Multipath Radiating Devices

Abstract

1. A radiation device for receiving pulsed input radiation and generating broadband output radiation, the radiation device comprising: an oscillation cavity including a first reflective surface and a second reflective surface; and a nonlinear medium located between the first reflective surface and the second reflective surface, the oscillation cavity being configured to receive the pulsed input radiation and cause it to oscillate therein, such that the pulsed input radiation undergoes a filamentation process to form one or more filaments in the nonlinear medium, and the pulsed input radiation passes through the nonlinear medium multiple times and is spectrally broadened by the nonlinear medium to form the broadband output radiation.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to European Patent Application No. 22195017.3, filed September 12, 2022, which is incorporated herein by reference in its entirety.

[0002] The present invention relates to multipath broadband radiation sources, and in particular to such broadband radiation sources associated with metrology applications in the manufacture of integrated circuits. [Background technology]

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

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

[0005]

[0004] Low k1 lithography can be used to process features with dimensions smaller than the classical resolution limit of a lithographic apparatus. In such processes, the resolution equation can be expressed as CD = k1 × λ / NA, where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection optics of the lithographic apparatus, CD is the "critical dimension" (generally the smallest feature size to be printed, in this case the half pitch), and k1 is an empirical resolution factor. In general, the smaller k1 is, the more difficult it is to reproduce on a substrate a pattern that resembles the shape and dimensions planned by a circuit designer to achieve a particular electrical functionality and performance. To overcome such difficulties, advanced fine-tuning steps can be applied to the lithographic projection apparatus and / or the design layout. Such steps include, but are not limited to, optimization of the NA, customization of the illumination scheme, use of phase-shift patterning devices, various optimizations of the design layout, such as optical proximity correction (OPC, sometimes also called "optical and process correction") in the design layout or other methods commonly defined as "resolution enhancement techniques" (RET). Alternatively, a strict control loop can be used to manage the stability of the lithographic apparatus to improve pattern replication at low k1.

[0006]

[0005] In many aspects of the IC manufacturing process, metrology tools are used, for example, as alignment tools for proper positioning of the substrate before exposure, as leveling tools for measuring the surface topology of the substrate for focus control, and as scatterometry-based tools for inspecting / measuring exposed and / or etched products in process control. In each case, a radiation source is required. For various reasons, including measurement robustness and accuracy, broadband or white light radiation sources are increasingly being used for such metrology applications. It would be desirable to improve this device for broadband radiation generation. Summary of the Invention

[0007]

[0006] According to a first aspect of the present invention, there is provided a radiation device for receiving pulsed input radiation and generating broadband output radiation, the radiation device comprising: an oscillation cavity including a first reflective surface and a second reflective surface; and a nonlinear medium located between the first reflective surface and the second reflective surface, wherein the oscillation cavity is configured to receive the pulsed input radiation and cause it to oscillate therein, so that the pulsed input radiation undergoes a filamentation process to form one or more filaments in the nonlinear medium, and the pulsed input radiation passes through the nonlinear medium multiple times and is spectrally broadened by the nonlinear medium to form the broadband output radiation.

[0008] According to a second aspect of the present invention, there is provided a radiation source comprising a radiation device according to the first aspect and a pump radiation source configured to output pulsed input radiation.

[0009]

[0008] According to a third aspect of the present invention, there is provided a method for configuring a radiation device according to the first aspect, the method comprising: supplying pulsed input radiation having an input spectrum; determining a target output spectrum; determining an amount of nonlinear spectral broadening required to widen the input spectrum to the target output spectrum via a nonlinear spectral broadening process; determining a total number of passes through a nonlinear medium to achieve the determined amount of nonlinear spectral broadening; determining an input direction such that the pulsed input radiation enters an oscillation cavity to achieve the determined total number of passes through the nonlinear medium; and directing the pulsed input radiation to follow the determined input direction.

[0010]

[0009] According to a fourth aspect of the present invention, there is provided a method for constructing a radiation source according to the second aspect, the method comprising: determining a critical power for self-focusing in a nonlinear medium; configuring the pump radiation source so that the pulsed input radiation has a peak power greater than or equal to the determined critical power for self-focusing; determining a focused beam size of the pulsed input radiation in the oscillating cavity having sufficient intensity to enable a filamentation process; determining a radius of curvature of the first reflective surface and / or a radius of curvature of the second reflective surface so that the pulsed input radiation is reflected by either the first reflective surface or the second reflective surface to be focused to have a beam size substantially the same as the determined focused beam size at the focusing surface of the oscillating cavity; and separating the first reflective surface and the second reflective surface along a common longitudinal axis by a distance equal to half the sum of the radius of curvature of the first reflective surface and the radius of curvature of the second reflective surface.

[0011]

[0010] According to a fifth aspect of the present invention, there is provided a method for generating broadband output radiation, the method comprising: generating pulsed input radiation; passing the pulsed input radiation through a nonlinear medium multiple times and repeatedly reflecting it back and forth so that the pulsed input radiation undergoes a filamentation process to form one or more filaments within the nonlinear medium, and the pulsed input radiation is spectrally broadened by the nonlinear medium to form broadband output radiation after passing through the nonlinear medium multiple times; and outputting the broadband output radiation.

[0012] Another aspect of the invention includes a metrology device including a radiation source according to the second aspect.

[0013]

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

[0014] [Figure 1] 1 depicts a schematic overview of a lithographic apparatus; [Figure 2]1 shows a schematic overview of a lithography cell. [Figure 3] A schematic diagram of holistic lithography is shown, illustrating the collaboration between three key technologies for optimizing semiconductor manufacturing. [Figure 4] 1 depicts a schematic diagram of a scatterometry apparatus for use as a metrology device, which may include a radiation source according to an embodiment of the present invention; [Figure 5] 1 shows a schematic diagram of a level sensor device that may include a radiation source according to an embodiment of the present invention; [Figure 6] 1 shows a schematic diagram of an alignment sensor apparatus that may include a radiation source according to an embodiment of the present invention; [Figure 7] 1 shows a schematic diagram of a prior art laser-driven radiation source; [Figure 8] 1 shows a schematic diagram of a prior art hollow-core photonic crystal fiber-based broadband radiation source. [Figure 9] 1 shows three filaments formed as a result of the filamentation process in a nonlinear medium. [Figure 10] 1 shows a schematic diagram of a gaseous nonlinear medium based multipath radiation device according to one embodiment; [Figure 11] 1 shows a mirror arrangement suitable for use in a multipath radiating device MPC (eg, as shown in FIG. 10). [Figure 12] 11 illustrates a flow diagram of a method for configuring a multipath radiating device (eg, as shown in FIG. 10) according to one embodiment. [Figure 13] 1 shows a schematic diagram of a multipath radiation device based on a solid nonlinear medium according to an embodiment; [Figure 14] 10 shows a flow diagram of a method for configuring a radiation source including a multipath radiation device (eg, as shown in FIG. 10 or FIG. 13). [Figure 15] FIG. 1 shows a block diagram of a computer system for controlling a broadband radiation source. DETAILED DESCRIPTION OF THE INVENTION

[0015]

[0013] In this document, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g., having wavelengths of 365, 248, 193, 157 or 126 nm) and EUV (e.g., extreme ultraviolet radiation having wavelengths in the range of about 5 to 100 nm).

[0016]

[0014] As used herein, the terms "reticle," "mask," or "patterning device" may be broadly interpreted to refer to a general patterning device that can be used to provide an incident radiation beam with a patterned cross-section that corresponds to the pattern to be created on a target portion of a substrate. The term "light valve" is sometimes also used in this context. In addition to classic masks (transmissive or reflective masks, binary masks, phase-shifting masks, hybrid masks, etc.), examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0017] 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes an illumination system IL (also called an illuminator) configured to condition a radiation beam B (e.g. UV radiation, DUV radiation or EUV radiation), a mask support (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to certain parameters, a substrate support (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support according to certain parameters, and a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0018]

[0016] In operation, the illumination system IL receives a radiation beam from a radiation source SO (e.g. via a beam delivery system BD). The illumination system IL may include various types of optical components for directing, shaping and / or controlling the radiation, 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 so that it has a desired spatial and angular intensity distribution in its cross-section in the plane of the patterning device MA.

[0019]

[0017] As used herein, the term "projection system" PS should be interpreted broadly to encompass various types of projection systems. Such systems may include refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as required by the exposure radiation being used and / or other factors (e.g., the use of an immersion liquid or a vacuum). Where the term "projection lens" is used herein, it may all be considered as synonymous with the more general term "projection system" PS.

[0020]

[0018] 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 a space between the projection system PS and the substrate W, which is also known as immersion lithography. Further details about immersion techniques are given in U.S. Patent No. 6,952,253, which is incorporated herein by reference.

[0021] The lithographic apparatus LA may be of a type having two or more substrate supports WT (also known as "dual stage"). In such a "multiple stage" machine, the substrate supports WT may be used in parallel, and / or a substrate W on one of the substrate supports WT may be used to expose a pattern thereon, while a procedure is being performed on another substrate W on the other substrate support WT in preparation for a subsequent exposure of that other substrate W.

