Radiation source

Abstract

A radiation source comprises: a hollow-core optical fiber, a working medium, and a pulsed pump radiation source. The hollow-core optical fiber includes a body and a hollow core. The working medium is disposed within the hollow core. The pulsed pump radiation source is configured to generate pulsed pump radiation, which is received by the hollow core and propagates from an input end through the hollow core to an output end. Parameters of the pulsed pump radiation, the optical fiber, and the working medium are configured to allow solitons in the pulsed pump radiation to self-compress, thereby changing the spectrum of the pulsed pump radiation to form output radiation. In some embodiments, the length of the optical fiber is such that the output end substantially coincides with a location where the temporal extent of the pulsed pump radiation is minimized.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP application 19188036.8 filed on July 24, 2019, EP application 19198105.9 filed on September 18, 2019, and EP application 20151889.1 filed on January 15, 2020, which are incorporated herein by reference in their entirety. Technical Field

[0003] The present invention relates to a radiation source. The radiation source may be a supercontinuum light source and may comprise an apparatus for receiving input radiation and broadening the frequency range of the input radiation to provide (broadband) output radiation. Background Art

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

[0005] To project a pattern onto a substrate, a lithographic apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features that can be formed on the substrate. Typical wavelengths currently used are 365nm (i-line), 248nm, 193nm, and 13.5nm. Compared to lithographic apparatuses using radiation with a wavelength of, for example, 193nm, lithographic apparatuses using extreme ultraviolet (EUV) radiation with a wavelength in the range of 4 to 20nm (e.g., 6.7nm or 13.5nm) can be used to form smaller features on a substrate.

[0006] Low-k1 lithography can be used to process features with dimensions smaller than the conventional resolution limit of the lithographic apparatus. In such processes, the resolution equation can be expressed as CD = k1 × λ / NA, where λ is the wavelength of the radiation employed, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the "critical dimension" (usually the smallest feature size to be printed, but in this case half the pitch), and k1 is the empirical resolution factor. In general, the smaller k1 is, the more difficult it is to replicate a pattern on a substrate with a shape and size similar to that designed by the circuit designer in order to achieve specific electrical functionality and performance. To overcome these difficulties, complex fine-tuning steps can be applied to the lithographic projection apparatus and / or the design layout. These include, 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, sometimes also referred to as "optical and process correction") in the design layout), or other methods generally defined as "resolution enhancement techniques" (RET). Alternatively, a tight control loop for controlling the stability of the lithographic apparatus can be used to improve pattern reproduction at low k1.

[0007] In the field of lithography, a number of measurement systems may be used both within and outside the lithographic apparatus. Typically, such a measurement system may use a radiation source to illuminate a target with radiation, and a detection system operable to measure at least one property of a portion of the incident radiation scattered from the target. An example of a measurement system external to the lithographic apparatus is an inspection device or metrology device that may be used to determine properties of a pattern pre-projected onto a substrate by the lithographic apparatus. Such an external inspection device may, for example, comprise a scatterometer. Examples of measurement systems that may be provided within the lithographic apparatus include: a topography measurement system (also known as a level sensor); a position measurement system (e.g. an interferometer device) for determining the position of a mask or wafer stage; and an alignment sensor for determining the position of alignment marks. These measurement devices may use electromagnetic radiation to perform measurements.

[0008] Different types of radiation can be used to interrogate different types of properties of the pattern. Some measurement systems can use a broadband radiation source. Such a broadband radiation source can be a supercontinuum source and can include an optical fiber with a nonlinear medium through which a pulsed pump radiation beam propagates to broaden the spectrum of the radiation.

[0009] It may be 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 so as to provide (broadband) output radiation which at least partially addresses one or more problems associated with the prior art whether referred to herein or otherwise. Summary of the Invention

[0010] According to a first aspect of the present invention, there is provided a radiation source comprising: a hollow-core optical fiber comprising 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, which is received by the hollow core and propagates from an input end through the hollow core to an output end; wherein 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 to change the spectrum of the pulsed pump radiation so as to form output radiation, and further wherein the length of the optical fiber is such that the output end substantially coincides with a position where the temporal extent of the output radiation is minimized.

[0011] According to a second aspect of the present invention, there is provided a radiation source comprising: a hollow-core optical fiber comprising 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, which is received by the hollow core and propagates from an input end through the hollow core to an output end; wherein 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 to change the spectrum of the pulsed pump radiation so as to form output radiation, and further wherein the length of the optical fiber is such that the output end substantially coincides with a position where the spectral width of the output radiation is maximum.

[0012] Radiation sources according to the first and second aspects of the invention are advantageous because they allow a broadband output radiation beam to be generated at the output. This may be useful for use within a metrology apparatus, such as within a lithographic apparatus.

[0013] Some known broadband radiation sources use an arrangement that produces spectral broadening of pulsed pump radiation, but in which the parameters of the pulsed pump radiation, the optical fiber, and the working medium are configured to allow modulation instabilities to produce spectral broadening. There are a number of reasons for using modulation instabilities to produce spectral broadening. First, modulation instabilities are known to produce broadband radiation with a relatively flat intensity-wavelength distribution. Such broadband radiation sources can be referred to as white light radiation sources (due to the relatively flat spectral intensity distribution). Second, modulation instabilities can be implemented using relatively economical laser sources as pump radiation sources.

[0014] On the other hand, soliton self-compression is a mechanism for generating an output laser beam with an offset wavelength from an input pump laser beam. For example, soliton self-compression is used to generate dispersive waves with different (offset) wavelengths. In the soliton self-compression mechanism (with a relatively low soliton number), the radiation pulse can undergo significant time compression, which is accompanied by spectral broadening. Eventually, the time compression will reach a maximum level (corresponding to the minimum time range of the pulse radiation), followed by the time broadening of the radiation. This time broadening is called soliton splitting because high-order solitons split into multiple individual solitons. When (high-order) solitons propagate along a hollow-core fiber, they can oscillate between the time compression and time broadening periods. After time broadening, other effects can cause the radiation spectrum to shift. For example, self-steepening (which can accompany and help soliton self-compression) can cause optical shocks that can trigger dispersive wave emission. By adjusting the parameters of the system, a specific desired wavelength can be generated. For example, a wavelength suitable for interacting with a specific molecule can be selected and used in exploratory experiments to study the molecule. Thus, soliton self-compression is a known mechanism for generating an output radiation beam having a second, offset wavelength from an input pump laser beam having a first wavelength.

[0015] The inventors of the present invention have recognized that during soliton self-compression, before temporal broadening (and before the formation of any dispersive waves), there is a (brief) transition period during which the radiation propagating through the hollow-core fiber is broadband radiation. Furthermore, the inventors have recognized that, despite the short lifetime of this broadband radiation, by selecting the length of the fiber so that the output end substantially coincides with the position where soliton self-compression has occurred but before subsequent temporal broadening (soliton splitting) and spectral shifting, this broadband radiation can be output from the fiber, thereby providing a particularly stable broadband radiation source.

[0016] In particular, if the length of the fiber is such that the output end substantially coincides with the location where the temporal extent of the pulsed pump radiation is at a minimum, a particularly stable broadband radiation source can be provided. It should be understood that, as used herein, if the temporal extent of the pulsed pump radiation at the output end is its minimum within the fiber, then the output end will substantially coincide with the location where the temporal extent of the pulsed pump radiation is at a minimum. That is, if the temporal extent of the pulsed pump radiation has not increased (due to soliton splitting), then the output end will substantially coincide with the location where the temporal extent of the pulsed pump radiation is at a minimum. It should be understood that for a sufficiently long hollow-core fiber, as radiation propagates through the hollow-core fiber, solitons can oscillate between periods of time compression and time stretching. Between each period of time compression and time stretching, a local minimum may exist in the temporal extent of the pulsed pump radiation. If the length of the hollow-core fiber is such that the output end is located anywhere up to or including a first local minimum, then the output end will substantially coincide with the location where the temporal extent of the pulsed pump radiation is at a minimum. Maximum spectral broadening can be achieved if the output end substantially coincides with the first local minimum.

[0017] Typically, after soliton self-compression, the spectral width of the pulsed pump radiation may decrease and / or gaps in the spectrum may form (e.g., when dispersive waves are emitted). Therefore, 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 at a maximum, a particularly stable broadband radiation source can be provided. It should be understood that, as used herein, the spectral width of the pulsed pump radiation (which may alternatively be referred to as the spectral bandwidth of the pulsed pump radiation) may be the spectral width at a threshold fraction of the power density above the maximum value. For example, the spectral width of the pulsed pump radiation may be the spectral width at a power density 0.0001 above the maximum value (i.e., spanning a spectrum of 40 dB). For example, the spectral width of the pulsed pump radiation may be the spectral width at a power density 0.001 above the maximum value (i.e., spanning a spectrum of 30 dB). For example, the spectral width of the pulsed pump radiation may be the spectral width at a power density 0.01 above the maximum value (i.e., spanning a spectrum of 20 dB). For example, the spectral width of the pulsed pump radiation may be the spectral width at a power density 0.1 above the maximum value (i.e., spanning a spectrum of 10 dB). Alternatively, if the length of the optical fibre is such that the output end substantially coincides with a position where the spectrum of the output radiation is substantially continuous, then a particularly stable broadband radiation source may be provided.

[0018] Compared to chaos-driven modulation-instability systems, the broadband radiation generated by this soliton self-compression will have no shot-to-shot variations. As a result, advantageously, a stable output spectrum can be generated using a single pulse (compared to the several pulses required to produce some stability in the output beam of a modulation-instability system).

[0019] It will be appreciated that as radiation propagates through a hollow-core fiber, solitons can 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 pulsed pump radiation. In principle, the length of the fiber can be such that the output end substantially coincides with any such location of a minimum temporal extent of the pulsed pump radiation. However, the most stable output spectrum (e.g., with respect to pulse-to-pulse variations) is provided when the output end coincides with the first location of a minimum temporal extent of the pulsed pump radiation.

