Radioactive Sources

Abstract

To provide a device and a method for use in a device for widening a frequency range of input radiation.SOLUTION: A radiation source 134 comprises: a hollow core optical fiber 102; a working medium 126; and a pulsed pump radiation source 136. The hollow core optical fiber comprises a body that has a hollow core. The working medium is disposed within the hollow core. The pulsed pump radiation source is arranged to produce pulsed pump radiation that is received by, and propagates through, the hollow core from an input end to an output end. Parameters of the pulsed pump radiation, the optical fiber and the working medium are configured to allow soliton self-compression of the pulsed pump radiation so as to change a spectrum of the pulsed pump radiation so as to form output radiation. In some embodiments, a length of the optical fiber is such that the output end substantially coincides with a position at which a temporal extent of the pulsed pump radiation is minimal.SELECTED DRAWING: Figure 8

Description

[Technical field]

[0001] [CROSS REFERENCE TO RELATED APPLICATIONS] This application claims priority to European Application No. 19188036.8, filed July 24, 2019, European Application No. 19198105.9, filed September 18, 2019, and European Application No. 20151889.1, filed January 15, 2020, which are incorporated herein by reference in their entireties.

[0002] [Technical field] The present invention relates to a radiation source. The radiation source may be a supercontinuum source and may include an apparatus for receiving input radiation and for broadening the frequency range of the input radiation to provide (broadband) output radiation. [Background technology]

[0003] A lithographic apparatus is a machine built 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 (also called a "design layout" or "design") in a patterning device (e.g., mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., wafer).

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

[0005] low k 1Lithography can be used to process features with dimensions smaller than the classical resolution limit of the lithography tool. In such processes, the equation for resolution is CD=k 1 × λ / NA, where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection optics of the lithography tool, CD is the "critical dimension" (usually the smallest feature size to be printed, in this case the half pitch), and k 1 is an empirical resolution factor. In general, k 1 The smaller the k, the more difficult it becomes to reproduce a pattern on the substrate that resembles the shape and dimensions planned by the circuit designer to achieve a particular electrical function and performance. To overcome these difficulties, advanced fine-tuning steps can be applied to the lithographic projection equipment and / or the design layout. These include, for example, but are not limited to, optimization of the NA, customized illumination schemes, use of phase-shifting patterning devices, various optimizations of the design layout such as optical proximity correction (OPC), also known as "optical and process correction", in the design layout, or other methods commonly defined as "resolution enhancement techniques" (RET). Alternatively, tight control loops to control the stability of the lithographic equipment can be used to achieve low k 1 This can improve the reproduction of patterns in the

[0006] In the field of lithography, many measurement systems can be used, both within the lithographic apparatus and external to the lithographic apparatus. In general, such measurement systems can use a radiation source to irradiate a target with radiation, and a detection system is operable to measure at least one property of a portion of the incident radiation that scatters from the target. An example of a measurement system that is external to the lithographic apparatus is an inspection or metrology apparatus, which can be used to determine properties of a pattern previously projected onto a substrate by the lithographic apparatus. Such an external inspection apparatus can comprise, for example, a scatterometer. Examples of measurement systems that can be provided within the lithographic apparatus include topography measurement systems (also known as level sensors), position measurement systems (e.g. interferometer apparatus) for determining the position of a reticle or wafer stage, and alignment sensors for determining the position of alignment marks. These measurement apparatus can perform measurements using electromagnetic radiation.

[0007] Different types of radiation can be used to characterize different types of patterns. Some measurement systems may use a broadband radiation source. Such a broadband radiation source may be a supercontinuum source and may include an optical fiber having a nonlinear medium through which a pulsed pump radiation beam is propagated to broaden the spectrum of the radiation. Summary of the Invention [Problem to be solved by the invention]

[0008] It is desirable to provide alternative apparatus and methods for use in an apparatus for receiving input radiation and broadening the frequency range of the input radiation to provide (broadband) output radiation that at least partially addresses one or more problems associated with the prior art, whether or not identified herein. [Means for solving the problem]

[0009] According to a first aspect of the present invention there is provided a radiation source comprising: a hollow core optical fibre including a body having a hollow core; a working medium disposed within the hollow core; and a pulsed pump radiation source arranged to generate pulsed pump radiation received by and propagating through the hollow core from an input end to an output end, wherein parameters of the pulsed pump radiation, the optical fibre and the working medium are configured to enable soliton self-compression of the pulsed pump radiation to modify a spectrum of the pulsed pump radiation to form an output radiation, and further wherein a length of the optical fibre is such that the output end substantially coincides with a position at which a temporal extent of the output radiation is minimum.

[0010] According to a second aspect of the present invention there is provided a radiation source comprising: a hollow core optical fibre including a body having a hollow core; a working medium disposed within the hollow core; and a pulsed pump radiation source arranged to generate pulsed pump radiation received by and propagating through the hollow core from an input end to an output end, wherein parameters of the pulsed pump radiation, the optical fibre and the working medium are configured to enable soliton self-compression of the pulsed pump radiation to modify a spectrum of the pulsed pump radiation to form an output radiation, and further wherein a length of the optical fibre is such that the output end substantially coincides with a position at which the spectral width of the output radiation is maximum.

[0011] Radiation sources according to the first and second aspects of the present invention are advantageous as they enable a broadband output radiation beam to be generated at the output end, which may be useful for use in a metrology apparatus, for example a lithographic apparatus.

[0012] Some known broadband radiation sources use configurations that generate a spectral broadening of the pulsed pump radiation, but the parameters of the pulsed pump radiation, the optical fiber, and the working medium are configured to allow modulation instability to generate the spectral broadening. There are several reasons why modulation instability is used to generate the spectral broadening. First, modulation instability is known to generate broadband radiation with a relatively flat intensity-wavelength distribution. Such broadband radiation sources are sometimes referred to as white light radiation sources (because of the relatively flat spectral intensity distribution). Second, modulation instability can be achieved using a relatively economical laser source as the pump radiation source.

[0013] Soliton self-compression, on the other hand, is a regime used to generate an output laser beam with a shifted wavelength from an input pump laser beam. For example, soliton self-compression is used to generate dispersive waves with different (shifted) wavelengths. In the soliton self-compression regime (where the number of solitons is relatively small), the pulse of radiation can undergo significant temporal compression, which is accompanied by spectral broadening. Eventually, the temporal compression reaches a maximum level (corresponding to the smallest time extent of the pulsed radiation), followed by temporal broadening of the radiation. This temporal broadening is called soliton splitting, because higher-order solitons split into multiple individual solitons. The (higher-order) solitons can oscillate between periods of temporal compression and temporal broadening as they propagate along the hollow-core optical fiber. Following the temporal broadening, other effects can lead to a shift in the spectrum of the radiation. For example, self-pulse steepening (which can accompany and assist soliton self-compression) can lead to optical shocks that can seed the dispersive-wave radiation. By tuning the parameters of the system, specific desired wavelengths can be generated. For example, the wavelength can be selected to be suitable for interacting with a particular molecule and used in research experiments to study said molecule. Soliton self-compression is thus a known regime for producing an output radiation beam having a second, shifted wavelength from an input pump laser beam having a first wavelength.

[0014] The inventors of the present invention have recognized that during soliton self-compression, there is a (short-lived) transition period during which the radiation propagating in the hollow-core fiber is broadband radiation, prior to the time broadening (and prior to the formation of a dispersive wave). Furthermore, the inventors have recognized that although this broadband radiation is short-lived, it can be output from the optical fiber to provide a particularly stable source of broadband radiation, by selecting the length of the optical fiber such that the output end substantially coincides with the location where the soliton self-compression occurred prior to the subsequent time broadening (soliton fission) and shift of the spectrum.

[0015] In particular, a particularly stable broadband radiation source can be provided if the length of the optical fiber is such that the output end substantially coincides with the location where the temporal extent of the pulsed pump radiation is minimum. As used herein, it will be understood that if the temporal extent of the pulsed pump radiation at the output end is minimum within the optical fiber, the output end substantially coincides with the location where the temporal extent of the pulsed pump radiation is minimum. That is, if the temporal extent of the pulsed pump radiation has not yet increased (due to soliton splitting), the output end substantially coincides with the location where the temporal extent of the pulsed pump radiation is minimum. It will be understood that for a sufficiently long hollow-core optical fiber, the soliton may oscillate between periods of temporal compression and temporal broadening as the radiation propagates through the hollow-core optical fiber. Between each period of temporal compression and temporal broadening, there may be a local minimum in the temporal extent of the pulsed pump radiation. If the length of the hollow-core optical fiber is such that the output end is located anywhere up to or including the first local minimum, the output end substantially coincides with the location where the temporal extent of the pulsed pump radiation is minimum. If the output end substantially coincides with the first local minimum, maximum spectral broadening may be achieved.

[0016] In general, after soliton self-compression, the spectral width of the pulsed pump radiation may decrease and / or spectral gaps may develop (e.g., when dispersive waves are emitted). Thus, a particularly stable broadband radiation source can be provided if the length of the optical fiber is such that the output end substantially coincides with the location where the spectral width of the pulsed pump radiation is maximum. As used herein, it will be understood that the spectral width of the pulsed pump radiation (sometimes referred to as the spectral bandwidth of the pulsed pump radiation) may be the width of the spectrum where the power density exceeds a threshold fraction of the maximum value. For example, the spectral width of the pulsed pump radiation may be the width of the spectrum where the power density exceeds 0.0001 of the maximum value (i.e., the spectrum spans 40 decibels). For example, the spectral width of the pulsed pump radiation may be the width of the spectrum where the power density exceeds 0.001 of the maximum value (i.e., the spectrum spans 30 decibels). For example, the spectral width of the pulsed pump radiation may be the width of the spectrum where the power density exceeds 0.01 of the maximum value (i.e., the spectrum spans 20 decibels). For example, the width of the spectrum of the pulsed pump radiation can be the width of the spectrum where the power density exceeds a maximum value of 0.1 (i.e., a spectrum spanning 10 decibels). Alternatively, if the length of the optical fiber is such that the output end is substantially coincident with a position where the spectrum of the output radiation is substantially continuous, a particularly stable broadband radiation source can be provided.

[0017] In contrast to chaotically driven modulational instability systems, the broadband radiation produced by such soliton self-compression will have no shot-to-shot variations. As a result, advantageously, a stable output spectrum can be produced using a single pulse (as opposed to several pulses that are required to produce a degree of stability in the output beam of a modulational instability system).