[0022] In addition to the substrate support WT, the lithographic apparatus LA may include a measurement stage. The measurement stage is configured to hold a sensor and / or a cleaning apparatus. The sensor may be configured to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold multiple sensors. The cleaning apparatus may be configured to clean part of the lithographic apparatus, for example part of the projection system PS or part of a system for supplying immersion liquid. The measurement stage may move below the projection system PS when the substrate support WT is spaced apart from the projection system PS.

[0023]

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

[0024] 2, the lithography apparatus LA may be part of a lithography cell LC (sometimes called a litho-cell or (litho)-cluster), which often also includes apparatus for performing pre-exposure and post-exposure processes on the substrate W. Conventionally, such apparatus include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH, and a bake plate BK (which, for example, adjust the temperature of the substrate W, e.g., to adjust the solvent in the resist layer). A substrate handler (i.e., robot) RO picks up substrates W from input / output ports I / O1, I / O2, moves them between various process tools, and delivers them to a loading bay LB of the lithography apparatus LA. The devices within a lithocell are often collectively referred to as a track and 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 (e.g., via a lithography control unit LACU).

[0025] To ensure that a substrate W to be exposed by lithographic apparatus LA is exposed accurately, it is desirable to inspect the substrate to measure properties of the patterned structures, such as overlay error between successive layers, line thickness, critical dimension (CD), etc. To that end, an inspection tool (not shown) may be included in lithocell LC. If an error is detected, adjustments may be made, for example, to the exposure of subsequent substrates or other process steps to be performed on substrate W, particularly if inspection is performed before other substrates W of the same batch or lot are subsequently exposed or processed.

[0026] Inspection apparatus, sometimes called metrology apparatus, are used to measure properties of substrates W, and in particular to measure how the properties of different substrates W vary, or how properties associated with different layers of the same substrate W vary from layer to layer. The inspection apparatus may alternatively be constructed to identify defects on substrates W and may, for example, be part of a lithocell LC, or integrated into a lithography apparatus LA, or may be a stand-alone apparatus. The inspection apparatus may measure properties related to a latent image (an image in a resist layer after exposure), or a semi-latent image (an image in a resist layer after a post-exposure bake step PEB), or a developed resist image (from which exposed or unexposed parts of the resist have been removed), or even a property related to an etched image (after a pattern transfer step such as etching).

[0027] Typically, the patterning process in a lithography apparatus LA is one of the most critical steps in processing, requiring high accuracy in the dimensioning and placement of structures on a substrate W. To ensure this high accuracy, three systems can be combined in a so-called "holistic" control environment, as shown schematically in FIG. 3. 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 a "holistic" environment is optimizing the coordination between these three systems to enforce the overall process window and achieve a tight control loop so that the patterning performed by the lithography apparatus LA stays within the process window. The process window defines the range of process parameters (e.g., dose, focus, overlay) within which a particular manufacturing process produces a specified result (e.g., a functioning semiconductor device), and typically within which the process parameters of a lithography process or patterning process can vary.

[0028]

[0026] The computer system CL is capable of predicting which resolution enhancement techniques should be used by using (part of) the design layout to be patterned, and is capable of performing computational lithography simulations and calculations to determine mask layouts and lithography apparatus settings that maximize the overall process window of the patterning process (shown in FIG. 3 by the double-headed arrow at the first scale SC1). Typically, resolution enhancement techniques are tailored to the patterning capabilities of the lithography apparatus LA. The computer system CL is further capable of predicting whether defects are likely to exist (e.g., due to suboptimal processing) by detecting where in the process window the lithography apparatus LA is currently operating (e.g., using input from the metrology tool MT) (shown in FIG. 3 by the arrow pointing to "0" at the second scale SC2).

[0029]

[0027] The metrology tool MT is capable of providing input to the computer system CL that enables accurate simulation and prediction, and is capable of providing feedback to the lithography apparatus LA that identifies possible drifts (e.g., in the calibration status of the lithography apparatus LA) (shown by multiple arrows at the third scale SC3 in Figure 3).

[0030]

[0028] In lithographic processes, it is desirable to frequently measure the structures produced (e.g., for process control and verification). Tools that perform such measurements are commonly called metrology tools MT. Various types of metrology tools MT that perform such measurements are known, such as scanning electron microscopes or various forms of scatterometer metrology tools MT. A scatterometer is a multipurpose instrument that allows measurements of parameters of the lithographic process to be made by having a sensor in the pupil or a conjugate plane to the pupil of the scatterometer objective (usually referred to as pupil-based measurements), or by having a sensor in the image plane or a conjugate plane to the image plane (in which case measurements are usually referred to as image-based or field-based measurements). Such scatterometers and associated measurement techniques are described in detail in U.S. Patent Application Publication Nos. 20100328655, 2011102753A1, 20120044470A, 20110249244, 20110026032, or EP 1,628,164A, which are incorporated by reference in their entireties. The scatterometers described above are capable of measuring gratings using soft x-rays and light in the visible to near-infrared wavelength range.

[0031] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, a reconstruction method may be applied to the measurement signal to reconstruct or calculate the properties of the grating. Such a reconstruction may, for example, be the result of simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measurement results. Parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from the real target.

[0032]

[0030] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, radiation emitted by a radiation source is directed towards a target, and radiation reflected or scattered from the target is directed towards a spectrometer detector, which measures the spectrum of the specularly reflected radiation (i.e., measures the intensity as a function of wavelength). From this data, it is possible to reconstruct the structure or profile of the target giving rise to the detected spectrum, for example by rigorous coupled wave theory and nonlinear regression, or by comparison with a library of simulated spectra.

[0033] In a third embodiment, the scatterometer MT is an ellipsometric scatterometer. An ellipsometric scatterometer makes it possible to determine parameters of a lithographic process by measuring scattered radiation for each polarization state. Such a metrology apparatus emits polarized light (e.g., linearly, circularly or elliptically polarized light), for example using appropriate polarizing filters in the illumination section of the metrology apparatus. A source suitable for the metrology apparatus can provide polarized radiation as well. Various embodiments of existing ellipsometric scatterometers are described in U.S. Patent Application Publication Nos. 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,449, 12 / 920,968, 12 / 922,587, 13 / 000,229, 13 / 033,135, 13 / 533,110, and 13 / 891,410, which are incorporated by reference herein in their entireties.

[0034] In one embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the overlay of two misaligned grating or periodic structures by measuring the asymmetry of the reflectance spectra and / or the detection configuration, where the asymmetry is related to the degree of overlay. The two (typically overlapping) grating structures can be applied in two different (not necessarily consecutive) layers and formed at substantially the same location on the wafer. The scatterometer can have a symmetric detection configuration, for example, as described in co-owned European Patent Application Publication No. 1628164A, so that any asymmetry can be clearly distinguished. This provides a straightforward method for measuring grating misalignment. Further examples for measuring the overlay error between two layers containing periodic structures when the target is measured through the asymmetry of the periodic structures can be obtained from PCT Patent Application Publication No. WO 2011 / 012624 or U.S. Patent Application No. 20160161863, which are incorporated herein by reference in their entirety.

[0035]

[0033] Other parameters of interest can be focus and dose. Focus and dose can be determined simultaneously by scatterometry (or alternatively by scanning electron microscopy), as described in U.S. Patent Application No. 2011-0249244, which is incorporated herein by reference in its entirety. A single structure can be used that has a unique combination of critical dimension and sidewall angle measurements for each point of the focus-energy matrix (FEM, also called focus-exposure matrix). If these unique combinations of critical dimension and sidewall angle are available, focus and dose values ​​can be uniquely determined from these measurements.

[0036]

[0034] A metrology target can be a collection of composite gratings, mostly formed by a lithography process in resist, but also formed after, for example, an etching process. Typically, the pitch and linewidth of the grating structures strongly depend on the measurement optics (specifically, the NA of the optics) so that the diffraction orders obtained from the metrology target can be captured. As previously shown, the diffraction signal can be used to determine the shift between two layers (also called "overlay") or to reconstruct at least a portion of the original grating as produced by the lithography process. This reconstruction can be used to provide guidance on the quality of the lithography process and can be used to control at least a portion of the lithography process. The target can have smaller subsegments configured to mimic the dimensions of the features of the design layout in the target. This subsegmentation causes the target to behave more similarly to the features of the design layout, so that all process parameter measurements closely resemble the features of the design layout. The target can be measured in underfill mode or overfill mode. In underfill mode, the measurement beam generates a spot that is smaller than the entire target. In overfill mode, the measurement beam generates a spot that is larger than the entire target. In such an overfill mode, it may be possible to simultaneously measure different targets, and therefore simultaneously determine different process parameters.

[0037] The overall measurement quality of a lithography parameter using a particular target depends, at least in part, on the measurement recipe used to measure that lithography parameter. The term “substrate measurement recipe” can include one or more parameters of the measurement itself, one or more parameters of the measured pattern(s), or both. For example, if the measurement used in the substrate measurement recipe is a diffraction-based optical measurement, one or more of the parameters of the measurement can include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation to the substrate, the orientation of the radiation relative to the pattern(s) on the substrate, etc. One of the criteria for selecting a measurement recipe can be, for example, the sensitivity of one of the measurement parameters to process variations. Further examples are described in U.S. Patent Application No. 2016-0161863 and published U.S. Patent Application No. 2016 / 0370717A1, which are incorporated herein by reference in their entireties.