[0020] The length of the optical fiber can be such that the output end substantially coincides with the first local minimum of the temporal extent of the pulsed pump radiation. It will be appreciated that the location of the first local minimum of the temporal extent of the pulsed pump radiation can depend on many factors, including, for example, parameters of the optical fiber (e.g., core diameter and length of the optical fiber), the working medium (e.g., gas type and pressure), and the pulsed pump radiation (e.g., pulse energy and pulse duration). To substantially coincide with the first local minimum of the temporal extent of the pulsed pump radiation, the output end can be positioned sufficiently close to the first local minimum of the temporal extent of the pulsed pump radiation such that the compressed pulse has not yet expanded or dispersed by 200% of the temporal extent of the pulsed pump radiation. For example, the output end can be positioned sufficiently close to the first local minimum of the temporal extent of the pulsed pump radiation such that the compressed pulse has not yet expanded or dispersed by 100% of the temporal extent of the pulsed pump radiation. For example, the output end can be positioned sufficiently close to the first local minimum of the temporal extent of the pulsed pump radiation such that the compressed pulse has not yet expanded or dispersed by 50% of the temporal extent of the pulsed pump radiation. For example, the output terminal can be arranged close enough to the first local minimum of the temporal extent of the pulsed pump radiation so that the compressed pulse does not extend or spread out by more than 10% of the first local minimum of the temporal extent of the pulsed pump radiation. For example, the output terminal can be arranged close enough to the first local minimum of the temporal extent of the pulsed pump radiation so that the compressed pulse does not extend or spread out by more than 5% of the first local minimum of the temporal extent of the pulsed pump radiation.

[0021] 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, such as approximately 150 fs.

[0022] 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, such as approximately 0.4 μJ.

[0023] The input pulsed pump radiation may have any desired wavelength. In some embodiments, the input pulsed pump radiation may have a wavelength of approximately 1 μm.

[0024] According to a third aspect of the present invention, there is provided a radiation source comprising: a hollow-core optical fiber comprising 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, which is received by the hollow core and propagates from an input end through the hollow core to an output end; wherein 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 to change the spectrum of the pulsed pump radiation, and further wherein the pulse duration of the input pulsed pump radiation is greater than 50 fs.

[0025] For example, the pulse duration of the input pulsed pump radiation may be greater than 100 fs, such as approximately 150 fs.

[0026] According to a fourth aspect of the present invention, there is provided a radiation source comprising: a hollow-core optical fiber comprising 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, which is received by the hollow core and propagates from an input end through the hollow core to an output end; wherein 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 to change the spectrum of the pulsed pump radiation, and further wherein the pulse energy of the input pulsed pump radiation is less than 1 μJ.

[0027] 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, such as approximately 0.4 μJ.

[0028] The radiation sources according to the third and fourth aspects of the invention are advantageous in that they allow a broadband output radiation beam to be generated at the output. This may be useful for use within a metrology apparatus, such as within a lithographic apparatus.

[0029] Compared to known broadband radiation sources which produce spectral broadening of the pulsed pump radiation under a modulation instability regime, the radiation sources according to the third and fourth aspects of the invention are more stable, eg with respect to pulse-to-pulse variations.

[0030] The soliton order N of the input pulsed pump radiation is a convenient parameter that can be used to distinguish between conditions where modulation instability dominates spectral broadening and conditions where soliton self-compression dominates spectral broadening. When N>>20, spectral broadening is generally dominated by modulation instability, while when N<<20, spectral broadening is generally dominated by soliton self-compression.

[0031] Therefore, for arrangements 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. Consequently, in prior art arrangements where soliton self-compression is dominant, the pulse duration of the input pulsed pump radiation is typically reduced to approximately 30 fs or less. To implement such arrangements, a high-power femtosecond fiber laser or a Ti:sapphire amplifier is typically used as the pulsed pump radiation source. The laser head is relatively bulky (a femtosecond fiber laser head has dimensions of, for example, 60 × 40 × 20 cm) and in most cases requires an external controller and a water cooler. Furthermore, such lasers are relatively expensive.

[0032] The inventors of the present invention have recognized 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, by reducing the pulse energy of the input pulsed pump radiation by a factor α, the same soliton order can be achieved using a pulse duration that is increased by the factor α. This reduction in pulse energy is contrary to the teachings of the prior art, which teach the use of increased pulse energy. In the art, radiation sources using soliton self-compression are often used in research applications such as atomic or molecular spectroscopy, where it is desirable to maximize the pulse energy of the radiation source.

[0033] In any of the first, second, third or fourth aspects of the present invention, the soliton order of the input pulsed pump radiation may be less than 20.

[0034] In any one of the first, second, third or fourth aspects of the present invention, the working medium may be configured to generate anomalous dispersion, that is, the working medium may have a negative group delay dispersion parameter.

[0035] In any of the first, second, third or fourth aspects of the present invention, the hollow core optical fiber may include a cladding portion surrounding the hollow core, and the cladding portion may include a plurality of anti-resonance elements for guiding radiation through the hollow core. Each anti-resonance element of the plurality of anti-resonance elements may include a capillary.

[0036] The plurality of anti-resonance elements of the cladding portion may be arranged in a ring structure surrounding the hollow core.

[0037] The plurality of anti-resonance elements may be arranged such that each of the anti-resonance elements does not contact any of the other anti-resonance elements.

[0038] In any one of the first, second, third or fourth aspects of the present invention, the working medium may include an inert gas, for example, one or more of argon, krypton, neon, helium and xenon.

[0039] In any one of the first, second, third or fourth aspects of the present invention, the working medium may include molecular gas. For example, the working medium may include one or more of N2, O2, CH4 and SF6.

[0040] According to a fifth aspect of the present invention, there is provided a measurement apparatus for determining a parameter of interest of a structure on a substrate, the measurement apparatus comprising: a radiation source according to any one of the first, second, third or fourth aspects of the present invention; an illumination subsystem for illuminating the structure on the substrate with 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 based on the portion of the radiation.

[0041] According to a sixth aspect of the present invention, there is provided a lithographic apparatus comprising a metrology device according to the fifth aspect of the present invention.

[0042] According to a seventh aspect of the present invention, there is provided a method for selecting a working mechanism of a radiation source, the radiation source comprising: a hollow core optical fiber comprising 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, which is received by the hollow core and propagates from an input end through the hollow core to an output end; wherein the method comprises: selecting parameters of one or more of the pulsed pump radiation, the optical fiber and the working medium so as to allow solitons of the pulsed pump radiation to self-compress to change the spectrum of the pulsed pump radiation so as to form output radiation, and further, wherein the parameters are selected so that the length of the optical fiber causes the output end to substantially coincide with the following positions: the time range of the output radiation is minimum; and / or the spectral width of the output radiation is maximum.

[0043] The method according to the eighth aspect of the present invention provides a method by which radiation sources according to the first, second and third aspects of the present invention may be designed.

[0044] In an initial application of the method, the parameters of the optical fiber can be selected. Once the optical fiber is manufactured, its parameters can be determined, for example by measurements, and its constraints can be input into a second application of the method.

[0045] The parameters of the optical fiber may be fixed, and the parameters of the pulsed pump radiation and / or the working medium may be selected. This may allow the operating parameters of the pulsed pump radiation and / or the working medium to be selected when the parameters of the optical fiber are fixed (e.g., once the optical fiber is manufactured). BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0047] Figure 1depicts a schematic overview of a lithographic apparatus;

[0048] Figure 2 A schematic overview of a lithography cell is depicted;

[0049] Figure 3 Depicted is a schematic representation of overall lithography, showing the collaboration between three key technologies used to optimize semiconductor manufacturing;

[0050] Figure 4 depicts a schematic overview of a scatterometer metrology tool;

[0051] Figure 5 A schematic overview of a level sensor measurement tool is depicted;

[0052] Figure 6 depicts a schematic overview of an alignment sensor metrology tool;

[0053] Figure 7 is a schematic cross-sectional view of a hollow-core optical fiber that may form part of a radiation source according to an embodiment in a transverse plane (ie, perpendicular to the axis of the optical fiber);

[0054] Figure 8 depicts a schematic representation of a radiation source according to an embodiment for providing broadband output radiation; and

[0055] Figure 9A and Figure 9B It shows that when the second output end of the optical fiber coincides with the position where the time range of the radiation is the minimum Figure 8 Simulation of the temporal and spectral evolution of a radiation pulse within a hollow-core optical fiber of the radiation source shown;

[0056] Figure 9C Shown with Figure 9A and Figure 9B Simulation of the output spectrum of the radiation source with the same simulation parameters as shown;

[0057] Figure 10A Shown Figure 8 a simulation of the spectral evolution of a radiation pulse within the hollow-core optical fiber of the radiation source shown, which spectral evolution would be experienced if the length of the optical fiber were increased so that the second output end of the optical fiber does not coincide with the position where the temporal extent of the radiation is a minimum; and

[0058] Figure 10B Shown with Figure 10A The simulation shown is a simulation of the output spectrum of the radiation source with the same simulation parameters. DETAILED DESCRIPTION

[0059] In this document, the terms "radiation" and "beam" are used to cover all types of electromagnetic radiation, including ultraviolet radiation (e.g., having a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultraviolet radiation, e.g., having a wavelength in the range of about 5-100 nm).

[0060] The terms "reticle," "mask," or "patterning device" as used herein should 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 corresponding to the pattern to be created in a target portion of the substrate. The term "light valve" may also be used in this context. In addition to classical masks (transmissive or reflective, binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0061] Figure 1 A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA comprises an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation), a mask support (e.g., mask table) T configured to support a patterning device (e.g., mask) MA and connected to a first positioner PM configured to precisely position the patterning device MA according to certain parameters, a substrate support (e.g., wafer stage) WT configured to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to precisely position the substrate support according to certain parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0062] In operation, the illumination system IL receives a radiation beam from a radiation source SO, for example, via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof, for directing, shaping, and / or controlling the radiation. The illuminator IL may be used to condition the radiation beam B so that it has a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0063] The term "projection system" PS as used herein should be broadly interpreted as covering various types of projection systems, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, appropriate to the exposure radiation used, and / or to other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0064] The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, such as water, so as to fill the space between the projection system PS and the substrate W, which is also known as immersion lithography. More information on immersion technology is given in US6952253, which is incorporated herein by reference.

[0065] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also referred to as "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or steps for preparing a substrate W for subsequent exposure may be performed on a substrate W on one of the substrate supports WT while another substrate W on another substrate support WT is being used to expose a pattern on the other substrate W.

