Supercontinuum radiation sources and associated measurement devices

Abstract

A supercontinuum radiation source is disclosed, comprising a pump laser arrangement for generating pump radiation and comprising a plurality of pump laser heads, a radiation combiner for combining respective pump radiation from each pump laser head, and a nonlinear fiber for receiving the pump radiation to excite a working medium in the nonlinear fiber to generate supercontinuum radiation. Each of the five pump laser heads has a dimension of 5 cm or less in any direction. Alternatively or additionally, the supercontinuum radiation source further comprises a control arrangement for controlling the pump laser arrangement, the control arrangement being configured for non-simultaneous emission of pulses from each pump laser head.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to European Application No. 22157166.4, filed February 17, 2022, which is incorporated by reference in its entirety into this specification.

[0002] The present invention relates to supercontinuum radiation sources (eg, photonic crystal fiber based supercontinuum radiation sources), and in particular to supercontinuum radiation sources relevant for metrology applications in integrated circuit manufacturing. [Background technology]

[0003]

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

[0004]

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

[0005]

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

[0006]

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

[0007]

[0007] In a first aspect of the present invention, there is provided a supercontinuum radiation source comprising a pump laser arrangement for generating pump radiation and comprising a plurality of pump laser heads, each pump laser head having a dimension of 5 cm or less in any direction, a radiation combiner for combining respective pump radiation from each pump laser head, and a nonlinear fibre for receiving the pump radiation so as to excite a working medium in the nonlinear fibre to generate supercontinuum radiation.

[0008]

[0008] In a second aspect of the present invention, there is provided a supercontinuum radiation source comprising a pump laser arrangement generating pump radiation and comprising a plurality of pump laser heads, a radiation combiner for combining respective pump radiation from each pump laser head, a nonlinear fibre for receiving the pump radiation so as to excite a working medium within the nonlinear fibre to generate supercontinuum radiation, and a control arrangement for controlling the pump laser arrangement, the control arrangement being configured for non-simultaneous emission of pulses from each pump laser head.

[0009] Further aspects include a metrology device including the supercontinuum source of the first or second aspect, and associated lithographic apparatus and lithocells. [Brief description of the drawings]

[0010]

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

[0011] [Figure 1] FIG. 1 depicts a schematic overview of a lithographic apparatus. [Diagram 2] FIG. 2 shows a schematic overview of a lithography cell. [Diagram 3] Figure 3 shows a schematic diagram of holistic lithography, illustrating the collaboration between three key technologies for optimizing semiconductor manufacturing. [Figure 4] FIG. 4 shows a schematic overview of a scatterometry apparatus used as a metrology device, which may include a radiation source according to an embodiment of the present invention. [Diagram 5] FIG. 5 shows a schematic overview of a level sensor apparatus which may include a radiation source according to an embodiment of the present invention. [Figure 6] FIG. 6 shows a schematic overview of an alignment sensor apparatus which may include a radiation source according to an embodiment of the present invention. [Figure 7a] FIG. 7(a) is a schematic diagram of a supercontinuum radiation source according to a first embodiment. [Figure 7b] FIG. 7(b) shows an alternative beam combining configuration for the supercontinuum source shown in FIG. 7(a). [Figure 7c] FIG. 7(c) shows an alternative beam combining configuration for the supercontinuum source shown in FIG. 7(a). [Figure 8] FIG. 8 is a schematic diagram of a pump laser head that can be used in the embodiments disclosed herein. [Figure 9] FIG. 9 is a schematic diagram of a supercontinuum radiation source according to a second embodiment. [Figure 10] FIG. 10 is a schematic cross-sectional view of a hollow core optical fiber that may form part of a radiation source according to an embodiment, in a transverse plane (ie perpendicular to the axis of the optical fiber). [Figure 11] FIG. 11 shows a schematic diagram of a radiation source according to an embodiment for providing broadband output radiation. [Figure 12(a)] FIG. 12(a) shows a schematic cross-sectional view of an example hollow-core photonic crystal fiber (HC-PCF) design for supercontinuum generation. [Figure 12(b)] FIG. 12(b) shows a schematic cross-sectional view of an example hollow-core photonic crystal fiber (HC-PCF) design for supercontinuum generation. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012]

[0011] In this document, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including ultraviolet (e.g., having wavelengths of 365, 248, 193, 157 or 126 nm) and EUV (e.g., extreme ultraviolet having wavelengths in the range of about 5 to 100 nm).

[0013]

[0012] As used herein, the terms "reticle", "mask" or "patterning device" may be broadly interpreted to mean a general patterning device that can be used to provide an incident radiation beam with a patterned cross-section that corresponds to a pattern to be created on a target portion of a substrate. The term "light valve" may also be used in this context. In addition to classical masks (transmissive or reflective masks, binary masks, phase-shifting masks, hybrid masks, etc.), examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0014]

[0013] Figure 1 illustrates diagrammatically a lithographic apparatus LA. The lithographic apparatus LA includes an illumination system IL (also called an illuminator) configured to condition a radiation beam B (e.g. UV, DUV or EUV radiation), a mask support (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to certain parameters, a substrate support (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support according to certain parameters, and a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0015]

[0014] In operation, the illumination system IL receives a radiation beam from a radiation source SO (e.g. via a beam delivery system BD). The illumination system IL may include various types of optical components for directing, shaping and / or controlling the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical components or any combination thereof. The illuminator IL may be used to condition the radiation beam B such that it has a desired spatial and angular intensity distribution in its cross-section in the plane of the patterning device MA.

[0016]

[0015] The term "projection system" PS as used herein should be broadly interpreted to encompass various types of projection systems. Such systems may include refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems or any combination thereof, as required by the exposure radiation being used and / or other factors (e.g., the use of an immersion liquid or the use of a vacuum). Where the term "projection lens" is used herein, it may all be considered as synonymous with the more general term "projection system" PS.

[0017]

[0016] The lithographic apparatus LA may be of a type in which at least a part of the substrate may be covered in a liquid having a relatively high refractive index (e.g. water) so as to fill a space between the projection system PS and the substrate W, which is also referred to as immersion lithography. Details of immersion techniques are given in US Pat. No. 6,952,253, which is incorporated herein by reference.

[0018] The lithographic apparatus LA may be of a type in which there is more than one substrate support WT (also referred to as "dual stage"). In such a "multi-stage" machine, the substrate supports WT can be used in parallel and / or preparation steps for a next exposure of a substrate W may be performed on one substrate support WT, while another substrate W on the other substrate support WT may be used for exposing a pattern on another substrate W.

[0019] 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 device. The sensor may be configured to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold a number of sensors. The cleaning device may be configured to clean a part of the lithographic apparatus, for example a part of the projection system PS or a part of a system for supplying immersion liquid. The measurement stage may move below the projection system PS when the substrate support WT is spaced apart from the projection system PS.

[0020]

[0019] In operation, a radiation beam B is incident on a patterning device (e.g. mask MA) held on a mask support MT and is patterned by a pattern (design layout) on the patterning device MA. After passing through the mask MA, the radiation beam B passes through a projection system PS, which focuses the beam onto a target portion C of a substrate W. Using the second positioner PW and the position measurement system IF, the substrate support WT can be accurately moved, for example to position different target portions C in the path of the radiation beam B at focused and aligned positions. Similarly, the patterning device MA can be accurately positioned with respect to the path of the radiation beam B using the first positioner PM and possibly further position sensors (not explicitly shown in Figure 1). The patterning device MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks P1, P2 as illustrated occupy dedicated target portions, the substrate alignment marks may be located in spaces between the target portions. When the substrate alignment marks P1, P2 are located between target portions C, they are known as scribe-lane alignment marks.