[0018] It will be appreciated that as radiation propagates through the hollow-core fiber, the solitons may oscillate between periods of time compression and time spreading. Between each period of time compression and time spreading, there may be a local minimum in the temporal extent of the pulsed pump radiation. In principle, the length of the optical fiber may be such that the output end substantially coincides with such a position of the minimum temporal extent of the pulsed pump radiation. However, the most stable output spectrum (e.g., with respect to pulse-to-pulse variations) may be provided when the output end coincides with a first position of the minimum temporal extent of the pulsed pump radiation.

[0019] The length of the optical fiber may be such that the output end substantially coincides with a first minimum of the time range of the pulsed pump radiation. The location of the first minimum of the time range of the pulsed pump radiation may depend on many factors, including, for example, parameters of the fiber (e.g., core diameter and fiber length), parameters of the working medium (e.g., type of gas and pressure), and parameters of the pulsed pump radiation (e.g., pulse energy and pulse duration). For the output end to substantially coincide with the first minimum of the time range of the pulsed pump radiation, the output end may be positioned sufficiently close to the first minimum of the time range of the pulsed pump radiation such that the compressed pulse has not yet been expanded or dispersed by 200% of the first minimum of the time range of the pulsed pump radiation. For example, the output end may be positioned sufficiently close to the first minimum of the time range of the pulsed pump radiation such that the compressed pulse has not yet been expanded or dispersed by 100% of the first minimum of the time range of the pulsed pump radiation. For example, the output end may be positioned sufficiently close to the first minimum of the time range of the pulsed pump radiation such that the compressed pulse has not yet been expanded or dispersed by 50% of the first minimum of the time range of the pulsed pump radiation. For example, the output end may be positioned close enough to a first minimum of the time range of the pulsed pump radiation such that the compressed pulse is not yet stretched or dispersed by 10% of the first minimum of the time range of the pulsed pump radiation. For example, the output end may be positioned close enough to a first minimum of the time range of the pulsed pump radiation such that the compressed pulse is not yet stretched or dispersed by 5% of the first minimum of the time range of the pulsed pump radiation.

[0020] The pulse duration of the input pulsed pump radiation may be greater than 50 fs, for example the pulse duration of the input pulsed pump radiation may be greater than 100 fs, for example of the order of 150 fs.

[0021] The pulse energy of the input pulsed pump radiation may be less than 1 μJ. For example, the pulse energy of the input pulsed pump radiation may be less than 0.75 μJ. For example, the pulse energy of the input pulsed pump radiation may be less than 0.5 μJ, for example on the order of 0.4 μJ.

[0022] The input pulsed pump radiation may have any desired wavelength. In some embodiments, the input pulsed pump radiation may have a wavelength of about 1 μm.

[0023] According to a third aspect of the present invention there is provided a radiation source comprising: a hollow core optical fibre including a body having a hollow core; a working medium disposed within the hollow core; and a pulsed pump radiation source arranged to generate pulsed pump radiation received by and propagating through the hollow core from an input end to an output end, wherein parameters of the pulsed pump radiation, the optical fibre and the working medium are configured to enable soliton self-compression of the pulsed pump radiation to modify a spectrum of the pulsed pump radiation to form an output radiation, and further wherein a pulse duration of the input pulsed pump radiation exceeds 50 fs.

[0024] For example, the pulse duration of the input pulsed pump radiation may be greater than 100 fs, for example on the order of 150 fs.

[0025] According to a fourth aspect of the present invention there is provided a radiation source comprising a hollow core optical fibre including a body having a hollow core, a working medium disposed within the hollow core, and a pulsed pump radiation source arranged to generate pulsed pump radiation received by and propagating through the hollow core from an input end to an output end, wherein parameters of the pulsed pump radiation, the optical fibre and the working medium are configured to enable soliton self-compression of the pulsed pump radiation to modify a spectrum of the pulsed pump radiation to form an output radiation, and further wherein a pulse energy of the input pulsed pump radiation is less than 1 μJ.

[0026] For example, the pulse energy of the input pulsed pump radiation may be less than 0.75 μJ. For example, the pulse energy of the input pulsed pump radiation may be less than 0.5 μJ, for example of the order of 0.4 μJ.

[0027] The radiation sources according to the third and fourth aspects of the present invention are advantageous as they enable a broadband output radiation beam to be generated at the output end, which may be useful for use in a metrology apparatus, for example a lithographic apparatus.

[0028] The radiation sources according to the third and fourth aspects of the present invention are, for example, more stable with respect to pulse-to-pulse variations than known broadband radiation sources which generate spectral broadening of the pulsed pump radiation under the modulation instability regime.

[0029] The soliton order N of the input pulsed pump radiation is a convenient parameter that can be used to distinguish between conditions where the spectral broadening is dominated by modulation instability and conditions where the spectral broadening is dominated by soliton self-compression. Spectral broadening is typically dominated by modulation instability for N>>20, whereas spectral broadening is typically dominated by soliton self-compression for N<<20.

[0030] Therefore, in configurations using soliton self-compression, it is desirable to generate input pulsed pump radiation with a low soliton order N. Furthermore, the soliton order of the input pulsed pump radiation is proportional to the pulse duration of the input pulsed pump radiation. Thus, in typical prior art configurations where soliton self-compression dominates, the pulse duration of the input pulsed pump radiation is typically reduced to the order of 30 fs or less. To realize such an arrangement, high-power femtosecond fiber lasers or Ti:Sapph amplifiers are typically used as pulsed pump radiation sources. The laser heads are relatively bulky (the dimensions of a femtosecond fiber laser head are, for example, 60 × 40 × 20 cm) and in most cases require external controllers and coolers. Furthermore, such lasers are relatively costly.

[0031] The inventors of the present invention have realized that the soliton order of the input pulsed pump radiation can alternatively be reduced by reducing the pulse energy of the input pulsed pump radiation. For example, if all other parameters remain constant, the same soliton order can be achieved using a pulse duration that is increased by a factor by reducing the pulse energy of the input pulsed pump radiation by a factor of α. This reduction in pulse energy is contrary to the teachings of the prior art, which teach to use increased pulse energies. In the art, radiation sources using soliton self-compression are typically used in research applications, such as, for example, atomic or molecular spectroscopy, where it is desirable to maximize the pulse energy of the radiation source.

[0032] In any of the first, second, third or fourth aspects of the invention, the soliton order of the input pulsed pump radiation may be less than 20.

[0033] In any of the first, second, third or fourth aspects of the present invention, the working medium may be configured to generate anomalous dispersion, i.e. the working medium may have a negative group delay dispersion parameter.

[0034] In any of the first, second, third or fourth aspects of the invention, the hollow-core optical fiber may include a cladding portion surrounding the hollow core, the cladding portion may include a plurality of anti-resonant elements for directing radiation through the hollow core, each of the plurality of anti-resonant elements may include a capillary.

[0035] A plurality of anti-resonant elements in the cladding portion may be arranged in a ring configuration around the hollow core.

[0036] Multiple anti-resonant elements can be arranged such that each anti-resonant element is not in contact with any of the other anti-resonant elements.

[0037] In any of the first, second, third or fourth aspects of the invention, the working medium may comprise a noble gas. For example, the working medium may comprise one or more of argon, krypton, neon, helium and xenon.

[0038] In any of the first, second, third or fourth aspects of the invention, the working medium may comprise a molecular gas. For example, the working medium may comprise N 2 , O 2 , C.H. 4 and S.F. 6 may include one or more of:

[0039] According to a fifth aspect of the present invention there is provided a metrology apparatus for determining a parameter of interest of a structure on a substrate comprising a radiation source of any of the first, second, third or fourth aspects, an illumination subsystem for illuminating the structure on the substrate using broadband output radiation, and a detection subsystem for detecting a portion of the radiation scattered and / or reflected by the structure and for determining the parameter of interest from the portion of the radiation.

[0040] According to a sixth aspect of the present invention, there is provided a lithographic apparatus including a metrology apparatus according to the fifth aspect of the present invention.

[0041] According to a seventh aspect of the present invention there is provided a method of selecting an operating regime of a radiation source comprising a hollow core optical fibre including a body having a hollow core, a working medium disposed within the hollow core, and a pulsed pump radiation source arranged to generate pulsed pump radiation received by and propagating through the hollow core from an input end to an output end, the method comprising selecting parameters of one or more of the pulsed pump radiation, the optical fibre and the working medium to enable soliton self-compression of the pulsed pump radiation to modify a spectrum of the pulsed pump radiation to form an output radiation, and further wherein the parameters are selected such that a length of the optical fibre substantially coincides with the following positions: a position at which the time extent of the output radiation is minimum, and / or a position at which the spectral width of the output radiation is maximum.

[0042] A method according to an eighth aspect of the invention provides a method by which a radiation source according to the first, second and third aspects of the invention can be designed.

[0043] In a first application of the method, parameters of the optical fiber can be selected. Once the optical fiber is manufactured, its parameters can be determined, for example by measurement, and input as constraints to a second application of the method.

[0044] The parameters of the optical fiber can be fixed and the parameters of the pulsed pump radiation and / or the working medium can be selected, such that when the parameters of the optical fiber are fixed (e.g., after the optical fiber is manufactured), operating parameters of the pulsed pump radiation and / or the working medium can be selected. [Brief description of the drawings]

[0045] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings. [Figure 1] 1 depicts a schematic diagram of a lithographic apparatus; [Diagram 2]FIG. 1 is a schematic diagram of a lithography cell. [Diagram 3] FIG. 1 is a schematic diagram of overall lithography illustrating the collaboration between three main technologies to optimize semiconductor manufacturing. [Figure 4] FIG. 1 is a schematic diagram of a scatterometer metrology tool; [Diagram 5] FIG. 1 is a schematic diagram of a level sensor metrology tool. [Figure 6] FIG. 1 is a schematic diagram of an alignment sensor metrology tool. [Figure 7] 1 is a schematic cross-section, in a transverse plane (i.e. perpendicular to the axis of the optical fiber), of a hollow core optical fiber forming part of a radiation source according to an embodiment; [Figure 8] FIG. 2 is a schematic diagram of a radiation source for providing broadband output radiation in accordance with an embodiment; [Figure 9A] FIG. 9 illustrates a simulation of the time evolution of a radiation pulse in the hollow core optical fiber of the radiation source shown in FIG. 8 when the second output end of the optical fiber coincides with the position where the temporal extent of the radiation is a minimum. [Figure 9B] FIG. 9 illustrates a simulation of the spectral evolution of a radiation pulse in the hollow core optical fiber of the radiation source shown in FIG. 8 when the second output end of the optical fiber coincides with the position where the temporal extent of the radiation is minimum. [Figure 9C] FIG. 9C shows a simulation of the output spectrum of a radiation source using the same parameters as the simulation shown in FIGS. 9A and 9B. [Figure 10A] FIG. 9 illustrates a simulation of the spectral evolution of a radiation pulse in a hollow core optical fiber of the radiation source shown in FIG. 8 as the length of the optical fiber is increased such that the second output end of the optical fiber does not coincide with the position where the temporal extent of the radiation is minimum. [Figure 10B] FIG. 10B shows a simulation of the output spectrum of the radiation source using the same parameters as the simulation shown in FIG. 10A. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0046] As used herein, 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 (extreme ultraviolet radiation, e.g., having wavelengths in the range of about 5 to 100 nm).

