Broadband radiation generator

Abstract

A broadband radiation generating device, comprising a fiber disposed in a housing, the housing comprising a main body and a heat transport region configured to transport heat from the fiber to the main body, the heat transport region comprising a first medium disposed between the fiber and the main body, and in at least one radial direction from the fiber, the heat transport region further comprising a second medium disposed between the first medium and the main body over a part of the housing, the second medium including a solid and / or a liquid, the radial distance between the fiber and the second medium being 2 mm or less, the second medium forming a slit over a part of the housing, the slit extending in a direction perpendicular to the longitudinal axis of the fiber, and the slit being wider than the diameter of the fiber.

Description

Technical Field

[0001] The present invention relates to a broadband radiation generating device comprising a hollow core fiber at least partially disposed within a housing.

Background Art

[0002] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus can project a pattern (also referred to as a “design layout” or “design”) of a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).

[0003] To project a pattern onto a substrate, a lithographic apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the features that can be formed on the substrate. Representative wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm, 13.5 nm. Using a lithographic apparatus that uses extreme ultraviolet (EUV) radiation having a wavelength in the range of 4 - 20 nm, for example 6.7 nm or 13.5 nm, smaller features can be formed on the substrate than with a lithographic apparatus that uses radiation having a wavelength of 193 nm.

[0004] Using low-k1 lithography, features of dimensions smaller than the classical resolution limit of a lithographic apparatus can be processed. In such a process, the resolution equation can be expressed as CD = k1 × λ / NA. Here, λ is the wavelength of the radiation used, NA is the numerical aperture of the projection optical system of the lithographic apparatus, CD is the "critical dimension" (usually the smallest feature size to be printed, but in this case the half-pitch), and k1 is an empirical resolution factor. Generally, the smaller k1 is, the more difficult it becomes to reproduce on a substrate a pattern similar to the shape and dimensions planned by a circuit designer to achieve a particular electrical function and performance. To overcome these difficulties, advanced fine-tuning steps can be applied to the lithographic projection apparatus and / or the design layout. These include, for example, various optimizations of the design layout such as optimization of NA, customized illumination schemes, use of phase-shifting patterning devices, optical proximity correction (OPC: also called "optical and process correction") in the design layout, or other methods generally defined as "resolution enhancement techniques" (RET), but are not limited thereto. Alternatively, a strict control loop for controlling the stability of the lithographic apparatus can be used to improve the reproduction of patterns at low k1. Summary of the Invention Problems to be Solved by the Invention

[0005] In the field of lithography, many measurement systems are used both inside and outside a lithographic apparatus. Generally, such measurement systems use a radiation source that irradiates a target and a detection system operable to measure at least one characteristic of a portion of the incident radiation scattered from the target. Examples of measurement systems outside the lithographic apparatus are inspection or metrology apparatuses, which can be used to determine the characteristics of patterns previously projected onto a substrate by the lithographic apparatus. Such external inspection apparatuses can include, for example, a scatterometer. Examples of measurement systems provided within the lithographic apparatus are a topography measurement system (also called a level sensor), a position measurement system (e.g., an interferometer device) for determining the position of a reticle or a wafer stage, and an alignment sensor for determining the position of alignment marks. These measurement devices use electromagnetic radiation to perform measurements.

[0006] Different types of radiation can be used to examine different types of characteristics of a pattern. In some measurement systems, a broadband radiation source is used. Such a broadband radiation source is a supercontinuum light source and can include an optical fiber with a non-linear medium through which a pulsed pump radiation beam propagates to broaden the spectrum of the radiation.

[0007] One issue associated with the implementation of such supercontinuum light sources is to extend the lifetime of the optical fiber by reducing existing failure mechanisms. During use, damage (such as glass deposition) has been observed at the fiber input and output ends. Furthermore, in the case of hollow-core photonic crystal fibers that include anti-resonant elements or capillaries that confine and propagate radiation around and through the core, these elements have been found to exhibit permanent mechanical deformation. Both of these effects can lead to a decrease in the output power of the supercontinuum light source and ultimately a change in the output spectrum, thus having an adverse effect on performance.

[0008] It is desirable to provide an alternative device (e.g., an optical fiber) and method for use in an apparatus for receiving an input radiation, broadening the frequency range of the input radiation, and providing a (broadband) output radiation that at least partially solves one or more problems related to the prior art, whether specified herein or otherwise. **Means for Solving the Problem**

[0009] According to a first aspect of the present invention, there is provided a broadband radiation generating apparatus, the apparatus comprising a hollow core fiber at least partially disposed within a housing, the housing comprising a body and a heat transport region configured to transport heat from the hollow core fiber to the body of the housing, the heat transport region comprising a first medium disposed between the hollow core fiber and the body of the housing, and in at least one radial direction from the hollow core fiber, the heat transport region further comprising a second medium disposed between the first medium and the body of the housing over a portion of the length of the housing, the second medium comprising a solid and / or a liquid. And the radial distance between the hollow core fiber (e.g., the surface of the hollow core fiber) and the second medium (e.g., the surface of the second medium) is 2 mm or less over a part, and the second medium forms a slit over at least a part of the length of the housing, the slit extending in a direction perpendicular to the longitudinal axis of the hollow core fiber, and the slit being wider than the diameter of the hollow core fiber.

[0010] The hollow core fiber (also referred to as "fiber") may be a hollow core photonic crystal fiber (HC-PCF). In some examples, the HC-PCF can include a hollow core extending axially along the HC-PCF, an inner cladding region including a plurality of capillaries surrounding the hollow core (each of the plurality of capillaries including a wall portion), and a jacket region surrounding the inner cladding region.

[0011] As used herein, it will be understood that the inner cladding region is intended to mean the region of the fiber for guiding radiation (i.e., confining the radiation within the hollow core) that propagates through the hollow core of the fiber. In particular, the inner cladding region is arranged to mainly confine the radiation propagating through the fiber within the hollow core and to guide the radiation along the fiber.

[0012] The wall portions facing the hollow cores of the plurality of capillaries can act as anti-resonant elements. It will be understood that an anti-resonant element means an element arranged to confine radiation within the hollow core mainly by anti-resonance. An optical fiber including an anti-resonant element or structure is known in the art as an anti-resonant fiber, a tubular fiber, a single-ring fiber, a negative-curvature fiber, or an Inhibited Coupling Fibers. Various different designs of such fibers are known in the art. In particular, the term anti-resonant element is not intended to include an element (such as a kagome photonic crystal fiber, etc.) arranged to confine radiation within the hollow core mainly by creating a photonic bandgap in the cladding region.

[0013] Generally, each capillary wall portion at least partially defines the hollow core and separates the hollow core from the cavity. For example, each capillary can include a cavity separated from the hollow core by a wall. It will be understood that the wall portion can function as an anti-resonant Fabry-Perot resonator for radiation propagating through the hollow core (and incident on the wall at grazing incidence angles). The thickness of the wall portion can be appropriate to ensure that reflection to the hollow core is generally enhanced and transmission to the cavity is generally suppressed.

[0014] The fiber can be suitable for use in an apparatus that receives an input radiation and broadens the frequency range of the input radiation to provide an output radiation. A fiber that uses an anti-resonant element to guide radiation can have a transmission window (i.e., a larger transmission bandwidth) wider than that of a photonic bandgap fiber. Advantageously, such a fiber may thus be more suitable for use in an apparatus (e.g., a supercontinuum light source) that receives an input radiation and broadens the frequency range of the input radiation to provide an output radiation.

[0015] Additionally or alternatively, the fiber can be suitable for beam delivery applications (where it is not necessary to broaden the frequency range of the radiation).

[0016] The fiber can be suitable for use as a fiber that delivers light from a light source to a sensor.

[0017] The housing, also referred to as, for example, a "reservoir", "housing", "container", or "gas cell", can be used to control, condition, and / or monitor the fiber and / or materials contained therein (e.g., solids, liquids, and / or gases). As used herein, the "body" of the housing generally refers to the edge region (e.g., the outer wall) of the housing through which heat is transported by a heat transport region. The heat transported to the body of the housing is then transported away from the housing by a cooling circuit and / or one or more cooling channels.

[0018] Although not wishing to be bound by theory, the failure of a broadband radiation (e.g., supercontinuum) generation system caused by fiber damage can be at least partially affected by the temperature of the fiber material, and thus it is desirable to improve the thermal management of the fiber and / or the nonlinear medium.

[0019] In known systems, the medium that promotes heat transfer between the fiber and the housing body is generally a gas, and in many cases, the same gas that forms a non-linear medium for broadband radiation generation. In contrast to such systems, the present invention provides a heat transport region that includes a solid and / or liquid medium, which has the advantage of improved thermal conductivity compared to a gas and thus improved heat transport from the fiber. In some examples, the thermal conductivity can be improved by one or several orders of magnitude.

[0020] The heat transport region can be configured to radially transport heat from the hollow core fiber to the housing body.