[0021]

[0020] As shown in Fig. 2, the lithographic apparatus LA may be part of a lithographic cell LC (sometimes called a litho-cell or (litho)-cluster), which often also includes apparatus for performing pre-exposure and post-exposure processes on the substrate W. Conventionally, such apparatus include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH and a bake plate BK for adjusting the temperature of the substrate W, e.g., to adjust the solvent in the resist layer. A substrate handler or robot RO picks up substrates W from input / output ports I / O1, I / O2, moves them between the various process devices and delivers them to a loading bay LB of the lithographic apparatus LA. The devices in a lithocell are often collectively referred to as a track and are typically under the control of a track control unit TCU, which may itself be controlled by a supervisory control system SCS, which may also control the lithographic apparatus LA (e.g. via a lithography control unit LACU).

[0022]

[0021] To ensure that a substrate W to be exposed by the lithographic apparatus LA is accurately exposed, it is desirable to inspect the substrate to measure properties of the patterned structures, such as overlay error between successive layers, line thickness, critical dimension (CD), etc. To that end, an inspection tool (not shown) may be included in the lithocell LC. If an error is detected, adjustments may be made, for example, to the exposure of subsequent substrates or other processing steps to be performed on the substrate W, especially if the inspection is performed before other substrates W of the same batch or lot are subsequently exposed or processed.

[0023]

[0022] The inspection apparatus, sometimes called metrology apparatus, is used to determine the properties of the substrate W, in particular how the properties of different substrates W vary, or how the 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 the substrate W and may for example be part of the lithocell LC, or integrated in the lithography apparatus LA, or may be a stand-alone device. The inspection apparatus may measure properties related to the latent image (image in the resist layer after exposure), or to the semi-latent image (image in the resist layer after a post-exposure bake step PEB), or to the developed resist image (where exposed or unexposed parts of the resist have been removed), or even to the etched image (after a pattern transfer step such as etching).

[0024] Typically, the patterning process in the lithography apparatus LA is one of the most critical steps of the process, requiring high accuracy in the dimensioning and placement of structures on the substrate W. To ensure this high accuracy, three systems may be combined as a so-called "holistic" controlled environment, as shown diagrammatically in FIG. 3. One of these systems is the lithography apparatus LA, which is (virtually) connected with a metrology tool MT (second system) and a computer system CL (third system). The key to such a "holistic" environment is to optimize the coordination between these three systems to enforce the overall process window and provide a tight control loop so that the patterning performed by the lithography apparatus LA stays within the process window. The process window defines the range of process parameters (e.g., dose, focus, overlay) within which a particular manufacturing process produces a defined result (e.g., a functioning semiconductor device) and within which the process parameters of the lithography process or patterning process may typically vary.

[0025]

[0024] The computer system CL can predict which resolution enhancement technique should be used by using (part of) the design layout to be patterned, and can perform computational lithography simulations and calculations to determine the mask layout and lithography device settings that achieve maximization of the overall process window of the patterning process (indicated by the double-headed arrow at the first scale SC1 in FIG. 3). Typically, the resolution enhancement technique is prepared to match the patterning capability of the lithography device LA. The computer system CL can further predict whether defects may be present (e.g., due to suboptimal processing) by detecting (e.g., using input from the metrology tool MT) where in the process window the lithography device LA is currently operating (indicated by the arrow pointing to "0" at the second scale SC2 in FIG. 3).

[0026]

[0025] The metrology tool MT is capable of providing input to the computer system CL enabling accurate simulation and prediction, and is capable of providing feedback to the lithographic apparatus LA identifying possible drifts (e.g. in the calibration status of the lithographic apparatus LA) (as shown by multiple arrows at the third scale SC3 in Figure 3).

[0027]

[0026] In lithographic processes, it is desirable to frequently measure the created structures (e.g. for process control and validation). Tools performing such measurements are commonly referred to as metrology tools MT. Different types of metrology tools MT performing such measurements are known, such as scanning electron microscopes or different forms of scatterometer metrology tools MT. A scatterometer is a multipurpose instrument that allows the measurement of parameters of the lithographic process, either by having a sensor in the pupil or in a conjugate plane to the pupil of the scatterometer objective (usually referred to as pupil-based metrology) or by having a sensor in the image plane or in a conjugate plane to the image plane (usually referred to as image-based or field-based metrology). 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, the entireties of which are incorporated by reference herein. The scatterometers described above are capable of measuring gratings using soft x-rays and light in the visible to near infrared wavelength range.

[0028]

[0027] In a first embodiment, the scatterometer MT is an angularly resolved scatterometer. In such a scatterometer, a reconstruction method can be applied to the measurement signal to reconstruct or calculate the properties of the grating. Such a reconstruction can for example be the result of simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measurement results. The parameters of the mathematical model are adjusted until the simulation of the interaction produces a diffraction pattern similar to the diffraction pattern observed from the real target.

[0029]

[0028] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, radiation emitted by a radiation source is directed to a target, and radiation reflected or scattered from the target is directed to a spectrometer detector, which measures the spectrum of the specularly reflected radiation (i.e., the intensity as a function of wavelength). From this data, it is possible to reconstruct the structure or profile of the target causing the detected spectrum, for example by rigorous coupled wave analysis and nonlinear regression, or by comparison with a library of simulated spectra.

[0030]

[0029] In a third embodiment, the scatterometer MT is an ellipsometric scatterometer. The ellipsometric scatterometer allows to determine the parameters of the lithographic process by measuring the scattered radiation for each of the polarization states. Such a metrology tool emits polarized light (e.g. linearly, circularly or elliptically polarized light), for example using a suitable polarizing filter in the illumination section of the metrology tool. A radiation source suitable for the metrology tool 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 in their entireties.

[0031]

[0030] A metrology device such as a scatterometer is shown in Figure 4. It includes a broadband (white light) radiation projector 2, which projects radiation onto a substrate W. The reflected or scattered radiation is sent to a spectrometer detector 4, which measures the spectrum 6 of the specularly reflected radiation (i.e. a measurement of the intensity as a function of wavelength). From this data, the structure or profile 8 giving rise to the detected spectrum can be reconstructed by a processing unit (PU), for example by rigorous coupled wave analysis and non-linear regression, or by comparison with a library of simulated spectra as shown in the lower part of Figure 3. Typically, for reconstruction, the general form of the structure is known and some parameters are assumed from knowledge of the process by which the structure was made, thereby leaving only a few parameters of the structure to be determined from the scatterometry data. Such a scatterometer may be configured as a normal incidence scatterometer or an oblique incidence scatterometer.

[0032]

[0031] The overall measurement quality of a lithography parameter through the measurement of a metrology target is determined, at least in part, by the measurement recipe used to measure this lithography parameter. The term "substrate measurement recipe" may encompass one or more parameters of the measurement itself, one or more parameters of the measured pattern or patterns, or both. For example, if the measurement used in the substrate measurement recipe is a diffraction-based optical measurement, one or more of the parameters of the measurement may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation to the substrate, the orientation of the radiation to the pattern on the substrate, etc. One of the criteria in selecting a measurement recipe may be, for example, the sensitivity of any measurement parameter to process variations. Further examples are described in U.S. Patent Application No. 2016-0161863 and published U.S. Patent Application No. 2016 / 0370717A1, which are incorporated herein by reference in their entirety.

[0033] Another type of metrology tool used in IC manufacturing is a topography measurement system, level sensor, or height sensor. Such a tool may be integrated into a lithography apparatus to measure the topography of the top surface of a substrate (or wafer). A map of the topography of the substrate (also called a height map) may be generated from these measurements showing the height of the substrate as a function of position on the substrate. This height map may later be used to correct the position of the substrate during transfer of a pattern on the substrate to provide an aerial image of the patterning device at the 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 optical system), and relative movement between the substrate and the optical system of the level or height sensor results in height measurements at locations across the substrate.