[0047] The terms "reticle," "mask," or "patterning device," as used herein, may be broadly interpreted to refer to a general patterning device that can be used to impart 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 standard masks (transmissive or reflective; binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0048] 1 illustrates diagrammatically a lithographic apparatus LA, comprising: an illumination system (also called illuminator) IL configured to condition a radiation beam B (e.g. UV, DUV or EUV radiation), a mask support (e.g. mask table) T constructed to support a patterning device (e.g. mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA according to certain parameters, a substrate support (e.g. wafer table) WT constructed to hold a substrate (e.g. resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support according to certain parameters, and a projection system (e.g. refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0049] In operation, the illumination system IL receives a beam from the radiation source SO, for example via the beam delivery system BD. The illumination system IL may include various optical elements, or any combination of these, such as refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical elements for directing, shaping and / or controlling the radiation. 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.

[0050] The term "projection system" PS as used herein should be interpreted broadly as covering any type of projection system including, for example, refractive, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation used and / or other factors such as the use of an immersion liquid or the use of a vacuum. When the term "projection lens" is used herein, it can be considered as synonymous with the more general term "projection system" PS.

[0051] The lithographic apparatus LA may be of a type in which at least a part of the substrate is covered by a liquid having a relatively high refractive index, e.g. water, so as to fill a gap between the projection system PS and the substrate W. This is also referred to as immersion lithography. Further details about immersion techniques are found in US Pat. No. 6,952,253, which is incorporated herein by reference.

[0052] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also called "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel and / or steps in preparation for subsequent exposure of the substrate W may be performed on a substrate W located on one of the substrate supports WT. The substrate W on the other substrate support WT is being used to expose a pattern on the other substrate W.

[0053] In addition to the substrate support WT, the lithographic apparatus LA may include a measurement stage. The measurement stage is arranged to hold a sensor and / or a cleaning device. The sensor may be arranged 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 providing immersion liquid. When the substrate support WT is away from the projection system PS, the measurement stage may be moved below the projection system PS.

[0054] In operation, the radiation beam B is incident on a patterning device, for example a mask MA, held on the mask support T, and is patterned according to a pattern (design layout) present on the patterning device MA. After passing the mask MA, the radiation beam B passes through a projection system PS which focuses the beam onto a target portion C of the substrate W. With the aid of a second positioner PW and a position measurement system IF, the substrate support WT can be precisely moved to position different target portions C in the path of the radiation beam B at focused and aligned positions. Similarly, the first positioner PM and possibly further position sensors (not explicitly shown in FIG. 1 ) can 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 can be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the illustrated substrate alignment marks P1, P2 occupy dedicated target portions, they may also be located in spaces between the target portions. The substrate alignment marks P1, P2, when they are located between target portions C, are known as scribe-lane alignment marks.

[0055] As shown in FIG. 2, the lithographic apparatus LA may form part of a lithographic cell LC, sometimes also referred to as a lithocell or (litho)cluster, which often also includes apparatus for performing pre-exposure and post-exposure processes on the substrate W. Conventionally, these include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a cooling plate CH, and a bake plate BK, for example to adjust the temperature of the substrate W, for example to adjust the solvent of the resist layer. A substrate handler or robot RO picks up the substrates W from input / output ports I / O1, I / O2, moves them between the different process apparatus, and delivers the substrates W to a loading bay LB of the lithographic apparatus LA. The apparatus of the lithocell (often also collectively referred to as the track) are typically under the control of a track control unit TCU, which is itself controlled by a supervisory control system SCS, which may control the lithographic apparatus LA, for example via a lithography control unit LACU.

[0056] In order to accurately and consistently expose an exposed substrate W by the lithographic apparatus LA, it is desirable to inspect the substrate to measure characteristics of the pattern structures such as overlay error between subsequent layers, line width, critical dimensions (CD), etc. For this purpose, an inspection tool (not shown) may be included in the lithocell LC. If an error is detected, for example, the exposure of a subsequent substrate or other processing steps performed on the substrate W may be adjusted, especially if inspection is done before other substrates W of the same batch or lot are yet to be exposed or processed.

[0057] 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. Alternatively, the inspection apparatus may be configured to identify defects on the substrate W and may for example be part of a lithocell LC or integrated in the lithography apparatus LA or may be a stand-alone apparatus. The inspection apparatus may measure properties of the latent image (image of the resist layer after exposure), or the semi-latent image (image of the resist layer after a post-exposure bake step PEB), or the developed resist image (image in which exposed or unexposed parts of the resist have been removed), or the etched image (after a pattern transfer step such as etching).

[0058] Typically, the patterning process in the lithography apparatus LA is one of the most critical steps in the process, which requires high accuracy of the dimensioning and placement of the structures on the substrate W. To guarantee this high accuracy, three systems can be combined in 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 to a metrology tool MT (second system) and a computer system CL (third system). The key to such a "holistic" environment is to optimize the cooperation between these three systems to enforce the overall process window and to provide a strict control loop that ensures that the patterning performed by the lithography apparatus LA stays within the process window. The process window defines the set of process parameters (e.g. dose, focus, overlay) within which a particular manufacturing process produces a defined result (e.g. a functional semiconductor device), typically within which the process parameters of the lithography process or patterning process are allowed to vary.

[0059] The computer system CL can use (part of) the design layout to be patterned to predict which resolution enhancement technique to use and perform computational lithography simulations and calculations to determine which mask layout and lithography apparatus settings will achieve the largest overall process window of the patterning process (indicated by the double arrow at the first scale SC1 in FIG. 3). Typically, the resolution enhancement technique is configured to match the patterning capabilities of the lithography apparatus LA. The computer system CL may also be used to detect where in the process window the lithography apparatus LA is currently operating (e.g., using input from the metrology tool MT) in order to predict whether defects are present due to suboptimal processing (indicated by the arrow pointing to "0" at the second scale SC2 in FIG. 3).

[0060] The metrology tool MT can provide input to the computer system CL to enable accurate simulations and predictions, and can provide feedback to the lithographic apparatus LA to identify possible drifts, for example in the calibration state of the lithographic apparatus LA (indicated by multiple arrows at a third scale SC3 in Figure 3). Different types of metrology tools MT for measuring one or more properties related to the lithographic apparatus and / or the substrate to be patterned will now be described.

[0061] In lithographic processes, it is frequently desirable to perform measurements of the created structures, e.g. for process control and verification. Tools for performing such measurements are typically called metrology tools MT. Various types of metrology tools MT for performing such measurements are known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. Scatterometers are versatile instruments that allow the measurement of parameters of the lithographic process by having a sensor in the pupil or in a plane conjugate to the pupil of the scatterometer objective lens (usually referred to as pupil-based measurements) or in the image plane or in a plane conjugate to the image plane (in this case the measurements are typically referred to as image or field-based measurements). Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP 1,628,164A, which are incorporated herein by reference in their entireties. The scatterometers described above can measure gratings using light from soft x-rays and light in the visible to near infrared wavelength range.

[0062] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, reconstruction methods can be applied to the measured signal in order to reconstruct or calculate the properties of the diffraction grating. Such a reconstruction can result, for example, from 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 simulated interaction produces a diffraction pattern similar to the one observed from the real target.

[0063] 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 reflected or scattered radiation from the target is directed towards a spectrometer detector which measures the spectrum of the specularly reflected radiation (i.e. a measurement of the intensity as a function of wavelength). From this data, the structure or profile of the target giving rise to the detected spectrum can be reconstructed, for example by rigorous coupled wave analysis and nonlinear regression, or by comparison with a library of simulated spectra.

[0064] In a third embodiment, the scatterometer MT is an ellipsometric scatterometer. The ellipsometric scatterometer allows to determine parameters of the lithographic process by measuring the scattered radiation of each polarization state. Such a metrology tool emits polarized light (such as linear, circular or elliptical), for example by using a suitable polarizing filter in the illumination section of the metrology tool. A radiation source suitable for the metrology tool may also provide polarized radiation. Various embodiments of existing ellipsometric scatterometers are described in U.S. patent application Ser. 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.

[0065] In one embodiment of the scatterometer MT, the scatterometer MT is configured to measure the overlay of two misaligned gratings or periodic structures by measuring the asymmetry of the reflection spectrum and / or the detection configuration. The asymmetry is related to the degree of overlay. The two (typically overlapping) grating structures may be applied to two different layers (not necessarily consecutive layers) and may be formed at substantially the same location on the wafer. The scatterometer may have a symmetric detection configuration so that the asymmetry can be clearly distinguished, for example as described in co-owned patent application EP 1,628,164 A. This allows the misalignment of the diffraction gratings to be easily measured. Further examples for measuring the overlay error between two layers containing periodic structures as targets measured via the asymmetry of the periodic structures can be found in International Patent Application Publication No. WO2011 / 012624 or US Patent Application No. US20160161863, which are incorporated herein by reference in their entirety.

[0066] Other parameters of interest may be focus and dose. Focus and dose may be determined simultaneously by a scatterometer (or alternatively by a scanning electron microscope) as described in US Patent Application US2011-0249244, which is incorporated herein by reference in its entirety. A single structure may be used, which 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, the values ​​of focus and dose may be uniquely determined from these measurements.

[0067] The metrology target may be a collection of composite gratings, often formed in the resist by the lithography process, but also formed after, for example, an etching process. Typically, the pitch and line width of the structures in the gratings strongly depend on the measurement optics (especially the NA of the optics) that can capture the diffraction orders coming from the metrology target. As indicated above, the diffracted signal may be used to determine the shift between the two layers (also called "overlay") and to reconstruct at least a portion of the original grating generated by the lithography process. This reconstruction may be used to provide an indication of the quality of the lithography process and may be used to control at least a portion of the lithography process. The target may have smaller sub-segmentations configured to mimic the dimensions of the features of the design layout in the target. Due to this sub-segmentation, the target will behave more similarly to the features of the design layout, such that the measurement results of the overall process parameters will closely resemble the features of the design layout. The target may be measured in an underfill mode or an overfill mode. In underfill mode, the measurement beam creates a spot that is smaller than the entire target. In overfill mode, the measurement beam creates a spot that is larger than the entire target. In such overfill mode, it may be possible to simultaneously measure different targets, and thus simultaneously determine different process parameters.