[0038] A metrology apparatus such as a scatterometer is shown in FIG. 4. It includes a broadband (white light) radiation projector 2 that projects radiation onto a substrate 6. Reflected or scattered radiation is sent to a spectrometer detector 4, which measures the spectrum 10 of the specularly reflected radiation (i.e., a measurement of intensity as a function of wavelength). From this data, the structure or profile giving rise to the detected spectrum can be reconstructed by a processing unit (PU), for example by rigorous coupled-wave analysis and nonlinear regression, or by comparison with a library of simulated spectra such as that shown at the bottom of FIG. 3. Typically, for reconstruction, the general form of the structure is known and some parameters are assumed from knowledge of the process by which the structure was created, thereby leaving only a few parameters of the structure to be determined from the scatterometry data. Such a scatterometer may be configured as a normal-incidence scatterometer or an oblique-incidence scatterometer.

[0039] The overall measurement quality of a lithography parameter through measurement of a metrology target depends, at least in part, on the measurement recipe used to measure the lithography parameter. The term “substrate measurement recipe” can encompass one or more parameters of the measurement itself, one or more parameters of one or more measured patterns, or both. For example, if the measurement performed in the substrate measurement recipe is a diffraction-based optical measurement, one or more parameters of the measurement can include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the orientation of the radiation relative to the patterns on the substrate, etc. One criterion for selecting a measurement recipe can be, for example, the sensitivity of any measurement parameter to process variations. Further examples are described in U.S. Patent Application No. 2016 / 0161863 and published U.S. Patent Application No. 2016 / 0370717A1, which are incorporated herein by reference in their entireties.

[0040] Another type of metrology tool used in IC manufacturing is a topography measurement system, level sensor, or height sensor. Such a tool may be incorporated into a lithography apparatus to measure the topography of the top surface of a substrate (or wafer). A map of the substrate's topography (also called a height map) can be generated from these measurements, showing the substrate's height as a function of position on the substrate. This height map can later be used to correct the position of the substrate during transfer of a pattern onto the substrate in order to provide an aerial image of the patterning device at the proper focus position on the substrate. It will be understood that "height" in this context refers broadly to the out-of-plane dimension relative to the substrate (also called the Z-axis). Typically, a level or height sensor performs measurements at a fixed position (relative to its own optics), and relative movement of the substrate and the optics of the level or height sensor results in height measurements at locations across the substrate.

[0041]

[0039] An example of a level or height sensor LS known in the art is shown schematically in Figure 5, which shows only the operating principle. In this example, the level sensor includes an optical system, which includes a projection unit LSP and a detection unit LSD. The projection unit LSP includes a radiation source LSO that provides a radiation beam LSB, which is provided by a projection grating PGR of the projection unit LSP. The radiation source LSO may be, for example, a narrowband or broadband light source, such as a polarized or unpolarized, pulsed or continuous (such as a polarized or unpolarized laser beam) supercontinuum light source. The radiation source LSO may include multiple radiation sources (such as multiple LEDs) with different colors or wavelength ranges. The radiation source LSO of the level sensor LS is not limited to visible radiation but may additionally or alternatively include UV and / or IR radiation, as well as any wavelength range suitable for reflection from the surface of the substrate.

[0042]

[0040] The projection grating PGR is a periodic grating comprising a periodic structure which results in a radiation beam BE1 with a periodically varying intensity. The radiation beam BE1 with a periodically varying intensity is directed towards a measurement location MLO on the substrate W with an angle of incidence ANG relative to an axis normal to the substrate surface of incidence (Z-axis) of between 0 and 90 degrees, typically between 70 and 80 degrees. At the measurement location MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and directed towards a detection unit LSD.

[0043]

[0041] To determine the height level at the measurement location MLO, the level sensor further includes a detection system including a detection grid DGR, a detector DET, and a processing unit (not shown) for processing an output signal of the detector DET. The detection grid DGR may be the same as the projection grid PGR. The detector DET generates a detector output signal indicative of the received light, for example indicative of the intensity of the received light (e.g., a photodetector) or representing the spatial distribution of the received intensity (e.g., a camera). The detector DET may include any combination of one or more types of detectors.

[0044]

[0042] Triangulation techniques allow the height level at the measurement location MLO to be determined. The detected height level is generally related to the signal intensity as measured by the detector DET, which has a periodicity that depends, among other things, on the design of the projection grating PGR and the (oblique) angle of incidence ANG.

[0045]

[0043] The projection unit LSP and / or the detection unit LSD may include further optical elements, such as lenses and / or mirrors, along the path of the patterned radiation beam between the projection grating PGR and the detection grating DGR (not shown).

[0046] In an embodiment, the detection grating DGR may be omitted and the detector DET may be placed at the position where the detection grating DGR is located. Such a configuration provides a more direct detection of the image of the projection grating PGR.

[0047]

[0045] In order to effectively cover the surface of the substrate W, the level sensor LS may be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating an array of measurement areas MLO or spots that cover a larger measurement range.

[0048] Various height sensors of the general type are disclosed, for example, in U.S. Patent Nos. 7,265,364 and 7,646,471, both of which are incorporated by reference. A height sensor that uses UV radiation instead of visible or infrared radiation is disclosed in incorporated U.S. Patent Application Publication No. 2010233600A1. In incorporated WO 2016102127A1, a miniature height sensor is described that uses a multi-element detector to detect and recognize the position of a grating image without the need for a detection grating.

[0049] Another type of metrology tool used in IC manufacturing is an alignment sensor. Therefore, an important aspect of the performance of a lithographic apparatus is the ability to correctly and accurately position a pattern applied relative to features built up on a previous layer (by the same apparatus or a different lithographic apparatus). For this purpose, the substrate is provided with one or more sets of marks or targets. Each mark is a structure whose position can be subsequently measured using a position sensor (typically an optical position sensor). The position sensor is sometimes called an "alignment sensor" and the marks are sometimes called "alignment marks".

[0050] A lithographic apparatus may include one or more (e.g., multiple) alignment sensors that can accurately measure the position of alignment marks provided on a substrate. Alignment (or position) sensors may use optical phenomena such as diffraction and interference to obtain position information from alignment marks formed on the substrate. One example of an alignment sensor used in current lithographic apparatuses is based on a self-referencing interferometer, such as that described in U.S. Patent No. 6,961,116. Various improvements and modifications to position sensors have been developed, as disclosed, for example, in U.S. Patent Application Publication No. 2015261097A1. The entire contents of these publications are incorporated herein by reference.

[0051] 6 is a schematic block diagram of an embodiment of a known alignment sensor AS, for example as described in the incorporated US Pat. No. 6,961,116. A radiation source RSO provides a radiation beam RB of one or more wavelengths, which is redirected by redirecting optics as an illumination spot SP onto a mark (such as a mark AM located on a substrate W). In this example, the redirecting optics include a spot mirror SM and an objective lens OL. The illumination spot SP (by which the mark AM is illuminated) may have a diameter slightly smaller than the width of the mark itself.

[0052]

[0050] Radiation diffracted by the alignment mark AM is collimated (in this example by an objective lens OL) into an information-bearing beam IB. The term "diffracted" is intended to include zeroth order diffraction from the mark (which is sometimes called reflection). A self-referencing interferometer SRI, for example of the type disclosed in the above-mentioned U.S. Patent No. 6,961,116, causes the beam IB to interfere with itself, after which the beam is received by a photodetector PD. Further optical components (not shown) may be included to provide separate beams if more than one wavelength is produced by the radiation source RSO. The photodetector may be a single element, or it may include several pixels, if required. The photodetector may include a sensor array.

[0053]

[0051] The redirecting optical element (in this example, the redirecting optical element includes a spot mirror SM) can also function to block zero-order radiation reflected from the mark AM so that the information-carrying beam IB contains only higher-order diffracted radiation from the mark AM (this is not essential for the measurement, but improves the signal-to-noise ratio).

[0054]

[0052] The intensity signal SI is fed to a processing unit PU. The combination of optical processing in block SRI and computational processing in unit PU outputs values ​​of the X and Y position on the substrate relative to a reference frame.

[0055] A single measurement of the type shown fixes the position of the mark only within a certain range corresponding to one pitch of the mark. In conjunction with this, coarser measurement techniques are used to identify which period of the sine wave contains the mark position. For improved accuracy and / or for robust detection of marks regardless of the material from which the mark is made and the material on which the mark is provided and / or below which it is provided, the same process can be repeated at coarser and / or finer levels with different wavelengths. Wavelengths may be optically multiplexed and demultiplexed to be processed simultaneously, and / or wavelengths may be multiplexed by time division or frequency division.

