height sensor performs measurement at a fixed position (relative to its own optical system), and the relative movement between the substrate and the optical system of the level or height sensor results in a height measurement at a position across the substrate.

[0056] exist Figure 5 An example of a level or height sensor LS as known in the art is schematically shown in FIG. Figure 5Only the operating principle is illustrated. In this example, the level sensor comprises an optical system comprising a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO, which provides a radiation radiation beam LSB imparted by a projection grating PGR of the projection unit LSP. The radiation source LSO can be, for example, a narrowband or broadband light source, such as a supercontinuum light source, a polarized or non-polarized, pulsed or continuous (e.g. polarized or non-polarized) laser radiation beam. The radiation source LSO can include a plurality of radiation sources with different colors or wavelength ranges, such as a plurality of LEDs. The radiation source LSO of the level sensor LS is not limited to visible radiation, but can additionally or alternatively include UV and / or IR radiation and any wavelength range suitable for reflection from the surface of the substrate.

[0057] The projection grating PGR is a periodic grating comprising a periodic structure, which causes the radiation beam BE1 to have a periodically varying intensity. The radiation beam BE1 with a periodically varying intensity is directed toward a measurement position MLO on the substrate W, the measurement position MLO having an incident angle ANG relative to an axis perpendicular to the incident substrate surface (Z axis), the incident angle ANG being between 0 and 90 degrees, typically between 70 and 80 degrees. At the measurement position MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and directed toward the detection unit LSD.

[0058] In order to determine the height level at the measuring position MLO, the level sensor further comprises a detection system comprising a detection grating DGR, a detector DET and a processing unit (not shown) for processing an output signal of the detector DET. The detection grating DGR may be identical to the projection grating PGR. The detector DET generates a detector output signal indicating the light received by, for example, a photodetector (for example, indicating the intensity of the received light) or representing the spatial distribution of the intensity received by, for example, a camera. The detector DET may comprise any combination of one or more detector types.

[0059] By means of triangulation techniques the height level at the measurement position MLO can be determined.The detected height level is typically related to the signal intensity as measured by the detector DET, which has a periodicity that depends inter alia on the design of the projection grating PGR and the (oblique) angle of incidence ANG.

[0060] The projection unit LSP and / or the detection unit LSD may comprise further optical elements (not shown), such as lenses and / or mirrors, along the path of the patterned radiation beam between the projection grating PGR and the detection grating DGR.

[0061] In an embodiment, the detection grating DGR may be omitted and the detector DET may be placed where the detection grating DGR is located. Such a configuration provides a more direct detection of the image of the projection grating PGR.

[0062] In order to effectively cover the surface of the substrate W, the level sensor LS may be configured to project an array of measurement radiation beams BE1 onto the surface of the substrate W, thereby generating an array of measurement areas MLO or radiation spots covering a larger measurement range.

[0063] For example, various height sensors of the general type are disclosed in US7265364 and US7646471, both of which are incorporated by reference. A height sensor using UV radiation rather than visible or infrared radiation is disclosed in US2010233600A1, incorporated by reference. In WO2016102127A1, incorporated by reference, a compact height sensor is described that uses a multi-element detector to detect and identify the position of a grating image without the need for a detection grating.

[0064] Another type of metrology tool used in IC manufacturing is an alignment sensor. Therefore, a key aspect of the performance of a lithographic apparatus is the ability to correctly and accurately place an applied pattern relative to features laid down in a previous layer (either 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 at a later time using a position sensor (typically, an optical position sensor). The position sensor may be referred to as an "alignment sensor" and the mark may be referred to as an "alignment mark".

[0065] The lithographic apparatus may include one or more (e.g., multiple) alignment sensors, by which the positions of alignment marks disposed on the substrate can be accurately measured. The alignment (or position) sensor may use optical phenomena such as diffraction and interference to obtain position information from alignment marks formed on the substrate. An example of an alignment sensor used in a current lithographic apparatus is based on a self-referencing interferometer as described in US6961116. Various enhancements and modifications of position sensors have been developed, for example, as disclosed in US2015261097A1. The contents of all these publications are incorporated herein by reference.

[0066] Figure 6is a schematic block diagram of an embodiment of a known alignment sensor AS, such as, for example, described in US6961116, and US6961116 is incorporated by reference. A radiation source RSO provides a radiation beam RB of one or more wavelengths, which is steered onto a mark (e.g., a mark AM located on a substrate W) as an illumination radiation spot SP by steering optics. In this example, the steering optics include a radiation spot reflector SM and an objective lens OL. The illumination radiation spot SP by which the mark AM is illuminated may be slightly smaller in diameter than the width of the mark itself.

[0067] Radiation diffracted by the alignment mark AM is collimated (in this example, via the objective lens OL) into an information-bearing radiation beam IB. The term "diffraction" is intended to include zero-order diffraction from the mark (which may be referred to as reflection). A self-referencing interferometer SRI, for example of the type disclosed in US6961116 mentioned above, causes the radiation beam IB to interfere with itself, after which the radiation beam is received by the photodetector PD. Additional optics (not shown) may be included to provide separate radiation beams in the event that more than one wavelength is created by the radiation source RSO. The photodetector may be a single element, or, if desired, it may comprise a plurality of pixels. The photodetector may comprise an array of sensors.

[0068] The steering optics comprising in this example a radiation spot mirror SM may also be used to block zeroth order radiation reflected from the mark so that the information-bearing radiation beam IB comprises only higher order diffracted radiation from the mark AM (this is not necessary for measurement but improves the signal-to-noise ratio).

[0069] The intensity signal SI is supplied to the processing unit PU. By a combination of optical processing in block SRI and computational processing in unit PU, the values ​​of the X position and Y position on the substrate relative to the reference system are output.

[0070] A single measurement of the type illustrated only fixes the position of the mark within a certain range corresponding to one pitch of the mark. A coarser measurement technique is used in conjunction with this to identify which period of the sine wave is the period containing the position of the mark. The same process at a coarser and / or finer level can be repeated at different wavelengths for improved accuracy and / or for robust detection of the mark, regardless of the material the mark is made of and the material on and / or below which the mark is placed. The wavelengths can be optically multiplexed and demultiplexed so as to be processed simultaneously, and / or they can be multiplexed by time division or frequency division.

[0071] In this example, the alignment sensor and the radiation spot SP remain stationary, while it is the substrate W that moves. Thus, the alignment sensor can be rigidly and accurately mounted to the reference frame, while effectively scanning the mark AM in a direction opposite to the direction of movement of the substrate W. The substrate W is controlled in this movement by mounting the substrate W on a substrate support and controlling the movement of the substrate support by a substrate positioning system. A substrate support position sensor (e.g., an interferometer) measures the position of a substrate support (not shown). In an embodiment, one or more (alignment) marks are provided on the substrate support. Measurement of the position of the mark provided on the substrate support allows calibration of the position of the substrate support determined by the position sensor (e.g., relative to the position of the frame to which the alignment system is connected). Measurement of the position of the alignment mark provided on the substrate allows determination of the position of the substrate relative to the substrate support.