[0034]

[0033] An example of a level or height sensor LS known in the art is shown diagrammatically in Fig. 5, which shows only the principle of operation. In this example, the level sensor includes an optical system, which includes a projection unit LSP and a detection unit LSD. The projection unit LSP includes a radiation source LSO, which provides a radiation beam LSB, 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.

[0035]

[0034] The projection grating PGR is a periodic grating including a periodic structure resulting in a radiation beam BE1 with a periodically varying intensity. The radiation beam BE1 with a periodically varying intensity is directed towards a measurement location MLO on the substrate W with an angle of incidence ANG relative to an axis normal to the incident substrate surface (Z-axis) between 0 degrees and 90 degrees, typically between 70 degrees and 80 degrees. At the measurement location MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and directed towards a detection unit LSD.

[0036]

[0035] To determine the height level at the measurement location MLO, the level sensor further comprises a detection system including a detection grid DGR, a detector DET and a processing unit (not shown) for processing an output signal of the detector DET. The detection grid DGR may be the same as the projection grid PGR. The detector DET generates a detector output signal indicative of the received light, for example indicative of the intensity of the received light (e.g. a photodetector) or representative of the spatial distribution of the received intensity (e.g. a camera). The detector DET may comprise any combination of one or more detector types.

[0037]

[0036] Triangulation techniques make it possible to determine the height level at the measurement location MLO. The detected height level is generally related to the signal intensity as measured by the detector DET, which has a periodicity that depends, among other things, on the design of the projection grating PGR and on the (oblique) angle of incidence ANG.

[0038]

[0037] 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).

[0039]

[0038] In an embodiment, the detection grid DGR may be omitted and the detector DET may be arranged at the position of the detection grid DGR. Such a configuration provides a more direct detection of the image of the projection grid PGR.

[0040]

[0039] In order to effectively cover the surface of the substrate W, the level sensor LS may be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating an array of measurement areas MLO or spots covering a larger measurement range.

[0041]

[0040] Various height sensors of the general type are disclosed, for example, in US Patent No. 7,265,364 and US Patent No. 7,646,471, both of which are incorporated by reference. A height sensor that uses UV radiation instead of visible or infrared radiation is disclosed in US Patent Publication No. 2010233600A1, which is incorporated by reference. WO 2016102127A1, which is incorporated by reference, describes a miniature height sensor that uses a multi-element detector to detect and recognize the position of a grid image without the need for a detection grid.

[0042]

[0041] 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 an applied pattern in relation to features built up in a previous layer (by the same apparatus or a different lithographic apparatus). For this purpose, the substrate is provided with one or more sets of marks or targets. Each mark is a structure whose position can be measured later using a position sensor (typically an optical position sensor). The position sensor may be called an "alignment sensor" and the marks may be called an "alignment mark".

[0043]

[0042] The lithographic apparatus may include one or more (e.g., a plurality of) alignment sensors, which may accurately measure the position of an alignment mark provided on a substrate. The alignment (or position) sensor may use optical phenomena such as diffraction and interference to obtain position information from an alignment mark formed on a substrate. One example of an alignment sensor used in current lithographic apparatus is based on a self-referencing interferometer, as described in US Pat. No. 6,961,116. Various improvements and modifications of position sensors have been developed, as disclosed, for example, in US Patent Application Publication No. 2015261097A1. The entire contents of these publications are incorporated herein by reference.

[0044]

[0043] Figure 6 is a schematic block diagram of an embodiment of a known alignment sensor AS, for example as described in US6961116, which is incorporated by reference. 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 includes a spot mirror SM and an objective lens OL. The illumination spot SP (by which the mark AM is illuminated) may be slightly smaller in diameter than the width of the mark itself.

[0045]

[0044] The radiation diffracted by the alignment mark AM is collimated (in this example via an objective lens OL) into an information-bearing beam IB. The term "diffracted" is intended to include zero order diffraction from the mark (which may be called reflection). A self-referencing interferometer SRI, for example of the type disclosed in the above-mentioned US Pat. No. 6,961,116, interferes the beam IB with itself, after which the beam is received by a photodetector PD. Further optics (not shown) may be included to provide separate beams if more than one wavelength is produced by the radiation source RSO. The photodetector may be a single element, or it may include several pixels, as required. The photodetector may include a sensor array.

[0046]

[0045] The redirecting optics (which in this example includes a spot mirror SM) may also function to block zero order radiation reflected from the mark so that the information-carrying beam IB contains only higher order diffracted radiation from the mark AM (this is not essential for the measurement, but improves the signal-to-noise ratio).

[0047]

[0046] The intensity signal SI is fed to a processing unit PU. The combination of optical processing in block SRI and computational processing in unit PU results in values ​​of the X and Y positions on the substrate relative to a reference frame.

[0048]

[0047] A single measurement of the type shown fixes the position of the mark only within a certain range corresponding to one pitch of the mark. In conjunction with this, coarser measurement techniques are used to identify which period of the sine wave contains the mark position. The same process can be repeated at coarser and / or finer levels with different wavelengths for improved accuracy and / or robust detection of the mark regardless of the material it is made of and the material on which and / or under which it is provided. The wavelengths may be optically multiplexed and demultiplexed to be processed simultaneously and / or the wavelengths may be multiplexed by time or frequency division.

[0049]

[0048] In this example, the alignment sensor and the spot SP remain stationary, and it is the substrate W that moves. The alignment sensor can therefore be rigidly and accurately mounted to a reference frame, effectively scanning the mark AM in a direction opposite to the direction of movement of the substrate W. The substrate W is controlled in this movement by a substrate positioning system, which controls its mounting on the substrate support and the movement of the substrate support. A substrate support position sensor (e.g. an interferometer) measures the position of the substrate support (not shown). In an embodiment, one or more (alignment) marks are provided on the substrate support. Measuring the position of the marks provided on the substrate support allows the position of the substrate support, as determined by the position sensor, to be calibrated (e.g. relative to a frame to which the alignment system is connected). Measuring the position of the alignment marks provided on the substrate allows the position of the substrate relative to the substrate support to be determined.

[0050]

[0049] For optical semiconductor metrology and inspection applications in any of the aforementioned metrology tools, a bright light source that outputs coherent radiation simultaneously covering a wide wavelength range (e.g., from UV to IR) is often preferred. Such a broadband light source can help improve the flexibility and robustness of the application by allowing wafers with different material properties to be optically interrogated in the same setup / system without the need for hardware modifications (e.g., changing the light source to have a specific wavelength). Also, being able to optimize the wavelength for a specific application means that the accuracy of the measurement can be further improved.

[0051]

[0050] Gas lasers based on gas discharge effects to emit multiple wavelengths simultaneously can be used for these applications. However, inherent problems such as high intensity instability and low spatial incoherence associated with gas lasers may make them unsuitable. Alternatively, the output from multiple lasers (e.g., solid-state lasers) with different wavelengths can be spatially combined into the optical path of the metrology or inspection system to provide a multiple wavelength source. The complexity and high implementation costs, which increase with the number of desired wavelengths, prevent such solutions from being widely used. In contrast, fiber-based broadband or white light lasers, also called supercontinuum lasers, can emit radiation with high spatial coherence and wide spectral coverage, for example from UV to IR, and are therefore a very attractive and practical option.

[0052]

[0051] A metrology tool MT, such as the scatterometer, topography measurement system or position measurement system described above, may use radiation originating from a radiation source to perform measurements. The characteristics of the radiation used by the metrology tool may affect the type and quality of measurements that may be performed. For some applications, it may be advantageous to use multiple radiation frequencies to measure the substrate, for example broadband radiation may be used. Multiple different frequencies may be capable of propagating, illuminating and scattering from the metrology target with no or minimal interference with other frequencies. Thus, for example, different frequencies may be used to simultaneously obtain more metrology data. Different radiation frequencies may also be capable of interrogating or discovering different characteristics of the metrology target. Broadband radiation may be useful in metrology systems MT, such as level sensors, alignment mark measurement systems, scatterometry tools or inspection tools. The broadband radiation source may be a supercontinuum source.