[0066] In addition to the substrate support WT, the lithographic apparatus LA can include a measurement stage. The measurement stage is arranged to hold sensors and / or cleaning devices. The sensors can be arranged to measure properties of the projection system PS or properties of the radiation beam B. The measurement stage can hold multiple sensors. The cleaning devices can be arranged to clean parts of the lithographic apparatus, such as a portion of the projection system PS or a portion of a system for providing immersion liquid. The measurement stage can be moved beneath the projection system PS when the substrate support WT is away from the projection system PS.

[0067] In operation, a radiation beam B is incident on a patterning device (e.g. a mask) MA held on a mask support T and is patterned by a pattern (design layout) present on the patterning device MA. After passing through the mask MA, the radiation beam B passes through a projection system PS which focuses the radiation beam onto a target portion C of a substrate W. With the aid of a second positioner PW and a position measurement system IF, the substrate support WT can be accurately moved, for example in order to position different target portions C in the path of the radiation beam B in a focused and aligned position. Similarly, a first positioner PM and possibly another position sensor ( Figure 1The patterning device MA may be precisely positioned relative to the path of the radiation beam B using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks P1, P2 are shown occupying dedicated target portions, they may be located in spaces between target portions. When the substrate alignment marks P1, P2 are located between target portions C, they are referred to as scribe-lane alignment marks.

[0068] like Figure 2 As shown, the lithography apparatus LA may form part of a lithography cell LC, sometimes also referred to as a lithography cell or (lithography) cluster, which typically also includes equipment for performing pre- and post-exposure processing on a substrate W. Conventionally, these include a spin coater SC to deposit a resist layer, a developer DE to develop the exposed resist, a cooling plate CH and a bake plate BK, e.g. for regulating the temperature of the substrate W, e.g. for regulating the dissolution of the resist layer. A substrate handler or robot RO picks up substrates W from input / output ports I / O1, I / O2, moves the substrates W between the different processing apparatuses, and transfers the substrates W to a loading bay LB of the lithography apparatus LA. The devices in the lithography cell, which are often also collectively referred to as tracks, are typically under the control of a track control unit TCU, which itself may be controlled by a monitoring system SCS, which may also control the lithography apparatus LA, e.g. via the lithography control unit LACU.

[0069] In order to ensure that substrates W exposed by the lithographic apparatus LA are correctly and consistently exposed, it is desirable to inspect the substrates to measure properties of the patterned structures, such as overlay errors between subsequent layers, line thickness, critical dimensions (CD), etc. For this purpose, an inspection tool (not shown) may be included in the lithography cell LC. If errors are detected, adjustments may be made to the exposure of subsequent substrates or to other processing steps to be performed on the substrate W, particularly if the inspection is performed before other substrates W of the same batch or lot are yet to be exposed or processed.

[0070] An inspection apparatus, which may also be referred to as a metrology apparatus, is used to determine properties of a substrate W, and in particular, how properties vary from one substrate W to another, or how properties associated with different layers of the same substrate W vary from layer to layer. Alternatively, the inspection apparatus may be configured to identify defects on the substrate W, and may, for example, be part of the lithography cell LC, or may be integrated into the lithography apparatus LA, or may even be a standalone device. The inspection apparatus may measure properties on a latent image (the image in the resist layer after exposure), or a semi-latent image (the image in the resist layer after a post-exposure bake step (PEB)), or a developed resist image (where exposed or unexposed portions of the resist have been removed), or even an etched image (after a pattern transfer step such as etching).

[0071] Typically, the patterning process in the lithography apparatus LA is one of the most critical steps in the process, which requires high precision in dimensioning and placement of structures on the substrate W. To ensure this high precision, three systems can be combined in a system such as Figure 3 In a so-called "holistic" control environment, schematically shown in FIG. One of these systems is a lithography apparatus LA that is (virtually) connected to a metrology tool MT (a second system) and a computer system CL (a third system). The key to this "holistic" environment is to optimize the collaboration between these three systems to enhance the overall process window and provide a tight control loop to ensure that the patterning performed by the lithography apparatus LA remains within the process window. The process window defines the range of process parameters (e.g., dose, focus, overlay accuracy) within which a particular manufacturing process produces a defined result (e.g., a functional semiconductor device) - typically, within this range, variations in process parameters in the lithography process or patterning process are allowed.

[0072] The computer system CL can use (part of) the design layout to be patterned to predict which resolution enhancement techniques to use and perform computational lithography simulations and calculations to determine which mask layout and lithographic equipment settings achieve the maximum total process window for the patterning process (in Figure 3 Typically, the resolution enhancement technique is set to match the patterning possibilities of the lithographic apparatus LA. The computer system CL may also be used to detect where within the process window the lithographic apparatus LA is currently operating (e.g. using input from the metrology tool MT) to predict whether resolution enhancement techniques are being employed due to, for example, suboptimal processing (e.g., in the process window). Figure 3 There is a defect (depicted by the arrow pointing to "0" in the second scale SC2).

[0073] The metrology tool MT may provide input to the computer system CL to enable accurate simulation and prediction, and may provide feedback to the lithographic apparatus LA to identify possible drift, for example, in its calibrated state. 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.

[0074] In photolithography processes, it is frequently desirable to measure the structures being created, for example for process control and verification. The tool used to perform such measurements is often referred to as a metrology tool MT. Different 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 parameters of the photolithography process to be measured by having a sensor in the pupil or a plane conjugate to the pupil of the scatterometer's objective, in which case the measurement is often referred to as pupil-based measurement, or by having a sensor in the image plane or a plane conjugate to the image plane, in which case the measurement is often referred to as image-based 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 scatterometers described above can measure gratings using light from soft X-rays and visible to near-infrared wavelengths.

[0075] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, reconstruction methods can be applied to the measurement signals to reconstruct or calculate the properties of the grating. For example, such a reconstruction can be obtained by simulating the interaction of the scattered radiation using 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 that observed from a real target.

[0076] 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 onto a target, and reflected or scattered radiation from the target is directed to a spectrometer detector, which measures the spectrum of the specularly reflected radiation (i.e., a measure of intensity as a function of wavelength). From this data, the structure or profile of the target that produced the detected spectrum can be reconstructed, for example, by rigorous coupled wave analysis and nonlinear regression or by comparison with a library of simulated spectra.

[0077] In a third embodiment, the scatterometer MT is an ellipsometer. This ellipsometer allows parameters of a photolithography process to be determined by measuring scattered radiation for each polarization state. Such a metrology device emits polarized light (such as linear, circular, or elliptical) by using, for example, appropriate polarization filters in the illumination region of the metrology device. A source suitable for the metrology device can also provide polarized radiation. Various embodiments of conventional ellipsometers are described in U.S. patent applications 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 herein by reference in their entireties.

[0078] In one embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the overlay of two misaligned gratings or periodic structures by measuring the reflection spectrum and / or an asymmetry in the detection configuration, which asymmetry is related to the degree of overlay. The two (usually overlapping) grating structures can be applied in two different layers (not necessarily continuous layers) and can be formed at substantially the same position on the wafer. The scatterometer can have a symmetrical detection configuration, such as described in the co-owned patent application EP1,628,164A, so that any asymmetry can be clearly distinguished. This provides a straightforward method for measuring misalignment in the gratings. Further examples for measuring the overlay error between two layers comprising a periodic structure when the target is measured by the asymmetry of the periodic structure can be found in PCT patent application publication number WO2011 / 012624 or in U.S. patent application US20160161863, which are incorporated herein by reference in their entirety.

[0079] Other parameters of interest may be focal length and dose. Focal length and dose may be determined simultaneously by scatterometry (or alternatively by scanning electron microscopy), as described in U.S. Patent Application US2011-0249244, the entire contents of which are incorporated herein by reference. A single structure may be used that has a unique combination of critical dimension and sidewall angle measurements for each point in the Focus Energy Matrix (FEM - also known as Focus Exposure Matrix). If these unique combinations of critical dimension and sidewall angle are available, then focal length and dose values may be uniquely determined from these measurements.

[0080] The metrology target can be a collection of composite gratings formed by a lithography process, primarily in resist, but also after, for example, an etching process. Typically, the pitch and linewidth of the structures in the gratings strongly depend on the measurement optics (particularly the NA of the optics) to be able to capture the diffraction orders from the metrology target. As previously mentioned, the diffraction signal can be used to determine the offset between two layers (also known as "overlay"), or to reconstruct at least a portion of the original grating produced by the lithography process. This reconstruction can be used to provide guidance on the quality of the lithography process and to control at least part of the lithography process. The target can have smaller sub-segments configured to mimic the dimensions of functional portions of the design layout in the target. Due to this sub-segmentation, the target will behave more similarly to the functional portions of the design layout, allowing the overall process parameter measurement to better resemble that of the functional portions of the design layout. The target can be measured in either underfill mode or overfill mode. In underfill mode, the measurement beam generates a spot that is smaller than the entire target. In overfill mode, the measurement beam generates a spot that is larger than the entire target. In this overfill mode, different targets can also be measured simultaneously, thereby determining different process parameters simultaneously.

[0081] The overall measurement quality of the lithographic parameters using a specific target is determined at least in part by the measurement recipe for measuring the lithographic parameters. The term "substrate measurement recipe" can include one or more parameters of the measurement itself, one or more parameters of the one or more patterns being measured, or both. For example, if the measurement used in the substrate measurement recipe is an optical measurement based on diffraction, the one or more parameters of the measurement can include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, etc. One of the criteria for selecting a measurement recipe can be, for example, the sensitivity of one of the measurement parameters to process variations. More examples are described in U.S. patent application US2016-0161863 and published U.S. patent application US2016 / 0370717A1, which are incorporated herein by reference in their entirety.

[0082] Figure 4 A metrology device such as a scatterometer SM1 is depicted. 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, which measures the spectrum 10 of the specularly reflected radiation (i.e., a measurement of the intensity In1 as a function of wavelength λ). From this data, the spectrum can be processed by a processing unit PU, for example by rigorous coupled wave analysis and nonlinear regression or by comparing the spectrum to the spectrum of the specularly reflected radiation. Figure 4The library of simulated spectra shown at the bottom is compared to reconstruct the structure or profile that produced the detected spectrum. Typically, for reconstruction, the general form of the structure is known, and some parameters are assumed based on knowledge of the process used to fabricate the structure, leaving only a few parameters of the structure to be determined from the scatterometry data. This scatterometer can be configured as either a normal-incidence scatterometer or an oblique-incidence scatterometer.