[0072] The measurement tool MT (for example, the scatterometer, topography system or position measurement system mentioned above) can use radiation originating from a radiation source to perform measurements. The properties of the radiation used by the measurement tool may affect the type and quality of the measurements that can be performed. For some applications, it may be advantageous to use multiple radiation frequencies to measure the substrate, for example, broadband radiation may be used. Multiple different frequencies may be able to propagate, illuminate and scatter the measurement target without or minimally interfering with other frequencies. Therefore, different frequencies may, for example, be used to obtain more measurement data at the same time. Different radiation frequencies may also be able to interrogate and discover different properties of the measurement target. Broadband radiation may be used in the measurement system MT (for example, for example, a level sensor, an alignment mark measurement system, a scattering measurement tool or an inspection tool). The broadband radiation source may be a supercontinuum source.

[0073] It may be difficult to produce high quality broadband radiation, for example, supercontinuum radiation. One method for producing broadband radiation may be to broaden high power narrowband or single frequency input radiation or pump radiation, for example, utilizing nonlinear higher order effects. The input radiation, which may be generated using a laser, may be referred to as pump radiation. Alternatively, the input radiation may be referred to as seed radiation. In order to obtain high power radiation for the broadening effect, the radiation may be confined to a small region so that strongly localized high intensity radiation is achieved. In those regions, the radiation may interact with the broadening structures and / or materials forming the nonlinear medium to produce broadband output radiation. In the high intensity radiation region, different materials and / or structures may be used to achieve and / or improve radiation broadening by providing a suitable nonlinear medium.

[0074] In some existing embodiments, broadband output radiation is produced by tightly focusing a laser radiation beam into an ionizable medium (e.g., an inert gas) contained in a specially constructed chamber. The laser radiation beam provides laser energy to the ionized medium in the chamber to maintain or generate a plasma that produces the broadband radiation. Such a radiation source is referred to as a laser driven radiation source.

[0075] Figure 7 A simplified schematic diagram of a prior art laser driven radiation source is shown. Further details on such radiation sources can be found in US Pat. No. 7,435,982 (B2), which is incorporated herein by reference. Figure 7 As shown in FIG, a chamber CB comprises an ionizable medium and an ignition source. At least a portion of the chamber wall is made of a material transparent to the input radiation IRD. The ignition source comprises two electrodes ET1, ET2 configured to ignite or ionize the ionizable medium. The input radiation is tightly focused into the ionized medium by a lens LEN so as to maintain or generate a localized high intensity plasma PS that generates a broadband output radiation ORD. The laser driven radiation source generates non-directional emission exceeding a solid angle of 4π.

[0076] In some existing embodiments, broadband output radiation is generated in a photonic crystal fiber (PCF), specifically a hollow core photonic crystal fiber (HC-PCF) including an anti-resonant structure for confining radiation. HC-PCF with anti-resonant structure is particularly suitable for strong radiation confinement mainly inside the hollow core of the fiber, thereby achieving high radiation intensity. The hollow core of the fiber can be filled with a gas that acts as a broadening medium for broadening the input radiation. Such an optical fiber and gas arrangement can be used to generate a supercontinuum radiation source.

[0077] Figure 8 A simplified schematic diagram of a prior art HC-PCF based broadband radiation source is shown. More details on such a radiation source can be found in US Pat. No. 11237486 (B2). Figure 8 As shown in , the HC-PCF includes a hollow core HC and a cladding portion surrounding the hollow core HC. The cladding portion includes a plurality of anti-resonance elements for guiding radiation through the hollow core HC. In this particular example, the cladding portion includes a single ring of six tubular capillaries (CAP). The hollow core HC is filled with a working medium (e.g., gas) under a certain pressure. After being coupled from one end of the optical fiber into the gas-filled hollow core HC, the narrowband input radiation IRD undergoes a nonlinear spectral broadening process and eventually becomes a broadband output radiation ORD that exits the opposite end of the optical fiber. The HC-PCF radiation source produces highly directional emission.

[0078] When compared to each other, both types of radiation sources have advantages and disadvantages. For example, laser-driven radiation sources generally have simpler system designs than HC-PCF-based radiation sources. This is achieved by Figure 7 and Figure 8 The two exemplary radiation sources shown in Figure 1 are compared to demonstrate this. Less complex system design enables reduced commodity costs and miniaturization. HC-PCF-based radiation sources typically have wider spectral coverage and directivity of the emitted broadband radiation, which increases the radiance of the system. At the same time, HC-PCF-based radiation sources require highly specialized optical fibers, which increases the complexity of the system.

[0079] Both types of radiation sources lack reliable pathways for power scalability and lifetime extension. In laser-driven radiation sources, the unidirectional emission of broadband output radiation and low collection efficiency require increasing the power of the input radiation for power scalability, leading to various lifetime issues. In HC-PCF-based radiation sources, power scaling is achieved by increasing the pulse repetition rate of the input radiation. Such an approach is problematic because it induces thermal effects in the fiber, which leads to lifetime shortening and output instability.

[0080] Multipass cells (or nonlinear multipass cavities) that include nonlinear media have been developed to provide alternative paths for nonlinear spectral broadening and / or nonlinear time compression of laser pulses. Allowing the laser pulse to pass through the nonlinear medium multiple times means that the nonlinearity required for a given amount of spectral broadening is distributed over multiple passes. Thus, to produce the same target spectrum, the peak intensity of the laser pulse required in a multipass cavity is much lower than that required in a single-pass configuration (with the same nonlinear medium). Although nonlinear multipass cavities have great potential for power scaling (e.g., average power scaling), current developments (e.g., work reported in the following scientific publications: Marc Hanna, Xavier Délen, Loïc Lavenu, Florent Guichard, Yoann Zaouter, Frédéric Druon, Patrick Georges, “Nonlinear temporal compression in multipass cells: Theory,” Journal of the Optical Society of America B, vol. 34, p. 1340, 2017; and Huabao Cao, Roland Nagymihály, Vladimir Chyvkov, Nikita Khodakovskiy, and Mikhail Kalashnikov, “Multipass-cell-based post-compression of radially and azimuthally polarized pulses to the sub-two-cycle regime”) have limited potential for power scaling (e.g., average power scaling). regime), Journal of the Optical Society of America B, vol. 36, pp. 2517-2525, 2019, both of which are incorporated herein by reference) mainly focus on further pulse energy scaling and / or further spectral coverage extension. However, the average power of existing nonlinear multi-pass cavities is still limited and insufficient for many measurement applications as described above. Therefore, it is desirable to better exploit the power scalability of nonlinear multi-pass cavities in order to produce high-power broadband radiation suitable for use in the aforementioned measurement systems.