[0021] Ideally, the fiber is fixed in direct contact with the second medium to provide the best possible thermal contact between the fiber and the second medium. However, fixing the fiber to a solid second medium causes stress in the fiber because the thermal expansion coefficients of the fiber and the solid medium are different, ultimately degrading the performance of the fiber. Thus, in the region of the second medium, the fiber can generally move freely due to thermal expansion (e.g., the thermal expansion of the housing and / or the fiber) and only partially contacts or does not directly contact the second medium at any given time. Generally, the fiber should not be attached to the second medium. The inventor has discovered that direct contact between the fiber and the second medium is not necessary to achieve sufficient heat transfer between the fiber and the second medium. To provide sufficient heat transfer between the fiber and the second medium, the radial distance between the fiber (e.g., the surface of the fiber) and the second medium (e.g., the surface of the second medium) over a portion of the housing that includes the second medium is preferably 1 mm or less at any given time.

[0022] Heat transfer over a non-zero distance between the fiber and the second medium is facilitated by a first medium that can include a gas and / or a liquid. For example, the first medium can include a gas (i.e., a non-linear medium, or a working medium, or a working gas) configured to generate broadband radiation. In some examples, the first medium can include helium gas, which can advantageously improve heat transfer between the fiber and the second medium as compared to a first medium consisting of only a non-linear medium. In some examples, the first medium can include a liquid. Advantageously, the liquid first medium improves heat transfer between the fiber and the second medium (as compared to a gaseous first medium) and at the same time allows for free movement of the fiber (e.g., as compared to a solid first medium).

[0023] The hollow core fiber can move freely within the first medium in at least one direction perpendicular to the longitudinal axis of the hollow core fiber.

[0024] Generally, the housing has a much larger coefficient of thermal expansion than the hollow core fiber. When the device is in a horizontal orientation, the fiber is generally fixed within the housing at both ends. For this reason, when the device is in an "off" or "cold" state, the fiber is bent in a direction perpendicular to the longitudinal axis of the fiber to compensate for the thermal expansion of the housing that can cause the fiber to straighten when the device reaches its operating temperature. In such cases, the configuration of the heat transport region needs to facilitate such thermal expansion by allowing the fiber to move in this direction.

[0025] In some examples, the radial distance between the hollow core fiber (e.g., the surface of the hollow core fiber) and the second medium (e.g., the surface of the second medium) varies by approximately + / - 1 mm in the azimuthal direction to facilitate free movement of the fiber.

[0026] In some examples, the radial distance between the hollow core fiber and the second medium is at least one radial distance. Alternatively, in the first radial direction, the first distance between the hollow core fiber and the second medium may be zero or greater than 1 mm, but in the second radial direction, the second distance between the hollow core fiber and the second medium may be less than 1 mm or zero.

[0027] In some examples, the apparatus further comprises a cooling channel, the cooling channel is outside the body, and the cooling channel is configured to cool the body during use. For example, the cooling channel may comprise a channel configured to flow a fluid (e.g., water) outside the housing as part of a cooling circuit.

[0028] In some examples, within the slit, the fiber can move freely within the slit in a direction perpendicular to the longitudinal axis of the fiber (e.g., due to thermal expansion of the housing), but the fiber is maintained in a state of sufficient adhesion to the second medium (i.e., the fiber is substantially completely surrounded by the second medium), enabling good heat transfer from the fiber. The slit may be filled with the first medium.

[0029] In some examples, the width of the slit is 2 mm or less, preferably 1 mm or less.

[0030] In some examples, a part of the length of the housing surrounds a part of the hollow core fiber, and the said part of the hollow core fiber is a part where broadband radiation generation occurs (i.e., a part or region of the hollow core fiber configured for broadband radiation generation). Since the part or region of the hollow core fiber where broadband radiation generation occurs is generally the part that reaches the highest temperature within the fiber, including the second medium in this region enables the most effective heat management compared to other parts of the fiber.

[0031] In some examples, a part of the length of the housing is at least 1 cm in length.

[0032] In some examples, a portion of the length of the housing is at least 2 cm in length, at least 5 cm in length, or at least 10 cm in length.

[0033] In some examples, a portion of the length of the housing is at least 2% of the total length of the fiber.

[0034] In some examples, a portion of the length of the housing is at least 5%, 10%, or 20% of the total length of the fiber.

[0035] In some examples, the heat transfer region contains the second medium only over a portion of the length of the housing that surrounds a portion of the hollow core fiber where broadband emission occurs (i.e., the second medium may not be present in other portions of the housing).

[0036] In some examples, the hollow core fiber is configured to hang vertically during use. For example, the housing can be arranged vertically so that the fiber hangs under gravity. In such a direction, the fiber is held at the uppermost end and the fiber can generally be placed in a position very close to the second medium (in some cases, the spatial separation is zero in at least one radial direction). In some examples, the vertically hanging fiber is at least partially in contact with the second medium. However, when hanging vertically, advantageously, it is possible to prevent the fiber from getting caught in the second medium (especially when the second medium contains a solid material). For example, it is advantageous when the second medium surrounds the fiber in all directions (i.e., in all radial directions) or surrounds more than half of the fiber.

[0037] In some examples, the hollow core fiber and the housing are arranged horizontally, for example, the fiber is clamped at both ends.

[0038] In some examples, the second medium includes one or more internal cooling channels extending in a direction parallel to the longitudinal axis of the hollow core fiber. For example, if the second medium includes a solid, the internal cooling channels may comprise holes passing through the second medium, and for example, as part of a cooling circuit, a cooling fluid (e.g., water) can be configured to flow through the holes. Alternatively, or additionally, if the second medium includes a liquid, the internal cooling channels can include pipes passing through the second medium. The pipes may be configured for the flow of a cooling fluid (e.g., water), for example, as part of a cooling circuit. In some examples, the apparatus comprises one or more internal cooling channels and cooling channels external to the body, as described herein.

[0039] In some examples, the first medium includes a gas. In some examples, the first medium can include a gas (i.e., a non-linear medium) configured to generate broadband radiation. In some examples, the first medium can include a gas mixture. In some examples, the first medium can include helium.

[0040] In some examples, the first medium includes a liquid (e.g., water, liquid gallium and / or another liquid metal, carbon disulfide (CS2), and / or tetrachloroethylene (C2Cl4)). Advantageously, the liquid first medium improves heat transfer between the fiber and the second medium (compared to a gaseous first medium) and also allows for free movement of the fiber (e.g., due to thermal expansion of the housing and / or the fiber). In some examples, both the first medium and the second medium include a liquid, such that the heat transport medium is at least partially or completely liquid in at least one radial direction between the fiber and the housing body.

[0041] In some examples, the second medium includes any of metal (e.g., steel, aluminum, invar, and / or their alloys), ceramic, and / or glass.

[0042] In some examples, in at least one radial direction, the radial distance between the hollow core fiber and the second medium is 200 μm, and preferably, the radial distance is 100 μm.

[0043] In some examples, at least one radial distance between the second medium and at least a portion of the length of the hollow core fiber is zero.

[0044] According to a second aspect of the present invention, a method for generating broadband radiation is provided. The method includes emitting input radiation from an input radiation source, receiving the input radiation by the apparatus described herein, and generating an output including broadband radiation using the apparatus.

Brief Description of the Drawings

[0045] Embodiments of the present invention are described by way of example only with reference to the accompanying schematic drawings.

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Mode for Carrying Out the Invention

[0046] As used herein, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation including ultraviolet radiation (e.g., having wavelengths of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultraviolet radiation, e.g., having wavelengths in the range of about 5 - 100 nm).

[0047] As used herein, the terms "reticle", "mask" or "patterning device" can be broadly interpreted to refer to a general patterning device that can be used to impart a patterned cross-section corresponding to a pattern to be created in a target portion of a substrate in an incident radiation beam. The term "light valve" can also be used in this context. Examples of such other patterning devices include programmable mirror arrays and programmable LCD arrays in addition to standard masks (transmission or reflection, binary, phase-shift, hybrid, etc.).

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

[0049] During operation, the illumination system IL receives a beam from the radiation source SO, for example via a beam delivery system BD. The illumination system IL may include various optical elements, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical elements, or any combination thereof, for directing, shaping, and / or controlling the radiation. The illuminator IL may be used to condition the radiation beam B such that it has a desired spatial and angular intensity distribution in its cross-section in the plane of the patterning device MA.

[0050] As used herein, the term "projection system" PS shall be construed broadly to encompass any type of projection system, including, for example, refractive optical systems, reflective optical systems, refractive reflective optical systems, anamorphic optical systems, magneto-optical systems, electro-optic systems, and / or electrostatic optical systems, or any combination thereof, depending on the exposure radiation used and other factors such as the use of an immersion liquid or a vacuum. When the term "projection lens" is used herein, this can be considered to be synonymous with the more general term "projection system" PS.

[0051] The lithographic apparatus LA may be of a type in which at least a portion of the substrate is covered by a liquid (e.g., water) having a relatively high refractive index so as to fill the gap between the projection system PS and the substrate W. This is also referred to as immersion lithography. Details of immersion techniques are described in U.S. Patent No. 6,952,253, which is incorporated herein by reference.

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

[0053] In addition to the substrate support WT, the lithographic apparatus LA may include a measurement stage. The measurement stage is arranged to hold a sensor and / or a cleaning device. The sensor can be arranged to measure the characteristics of the projection system PS or the radiation beam B. The measurement stage can hold a plurality of sensors. The cleaning device can be configured to clean a part of the lithographic apparatus, for example, a part of the projection system PS or a part of the system that provides the immersion liquid. When the substrate support WT is away from the projection system PS, the measurement stage can move under the projection system PS.