[0056] In this example, the alignment sensor and spot SP remain stationary, and it is the substrate W that moves. The alignment sensor can therefore be rigidly and accurately mounted to a reference frame, effectively scanning the mark AM in a direction opposite to the direction of movement of the substrate W. The substrate W is controlled in this movement by a substrate positioning system that controls its mounting on the substrate support and the movement of the substrate support. A substrate support position sensor (e.g., an interferometer) measures the position of the substrate support (not shown). In an embodiment, one or more (alignment) marks are provided on the substrate support. Measuring the position of the marks provided on the substrate support allows the position of the substrate support, as determined by the position sensor, to be calibrated (e.g., relative to a frame to which the alignment system is connected). Measuring the position of the alignment marks provided on the substrate allows the position of the substrate relative to the substrate support to be determined.

[0057]

[0055] Metrology tools MT, such as the scatterometers, topography measurement systems, or position measurement systems described above, may use radiation generated by a radiation source to perform measurements. The characteristics of the radiation used by the metrology tool may affect the type and quality of measurements that 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 capable of propagating, irradiating, and scattering from the metrology target with no or minimal interference with other frequencies. Thus, for example, different frequencies may be used to simultaneously obtain more metrology data. Different radiation frequencies may also be capable of interrogating or discovering different characteristics of the metrology target. Broadband radiation may be useful in metrology systems MT, for example, level sensors, alignment mark measurement systems, scatterometry tools, or inspection tools. The broadband radiation source may be a supercontinuum light source.

[0058]

[0056] High-quality broadband radiation (e.g., supercontinuum radiation) can be difficult to generate. One way to generate broadband radiation can be to broaden high-power narrowband or single-frequency input radiation or pump radiation, for example, by utilizing nonlinear and higher-order effects. The input radiation (which can be generated using a laser) can be referred to as pump radiation. Alternatively, the input radiation can be referred to as seed radiation. To obtain high-power radiation due to the broadening effect, the radiation can be confined within a small area so that strong, localized, high-intensity radiation is achieved. In these areas, the radiation can interact with broadening structures and / or materials that form a nonlinear medium, and broadband output radiation can be produced. In high-intensity radiation areas, different materials and / or structures can be used to enable and / or improve radiation broadening by providing a suitable nonlinear medium.

[0059] In some existing implementations, broadband output radiation is generated by tightly focusing a laser beam onto an ionizable medium (e.g., a noble gas) contained within a specially constructed chamber. The laser beam supplies laser energy to the ionizable medium within the chamber to sustain or create a plasma that generates the broadband radiation. Such radiation sources are known as laser-driven radiation sources.

[0060]

[0058] Figure 7 shows a simplified schematic diagram of a prior art laser-driven radiation source. Further details regarding such radiation sources can be found in U.S. Patent No. 7,435,982 (B2), which is incorporated herein by reference. As shown in Figure 7, a chamber CB contains an ionizable medium and an ignition source. At least a portion of the chamber walls is made of a material transparent to the input radiation IRD. The ignition source includes two electrodes ET1, ET2 configured to ignite or ionize the ionizable medium. The input radiation is tightly focused by a lens LEN onto the ionizable medium so as to sustain or create a localized high-intensity plasma PS that generates broadband output radiation ORD. The laser-driven radiation source produces omnidirectional emission over a 4π solid angle.

[0061]

[0059] In some existing implementations, broadband output radiation is generated in a photonic crystal fiber (PCF), specifically a hollow-core photonic crystal fiber (HC-PCF) that includes an anti-resonant structure to confine the radiation. HC-PCFs with anti-resonant structures are particularly suitable for strong radiation confinement, primarily inside the hollow core of the fiber, to achieve high radiation intensity. The hollow core of the fiber can be filled with a gas, which acts as a spreading medium to broaden the input radiation. Such a fiber and gas arrangement can be used to generate a supercontinuum radiation source.

[0062] FIG. 8 shows a simplified schematic diagram of a prior art HC-PCF-based broadband radiation source. Further details regarding such radiation sources can be found in U.S. Pat. No. 1,123,7486 (B2). As shown in FIG. 8, the HC-PCF includes a hollow core HC and a cladding portion surrounding the hollow core HC. The cladding portion includes multiple anti-resonant elements for guiding radiation through the hollow core HC. In this specific 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) at a certain pressure. After being coupled into the gas-filled hollow core HC from one end of the fiber, narrowband input radiation IRD undergoes a nonlinear spectral broadening process and ultimately becomes broadband output radiation ORD, which exits from the opposite end of the fiber. The HC-PCF radiation source produces highly directional emission.

[0063]

[0061] When compared with 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 evident when comparing the two radiation source examples shown in Figures 7 and 8, respectively. A less complex system design allows for lower cost and smaller products. HC-PCF-based radiation sources generally have a wider spectral range and directionality of the emitted broadband radiation, improving the radiance of the system. At the same time, HC-PCF-based radiation sources require highly specialized fibers, increasing the system complexity.

[0064]

[0062] Both types of radiation sources suffer from the lack of reliable means for power scalability and lifetime extension. In some laser-driven radiation sources, the unidirectional emission of broadband output radiation and low collection efficiency necessitate increasing the power of the input radiation for power scalability, resulting in various lifetime issues. In HC-PCF-based radiation sources, power extension is achieved by increasing the pulse repetition rate of the input radiation. Such an approach is problematic because it causes thermal effects in the fiber, leading to reduced lifetime and output instability.

[0065]

[0063] Multipass cells (or nonlinear multipass cells) containing nonlinear media have been developed to provide an alternative means for nonlinear spectral broadening and / or nonlinear time compression of laser pulses. Allowing a laser pulse to pass multiple times through a nonlinear medium means that the nonlinearity required for a given amount of spectral broadening is distributed over the multiple passes. Therefore, to produce the same target spectrum, the peak intensity of the laser pulse required in a multipass cell is much lower than that required in a single-pass configuration (using the same nonlinear medium). Nonlinear multipass cells have great potential for power enhancement (e.g., average power enhancement), but current developments (e.g., studies reported in scientific publications, such as “Nonlinear temporal compression in multipass cells: Theory,” Marc Hanna, Xavier Delen, Loic Lavenu, Florent Guichard, Yoann Zaouter, Frederic Druon, Patrick Georges, Journal of the Optical Society of America B, Vol. 34, Page 1340, 2017, and “Multipass-cell-based post-compression of radially and azimuthally polarized pulses to the sub-two-cycle regime,” Huabao Cao, Roland Nagymihaly, Vladimir Chyvkov, Nikita Khodakovskiy, and Mikhail Kalashnikov, Journal of the Optical Society of America B. Vol. 36, pp. 2517-2525, 2017) suggest that they offer significant potential for power enhancement (e.g., average power enhancement). 2019, both of which are incorporated herein by reference), have focused primarily on further pulse energy extension and / or further broadening of the spectral range. However, the average power of existing nonlinear multipass cells remains limited and insufficient for many metrology applications such as those mentioned above.It is therefore desirable to better exploit the power scalability of nonlinear multipass cells to produce high-power broadband radiation suitable for use in the aforementioned metrology systems.

[0066]

[0064] According to a first aspect of the present disclosure, there is provided a radiation device for receiving pulsed input radiation and generating broadband output radiation, the radiation device comprising: an oscillation cavity including a first reflective surface and a second reflective surface; and a nonlinear medium located between the first reflective surface and the second reflective surface, wherein the oscillation cavity is configured to receive the pulsed input radiation and cause it to oscillate therein, such that the pulsed input radiation undergoes a filamentation process to form one or more filaments in the nonlinear medium, and the pulsed input radiation passes through the nonlinear medium multiple times and is spectrally broadened by the nonlinear medium to form the broadband output radiation.

[0067]

[0065] It should be noted that the above-referenced lasing cavity should be distinguished from a laser cavity in which a laser pulse travels back and forth on the same optical path for an infinite number of bounces. In the context of the present disclosure, an lasing cavity is defined as a space surrounded by a first reflective surface and a second reflective surface. Within such a space, the laser pulse travels back and forth for a finite number of bounces, and after each bounce from one of the reflective surfaces, the laser pulse may travel a different path to reach the other reflective surface.

[0068]

[0066] In contrast to many existing nonlinear multipass cells (e.g., European Patent Application Publication No. 3143669A1, incorporated herein by reference), in which the peak power of the input radiation is intentionally kept below the critical power for self-focusing to prevent the self-focusing effect (also known as the Kerr effect) from adversely affecting the spectral broadening, the inventors have found that as long as the pulsed input radiation undergoes a filamentation process each time it passes through the nonlinear medium, the self-focusing effect can be beneficial to increasing the efficiency of the nonlinear spectral broadening process in the nonlinear multipass cell and thus increasing the power of the broadband output radiation. As described in the scientific publication "Ultrafast supercontinuum generation in bulk condensed media," Audrius Dubietis, Gintaras Tamosauskas, Rosvaldas Suminas, Vytautas Jukna, Lithuanian Journal of Physics, Vol. 57, No. 3, pp. 113-157 (2017), which is incorporated herein by reference, the filamentation process "results from the interplay between self-focusing, self-phase modulation, and multiphoton / ionization-induced free electron plasma, resulting in the emergence of dynamic structures with intense cores that can propagate over distances much longer than typical diffraction lengths while maintaining a narrow beam size without the aid of external guidance mechanisms."