[0081] According to a first aspect of the present disclosure, there is provided a radiation device for generating broadband output radiation upon receiving pulsed input radiation, the radiation device comprising: an oscillation cavity comprising a first reflection surface and a second reflection surface; and a nonlinear medium located between the first reflection surface and the second reflection surface; wherein the oscillation cavity is configured to receive and oscillate the pulsed input radiation therein, so that the pulsed input radiation undergoes a filamentation process, thereby causing one or more filaments to be formed in the nonlinear medium, and so that the pulsed input radiation undergoes multiple passes through the nonlinear medium, so that the pulsed input radiation is spectrally broadened by the nonlinear medium to form broadband output radiation.

[0082] Note that the oscillation cavity mentioned above should be distinguished from the laser cavity, in which the laser pulse travels back and forth on the same optical path for an infinite number of bounces. In the context of the present disclosure, the oscillation cavity is defined as a space surrounded by a first reflective surface and a second reflective surface. In such a space, the laser pulse travels back and forth for a finite number of bounces, wherein after each bounce from one of the reflective surfaces, the laser pulse may travel a different path to reach the other reflective surface.

[0083] Compared to many existing nonlinear multi-pass cavities (e.g., EP3143669A1 incorporated herein by reference) in which the peak power of the input radiation is intentionally kept below the critical power for self-focusing in order to prevent the self-focusing effect (also known as the Kerr effect) from negatively affecting spectral broadening, the inventors have found that as long as the pulsed input radiation undergoes a filamentation process during each pass through the nonlinear medium, the self-focusing effect can be beneficial in increasing the efficiency of the nonlinear spectral broadening process in the nonlinear multi-pass cavity and thereby increasing the power of the broadband output radiation. As described in the scientific publication Audrius Dubietis, Gintaras Tamosauskas, Rosvaldas Šuminas, Vytautas Jukna, “Ultrafast supercontinuum generation in bulk condensed media”, Lithuanian Journal of Physics, Vol. 57, No. 3, pp. 113-157 (2017), which is incorporated herein by reference, the filamentation process “arises from the interaction between self-focusing, self-phase modulation, multiphoton / ionization induced free electron plasma”, leading to “the emergence of dynamic structures with strong cores that are able to propagate over extended distances much larger than typical diffraction lengths while maintaining a narrow radiation beam size without the help of any external guiding mechanism”.

[0084] The self-focusing effect is caused by the intensity-dependent refractive index n, which is expressed as:

[0085] n=n0+n2I,[1]

[0086] Where n0 is the linear refractive index, n2 is the nonlinear refractive index related to the third-order optical susceptibility of the material, and I is the intensity of the pulsed input radiation. The induced change in refractive index is proportional to the local intensity and is therefore higher in the central region of the pulsed input radiation beam and lower in the peripheral region. The nonlinear medium thus acts like a lens that forces the pulsed input radiation beam to self-focus.

[0087] Self-phase modulation (SPM) is a well-known nonlinear effect that results from a nonlinear change in the phase of a pulse imparted by a time-varying refractive index, as expressed in equation [1]. The nonlinear phase change results in a frequency change that results in a time-varying instantaneous frequency and, therefore, in a spectral broadening of the pulse. The time-varying instantaneous frequency δω(t) is expressed as (assuming a Gaussian laser pulse):

[0088] , [2]

[0089] in is the pulse duration of the Gaussian pulse, ω0 is the carrier frequency, and z is the propagation distance, and t is the time.

[0090] The requirement for starting the filamentation process is that the peak power of the input pulse reaches the self-focusing threshold or the critical power for self-focusing P cr :

[0091] P cr =3.72λ 2 / 8πn0n2, [3]

[0092] where λ is the wavelength of the pulsed input radiation. crAt , the effect of self-focusing accurately balances the diffractive spread of the laser radiation beam. This can be understood as follows: when the laser radiation beam self-focuses, the intensity of the laser radiation beam increases, which further strengthens the self-focusing effect; however, the increase in laser intensity is not infinite and will end after the pulse intensity becomes strong enough to start multiphoton absorption and ionization in the nonlinear medium; multiphoton absorption and ionization produce energy losses and generate free electron plasma, which further absorbs and defocuses the laser radiation beam. As the laser intensity decreases, multiphoton absorption and ionization lose their effects. Therefore, the defocused laser radiation beam will self-focus again. Such a focusing-defocusing-refocusing process will continue as long as the conditions for the filamentation process are still met (for example, the critical power P for self-focusing cr ). The filamentation process described above will result in the formation of one or more filaments in the nonlinear medium.

[0093] Fig. 9 Three exemplary filaments formed due to the filamentation process in a nonlinear medium are schematically depicted. Fig. 9 As shown in FIG, the pulse input radiation IRD is focused into the nonlinear medium NLM. The peak power of the pulse input radiation IRD is equal to or greater than the critical power P for self-focusing. cr . When propagating through the nonlinear medium NLM, the pulsed input radiation IRD undergoes a filamentation process as well as a nonlinear spectral broadening process (e.g., SPM). In this way, the pulsed input radiation IRD is repeatedly focused and defocused in the nonlinear medium NLM until the peak power of the pulsed input radiation IRD no longer reaches the self-focusing threshold. In this exemplary embodiment, the filamentation process results in the formation of three filaments along the propagation direction of the pulsed input radiation IRD. At the same time, the pulsed input radiation IRD is spectrally broadened by the nonlinear spectral broadening process and eventually exits the nonlinear medium NLM as broadband output radiation ORD.

[0094] Embodiments of the proposed multi-pass radiation device will be described in detail below. Fig.10 A multi-pass radiation device MPC based on a gaseous nonlinear medium according to an embodiment is schematically depicted. Fig.10, the nonlinear medium NLM may be a gas, which may be, for example, one or more of the following: helium, neon, argon, krypton, xenon and radon. The gas may substantially fill the space of the oscillation cavity OC, i.e., the space between the first reflection surface RS1 and the second reflection surface RS2. The multi-pass radiation device MPC may include a chamber CB, which is arranged to at least partially surround the nonlinear medium NLM, the first reflection surface RS1, the second reflection surface RS2 and the space therebetween. The chamber CB may include an input window IW configured to substantially transmit the pulsed input radiation IRD and an output window OW configured to substantially transmit the broadband output radiation ORD.