[0053]

[0052] One of the ways to generate broadband radiation can be to broaden high-power narrowband or single-frequency input radiation, for example, by utilizing nonlinear and higher-order effects. The input radiation (which can be generated using a laser) can be called pump laser radiation. Alternatively, the input radiation can be called seed radiation. To obtain high-power radiation for broadening effects, the radiation can be confined within a small area such that strong localized high-intensity radiation is achieved. In those areas, the radiation can interact with broadening structures and / or materials that form a nonlinear medium, and broadband output radiation can be produced. In the 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.

[0054]

[0053] In some implementations, broadband output radiation is produced in a photonic crystal fiber (PCF). In some embodiments, such a photonic crystal fiber has a microstructure around its fiber core to help confine radiation traveling in the fiber core through the fiber. The fiber core may be made of a solid material that has nonlinear properties and allows the generation of broadband radiation when high-intensity pump laser radiation is transmitted through the fiber core. Alternatively, a hollow-core photonic crystal fiber (HC-PCF) comprises a central hollow core region and an inner cladding structure surrounding the hollow core, both extending axially along the entire fiber. The light guidance mechanism is enabled by the inner cladding waveguiding structure, which may include, for example, a thin glass element. Thus, the radiation is mainly confined inside the hollow core and propagates along the fiber in the form of transverse core modes.

[0055]

[0054] Current commercial supercontinuum sources typically use high-power ultrashort pulse fiber lasers to generate pump radiation. These pump lasers are relatively expensive. To generate supercontinuum light, the pump laser should generate short pulses (e.g., in the femtosecond fs to picosecond ps range). This requires high energy per pulse to obtain a relatively high peak power given a certain repetition rate.

[0056]

[0055] The pump laser for such a supercontinuum source typically consists of a mode-locked seed laser, a preamplifier and a power amplifier. To avoid damage, the power amplifier requires a special large mode area fiber. The cost of the large mode area fiber is high and its bending properties set a limit to the total volume of the pump laser. Supercontinuum radiation is typically produced commercially with pump wavelengths of 1030 nm or 1064 nm, and to extend the wavelength range to the UV, the nonlinear fiber needs to be specially configured (e.g., by tapering) or pumped with high pulse energy. This can shorten the fiber's lifetime. Also, in many cases only a small portion of the wavelength range is required, and most of the generated light is therefore discarded, making it very inefficient.

[0057]

[0056] To address one or more of these problems, a supercontinuum source is proposed, comprising a pump laser arrangement including a plurality of pump laser heads generating pump radiation and having a dimension of 5 cm or less (optionally smaller than 4 cm, 3 cm, 2 cm or 1 cm) in any direction, a radiation combiner for combining the respective pump radiation from each pump laser head, and a nonlinear fiber for receiving the pump radiation and exciting a working medium in the nonlinear fiber to generate supercontinuum radiation. As an alternative or in addition to each pump laser head having a dimension of 5 cm or less (optionally smaller than 4 cm, 3 cm, 2 cm or 1 cm) in any direction, the supercontinuum source may comprise a control arrangement for the pump laser arrangement configured for non-simultaneous emission of pulses from each pump laser head.

[0058]

[0057] The peak power of each laser head may be greater than 1 kW, greater than 5 kW, greater than 10 kW, greater than 15 kW, or greater than 20 kW.

[0059]

[0058] In one embodiment, each pump laser head can include a microchip laser. Microchip laser is an established technology based on a very thin (e.g. solid) gain medium in a resonator cavity, which is pumped by a pump laser. The pump laser can include a diode laser, but other suitable miniaturized laser technologies are also possible, such as VCSEL (Vertical Cavity Surface Emitting Laser) or VECSEL (Vertical External Cavity Surface Emitting Laser). Alternatively, a VCSEL or VECSEL can be used as a pump laser head without an additional cavity.

[0060]

[0059] The pump laser configuration may have, for example, more than 3, more than 4, more than 5, more than 6, more than 10, more than 15, more than 20, or more than 30 pump laser heads.

[0061]

[0060] Fig. 7(a) is a schematic diagram of a supercontinuum source according to an embodiment. The supercontinuum source comprises a number of pump laser heads controlled by a control unit CON. In this specific embodiment, each pump laser head is a microchip laser ML1 to ML6 (the number of microchip lasers can of course differ from the six shown here), although other laser technologies can be used for the pump laser heads. The outputs of the microchip lasers ML1 to ML6 are combined with a coupling optics, more specifically, in this example transported by respective fibers to a fiber-based beam combiner BC. The combined output is then used to pump a working medium in a nonlinear fiber NLF (e.g., a hollow-core or solid-core PCF or any other nonlinear medium), generating a broadband spectrum. For example, the combined output can be focused on the input facet of the nonlinear fiber NLF. In this way, the radiation source can be operated at a relatively high repetition rate and each pulse of the microchip lasers ML1 to ML6 can be converted into a pulse of supercontinuum radiation.

[0062]

[0061] The process and fiber used to generate broadband light should be such that short pulses of relatively low pulse energy can be combined and sent into the fiber to generate supercontinuum radiation. Each of the microchip lasers ML1 to ML6 has a respective pump pulse P ML1 -P ML6are fired non-simultaneously (e.g., sequentially or staggered). This is shown in the figure by the position of the pulses with respect to the time axis t (showing pre-combination by the beam combiner BC and post-combination by the beam combiner BC). There may (e.g., at least usually) be no overlap in time between the pulses from the different microchip lasers. However, for simplicity of design, the microchip laser firings may be relatively loosely controlled (the exact timing is not critical), and thus some pulse overlap is possible. However, the control of each microchip laser is such that pulses are fired non-simultaneously from each microchip laser, e.g., without synchronization between the laser firings. Thus, the pulse repetition rate of each microchip laser (or more generally the pump laser head) may be less than 5 MHz or less than 1 MHz, while the pulse repetition rate of the combined output of the microchip lasers ML1 to ML6 (i.e., of the entire pump laser arrangement) may be greater than 1 MHz or 10 MHz.

[0063]

[0062] The total output power and / or repetition rate of the pump laser configuration can be controlled or scaled by adding / removing microchip lasers ML1 to ML6 to the multiple microchip lasers or microchip laser array, or by activating / switching on / off individual microchip lasers ML1 to ML6 of the multiple microchip lasers / microchip laser array.

[0064]

[0063] The different microchip lasers ML1-ML6 may all output radiation at the same wavelength, or one or more of the different microchip lasers ML1-ML6 may emit radiation at different wavelengths. Using multiple wavelengths for pump radiation can improve the overall efficiency of supercontinuum generation and / or generate different output spectra by integrating over time and / or by allowing the selection of various output combinations (e.g., via individual control / activation of the microchip lasers ML1-ML6 as just described).

[0065]

[0064] The combining optics can be different from the fiber-based beam combiner BC shown in this figure. Figures 7(b) and 7(c) show two alternative free-space optical concepts for combining the outputs of the microchip lasers ML1-ML6. The configuration in Figure 7(b) includes directing elements or mirrors / prisms DE to direct the respective input beams IN1-IN4 from each microchip laser into a single beam, as shown. These directing elements DE are placed slightly offset (or progressively smaller) in the direction perpendicular to the beam propagation direction, so that the directing elements do not disturb the previously combined input beams. Such an approach is suitable for input beams IN1-IN4 wavelengths that are all the same or at least one different.