[0068] The overall quality of a measurement of a lithography parameter using a particular target is determined at least in part by the measurement recipe used to measure that lithography parameter. The term "substrate measurement recipe" may include one or more parameters of the measurement itself, or may be used to measure one or more parameters of the measured lithography parameter. The measurement recipe may include one or more parameters of the pattern, or may include both. For example, if the measurement used in a substrate measurement recipe is a diffraction-based optical measurement, the one or more 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 with respect to the pattern on the substrate, etc. One of the criteria for selecting a measurement recipe may be, for example, the sensitivity of one measurement parameter to process variations. More examples are described in U.S. Patent Application No. US2016-0161863 and published U.S. Patent Application No. US2016 / 0370717, which are incorporated by reference herein in their entirety.

[0069] A metrology device such as a scatterometer SM1 is shown in FIG. 4. It comprises a broadband (white light) radiation projector 2 that projects radiation onto a substrate 6. The reflected or scattered radiation is passed to a spectrometer detector 4. This measures the spectrum 10 of the specularly reflected radiation (i.e. the measurement of the intensity In1 as a function of wavelength λ). From this data, the structure or profile giving rise to the detected spectrum can be reconstructed by the processing unit PU, for example by rigorous coupled wave analysis and nonlinear regression, or by comparison with a library of simulated spectra as shown in the lower part of FIG. 4. Typically, in the reconstruction, the general form of the structure is known and some parameters are inferred from knowledge of the process by which the structure was created, leaving only a few parameters of the structure to be determined from the scatterometry data. Such a scatterometer can be configured as a normal incidence scatterometer or an oblique incidence scatterometer.

[0070] In lithographic processes, it is frequently desirable to perform measurements of the created structures, for example for process control and verification. Various tools are known for performing such measurements, including various forms of metrology devices such as scanning electron microscopes or scatterometers. Known examples of scatterometers often rely on the provision of dedicated metrology targets, such as underfilled targets (targets in the form of simple gratings or overlapping gratings of different layers, large enough for the measurement beam to generate a spot smaller than the grating) or overfilled targets (whereby the illumination spot partially or completely contains the target). Furthermore, the use of metrology tools, for example angle resolved scatterometers illuminating underfilled targets such as gratings, allows the use of so-called reconstruction methods, which allow the properties of the diffraction grating to be calculated by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measured results. The parameters of the model are adjusted until the simulated interaction produces diffraction turns similar to those observed from the real target.

[0071] Scatterometers are versatile instruments that allow for the measurement of parameters of lithography processes by having a sensor at the pupil or a conjugate plane of the scatterometer objective (usually referred to as pupil-based measurement) or by placing a sensor at the image plane or a plane conjugate to the image plane (in which case the measurement is typically referred to as image or field-based measurement). Such scatterometers and related measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP1,628,164A, which are incorporated herein by reference in their entirety. The aforementioned scatterometers can measure multiple targets from multiple gratings in one image using soft x-rays and light in the visible to near infrared wavelength range.

[0072] A topography measurement system, level sensor or height sensor, which may be integrated in a lithographic apparatus, is arranged 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, indicating the height of the substrate as a function of position on the substrate. This height map may then be used to correct the position of the substrate during transfer of a pattern to the substrate, to provide an aerial image of the patterning device at the correct focused position on the substrate. In this context, it will be understood that "height" refers generally to the out-of-plane dimension (also called the Z-axis) relative to the substrate. Typically, a level or height sensor performs measurements at a fixed location (relative to its optics), and relative movement between the substrate and the optics of the level or height sensor results in height measurements at each location across the substrate.

[0073] An example of a level or height sensor LS known in the art is shown diagrammatically in FIG. 5. The figure illustrates only the principle of operation. In this example, the level sensor comprises an optical system including a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO providing a radiation beam LSB provided by a projection grating PGR of the projection unit LSP. The radiation source LSO may be, for example, a narrowband or broadband radiation source, such as a supercontinuum source, polarized or unpolarized, such as a polarized or unpolarized laser beam, pulsed or continuous. The radiation source LSO may include multiple radiation sources with different colors or wavelength ranges, such as multiple LEDs. The radiation source LSO of the level sensor LS is not limited to visible radiation, but may additionally or alternatively encompass UV and / or IR radiation and any range of wavelengths suitable for reflection from the surface of the substrate.

[0074] The projection grating PGR is a periodic grating with a periodic structure resulting in a radiation beam BE1 having a periodically varying intensity. The radiation beam BE1 having a periodically varying intensity is directed towards a measurement location MLO on the substrate W with an angle of incidence ANG of 0 to 90 degrees, typically 70 to 80 degrees, relative to an axis perpendicular to the incident substrate surface (Z-axis). 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.

[0075] To determine the height level at the measurement location MLO, the level sensor further comprises a detection system comprising 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 identical to the projection grid PGR. The detector DET generates a detector output signal indicative of the received light, e.g. indicative of the intensity of the received light as in a photodetector or representative of the spatial distribution of the received intensity as in a camera. The detector DET may comprise any combination of one or more detector types.

[0076] By means of triangulation techniques the height level at the measurement location MLO can be determined. The detected height level is typically related to the signal intensity measured by the detector DET, which has a periodicity that depends inter alia on the design of the projection grating PGR and on the (oblique) angle of incidence ANG.

[0077] 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).

[0078] In one embodiment, the detection grid DGR may be omitted and the detector DET may be placed at the location of the detection grid DGR, such a configuration providing a more direct detection of the image of the projection grid PGR.

[0079] 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.

[0080] Various height sensors of the general type are disclosed, for example, in U.S. Patent Nos. 7,265,364 and 7,646,471, which are incorporated by reference. A height sensor using UV radiation instead of visible or infrared radiation is disclosed in U.S. Patent Publication No. 2010233600A1, which is incorporated by reference. WO 2016102127A1, which is incorporated by reference, describes a compact 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.

[0081] The position measurement system PMS may include any type of sensor suitable for determining the position of the substrate support WT. The position measurement system PMS may include any type of sensor suitable for determining the position of the mask support MT. The sensor may be an optical sensor, such as an interferometer or an encoder. The position measurement system PMS may comprise a combined interferometer and encoder system. The sensor may be another type of sensor, such as a magnetic sensor, a capacitive sensor or an inductive sensor. The position measurement system PMS may determine a position relative to a reference, e.g. the metrology frame MF or the projection system PS. The position measurement system PMS may determine the position of the substrate table WT and / or the mask support MT by measuring the position or by measuring a time derivative of the position, such as the velocity or acceleration.

[0082] The position measurement system PMS may comprise an encoder system. An encoder system is known, for example, from US patent application 2007 / 0058173 A1, filed 7 September 2006, which is incorporated herein by reference. The encoder system comprises an encoder head, a grating and a sensor. The encoder system can receive a primary radiation beam and a secondary radiation beam. Both the primary radiation beam and the secondary radiation beam originate from the same radiation beam, i.e. the original radiation beam. At least one of the primary radiation beam and the secondary radiation beam is generated by diffracting the original radiation beam with a diffraction grating. If both the primary radiation beam and the secondary radiation beam are generated by diffracting the original radiation beam with a diffraction grating, the primary radiation beam should have a different diffraction order than the secondary radiation beam. The different diffraction orders are, for example, +1, -1, +2 and -2 orders. The encoder system optically combines the primary radiation beam and the secondary radiation beam into a combined radiation beam. A sensor in the encoder head determines the phase or phase difference of the combined radiation beam. The sensor generates a signal based on the phase or phase difference. The signal is indicative of the position of the encoder head relative to the grating. The encoder head and one of the gratings may be located on the substrate structure WT. The other of the encoder head and grating may be located on the metrology frame MF or the base frame BF. For example, multiple encoder heads are located on the metrology frame MF while the gratings are located on a top surface of the substrate support WT. In another example, the gratings are located on a bottom surface of the substrate support WT and the encoder heads are located below the substrate support WT.

[0083] The position measurement system PMS may include an interferometer system. An interferometer system is known, for example, from US Pat. No. 6,020,964, filed 13 July 1998, which is incorporated herein by reference. The interferometer system may comprise a beam splitter, a mirror, a reference mirror, and a sensor. A beam of radiation is split into a reference beam and a measurement beam by the beam splitter. The measurement beam propagates to the mirror and is reflected by the mirror back to the beam splitter. The reference beam propagates to the reference mirror and is reflected by the reference mirror back to the beam splitter. At the beam splitter, the measurement beam and the reference beam are combined into a combined radiation beam. The combined radiation beam is incident on a sensor. The sensor determines a phase or a frequency of the combined radiation beam. The sensor generates a signal based on the phase or frequency. The signal is representative of the displacement of the mirror. In an embodiment, the mirror is connected to the substrate support WT. The reference mirror may be connected to a metrology frame MF. In one embodiment, the measurement beam and the reference beam are combined into a combined radiation beam by additional optical components instead of a beam splitter.

[0084] In the manufacture of complex devices, typically many lithographic patterning steps are performed, which result in the formation of functional features in successive layers on a substrate. Therefore, an important aspect of the performance of a lithographic apparatus is the ability to correctly and accurately position an applied pattern with respect to features defined in a previous layer (by the same or a different lithographic apparatus). For this purpose, one or more sets of alignment marks are provided on the substrate. Each mark is a structure whose position can be subsequently measured by 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". The marks are sometimes called metrology targets.

[0085] A lithographic apparatus may include one or more (e.g., multiple) alignment sensors, by which the position of an alignment mark provided on a substrate can be accurately measured. The alignment (or position) sensor may use optical phenomena such as diffraction and interference to obtain position information from an alignment mark formed on the substrate. An example of an alignment sensor used in current lithographic apparatus is based on a self-referencing interferometer, as described in US Patent 6,961,115. Various extensions and modifications of the position sensor have been developed, as disclosed, for example, in US Patent Publication 2015261097A1. The contents of all these publications are incorporated herein by reference.

[0086] The mark or alignment mark may include a series of bars formed on or in a layer provided on or formed (directly) in the substrate. The bars are spaced at regular intervals to act as grating lines, so that the mark can be considered as a diffraction grating with a known spatial period (pitch). Depending on the orientation of the grating lines, the mark can be designed to allow measurement of position along the X-axis or the Y-axis (oriented substantially perpendicular to the X-axis). Marks including bars aligned at +45 degrees and / or -45 degrees to both the X-axis and the Y-axis allow combined X and Y measurements using techniques such as those described in US Patent Publication No. 2009 / 195768A, which is incorporated herein by reference.