[0095] The multi-pass radiation device MPC may include a first reflector CM1 and a second reflector CM2 configured to provide a first reflective surface RS1 and a second reflective surface RS2, respectively. Fig.11 The diagram shows a method suitable for use in the proposed multi-pass radiation device MPC (e.g., Fig.10 ). Fig.11 As shown in , the first reflecting surface RS1 and the second reflecting surface RS2 may be curved inwardly or concavely, and may have a first radius of curvature ROC1 and a second radius of curvature ROC2, respectively. Correspondingly, the first reflector CM1 and the second reflector CM2 may both be considered as concave reflectors. The first reflecting surface RS1 and the second reflecting surface RS2 may be symmetrical with respect to a common principal axis PA (or longitudinal axis), and may be separated by a distance DT along the common principal axis PA. The distance DT is equal to half of the sum of the first radius of curvature ROC1 and the second radius of curvature ROC2. For a concave reflector, it is known that the focal length FL and the radius of curvature ROC of the reflector satisfy the following relationship:

[0096] FL=ROC / 2. [4]

[0097] Therefore, the spacing distance DT between the two reflecting surfaces RS1 and RS2 is also equal to the sum of the first focal length FL1 of the first reflector CM1 and the second focal length FL2 of the second reflector CM2. In this way, the main focal points of the first reflector CM1 and the second reflector CM2 overlap spatially at the intracavity focusing plane FP of the oscillation cavity OC. In an embodiment, the first radius of curvature ROC1 and the second radius of curvature ROC2 may be equal to each other. In such a case, the spacing distance DT between the two reflectors is equal to the curvature radii ROC1 and ROC2. In an embodiment, the first radius of curvature ROC1 and the second radius of curvature ROC2 may be different from each other.

[0098] In an embodiment, the multi-pass radiation device MPC may include an input mode matching device (not shown) for optimally coupling the pulsed input radiation IRD into the oscillation cavity OC. The input mode matching device may be configured to adjust the radiation beam size and / or divergence of the pulsed input radiation IRD so that the resulting focusing position of the pulsed input radiation IRD in the oscillation cavity OC substantially overlaps with the focal plane FP. In an embodiment, the input mode matching device may include one or more lenses (e.g., a mode matching telescope). In an embodiment, the input mode matching device may include one or more curved mirrors. In an embodiment, the multi-pass radiation device MPC may not include an input mode matching device. In such a case, the mode characteristics of the pulsed input radiation IRD may be adjusted by an external mode matching device located inside or outside the pump radiation source that emits the pulsed input radiation IRD.

[0099] In an embodiment, the multi-pass radiation device MPC may include an output mode matching device, which is configured to adjust the radiation beam size and / or divergence of the broadband output radiation ORD so as to meet the mode requirements of any application (for example, any of the aforementioned measurement applications). In an embodiment, the output mode matching device may include one or more lenses (for example, a mode matching telescope). In an embodiment, the output mode matching device may include one or more curved reflectors. In an embodiment, the multi-pass radiation device MPC may not include an output mode matching device. In such a case, the mode characteristics of the broadband input radiation IRD may be adjusted by an external mode matching device located outside the multi-pass radiation device (for example, between the multi-pass radiation device MPC and the objective lens of the scatterometer).

[0100] In an embodiment, the multi-pass radiation device MPC may include one or more input steering mirrors SM1 arranged to direct the pulsed input radiation towards one of the mirrors. For a given pair of mirrors CM1, CM2, the input direction of the pulsed input radiation IRD relative to the main axes of the mirrors CM1, CM2 may determine the travel path of the pulsed input radiation within the oscillation cavity OC, which determines the total number of passes through the nonlinear medium NLM and therefore the total amount of nonlinear spectral broadening that the pulsed input radiation IRD will experience. In an embodiment, ray tracing software (e.g., OpticStudio sold by Zemax) may be used to determine the input direction and resulting travel path of the pulsed input radiation IRD within the oscillation cavity OC.

[0101] Therefore, according to another aspect of the present disclosure, a method for constructing a multi-pass radiation device MPC is provided (e.g., Fig.10 ). Fig.12 , the method may include, for example, the following six steps.

[0102] Step 1210: Provide a pulsed input radiation IRD having an input spectrum (eg, a narrowband spectrum).

[0103] Step 1220: Define a target output spectrum (eg, a wideband spectrum).

[0104] Step 1230: Determine the amount of nonlinear spectral stretching required to stretch the input spectrum to the target output spectrum.

[0105] Step 1240: Determine the total number of passes through the nonlinear medium NLM to achieve the determined amount of nonlinear spectral broadening.

[0106] Step 1250: Determine the input direction of the pulse input radiation IRD into the oscillator cavity OC so as to achieve the determined total number of passes through the nonlinear medium NLM.

[0107] Step 1260: Directing the pulsed input radiation IRD to follow the determined input direction.

[0108] In an embodiment, one or more input turning mirrors SM1 may be curved mirrors and may thus simultaneously serve as input pattern matching devices.

[0109] In an embodiment, the multi-pass radiation device MPC may comprise one or more output steering mirrors SM2 arranged to direct the broadband output radiation ORD towards the output window OW of the chamber CB. In an embodiment, the one or more output steering mirrors SM2 may be curved mirrors and may simultaneously serve as output mode matching device.

[0110] Return to reference Fig.10 , the irradiation device may be configured to ensure that the pulsed input radiation passes through the nonlinear medium NLM two or more times, and each of the two or more passes through the nonlinear medium NLM may follow a different path. In some embodiments, during each pass, the pulsed input radiation IRD may undergo the aforementioned filamentation process, thereby causing one or more filaments to be formed in the nonlinear medium NLM (see Fig. 9). For a given set of parameters of the pulsed input radiation IRD (e.g., pulse duration, pulse peak power, pulse repetition rate), the parameters of the oscillation cavity OC (e.g., the radii of curvature ROC1, ROC2 of the first reflection surface RS1 and the second reflection surface RS2, the spacing distance between the two reflection surfaces RS1, RS2), and the parameters of the gaseous nonlinear medium NLM (e.g., gas type, gas pressure) can be configured so that each pass of the pulsed input radiation IRD through the nonlinear medium NLM can undergo a filamentation process. In some embodiments, during some of the two or more passes, the pulsed input radiation IRD may undergo the aforementioned filamentation process, resulting in the formation of one or more filaments in the nonlinear medium NLM. The filamentation process enhances the effect of nonlinear spectral broadening in the nonlinear medium at each pass, and thus improves the efficiency of converting the pulsed input radiation IRD into the target broadband output radiation ORD.

[0111] Those skilled in the art will appreciate that the nonlinear medium NLM is not limited to a gaseous nonlinear medium, but may be any nonlinear medium (eg, solid, liquid) capable of achieving nonlinear spectral broadening and filamentation of the pulsed input radiation IRD. Fig.13 A multi-pass radiation device MPC' based on a solid nonlinear medium NLM' according to an embodiment is schematically depicted. The solid nonlinear medium NLM' may be, for example, a yttrium aluminum garnet (YAG) crystal or a fused silica crystal. The solid nonlinear medium NLM' may be placed at a position substantially at the focal plane FP' of the oscillation cavity to benefit from a higher peak intensity of the pulsed input radiation IRD'. Note that Fig.11 The mirror arrangement shown in is also suitable for Fig.13 The embodiment shown in .

[0112] exist Fig.13 In an embodiment, the multi-pass radiation device MPC' may not include the chamber CB. The pulsed input radiation IRD' may be directed into the oscillation cavity OC' by one or more input steering mirrors SM1'. The one or more input steering mirrors SM1' may be curved mirrors and may also serve as input mode matching means (e.g., as described above with respect to Fig.10 In an embodiment, the input mode matching device may be placed in the radiation beam path of the pulsed input radiation IRD′ upstream of one or more input steering mirrors SM1 ′ or in between two input steering mirrors.