[0069] The self-focusing effect results from the intensity-dependent refractive index n, which can be expressed as: n=n0+n2I [1] where n0 is the linear refractive index, n2 is the nonlinear refractive index associated with the third-order optical susceptibility of the material, and I is the intensity of the pulsed input radiation. The induced refractive index change is proportional to the local intensity and is therefore higher in the center of the pulsed input radiation beam and lower at the periphery. The nonlinear medium therefore acts like a lens that forces the pulsed input radiation beam to self-focus.

[0070]

[0068] Self-phase modulation (SPM) is a well-known nonlinear effect that results from the nonlinear change in the phase of a pulse caused by the time-varying refractive index, as expressed in equation [1]. The nonlinear phase change leads to a frequency change, which results in a time-varying instantaneous frequency and thus a spectral broadening of the pulse. The time-varying instantaneous frequency δω(t) can be expressed (assuming a Gaussian laser pulse) as follows:

number

number

[0071] The requirement for initiating the filamentation process is that the peak power of the input pulse must exceed the self-focusing threshold or critical power for self-focusing P cr The goal is to reach this goal. P cr =3.72λ 2 / 8πn0n2[3] where λ is the wavelength of the pulsed input radiation. cr In the laser beam, the self-focusing effect precisely balances the diffraction and diffusion of the laser beam. This can be understood as follows: When a laser beam self-focuses, the intensity of the laser beam is increased, and the self-focusing effect becomes stronger. However, the increase in laser intensity does not last indefinitely, but ends after the intensity of the pulse becomes strong enough to initiate multiphoton absorption and ionization in the nonlinear medium. Multiphoton absorption and ionization cause energy loss and generate free electron plasma, which further absorbs and defocuses the laser beam. When the laser intensity decreases, multiphoton absorption and ionization lose their effect. As a result, the defocused laser beam self-focuses again. The conditions for the filamentation process (e.g., the critical power P for self-focusing) cr) is still satisfied, the focusing-defocusing-refocusing process will continue. The filamentation process described above will result in the formation of one or more filaments within the nonlinear medium.

[0072]

[0070] Figure 9 shows a schematic example of three filaments formed as a result of the filamentation process in a nonlinear medium. As shown in Figure 9, pulsed input radiation IRD is focused into the nonlinear medium NLM. The peak power of the pulsed input radiation IRD is equal to the critical power P for self-focusing. cr That's it. During propagation through the nonlinear medium NLM, the pulsed input radiation IRD undergoes not only a filamentation process but also a nonlinear spectral broadening process (e.g., SPM). Therefore, the pulsed input radiation IRD repeatedly focuses and defocuses within the nonlinear medium NLM until the peak power of the pulsed input radiation IRD no longer reaches the self-focusing threshold. In this implementation example, the filamentation process results in the formation of three filaments along the propagation direction of the pulsed input radiation IRD. Meanwhile, the pulsed input radiation IRD is spectrally broadened by the nonlinear spectral broadening process and finally exits the nonlinear medium NLM as broadband output radiation ORD.

[0073]

[0071] Embodiments of the proposed multipath radiation device are described in detail below. FIG. 10 schematically illustrates a multipath radiation device MPC based on a gaseous nonlinear medium according to one embodiment. With reference to FIG. 10, the nonlinear medium NLM may be a gas. The gas may be, for example, one or more of 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 reflecting surface RS1 and the second reflecting surface RS2. The multipath radiation device MPC may include a chamber CB arranged to at least partially enclose the nonlinear medium NLM, the first reflecting surface RS1, the second reflecting 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.

[0074] The multipath radiating device MPC may include a first reflecting mirror CM1 and a second reflecting mirror CM2 configured to provide a first reflecting surface RS1 and a second reflecting surface RS2, respectively. FIG. 11 shows a mirror arrangement suitable for use in the proposed multipath radiating device MPC (e.g., as shown in FIG. 10). As shown in FIG. 11, both the first reflecting surface RS1 and the second reflecting surface RS2 may be inwardly or concavely curved and may have a first radius of curvature ROC1 and a second radius of curvature ROC2, respectively. Accordingly, both the first reflecting mirror CM1 and the second reflecting mirror CM2 may be considered concave mirrors. The first reflecting surface RS1 and the second reflecting surface RS2 may be symmetrical about a common major axis PA (or longitudinal axis) and may be separated by a distance DT along the common major axis PA. The distance DT is equal to one-half the sum of the first radius of curvature ROC1 and the second radius of curvature ROC2. For a concave mirror, it is known that the focal length FL and radius of curvature ROC of the mirror satisfy the following relationship: FL=ROC / 2 [4] Therefore, the separation 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 reflecting mirror CM1 and the second focal length FL2 of the second reflecting mirror CM2. In this manner, the principal foci of the first reflecting mirror CM1 and the second reflecting mirror CM2 spatially overlap at the intra-cavity focusing plane FP of the oscillation cavity OC. In some embodiments, the first radius of curvature ROC1 and the second radius of curvature ROC2 may be equal to each other. In such a case, the separation distance DT between the two reflecting mirrors is equal to the radii of curvature ROC1 and ROC2. In some embodiments, the first radius of curvature ROC1 and the second radius of curvature ROC2 may be different from each other.

[0075] In an embodiment, the multi-path radiation device MPC may include an input mode matching mechanism (not shown) for optimally coupling the pulsed input radiation IRD into the oscillating cavity OC. The input mode matching mechanism may be configured to adjust the beam size and / or divergence of the pulsed input radiation IRD such that the resulting focused position of the pulsed input radiation IRD in the oscillating cavity OC substantially overlaps with the focusing plane FP. In an embodiment, the input mode matching mechanism may include one or more lenses (e.g., mode-matching telescopes). In an embodiment, the input mode matching mechanism may include one or more curved mirrors. In an embodiment, the multi-path radiation device MPC may not include an input mode matching mechanism. In such a case, the modal characteristics of the pulsed input radiation IRD may be adjustable by an external mode matching mechanism located either inside or outside the pump radiation source emitting the pulsed input radiation IRD.

[0076] In some embodiments, the multipath radiation device MPC may include an output mode-matching mechanism configured to adjust the beam size and / or divergence of the broadband output radiation ORD to meet the modal requirements of any application (e.g., any of the aforementioned metrology applications). In some embodiments, the output mode-matching mechanism may include one or more lenses (e.g., mode-matching telescopes). In some embodiments, the output mode-matching mechanism may include one or more curved mirrors. In some embodiments, the multipath radiation device MPC may not include an output mode-matching mechanism. In such cases, the modal characteristics of the broadband input radiation IRD may be adjustable by an external mode-matching mechanism located outside the multipath radiation device, for example, between the multipath radiation device MPC and the objective lens of the scatterometer.

[0077]

[0075] In an embodiment, the multi-path radiation device MPC may include one or more input steering mirrors SM1 arranged to direct the pulsed input radiation towards one of the reflecting mirrors. For a given pair of reflecting mirrors CM1, CM2, the input direction of the pulsed input radiation IRD relative to the major axes of the reflecting mirrors CM1, CM2 may determine the travel path of the pulsed input radiation within the oscillating cavity OC, which in turn may determine the total number of passes through the nonlinear medium NLM and thus the total amount of nonlinear spectral broadening experienced by the pulsed input radiation IRD. In an embodiment, the input direction of the pulsed input radiation IRD and the resulting travel path within the oscillating cavity OC may be determined using ray tracing software (e.g., OpticStudio, commercially available from Zemax Inc.).

[0078]

[0076] Therefore, according to another aspect of the present disclosure, there is provided a method for configuring a multipath radiating device MPC (e.g., as shown in Figure 10). With reference to Figure 12, the method may include, for example, the following six steps:

[0079] Step 1210: Providing pulsed input radiation IRD having an input spectrum (eg, a narrowband spectrum).

[0080] Step 1220: Defining a target output spectrum (eg, a broadband spectrum).

[0081] Step 1230: Determining the amount of nonlinear spectral broadening required to broaden the input spectrum to the target output spectrum.

[0082] Step 1240: Determining the total number of passes through the nonlinear medium NLM to achieve the determined amount of nonlinear spectral broadening.

[0083] Step 1250: Determining an input direction for the pulsed input radiation IRD to enter the oscillating cavity OC to achieve a determined total number of passes through the nonlinear medium NLM.

[0084] Step 1260: Directing the pulsed input radiation IRD to follow the determined input direction.

[0085]

[0083] In some embodiments, the input steering mirror(s) SM1 may be curved mirrors and therefore may simultaneously function as input mode matching mechanisms.

[0086] In some embodiments, the multi-path radiation device MPC may include one or more output steering mirrors SM2 positioned to direct the broadband output radiation ORD towards an output window OW of the chamber CB. In some embodiments, the one or more output steering mirrors SM2 may be curved mirrors and may simultaneously function as output mode matching mechanisms.