[0083] In photolithographic processes, it is frequently desirable to measure the structures being created, for example for process control and verification. Various tools are known for performing such measurements, including scanning electron microscopes or various forms of metrology equipment, such as scatterometers. Examples of known scatterometers typically rely on the provision of dedicated metrology targets, such as underfilled targets (targets in the form of simple gratings or overlapping gratings in different layers, which are large enough to allow the measurement beam to generate a spot smaller than the grating) or overfilled targets (whereby the illumination spot partially or completely encompasses the target). Furthermore, the use of metrology tools (e.g., angle-resolved scatterometers that illuminate underfilled targets such as gratings) allows the use of so-called reconstruction methods, in which the properties of the grating can be calculated by simulating the interaction of scattered radiation using a mathematical model of the target structure and comparing the simulation results with the measurement results. The parameters of the model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from a real target.

[0084] Scatterometers are versatile instruments that allow for the measurement of parameters of a lithographic process by having a sensor in the pupil or in a plane conjugate to the pupil of the scatterometer's objective, a measurement often referred to as pupil-based measurement, or by having a sensor in the image plane or in a plane conjugate to the image plane, in which case the measurement is often referred to as image-based 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 entireties. The aforementioned scatterometers can measure multiple targets from multiple gratings in a single image using light from soft x-rays and the visible to near-IR wave range.

[0085] A topography measurement system that can be integrated into a lithographic apparatus, a level sensor or height sensor is arranged to measure the topography of the top surface of a substrate (or wafer). A topography map (also called a height map) of the substrate can be generated based on these measurement results, which indicate the height of the substrate as a function of the position on the substrate. The height map can then be used to correct the position of the substrate during pattern transfer on the substrate so as to provide an aerial image of the pattern forming device at an appropriate focus position on the substrate. It should be understood that "height" in this article refers to the broad dimension from the plane to the substrate (also called the Z-axis). Typically, the level or height sensor performs measurements at a fixed position (relative to its own optical system), and the relative movement of the level or height sensor between the substrate and the optical system results in height measurements at positions across the substrate.

[0086] exist Figure 5 An example of a level or height sensor LS known in the art is schematically shown in FIG. Figure 5 Only the operating principle is shown. In this example, the level sensor includes an optical system comprising a projection component LSP and a detection component LSD. The projection component LSP includes a radiation source LSO that provides a radiation beam LSB, which is provided by a projection grating PGR of the projection component LSP. The radiation source LSO can be, for example, a narrowband or broadband radiation source, such as a supercontinuum light source, polarized or unpolarized, pulsed or continuous, such as a polarized or unpolarized laser beam. The radiation source LSO can 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 can also or alternatively include UV and / or IR radiation, as well as any wavelength range suitable for reflection from the substrate surface.

[0087] Projection grating PGR is a periodic grating comprising a periodic structure that causes radiation beam BE1 to have a periodically varying intensity. The periodically varying intensity radiation beam BE1 is directed toward a measurement position MLO on substrate W at an angle of incidence ANG between 0 and 90 degrees, typically between 70 and 80 degrees, relative to an axis perpendicular to the substrate surface (the Z-axis). At measurement position MLO, the patterned radiation beam BE1 is reflected by substrate W (indicated by arrow BE2) and directed toward detection device LSD.

[0088] To determine the height level at the measurement position MLO, the level sensor further comprises a detection system comprising a detection grating DGR, a detector DET, and processing means (not shown) for processing an output signal of the detector DET. The detection grating DGR may be identical to the projection grating PGR. The detector DET generates a detector output signal indicative of the received light (e.g., the intensity of the received light), such as a photodetector, or a spatial distribution of the received intensity, such as a camera. The detector DET may comprise any combination of one or more detector types.

[0089] By triangulation techniques the height level at the measurement position 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 the (oblique) angle of incidence ANG.

[0090] The projection component LSP and / or the detection component LSD may comprise further optical elements such as lenses and / or mirrors (not shown) along the path of the patterned radiation beam between the projection grating PGR and the detection grating DGR.

[0091] In one embodiment, the detection grating DGR can be omitted and the detector DET can be placed where the detection grating DGR is located. Such a configuration provides a more direct detection of the image of the projection grating PGR.

[0092] 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 or spots of measurement areas MLO covering a large measurement range.

[0093] Various height sensors of the general type are disclosed, for example, in US7265364 and US7646471, both of which are incorporated herein by reference. A height sensor that uses UV radiation instead of visible light or infrared radiation is disclosed in US2010233600A1, which is incorporated herein by reference. In WO2016102127A1, incorporated by reference, a compact height sensor is described that uses a multi-element detector to detect and identify the position of a grating image without requiring a detection grating.

[0094] The position measurement system PMS may comprise any type of sensor suitable for determining the position of the substrate support WT. The position measurement system PMS may comprise 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 system of an interferometer and an encoder. 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 the position relative to a reference, such as a metrology frame MF or a 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 a velocity or an acceleration.

[0095] The position measurement system (PMS) may include an encoder system. For example, an encoder system is known from U.S. patent application US2007 / 0058173A1 filed on September 7, 2006, which is incorporated herein by reference. The encoder system includes an encoder head, a grating, and a sensor. The encoder system can receive a primary radiation beam and a secondary radiation beam. The primary radiation beam and the secondary radiation beam both originate from the same radiation beam, namely the primary radiation beam. At least one of the primary radiation beam and the secondary radiation beam is generated by diffracting the primary radiation beam using a grating. If the primary radiation beam and the secondary radiation beam are generated by diffracting the primary radiation beam using a grating, the primary radiation beam needs to have different diffraction orders than the secondary radiation beam. Different diffraction orders are, for example, +1, -1, +2, and -2. 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 represents the position of the encoder head relative to the grating. One of the encoder head and the grating can be arranged on the substrate structure WT. The other of the encoder head and the grating can be arranged on the measurement frame MF or the base frame BF. For example, a plurality of encoder heads are arranged on the measurement frame MF, while the grating is arranged on the top surface of the substrate support WT. In another example, the grating is arranged on the bottom surface of the substrate support WT, and the encoder head is arranged below the substrate support WT.

[0096] The position measurement system PMS may include an interferometer system. An interferometer system may be known from, for example, U.S. Pat. No. 6,020,964, filed on July 13, 1998, which is incorporated herein by reference. The interferometer system may include a beam splitter, a reflector, a reference reflector, and a sensor. The radiation beam is split into a reference beam and a measurement beam by the beam splitter. The measurement beam propagates to the reflector and is reflected back to the beam splitter by the reflector. The reference beam propagates to the reference reflector and is reflected back to the beam splitter by the reference reflector. 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 the sensor. The sensor determines the phase or frequency of the combined radiation beam. The sensor generates a signal based on the phase or frequency. The signal represents the displacement of the reflector. In one embodiment, the reflector is connected to the substrate support WT. The reference reflector can be connected to the measurement frame MF. In one embodiment, the measurement beam and the reference beam are combined into a combined radiation beam by an additional optical component instead of a beam splitter.

[0097] In the manufacture of complex devices, many lithographic patterning steps are typically performed to form functional features in successive layers on a substrate. Therefore, a key aspect of the performance of a lithographic apparatus is the ability (by the same apparatus or a different lithographic apparatus) to correctly and accurately place the applied pattern relative to the features laid down in the previous layer. To this end, the substrate has one or more sets of marks. Each mark is a structure whose position can be measured at a later time using a position sensor (usually an optical position sensor). The position sensor may be referred to as an "alignment sensor" and the mark may be referred to as an "alignment mark". The mark may also be referred to as a measurement target.

[0098] The 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 a substrate. An example of an alignment sensor used in a current lithographic apparatus is based on a self-referencing interferometer as described in US6961116. Various enhancements and modifications of position sensors have been developed, such as those disclosed in US2015261097A1. All of these disclosed contents are incorporated herein by reference.

[0099] The mark or alignment mark may comprise a series of strips formed on or in a layer provided on the substrate or formed (directly) in the substrate. These strips may be regularly spaced and used as grating lines so that the mark can be regarded as a diffraction grating with a known spatial period (pitch). Depending on the orientation of these grating lines, the mark can be designed to allow measurement of the position along the X-axis or along the Y-axis (whose orientation is substantially perpendicular to the X-axis). Marks comprising strips arranged at +45 degrees and / or -45 degrees relative to the X- and Y-axes allow combined X- and Y-measurements to be performed using techniques as described in US2009 / 195768A, which is incorporated herein by reference.

[0100] 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 analyzed to determine the position of the mark and, therefore, the position of the substrate relative to the alignment sensor, which is fixed relative to the reference frame of the lithographic apparatus. So-called coarse and fine marks associated with different (coarse and fine) mark sizes can be provided, allowing the alignment sensor to distinguish between different periods of the periodic signal and the precise position (phase) within a period. Marks with different spacing can also be used for this purpose.

[0101] Measuring the position of the mark can also provide information about the deformation of the substrate on which the mark is provided, for example in the form of a wafer grid. The deformation of the substrate can occur by, for example, electrostatically clamping the substrate to the substrate table and / or heating the substrate when the substrate is exposed to radiation.

[0102] Figure 6 FIG6 is a schematic block diagram of an embodiment of a known alignment sensor AS, such as that described in US Pat. No. 6,961,116, which is incorporated herein by reference. A radiation source RS0 provides a radiation beam RB of one or more wavelengths, which is steered by steering optics onto a mark (such as a mark AM located on a substrate W) as an illumination spot SP. In this example, the steering optics include a spot mirror SM and an objective lens OL. The diameter of the illumination spot SP illuminating the mark AM can be slightly smaller than the width of the mark itself.

[0103] Radiation diffracted by the mark AM (in this example via the objective lens OL) is collimated into an information-bearing beam IB. The term "diffraction" is intended to include zero-order diffraction from the mark (which may be referred to as reflection). A self-referencing interferometer SRI (for example of the type disclosed in the aforementioned US Pat. No. 6,961,116) causes the beam IB to interfere with itself, after which the beam is received by a photodetector PD. Additional optics (not shown) may be included to provide separate beams in the event that the radiation source RSO generates more than one wavelength. If desired, the photodetector may be a single element, or it may comprise a plurality of pixels. The photodetector may comprise an array of sensors.

[0104] The steering optics, which in this example comprise a spot mirror SM, may also be used to block zeroth order radiation reflected from the mark so that the information-bearing beam IB comprises only higher order diffracted radiation from the mark AM (this is not necessary for the measurement, but improves the signal-to-noise ratio).