[0113] like Fig.13As shown in , the pulsed input radiation IRD' can be focused directly into the solid nonlinear medium NLM'. The pulsed input radiation IRD' can follow a predetermined input direction having an inclination angle relative to the main axis PA' of the reflection surfaces RS1', RS2' (or the reflection mirrors CM1', CM2'). Once it passes through the nonlinear medium NLM', it can continue to travel until it hits the second reflection surface RS2' provided by the second reflection mirror CM2'. The second reflection surface RS2' can reflect the pulsed input radiation IRD' back to the nonlinear medium NLM'. Since the nonlinear medium NLM' is placed at the focal plane FP' of the oscillation cavity OC', the spectrally broadened pulsed input radiation IRD' is refocused into the nonlinear medium NLM' by the second reflection surface RS2'. Once it passes through the nonlinear medium NLM', it can continue to travel until it hits the first reflection surface RS1' provided by the second reflection mirror CM1'. Similar to the second reflection surface RS2', the first reflection surface RS1' can reflect and refocus the spectrally broadened pulsed input radiation IRD' back into the nonlinear medium NLM'. Such a reflection-focusing-reflection process can continue until the spectrally broadened pulse input radiation IRD' finally leaves the oscillation cavity OC', ie, the spectrally broadened pulse input radiation IRD' no longer strikes any reflecting surface RS1', RS2'.

[0114] After exiting the oscillating cavity OC, the spectrally broadened pulsed input radiation IRD' can be regarded as broadband output radiation ORD'. The multi-pass radiation device MPC' may include one or more output steering mirrors SM2' arranged to guide the broadband output radiation ORD'. In an embodiment, one or more output steering mirrors SM2 may be curved mirrors and may simultaneously serve as output mode matching devices.

[0115] Similar to Fig.10 In the embodiment shown in , the total number of passes through the solid nonlinear medium NLM' may depend on the amount of spectral broadening required to broaden the spectrum of the pulsed input radiation IRD' to a target broadband spectrum. In some embodiments, during each of the multiple passes through the solid nonlinear medium NLM', the pulsed input radiation IRD' may undergo a nonlinear spectral broadening process (e.g., SPM) as well as a filamentation process. Thus, during each pass through the solid nonlinear medium NLM', one or more filaments may be formed inside the nonlinear medium NLM' along the propagation direction of the pulsed input radiation IRD'. In some embodiments, during some of the multiple passes through the solid nonlinear medium NLM', in addition to the nonlinear spectral broadening process, the pulsed input radiation IRD' may also undergo a filamentation process.

[0116] Since both the linear and nonlinear refractive indices of solids are much higher than those of gases, the critical power for self-focusing in solids is much lower than in gases according to equation [3]. This means that the focusing requirements to achieve the critical power for self-focusing can be relaxed, and thus the filamentation process can be started more easily in solid nonlinear media than in gaseous nonlinear media.

[0117] The pulsed input radiation IRD, IRD' may have a pulse duration of less than 10000 fs, 5000 fs or 1000 fs; for example, a pulse duration between 1 fs and 10000 fs, or between 1 fs and 5000 fs, or between 1 fs and 1000 fs, or between 10 fs and 500 fs, or between 50 fs and 500 fs, or between 50 fs and 200 fs, or between 100 fs and 200 fs. The pulsed input radiation IRD, IRD' may have a central wavelength in a wavelength range between 400 nm and 2000 nm, or between 400 nm and 1600 nm, or between 400 nm and 1200 nm, or between 600 nm and 1000 nm.

[0118] The broadband output radiation ORD, ORD' may have a spectrum spanning from 200 nm to 2000 nm, or from 200 nm to 1600 nm, or from 200 nm to 1200 nm, or from 400 nm to 1000 nm.

[0119] According to another different aspect of the present disclosure, there is provided a method for constructing a radiation source, the radiation source comprising a multi-pass radiation device MPC, MPC' and a pump radiation source configured to emit a pulsed input radiation IRD. Fig.14 , the method may include, for example, the following six steps.

[0120] Step 1410: Provide a nonlinear medium NLM, NLM' for spectrum broadening of the pulse input radiation IRD, IRD'.

[0121] Step 1420: Determine the critical power for self-focusing in the nonlinear medium NLM, NLM'.

[0122] Step 1430: constructing the pump radiation source such that the pulsed input radiation IRD, IRD′ has a peak power equal to or greater than the determined critical power for self-focusing.

[0123] Step 1440: Determine a focused radiation beam size of the pulsed input radiation IRD, IRD' in the oscillation cavity, the focused radiation beam size having an intensity sufficient to achieve the filamentation process.

[0124] Step 1450: Determine the radius of curvature of the first reflection surface RS1, RS1' and the radius of curvature of the second reflection surface RS2, RS2', so that when reflected by any of the first reflection surface RS1, RS1' and the second reflection surface RS2, RS2', the pulsed input radiation IRD, IRD' is focused into a radiation beam size substantially the same as the determined focused radiation beam size at the focusing plane FP, FP' of the oscillation cavity OC, OC'.

[0125] Step 1460: Separate the first and second reflective surfaces RS1, RS1' and RS2, RS2' along the common longitudinal axis PA, PA' by a distance equal to half the sum of the radius of curvature of the first reflective surface RS1, RS1' and the second radius of curvature of the second reflective surface RS2, RS2'.

[0126] In an embodiment, the method may further comprise: adjusting a radiation beam size and / or a divergence of the pulsed input radiation such that a focusing position of the pulsed input radiation in the oscillation cavity substantially overlaps with an intra-cavity focusing plane of the oscillation cavity.

[0127] In an embodiment, the nonlinear medium may be a solid state medium, and the method may further comprise placing the nonlinear medium at a position substantially overlapping a focal plane of the oscillating cavity.

[0128] It will be appreciated that the above embodiments are provided for the purpose of describing the proposed concepts and are not intended to limit the scope of the present disclosure. Of course, other embodiments may be envisaged.

[0129] For example, in an embodiment, the multi-pass radiation device MPC, MPC' may include a transmission optical fiber configured to transmit broadband output radiation ORD, ORD' to different locations (e.g., one of the aforementioned measurement tools). The transmission optical fiber may be, for example, a single-mode photonic crystal fiber, such as HC-PCF. In an embodiment, one or both of the first reflector CM1, CM1' and the second reflector CM2, CM2' may include a dispersion compensation coating, which is configured to provide a high reflection coefficient for the pulse input radiation IRD, IRD' while substantially compensating for the dispersion accumulated by the pulse input radiation IRD, IRD' by traveling in the oscillation cavity OC, OC'. In addition or optionally, one or more of the input steering mirrors SM1, SM1' and / or the output steering mirrors SM2, SM2' may also include a dispersion compensation coating. In this way, the broadband output radiation ORD, ORD' may have both a wide spectral profile and a substantially compressed time profile.