[0087] 10 , the radiation device may be configured to ensure that the pulsed input radiation IRD passes through the nonlinear medium NLM two or more times, with each of the two or more passes through the nonlinear medium NLM following a different path. In some embodiments, with each pass, the pulsed input radiation IRD may undergo the aforementioned filamentation process, resulting in the formation of one or more filaments (see the example filaments shown in FIG. 9 ) within the nonlinear medium NLM. For a given set of parameters of the pulsed input radiation IRD (e.g., pulse length, pulse peak power, pulse repetition rate), the parameters of the oscillation cavity OC (e.g., the radii of curvature ROC1 of the first reflecting surface RS1 and the radii of curvature ROC2 of the second reflecting surface RS2, the separation between the two reflecting surfaces RS1, RS2) and the parameters of the gaseous nonlinear medium NLM (e.g., gas type, gas pressure) may be configured to enable a filamentation process each time the pulsed input radiation IRD passes through the nonlinear medium NLM. 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 within the nonlinear medium NLM, which with each pass intensifies the effect of nonlinear spectral broadening in the nonlinear medium, thus increasing the efficiency of converting the pulsed input radiation IRD into the target broadband output radiation ORD.

[0088]

[0086] It will be understood by those skilled in the art that the nonlinear medium NLM is not limited to a gaseous nonlinear medium, but may be any nonlinear medium (e.g., solid, liquid) that can enable nonlinear spectral broadening and filamentation of the pulsed input radiation IRD. FIG. 13 schematically shows a multi-path radiation device MPC' based on a solid nonlinear medium NLM' according to one embodiment. The solid nonlinear medium NLM' may be, for example, an yttrium aluminum garnet (YAG) crystal or a fused silica crystal. The solid nonlinear medium NLM' may be located substantially at the focusing plane FP' of the oscillating cavity to benefit from a higher peak intensity of the pulsed input radiation IRD'. It should be noted that the mirror arrangement shown in FIG. 11 is equally suitable for the embodiment shown in FIG. 13.

[0089] 13, the multi-path radiation device MPC′ may not include a chamber CB. The pulsed input radiation IRD′ may be directed into the oscillating cavity OC′ by one or more input steering mirrors SM1′. The one or more input steering mirrors SM1′ may be curved mirrors and may simultaneously function as input mode matching mechanisms (e.g., as described above with respect to the embodiment shown in FIG. 10). In an embodiment, the input mode matching mechanism may be located in the beam path of the pulsed input radiation IRD′ either upstream of the one or more input steering mirrors SM1′ or intermediate between two input steering mirrors.

[0090]

[0088] As shown in Figure 13, the pulsed input radiation IRD' may be directly focused into the solid nonlinear medium NLM'. The pulsed input radiation IRD' may follow a predetermined input direction having an inclined angle with respect to the main axis PA' of the reflecting surfaces RS1', RS2' (or reflecting mirrors CM1', CM2'). After passing through the nonlinear medium NLM', the pulsed input radiation IRD' may continue traveling until it hits a second reflecting surface RS2' provided by a second reflecting mirror CM2'. The second reflecting surface RS2' may reflect the pulsed input radiation IRD' into the nonlinear medium NLM'. Since the nonlinear medium NLM' is placed at the focusing plane FP' of the oscillating cavity OC', the spectrally broadened pulsed input radiation IRD' is again focused into the nonlinear medium NLM' by the second reflecting surface RS2'. After passing through the nonlinear medium NLM', the pulsed input radiation IRD' may continue traveling until it strikes a first reflecting surface RS1' provided by a second reflecting mirror CM1'. Similar to the second reflecting surface RS2', the first reflecting surface RS1' may reflect and refocus the spectrally broadened pulsed input radiation IRD' into the nonlinear medium NLM'. Such a reflection-focusing-reflection process may continue until the spectrally broadened pulsed input radiation IRD' finally leaves the oscillating cavity OC', i.e., when the spectrally broadened pulsed input radiation IRD' no longer strikes either reflecting surface RS1', RS2'.

[0091] After emerging from the oscillating cavity OC, the spectrally broadened pulsed input radiation IRD' may be regarded as broadband output radiation ORD'. The multi-path radiation device MPC' may include one or more output steering mirrors SM2' arranged to direct the broadband output radiation ORD'. In an embodiment, the one or more output steering mirrors SM2 may be curved mirrors and may simultaneously function as output mode matching mechanisms.

[0092] 10 , the total number of passes through the solid nonlinear medium NLM′ may depend on the amount of spectral broadening required to widen the spectrum of the pulsed input radiation IRD′ to the 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 not only a nonlinear spectral broadening process (e.g., SPM) but also a filamentation process. Thus, during each pass through the solid nonlinear medium NLM′, one or more filaments may be formed along the propagation direction of the pulsed input radiation IRD′ inside the nonlinear medium NLM′. In some embodiments, during some of the multiple passes through the solid nonlinear medium NLM′, the pulsed input radiation IRD′ may undergo a filamentation process in addition to a nonlinear spectral broadening process.

[0093]

[0091] Because both the linear and nonlinear refractive indices of a solid are much higher than those of a gas, the critical power for self-focusing in a solid is much lower than that in a gas, according to equation [3]. This means that the focusing requirements to reach the critical power for self-focusing may be relaxed, and therefore it may be easier to initiate the filamentation process in a solid nonlinear medium than in a gaseous nonlinear medium.

[0094] The pulsed input radiation IRD, IRD' may have a pulse length of 10,000 fs, 5,000 fs or less than 1,000 fs, for example between 1 and 10,000 fs, alternatively between 1 and 5,000 fs, alternatively between 1 and 1,000 fs, alternatively between 10 and 500 fs, alternatively between 50 fs and 500 fs, alternatively between 50 fs and 200 fs, alternatively between 100 fs and 200 fs. The pulsed input radiation IRD, IRD' may have a central wavelength in the wavelength range between 400 nm and 2,000 nm, alternatively between 400 nm and 1,600 nm, alternatively between 400 nm and 1,200 nm, alternatively between 600 nm and 1,000 nm.

[0095]

[0093] The broadband output radiation ORD, ORD' may have a spectrum ranging from 200 nm to 2000 nm, alternatively from 200 nm to 1600 nm, alternatively from 200 nm to 1200 nm, alternatively from 400 nm to 1000 nm.

[0096] According to another different aspect of the present disclosure, there is provided a method for configuring a radiation source including a multi-path radiation device MPC, MPC′ and a pump radiation source configured to emit pulsed input radiation IRD. With reference to Figure 14, the method may for example include the following six steps:

[0097] Step 1410: Providing a nonlinear medium NLM, NLM' for spectrally broadening the pulsed input radiation IRD, IRD'.

[0098] Step 1420: Determining the critical power for self-focusing in the nonlinear medium NLM, NLM′.

[0099] Step 1430: Configuring the pump radiation source so that the pulsed input radiation IRD, IRD' has a peak power greater than or equal to the determined critical power for self-focusing.

[0100] Step 1440: Determining a focused beam size of pulsed input radiation IRD, IRD' in the oscillation cavity that has sufficient intensity to enable the filamentation process.

[0101]

[0099] Step 1450: Determine the radius of curvature of the first reflecting surface RS1, RS1' and the radius of curvature of the second reflecting surface RS2, RS2' so that the pulsed input radiation IRD, IRD' is reflected by either the first reflecting surface RS1, RS1' or the second reflecting surface RS2, RS2' and focused to have a beam size substantially the same as the focused beam size at the determined focusing surface FP, FP' of the oscillation cavity OC, OC'.

[0102] [000100] Step 1460: Separating the first reflective surface RS1, RS1′ and the second reflective surface RS2, RS2′ along the common longitudinal axis PA, PA′ by a distance equal to one-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′.

[0103] [000101] In an embodiment, the method may further include adjusting a beam size and / or divergence of the pulsed input radiation such that a focusing position of the pulsed input radiation within the oscillating cavity substantially overlaps with an intra-cavity focusing plane of the oscillating cavity.

[0104] [000102] In an embodiment, the nonlinear medium may be a solid medium, and the method may further include positioning the nonlinear medium substantially overlapping with a focusing surface of the oscillating cavity.

[0105] [000103] It is understood that the above embodiments are provided for the purpose of illustrating the proposed concepts and are not intended to limit the scope of the present disclosure. Of course, other embodiments can be envisioned.

[0106] [000104] For example, in an embodiment, the multi-path radiation device MPC, MPC' may include a transmission fiber configured to transmit the broadband output radiation ORD, ORD' to a different location (e.g., one of the metrology tools mentioned above). The transmission fiber may be, for example, a single-mode photonic crystal fiber such as HC-PCF. In an embodiment, one or both of the first reflecting mirror CM1, CM1' and the second reflecting mirror CM2, CM2' may include a dispersion-compensating coating configured to provide a high reflection coefficient for the pulsed input radiation IRD, IRD' while substantially compensating for dispersion accumulated by the pulsed input radiation IRD, IRD' through its travel within the oscillation cavity OC, OC'. Additionally or optionally, one or more of the input steering mirror SM1, SM1' and / or the output steering mirror SM2, SM2' may also include a dispersion-compensating coating. In this way, the broadband output radiation ORD, ORD' may simultaneously have a broad spectral profile and a substantially compressed temporal profile.

[0107] [000105] After time compression or dispersion compensation, the broadband output radiation ORD, ORD' may have a pulse length between 1 and 10000 fs, alternatively between 1 and 5000 fs, alternatively between 1 and 1000 fs, alternatively between 10 and 500 fs, alternatively between 50 fs and 500 fs, alternatively between 50 fs and 200 fs, alternatively between 100 fs and 200 fs.