[0066]

[0065] Figure 7(c) includes multiple different dichroic mirrors DM1-DM3, each of which transmits the wavelengths of the beams (e.g., the wavelengths of the previously combined beam(s)) on its input face but reflects the wavelengths of its respective input beam. The first input beam IN1 can be guided by a guide element DE or a conventional mirror / prism. As an example, the dichroic mirror DM1 may transmit the wavelengths of the input beam IN1 and reflect the wavelengths of the input beam IN2. Similarly, the dichroic mirror DM1 may transmit the wavelengths of the combined input beams IN1, IN2 and reflect the wavelengths of the input beam IN3.

[0067]

[0066] Figure 8 is a more detailed schematic diagram of a single pump laser head, more specifically a single microchip laser or microchip laser head. The microchip laser includes a pump laser PL, such as a laser diode or VCSEL, which outputs pump radiation PR to a laser cavity CAV to generate output pump radiation OPR for combining with the output from other microchip lasers. The laser cavity CAV can be defined by a pair of dielectric mirrors DEM separated by a (e.g., solid-state) gain medium GM.

[0068]

[0067] Fig. 9 shows an example of a pump laser arrangement for further improving integration and space efficiency. In this embodiment, the pump laser heads are integrated into an integrated array (e.g., 1D or 2D array). In the illustrated embodiment, an array PLA of pump lasers PL (e.g., laser diodes or VCSEL / VECSEL) is provided, for example arranged or integrated on a substrate (laser diodes or VCSEL / VECSEL can be integrated at wafer level). The gain medium and cavity CAV may include a common gain medium and cavity CAV for all pump lasers PL of the array PLA (as shown here). Alternatively, this embodiment may include a separate but integrated gain medium and cavity CAV for each microchip seed laser MSL or group thereof. The pump radiation from each pump laser PL can then be collected by a respective lens LN (e.g., microlens) of a lens array LNA (e.g., fiber-coupled) or microlens array for combination by beam combining optics (not shown).

[0069]

[0068] The concept disclosed herein allows for a low-cost and very compact design compared to current pump laser configurations. Moreover, each laser head beam can be selected and controlled individually, thus providing flexibility for different applications with a common design. For example, if more power is required, all pump laser heads can be set to output at the same wavelength. Because there are multiple lasers, the effective repetition rate is also a multiple of the repetition rate of the individual lasers, which helps to reduce the integrated noise. For different applications, different laser heads can be configured to have different wavelengths if the required wavelength range is different.

[0070]

[0069] In some implementations, as further discussed below with reference to FIG. 11, the method and apparatus for spreading the input radiation can use a fiber for confining the input radiation and spreading the input radiation into an output broadband radiation. Thus, the fiber can be a nonlinear fiber receiving pump radiation to excite a working medium in the nonlinear fiber to generate broadband or supercontinuum radiation. The fiber can be a hollow core fiber and can include an internal structure to achieve effective guiding and confinement of radiation in the fiber. The fiber can be a hollow core photonic crystal fiber (HC-PCF), which is particularly suitable for strong radiation confinement mainly inside the hollow core of the fiber and can achieve high radiation intensity. The hollow core of the fiber can be filled with a gas that acts as a spreading medium to spread the input radiation. Such a fiber and gas configuration can be used to create a supercontinuum radiation source. The radiation input to the fiber can be electromagnetic radiation, for example, radiation in one or more of the infrared, visible, UV and extreme UV spectrum. The output radiation may consist of or include broadband radiation, which may be referred to herein as white light.

[0071]

[0070] Some embodiments relate to a new design of such a broadband radiation source that includes an optical fiber. The optical fiber is a hollow core photonic crystal fiber (HC-PCF). In particular, the optical fiber can be a type of hollow core photonic crystal fiber that includes an anti-resonant structure for confinement of radiation. Such fibers that include an anti-resonant structure are known in the art as anti-resonant fibers, tubular fibers, single ring fibers, negative curvature fibers, or suppressive coupling fibers. Various different designs of such fibers are known in the art. Alternatively, the optical fiber can be a photonic band gap fiber (HC-PBF, e.g., Kagome fiber).

[0072]

[0071] Several types of HC-PCFs can be engineered, each based on a different physical guidance mechanism. Two such HC-PCFs include hollow-core photonic bandgap fiber (HC-PBF) and hollow-core anti-resonant reflecting fiber (HC-ARF). Details regarding the design and fabrication of HC-PCFs can be obtained from U.S. Patent No. 2004 / 015085A1 (for HC-PBF) and International PCT Patent Application Publication No. WO 2017 / 032454A1 (for hollow-core anti-resonant reflecting fiber), which are incorporated herein by reference. Figure 12(a) shows a Kagome fiber including a Kagome lattice structure.

[0073]

[0072] An example of an optical fiber for use in a radiation source will now be described with reference to Fig. 10. Fig. 10 is a schematic cross-section of an optical fiber OF in a transverse section. Further embodiments similar to the application of the fiber of Fig. 10 are disclosed in WO 2017 / 032454 A1.

[0074]

[0073] The optical fiber OF includes an elongated body, with one dimension of the fiber OF being longer than the other two dimensions. This longer dimension may be referred to as the axial direction and may define the axis of the optical fiber OF. The other two dimensions define a plane that may be referred to as the transverse plane. Figure 10 shows a cross-section of the optical fiber OF in this transverse plane (i.e., perpendicular to the axis), which is labeled the xy plane. The transverse plane of the optical fiber OF may be substantially constant along the fiber axis.

[0075]

[0074] It will be understood that the optical fiber OF has a degree of flexibility, and thus the axial direction will generally not be uniform along the length of the optical fiber OF. Terms such as optical axis, cross section, and the like will be understood to mean local optical axis, local cross section, etc. Furthermore, where a component is described as being cylindrical or tubular, it will be understood that these terms encompass shapes that would be distorted if the optical fiber OF were bent.

[0076]

[0075] It will be understood that the optical fiber OF can have any length, and the length of the optical fiber OF can depend on the application. The optical fiber OF can have a length of 1 cm to 10 cm, for example, the optical fiber OF can have a length of 10 cm to 100 cm.

[0077]

[0076] The optical fiber OF includes a hollow core COR, a cladding portion surrounding the hollow core COR, and a support portion SP surrounding and supporting the cladding portion. The optical fiber OF can be considered to include a body (including a cladding portion and a support portion SP) having a hollow core COR. The cladding portion includes a plurality of anti-resonant elements for guiding radiation through the hollow core COR. Specifically, the plurality of anti-resonant elements are arranged to confine radiation propagating through the optical fiber OF mainly within the hollow core HC and guide the radiation along the optical fiber OF. The hollow core HC of the optical fiber OF can be substantially arranged in a central region of the optical fiber OF, whereby the axis of the optical fiber OF can also define the axis of the hollow core HC of the optical fiber OF.

[0078]

[0077] The cladding portion includes a plurality of anti-resonant elements for guiding radiation propagating through the optical fiber OF. Specifically, in this embodiment, the cladding portion includes a single ring of six tubular capillary CAPs. Each of the tubular capillary CAPs acts as an anti-resonant element.

[0079]

[0078] The capillary CAP may also be referred to as a tube. The capillary CAP may be circular in cross section or have another shape. Each capillary CAP includes a generally cylindrical wall portion WP, which at least partially defines the hollow core HC of the optical fiber OF and separates the hollow core HC from the capillary cavity CC. It will be appreciated that the wall portion WP may act as an anti-reflection Fabry-Perot resonator for radiation propagating through the hollow core HC (which may be incident on the wall portion WP at an oblique incidence angle). The thickness of the wall portion WP may be suitable to ensure that transmission into the capillary cavity CC is generally suppressed, while reflection back into the hollow core HC is generally enhanced. In some embodiments, the capillary wall portion WP may have a thickness of 0.01 to 10.0 μm.

[0080]

[0079] It will be understood that the term cladding portion, as used herein, is intended to mean a portion of the optical fiber OF for guiding radiation propagated through the optical fiber OF (i.e., the capillary CAP that confines the radiation within the hollow core COR). The radiation can be confined in the form of a transverse mode (propagating along the fiber axis).