[0105] The intensity signal SI is provided to the processing unit PU. Through a combination of optical processing in block SRI and computational processing in unit PU, the values of the X- and Y-position on the substrate relative to the reference frame are output.

[0106] A single measurement of the type shown only fixes the position of the mark within a specific range corresponding to one pitch of the mark. A coarser measurement technique is used in conjunction with this to identify which period of the sine wave contains the position of the mark. The same process at coarser and / or finer levels can be repeated at different wavelengths to improve the accuracy of the mark and / or to robustly detect the mark regardless of the material from which the mark is made and the material above and / or below it. The wavelengths can be multiplexed and demultiplexed for simultaneous processing, and / or they can be multiplexed using time or frequency division.

[0107] In this example, the alignment sensor and the spot SP remain stationary while the substrate W moves. Thus, the alignment sensor can be firmly and accurately mounted on the reference frame while effectively scanning the mark AM in a direction opposite to the direction of movement of the substrate W. The movement of the substrate W is controlled by mounting the substrate W on a substrate support and a substrate positioning system that controls 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 one embodiment, one or more (alignment) marks are provided on the substrate support. Measurement of the position of the mark provided on the substrate support allows calibration of the position of the substrate support determined by the position sensor (e.g., relative to a frame to which the alignment system is connected). Measurement of the position of the alignment mark provided on the substrate allows determination of the position of the substrate relative to the substrate support.

[0108] A measurement tool MT, such as a scatterometer, a topography measurement system or a position measurement system as described above, can perform measurements using radiation originating from a radiation source. The nature of the radiation used by the measurement tool may influence the type and quality of the measurements that can be performed. For some applications, it may be advantageous to use multiple radiation frequencies to measure the substrate, for example broadband radiation may be used. A plurality of different frequencies can propagate, illuminate and scatter away from the measurement target with no or minimal interference with other frequencies. Thus, for example, different frequencies can be used to obtain more measurement data simultaneously. Different radiation frequencies can also interrogate and discover different properties of the measurement target. Broadband radiation can be used for a measurement system MT, such as, for example, a level sensor, an alignment mark measurement system, a scatterometry tool or an inspection tool. The broadband radiation source may be a supercontinuum light source.

[0109] It may be difficult to generate high-quality broadband radiation (e.g., supercontinuum radiation). One method for generating broadband radiation may be to broaden high-power narrowband or single-frequency input radiation (e.g., using nonlinear, higher-order effects). The input radiation (which may be generated using a laser) may be referred to as pump radiation. Alternatively, the input radiation may be referred to as seed radiation. In order to obtain high-power radiation for the broadening effect, the radiation may be confined to a small region, thereby achieving strongly localized high-intensity radiation. In these regions, the radiation may interact with the broadening structures and / or materials that form the nonlinear medium to produce broadband output radiation. In the high-intensity radiation region, different materials and / or structures may be used to achieve and / or improve radiation broadening by providing a suitable nonlinear medium.

[0110] In some implementations, such as the following reference Figure 8 As further discussed, methods and apparatus for broadening input radiation may use an optical fiber for confining the input radiation and for broadening the input radiation into output broadband radiation. The optical fiber may be a hollow core optical fiber and may include an internal structure to enable efficient guiding and confinement of radiation in the optical fiber. The optical fiber may be a hollow core photonic crystal fiber (HC-PCF), which is particularly suitable for achieving strong radiation confinement with high radiation intensity primarily within the hollow core of the optical fiber. The hollow core of the optical fiber may be filled with a gas that serves as a broadening medium for broadening the input radiation. Such an optical fiber and gas arrangement may be used to generate a supercontinuum radiation source. The radiation input to the optical fiber may be electromagnetic radiation, such as radiation in one or more of the infrared, visible, UV, and extreme UV spectra. The output radiation may consist of or include broadband radiation, which may be referred to herein as white light.

[0111] Some embodiments relate to novel designs of such broadband radiation sources comprising 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 antiresonant structure for confining radiation. Such optical fibers including antiresonant structures are known in the art as antiresonant fibers, tubular fibers, single-ring fibers, negative curvature fibers, or suppressed 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., a Kagome fiber).

[0112] Now refer to Figure 7 Describe examples of optical fibers used in radiation sources, Figure 7 is a schematic cross-sectional view of the optical fiber 100 in a transverse plane.

[0113] Optical fiber 100 includes an elongated body that is longer in one dimension than in the other two dimensions of optical fiber 100. This longer dimension may be referred to as an axial direction and may define the axis of optical fiber 100. The two other dimensions define a plane that may be referred to as a transverse plane. Figure 7 A cross section of the optical fiber 100 is shown in this transverse plane (ie 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.

[0114] It will be understood that the optical fiber 100 has a certain degree of flexibility, and therefore the direction of the axis is generally not uniform along the length of the optical fiber 100. Terms such as optical axis, transverse cross-section, etc. will be understood to refer to a local optical axis, a local transverse cross-section, etc. Furthermore, where components are described as cylindrical or tubular, these terms will be understood to encompass shapes that may have deformed when the optical fiber 100 is bent.

[0115] The optical fiber 100 can have any length, and it should be understood that 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 for example, the optical fiber 100 can have a length between 10 cm and 100 cm.

[0116] The optical fiber 100 includes: 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 include a body (including the cladding portion and the support portion 108) having the hollow core 102. The cladding portion includes a plurality of antiresonant elements for guiding radiation through the hollow core 102. In particular, the plurality of antiresonant elements are arranged to confine radiation propagating through the optical fiber 100 primarily within the hollow core 102 and guide the radiation along the optical fiber 100. The hollow core 102 of the optical fiber 100 can be substantially disposed in a central region of the optical fiber 100, such that the axis of the optical fiber 100 can also define the axis of the hollow core 102 of the optical fiber 100.

[0117] The cladding portion includes a plurality of antiresonant elements for guiding radiation propagating through the optical fiber 100. In particular, in this embodiment, the cladding portion includes a single ring composed of six tubular capillaries 104. Each of the tubular capillaries 104 functions as an antiresonant element.

[0118] Capillary 104 can also be referred to as tube. The cross section of capillary 104 can be circular, or can have other shapes. Each capillary 104 includes a generally cylindrical wall portion 105, which at least partially defines the hollow core 102 of optical fiber 100 and separates hollow core 102 from capillary cavity 106. It should be understood that wall portion 105 can be used as an anti-reflection Fabry-Perot resonator for propagating radiation through hollow core 102 (and which can be incident on wall portion 105 at a grazing angle of incidence). The thickness of wall portion 105 can be suitable, so as to ensure that the reflection back into hollow core 102 is generally enhanced, while the transmission into capillary cavity 106 is generally suppressed. In some embodiments, capillary wall portion 105 can have a thickness between 0.01-10.0 μm.

[0119] It should be understood that, as used herein, the term cladding portion is intended to refer to the portion of the optical fiber 100 that is used to guide radiation propagating through the optical fiber 100 (i.e., the capillaries 104 that confine the radiation within the hollow core 102). The radiation may be confined in the form of transverse modes, propagating along the axis of the optical fiber.

[0120] The support portion is generally tubular and supports the six capillaries 104 of the cladding portion. The six capillaries 104 are evenly distributed around the inner surface of the inner support portion 108. The six capillaries 104 can be described as being arranged in a generally hexagonal formation.

[0121] The capillaries 104 are arranged so that each capillary does not contact any of the other capillaries 104. Each of the capillaries 104 is in contact with the inner support portion 108 and is spaced apart from adjacent capillaries 104 in the ring structure. This arrangement can be beneficial because it can increase the transmission bandwidth of the optical fiber 100 (e.g., relative to an arrangement in which the capillaries are in contact with each other). Alternatively, in some embodiments, each of the capillaries 104 can be in contact with adjacent capillaries 104 in the ring structure.

[0122] The six capillaries 104 of the cladding portion are arranged in a ring structure around the hollow core 102. The inner surface of the ring structure of capillaries 104 at least partially defines the hollow core 102 of the optical fiber 100. The diameter of the hollow core 102 (which can be defined as the smallest dimension between opposing capillaries, as indicated by arrows 114) can be between 10 and 1000 μm. The diameter 114 of the hollow core 102 can affect the mode field diameter, impact loss, dispersion, mode number, and nonlinear properties of the hollow core optical fiber 100.

[0123] In this embodiment, the cladding portion comprises a single ring arrangement (which acts as an antiresonant element) of capillaries 104. Thus, a line from the center of the hollow core 102 to the exterior of the optical fiber 100 passes through no more than one capillary 104 in any radial direction.

[0124] It will be appreciated that other embodiments may have different anti-resonance element arrangements. These may include arrangements with multiple anti-resonance element rings and arrangements with nested anti-resonance elements. Furthermore, although Figure 7 The illustrated embodiment includes a ring of six capillaries, but in other embodiments, one or more rings including any number of antiresonant elements (e.g., 4, 5, 6, 7, 8, 9, 10, 11, or 12 capillaries) may be provided 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.

[0125] Figure 8 A radiation source 134 is depicted for providing broadband output radiation. The radiation source 134 comprises: a pulsed pump radiation source 136; an optical fiber 100 having a hollow core 102 ( Figure 7 and a working medium 126 (e.g., gas) disposed within the hollow core 102. Figure 8 The radiation source 134 includes Figure 7 Optical fiber 100 is shown, but other types of hollow core optical fibers may be used in alternative embodiments.

[0126] 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 to stretch the input radiation 122 to provide output radiation 124. The working medium 126 achieves a broadening of the frequency range of the received input radiation 122 to provide broadband output radiation 124.

[0127] Radiation source 134 also includes a reservoir 128. Optical fiber 100 is disposed within reservoir 128. Reservoir 128 may also be referred to as a housing or container. Reservoir 128 is configured to contain a working medium 126. Reservoir 128 may include one or more features known in the art for controlling, adjusting, and / or monitoring the composition of working medium 126 (which may be a gas) within reservoir 128. Reservoir 128 may include a first transparent window 130. In use, optical fiber 100 is disposed within reservoir 128 such that first transparent window 130 is located near an input end of optical fiber 100. First transparent window 130 may form part of a wall of reservoir 128. First transparent window 130 may be transparent to at least the received input radiation frequency, such that received input radiation 122 (or at least a substantial portion thereof) may be coupled into optical fiber 100 within reservoir 128. It should be understood that optical components (not shown) may be provided for coupling input radiation 122 into optical fiber 100.