[0087] The alignment sensor optically scans each mark with a radiation spot to obtain a periodically varying signal, such as a sine wave. The phase of this signal is analysed to determine the position of the mark relative to the alignment sensor, and thus the position of the substrate. The alignment sensor is fixed with respect to the reference frame of the lithographic apparatus. By providing so-called coarse and fine marks associated with different (coarse and fine) mark dimensions, the alignment sensor is able to distinguish between different cycles of the periodic signal and to identify the exact position (phase) within one cycle. For this purpose marks of different pitches may be used.

[0088] Measuring the position of the marks can also provide information about deformations of the substrate on which the marks are provided, e.g. in the form of a wafer grid, which may be caused, for example, by electrostatic clamping of the substrate to the substrate table and / or by heating of the substrate when exposing it to radiation.

[0089] 6 is a schematic block diagram of an embodiment of a known alignment sensor AS, for example as described in US Pat. No. 6,961,116, which is incorporated herein by reference. A radiation source RSO provides a radiation beam RB of one or more wavelengths, which is diverted by diverting optics onto a mark, such as a mark AM, positioned on a substrate W, as an illumination spot SP. In this example, the diverting optics includes a spot mirror SM and an objective lens OL. The illumination spot SP illuminating the mark AM may be of a diameter slightly smaller than the width of the mark itself.

[0090] The radiation diffracted by the 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 (which may be referred to as reflection) from the mark. 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, which is then received by a photodetector PD. If more than one wavelength is produced by the radiation source RSO, additional optics (not shown) may be included to provide separate beams. The photodetector may be a single element or may include many pixels, if desired. The photodetector may include a sensor array.

[0091] Guiding optics, which in this example include a spot mirror SM, may serve to block zero order radiation reflected from the mark, so that the information-bearing 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).

[0092] 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 ​​for the X and Y position on the substrate relative to a reference frame.

[0093] A single measurement of the type shown only fixes the position of the mark within a certain range corresponding to one pitch of the mark. to identify the period of the sine wave that contains this mark location. The same process can be repeated at different wavelengths and at coarse and / or fine levels for improved accuracy and / or robust detection of the mark, independent of the material the mark is made of and the material provided below and / or above the mark. The wavelengths can be optically multiplexed and demultiplexed and / or multiplexed by time or frequency division to be processed simultaneously.

[0094] In this example, the alignment sensor and the spot SP remain fixed, and it is the substrate W that moves. The alignment sensor can therefore scan the mark AM in a direction substantially opposite to the direction of movement of the substrate W, while being rigidly and precisely mounted to a reference frame. This movement of the substrate W is controlled by the substrate W being mounted on a substrate support and a substrate positioning system controlling 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. By measuring the positions of the marks provided on the substrate support, the position of the substrate support as determined by the position sensor can be calibrated (e.g. relative to a frame to which the alignment system is connected). By measuring the positions of the alignment marks provided on the substrate, the position of the substrate can be determined relative to the substrate support.

[0095] A metrology tool MT, such as the scatterometer, topography measurement system, or position measurement system mentioned above, can use radiation originating from a radiation source to perform measurements. The characteristics of the radiation used in the metrology tool can affect the type and quality of measurements that may be performed. In some applications, it may be advantageous to use multiple radiation frequencies to measure the substrate. For example, broadband radiation can be used. Multiple different frequencies may be able to propagate, illuminate, and scatter the metrology target with no or minimal interference with other frequencies. Thus, for example, more metrology data may be acquired simultaneously using different frequencies. Different radiation frequencies may also be able to probe and discover 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.

[0096] High quality broadband radiation, e.g., supercontinuum radiation, can be difficult to generate. One way to generate broadband radiation is to broaden high power narrowband or single frequency input radiation, e.g., by utilizing nonlinear higher order effects. The input radiation, which may be generated using a laser, may be referred to as pump radiation. Alternatively, the input radiation may be referred to as seed radiation. To obtain high power radiation for broadening effects, the radiation may be confined to a small region to achieve a strongly localized high intensity radiation. In these regions, the radiation may interact with broadening structures and / or materials that form a nonlinear medium to generate broadband output radiation. In high intensity radiation regions, different materials and / or structures may be used to enable and / or improve the broadening of the radiation by providing a suitable nonlinear medium.

[0097] In some implementations, as further described below with reference to FIG. 8, the method and apparatus for spreading input radiation may use a fiber to confine the input radiation and to spread the input radiation to output broadband radiation. The fiber may be a hollow core fiber and may include an internal structure to achieve effective guiding and confinement of the radiation within the fiber. The fiber may 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, achieving high radiation intensity. The hollow core of the fiber may be filled with a gas that acts as a spreading medium to spread the input radiation. Such a fiber and gas arrangement may be used to create a supercontinuum radiation source. The radiation input to the fiber may be electromagnetic radiation, for example, one or more radiation in 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.

[0098] Some embodiments relate to new designs of such broadband radiation including optical fibers. The optical fiber is a hollow-core photonic crystal fiber (HC-PCF). In particular, the optical fiber can be a hollow-core photonic crystal fiber of the type that includes an anti-resonant structure to confine the 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 inhibited coupling fibers. Various different designs of such fibers are known in the art. Alternatively, the optical fiber can be a photonic bandgap fiber (e.g., Kagome fiber).

[0099] An example of an optical fiber for use in a radiation source will now be described with reference to Figure 7. Figure 7 is a schematic cross-sectional view of an optical fiber 100 in transverse cross-section.

[0100] The optical fiber 100 includes an elongated body that is longer in one dimension compared to the other two dimensions of the optical fiber 100. This longer length may be referred to as the axial direction and define the axis of the optical fiber 100. The other two dimensions define a plane called the transverse plane. Figure 7 shows a cross-section of the optical fiber 100 in this transverse plane (i.e., perpendicular to the axis), which is labeled the xy plane. The transverse cross-section of the optical fiber 100 may be substantially constant along the fiber axis.

[0101] It will be understood that the optical fiber 100 has some flexibility, and thus the axial orientation will generally not be uniform along the length of the optical fiber 100. Terms such as optical axis, cross section, etc. will be understood to mean the local optical axis, local cross section, etc. Additionally, when a component is described as being cylindrical or tubular, these terms will be understood to encompass the shape that the optical fiber 100 is distorted into when it is bent.

[0102] It will be appreciated that the optical fiber 100 can have any length and the length of the optical fiber 100 can depend on the application. The optical fiber 100 can have a length between 1 cm and 20 m, for example, the optical fiber 100 can have a length between 1 cm and 10 m, or the optical fiber 100 can have a length between 10 cm and 100 cm.

[0103] The optical fiber 100 comprises a hollow core 102, a cladding portion surrounding the hollow core 102, and a support portion 108 surrounding and supporting the cladding portion. The optical fiber 100 can be considered to comprise a body (including the cladding portion and the support portion 108) having the hollow core 102. The cladding portion comprises a plurality of anti-resonant elements for guiding radiation through the hollow core 102. In particular, the plurality of anti-resonant elements are arranged to confine radiation propagating through the optical fiber 100 primarily inside the hollow core 102 and guide the radiation along the optical fiber 100. The hollow core 102 of the optical fiber 100 may be disposed substantially in a central region of the optical fiber 100. The axis of the optical fiber 100 may also define an axis of the hollow core 102 of the optical fiber 100.

[0104] The cladding portion includes a plurality of anti-resonant elements for guiding radiation propagating through the optical fiber 100. In particular, in this embodiment, the cladding portion includes a single ring of six tubular capillaries 104. Each of the tubular capillaries 104 functions as an anti-resonant element.

[0105] The capillaries 104 are sometimes referred to as tubes. The capillaries 104 may be circular in cross section or may have another shape. Each capillary 104 comprises a generally cylindrical wall portion 105 that at least partially defines the hollow core 102 of the optical fiber 100 and separates the hollow core 102 from a capillary cavity 106. It will be appreciated that the wall portion 105 may function as an anti-reflective Fabry-Perot resonator for radiation propagating through the hollow core 102 (and incident on the wall portion 105 at a grazing incidence angle). The thickness of the wall portion 105 may be appropriated to ensure that reflection into the hollow core 102 is generally enhanced while transmission into the capillary cavity 106 is generally suppressed. In some embodiments, the capillary wall portion 105 may have a thickness between 0.01 and 10.0 μm.

[0106] It will be understood that, as used herein, the term cladding portion is intended to mean the portion of the optical fiber 100 intended to guide the radiation propagating therethrough (i.e. the capillary 104 confining said radiation within the hollow core 102). The radiation is confined in the form of transverse modes and propagates along the fiber axis.

[0107] The support portion is generally tubular and supports the cladding portion's six capillaries 104. The six capillaries 104 are evenly distributed around the inner surface in the case of the inner support portion 108. The six capillaries 104 can be described as being generally arranged in a hexagonal formation.

[0108] The capillaries 104 are arranged such that each capillary does not contact any of the other capillaries 104. Each of the capillaries 104 contacts the inner support portion 108 and is spaced apart from adjacent capillaries 104 in the ring structure. Such an arrangement may be beneficial (e.g., compared to an arrangement in which the capillaries are in contact with each other) as it may increase the transmission bandwidth of the optical fiber 100. Alternatively, in some embodiments, each of the capillaries 104 may contact adjacent capillaries 104 in the ring structure.

[0109] The six capillaries 104 of the cladding section are arranged in a ring structure around the hollow core 102. The inner surface of the ring structure of the capillaries 104 at least partially defines the hollow core 102 of the optical fiber 100. The diameter 114 of the hollow core 102 (which may be defined as the smallest dimension between opposing capillaries, indicated by arrows 114) may be between 10 and 1000 μm. The diameter 114 of the hollow core 102 may affect the mode field diameter, collisional loss, dispersion, modal plurality, and nonlinear properties of the hollow-core optical fiber 100.

[0110] In this embodiment, the cladding portion includes a single ring arrangement of capillaries 104 (which act as anti-resonant elements). Thus, any radial line from the center of the hollow core 102 to the outside of the optical fiber 100 passes through only one capillary 104.

[0111] It will be appreciated that other embodiments may be provided with different arrangements of anti-resonant elements. These may include arrangements having multiple rings of anti-resonant elements, and arrangements having nested anti-resonant elements. Additionally, while the embodiment shown in FIG. 7 includes six rings of capillaries, other embodiments may provide one or more rings including any number of anti-resonant elements (e.g., 4, 5, 6, 7, 8, 9, 10, 11, or 12 capillaries) in the cladding portion. Optionally, the support portion 108 may include a deformable portion to at least partially isolate the cladding portion from external stresses.

[0112] Figure 8 shows a radiation source 134 for providing broadband output radiation. The radiation source 134 comprises a pulsed pump radiation source 136, an optical fiber 100 (of the type shown in Figure 7) with a hollow core 102, and a working medium 126 (e.g. a gas) disposed within the hollow core 102. In Figure 8, the radiation source 134 includes the optical fiber 100 shown in Figure 7, although in alternative embodiments other types of hollow core optical fibers could be used.