[0081]

[0080] The support portion is generally tubular and supports six capillary CAPs of the cladding portion. The six capillary CAPs are evenly distributed around the inner surface in the case of the inner support portion SP. The six capillary CAPs may be described as being arranged in a general hexagonal configuration.

[0082]

[0081] The capillary CAPs are arranged such that each capillary does not contact any other capillary CAP. Each capillary CAP contacts the inner support portion SP and is separated from adjacent capillary CAPs in a ring structure. Such a configuration may be beneficial because it may increase the transmission bandwidth of the optical fiber OF (e.g., compared to a configuration in which the capillaries are in contact with each other). Alternatively, in some embodiments, each capillary CAP may contact adjacent capillary CAPs in a ring structure.

[0083]

[0082] The six capillaries CAP of the cladding section are arranged in a ring structure around the hollow core COR. The inner surface of the ring structure of the capillary CAPs at least partially defines the hollow core HC of the optical fiber OF. The diameter d of the hollow core HC (which may be defined as the smallest dimension between the opposing capillaries and is indicated by the arrow d) may be 10 to 1000 μm. The diameter d of the hollow core HC may affect the mode field diameter, collision loss, dispersion, mode plurality and nonlinearity characteristics of the hollow core optical fiber OF.

[0084]

[0083] In this embodiment, the cladding portion includes a single ring arrangement of capillary CAPs (acting as an anti-resonant element), so that any radial line from the center of the hollow core HC to the outside of the optical fiber OF passes through only one capillary CAP.

[0085] It will be appreciated that other embodiments can be provided with different configurations of anti-resonant elements. These can include configurations with multiple rings of anti-resonant elements, or configurations with nested anti-resonant elements. Furthermore, while the embodiment shown in FIG. 10 includes six rings of capillaries, in other embodiments the cladding portion can be provided with one or more rings including any number of anti-resonant elements (e.g., 4, 5, 6, 7, 8, 9, 10, 11 or 12 capillaries).

[0086]

[0085] Figure 12(b) shows a modified embodiment of the HC-PCF discussed above with a single ring of tubular capillary. In the example of Figure 12(b), there are two concentric rings of tubular capillary 21. The HC-PCF may be provided with a support tube ST to hold the inner and outer rings of tubular capillary 21. The support tube may be made of silica.

[0087]

[0086] The tubular capillary of the example of Figure 10 and Figures 12(a) and (b) may have a circular cross-sectional shape. Other shapes such as elliptical or polygonal cross-sections are also possible for the tubular capillary. Furthermore, the solid material of the tubular capillary of the example of Figure 10 and Figures 12(a) and (b) may include a plastic material such as PMA, a glass such as silica, or a soft glass.

[0088]

[0087] Figure 11 shows a radiation source RDS for providing broadband output radiation. The radiation source RDS includes a pulsed pump radiation source PRS including any pump laser configuration disclosed and / or described herein, an optical fiber OF (e.g. of the type shown in Figure 10) having a hollow core COR, and a working medium WM (e.g. a gas) disposed within the hollow core COR. In Figure 11, the radiation source RDS includes the optical fiber OF shown in Figure 10, although in alternative embodiments other types of hollow core optical fibers OF can be used.

[0089]

[0088] The pulsed pump radiation source PRS is configured to provide an input radiation IRD. The hollow core HC of the optical fiber OF is arranged to receive the input radiation IRD from the pulsed pump radiation source PRS and to spread the input radiation IRD to provide an output radiation ORD. The working medium WM enables spreading a frequency range of the received input radiation IRD to provide a broadband output radiation ORD.

[0090]

[0089] The radiation source RDS further includes a reservoir RSV. The optical fiber OF is disposed within the reservoir RSV. The reservoir RSV may also be referred to as a housing, a container, or a gas cell. The reservoir RSV is configured to contain a working medium WM. The reservoir RSV may include one or more features known in the art for controlling, regulating, and / or monitoring the composition of the working medium WM (which may be a gas) within the reservoir RSV. The reservoir RSV may include a first transparent window TW1. In use, the optical fiber OF is disposed within the reservoir RSV such that the first transparent window TW1 is located immediately adjacent to the input end IE of the optical fiber OF. The first transparent window TW1 may form a part of the wall of the reservoir RSV. The first transparent window TW1 may be transparent to at least the received input radiation frequency, thereby allowing the received input radiation IRD (or at least a majority thereof) to be coupled to the optical fiber OF located within the reservoir RSV. It will be appreciated that optics (not shown) can be provided for coupling the input radiation IRD into the optical fiber OF.

[0091]

[0090] The reservoir RSV includes a second transparent window TW2 forming part of the wall of the reservoir RSV. In use, when the optical fiber OF is placed in the reservoir RSV, the second transparent window TW2 is located in close proximity to the output end OE of the optical fiber OF. The second transparent window TW2 may be transparent at least to the frequency of the broadband output radiation ORD of the device 120.

[0092]

[0091] Alternatively, in another embodiment, both ends of the optical fiber OF can be placed in different reservoirs. The optical fiber OF can include a first end section configured to receive the input radiation IRD and a second end section for outputting the broadband output radiation ORD. The first end section can be placed in a first reservoir containing the working medium WM. The second end section can be placed in a second reservoir, which can also contain the working medium WM. The function of the reservoirs can be as described with respect to FIG. 11 above. The first reservoir can include a first transparent window configured to be transparent to the input radiation IRD. The second reservoir can include a second transparent window configured to be transparent to the broadband output broadband radiation ORD. The first and second reservoirs can also include sealable openings to allow the optical fiber OF to be placed partially inside and partially outside the reservoirs, thereby sealing the gas in the reservoirs. The optical fiber OF may further include an intermediate section that is not contained within the reservoir. Such a configuration using two separate gas reservoirs may be particularly convenient for embodiments in which the optical fiber OF is relatively long (e.g., when the length is greater than 1 m). In such a configuration using two separate gas reservoirs, it will be understood that the two reservoirs (which may include one or more features known in the art for controlling, regulating and / or monitoring the composition of the gas in the two reservoirs) are considered to provide a device for providing the working medium WM within the hollow core HC of the optical fiber OF.

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

[0094]

[0093] Both the first transparent window TW1 and the second transparent window TW2 can form an airtight seal in the wall of the reservoir RSV, thereby allowing the working medium WM (which can be a gas) to be contained within the reservoir RSV. It will be appreciated that the gas WM can be contained within the reservoir RSV at a pressure different from the ambient pressure of the reservoir RSV.

[0095]

[0094] The working medium WM may include noble gases such as argon, krypton and xenon, Raman active gases such as hydrogen, deuterium and nitrogen, or gas mixtures such as argon / hydrogen mixtures, xenon / deuterium mixtures, krypton / nitrogen mixtures or nitrogen / hydrogen mixtures. Depending on the type of filling gas, nonlinear optical processes may include modulation instability (MI), soliton self-compression, soliton splitting, Kerr effect, Raman effect and dispersive wave generation, the details of which are described in WO 2018 / 127266 A1 and US 9160137 B1 (both of which are incorporated herein by reference). The dispersion of the filling gas can be adjusted by varying the pressure of the working medium WM in the reservoir RSR (i.e., the gas cell pressure), so that the resulting broadband pulse dynamics and associated spectral broadening characteristics can be adjusted to optimize frequency conversion.

[0096]

[0095] In one implementation, the working medium WM may be located within the hollow core HC at least while the optical fiber OF receives the input radiation IRD for generating the broadband output radiation ORD. It will be appreciated that the gas WM may be totally or partially absent within the hollow core HC while the optical fiber OF does not receive the input radiation IRD for generating the broadband output radiation.