[0054] During operation, the radiation beam B is incident on a patterning device, such as a mask MA, held by the mask support MT, and is patterned by the 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 that focuses the beam onto a target portion C of the substrate W. With the aid of the second positioning device PW and the position measurement system IF, the substrate support WT can be accurately moved so that different target portions C in the path of the radiation beam B are located at focused and aligned positions. Similarly, the first positioning device PM and optionally another position sensor (not explicitly shown in FIG. 1) can be used to accurately position the patterning device MA with respect to the path of the radiation beam B. The patterning device MA and the substrate W can be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. The illustrated substrate alignment marks P1, P2 occupy dedicated target portions, but they may also be arranged in the spaces between the target portions. The substrate alignment marks P1, P2 are known as scribe lane alignment marks when they are arranged between the target portions C.

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

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

[0057] An inspection device, which may also be called a measurement device, is used to determine the characteristics of a substrate W, in particular, how the characteristics of different substrates W change, or how the characteristics related to different layers of the same substrate W change from layer to layer. Alternatively, the inspection device may be configured to identify defects on the substrate W, for example, it may be part of a litho cell LC, or may be integrated into a lithography apparatus LA, or may be a stand-alone device. The inspection device can measure the characteristics of a latent image (an image of the resist layer after exposure), or a semi-latent image (an image of the resist layer after a post-exposure bake step PEB), or a developed resist image (an image in which the exposed or unexposed portions of the resist have been removed), or an etched image (after a pattern transfer step such as etching).

[0058] Typically, the patterning process in a lithography apparatus LA is one of the most important steps in a process that requires high precision in the dimensioning and placement of structures on a substrate W. To ensure this high precision, as schematically shown in FIG. 3, three systems can be combined in a so-called "overall" control environment. One of these systems is a lithography apparatus LA that is (substantially) connected to a measurement tool MT (the second system) and a computer system CL (the third system). The key to such an "overall" environment is to optimize the cooperation between these three systems to enhance the entire process window and provide a tight control loop that ensures that the patterning performed by the lithography apparatus LA remains within the process window. The process window defines a set of process parameters (such as dose, focus, overlay) for a particular manufacturing process to produce a defined result (such as a functional semiconductor device), within which the process parameters of the lithography process or patterning process are usually permitted to vary.

[0059] Computer system CL can predict which resolution improvement technique to use using a (portion of) the patterned design layout, and perform computerized lithography simulation and calculations to determine which mask layout and lithography apparatus settings achieve the maximum overall process window for the patterning process (indicated by the double arrow of the first scale SC1 in FIG. 3). Usually, the resolution improvement technique is configured to match the patterning capabilities of the lithography apparatus LA. Computer system CL may also be used to detect where in the process window the lithography apparatus LA is currently operating (e.g., using the input from the measurement tool MT) to predict whether there are defects, for example, by second-best processing (indicated by the arrow pointing to "0" of the second scale SC2 in FIG. 3).

[0060] Measurement tool MT can provide input to computer system CL to enable accurate simulation and prediction, and provide feedback to lithography apparatus LA to identify, for example, drifts that can occur in the calibrated state of lithography apparatus LA (indicated by the multiple arrows of the third scale SC3 in FIG. 3). Next, various types of measurement tools MT for measuring one or more characteristics related to the lithography apparatus and / or the substrate to be patterned will be described.

[0061] In a lithography process, for example for process control and verification, it is desirable to frequently measure the created structures. Tools for performing such measurements are usually called metrology tools MT. Various types of metrology tools MT for performing such measurements are known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. A scatterometer enables measurement of parameters of the lithography process (in this case, the measurement is usually called image or field-based measurement) by having a sensor in a conjugate plane with the pupil or the pupil of the objective lens of the scatterometer (a measurement usually called pupil-based measurement), or by having a sensor in the image plane or a conjugate plane with the image plane, and is a highly versatile instrument. Such scatterometers and related measurement techniques are further described in Patent Application Publication No. 20100328655, US Patent Application Publication No. 2011102753, US Patent Application Publication No. 20120044470, US Patent Application Publication No. 20110249244, US Patent Application Publication No. 20110026032, or European Patent Application Publication No. 1,628,164, which are hereby incorporated by reference in their entirety. The above-described scatterometer can measure gratings using light from soft X-rays and light in the wavelength range from visible to near infrared.

[0062] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, a reconstruction method can be applied to the measured signal to reconstruct or calculate the characteristics of the diffraction grating. Such a reconstruction can result, for example, from simulating the interaction between the scattered radiation and 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 generates a diffraction pattern similar to that observed from the actual target.

[0063] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, radiation emitted from a radiation source is directed towards a target, and the reflected or scattered radiation from the target is directed towards a spectrometer detector that measures the spectrum of the specularly reflected radiation (i.e., measures the intensity as a function of wavelength). From this data, the structure or profile of the target that gives rise to the detected spectrum can be reconstructed, for example, by rigorous coupled-wave analysis and non-linear regression, or by comparison with a library of simulated spectra.

[0064] In a third embodiment, the scatterometer MT is an ellipsometric scatterometer. An ellipsometric scatterometer makes it possible to determine the parameters of a lithography process by measuring the scattered radiation for each polarization state. Such a measuring device emits polarized light (such as linearly, circularly, or elliptically polarized light), for example, by using a suitable polarization filter in the illumination section of the measuring device. A radiation source suitable for the measuring device may also provide polarized radiation. Various embodiments of existing ellipsometric scatterometers are described in U.S. Patent Applications 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, the entire contents of which are incorporated herein by reference.

[0065] In one embodiment of the scatterometer MT, the scatterometer MT is configured to measure the overlay of two misaligned gratings or periodic structures by measuring the reflection spectrum and / or the asymmetry of the detection configuration. The asymmetry is related to the degree of overlay. The two (typically overlapping) grating structures may be applied to two different layers (not necessarily contiguous layers) or may be formed at substantially the same location on the wafer. The scatterometer can have a symmetric detection configuration so that the asymmetry can be clearly distinguished, as described, for example, in co-owned patent application EP1,628,164A. This allows for easy measurement of the misalignment of the diffraction grating. Further examples for measuring the overlay error between two layers including a periodic structure as a target measured via the asymmetry of the periodic structure can be found in International Patent Application Publication No. 2011 / 012624 or U.S. Patent Application Publication No. 20160161863, which are hereby incorporated by reference in their entirety.

[0066] Other parameters of interest may be focus and dose. Focus and dose can be determined simultaneously by a scatterometer (or alternatively by a scanning electron microscope) as described in U.S. Patent Application US2011-0249244, which is hereby incorporated by reference in its entirety. A single structure may be used, which has a unique combination of measured critical dimensions and sidewall angles for each point of a focus energy matrix (FEM, also called a focus exposure matrix). If these unique combinations of critical dimensions and sidewall angles are available, the focus and dose values can be uniquely determined from these measurements.

[0067] The measurement target may be an aggregate of composite gratings, which are often formed in a resist by a lithography process, but may also be formed, for example, after an etching process. Typically, the pitch and linewidth of the structures within the grating strongly depend on the measurement optical system (especially the NA of the optical system) capable of capturing the diffraction orders coming from the measurement target. As shown previously, the diffracted signal may be used to determine the shift (also referred to as "overlay") between two layers, or may be used to reconstruct at least a portion of the original grating generated by the lithography process. This reconstruction may be used to provide an indication of the quality of the lithography process and may also be used to control at least a part of the lithography process. The target may have smaller sub-segmentations (sub-divisions) configured to mimic the dimensions of the functional part of the design layout within the target. Due to this sub-segmentation, the target will behave more similarly to the functional part of the design layout such that the measurement results of the overall process parameters will closely resemble those of the functional part of the design layout. The target may be measured in underfill mode or overfill mode. In underfill mode, the measurement beam generates a spot smaller than the entire target. In overfill mode, the measurement beam generates a spot larger than the entire target. In such an overfill mode, it may be possible to measure different targets simultaneously, and thus determine different process parameters simultaneously.

[0068] The overall measurement quality of the lithography parameters using a particular target is at least partially determined by the measurement recipe used for the measurement of these lithography parameters. The term "substrate measurement recipe" may include one or more parameters of the measurement itself, or the one It may include one or more parameters of the above patterns, or may include both. For example, when the measurement used in a certain substrate measurement recipe is a diffraction-based optical measurement, one or more parameters of the measurement may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation on the substrate, the direction of the radiation with respect to the pattern on the substrate, etc. One of the criteria for selecting a measurement recipe may be, for example, the sensitivity of one measurement parameter to process variations. More examples are described in U.S. Patent Application Publication No. 2016-0161863 and Published U.S. Patent Application Publication 2016 / 0370717, the entire contents of which are incorporated herein by reference.

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

[0070] In a lithography process, it is desirable to frequently measure the fabricated structures, for example, for process control and verification. Various tools for such measurements are known, such as various forms of measurement devices like scanning electron microscopes and scatterometers. Examples of known scatterometers often rely on the provision of dedicated measurement targets such as underfilled targets (targets in the form of simple gratings or gratings with overlapping different layers, of a size sufficient to generate a spot smaller than the grating for the measurement beam) or overfilled targets (where the illumination spot partially or completely encompasses the target). Further, when using a measurement tool such as an angular - resolved scatterometer that illuminates an underfilled target such as a grating, a so - called reconstruction method can be used. In this reconstruction method, the characteristics of the grating can be calculated by simulating the interaction between the scattered radiation and a mathematical model of the target structure and comparing the simulation results with the measurement results. The parameters of the model are adjusted until a diffraction pattern similar to the diffraction pattern observed from the actual target is generated by the simulated interaction.