[0130] After time compression or dispersion compensation, the broadband output radiation ORD, ORD′ may have a pulse duration between 1 fs and 10000 fs, or between 1 fs and 5000 fs, or between 1 fs and 1000 fs, or between 10 fs and 500 fs, or between 50 fs and 500 fs, or between 50 fs and 200 fs, or between 100 fs and 200 fs.

[0131] In an embodiment, the multi-pass radiation device MPC, MPC' may include a solid monolithic element having a first end section configured to provide a first reflection surface RS1, RS1', a second end section configured to provide a second reflection surface RS2, RS2' and a central section serving as a nonlinear medium NLM, NLM'. The three sections may be substantially transparent to the pulsed input radiation IRD, IRD' and the broadband output radiation ORD, ORD'. The first section and the second section may be made of, for example, fused silica, and the central section may be made of YAG. The three sections may be joined together to form a monolithic component, wherein the central section is clamped by the first end section and the second end section. The first reflection surface RS1, RS1' may be the outer surface of the first end section, and the second reflection surface RS2, RS2' may be the outer surface of the second end section. Both the first reflection surface RS1, RS1' and the second reflection surface RS2, RS2' may be curved surfaces (e.g., curved inwardly from the central section).

[0132] Fig.15 1 is a block diagram illustrating a computer system 1500 that can assist in implementing the methods and processes disclosed herein. The computer system 1500 includes: a bus 1502 or other communication mechanism for communicating information; and a processor 1504 (or multiple processors 1504 and 1505) coupled to the bus 1502 for processing information. The computer system 1500 also includes a main memory 1506 (e.g., a random access memory (RAM) or other dynamic storage device) coupled to the bus 1502 for storing information and instructions to be executed by the processor 1504. The main memory 1506 can also be used to store temporary variables or other intermediate information during the execution of instructions to be executed by the processor 1504. The computer system 1500 further includes a read-only memory (ROM) 1508 or other static storage device coupled to the bus 1502 for storing static information and instructions for the processor 1504. A storage device 1510 (e.g., a magnetic disk or optical disk) is provided and coupled to the bus 1502 for storing information and instructions.

[0133] The computer system 1500 can be connected to a display 1512, such as a cathode ray tube (CRT) or a flat panel or touch panel display, via the bus 1502 for displaying information to a computer user. An input device 1514 including alphanumeric keys and other keys is connected to the bus 1502 for transmitting information and command selections to the processor 1504. Another type of user input device is a cursor control 1516, such as a mouse, a trackball, or cursor direction keys, for transmitting direction information and command selections to the processor 1504 and for controlling cursor movement on the display 1512. The input device typically has two degrees of freedom on two axes, a first axis (e.g., x) and a second axis (e.g., y), which allows the device to specify a position in a plane. A touch panel (screen) display can also be used as an input device.

[0134] In response to processor 1504 executing one or more sequences of one or more instructions contained in main memory 1506, computer system 1500 can perform one or more methods described herein. Such instructions can be read from another computer-readable medium (e.g., storage device 1510) into main memory 1506. The execution of the sequence of instructions contained in main memory 1506 causes processor 1504 to perform the processing steps described herein. One or more processors in a multi-processing arrangement can also be used to execute the sequence of instructions contained in main memory 1506. In alternative embodiments, hard-wired circuits can be used to replace software instructions or to combine with software instructions. Therefore, the description herein is not limited to any specific combination of hardware circuits and software.

[0135] As used herein, the term "computer-readable medium" refers to any medium that participates in providing instructions to the processor 1504 for execution. Such media can 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 1510. Volatile media include dynamic memory, such as main memory 1506. Transmission media include coaxial cables, copper wires, and optical fibers, including wires that include bus 1502. Transmission media can also take the form of sound waves 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, floppy disks, flexible disks, hard disks, magnetic tapes, any other magnetic media, CD-ROMs, DVDs, any other optical media, punch cards, paper tapes, any other physical media with hole patterns, RAM, PROMs and EPROMs, FLASH-EPROMs, any other memory chips or cassettes, carriers as described below, or any other media from which a computer can read.

[0136] Various forms of computer readable media may be involved in carrying one or more sequences of one or more instructions to the processor 1504 for execution. For example, the instructions may initially be carried on a 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 1500 may receive the data on the telephone line and convert the data into an infrared signal using an infrared transmitter. An infrared detector connected to the bus 1502 may receive the data carried in the infrared signal and place the data on the bus 1502. The bus 1502 carries the data to the main memory 1506, from which the processor 1504 retrieves and executes the instructions. The instructions received by the main memory 1506 may optionally be stored on the storage device 1510 before or after being executed by the processor 1504.

[0137] Computer system 1500 also preferably includes a communication interface 1518 connected to bus 1502. Communication interface 1518 provides bidirectional data communication connected to a network link 1520 connected to a local network 1522. For example, communication interface 1518 can be an integrated services digital network (ISDN) card or a modem to provide a data communication connection to a telephone line of a corresponding type. As another example, communication interface 1518 can be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links can also be implemented. In any such embodiment, communication interface 1518 sends and receives electrical signals, electromagnetic signals, or optical signals that carry digital data streams representing various types of information.

[0138] The network link 1520 typically provides data communication to other data devices through one or more networks. For example, the network link 1520 can provide a connection to a host computer 1524 or to data equipment operated by an Internet Service Provider (ISP) 1526 through a local network 1522. ISP 1526, in turn, provides data communication services through a global packet data communication network (now commonly referred to as the "Internet" 1528). Both the local network 1522 and the Internet 1528 use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on the network link 1520 and through the communication interface 1518 (which carry the digital data to and from the computer system 1500) are exemplary forms of carrier waves that transport the information.

[0139] The computer system 1500 can send messages and receive data, including program code, through (multiple) networks, network links 1520, and communication interfaces 1518. In the Internet example, a server 1530 can transmit a request code for an application through the Internet 1528, an ISP 1526, a local network 1522, and a communication interface 1518. For example, one such downloaded application can provide one or more of the technologies described herein. The received code can be executed by the processor 1504 when it is received, and / or stored in the storage device 1510 or other non-volatile memory for later execution. In this way, the computer system 1500 can obtain application code in the form of a carrier wave.

[0140] Further embodiments are disclosed in the following list of numbered aspects:

[0141] 1. A radiating device for generating broadband output radiation upon receiving pulsed input radiation, the radiating device comprising:

[0142] an oscillating cavity comprising a first reflecting surface and a second reflecting surface; and

[0143] a nonlinear medium, the nonlinear medium being located between the first reflecting surface and the second reflecting surface;

[0144] Wherein, the oscillation cavity is configured to receive and oscillate the pulse input radiation therein, so that the pulse input radiation undergoes a filamentation process, thereby causing one or more filaments to be formed in the nonlinear medium, and so that the pulse input radiation undergoes multiple passes through the nonlinear medium, so that the pulse input radiation is spectrally broadened by the nonlinear medium to form the broadband output radiation.