[0097]

[0096] To achieve frequency spreading, high intensity radiation may be desired. The advantage of having a hollow core optical fiber OF is that it allows high intensity radiation to be achieved through strong spatial confinement of the radiation propagating through the optical fiber OF, and high local radiation intensity can be achieved. The radiation intensity in the optical fiber OF can be high, for example, due to the high input radiation intensity received and / or the strong spatial confinement of the radiation in the optical fiber OF. The advantage of hollow core optical fibers is that they can guide radiation with a wider wavelength range, such that solid core fibers, and specifically hollow core optical fibers, can guide radiation in both the ultraviolet and infrared ranges.

[0098]

[0097] The advantage of using a hollow core optical fiber OF may be that most of the radiation guided in the optical fiber OF is confined to the hollow core COR. Therefore, most of the interaction of the radiation in the optical fiber OF occurs with the working medium WM provided in the hollow core HC of the optical fiber OF. As a result, the spreading effect of the radiation by the working medium WM can be enhanced.

[0099]

[0098] The received input radiation IRD may be electromagnetic radiation. The input radiation IRD may be received as pulsed radiation. For example, the input radiation IRD may comprise an ultrafast pulse, for example generated by a laser.

[0100]

[0099] The input radiation IRD may be coherent radiation. The input radiation IRD may be collimated radiation, the advantage of which may be to facilitate and improve the efficiency of coupling the input radiation IRD into the optical fiber OF. The input radiation IRD may include a single frequency, or a narrow range of frequencies. The input radiation IRD may be generated by a laser. Similarly, the output radiation ORD may be collimated and / or coherent.

[0101] [000100] The broadband range of the output radiation ORD may be a continuous range including a continuous range of radiation frequencies. The output radiation ORD may include supercontinuum radiation. The continuous radiation may be beneficial for use in many applications, such as metrology applications. For example, a continuous range of frequencies may be used to probe a large number of properties. A continuous range of frequencies may be used, for example, to determine and / or remove frequency dependence of a measured property. The supercontinuum output radiation ORD may include electromagnetic radiation over a wavelength range of, for example, 100 nm to 4000 nm. The broadband output radiation ORD frequency range may be, for example, 400 nm to 900 nm, 500 nm to 900 nm, or 200 nm to 2000 nm. The supercontinuum output radiation ORD may include white light.

[0102] [000101] The input radiation IRD provided by the pulsed pump radiation source PRS may be pulsed. The input radiation IRD may comprise electromagnetic radiation at one or more frequencies between 200 nm and 2 μm. The input radiation IRD may comprise electromagnetic radiation having a wavelength of, for example, 1.03 μm. The repetition rate of the pulsed radiation IRD may be on the order of magnitude of 1 kHz to 100 MHz. The pulse energy may have an order of magnitude of 0.1 μJ to 100 μJ, for example on the order of magnitude of 1 to 10 μJ. The pulse duration for the input radiation IRD may be between 10 fs and 10 ps, ​​for example 300 fs. The average power of the input radiation IRD may be between 100 mW and several hundreds of W. The average power of the input radiation IRD may be, for example, 20 to 50 W.

[0103] [000102] The pulsed pump radiation source PRS may be a laser. The spatiotemporal transmission characteristics of such laser pulses transmitted along the optical fiber OF, such as their spectral amplitude and phase, can be varied and adjusted through adjustment of (pump) laser parameters, working component WM variations, and optical fiber OF parameters. The above-mentioned spatiotemporal transmission characteristics can include one or more of the following: output power, output mode profile, output temporal profile, width of the output temporal profile (or output pulse width), output spectral profile, and bandwidth of the output spectral profile (or output spectral bandwidth). The above-mentioned pulsed pump radiation source PRS parameters can include one or more of the following: pump wavelength, pump pulse energy, pump pulse width, pump pulse repetition rate, and can be changed by changing any one or more parameters of the pump laser head. The above-mentioned optical fiber OF parameters can include one or more of the following: length of the optical fiber, size and shape of the hollow core 101, size and shape of the capillary, and thickness of the wall of the capillary surrounding the hollow core. For the above mentioned operating components WM, e.g., fill gas, the parameters may include one or more of gas type, gas pressure, and gas temperature.

[0104] [000103] The broadband output radiation ORD provided by the radiation source RDS may have an average output power of at least 1 W. The average output power may be at least 5 W. The average output power may be at least 10 W. The broadband output radiation ORD may be a pulsed broadband output radiation ORD. The broadband output radiation ORD may have a power spectral density within the full wavelength band of the output radiation of at least 0.01 mW / nm. The power spectral density within the full wavelength band of the broadband output radiation may be at least 3 mW / nm.