[0071] A scatterometer is a multi - purpose instrument that enables the measurement of parameters of a lithography process by placing a sensor at the pupil of the objective lens of the scatterometer or at a plane conjugate to the pupil (this measurement is usually called pupil - based measurement), or by placing the sensor at the image plane or at a plane conjugate to the image plane (in this case, the measurement is usually called image - based or field - based measurement). Such scatterometers and related measurement techniques are further described in U.S. Patent Application Publication No. 20100328655, U.S. Patent Application Publication No. 2011102753, U.S. Patent Application Publication No. 20120044470, U.S. Patent Application Publication No. 20110249244, U.S. Patent Application Publication No. 20110026032, or European Patent Application Publication No. 1,628,164, the entire contents of which are incorporated herein by reference. The aforementioned scatterometer can measure multiple targets from multiple gratings in one image using light in the wavelength range from soft X - rays to near - infrared light from visible light.

[0072] A topography measurement system, a level sensor, or a height sensor can be integrated into a lithographic apparatus and is arranged to measure the topography of the upper surface of a substrate (or wafer). A map of the topography of the substrate, also referred to as a height map, can be generated from these measurements that indicate the height of the substrate as a function of the position on the substrate. This height map can then be used to correct the position of the substrate during the transfer of the pattern onto the substrate in order to provide a spatial image of the patterning device at the proper focus position on the substrate. In this context, it will be understood that "height" refers generally to a dimension out of the plane with respect to the substrate (also referred to as the Z-axis). Typically, the level or height sensor performs the measurement at a fixed location (with respect to its optical system), and the relative movement between the substrate and the optical system of the level or height sensor results in height measurements at each location across the substrate.

[0073] An example of a level or height sensor LS known in the art is schematically shown in FIG. 5. This figure only illustrates the principle of operation. In this example, the level sensor comprises an optical system including a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO that provides a radiation beam LSB imparted by a projection grating PGR of the projection unit LSP. The radiation source LSO can be, for example, a narrow-band or wide-band radiation source such as a supercontinuum light source, polarized or non-polarized, pulsed or continuous, such as a polarized or non-polarized laser beam. The radiation source LSO can include a plurality of radiation sources having different colors or wavelength ranges, such as a plurality of LEDs. The radiation source LSO of the level sensor LS is not limited to visible radiation and can additionally or alternatively include UV and / or IR radiation and any range of wavelengths suitable for reflection from the surface of the substrate.

[0074] The projection grating PGR is a periodic grating with a periodic structure that provides a radiation beam BE1 having a periodically varying intensity. The radiation beam BE1 having a periodically varying intensity is directed towards the measurement location MLO on the substrate W at an incident angle ANG of 0 degrees to 90 degrees, typically 70 degrees to 80 degrees, with respect to an axis (Z-axis) perpendicular to the incident substrate surface. At the measurement location MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and directed towards the detection unit LSD.

[0075] To determine the height level at the measurement location MLO, the level sensor further comprises a detection system including a detection grating DGR, a detector DET, and a processing unit (not shown) for processing the output signal of the detector DET. The detection grating DGR may be identical to the projection grating PGR. The detector DET generates a detector output signal indicating the received light, for example, indicating the intensity of the received light like a photodetector, or representing the spatial distribution of the intensity of the received light like a camera. The detector DET may comprise any combination of one or more detector types.

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

[0077] The projection unit LSP and / or the detection unit LSD may include additional 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.

[0078] In one embodiment, the detection grating DGR may be omitted, and the detector DET may be installed at the position where the detection grating DGR was located. Such a configuration provides a more direct detection of the image of the projection grating PGR.

[0079] To effectively cover the surface of the substrate W, the level sensor LS may be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating an array of measurement regions MLO or spots that cover a larger measurement range.

[0080] Various height sensors of a general type are disclosed, for example, in U.S. Patent No. 7,265,364 and U.S. Patent No. 7,646,471, which are incorporated by reference. A height sensor using UV radiation instead of visible or infrared radiation is disclosed in U.S. Patent Application Publication No. 2010 / 233,600, which is incorporated by reference. International Publication No. 2016 / 102,127, which is incorporated by reference, describes a small height sensor that uses a multi-element detector to detect and recognize the position of a grating image without the need for a detection grating.

[0081] The position measurement system PMS may include any type of sensor suitable for determining the position of the substrate support WT. The position measurement system PMS may include any type of sensor suitable for determining the position of the mask support MT. The sensor may be an optical sensor such as an interferometer or an encoder. The position measurement system PMS may include a system that combines an interferometer and an encoder. The sensor may be another type of sensor such as a magnetic sensor, a capacitance sensor, or an inductive sensor. The position measurement system PMS may determine the position relative to a reference, such as a measurement 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 the time derivative of the position, such as velocity or acceleration.

[0082] The position measurement system PMS may include an encoder system. The encoder system is known, for example, from U.S. Patent Application Publication No. 2007 / 0058173, filed on September 7, 2006 and 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. Both the primary radiation beam and the secondary radiation beam are generated from the same radiation beam, i.e., the original radiation beam. At least one of the primary radiation beam and the secondary radiation beam is generated by diffracting the original radiation beam with the grating. When both the primary radiation beam and the secondary radiation beam are generated by diffracting the original radiation beam with the grating, the primary radiation beam needs to have a different diffraction order from the secondary radiation beam. Different diffraction orders are, for example, +1st order, -1st order, +2nd order, and -2nd order. The encoder system optically combines the primary radiation beam and the secondary radiation beam into a combined radiation beam. The 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 disposed on the substrate structure WT. The other of the encoder head and the grating can be disposed on the measurement frame MF or the base frame BF. For example, a plurality of encoder heads are disposed on the measurement frame MF, while the grating is disposed on the upper surface of the substrate support WT. In another example, the grating is disposed on the bottom surface of the substrate support WT, and the encoder head is disposed below the substrate support WT.

[0083] The position measurement system PMS can include an interferometer system. The interferometer system is known, for example, from U.S. Patent No. 6,020,964, filed July 13, 1998, which is incorporated herein by reference. The interferometer system may include a beam splitter, mirrors, a reference mirror, and a sensor. The radiation beam is split by the beam splitter into a reference beam and a measurement beam. The measurement beam propagates to the mirror and is reflected by the mirror back to the beam splitter. The reference beam propagates to the reference mirror and is reflected by the reference mirror back to the beam splitter. At the beam splitter, the measurement beam and the reference beam are combined to form 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 mirror. In one embodiment, the mirror is connected to the substrate support WT. The reference mirror may be connected to the metrology frame MF. In one embodiment, the measurement beam and the reference beam are combined by additional optical components instead of the beam splitter to form a combined radiation beam.

[0084] In the manufacture of complex devices, typically a number of lithographic patterning steps are performed, whereby functional features are formed in successive layers on a substrate. Thus, an important aspect of the performance of a lithographic apparatus is the ability to place the imparted pattern correctly and precisely with respect to features defined in previous layers (by the same apparatus or a different lithographic apparatus). For this purpose, one or more sets of marks are provided on the substrate. Each mark is a structure whose position can later be measured by a position sensor, typically an optical position sensor. The position sensor is called an "alignment sensor" and the mark may be called an "alignment mark". The mark may also be called a metrology target.

[0085] A lithographic apparatus can include one or more (e.g., multiple) alignment sensors, thereby enabling accurate measurement of the position of alignment marks provided on a substrate. The alignment (or position) sensor can obtain position information from alignment marks formed on the substrate using optical phenomena such as diffraction or interference. An example of an alignment sensor used in current lithographic apparatuses is based on a self-referencing interferometer described in U.S. Patent No. 6,961,115. Various extensions and modifications of the position sensor have been developed, as disclosed, for example, in U.S. Patent Publication No. 2015 / 261097A1. The content of all these published documents is incorporated herein by reference.

[0086] A mark, or alignment mark, can be composed of a series of bars formed on or within a layer provided on the substrate or (directly) within the substrate. The bars are regularly spaced and function as grid lines, so the mark can be regarded as a diffraction grating having a well-known spatial period (pitch). Depending on the orientation of these grid lines, the mark can be designed to enable measurement of the position along the X-axis or the position along the Y-axis (oriented substantially perpendicular to the X-axis). A mark including bars arranged at +45 degrees and / or -45 degrees with respect to both the X-axis and the Y-axis enables combined measurement of the X-axis and the Y-axis using the technique described in U.S. Patent Application Publication No. 2009 / 195768, which is incorporated herein by reference.

[0087] The alignment sensor optically scans each mark using a radiation spot and acquires a periodically varying signal such as a sine wave. The phase of this signal is analyzed to determine the position of the mark and hence the position of the substrate relative to the alignment sensor. The alignment sensor is fixed relative to the reference frame of the lithographic apparatus. Sometimes so-called coarse and fine marks are provided, related to different (coarse) and fine mark dimensions, so that the alignment sensor can distinguish between different cycles of the periodic signal and the exact position (phase) within a cycle. For this purpose, marks with different pitches can also be used.