[0128] The reservoir 128 includes a second transparent window 132 that forms 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 located near the output end of the optical fiber 100. The second transparent window 132 may be transparent to at least the frequencies of the broadband output radiation 124 of the device 120.

[0129] Alternatively, in another embodiment, the two opposite ends of the optical fiber 100 can be placed in different reservoirs. The optical fiber 100 can include a first end region configured to receive input radiation 122 and a second end region for outputting broadband output radiation 124. The first end region can be placed in a first reservoir including a working medium 126. The second end region can be placed in a second reservoir, wherein the second reservoir can also include a working medium 126. The function of the reservoir can be as described above with respect to Figure 8As described. 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 to allow the optical fiber 100 to be partially placed inside the reservoir and partially placed outside the reservoir so that the gas can be sealed inside the reservoir. The optical fiber 100 may also include an intermediate region that is not contained within the reservoir. For embodiments in which the optical fiber 100 is relatively long (for example, when the length is greater than 1 m), this arrangement using two separate gas reservoirs may be particularly convenient. It should be understood that for this arrangement using two separate gas reservoirs, the two reservoirs (which may include one or more features known in the art for controlling, adjusting and / or monitoring the composition of the gas within the two reservoirs) can be considered to provide a device for providing a working medium 126 within the hollow core 102 of the optical fiber 100.

[0130] In this context, a window may be transparent to a frequency if at least 50%, 75%, 85%, 90%, 95% or 99% of incident radiation of that frequency on the window is transmitted through the window.

[0131] The first transparent window 130 and the second transparent window 132 can each 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. It should be understood that the gas 126 can be contained within the reservoir 128 at a pressure that is different from the ambient pressure of the reservoir 128.

[0132] The working medium 126 may include an inert gas. The working medium 126 may include one or more of argon, krypton, neon, helium, and xenon. As an alternative to or in addition to the inert gas, the working component may include a molecular gas (e.g., H2, N2, O2, CH4, SF6). The working medium 126 may also include a mixture of two or more of argon, krypton, neon, helium, xenon, and a molecular gas (e.g., H2, N2, O2, CH4, SF6). The working medium 126 may be configured to generate anomalous dispersion, and optionally, 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.

[0133] In one implementation, the working medium 126 can be disposed within the hollow core 102 at least during the period of receiving the input radiation 122 for generating the broadband output radiation 124. It should be understood that the gas 126 can be completely or partially absent from the hollow core 102 when the optical fiber 100 is not receiving the input radiation 122 for generating the broadband output radiation.

[0134] To achieve frequency broadening, high-intensity radiation may be desirable. An advantage of having a hollow-core fiber 100 is that it can achieve high-intensity radiation by providing strong spatial confinement of radiation propagating through the fiber 100, thereby achieving a highly localized radiation intensity. The radiation intensity within the fiber 100 can be high, for example, due to a high received input radiation intensity and / or due to the strong spatial confinement of the radiation within the fiber 100. An advantage of hollow-core fibers is that they can guide radiation having a wider range of wavelengths than solid-core fibers. In particular, hollow-core fibers can guide radiation in the ultraviolet and infrared ranges.

[0135] An advantage of using a hollow core optical fiber 100 can be that most of the radiation guided within the optical fiber 100 is confined within the hollow core 102. Therefore, most of the interaction of the radiation within the optical fiber 100 is with the working medium 126, which is disposed within the hollow core 102 of the optical fiber 100. As a result, the broadening effect of the working medium 126 on the radiation can be increased.

[0136] 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.

[0137] Input radiation 122 can be coherent radiation. Input radiation 122 can be collimated radiation, which can facilitate and improve the efficiency of coupling input radiation 122 into optical fiber 100. Input radiation 122 can include a single frequency or a narrow range of frequencies. Input radiation 122 can be generated by a laser. Similarly, output radiation 124 can be collimated and / or coherent.

[0138] The broadband range of output radiation 124 can be a continuous range, including a continuous range of radiation frequencies. Output radiation 124 can include supercontinuum radiation. Continuous radiation can be beneficial for use in many applications, such as metrology applications. For example, a continuous frequency range can be used to interrogate a large number of properties. A continuous frequency range can be used, for example, to determine and / or eliminate frequency dependence of a measured property. Supercontinuum output radiation 124 can include, for example, electromagnetic radiation in a wavelength range of 100 nm to 4000 nm. The frequency range of broadband output radiation 124 can be, for example, 400 nm to 900 nm, 500 nm to 900 nm, or 200 nm to 2000 nm. Supercontinuum output radiation 124 can include white light.

[0139] The input radiation 122 provided by the pulsed pump radiation source 136 can be pulsed. The input radiation 122 can include electromagnetic radiation at one or more frequencies between 200 nm and 2 μm. The input radiation 122 can, for example, include electromagnetic radiation having a wavelength of 1.03 μm. The repetition rate of the pulsed radiation 122 can be on the order of 1 kHz to 100 MHz. The pulse energy can be on the order 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 can be between 10 fs and 10 ps, for example, 300 fs. The average power of the input radiation 122 can be between 100 mW and several 100 W. The average power of the input radiation 122 can be, for example, 20-50 W.

[0140] The broadband output radiation 124 provided by the radiation source 134 can have an average output power of at least 1 W. The average output power can be at least 5 W. The average output power can be at least 10 W. The broadband output radiation 124 can be pulsed broadband output radiation 124. The broadband output radiation 124 can have a power spectral density of at least 0.01 mW / nm across the entire wavelength band of the output radiation. The power spectral density of the broadband output radiation across the entire wavelength band can be at least 3 mW / nm.

[0141] Some embodiments relate to Figure 8 A radiation source in the form of radiation source 134 is shown, comprising a hollow core optical fiber 100; a working medium 126 disposed within the hollow core; and a pulsed pump radiation source 136 arranged to generate pulsed pump radiation 122 that is received by the hollow core and propagates through the hollow core from the input end 110 to the output end 112. In particular, some embodiments relate to such a radiation source wherein parameters of the pulsed pump radiation 122, the optical fiber 100, and the working medium 126 are configured to allow solitons of the pulsed pump radiation 122 to self-compress to alter the spectrum of the pulsed pump radiation 122 to form output radiation 124.

[0142] Some known broadband radiation sources use an arrangement that produces spectral broadening of pulsed pump radiation, but in which the parameters of the pulsed pump radiation, the optical fiber, and the working medium are configured to allow modulation instabilities to produce spectral broadening. There are many reasons for using modulation instabilities to produce spectral broadening. First, assuming a sufficient number of pulses are averaged, modulation instabilities are known to produce broadband radiation with a relatively flat intensity-wavelength distribution. Such broadband radiation sources can be referred to as white light radiation sources (due to the relatively flat spectral intensity distribution). Second, modulation instabilities can be implemented using relatively economical laser sources as pump radiation sources.

[0143] On the other hand, in the soliton self-compression mechanism, the input pulse undergoes compression in the time domain, which is accompanied by an increase in the spectral width. After soliton self-compression, the compressed pulse undergoes soliton fission, in which the pulse splits into multiple solitons. This soliton fission leads to temporal broadening of the radiated pulse and a shift in its spectrum.

[0144] In some embodiments, the length of the optical fiber 100 is such that the output end 112 substantially coincides with the location where soliton self-compression occurs but before soliton splitting sets in. This provides a particularly stable source of broadband radiation that is relatively smooth and lacks any significant gaps in its spectrum.

[0145] In some embodiments, the length of optical fiber 100 is such that output end 112 substantially coincides with a location where the temporal extent of output radiation 124 is less than a first threshold (or, alternatively, where the temporal extent of output radiation 124 is at a minimum). The first threshold can be selected so that a sufficiently broad output radiation spectrum is obtained. In practice, the compressed pulses exiting output end 112 of optical fiber 100 are temporarily broadened by second transparency window 132 due to bulk material dispersion. Consequently, the duration of the pulses of output radiation 124 after second transparency window 132 can be relatively large.

[0146] In some embodiments, the length of optical fiber 100 is such that output end 112 substantially coincides with a location where the spectral width of output radiation 124 is greater than a second threshold (or, alternatively, where the spectral width of output radiation 124 is at a maximum). The second threshold can be selected so that the spectral width is large enough to meet the bandwidth requirements of a particular application, such as a measurement sensor.

[0147] 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.

[0148] Such radiation sources are advantageous because they allow, as will now be discussed, the generation of a stable broadband output radiation beam 124 at the output end 112. Such a stable broadband output radiation beam 124 may be used within a metrology apparatus, such as a lithographic apparatus.

[0149] In the soliton self-compression mechanism (with a relatively low soliton number), the radiation pulse can undergo significant time compression, which is accompanied by spectral broadening. Eventually, the time compression will reach a maximum level (corresponding to the minimum time range of the pulse radiation), followed by the time broadening of the radiation (soliton splitting). When the (high-order) soliton propagates along the hollow core fiber, it can oscillate between the time compression and time broadening periods. After the time broadening, other effects can cause the shift of the radiation spectrum. For example, self-steepening (which can accompany and help soliton self-compression) can cause optical shocks, which can trigger dispersive wave emission. By adjusting the parameters of the system, a specific desired wavelength can be generated. For example, a wavelength suitable for interacting with a specific molecule can be selected and used in research experiments to study the molecule. Therefore, soliton self-compression is a known mechanism for generating an output radiation beam with a second offset wavelength from an input pump laser beam with a first wavelength.

[0150] The inventors have recognized that during soliton self-compression, before temporal broadening (and before any dispersive waves are formed), there is a (brief) transition period during which the radiation propagating through the hollow-core fiber is broadband radiation (i.e., having a broad, relatively flat spectrum without significant gaps in the spectral density spectrum). Furthermore, the inventors have recognized that, despite the short lifetime of this broadband radiation, by selecting the length of the fiber 100 so that the output end 112 substantially coincides with the location where soliton self-compression has occurred but before subsequent temporal broadening and spectral shifting, this broadband radiation can be output from the fiber 100 to provide a particularly stable broadband radiation source 134.

[0151] In particular, if 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 radiation is a minimum, a particularly stable broadband radiation source 134 may be provided.