[0113] The pulsed pump radiation source 136 is configured to provide input radiation 122. The hollow core 102 of the optical fiber 100 is configured to receive the input radiation 122 from the pulsed pump radiation source 136 and broaden it to provide output radiation 124. The working medium 126 enables broadening the frequency range of the received input radiation 122 to provide the broadband output radiation 124.

[0114] The radiation source 134 further includes a reservoir 128. The optical fiber 100 is disposed inside the reservoir 128. The reservoir 128 may also be referred to as a housing or container. The reservoir 128 is configured to contain the working medium 126. The reservoir 128 may include one or more features known in the art for controlling, regulating, and / or monitoring a composition of the working medium 126 (which may be a gas) inside the reservoir 128. The reservoir 128 may include a first transparent window 130. In use, the optical fiber 100 is disposed inside the reservoir 128 such that the first transparent window 130 is disposed proximate to an input end of the optical fiber 100. The first transparent window 130 may form part of a wall of the reservoir 128. The first transparent window 130 may be transparent to at least the received input radiation frequency such that the received input radiation 122 (or at least a majority thereof) may be coupled into the optical fiber 100 disposed inside the reservoir 128. It will be appreciated that optics (not shown) may be provided to couple the input radiation 122 into the optical fiber 100.

[0115] The reservoir 128 includes a second transparent window 132 forming part of a wall of the reservoir 128. In use, when the optical fiber 100 is disposed within the reservoir 128, the second transparent window 132 is disposed proximate to the output end. The second transparent window 132 may be transparent to at least the frequencies of the broadband output radiation 124 of the device 120.

[0116] Alternatively, in another embodiment, the two opposing ends of the optical fiber 100 may be disposed in different reservoirs. The optical fiber 100 may include a first end section configured to receive the input radiation 122 and a second end section for outputting the broadband output radiation 124. The first end section may be disposed in a first reservoir containing the working medium 126. The second end section may be disposed inside a second reservoir, which may also contain the working medium 126. The function of the reservoirs may be as described in relation to FIG. 8 above. The first reservoir may include a first transparent window configured to be transparent to the input radiation 122. The second reservoir may include a second transparent window configured to be transparent to the broadband output broadband radiation 124. The first and second reservoirs may also include sealable openings that allow the optical fiber 100 to be disposed partially inside and partially outside the reservoirs, such that gas can be sealed inside the reservoirs. The optical fiber 100 may further include an intermediate section that is not contained within a reservoir. Such an arrangement using two separate gas reservoirs is particularly convenient for embodiments in which the optical fiber 100 is relatively long (e.g., greater than 1 m in length). In such a configuration using two separate gas reservoirs, the two reservoirs (which may include one or more features known in the art for controlling, regulating, and / or monitoring the composition of the gases in the two reservoirs) may be considered to provide an apparatus for providing the working medium 126 within the hollow core 102 of the optical fiber 100.

[0117] In this context, a window may be transparent to a frequency if at least 50%, 75%, 85%, 90%, 95%, or 99% of radiation at that frequency incident on the window is transmitted through the window.

[0118] Both the first transparent window 130 and the second transparent window 132 can form an airtight seal within the wall of the reservoir 128 such that the working medium 126, which can be a gas, can be contained within the reservoir 128. The gas 126 can be contained within the reservoir 128 at a pressure that is different from the ambient pressure of the reservoir 128.

[0119] The working medium 126 may include a noble gas. The working medium 126 may include one or more of argon, krypton, neon, helium, and xenon. As an alternative or in addition to a noble gas, the working component may be a molecular gas (e.g., H 2 , N 2 , O 2 , C.H. 4 ,SCIENCE FICTION 6 The working medium 126 may also include argon, krypton, neon, helium, xenon, and molecular gases (e.g., H 2 , N 2 , O 2 , C.H. 4 ,SCIENCE FICTION 6 ) The working medium 126 may be configured to generate anomalous dispersion, and optionally may be configured to generate anomalous dispersion at the wavelength of the input radiation 122. That is, the working medium 126 may have a negative group delay dispersion parameter.

[0120] In one embodiment, the working medium 126 may be disposed within the hollow core 102 at least during receipt of the input radiation 122 to generate the broadband output radiation 124. It will be appreciated that the gas 126 may be completely or partially absent from the hollow core 102 while the optical fiber 100 is not receiving input radiation 122 to generate the broadband output radiation.

[0121] In order to spread the frequency, high intensity radiation may be desired. An advantage of having a hollow core optical fiber 100 is that high intensity radiation can be achieved due to the strong spatial confinement of the radiation propagating through the optical fiber 100, and high local radiation intensity can be achieved. The radiation intensity within the optical fiber 100 may be high, for example, due to high received input radiation intensity and / or due to the strong spatial confinement of the radiation within the optical fiber 100. An advantage of hollow core optical fibers is that they can guide radiation in a wider wavelength range than solid core optical fibers, and in particular, hollow core optical fibers can guide radiation in both the ultraviolet and infrared ranges.

[0122] An advantage of using a hollow core optical fiber 100 is that most of the radiation guided into the optical fiber 100 is confined to the hollow core 102. Thus, most of the interactions of the radiation in the optical fiber 100 are with the working medium 126 provided inside the hollow core 102 of the optical fiber 100. As a result, the spreading effect of the working medium 126 on the radiation may be increased.

[0123] The received input radiation 122 may be electromagnetic radiation. The input radiation 122 may be received as pulsed radiation. For example, the input radiation 122 may include ultrafast pulses.

[0124] The input radiation 122 may be coherent radiation. The input radiation 122 may be collimated radiation, the advantage of which may be to facilitate and improve the efficiency of coupling the input radiation 122 into the optical fiber 100. The input radiation 122 may include a single frequency or a narrow range of frequencies. The input radiation 122 may be generated by a laser. Similarly, the output radiation 124 may be collimated and / or coherent.

[0125] The broadband range of the output radiation 124 may be a continuous range, including a continuous range of radiation frequencies. The output radiation 124 may include supercontinuum radiation. Continuum radiation may be useful for use in many applications, for example, 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 eliminate frequency dependence of a measured property. The supercontinuum output radiation 124 may include electromagnetic radiation over a wavelength range, for example, from 100 nm to 4000 nm. The frequency range of the broadband output radiation 124 may be, for example, from 400 nm to 900 nm, 500 nm to 900 nm, or 200 nm to 2000 nm. The supercontinuum output radiation 124 may include white light.

[0126] The input radiation 122 provided by the pulsed pump radiation source 136 may be pulsed. The input radiation 122 may include electromagnetic radiation at one or more frequencies between 200 nm and 2 μm. The input radiation 122 may include electromagnetic radiation having a wavelength of, for example, 1.03 μm. The repetition rate of the pulsed radiation 122 may be on the order of magnitude of 1 kHz to 100 mJ. The pulse energy may have a magnitude on the order of magnitude of 0.01 μJ to 100 μJ, for example 0.1 μJ to 100 μJ, or for example 1-10 μJ. The pulse duration of the input radiation 122 may be between 10 fs and 10 ps, ​​for example 300 fs. The average power of the input radiation 122 may be between 100 mW and several hundreds of W. The average power of the input radiation 122 may be, for example, 20-50 W.

[0127] The broadband output radiation 124 provided by the radiation source 134 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 124 may be pulsed broadband output radiation 124. The broadband output radiation 124 may have a power spectral density over the full wavelength band of the output radiation of at least 0.01 mW / nm. The power spectral density over the full wavelength band of the broadband output radiation may be at least 3 mW / nm.

[0128] Some embodiments relate to a radiation source in the form of a radiation source 134 shown in Figure 8 comprising a hollow core optical fiber 100, a working medium 126 disposed within the hollow core, and a pulsed pump radiation source 136 configured to generate pulsed pump radiation 122 that is received by and propagates through the hollow core from an input end 110 to an output end 112. In particular, some embodiments relate to a radiation source in which parameters of the pulsed pump radiation 122, the optical fiber 100 and the working medium 126 are configured to enable soliton self-compression of the pulsed pump radiation 122 to change the spectrum of the pulsed pump radiation 122 to form an output radiation 124.

[0129] Some known broadband radiation sources generate a spectral broadening of the pulsed pump radiation, but the parameters of the pulsed pump radiation, the optical fiber, and the working medium are configured to allow modulation instability to generate the spectral broadening. There are several reasons why modulation instability is used to generate the spectral broadening. First, modulation instability is known to generate broadband radiation with a relatively flat intensity-wavelength distribution, provided that a sufficient number of pulses are averaged. Such broadband radiation sources are sometimes referred to as white light radiation sources (because of the relatively flat spectral intensity distribution). Second, modulation instability can be achieved using a relatively economical laser source as the pump radiation source.

[0130] On the other hand, in the soliton self-compression regime, the input pulse undergoes compression in the time domain, which is accompanied by an increase in its spectral width. Following soliton self-compression, the compressed pulse undergoes soliton fission, in which the pulse splits into multiple solitons. This soliton fission leads to a temporal broadening and a spectral shift of the emitted pulse.

[0131] In some embodiments, the length of optical fiber 100 is such that output end 112 is substantially coincident with the position at which soliton self-compression occurs but before soliton splitting begins, providing a particularly stable broadband radiation source that is relatively smooth and free of significant gaps in the spectrum.

[0132] In some embodiments, the length of the optical fiber 100 is such that the output end 112 substantially coincides with the position where the temporal extent of the output radiation 124 is less than the first threshold value, or optionally, the position where the temporal extent of the output radiation 124 is at a minimum. The first threshold value can be selected to obtain a sufficiently broad output radiation spectrum. In practice, the compressed pulse leaving the output end 112 of the optical fiber 100 is broadened in time by the second transparent window 132 as a result of bulk material dispersion. As a result, the duration of the pulse of output radiation 124 after the second transparent window 132 can be relatively long.

[0133] In some embodiments, the length of optical fiber 100 is such that output end 112 is substantially coincident with a location where the spectral width of output radiation 124 is greater than a second threshold, or optionally, a location where the spectral width of output radiation 124 is at a maximum. The second threshold may be selected such that the spectral width is large enough to meet the bandwidth requirements of a particular application, such as the requirements of a metrology sensor.

[0134] In some embodiments, the length of optical fiber 100 is such that output end 112 substantially coincides with a location where the spectrum of output radiation 124 is substantially continuous.

[0135] Such a radiation source is advantageous, as discussed herein, as it enables a stable broadband output radiation beam 124 to be generated at the output end 112. Such a stable broadband output radiation beam 124 may be useful for use in a metrology apparatus, for example a lithography apparatus.