[0088] When measuring the position of the mark, information about deformation of the substrate on which the mark is provided (e.g., in the form of a wafer grid) is also obtained. Substrate deformation can occur, for example, by electrostatically clamping the substrate to the substrate table or by heating of the substrate when it is exposed to radiation.

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

[0090] The radiation diffracted by mark AM is collimated into information-bearing beam IB (in this example via objective lens OL). The term "diffracted" is intended to include the zero-order diffraction from the mark (which can be called reflection). For example, a self-referencing interferometer SRI of the type disclosed in U.S. Patent No. 6,961,116 above interferes beam IB with itself, and then the beam is received by photodetector PD. If two or more wavelengths are generated by radiation source RSO, an additional optical system (not shown) may be included to provide separate beams. The photodetector can be a single element or, if desired, can include many pixels. The photodetector can include a sensor array.

[0091] In this example, an inductive optical system including spot mirror SM can function to block the zero-order radiation reflected from the mark, so that information-bearing beam IB includes only the higher-order diffracted radiation from mark AM (this is not essential for measurement but improves the signal-to-noise ratio).

[0092] An intensity signal SI is supplied to processing unit PU. A combination of the optical processing in block SRI and the computational processing in unit PU outputs values of the X and Y positions on the substrate relative to a reference frame.

[0093] A single measurement of the type shown only fixes the position of the mark within a specific range corresponding to one pitch of the mark. Associated with this, a coarser measurement technique is used to identify the period of the sine wave including this mark position. Regardless of the material on which the mark is fabricated and the materials provided under and / or above the mark, the same process can be repeated at various wavelengths, at a coarse level and / or a fine level, for improved accuracy and / or robust detection of the mark. The wavelengths can be optically multiplexed and demultiplexed to be processed simultaneously, and / or multiplexed by time division or frequency division.

[0094] In this example, the alignment sensor and the spot SP remain fixed, and it is the substrate W that moves. Therefore, while the alignment sensor is firmly and highly accurately mounted on the reference frame, it can scan the mark AM substantially in the direction opposite to the moving direction of the substrate W. This movement of the substrate W is controlled by mounting the substrate W on the substrate support and by the substrate positioning system controlling the movement of the substrate support. A substrate support position sensor (e.g., an interferometer) measures the position of the substrate support (not shown). In one embodiment, one or more (alignment) marks are provided on the substrate support. By measuring the position of the marks provided on the substrate support, the position of the substrate support determined by the position sensor can be calibrated (e.g., with respect to the frame to which the alignment system is connected). By measuring the position of the alignment marks provided on the substrate, the position of the substrate with respect to the substrate support can be determined.

[0095] Measurement tools MT such as the above-mentioned scatterometer, topography measurement system, or position measurement system can use radiation generated from a radiation source to perform measurements. The characteristics of the radiation used in the measurement tool can affect the type and quality of the measurements that can be performed. In some applications, it may be advantageous to use multiple radiation frequencies to measure the substrate. For example, broadband radiation can be used. Multiple different frequencies may be able to propagate, irradiate, and scatter the measurement target without or with minimal interference with other frequencies. Therefore, for example, different frequencies can be used to simultaneously acquire more measurement data. Different radiation frequencies may also be able to investigate and discover different characteristics of the measurement target. Broadband radiation can be useful in measurement systems MT such as, for example, level sensors, alignment mark measurement systems, scatterometry tools, or inspection tools. The broadband radiation source can be a supercontinuum light source.

[0096] High-quality broadband radiation, such as supercontinuum radiation, can be difficult to generate. One way to generate broadband radiation is to broaden a high-power narrowband or single-frequency input radiation, for example, by utilizing non-linear higher-order effects. The input radiation (which can be generated using a laser) may be referred to as pump radiation. To obtain high-power radiation for broadening the effect, the radiation can be confined to a small area to achieve strongly localized high-intensity radiation. In these areas, the radiation can interact with an extended structure and / or material that forms a non-linear medium to generate broadband output radiation. In the high-intensity radiation area, different materials and / or structures can be used to enable and / or improve the broadening of the radiation by providing an appropriate non-linear medium.

[0097] In some implementations, as further described below with reference to FIGS. 9 to 11, the method and apparatus for broadening the input radiation may use a fiber to confine the input radiation and to broaden the input radiation to output broadband radiation. The fiber can be a hollow-core fiber and can include an internal structure to achieve effective guiding and confinement of the radiation within the fiber. The fiber can be a hollow-core photonic crystal fiber (HC-PCF). This is particularly suitable for strong radiation confinement mainly inside the hollow core of the fiber and realizes high radiation intensity. The hollow core of the fiber can be filled with a gas that functions as an expanding medium for broadening the input radiation. Such an arrangement of the fiber and the gas can be used to create a supercontinuum radiation source. The radiation input to the fiber can be electromagnetic radiation, for example, one or more radiations in the infrared, visible, UV, and extreme UV spectra. The output radiation can consist of or include broadband radiation, which may be referred to herein as white light.

[0098] Embodiments of the present invention relate to a new design of an optical fiber used in such a broadband radiation source and a broadband radiation source comprising such a new optical fiber. This new optical fiber is a hollow-core photonic crystal fiber (HC-PCF). In particular, this new optical fiber can be a type of hollow-core photonic crystal fiber that includes an anti-resonant structure for confining radiation. Such a fiber including an anti-resonant structure is known in the art as an anti-resonant fiber, a tubular fiber, a single-ring fiber, a negative-curvature fiber, or an inhibited coupling fiber. Various different designs of such fibers are known in the art.

[0099] It will be understood that the anti-resonant element mainly means an element arranged to confine radiation in the hollow core mainly by anti-resonance. In particular, the term anti-resonant element is not intended to include an element (such as a kagome photonic crystal fiber) arranged to confine radiation in the hollow core mainly by creating a photonic bandgap in the cladding portion. A pure photonic bandgap fiber provides very low loss over a very limited bandwidth. An optical fiber that uses an anti-resonant element to guide radiation can have a wider transmission window (i.e., a larger transmission bandwidth) than a photonic bandgap fiber. Advantageously, such a fiber may thus be more suitable for use in a device (such as a supercontinuum source) that receives input radiation and broadens the frequency range of the input radiation to provide output radiation.

[0100] FIG. 7 and FIG. 8 are schematic cross-sectional views of an example of a hollow-core fiber (i.e., a hollow-core optical fiber) 100 in two planes orthogonal to each other (the x-y plane and the y-z plane, respectively).

[0101] The optical fiber 100 includes an elongated body having one dimension that is longer compared to the other two dimensions of the optical fiber 100. This longer dimension is referred to as the axial direction and can define the axis 101 (also referred to as the longitudinal axis) of the optical fiber 100. The other two dimensions define a plane called the cross-section. FIG. 7 shows a cross-section of the optical fiber 100 in this cross-section (i.e., perpendicular to the axis 101), which is shown as the x-y plane. FIG. 8 shows a cross-section of the optical fiber 100 in a plane including the axis 101, particularly the x-Z plane. The cross-section of the optical fiber 100 can be substantially constant along the fiber axis 101.

[0102] The optical fiber 100 has a certain degree of flexibility, and thus it will be understood that the direction of the axis 101 is generally not uniform along the length of the optical fiber 100. Terms such as the optical axis 101 and the cross-section are understood to mean a local optical axis 101, a local cross-section, etc. Further, when a component is described as being cylindrical or tubular, these terms are understood to include shapes that are distorted when the optical fiber 100 is bent.

[0103] The optical fiber 100 can have any length, and it will be understood that the length of the optical fiber 100 can depend on the application (e.g., the amount of spectral broadening desired in an application within a supercontinuum radiation source). The optical fiber 100 can have a length between 1 cm and 10 m. For example, the optical fiber 100 can have a length between 10 cm and 100 cm.

[0104] The optical fiber 100 includes a hollow core 102, an inner cladding region surrounding the hollow core 102, and a jacket region 110 surrounding and supporting the inner cladding region. The inner cladding region includes a plurality of anti-resonance elements for guiding radiation through the hollow core 102. In particular, the plurality of anti-resonance elements are arranged to mainly confine the radiation propagating through the optical fiber 100 inside 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 arranged in the central region of the optical fiber 100. The axis of the optical fiber 100 can also define the axis of the hollow core 102 of the optical fiber 100.

[0105] The inner cladding region includes a plurality of capillaries 104 surrounding the hollow core 102, such as tubular capillaries. In particular, in the examples shown in FIGS. 7 and 8, the inner cladding region includes a single ring of six tubular capillaries 104.

[0106] The capillary 104 may also be referred to as a tube. The capillary 104 may have a circular cross-section or may have another shape. Each capillary 104 includes a substantially cylindrical wall portion 105 that at least partially defines the hollow core 102 of the optical fiber 100 and separates the hollow core 102 from the cavity 106. Each of the capillary wall portions 105 facing the hollow core functions as an anti-resonance element for guiding the radiation propagating through the optical fiber 100. It will be appreciated that the wall portion 105 can function as an anti-reflection Fabry-Perot resonator for the radiation propagating through the hollow core 102 (and incident on the wall portion 105 at grazing incidence angles). The thickness 160 of the wall portion 105 can be appropriately made to ensure that reflection to the hollow core 102 is generally enhanced while transmission to the cavity 106 is generally suppressed. In some embodiments, the thickness 160 of the capillary wall portion 105 may be less than 400 nm, less than 300 nm, or less than 150 nm.