[0108] [000106] In some embodiments, the multipath radiation device MPC, MPC' may include a solid monolithic element having a first end segment configured to provide a first reflecting surface RS1, RS1', a second end segment configured to provide a second reflecting surface RS2, RS2', and a central segment acting as the nonlinear medium NLM, NLM'. The three segments may be substantially transparent to the pulsed input radiation IRD, IRD' and the broadband output radiation ORD, ORD'. The first and second segments may be made of fused silica, for example, and the central segment may be made of YAG. The three segments may be joined to form a monolithic part, with the central segment sandwiched between the first and second end segments. The first reflecting surface RS1, RS1' may be an outer surface of the first end segment, and the second reflecting surface RS2, RS2' may be an outer surface of the second end segment. The first reflecting surface RS1, RS1' and the second reflecting surface RS2, RS2' may both be curved (eg, curved inward from a central segment).

[0109] 15 is a block diagram illustrating a computer system 1500 that may assist in implementing the methods and flows 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, 1505) coupled with the bus 1502 for processing information. The computer system 1500 also includes a main memory 1506 (such as a random access memory (RAM) or other dynamic storage device) coupled to the bus 1502 for storing information and instructions for execution by the processor 1504. The main memory 1506 may also be used for storing temporary variables or other intermediate information during execution of instructions 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 (such as a magnetic or optical disk) is provided and coupled to the bus 1502 for storing information and instructions.

[0110] [000108] Computer system 1500 can be coupled via bus 1502 to a display 1512 (such as a cathode ray tube (CRT) or flat-panel or touch-panel display) for displaying information to a computer user. Input devices 1514 (including alphanumeric and other keys) are coupled to bus 1502 for communicating information and command selections to processor 1504. Another type of user input device is a cursor control 1516 (such as a mouse, trackball, or cursor direction keys) for communicating directional information and command selections to processor 1504 and for controlling cursor movement on display 1512. This input device typically has two degrees of freedom in two axes, i.e., a first axis (e.g., x) and a second axis (e.g., y), allowing the device to specify a position in a plane. Touch-panel (screen) displays can also be used as input devices.

[0111] [000109] One or more of the methods described herein may be performed by computer system 1500 in response to processor 1504 executing one or more sequences of one or more instructions contained in main memory 1506. Such instructions may be read into main memory 1506 from another computer-readable medium, such as storage device 1510. Execution of the sequences of instructions contained in main memory 1506 causes processor 1504 to perform the process steps described herein. One or more processors in a multi-processing arrangement may also be used to execute the sequences of instructions contained in main memory 1506. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions. Thus, the description herein is not limited to any specific combination of hardware circuitry and software.

[0112] The term "computer-readable medium," as used herein, refers to any medium that participates in providing instructions to processor 1504 for execution. Such media may take many forms, including but not limited to, non-volatile media, volatile media, and transmission media. Non-volatile media include, for example, optical or magnetic disks, such as storage device 1510. Volatile media include dynamic memory, such as main memory 1506. Transmission media include coaxial cables, copper wire and fiber optics, including the wires that comprise bus 1502. Transmission media may also take the form of acoustic or light waves, such as those generated during radio frequency (RF) and infrared (IR) data communications. Common forms of computer-readable media include, for example, a floppy disk, a flexible disk, a hard disk, magnetic tape, any other magnetic medium, a CD-ROM, a DVD, any other optical medium, punch cards, paper tape, any other physical medium with a pattern of holes, a RAM, a PROM, an EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave as described below, or any other medium from which a computer can read.

[0113] Various forms of computer-readable media may be involved in carrying one or more sequences of one or more instructions to processor 1504 for execution. For example, the instructions may initially reside on a magnetic disk of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to computer system 1500 can receive the data on the telephone line and use an infrared transmitter to convert the data to an infrared signal. An infrared detector coupled to the bus can receive the data carried in the infrared signal and place the data on bus 1502. Bus 1502 carries the data to main memory 1506, from which processor 1504 retrieves and executes the instructions. The instructions received by main memory 1506 may optionally be stored on storage device 1510 either before or after execution by processor 1504.

[0114] Computer system 1500 also preferably includes a communication interface 1518 coupled to bus 1502. The communication interface 1518 provides a two-way data communication coupling to a network link 1520 that is connected to a local network 1522. For example, communication interface 1518 may be an Integrated Services Digital Network (ISDN) card or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, communication interface 1518 may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In any such implementation, communication interface 1518 sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information.

[0115] [000113] Network link 1520 typically provides data communication through one or more networks to other data devices. For example, network link 1520 may provide a connection through local network 1522 to a host computer 1524 or to data equipment operated by an Internet Service Provider (ISP) 1526. ISP 1526, in turn, provides data communication services through the world wide packet data communication network (now commonly referred to as the "Internet" 1528). Local network 1522 and the Internet 1528 both use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on network link 1520 and through communication interface 1518 (which carry the digital data to and from computer system 1500) are exemplary forms of carrier waves transporting the information.

[0116] [000114] Computer system 1500 can send messages and receive data, including program code, through the network(s), network link 1520 and communication interface 1518. In the Internet example, a server 1530 might send a requested code for an application program through the Internet 1528, ISP 1526, local network 1522 and communication interface 1518. One such downloaded application may, for example, comprise one or more of the techniques described herein. The received code may be executed by processor 1504 upon receipt and / or stored in storage device 1510 or other non-volatile storage for later execution. In this manner, computer system 1500 can obtain application code in the form of a carrier wave.