[0152] Typically, after soliton self-compression, the spectral width of the pulsed radiation may decrease and / or gaps may form in the spectrum (e.g., as solitons evolve and as dispersive waves are emitted). Therefore, if the length of optical fiber 100 is such that output end 112 substantially coincides with the location where the spectral width of the radiation is maximum, a particularly stable and flat broadband radiation source may be provided. Alternatively, if the length of optical fiber 100 is such that output end 112 substantially coincides with the location where the radiation spectrum is substantially continuous, a particularly stable and flat broadband radiation source may be provided.

[0153] Compared to noise-induced modulation instability systems, the broadband radiation generated by this soliton self-compression will have essentially no shot-to-shot variation. As a result, a stable output spectrum can be generated using a single pulse. In contrast, several pulses are required to produce some stability in the output beam of a modulation instability system.

[0154] It will be appreciated that as radiation propagates through the hollow-core fiber 100, 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 fiber 100 can be such that the output end 112 substantially coincides with any such location of minimum temporal extent of the radiation. However, when the output end 112 coincides with the first location of minimum temporal extent of the radiation, the most stable (e.g., with respect to pulse-to-pulse variations) and flattest output spectrum can be provided.

[0155] Some embodiments relate to a radiation source that utilizes soliton self-compression and wherein the pulse duration of the input pulsed pump radiation 122 is greater than 50 fs, and optionally, 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 can be greater than 100 fs, such as approximately 150 fs.

[0156] Some embodiments relate to a radiation source that utilizes soliton self-compression and 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 greater than or equal to 0.1 μJ.

[0157] The soliton order N of the input pulsed pump radiation 122 is a convenient parameter that can be used to distinguish between conditions where modulation instability dominates the spectral broadening and conditions where soliton self-compression dominates the spectral broadening. The soliton order N of the input pulsed pump radiation 122 is given by:

[0158]

[0159] Where γ is the nonlinear phase (or nonlinear parameter); P p is the pump peak power of the input pulsed pump radiation 122 ; τ is the pump pulse duration of the input pulsed pump radiation 122 ; and β2 is the group velocity dispersion of the working medium 126 .

[0160] When N>>20, the spectral broadening is usually dominated by modulation instability, while when N<<20, the spectral broadening is usually dominated by soliton self-compression.

[0161] Therefore, for an arrangement using soliton self-compression, it is desirable to generate an input pulsed pump radiation 122 having a low soliton order number N. Furthermore, it can be seen from equation (1) that the soliton order of the input pulsed pump radiation 122 is proportional to the pulse duration τ of the input pulsed pump radiation 122. Therefore, in prior art arrangements where soliton self-compression is dominant, the pulse duration of the input pulsed pump radiation 122 is typically reduced to about 30 fs or less. To implement such an arrangement, a high-power femtosecond fiber laser or a Ti: sapphire amplifier, which is typically compressed, is used as the pulsed pump radiation source 136. The Ti: sapphire amplifier can generate pulsed radiation having a pulse duration of about 30 fs or less. High-power femtosecond fiber lasers typically have a pulse duration of 300 fs, so known soliton self-compression arrangements using femtosecond fiber lasers also include a system for compressing this pulse duration to, for example, 30 fs. Such a system for compressing the pulse duration can, for example, use spectral broadening based on self-phase modulation and phase compression using a chirped mirror or grating. The laser head is relatively bulky (a femtosecond fiber laser head has dimensions of, for example, 60×40×20 cm) and in most cases requires an external controller and a water cooler. Furthermore, such lasers are relatively expensive.

[0162] The inventors have realized that the soliton order of the input pulsed pump radiation can alternatively be reduced by reducing the pulse energy E of the input pulsed pump radiation. p To reduce (where E p =P p τ). For example, if all other parameters remain constant, by reducing the pulse energy of the input pulsed pump radiation by a factor α, the same soliton order can be achieved using a pulse duration that is increased by a factor α. This reduction in pulse energy is contrary to the teachings of the prior art, which use increased pulse energy. In the prior art, radiation sources using soliton self-compression are often used in research applications such as atomic or molecular spectroscopy, where it is desirable to maximize the pulse energy of the radiation source.

[0163] Now refer to 9A to 10B Discuss exemplary embodiments.

[0164] Figure 9A and Figure 9B The simulation of the temporal and spectral evolution of a radiation pulse in a hollow core fiber 100 is shown. The hollow core 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 of 150 fs, a pulse energy E of 0.4 μJ and an energy of 0.5 μm. p and a wavelength of 1030 nm. The pulse energy E pThis is about an order of magnitude lower than the pulse energy currently used in broadband sources driven by modulation instabilities. This configuration allows pumping in the anomalous dispersion regime (β2 = -6.3 fs at a pump wavelength of 1030 nm). 2 / cm). The soliton order number N=17 allows solitons of the pulsed pump radiation 122 to self-compress to change the spectrum of the pulsed pump radiation to form output radiation 124.

[0165] In the first section of the optical fiber 100, the radiation undergoes self-phase modulation 140. This is followed by soliton self-compression 142, and at a distance of about 110 cm from the first end 112 of the optical fiber 100, the temporal extent of the radiation is minimal (see Figure 9A ).like Figure 9B As shown, this soliton self-compression is accompanied by a significant broadening of the radiation spectrum 144. The second end 114 of the optical fiber 100 coincides with the position 142 where the temporal extent of the radiation is a minimum.

[0166] Figure 9C The output spectrum 146 of the radiation source 134 is shown. Also shown is the spectrum 148 of the input pulsed pump radiation 122. As can be seen, a flatness of approximately 10 dB is achieved in the 500-900 nm band. Note that the output spectrum 146 is calculated for a single shot, and the smoothness results from stability against small perturbations.

[0167] Note that the pulses used in the exemplary embodiments described above have a pulse duration τ of 150 fs, which is significantly longer than the typical pulse duration for soliton self-compression (>30 fs). Furthermore, such pulses can be readily generated by new lasers that offer pulse durations between those of Ti:sapphire amplifiers (30 fs) and high-power fiber lasers (300 fs), and have recently been made available at significantly reduced size or cost. An example of a suitable laser is the laser sold as Goji by the French company Amplitudes.Systemes.SA.

[0168] Figure 10A A simulation of the spectral evolution of a radiation pulse within the hollow core optical fiber 100 is shown if the length of the optical fiber is increased to 150 cm (ie such that the second end 114 of the optical fiber 100 no longer coincides with the position 142 where the temporal extent of the radiation is a minimum). Figure 10B The output spectrum 150 of the radiation source 134 with this increased length of optical fiber is shown. The spectrum 148 of the input pulsed pump radiation 122 is also shown.

[0169] It can be seen that the spectrum of the radiation undergoes a number of changes following soliton self-compression and associated spectral broadening 144. For example, dispersive waves 152 are emitted, and the radiation oscillates between periods of spectral compression and spectral broadening.

[0170] In addition, from Figure 10B As can be clearly seen in the output spectrum 150 in FIG. 1 , the flatness of the output spectrum is lost once the second end 114 of the optical fiber 100 no longer coincides with the location 142 where the temporal extent of the radiation is a minimum. The spectrum is no longer smooth, but has many peaks and valleys.

[0171] According to some embodiments, there is also provided an option Figure 8 The method may include: selecting the pulsed pump radiation 122 (eg, the pump pulse duration τ and / or the pulse energy E p ), one or more parameters of the optical fiber 100 (e.g., geometry, core diameter, etc.) and the working medium 126 (e.g., gas composition, pressure, etc.) so as to allow soliton self-compression and to make the output end of the optical fiber 100 substantially coincide with the position where the time range of the radiation is minimum.

[0172] In an initial application of the method, the parameters of the optical fiber 100 can be selected. Once the optical fiber 100 is manufactured, its parameters can be determined, for example, by measurement, and can be input as constraints in a second application of the method. This can allow the operating parameters of the pulsed pump radiation 122 and / or the working medium 126 to be selected when the parameters of the optical fiber 100 are fixed (e.g., once the optical fiber is manufactured).

[0173] The radiation source 134 can be configured as part of a metrology apparatus for determining a parameter of interest of a structure on a substrate. The structure on the substrate can be, for example, a photolithographic pattern applied to the substrate. The metrology apparatus can further include an illumination subsystem for illuminating the structure on the substrate. The metrology apparatus can further include a detection subsystem for detecting the portion of the radiation scattered and / or reflected by the structure. The detection subsystem can also determine a parameter of interest of the structure based on the portion of the radiation scattered and / or reflected by the structure. The parameter can be, for example, overlay, alignment, or leveling data of the structure on the substrate.

[0174] The metrology apparatus described above may form part of a metrology apparatus MT.The metrology apparatus described above may form part of an inspection apparatus.The metrology apparatus described above may be comprised in a lithographic apparatus LA.

[0175] Further embodiments are disclosed in the following numbered clauses:

[0176] 1. A radiation source comprising:

[0177] A hollow core optical fiber includes a body having a hollow core;

[0178] a working medium disposed within the hollow core; and

[0179] a pulsed pump radiation source arranged to generate pulsed pump radiation that is received by the hollow core and propagates through the hollow core from the input end to the output end;

[0180] The 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 to change the spectrum of the pulsed pump radiation to form output radiation, and further, the length of the optical fiber is such that the output end substantially coincides with the position where the temporal extent of the output radiation is minimized.

[0181] 2. A radiation source comprising:

[0182] A hollow core optical fiber includes a body having a hollow core;

[0183] a working medium disposed within the hollow core; and

[0184] a pulsed pump radiation source arranged to generate pulsed pump radiation that is received by the hollow core and propagates through the hollow core from the input end to the output end;

[0185] The 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 to change the spectrum of the pulsed pump radiation so as to form output radiation, and further, the length of the optical fiber is such that the output end substantially coincides with the position where the width of the spectrum of the output radiation is maximum.

[0186] 3. The radiation source according to any of the preceding clauses, wherein the length of the optical fiber is such that the output end substantially coincides with a first local minimum of the temporal extent of the pulsed pump radiation.

[0187] 4. The radiation source according to any of the preceding clauses, 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.

[0188] 5. The radiation source according to any of the preceding clauses, 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 greater than or equal to 0.01 μJ.

[0189] 6. A radiation source comprising:

[0190] A hollow core optical fiber includes a body having a hollow core;

[0191] a working medium disposed within the hollow core; and

[0192] a pulsed pump radiation source arranged to generate pulsed pump radiation that is received by the hollow core and propagates through the hollow core from the input end to the output end;

[0193] wherein the 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 to change the spectrum of the pulsed pump radiation, and further 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.