[0107] As used herein, the term inner cladding region is intended to mean the region of the optical fiber 100 for guiding the radiation propagating through the optical fiber 100 (i.e., the capillary 104 that confines the radiation within the hollow core 102). The radiation is confined in the form of a transverse mode and propagates along the fiber axis 101.

[0108] The jacket region 110 is generally tubular and supports the capillary 104 of the inner cladding region. The capillary 104 is evenly distributed around the inner surface of the jacket region 110. It can be described that the six capillaries 104 surround the hollow core 102 in a symmetric arrangement. In an embodiment including six capillaries 104, the capillaries 104 can be described as being generally arranged in a hexagonal shape.

[0109] The capillaries 104 are arranged such that each capillary does not contact any of the other capillaries 104. Each of the capillaries 104 contacts the jacket region 110 and is spaced apart from the adjacent capillaries 104 within the ring structure. Such an arrangement can be beneficial as it can increase the transmission bandwidth of the optical fiber 100 (e.g., compared to an arrangement where the capillaries are in contact with each other). Alternatively, in some embodiments, each of the capillaries 104 may be in contact with the adjacent capillaries 104 within the ring structure.

[0110] The six capillaries 104 of the inner cladding region are arranged in a ring structure around the hollow core 102. The inner surface of the ring structure of the capillaries 104 at least partially defines the hollow core 102 of the optical fiber 100. In some embodiments, the diameter of the hollow core 102 (which may be defined as the minimum dimension between opposing capillaries indicated by arrow 114) may be between 5 μm and 100 μm. In some embodiments, the diameter 114 of the hollow core 102 may be between 5 μm and 50 μm. In some embodiments, the diameter 114 of the hollow core 102 may be between 30 μm and 40 μm. The diameter 114 of the hollow core 102 can affect the mode field parameters, impact loss, dispersion, modal plurality, and non-linear characteristics of the hollow core optical fiber 100.

[0111] In the embodiments shown in FIGS. 7 and 8, the inner cladding region includes a single ring arrangement of capillaries 104 (wall portion 105 facing the hollow core that functions as an anti-resonant element). Thus, any radial line from the center of the hollow core 102 to the outside of the optical fiber 100 passes through only one capillary 104.

[0112] It will be understood that in other embodiments, different arrangements of the anti-resonant elements may be provided. These may include arrangements having multiple rings of anti-resonant elements and arrangements having nested anti-resonant elements. Further, the embodiments shown in FIGS. 7 and 8 include a ring of six capillaries 104 having wall portions 105, but in other embodiments, one or more rings with any number of anti-resonant elements (e.g., 4, 5, 6, 7, 8, 9, 10, 11, or 12 capillaries) can be provided in the inner cladding region.

[0113] In the embodiments shown in FIGS. 7 and 8, the inner cladding region has a circular cross-section. However, it will be understood that in other embodiments, an inner cladding region having a cross-section of a shape other than circular may be provided. For example, in one embodiment of the present invention, the inner cladding region may have a hexagonal cross-section. A hexagonal cross-section has the advantage of facilitating the easier placement of the capillary 104 in a symmetric arrangement. For example, six capillaries 104 can be placed at the vertices of the hexagonal cross-section respectively to provide an arrangement of capillaries 104 having hexagonal symmetry.

[0114] The optical fiber 100 may be referred to as a hollow-core photonic crystal fiber (HC-PCF). Generally, such a hollow-core photonic crystal fiber includes an inner cladding region (e.g., including anti-resonant elements) for guiding radiation within the fiber and a jacket region. The jacket region is typically a jacket or tube of material that supports the inner cladding region.

[0115] FIG. 9 schematically shows a general setup of an apparatus 120 that receives an input radiation 122 and broadens the frequency range of the input radiation 122 to provide a broadband output radiation 124. The apparatus 120 includes an optical fiber 100 and a hollow core 102 for guiding the radiation propagating through the optical fiber 100. For clarity of the figure, it will be understood that only the hollow core 102 of the optical fiber 100 is shown in FIG. 9 (the cladding portion and the support portion are not distinguished). The apparatus 120 further includes a gas 126 disposed within the hollow core 102, and this gas includes operating components for broadening the frequency range of the received input radiation 126 to enable the provision of the broadband output radiation 124.

[0116] The operating component of the gas 126 may be a noble gas. The operating component may include one or more of argon, krypton, neon, helium, and xenon. Instead of or in addition to the noble gas, the operating component may include molecular gases (e.g., H2, N2, O2, CH4, SF6).

[0117] In one embodiment, the gas 126 can be disposed within the hollow core 102 during reception of the input radiation 122 for generating at least the broadband output radiation 124. It will be appreciated that the gas 126 may be completely or partially absent from the hollow core 102 while the device 120 is not receiving the input radiation 122 for generating the broadband output radiation. Generally, the device 120 comprises a device for providing the gas 126 within the hollow core 102 of the optical fiber 100. Such a device for providing the gas 126 within the hollow core 102 of the optical fiber 100 can comprise a reservoir, as will be described herein with reference to FIG. 10.

[0118] FIG. 10 shows that the device 120 shown in FIG. 9 further comprises a housing 128. The optical fiber 100 is disposed within the housing 128. The housing 128 is also referred to as a reservoir, a housing, or a container. The housing 128 comprises a heat transport region 127. The heat transport region 127 can comprise, for example, the same gas as the gas 126 within the hollow core 102 of the optical fiber 100 and / or helium. As described herein, the heat transport region 127 further comprises a second medium comprising a solid and / or a liquid to improve heat conduction between the optical fiber 100 and the body or exterior of the housing 128.

[0119] The housing 128 may comprise one or more functions known in the art for controlling, regulating, and / or monitoring the composition of the gas within the housing 128. The reservoir may comprise a first transparent window 130. In use, the optical fiber 100 is disposed within the housing 128 such that the first transparent window 130 is disposed near the input end of the optical fiber 100. The first transparent window 130 may form part of the wall of the housing 128. The first transparent window 130 is at least transparent to the received input radiation frequency, and the received input radiation 122 (or at least a majority thereof) may be coupled to the optical fiber 100 disposed within the housing 128. The housing 128 may comprise a second transparent window 132 that forms part of the wall of the housing 128. When the optical fiber 100 is disposed within the housing 128 in use, the second transparent window 132 is disposed near the output end of the optical fiber 100. The second transparent window 132 may be at least transparent to the frequency of the broadband output radiation 124 of the device 120.

[0120] Alternatively, in another embodiment, the two opposing ends of the optical fiber 100 can be disposed in different enclosures. The optical fiber 100 can include a first end section configured to receive the input radiation 122 and a second end section for outputting the broadband output radiation 124. The first end section can be disposed in a first enclosure that includes a heat transport region 127. The second end section can be disposed inside a second enclosure, which can also include a heat transport region 127. The functions of the enclosures can be as described in relation to FIG. 10 above. The first enclosure can include a first transparent window configured to be transparent to the input radiation 122. The second enclosure can include a second transparent window configured to be transparent to the broadband output broadband radiation 124. The first and second enclosures can also include a sealable opening that allows the optical fiber 100 to be disposed partially inside and partially outside the enclosures, such that a gas can be sealed inside the enclosures. The optical fiber 100 can further include an intermediate section that is not contained within the enclosures. Such an arrangement using two separate enclosures is particularly advantageous for embodiments where the optical fiber 100 is relatively long (e.g., greater than 1 m in length). In the case of such a configuration using two separate gas enclosures, the two enclosures (which can include one or more features known in the art for controlling, regulating, and / or monitoring the gas composition within the two enclosures) can be considered to provide a means for providing the gas 126 within the hollow core 102 of the optical fiber 100.

[0121] In this context, a window can be transparent to a frequency if at least 50%, 75%, 85%, 90%, 95%, or 99% of the incident radiation at that frequency on the window is transmitted through the window.

[0122] Both the first transparent window 130 and the second transparent window 132 can form an airtight seal within the wall of the enclosure 128, such that the gas 126, and the gas or liquid in the heat transport region 127 can be contained within the enclosure 128. It will be understood that the gas or liquid can be contained within the enclosure 128 at a pressure different from the ambient pressure of the enclosure 128.

[0123] In order to broaden the frequency, high-intensity radiation may be desirable. The advantage of having the hollow-core optical fiber 100 is that high-intensity radiation can be achieved by the strong spatial confinement of the radiation propagating through the optical fiber 100, and a high local radiation intensity can be achieved. Furthermore, the hollow-core design (e.g., as compared to a solid-core design) can result in a higher-quality transmission mode (e.g., a high proportion of single-mode transmission). The radiation intensity within the optical fiber 100 may 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 optical fiber 100.

[0124] The advantage of using the hollow-core optical fiber 100 is that most of the radiation guided into the optical fiber 100 is confined within the hollow core 102. Thus, most of the interaction of the radiation within the optical fiber 100 is with the gas 126 provided inside the hollow core 102 of the optical fiber 100. As a result, the amplification effect of the active component of the gas 126 on the radiation can be increased.