[0145] 2. The radiation device of clause 1, wherein each of said multiple passes through said non-linear medium follows a different path.

[0146] 3. The radiation device according to aspects 1 or 2, wherein the pulsed input radiation undergoes the filamentation process during two or more of the plurality of passes through the nonlinear medium.

[0147] 4. The radiation device according to any preceding aspect, wherein the nonlinear spectrum broadening process comprises a self-phase modulation process.

[0148] 5. The radiation device according to any preceding aspect, wherein the first reflecting surface and the second reflecting surface are both concavely curved.

[0149] 6. The radiation device according to any preceding aspect, wherein the first reflective surface and the second reflective surface each define a common intracavity focusing plane, and the pulsed input radiation is focused at the common intracavity focusing plane after each pass in the oscillation cavity.

[0150] 7. The radiation device of clause 6, wherein the nonlinear medium is at least partially comprised at the intracavity focusing plane.

[0151] 8. The radiation device according to any of the preceding aspects, further comprising a mode matching device, the mode matching device being configured to adjust the radiation beam size and / or divergence of the pulse input radiation so that the focusing position of the pulse input radiation in the oscillation cavity is substantially at the intra-cavity focusing plane.

[0152] 9. The radiation device according to aspect 8, wherein the mode matching device comprises one or more lenses, one or more curved mirrors or a combination thereof.

[0153] 10. The radiation device of any preceding aspect, wherein the first reflective surface has a first radius of curvature and the second reflective surface has a second radius of curvature; and wherein the first reflective surface and the second reflective surface are symmetrical about a common longitudinal axis and are separated along the common longitudinal axis by a distance equal to half the sum of the first radius of curvature and the second radius of curvature.

[0154] 11. The radiation device of clause 10, wherein the first radius of curvature and the second radius of curvature are equal.

[0155] 12. The radiation device of clause 10, wherein the first radius of curvature and the second radius of curvature are different.

[0156] 13. The radiation device according to any one of aspects 10 to 12, wherein the first radius of curvature and the second radius of curvature are such that when reflected by either the first reflective surface or the second reflective surface, the pulsed input radiation is focused to a radiation spot having a sufficient peak intensity for achieving the filamentation process.

[0157] 14. The radiation device according to any preceding aspect, further comprising a first reflector configured to provide the first reflective surface and a second reflector configured to provide the second reflective surface.

[0158] 15. The radiation device of clause 14, wherein one or both of the first reflector and the second reflector comprises a dispersion compensating coating configured to provide a high reflectivity for the pulsed input radiation while substantially compensating for dispersion accumulated by the pulsed input radiation through its travel within the oscillating cavity.

[0159] 16. The radiation device according to any preceding aspect, wherein the nonlinear medium is a gas.

[0160] 17. The radiation apparatus of clause 16, wherein the nonlinear medium is one or more of: helium, neon, argon, krypton, xenon, and radon.

[0161] 18. The radiation device according to any one of clauses 1 to 15, wherein the non-linear medium is a liquid.

[0162] 19. The radiation device according to any one of clauses 1 to 15, wherein the nonlinear medium is a solid.

[0163] 20. The radiation device according to clause 19, wherein the nonlinear medium is a yttrium aluminum garnet (YAG) crystal or a fused silica crystal.

[0164] 21. The radiation device according to any preceding aspect, further comprising a chamber arranged to at least partially surround the first reflecting surface, the second reflecting surface and the nonlinear medium.

[0165] 22. The radiation apparatus of clause 21, wherein the chamber comprises an input window configured to substantially transmit the pulsed input radiation and an output window configured to substantially transmit the broadband output radiation.

[0166] 23. The radiation device of any preceding aspect, wherein the pulsed input radiation has a pulse duration between 1 fs and 10000 fs.

[0167] 24. A radiation device according to any preceding aspect, wherein the pulsed input radiation has a central wavelength between 400 nm and 2000 nm.

[0168] 25. A radiation device according to any preceding aspect, wherein the broadband output radiation has a spectrum at least partially overlapping with a spectral range from 200 nm to 2000 nm, or alternatively, the broadband radiation has a spectrum spanning from 200 nm to 2000 nm.

[0169] 26. The radiation device according to any preceding aspect, further comprising a transmission optical fiber for transmitting the broadband output radiation.

[0170] 27. The radiation device according to clause 26, wherein the transmission optical fiber is a single mode optical fiber photonic crystal fiber (PCF).

[0171] 28. The radiation device of clause 27, wherein the transmission optical fiber is a single-mode hollow core photonic crystal fiber (HC-PCF).

[0172] 29. A radiation source comprising:

[0173] A radiation device, the radiation device being a radiation device as described in any of the preceding aspects, and

[0174] A pump radiation source is configured to output the pulsed input radiation.

[0175] 30. A measurement device comprising a radiation source according to any preceding aspect.

[0176] 31. A metrology apparatus according to clause 30, comprising a scatterometry device, a level sensor or an alignment sensor.

[0177] 32. A method for constructing a radiation device according to any one of aspects 1 to 28, comprising:

[0178] providing pulsed input radiation having an input spectrum;

[0179] Define the target output spectrum;

[0180] determining an amount of nonlinear spectral broadening required to broaden the input spectrum to the target output spectrum via the nonlinear spectral broadening process;

[0181] determining a total number of passes through the nonlinear medium to achieve the determined amount of nonlinear spectral broadening;

[0182] determining an input direction of the pulsed input radiation into the oscillator cavity so as to achieve a determined total number of passes through the nonlinear medium; and

[0183] The pulsed input radiation is directed to follow the determined input direction.

[0184] 33. A method for constructing a radiation source according to aspect 29, comprising:

[0185] determining a critical power for self-focusing in said nonlinear medium;

[0186] configuring the pump radiation source so that the pulsed input radiation has a peak power equal to or greater than the determined critical power for self-focusing;

[0187] Determining a focused radiation beam size of the pulsed input radiation in the oscillation cavity, wherein the focused radiation beam size has an intensity sufficient to achieve a filamentation process;

[0188] determining a radius of curvature of the first reflective surface and / or a radius of curvature of the second reflective surface such that, when reflected by any of the first reflective surface and the second reflective surface, the pulsed input radiation is focused to have a radiation beam size substantially the same as the determined focused radiation beam size at a focal plane of the oscillation cavity; and

[0189] The first reflective surface is separated from the second reflective surface along a common longitudinal axis by a distance equal to half a sum of the radius of curvature of the first reflective surface and the radius of curvature of the second reflective surface.

[0190] 34. The method of clause 33, further comprising: adjusting the radiation beam size and / or divergence of the pulsed input radiation so that the focusing position of the pulsed input radiation in the oscillation cavity substantially overlaps with the intra-cavity focusing plane of the oscillation cavity.