[0105] [000104] Further embodiments are disclosed in the following numbered clause listing: 1. A supercontinuum radiation source comprising: a pump laser arrangement for generating pump radiation and comprising a plurality of pump laser heads, each pump laser head having a dimension in any direction of 5 cm or less; a radiation combiner for combining respective pump radiation from each pump laser head; a nonlinear fiber for receiving pump radiation to excite a working medium within the nonlinear fiber to generate supercontinuum radiation; A supercontinuum radiation source comprising: 2. A supercontinuum radiation source as described in clause 1, comprising a control arrangement for controlling the pump laser heads, the control arrangement being configured for non-simultaneous emission of pulses from each pump laser head. 3. A supercontinuum radiation source as described in clause 2, wherein the controller is operable to selectively activate individual pump laser heads. 4. A supercontinuum radiation source as described in any of clauses 1 to 3, wherein each pump laser head has a dimension of 1 cm or less in any direction. 5. A supercontinuum radiation source according to any one of clauses 1 to 4, wherein the peak power of each pump laser head is greater than 1 kW. 6. A supercontinuum radiation source according to any one of clauses 1 to 5, wherein the peak power of each pump laser head is greater than 10 kW. 7. Each pump laser head includes a microchip laser, the microchip laser being A pump laser; A laser cavity; and a gain medium within the laser cavity. 8. The supercontinuum radiation source of clause 7, wherein each pump laser comprises one of a diode laser, a vertical cavity surface emitting laser, or a vertical external cavity surface emitting laser. 9. A supercontinuum radiation source according to clause 7 or 8, wherein the pump laser heads are integrated into an integrated array. 10. A supercontinuum radiation source as claimed in clause 9, comprising a common gain medium and / or a common laser cavity for some or all of the pump lasers. 11. A supercontinuum radiation source according to clause 9 or 10, wherein the pump lasers are integrated on a common substrate. 12. A supercontinuum radiation source according to any of clauses 1 to 11, comprising a lens array having a lens for collecting pump radiation from each of the plurality of pump laser heads. 13. A supercontinuum radiation source according to any of clauses 1 to 12, wherein the pump laser arrangement comprises more than three pump laser heads. 14. A supercontinuum radiation source according to any of clauses 1 to 13, wherein the pump laser configuration comprises more than five pump laser heads. 15. A supercontinuum radiation source according to any of clauses 1 to 14, wherein the pump laser configuration comprises more than 10 pump laser heads. 16. A supercontinuum radiation source according to any of clauses 1 to 15, wherein the pump laser configuration outputs a train of laser pulses having a pulse repetition rate of more than 10 MHz. 17. A supercontinuum radiation source according to any one of clauses 1 to 16, wherein the pulse repetition rate of each pump laser head is less than 5 MHz. 18. A supercontinuum radiation source according to any one of clauses 1 to 17, wherein the pulse repetition rate of each pump laser head is less than 1 MHz. 19. A supercontinuum radiation source as described in any of clauses 1 to 18, wherein each pump laser head outputs radiation having the same wavelength. 20. A supercontinuum radiation source according to any of clauses 1 to 18, wherein the multiple pump laser heads include at least two pump laser heads that output radiation having different wavelengths. 21. A supercontinuum radiation source according to any one of clauses 1 to 20, wherein the nonlinear fibre is a hollow-core or solid-core photonic crystal fibre. 22. A supercontinuum radiation source according to any of clauses 1 to 21, comprising a focusing arrangement for focusing the combined radiation from the radiation combiner onto an input facet of the nonlinear fibre. 23. A supercontinuum radiation source comprising: a pump laser arrangement for generating pump radiation and comprising a plurality of pump laser heads; a radiation combiner for combining respective pump radiation from each pump laser head; a nonlinear fiber for receiving pump radiation to excite a working medium within the nonlinear fiber to generate supercontinuum radiation; a control arrangement for controlling the pump laser arrangement, the control arrangement being configured for non-simultaneous emission of pulses from each pump laser head; A supercontinuum radiation source comprising: 24. A supercontinuum radiation source as described in clause 23, wherein the controller is operable to selectively activate individual pump laser heads. 25. A supercontinuum radiation source as described in clause 23 or 24, wherein each pump laser head has a dimension in any direction of 1 cm or less. 26. A supercontinuum radiation source according to any one of clauses 23 to 25, wherein the peak power of each pump laser head is greater than 1 kW. 27. A supercontinuum radiation source according to any of clauses 23 to 26, wherein the peak power of each pump laser head is greater than 10 kW. 28. Each pump laser head includes a microchip laser, the microchip laser comprising: A pump laser; A laser cavity; 28. A supercontinuum radiation source according to any of clauses 23 to 27, comprising a gain medium within the laser cavity. 29. The supercontinuum radiation source of clause 28, wherein each pump laser comprises one of a diode laser, a vertical cavity surface emitting laser, or a vertical external cavity surface emitting laser. 30. A supercontinuum radiation source according to clause 28 or 29, wherein the pump laser heads are integrated into an integrated array. 31. A supercontinuum radiation source as claimed in clause 30, comprising a common gain medium and / or a common laser cavity for some or all of the pump lasers. 32. A supercontinuum radiation source according to clause 30 or 31, wherein the pump lasers are integrated on a common substrate. 33. A supercontinuum radiation source according to any of clauses 23 to 32, comprising a lens array having a lens for collecting pump radiation from each of the plurality of pump laser heads. 34. A supercontinuum radiation source according to any of clauses 23 to 33, wherein the pump laser configuration includes more than three pump laser heads. 35. A supercontinuum radiation source according to any of clauses 23 to 34, wherein the pump laser arrangement comprises more than five pump laser heads. 36. A supercontinuum radiation source according to any of clauses 23 to 35, wherein the pump laser configuration includes more than 10 pump laser heads. 37. A supercontinuum radiation source according to any of clauses 23 to 36, wherein the pump laser configuration outputs a train of laser pulses having a pulse repetition rate of more than 10 MHz. 38. A supercontinuum radiation source according to any of clauses 23 to 37, wherein the pulse repetition rate of each pump laser head is less than 5 MHz. 39. A supercontinuum radiation source according to any of clauses 23 to 38, wherein the pulse repetition rate of each pump laser head is less than 1 MHz. 40. A supercontinuum radiation source according to any of clauses 23 to 39, wherein each pump laser head outputs radiation having the same wavelength. 41. A supercontinuum radiation source according to any of clauses 23 to 39, wherein the multiple pump laser heads include at least two pump laser heads that output radiation having different wavelengths. 42. A supercontinuum radiation source according to any one of clauses 23 to 41, wherein the nonlinear fibre is a hollow-core or solid-core photonic crystal fibre. 43. A supercontinuum radiation source according to any of clauses 23 to 42, comprising a focusing arrangement for focusing the combined radiation from the radiation combiner onto an input facet of the nonlinear fibre. 44. A substrate support for supporting a substrate; A supercontinuum radiation source according to any one of clauses 1 to 43, an optical system operable to direct the supercontinuum radiation from the supercontinuum radiation source to the substrate; A measurement device comprising: 45. A metrology device as described in clause 44, wherein the metrology device is operable as a scatterometer metrology tool. 46. ​​A measuring device as described in clause 44, wherein the measuring device is operable as a level sensor or an alignment sensor. 47. A lithographic apparatus comprising at least one metrology device according to clause 46 for performing alignment and / or levelling metrology. 48. A lithographic cell comprising a lithographic apparatus according to clause 47 and a metrology device according to clause 45.

[0106] [000105] Although specific reference is made in this specification to the use of the lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0107] [000106] Although embodiments of the invention may be specifically referenced herein in connection with lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus or any apparatus that measures or processes objects such as wafers (or other substrates) or masks (or other patterning devices). Collectively, these apparatus may be referred to as lithography tools. Such lithography tools may use vacuum or ambient (non-vacuum) conditions.

[0108] [000107] Although specific reference has been made above to the use of embodiments of the invention in connection with optical lithography, it will be appreciated that the invention, where the context permits, is not limited to optical lithography and may be used in other applications, for example in imprint lithography.

[0109] [000108] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The above description is intended to be illustrative rather than limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

1. 1. A supercontinuum radiation source, comprising: a pump laser arrangement for generating pump radiation and comprising a plurality of pump laser heads, each pump laser head having a dimension in any direction of 5 cm or less; a radiation combiner for combining the respective pump radiation from each pump laser head; a nonlinear fiber for receiving said pump radiation so as to excite a working medium within said nonlinear fiber to generate supercontinuum radiation; A supercontinuum radiation source comprising:

2. 2. A supercontinuum radiation source according to claim 1, comprising a control arrangement for controlling the pump laser arrangement, the control arrangement being configured for non-simultaneous emission of pulses from each pump laser head.

3. 3. A supercontinuum radiation source as claimed in claim 2, wherein the controller is operable to selectively activate individual pump laser heads.

4. A supercontinuum radiation source according to any of claims 1 to 3, wherein the peak power of each pump laser head is greater than 1 kW.

5. Each pump laser head includes a microchip laser, the microchip laser having: A pump laser; A laser cavity; A supercontinuum radiation source according to any preceding claim, comprising a gain medium within the laser cavity.

6. 6. The supercontinuum radiation source of claim 5, wherein each pump laser comprises one of a diode laser, a vertical cavity surface emitting laser, or a vertical external cavity surface emitting laser.

7. 7. A supercontinuum radiation source according to claim 5 or 6, wherein the pump laser heads are integrated into an integrated array.

8. 8. A supercontinuum radiation source according to claim 7, comprising a common gain medium and / or a common laser cavity for some or all of the pump lasers.

9. 9. A supercontinuum radiation source according to claim 7 or 8, wherein the pump lasers are integrated on a common substrate.

10. A supercontinuum radiation source according to any preceding claim, comprising a lens array having a lens for collecting pump radiation from each pump laser head of the plurality of pump laser heads.

11. A supercontinuum radiation source according to any of the preceding claims, wherein the pump laser arrangement comprises more than three pump laser heads.

12. A supercontinuum radiation source according to any of the preceding claims, wherein the pump laser arrangement outputs a train of laser pulses having a pulse repetition rate above 10 MHz.

13. A supercontinuum radiation source according to any preceding claim, wherein the pulse repetition rate of each pump laser head is less than 5 MHz.

14. 1. A supercontinuum radiation source, comprising: a pump laser arrangement for generating pump radiation and comprising a plurality of pump laser heads; a radiation combiner for combining the respective pump radiation from each pump laser head; a nonlinear fiber for receiving the pump radiation to excite a working medium within the nonlinear fiber to generate supercontinuum radiation; a control arrangement for controlling the pump laser arrangement, the control arrangement being configured for non-simultaneous emission of pulses from each pump laser head; A supercontinuum radiation source comprising:

15. A substrate support for supporting a substrate; A supercontinuum radiation source according to any one of claims 1 to 14, an optical system operable to direct supercontinuum radiation from the supercontinuum radiation source to the substrate; A measurement device comprising:

Images