[0125] 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 ultra-fast pulses. The mechanism of spectral broadening when the radiation interacts with the gas 126 may be, for example, one or more of four-wave mixing, modulation instability, ionization of the active gas, Raman effect, Kerr non-linearity, soliton formation, or soliton fission. In particular, the spectral broadening may be achieved by one or both of soliton formation or soliton fission.

[0126] The input radiation 122 can be coherent radiation. The input radiation 122 can be collimated radiation. The advantage can be to facilitate and improve the efficiency of coupling the input radiation 122 into the optical fiber 100. The input radiation 122 can include a single frequency or a narrow range of frequencies. The input radiation 122 can be generated by a laser. Similarly, the output radiation 124 can be collimated and / or can be coherent.

[0127] The broadband range of the output radiation 124 can be a continuous range including a continuous range of radiation frequencies. The output radiation 124 can include supercontinuum radiation. The continuous radiation may be useful for many applications, such as measurement applications. For example, a continuous range of frequencies can be used to examine a number of characteristics. The continuous range of frequencies can be used, for example, to determine and / or eliminate the frequency dependence of the measured characteristics. The supercontinuum output radiation 124 can include, for example, electromagnetic radiation over a wavelength range of 100 nm to 4000 nm. The frequency range of the broadband output radiation 124 can be, for example, 400 nm to 900 nm, 500 nm to 900 nm, or 200 nm to 2000 nm. The supercontinuum output radiation 124 can include white light.

[0128] FIG. 11 shows a radiation source 134 for providing broadband output radiation. The radiation source 134 comprises an apparatus 120 as described above with reference to FIG. 10. The radiation source 134 further comprises an input radiation source 136 configured to provide the input radiation 122 to the apparatus 120. The apparatus 120 can receive the input radiation 122 from the input radiation source 136 and expand it to provide the output radiation 124.

[0129] The input radiation 122 supplied by the input radiation source 136 may be pulsed. The input radiation 122 may include electromagnetic radiation of one or more frequencies between 200 nm and 2 μm. The input radiation 122 may include, for example, electromagnetic radiation having a wavelength of 1.03 μm. The repetition rate of the pulsed radiation 122 may be on the order of 1 kHz to 100 mM. The pulse energy may be on the order of 0.1 μJ to 100 μJ, for example, 1 to 10 μJ. The pulse duration of the input radiation 122 may be between 10 fs and 10 ps, for example, 300 fs. The average power of the input radiation 122 may be between 100 mW and several 100 W. The average power of the input radiation 122 may be, for example, 20 to 50 W.

[0130] The broadband output radiation 124 provided by the radiation source 134 may have an average output power of at least 1 W. The average output power may be at least 5 W. The average output power may be at least 10 W. The broadband output radiation 124 may be pulsed broadband output radiation 124. The broadband output radiation 124 may have a power spectral density in the entire wavelength band of the output radiation of at least 0.01 mW / nm. The power spectral density in the entire wavelength band of the broadband output radiation may be at least 3 mW / nm.

[0131] The above-described radiation source 134 may be provided as part of a measuring device for determining the parameters of interest of the structure on the substrate. The structure on the substrate may be, for example, a lithography pattern applied to the substrate. The measuring device may further include an illumination subsystem for illuminating the structure on the substrate. The measuring device may further include a detection subsystem for detecting a part of the radiation scattered and / or reflected by the structure. The detection subsystem may further determine the parameters of interest on the structure from a part of the radiation scattered and / or reflected by the structure. The parameters may be, for example, overlay, alignment, or leveling data of the structure on the substrate.

[0132] The above measurement device may form part of the measurement device MT. The above measurement device may form part of the inspection device. The above measurement device may be included in the lithography apparatus LA.

[0133] It will be appreciated that the devices 120 and the radiation source 134 shown in FIGS. 9 to 11 are shown in cross-section in the y-z plane.

[0134] The non-linear white light generation process is not without losses, as the strong pump pulse may partially ionize the working components of the gas 126. During the subsequent plasma relaxation process, the lost power is converted into heat, increasing the temperature inside the fiber 100 and the fiber 100 itself. This heat is transported to the (usually metal) body of the housing 128 via the surrounding heat transport medium 127 (usually a gas), where it is dissipated to the cooling circuit. As an example, the power accumulated in the system can reach several watts per meter.

[0135] Generally, the working components of the gas 126 are not optimal heat conductors, for example, having a thermal conductivity of about 10E-3 W / mK in the case of argon and 9E-3 W / mK in the case of krypton, resulting in a relatively high peak temperature inside the fiber. Numerical simulations have shown that when several W / m of energy is accumulated in the fiber, the temperature inside the HC-PCF can reach about 1000 °C. Without being bound by theory, such high temperatures can damage the fiber in the form of mechanical deformation of the fiber (e.g., capillary) or deposition of material at the input and output ends of the fiber 100.

[0136] Therefore, in order to mitigate the above effects, it is necessary to improve the heat transport from the fiber 100 to the body of the housing 128 (i.e., the conductivity of heat by conduction, convection, and / or radiation).

[0137] FIG. 12 shows a cross-section of the broadband radiation generating device 120, which generally corresponds to the device 120 shown in FIGS. 10 and 11 and is shown in the x-y plane (i.e., the plane perpendicular to the longitudinal axis 101 of the fiber 100). The fiber 100 is disposed within the housing 128 as described above. The fiber 100 shown in FIG. 12 passes through the center (i.e., coaxial) of the housing 128, but it should be understood that it is not necessarily so, and the fiber 100 can also be disposed at a position offset from the center.

[0138] When the device 120 is in use (i.e., when the input radiation is input to the fiber 100 for broadband radiation generation), the fiber 100 is heated as described above. Heat is typically transported radially from the fiber 100 to the main body 202 of the housing 128 through the heat transport region 127, as indicated by the dashed arrows in FIG. 12. Ideally, the fiber 100 is completely surrounded by a solid material having good thermal conductivity, for example, by soldering the fiber 100 to the inner surface of the main body 202 of the housing 128. However, if the thermal expansion coefficients of the main body 202 (usually metal, such as steel and / or aluminum) and the fiber 100 (usually fused silica) are different, stress will occur in the fiber 100, ultimately leading to damage and / or performance degradation of the fiber 100.

[0139] Instead, heat is transported from the fiber 100 to the body 202 via a heat transport region 127 that includes the same working medium (i.e., gas 126) as that typically used to generate broadband radiation in conventional systems. Recently, it has been demonstrated that including helium as a component of the working medium 126 improves the thermal conductivity of the working medium 126, which is described in International Publication No. 2022 / 008164, the content of which is incorporated herein by reference in its entirety. Helium is characterized by a thermal conductivity that is approximately an order of magnitude higher than that of krypton or argon, and when mixed in a sufficient ratio, for example, a ratio where the helium content exceeds 20%, the thermal conductivity of the working medium or working gas 126 can be efficiently improved. Including helium in the working medium 126 may improve the life of the fiber by about three months, but further reducing the thermal resistance of the heat transport region 127 may further improve the life of the fiber 100.

[0140] FIG. 13 shows a numerical simulation of a fiber 100 disposed within a gas medium 204 (e.g., krypton gas), where the gas medium 204 is housed within a solid medium 206 (specifically, stainless steel). The box on the right side of FIG. 24 corresponds to an enlarged portion of the box on the left side. The model is shown in the x-y plane (i.e., a plane perpendicular to the longitudinal axis 101 of the fiber 100) and can be considered to be generally similar to the housing 128 of the apparatus 120 shown in FIGS. 9-12. The modeled fiber 100 is an HC-PCF with an outer diameter of approximately 180 μm. The cross-sectional area of the modeled portion of the housing containing the gas medium 204 is 15 mm × 18 mm. The distance s between the fiber 100 and the solid medium 206 varies between 0 and 7.5 mm.

[0141] FIG. 14 shows the results of the numerical simulation shown in FIG. 13. That is, FIG. 14 shows the calculated peak temperature reached inside the fiber 100 as a function of the power accumulated in the fiber core for each simulated distance s. The lines 1402, 1404, 1406, and 1408 of the graph in FIG. 14 correspond to simulated distances s of 7.5 mm, 1.0 mm, 0.2 mm, and 0 mm, respectively. As shown in FIG. 14, when s is shortened to 0.2 mm, the fiber temperature drops by more than 30% compared to the case where the fiber is in the center of the gas medium 204. When the distance is shortened to zero (i.e., contact), a reduction of more than 50% is achieved.

[0142] The coefficient of thermal expansion of stainless steel is about 30 times that of the fused silica fiber. Generally, the fiber 100 is held within the housing 128 by being clamped or otherwise held at the opposite end (i.e., when the device 120 is in the horizontal orientation). As a result, when the temperature rises, the gas cell expands more than the fiber, creating a risk that the fiber will be pulled out of the holder or damaged. For this reason, when the device 120 is in the "off" or "cold" state, the fiber 100 is typically held in the initially bent configuration to compensate for the thermal expansion of the housing 128 when the device 120 is heated to the operating temperature, after which the fiber may reach a substantially straight state.

[0143] Generally, since the fiber 100 needs to be able to move freely within the housing 120 to accommodate thermal expansion of the housing 120 (and / or the fiber 100), the minimum distance between the fiber 100 and the solid medium 206 needs to be non-zero along at least a portion of the length of the fiber 100, thereby avoiding the risk that the fiber catches on the solid medium 206 and is damaged by stress as described above.