[0117] [000115] Further embodiments are disclosed in the following list of numbered clauses: 1. A radiating device for receiving pulsed input radiation and generating broadband output radiation, comprising: an oscillating cavity including a first reflecting surface and a second reflecting surface; a nonlinear medium located between the first reflecting surface and the second reflecting surface; Including, A radiation device having an oscillating cavity configured to receive pulsed input radiation and oscillate therein, such that the pulsed input radiation undergoes a filamentation process to form one or more filaments in the nonlinear medium, and the pulsed input radiation passes through the nonlinear medium multiple times and is spectrally broadened by the nonlinear medium to form broadband output radiation. 2. A radiating device as defined in clause 1 in which multiple passes through a nonlinear medium each follow a different path. 3. A radiating device as defined in clause 1 or 2 in which the pulsed input radiation undergoes a filamentation process during two or more of its multiple passes through the nonlinear medium. 4. A radiating device as defined in any one of the preceding clauses, wherein the nonlinear spectral broadening process comprises a self-phase modulation process. 5. A radiating device as defined in any one of the preceding clauses, wherein the first reflective surface and the second reflective surface are both concavely curved. 6. A radiation device as defined in any one of the preceding clauses, wherein the first reflective surface and the second reflective surface each define a common intra-cavity focusing surface onto which pulsed input radiation is focused each time it passes through the oscillating cavity. 7. A radiating device as defined in clause 6, wherein the nonlinear medium is at least partially configured in an intracavity focusing surface. 8. A radiation device as defined in any one of the preceding clauses, further comprising a mode-matching mechanism configured to adjust the beam size and / or divergence of the pulsed input radiation such that the focusing position of the pulsed input radiation within the oscillating cavity is substantially at an intra-cavity focusing plane. 9. A radiating device as defined in clause 8, wherein the mode-matching mechanism comprises one or more lenses, one or more curved mirrors, or a combination thereof. 10. A radiating device as defined in any one of the preceding clauses, 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 and second reflective surfaces are symmetrical about a common longitudinal axis and are separated along the common longitudinal axis by a distance equal to one-half the sum of the first and second radii of curvature. 11. A radiating device as defined in clause 10, wherein the first radius of curvature and the second radius of curvature are equal. 12. A radiating device as defined in clause 10, wherein the first radius of curvature and the second radius of curvature are different. 13. A radiation device as defined in any one of clauses 10 to 12, wherein the first radius of curvature and the second radius of curvature are reflected by one of the first reflective surface and the second reflective surface such that the pulsed input radiation is focused into a spot having a peak intensity sufficient to enable the filamentation process. 14. A radiation device as defined in any one of the preceding clauses, further comprising a first reflecting mirror configured to provide a first reflecting surface and a second reflecting mirror configured to provide a second reflecting surface. 15. A radiation device as defined in clause 14, wherein one or both of the first reflecting mirror and the second reflecting mirror includes a dispersion compensation coating configured to provide a high reflection coefficient for the pulsed input radiation while substantially compensating for dispersion accumulated by the pulsed input radiation throughout its travel within the oscillation cavity. 16. A radiating device as defined in any one of the preceding clauses, wherein the nonlinear medium is a gas. 17. A radiative device as defined in clause 16, wherein the nonlinear medium is one or more of helium, neon, argon, krypton, xenon, and radon. 18. A radiating device as defined in any one of clauses 1 to 15, wherein the nonlinear medium is a liquid. 19. A radiating device as defined in any one of clauses 1 to 15, wherein the nonlinear medium is a solid. 20. A radiating device as defined in clause 19, in which the nonlinear medium is an yttrium aluminum garnet (YAG) crystal or a fused silica crystal. 21. A radiation device as defined in any one of the preceding clauses, further comprising a chamber arranged to at least partially enclose the first reflective surface, the second reflective surface, and the nonlinear medium. 22. A radiation device as defined in clause 21, wherein the chamber includes an input window configured to substantially transmit pulsed input radiation and an output window configured to substantially transmit broadband output radiation. 23. A radiation device as defined in any one of the preceding clauses, wherein the pulsed input radiation has a pulse length between 1 and 10,000 fs. 24. A radiation device as defined in any one of the preceding clauses, wherein the pulsed input radiation has a central wavelength between 400 nm and 2000 nm. 25. A radiation device as defined in any one of the preceding clauses, wherein the broadband output radiation has a spectrum that at least partially overlaps the spectral range from 200 nm to 2000 nm, or optionally, the broadband radiation has a spectrum spanning the range from 200 nm to 2000 nm. 26. A radiating device as defined in any one of the preceding clauses, further comprising a transmission fiber for transmitting the broadband output radiation. 27. A radiating device as defined in clause 26, wherein the transmission fiber is a single-mode fiber photonic crystal fiber (PCF). 28. A radiating device as defined in clause 27, wherein the transmission fiber is a single-mode hollow-core photonic crystal fiber (HC-PCF). 29. A radiating device as defined in any one of the preceding clauses; a pump radiation source configured to output pulsed input radiation; A radiation source including: 30. A metrology device including a radiation source as defined in any one of the preceding clauses. 31. A metrology device as defined in clause 30, including a scatterometer metrology instrument, level sensor or alignment sensor. 32. A method for constructing a radiation device as defined in any one of clauses 1 to 28, comprising: providing pulsed 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 a target output spectrum via a nonlinear spectral broadening process; determining the total number of passes through the nonlinear medium to achieve the determined amount of nonlinear spectral broadening; determining an input direction for the pulsed input radiation to enter the oscillation cavity to achieve the determined total number of passes through the nonlinear medium; directing the pulsed input radiation to follow the determined input direction; A method comprising: 33. A method for constructing a radiation source as defined in Article 29, comprising: Determining the critical power for self-focusing in a nonlinear medium; configuring a pump radiation source such that the pulsed input radiation has a peak power greater than or equal to the determined critical power for self-focusing; determining a focused beam size of pulsed input radiation within an oscillation cavity having sufficient intensity to enable a filamentation process; determining a radius of curvature of the first reflecting surface and / or a radius of curvature of the second reflecting surface such that the pulsed input radiation is reflected by either the first reflecting surface or the second reflecting surface to be focused to have substantially the same beam size as the determined focused beam size at a focusing surface of the oscillating cavity; Separating the first and second reflective surfaces along a common longitudinal axis by a distance equal to one-half the sum of the radii of curvature of the first and second reflective surfaces; A method comprising: 34. The method defined in clause 33, further comprising adjusting the beam size and / or divergence of the pulsed input radiation such that a focusing position of the pulsed input radiation within the oscillating cavity substantially overlaps with an intra-cavity focusing surface of the oscillating cavity. 35. A method as defined in clause 33 or 34, wherein the nonlinear medium is a solid medium; The method further comprising positioning the nonlinear medium substantially overlapping the focusing surface of the oscillating cavity. 36. A method for generating broadband output radiation, comprising: generating a pulsed input radiation; passing the pulsed input radiation through the nonlinear medium multiple times and repeatedly reflecting it back and forth, such that the pulsed input radiation undergoes a filamentation process to form one or more filaments within the nonlinear medium, and the pulsed input radiation is spectrally broadened by the nonlinear medium to form broadband output radiation after passing through the nonlinear medium multiple times; outputting a broadband output radiation; A method comprising: 37. The method defined in clause 36, wherein each reflection of the repeatedly reflecting step includes focusing the pulsed input radiation at a focusing surface, and wherein the nonlinear medium is configured at least partially at the focusing surface. 38. A method as defined in clause 36 or 37, wherein each reflection of the repeatedly reflecting step includes compensating for dispersion of the pulsed input radiation during multiple passes through the nonlinear medium. 39. A method as defined in any one of clauses 36 to 38, in which each of the multiple passes through the nonlinear medium follows a different path. 40. A method as defined in any one of clauses 36 to 39, wherein the pulsed input radiation undergoes a filamentation process each time it passes through the nonlinear medium multiple times. 41. The method defined in any one of clauses 36 to 40, wherein the nonlinear spectral broadening process comprises a self-phase modulation process. 42. A method as defined in any one of clauses 37 to 41, wherein the step of receiving pulsed input radiation comprises adjusting a beam size and / or divergence of the pulsed input radiation such that the focal position of the pulsed input radiation is substantially at the focal plane. 43. The method defined in any one of the preceding clauses, wherein the outputting step includes coupling the broadband output radiation into a transmission fiber.

[0118] [000116] Although specific reference is made in this specification to uses of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optics, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0119] [000117] Although embodiments of the invention may be specifically referenced herein in connection with lithography apparatus, they may be used in other apparatus. They may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes objects such as wafers (or other substrates) or masks (or other patterning devices). These apparatuses may collectively be referred to as lithography tools. Such lithography tools may use vacuum or ambient (non-vacuum) conditions.

[0120] [000118] Although specific reference has been made above to the use of embodiments of the invention in connection with optical lithography, it will be understood that the invention is not limited to optical lithography and may be used in other applications, for example in imprint lithography, where the context allows.

[0121] [000119] While specific embodiments of the present invention have been described above, it will be understood that the invention may be practiced otherwise than as described. The foregoing description is intended to be illustrative and not limiting. Accordingly, it will be apparent to those skilled in the art that modifications can be made to the invention as described without departing from the scope of the claims set forth below.

Claims

1. 1. A radiation device for receiving pulsed input radiation and generating broadband output radiation, comprising: an oscillating cavity including a first reflecting surface and a second reflecting surface; a nonlinear medium located between the first reflecting surface and the second reflecting surface; Including, a radiation device wherein the oscillating cavity is configured to receive the pulsed input radiation and oscillate therein, such that the pulsed input radiation undergoes a filamentation process to form one or more filaments in the nonlinear medium, and the pulsed input radiation passes through the nonlinear medium multiple times and is spectrally broadened by the nonlinear medium to form the broadband output radiation.

2. 10. The radiating device of claim 1, wherein each of the multiple passes through the nonlinear medium follows a different path.

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

4. A radiating device according to any one of claims 1 to 3, wherein the nonlinear spectral broadening process comprises a self-phase modulation process.

5. 5. The radiating device according to claim 1, wherein the first reflecting surface and the second reflecting surface are both concavely curved.

6. 6. A radiating device according to claim 1, wherein the first reflecting surface and the second reflecting surface each define a common intra-cavity focusing surface onto which the pulsed input radiation is focused each time it passes through the oscillating cavity, and optionally the nonlinear medium is at least partially arranged in the intra-cavity focusing surface.

7. 7. A radiation device according to any one of claims 1 to 6, further comprising a mode matching mechanism configured to adjust a beam size and / or divergence of the pulsed input radiation within the oscillating cavity such that a focusing position of the pulsed input radiation within the oscillating cavity is substantially at the intra-cavity focusing plane, optionally the mode matching mechanism comprising one or more lenses, one or more curved mirrors, or a combination thereof.

8. 8. A radiating device according to any one of claims 1 to 7, wherein the first reflective surface has a first radius of curvature and the second reflective surface has a second radius of curvature, the first and second reflective surfaces being symmetrical about a common longitudinal axis and separated along the common longitudinal axis by a distance equal to half the sum of the first and second radii of curvature.

9. 9. The radiation device of claim 8, wherein the first and second radii of curvature are reflected by one of the first and second reflective surfaces to focus the pulsed input radiation into a spot having a peak intensity sufficient to enable the filamentation process.

10. 10. A radiating device according to any one of claims 1 to 9, further comprising: a first reflective mirror configured to provide the first reflective surface; and a second reflective mirror configured to provide the second reflective surface, optionally one or both of the first reflective mirror and the second reflective mirror comprising a dispersion compensation coating configured to provide a high reflection coefficient to the pulsed input radiation while substantially compensating for dispersion accumulated by the pulsed input radiation throughout its travel within an oscillating cavity.

11. A radiation device according to any one of claims 1 to 10, 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 an yttrium aluminum garnet (YAG) crystal or a fused silica crystal; The radiation device is at least one of:

12. 12. A radiation device according to any one of claims 1 to 11, further comprising a chamber arranged to at least partially enclose the first reflective surface, the second reflective surface, and the nonlinear medium, optionally comprising 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 one of claims 1 to 12.

14. 14. The metrology device of claim 13, comprising a scatterometer metrology tool, 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 the steps of: providing pulsed input radiation having an input spectrum; defining a target output spectrum; determining an amount of nonlinear spectral broadening required to widen the input spectrum to the target output spectrum via a nonlinear spectral broadening process; determining a total number of passes through a nonlinear medium to achieve the determined amount of nonlinear spectral broadening; determining an input direction for the pulsed input radiation to enter an oscillation cavity to achieve the determined total number of passes through the nonlinear medium; directing the pulsed input radiation to follow the determined input direction; A method comprising:

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