[0136] In the soliton self-compression regime (where the soliton number is relatively low), the pulse of radiation may undergo significant temporal compression accompanied by spectral broadening. Eventually, the temporal compression reaches a maximum level (corresponding to the minimum temporal extent of the pulse radiation) followed by temporal broadening of the radiation (soliton splitting). The (higher order) solitons may oscillate between periods of temporal compression and temporal broadening as they propagate along the hollow-core optical fiber. Following the temporal broadening, other effects may lead to a shift in the spectrum of the radiation. For example, self-pulse steepening (which may accompany and assist soliton self-compression) may lead to optical shocks that may seed dispersive-wave radiation. By tuning the parameters of the system, specific desired wavelengths may be generated. For example, the wavelength may be selected to be suitable for interacting with a particular molecule and may be used in research experiments studying said molecule. Soliton self-compression is thus a known regime for generating an output radiation beam with a second, shifted wavelength from an input pump laser beam with a first wavelength.

[0137] The inventors have recognized that during soliton self-compression, prior to the time broadening (and prior to the formation of a dispersive wave), there is a (short-lived) transition period during which the radiation propagating through the hollow-core fiber is broadband radiation (i.e., has a broad, relatively flat spectrum without significant gaps in the spectral density spectrum). Furthermore, the inventors have recognized that although this broadband radiation is short-lived, by selecting the length of the optical fiber 100 such that the output end 112 substantially coincides with the location where soliton self-compression occurred prior to the subsequent time broadening and shift in the spectrum, this broadband radiation can be output from the optical fiber 100 to provide a particularly stable broadband radiation source 134.

[0138] In particular, if the length of optical fiber 100 is such that output end 112 substantially coincides with the location where the temporal extent of the radiation is a minimum, a particularly stable broadband radiation source 134 can be provided.

[0139] In general, after soliton self-compression, the width of the spectrum of the pulsed radiation may decrease and / or a spectral gap may develop (e.g., as the soliton evolves and dispersive waves are emitted). Thus, a particularly stable and flat broadband radiation source can be provided if the length of the optical fiber 100 is such that the output end 112 substantially coincides with a position where the width of the spectrum of the radiation is maximum. Alternatively, a particularly stable and flat broadband radiation source can be provided if the length of the optical fiber 100 is such that the output end 112 substantially coincides with a position where the spectrum of the radiation is substantially continuous.

[0140] In contrast to noise-seeded modulational instability systems, the broadband radiation produced by such soliton self-compression is substantially free of shot-to-shot variations. As a result, advantageously, a stable output spectrum can be produced using a single pulse. In contrast, several pulses are required to provide some stability to the output beam of a modulational instability system.

[0141] It will be appreciated that as radiation propagates through the hollow core optical fiber 100, the solitons may oscillate between periods of time compression and time expansion. Between each period of time compression and time expansion, there may be a local minimum in the temporal extent of the radiation. In principle, the length of the optical fiber 100 may be such that the output end 112 substantially coincides with such a position of the minimum temporal extent of the radiation. However, the most stable (e.g., with respect to pulse-to-pulse variations) and flattest output spectrum may be provided by when the output end 112 coincides with a first position of the minimum temporal extent of the radiation.

[0142] Some embodiments relate to a radiation source that utilizes soliton self-compression and where the pulse duration of the input pulsed pump radiation 122 is greater than 50 fs, and optionally where the pulse duration of the input pulsed pump radiation 122 is less than or equal to 400 fs. For example, the pulse duration of the input pulsed pump radiation 122 may be greater than 100 fs, e.g., on the order of 150 fs.

[0143] Some embodiments relate to a radiation source utilising soliton self-compression, wherein the pulse energy of the input pulsed pump radiation is less than 1 μJ, optionally wherein the pulse energy of the input pulsed pump radiation is equal to or greater than 0.1 μJ.

[0144] The soliton order N of the input pulsed pump radiation 122 is a convenient parameter that can be used to distinguish between conditions where the spectral broadening is dominated by modulation instability and conditions where the spectral broadening is dominated by soliton self-compression. The soliton order N of the input pulsed pump radiation 122 is given by:

number

[0145] The spectral broadening is usually dominated by modulation instability when N>>20, whereas the spectral broadening is usually dominated by soliton self-compression when N<<20.

[0146] Therefore, in a configuration using soliton self-compression, it is desirable to generate input pulsed pump radiation 122 with a low soliton order N. Furthermore, as can be seen from equation (1), the soliton order of the input pulsed pump radiation 122 is proportional to the pulse duration τ of the input pulsed pump radiation 122. Thus, a typical conventional configuration in which soliton self-compression dominates typically reduces the pulse duration τ of the input pulsed pump radiation 122 to the order of 30 fs or less. To realize such a configuration, a compressed high-power femtosecond fiber laser or a Ti:Sapph amplifier is typically used as the pulsed pump radiation source 136. A Ti:Sapph amplifier can generate pulsed radiation with a pulse duration τ of the order of 30 fs or less. Since the pulse duration of a high-power femtosecond fiber laser is typically 300 fs, known soliton self-compression configurations using femtosecond fiber lasers also include a system for compressing this pulse duration to, for example, 30 fs. Such systems for compressing the pulse duration can use, for example, spectral broadening based on self-phase modulation and phase compression using chirped mirrors or gratings. The laser heads are relatively bulky (the dimensions of a femtosecond fiber laser head are, for example, 60 x 40 x 20 cm) and in most cases require an external controller and water cooler. Moreover, such lasers are relatively costly.

[0147] We define the pulse energy Ep of the input pulsed pump radiation (where E p =P p We have noticed that the soliton order of the input pulsed pump radiation can alternatively be reduced by reducing the pulse energy E pBy reducing the pulse energy by a factor of α, the same soliton order can be achieved using a pulse duration that increases by a factor of α. This reduction in pulse energy is contrary to the teachings of the prior art, which teach to use increased pulse energies. In the prior art, radiation sources using soliton self-compression are typically used in research applications, such as, for example, atomic or molecular spectroscopy, where it is desirable to maximize the pulse energy of the radiation source.

[0148] An exemplary embodiment will now be described with reference to Figures 9A to 10B.

[0149] 9A and 9B show simulations of the temporal and spectral evolution of a radiation pulse in a hollow-core optical fiber 100. The hollow-core optical fiber 100 has a core diameter of 32.5 μm and is filled with a working medium 126 of krypton gas at a pressure of 10 bar. The input pulsed pump radiation 122 has a pump pulse duration τ of 150 fs, a pulse energy E of 0.4 μJ energy, p , and a wavelength of 1030 nm. This pulse energy E p is about an order of magnitude lower than currently used in modulation instability-driven broadband sources. This configuration enables pumping in the anomalous dispersion regime (β 2 =-6.3fs 2 The soliton order of N=17 allows for soliton self-compression of the pulsed pump radiation 122 to change the spectrum of the pulsed pump radiation to form output radiation 124.

[0150] In the first portion of the optical fiber 100, the radiation undergoes self-phase modulation 140. This is followed by soliton self-compression 142, with the temporal extent of the radiation being a minimum at a distance of about 110 cm from the first end 112 of the optical fiber 100 (see FIG. 9A). As can be seen in FIG. 9B, this soliton self-compression is accompanied by a significant broadening 144 of the radiation spectrum. The second end 114 of the optical fiber 100 coincides with the position 142 where the temporal extent of the radiation is a minimum.

[0151] 9C shows the output spectrum 146 of the radiation source 134. Also shown is the spectrum 148 of the input pulsed pump radiation 122. It can be seen that a flatness of about 10 dB is achieved in the band from 500 to 900 nm. Note that the output spectrum 146 is calculated for a single shot, and the smoothness is due to small perturbations.

[0152] It should be noted that the pulses used in the above exemplary embodiment have a pulse duration τ of 150 fs, which is significantly larger than the typical pulse durations used for soliton self-compression (>30 fs). Moreover, such pulses can be easily generated by a new class of lasers offering pulse durations between those of Ti:Sapph amplifiers (30 fs) and high-power fiber lasers (300 fs), which have been dramatically reduced in volume or cost in recent years. An example of a suitable laser is the laser sold as Goji by Amplitude Systems SA, a company established in France.

[0153] Figure 10A shows a simulation of the spectral evolution of a radiation pulse in a hollow core optical fiber 100 experienced when the length of the optical fiber is increased to 150 cm (i.e. when the second end 114 of the optical fiber 100 no longer coincides with the location 142 where the temporal extent of the radiation is minimal). Figure 10B shows the output spectrum 150 of a radiation source 134 using an optical fiber of this increased length. Also shown is the spectrum 148 of the input pulsed pump radiation 122.

[0154] It can be seen that following soliton self-compression and the associated spectral broadening 144, the spectrum of the radiation undergoes many changes. For example, a dispersive wave 152 is emitted and the radiation oscillates between periods of spectral compression and spectral broadening.

[0155] 10B, it can be clearly seen that the flatness of the output spectrum is lost when the second end 114 of the optical fiber 100 no longer coincides with the location 142 where the time extent of the emission is at a minimum. The spectrum is no longer smooth, but instead has many peaks and valleys.

[0156] According to some embodiments, there is also provided a method of selecting an operating regime for a radiation source 134 of the type shown in Figure 8. The method comprises selecting the pulsed pump radiation 122 (e.g., pump pulse duration τ and / or pulse energy E) such that soliton self-compression is possible and the output end of the optical fiber 100 substantially coincides with a position where the temporal extent of the radiation is a minimum. p ), the optical fiber 100 (e.g., geometry, core diameter, etc.), and one or more parameters of the working medium 126 (e.g., gas composition, pressure, etc.).

[0157] In a first application of the method, parameters of the optical fiber 100 may be selected. Once the optical fiber 100 is manufactured, the parameters may be determined, for example, by measurement, and input as constraints to a second application of the method, whereby operating parameters of the pulsed pump radiation 122 and / or the working medium 126 may be selected when the parameters of the optical fiber 100 are fixed (e.g., after the optical fiber is manufactured).

[0158] The radiation source 134 described above may be provided as part of a metrology apparatus for determining a parameter of interest of a structure on a substrate. The structure on the substrate may be, for example, a lithography pattern applied to the substrate. The metrology apparatus may further include an illumination subsystem for illuminating the structure on the substrate. The metrology apparatus may further include a detection subsystem for detecting a portion of the radiation scattered and / or reflected by the structure. The detection subsystem may further determine a parameter of interest of the structure from the portion of the radiation scattered and / or reflected by the structure. The parameter may be, for example, overlay, alignment, or leveling data of the structure on the substrate.

[0159] The metrology apparatus may form part of a metrology apparatus MT. The metrology apparatus may form part of an inspection apparatus. The metrology apparatus may be included within a lithographic apparatus LA.