[0194] 7. A radiation source comprising:

[0195] A hollow core optical fiber includes a body having a hollow core;

[0196] a working medium disposed within the hollow core; and

[0197] a pulsed pump radiation source arranged to generate pulsed pump radiation that is received by the hollow core and propagates through the hollow core from the input end to the output end;

[0198] wherein the 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 to change the spectrum of the pulsed pump radiation, and further 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 greater than or equal to 0.01 μJ.

[0199] 8. The radiation source according to any of the preceding clauses, wherein the number of soliton orders of the input pulsed pump radiation is less than 20.

[0200] 9. The radiation source according to any of the preceding clauses, wherein the working medium is configured to generate anomalous dispersion, and optionally the working medium is configured to generate anomalous dispersion at least at the wavelength of the pulsed pump radiation.

[0201] 10. The radiation source of any of the preceding clauses, 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.

[0202] 11. The radiation source according to clause 10, wherein the plurality of anti-resonance elements of the cladding portion are arranged in a ring structure surrounding the hollow core.

[0203] 12. The radiation source of clause 10 or clause 11, wherein the plurality of anti-resonance elements are arranged such that each of the anti-resonance elements does not contact any of the other anti-resonance elements.

[0204] 13. The radiation source according to any of the preceding clauses, wherein the working medium comprises an inert gas.

[0205] 14. The radiation source according to any of the preceding clauses, wherein the working medium comprises a molecular gas.

[0206] 15. A metrology apparatus for determining a parameter of interest of a structure on a substrate, the metrology apparatus comprising:

[0207] Radiation sources of any of the preceding clauses;

[0208] an illumination subsystem for illuminating structures on the substrate with broadband output radiation; and

[0209] A detection subsystem is configured to detect a portion of the radiation scattered and / or reflected by the structure, and to determine a parameter of interest based on the portion of the radiation.

[0210] 16. A lithographic apparatus comprising a metrology device according to clause 15.

[0211] 17. A method for selecting an operating mechanism of a radiation source, the radiation source comprising:

[0212] A hollow core optical fiber includes a body having a hollow core;

[0213] a working medium disposed within the hollow core; and

[0214] a pulsed pump radiation source arranged to generate pulsed pump radiation that is received by the hollow core and propagates through the hollow core from the input end to the output end;

[0215] The method includes:

[0216] Parameters of one or more of the pulsed pump radiation, the optical fiber, and the working medium are selected to allow soliton self-compression of the pulsed pump radiation to change the spectrum of the pulsed pump radiation to form output radiation, and further wherein the parameters are selected such that the length of the optical fiber is such that the output end substantially coincides with the location where:

[0217] The time range of output radiation is less than a first threshold; and / or

[0218] The width of the spectrum of the output radiation is greater than a second threshold.

[0219] 18. The method according to clause 17, wherein the parameters of the optical fiber are fixed, and wherein the parameters of the pulsed pump radiation and / or the working medium are selected.

[0220] 19. A method according to clause 17, wherein the parameters are chosen so that the length of the optical fibre is such that the output end substantially coincides with:

[0221] The time span over which the output radiation is minimized; and / or

[0222] The spectral width of the output radiation is the largest.

[0223] Although specific reference may be made herein to the use of lithographic apparatus in IC manufacturing, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

[0224] Although specific reference may be made herein to embodiments of the present invention in the context of lithographic equipment, embodiments of the present invention may be used in other equipment. Embodiments of the present invention may form part of mask inspection equipment, metrology equipment, or any equipment that measures or processes objects such as wafers (or other substrates) or masks (or other patterning devices). These equipment are generally referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0225] Although specific reference has been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention is not limited to optical lithography and may be used in other applications, such as imprint lithography, where the context permits.

[0226] Although specific reference is made to a "metrology device / tool / system" or an "inspection device / tool / system," these terms may refer to the same or similar types of tools, devices, or systems. For example, an inspection or metrology device including embodiments of the present invention may be used to determine characteristics of structures on a substrate or on a wafer. For example, an inspection or metrology device including embodiments of the present invention may be used to detect defects in a substrate or a structure on a substrate or on a wafer. In such embodiments, the characteristic of interest in a structure on a substrate may relate to a defect in the structure, the absence of a particular portion of the structure, or the presence of an unwanted structure on the substrate or on the wafer.

[0227] While specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be practiced otherwise than as described. The foregoing description is intended to be illustrative and not limiting. Therefore, it will be apparent to those skilled in the art that modifications may be made to the described invention without departing from the scope of the claims set forth below.

Claims

1. A radiation source comprising: a hollow core optical fiber comprising a body having a hollow core, the hollow core optical fiber including a cladding portion surrounding the hollow core, the cladding portion including a plurality of antiresonant elements for guiding radiation through the hollow core; A working medium is disposed in the hollow core; as well as a pulsed pump radiation source arranged to generate pulsed pump radiation that is received by the hollow core and propagates through the hollow core from the input end to the output end; The parameters of the pulsed pump radiation, the optical fiber, and the working medium are configured to allow solitons of the pulsed pump radiation to self-compress to change the spectrum of the pulsed pump radiation so as to form output radiation, and further, the length of the optical fiber is such that the output end substantially coincides with the position where the temporal extent of the output radiation is minimum.

2. A radiation source comprising: a hollow core optical fiber comprising a body having a hollow core, the hollow core optical fiber including a cladding portion surrounding the hollow core, the cladding portion including a plurality of antiresonant elements for guiding radiation through the hollow core; A working medium is disposed in the hollow core; as well as a pulsed pump radiation source arranged to generate pulsed pump radiation that is received by the hollow core and propagates through the hollow core from the input end to the output end; The parameters of the pulsed pump radiation, the optical fiber, and the working medium are configured to allow solitons of the pulsed pump radiation to self-compress to change the spectrum of the pulsed pump radiation so as to form output radiation, and further, the length of the optical fiber is such that the output end substantially coincides with a position where the width of the spectrum of the output radiation is maximum.

3. The radiation source according to any of the preceding claims, wherein the length of the optical fiber is such that the output end substantially coincides with a first local minimum of the temporal extent of the pulsed pump radiation.

4. The radiation source of any one of the preceding claims, wherein the input pulsed pump radiation has a pulse duration greater than 50 fs.

5. The radiation source of claim 4, wherein the pulse duration of the input pulsed pump radiation is less than or equal to 400 fs.

6. The radiation source of any preceding claim, wherein the input pulsed pump radiation has a pulse energy of less than 1 μJ.

7. The radiation source of claim 6, wherein the pulse energy of the input pulsed pump radiation is greater than or equal to 0.01 μJ.

8. A radiation source comprising: a hollow core optical fiber comprising a body having a hollow core, the hollow core optical fiber including a cladding portion surrounding the hollow core, the cladding portion including a plurality of antiresonant elements for guiding radiation through the hollow core; A working medium is disposed in the hollow core; as well as a pulsed pump radiation source arranged to generate pulsed pump radiation that is received by the hollow core and propagates through the hollow core from the input end to the 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 to change the spectrum of the pulsed pump radiation, and further, the pulse duration of the input pulsed pump radiation is greater than 50 fs.

9. The radiation source of claim 8, wherein the pulse duration of the input pulsed pump radiation is less than or equal to 400 fs.

10. A radiation source comprising: a hollow core optical fiber comprising a body having a hollow core, the hollow core optical fiber including a cladding portion surrounding the hollow core, the cladding portion including a plurality of antiresonant elements for guiding radiation through the hollow core; A working medium is disposed in the hollow core; as well as a pulsed pump radiation source arranged to generate pulsed pump radiation that is received by the hollow core and propagates through the hollow core from the input end to the 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 to change the spectrum of the pulsed pump radiation, and further, the pulse energy of the input pulsed pump radiation is less than 1 μJ.

11. The radiation source of claim 10, wherein the pulse energy of the input pulsed pump radiation is greater than or equal to 0.01 μJ.

12. The radiation source of any preceding claim, wherein the input pulsed pump radiation has a soliton order number of less than 20.

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

14. The radiation source of claim 13, wherein the working medium is configured to generate anomalous dispersion at least at the wavelength of the pulsed pump radiation.

15. The radiation source of claim 10, wherein the plurality of antiresonant elements of the cladding portion are arranged in a ring structure surrounding the hollow core. 16 . The radiation source of claim 10 , wherein the plurality of anti-resonance elements are arranged such that each of the anti-resonance elements does not contact any of the other anti-resonance elements.

17. The radiation source of any preceding claim, wherein the working medium comprises an inert gas.

18. The radiation source of any preceding claim, wherein the working medium comprises a molecular gas.

19. A metrology apparatus for determining a parameter of interest of a structure on a substrate, the metrology apparatus comprising: A radiation source as claimed in any preceding claim; an illumination subsystem for illuminating the structure on the substrate with broadband output radiation; as well as A detection subsystem is provided for detecting a portion of the radiation scattered and / or reflected by the structure and for determining the parameter of interest based on the portion of the radiation.

20. A lithographic apparatus comprising the metrology device according to claim 19.

21. A method for selecting a working mechanism of a radiation source, the radiation source comprising: a hollow core optical fiber comprising a body having a hollow core, the hollow core optical fiber including a cladding portion surrounding the hollow core, the cladding portion including a plurality of antiresonant elements for guiding radiation through the hollow core; A working medium is disposed in the hollow core; as well as a pulsed pump radiation source arranged to generate pulsed pump radiation that is received by the hollow core and propagates through the hollow core from the input end to the output end; The method comprises: Parameters of one or more of the pulsed pump radiation, the optical fiber, and the working medium are selected to allow solitons of the pulsed pump radiation to self-compress to change the spectrum of the pulsed pump radiation to form output radiation, and further wherein the parameters are selected such that the length of the optical fiber is such that the output end substantially coincides with the location of: The time range of the output radiation is less than a first threshold; and / or The width of the spectrum of the output radiation is greater than a second threshold.

22. The method according to claim 21, wherein parameters of the optical fiber are fixed and wherein parameters of the pulsed pump radiation and / or the working medium are selected.

23. The method of claim 21 , wherein the parameters are selected such that the length of the optical fiber is such that the output end substantially coincides with the location of: The time range of the output radiation is minimized; and / or The spectrum of the output radiation has a maximum width.

Images