[0191] 35. A method according to clause 33 or 34, wherein the nonlinear medium is a solid state medium, and the method further comprises:

[0192] The nonlinear medium is placed at a position substantially overlapping the focusing plane of the oscillation cavity.

[0193] 36. A method for generating broadband output radiation, comprising:

[0194] generating pulsed input radiation;

[0195] repeatedly reflecting the pulsed input radiation back and forth in multiple passes through a nonlinear medium such that the pulsed input radiation undergoes a filamentation process resulting in the formation of one or more filaments in the nonlinear medium, and such that after the multiple passes through the nonlinear medium, the pulsed input radiation is spectrally broadened by the nonlinear medium to form the broadband output radiation; and

[0196] The broadband output radiation is output.

[0197] 37. The method of clause 36, wherein each reflection of the repeating reflecting step comprises focusing the pulsed input radiation to a focal plane, and wherein the nonlinear medium is at least partially contained at the focal plane.

[0198] 38. A method according to clause 36 or 37, wherein each reflection of said repeatedly reflecting step comprises compensating for dispersion of said pulsed input radiation during said multiple passes through said nonlinear medium.

[0199] 39. A method according to any one of clauses 36 to 38, wherein each of said plurality of passes through said non-linear medium follows a different path.

[0200] 40. A method according to any one of clauses 36 to 39, wherein the pulsed input radiation undergoes the filamentation process during each of the plurality of passes through the nonlinear medium.

[0201] 41. A method according to any one of clauses 36 to 40, wherein the nonlinear spectrum broadening process comprises a self-phase modulation process.

[0202] 42. A method according to any one of clauses 37 to 41, wherein the receiving step comprises adjusting a radiation beam size and / or divergence of the pulsed input radiation to cause a focus position of the pulsed input radiation to be substantially at the focus plane.

[0203] 43. The method of any preceding aspect, wherein the outputting step comprises coupling the broadband output radiation into a transmission optical fiber.

[0204] Although specific reference may be made herein to the use of lithographic apparatus in IC manufacturing, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0205] Although embodiments of the present invention may be specifically referenced herein in the context of lithographic equipment, embodiments of the present invention may be used in other equipment. Embodiments of the present invention may form part of a mask inspection equipment, a metrology equipment, or any equipment that measures or processes an object such as a wafer (or other substrate) or a mask (or other patterning device). These equipment may generally be referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0206] Although specific reference has been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention is not limited to optical lithography and may be used in other applications (for example imprint lithography) where the context permits.

[0207] Although specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be practiced in a manner other than as described. The above description is intended to be illustrative rather than restrictive. Therefore, it will be apparent to those skilled in the art that modifications may be made to the present invention as described without departing from the scope of the appended claims.

Claims

1. A radiating device for generating broadband output radiation upon receiving pulsed input radiation, the radiating device comprising: an oscillating cavity, the oscillating cavity comprising a first reflecting surface and a second reflecting surface; and a nonlinear medium, the nonlinear medium being located between the first reflecting surface and the second reflecting surface; Wherein, the oscillation cavity is configured to receive the pulse input radiation and cause the pulse input radiation to oscillate within the oscillation cavity, so that the pulse input radiation undergoes a filamentation process, thereby causing one or more filaments to be formed in the nonlinear medium, and so that the pulse input radiation undergoes multiple passes through the nonlinear medium, so that the pulse input radiation is spectrally broadened by the nonlinear medium to form the broadband output radiation.

2. The radiation device according to claim 1, wherein: Each of the multiple passes through the nonlinear medium follows a different path.

3. The radiation device according to claim 1 or 2, wherein: The pulsed input radiation undergoes the filamentation process during two or more of the plurality of passes through the nonlinear medium.

4. A radiation device according to any preceding claim, wherein: The nonlinear spectrum broadening process includes a self-phase modulation process.

5. A radiation device according to any preceding claim, wherein: Both the first reflecting surface and the second reflecting surface are concavely curved.

6. A radiation device according to any preceding claim, wherein: The first and second reflective surfaces each define a common intracavity focusing plane, the pulsed input radiation is focused at the intracavity focusing plane after each pass within the oscillation cavity, and optionally, the nonlinear medium is at least partially included at the intracavity focusing plane.

7. The radiation device according to any of the preceding claims further comprises a mode matching device, which is configured to adjust the radiation beam size and / or divergence of the pulsed input radiation so that the focusing position of the pulsed input radiation in the oscillation cavity is substantially at the focusing plane in the cavity, and optionally, the mode matching device comprises one or more lenses, one or more curved mirrors or a combination thereof.

8. A radiation device according to any preceding claim, wherein: The first reflective surface has a first radius of curvature and the second reflective surface has a second radius of curvature; and wherein the first reflective surface and the second reflective surface are symmetrical with respect to a common longitudinal axis and are separated along the common longitudinal axis by a distance equal to half the sum of the first radius of curvature and the second radius of curvature.

9. The radiation device according to claim 8, wherein: The first radius of curvature and the second radius of curvature are such that, when reflected by either of the first reflective surface and the second reflective surface, the pulsed input radiation is focused into a radiation spot having a sufficient peak intensity to achieve the filamentation process.

10. The radiation device according to any preceding claim, further comprising a first reflector configured to provide the first reflective surface and a second reflector configured to provide the second reflective surface, and optionally, one or both of the first reflector and the second reflector comprises a dispersion compensating coating, wherein the dispersion compensating coating is configured to provide a high reflection coefficient for the pulsed input radiation while substantially compensating for the dispersion accumulated by the pulsed input radiation through the travel of the pulsed input radiation within the oscillation cavity.

11. A radiation device according to any preceding claim, wherein: At least one of the following conditions is met: - the nonlinear medium is a gas, optionally one or more of helium, neon, argon, krypton, xenon and radon, - the nonlinear medium is a liquid, and - The nonlinear medium is a solid, optionally a yttrium aluminum garnet (YAG) crystal or a fused silica crystal.

12. The radiation device according to any preceding claim, further comprising a chamber arranged to at least partially surround the first reflective surface, the second reflective surface and the nonlinear medium, and optionally, the chamber comprises an input window configured to substantially transmit the pulsed input radiation and an output window configured to substantially transmit the broadband output radiation.

13. A metrology device comprising a radiation source according to any preceding claim.

14. A metrology arrangement according to claim 13, comprising a scatterometer measurement device, a level sensor or an alignment sensor.

15. A method for constructing a radiation device according to any one of claims 1 to 12, comprising: providing pulsed input radiation having an input spectrum; Define the target output spectrum; determining an amount of nonlinear spectral broadening required to broaden the input spectrum to the target output spectrum via the nonlinear spectral broadening process; determining a total number of passes through the nonlinear medium to achieve the determined amount of nonlinear spectral broadening; determining an input direction of the pulsed input radiation into the oscillator cavity so as to achieve a determined total number of passes through the nonlinear medium; and The pulsed input radiation is directed to follow the determined input direction.

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