[0160] Further embodiments are disclosed in the following numbered paragraphs: 1. A hollow-core optical fiber including a body having a hollow core; a working medium disposed within the hollow core; a pulsed pump radiation source arranged to generate pulsed pump radiation received by and propagating through said hollow core from an input end to an output end; Equipped with a radiation source, wherein parameters of the pulsed pump radiation, the optical fiber, and the working medium are configured to enable soliton self-compression of the pulsed pump radiation to change a spectrum of the pulsed pump radiation to form an output radiation, and further wherein a length of the optical fiber is such that the output end substantially coincides with a position at which a time extent of the output radiation is minimum. 2. A hollow-core optical fiber including a body having a hollow core; a working medium disposed within the hollow core; a pulsed pump radiation source arranged to generate pulsed pump radiation received by and propagating through said hollow core from an input end to an output end; Equipped with a radiation source, wherein parameters of the pulsed pump radiation, the optical fiber, and the working medium are configured to enable soliton self-compression of the pulsed pump radiation to change a spectrum of the pulsed pump radiation to form output radiation, and further wherein a length of the optical fiber is such that the output end substantially coincides with a position at which a spectral width of the output radiation is maximized. 3. The radiation source according to item 1 or 2, wherein the length of the optical fiber is such that the output end substantially coincides with a first minimum in the time range of the pulsed pump radiation. 4. A radiation source according to any of clauses 1 to 3, wherein the pulse duration of the input pulsed pump radiation is greater than 50 fs, and optionally the pulse duration of the input pulsed pump radiation is less than or equal to 400 fs. 5. A radiation source according to any of clauses 1 to 4, wherein the pulse energy of the input pulsed pump radiation is less than 1 μJ, and optionally the pulse energy of the input pulsed pump radiation is 0.01 μJ or more. 6. A hollow-core optical fiber including a body having a hollow core; a working medium disposed within the hollow core; a pulsed pump radiation source arranged to generate pulsed pump radiation received by and propagating through said hollow core from an input end to an output end; Equipped with a radiation source, wherein parameters of the pulsed pump radiation, the optical fiber and the working medium are configured to enable soliton self-compression of the pulsed pump radiation to change a spectrum of the pulsed pump radiation to form an output radiation, and further wherein a pulse duration of the input pulsed pump radiation exceeds 50 fs, and optionally a pulse duration of the input pulsed pump radiation is less than or equal to 400 fs. 7. A hollow-core optical fiber including a body having a hollow core; a working medium disposed within the hollow core; a pulsed pump radiation source arranged to generate pulsed pump radiation received by and propagating through said hollow core from an input end to an output end; Equipped with a radiation source, wherein parameters of the pulsed pump radiation, the optical fiber and the working medium are configured to enable soliton self-compression of the pulsed pump radiation to change a spectrum of the pulsed pump radiation to form an output radiation, and further wherein a pulse energy of the input pulsed pump radiation is less than 1 μJ, and optionally a pulse energy of the input pulsed pump radiation is less than 0.01 μJ or more. 8. A radiation source according to any one of items 1 to 7, wherein the soliton order of the input pulsed pump radiation is less than 20. 9. The radiation source according to any of clauses 1 to 8, wherein the working medium is configured to generate anomalous dispersion, the working medium being configured to generate anomalous dispersion at least at a wavelength of the pulsed pump radiation. 10. A radiation source according to any of clauses 1 to 9, wherein the hollow-core optical fibre comprises a cladding portion surrounding the hollow core, the cladding portion comprising a plurality of anti-resonant elements for guiding radiation through the hollow core. 11. The radiation source according to claim 10, wherein the plurality of anti-resonant elements of the cladding portion are arranged in a ring structure around the hollow core. 12. The radiation source according to clause 10 or 11, wherein the plurality of anti-resonant elements are arranged such that each of the anti-resonant elements is not in contact with any of the other anti-resonant elements. 13. The radiation source according to any one of claims 1 to 12, wherein the working medium comprises a noble gas. 14. The radiation source according to any one of claims 1 to 13, wherein the working medium comprises a molecular gas. 15. A metrology apparatus for determining a parameter of interest of a structure on a substrate, comprising: A radiation source according to any one of items 1 to 14, an illumination subsystem for illuminating the structure on the substrate using broadband output radiation; a detection subsystem for detecting a portion of the radiation scattered and / or reflected by the structure and for determining the parameter of interest from the portion of the radiation; A measuring device comprising: 16. A lithographic apparatus comprising the metrology apparatus according to item 15. 17. A method for selecting an operating regime for a radiation source, comprising the steps of: The radiation source comprises: a hollow core optical fiber including a body having a hollow core; a working medium disposed within the hollow core; a pulsed pump radiation source arranged to generate pulsed pump radiation received by and propagating through said hollow core from an input end to an output end; Equipped with The method comprises: selecting parameters of one or more of the pulsed pump radiation, the optical fiber, and the working medium to enable soliton self-compression of the pulsed pump radiation to change a spectrum of the pulsed pump radiation to form output radiation; Further, the parameters include a length of the optical fiber, a position of the output end: a position where the time range of the output emission is less than a first threshold value; and / or a position where the spectral width of the output radiation is greater than a second threshold; The method of claim 1, wherein the length is selected to be substantially equal to 18. The method of claim 17, wherein parameters of the optical fiber are fixed and parameters of the pulsed pump radiation and / or the working medium are selected. 19. The parameter is such that the length of the optical fiber is such that the output end is at the following position: a position where the time extent of said output emission is minimal; and / or a position at which the spectral width of the output radiation is maximum; Item 18. The method of item 17, wherein the length is selected to be substantially equal to

[0161] Although specific reference may be made herein to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, including the manufacture of guidance and detection patterns for integrated optical systems, magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0162] Although specific reference may be made herein to embodiments of the invention in relation to lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object, such as a wafer (or other substrate) or a mask (or other patterning device). These apparatus may be generally referred to as lithography tools. Such lithography tools may use vacuum or ambient (non-vacuum) conditions.

[0163] Although specific reference has been made above to the use of embodiments of the invention in the context of optical lithography, the invention is not limited to optical lithography and may also be used in other applications, for example imprint lithography, where the context permits.

[0164] Although specific reference is made to a "metrology apparatus / tool / system" or an "inspection apparatus / tool / system," these terms may refer to the same or similar types of tools, apparatus, or systems. For example, an inspection or metrology apparatus that includes an embodiment of the present invention may be used to determine characteristics of structures on a substrate or wafer. For example, an inspection or metrology apparatus that includes an embodiment of the present invention may be used to detect defects in the substrate or in structures on the substrate or wafer. In such embodiments, the characteristics of interest of the structures on the substrate may relate to defects in the structure, absence of a particular portion of the structure, or the presence of an undesirable structure on the substrate or wafer.

[0165] 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 and not limiting. Thus, it will be apparent to one skilled in the art that modifications can be made to the invention as described without departing from the scope of the claims set out below.

Claims

1. a hollow core optical fiber including an input end, an output end, and a body having a hollow core; a working medium, the working medium being a gas, disposed within the hollow core; a pulsed pump radiation source arranged to generate pulsed pump radiation received by and propagating through the hollow core from the input end to the output end; Equipped with a radiation source, wherein parameters of the pulsed pump radiation, the optical fiber, and the working medium are configured to enable soliton self-compression of the pulsed pump radiation to change a spectrum of the pulsed pump radiation to form an output radiation, and further wherein a length of the optical fiber from the input end to the output end is such that the output end substantially coincides with a position at which a time extent of the output radiation is minimum.

2. a hollow core optical fiber including an input end, an output end, and a body having a hollow core; a working medium, the working medium being a gas, disposed within the hollow core; a pulsed pump radiation source arranged to generate pulsed pump radiation received by and propagating through the hollow core from the input end to the output end; Equipped with a radiation source, wherein parameters of the pulsed pump radiation, the optical fiber, and the working medium are configured to enable soliton self-compression of the pulsed pump radiation to change a spectrum of the pulsed pump radiation to form an output radiation, and further wherein a length of the optical fiber from the input end to the output end is such that the output end substantially coincides with a position at which the spectral width of the output radiation is maximized.

3. 3. A radiation source according to claim 1 or 2, wherein the length of the optical fibre is such that the output end substantially coincides with a first minimum in the time range of the pulsed pump radiation.

4. 4. A radiation source according to any preceding claim, wherein the input pulsed pump radiation has a pulse duration greater than 50 fs.

5. 5. A radiation source according to any preceding claim, wherein the pulsed pump radiation has a pulse energy of less than 1 μJ.

6. 6. A radiation source according to any preceding claim, wherein the input pulsed pump radiation has a soliton order of less than 20.

7. 7. The radiation source of any preceding claim, wherein the working medium is configured to produce anomalous dispersion.

8. 8. The radiation source according to any of claims 1 to 7, wherein the hollow-core optical fiber comprises a cladding portion surrounding the hollow core, the cladding portion comprising a plurality of anti-resonant elements for guiding radiation through the hollow core, optionally the plurality of anti-resonant elements of the cladding portion being arranged in a ring structure around the hollow core.

9. 9. The radiation source of claim 8, wherein the plurality of anti-resonant elements are arranged such that each of the anti-resonant elements is not in contact with any of the other anti-resonant elements.

10. 10. The radiation source of any of the preceding claims, wherein the working medium comprises at least one of: i) a noble gas; and ii) a molecular gas.

11. 1. A metrology apparatus for determining a parameter of interest of a structure on a substrate, comprising: A radiation source according to any one of claims 1 to 10; an illumination subsystem for illuminating the structure on the substrate using broadband output radiation; a detection subsystem for detecting a portion of the radiation scattered and / or reflected by the structure and for determining the parameter of interest from the portion of the radiation; A measuring device comprising:

12. A lithographic apparatus comprising the metrology apparatus according to claim 11.

13. 1. A method for selecting an operating regime for a radiation source, comprising the steps of: The radiation source includes: a hollow core optical fiber including an input end, an output end, and a body having a hollow core; a working medium, the working medium being a gas, disposed within the hollow core; a pulsed pump radiation source arranged to generate pulsed pump radiation received by and propagating through the hollow core from the input end to the output end; Equipped with The method comprises: selecting parameters of one or more of the pulsed pump radiation, the optical fiber, and the working medium to enable soliton self-compression of the pulsed pump radiation to change a spectrum of the pulsed pump radiation to form output radiation; Further, the parameter is a length from the input end to the output end of the optical fiber, the output end being at the following position: a position where the time extent of said output emission is minimal; and / or a position at which the spectral width of the output radiation is maximized; The method of claim 1, wherein the length is selected to be substantially equal to

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