[0144] FIG. 15 shows an example of an apparatus 120 according to the present invention. The apparatus 120 includes a hollow-core optical fiber 100 disposed within a housing 128. It can be seen that the apparatus 120 is shown in a cross-section of the x-y plane (i.e., a plane perpendicular to the longitudinal axis 101 of the fiber 100, with the fiber 100 extending longitudinally within the page in the z-direction) in FIG. 15. A heat transfer medium 127 is located between the fiber 100 and the body 202 of the housing 128, and the heat transfer medium 127 includes a first medium 212 and a second medium 214. The first medium 212 can include, for example, a gas containing an operating component of a gas 126 disposed within the core of the fiber 100, and / or a gas containing helium, such as a gas containing a helium content exceeding 20%.

[0145] The second medium 214 can be composed of a solid, such as a metal (e.g., steel), a ceramic, and / or a glass. The second medium 214 is disposed between the fiber 100 and the body 202 of the housing 128 in at least one radial direction (i.e., radially from the fiber 100). In the example shown in FIG. 15, the second medium 214 completely fills the housing 128 on one side of the fiber 100. The fiber 100 is spatially separated from the second medium 214 by a radial distance 250 (i.e., radially from the fiber). The radial distance 250 can be 1 mm or less over at least a part of the length of the fiber. In some examples, the radial distance 250 is 200 μm or less. In some examples, the radial distance 250 is 100 μm or less. Generally, it will be understood that the radial distance 250 needs to be small enough to maximize heat transfer from the fiber 100 to the body 202 of the housing 128 through the second medium 214, but large enough to accommodate free movement of the fiber 100 due to thermal expansion. In some examples, the fiber 100 may be in direct contact with the second medium 214 over at least a part of the length of the fiber 100.

[0146] In the case of a horizontal device 120 in which the fiber 100 is held at both ends, the fiber 100 must also be able to move freely in the vertical direction (i.e., the y-direction). This is, for example, to enable the fiber to move from an initially bent configuration to a substantially straight state as the device 120 is heated, as described above. As an example, a temperature change of 100 K of the fiber 100 may require a vertical movement of about 1 cm. The arrangement of the second medium 214 in FIG. 15 corresponds to this vertical movement because the fiber can move freely vertically across the inner diameter of the housing 128 (i.e., within the main body 202).

[0147] In some examples, one or more of the first medium 212 and the second medium 214 are composed of a liquid, for example, water, liquid gallium or another liquid metal, liquid nitrogen, carbon disulfide CS2, carbon tetrachloride, and / or carbon tetrachloride CCl4.

[0148] When the first medium 212 is composed of a liquid rather than a gas, the thermal conductivity of the liquid is generally higher than that of the gas, so the thermal conductivity between the fiber 100 and the second medium 214 can be further improved.

[0149] In one example, both the first medium 212 and the second medium 214 contain a liquid. For example, the device 120 is similar to the example shown in FIG. 12, and the heat transport region 127 contains a liquid that completely surrounds the fiber 100.

[0150] The liquid heat transport region 127 and / or the liquid first medium 212 have the advantage of allowing free movement of the fiber 100 while also providing direct contact between the fiber 100 and the heat transport medium with significantly improved thermal conductivity compared to the gas in prior art systems.

[0151] In the case of the liquid first medium 212 and / or the second medium 214, it will generally be understood that the liquid medium needs to be reliably sealed from the gas forming the working medium 126 for broadband radiation generation. Appropriate sealing means can withstand the operating pressure of the working medium 126, which is on the order of 10 to 100 bar.

[0152] FIG. 16 shows a further example of the apparatus 120 according to the present invention. In FIG. 16, a second medium 214 is disposed on both sides of the fiber 100, a slit is formed in the main body 202 of the housing 128, the slit is filled with a first medium 212, and the fiber 100 is disposed in the slit. The slit configuration in FIG. 16 has the advantage that the heat transport between the fiber 100 and the main body 202 of the housing 128 can be improved because the fiber 100 is surrounded by the second medium 214 in almost all radial directions, and at the same time the fiber 100 can move freely in the vertical direction. The slit width d generally needs to be as narrow as possible to maximize the heat transfer efficiency between the fiber 100 and the main body 202 of the housing 128, but on the other hand, it is also necessary to ensure that the fiber 100 can move freely. In one example (for example, in the case of a fiber 100 with a diameter of about 100 μm to 200 μm), d may be 2 mm or less. In another example, d may be 1 mm or less.

[0153] FIG. 17 shows an example of the apparatus 120 according to the present invention, and this apparatus further includes a cooling channel 302 outside the main body 202 of the housing 128. For example, the housing 128 can be disposed in a channel 302 configured to cool the main body 202 of the housing 128 by flowing a fluid such as water, helium (gas or liquid), and / or oil over the main body 202 of the apparatus 120 during use of the apparatus 120. The cooling channel 302 is connected to a cooling circuit (for example, a cooling system) for cooling the apparatus. The cooling circuit, for example, takes away the heat transported from the fiber 100 to the main body 202 of the housing 128 through the heat transport region 127 from the main body 202.

[0154] FIG. 18 shows a further example of the apparatus 120 according to the present invention. The apparatus 120 includes a second medium 214, and the second medium 214 includes one or more internal cooling channels 304 that extend in a direction parallel to the longitudinal axis 101 of the fiber 100 (i.e., the z direction). The one or more internal cooling channels 304 may include holes that penetrate the solid second medium 214 and / or pipes that penetrate the liquid second medium 214. The internal cooling channels 304 may be configured to cool the body 202 of the housing 128 and / or the second medium 214 by allowing a fluid such as water, helium (gas or liquid), and / or oil to flow through the second medium 214 during use of the apparatus 120, instead of or in addition to the cooling channels 302 as shown in FIG. 17. The cooling channels 304 may be connected to a cooling circuit (e.g., a refrigeration system) to cool the apparatus. The cooling circuit, for example, removes heat transported from the fiber 100 to the body 202 of the housing 128 through the heat transport region 127 from the body 202. The internal cooling channels 304 shown in FIG. 18 are shown as cylindrical holes or pipes passing through the second medium 214. However, it will be understood that the internal cooling channels 304 can have any shape suitable for transporting the cooling fluid along the length of the apparatus 120. For example, the one or more cooling channels 304 can have a helical or corkscrew configuration that penetrates the second medium 214 in the z direction.

[0155] It will be understood that the second medium 214 does not necessarily need to extend along the entire length of the housing 128. In other words, as shown in FIG. 19, the second medium 214 may extend only over a part of the length of the housing 128 in the z direction. The portion where the second medium 214 extends in the z direction is shown as length 402 in FIG. 19. It can be seen that FIG. 19 shows a cross-section of the y-z plane of a device similar to the device shown in FIG. 18. In some examples, the second medium 214 may extend over a distance 402 of about 1 cm in the z direction. In some examples, the second medium 214 may extend only in the z direction over a portion of the optical fiber 100 where broadband radiation generation occurs. This is because this portion generally reaches the highest operating temperature of the optical fiber 100. In other words, the distance 402 may cover only the portion of the optical fiber 100 where broadband radiation occurs.

[0156] In some cases, the second medium 214 may be present over most or all of the length of the housing 128.

[0157] FIG. 20 shows a further example of the device 120 according to the present invention, shown in a cross-section of the y-z plane. It can be seen that the device 120 shown in FIG. 20 is arranged perpendicular to the examples shown in FIGS. 9 to 11 and FIG. 19. In other words, the device 120 is arranged vertically rather than horizontally (i.e., with respect to the earth), and the length of the fiber 100 is along the y direction rather than the z direction. In the vertical arrangement, the fiber 100 hangs down within the housing 128 (for example, the fiber 100 can be clamped or held at the upper end), and the orientation of the fiber 100 within the housing 128 is determined by gravity. In such a vertical arrangement, the fiber 100 is surrounded by the solid second medium 214 on all sides (i.e., in all radial directions), and since the fiber 100 does not hit the second medium 214 and come to rest due to gravity as in the case of the horizontal arrangement, it is prevented from being caught by the second medium 214. Therefore, in the vertical arrangement, since all sides of the fiber 100 are surrounded by the second medium 214, the thermal conductivity between the fiber 100 and the main body 202 of the housing 128 can be further increased.

[0158] In the example where the device 120 is vertically arranged, it will be understood that heat transfer also occurs along the length of the device 120 due to convection.

[0159] FIG. 21 schematically shows a method for generating broadband radiation according to the present invention. In step 2102 of the method, input radiation is emitted from an input radiation source, for example, an input radiation source 136 of the type shown in FIG. 11 and described herein. The input radiation corresponds to the input radiation 122 shown in FIG. 11 and described herein. In step 2104 of the method, the input radiation is received by the device 120 described herein. In step 2106 of the method, an output including broadband radiation is generated. The output radiation corresponds to the broadband output radiation 124 shown in FIG. 11 and described herein. The output radiation may be generated according to any of the broadband output radiation generation processes described herein.

[0160] Although specific reference may be made herein to the use of a lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other uses. Other possible uses include the manufacture of guidance and detection patterns for integrated optical systems, magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

[0161] Although embodiments of the present invention may be specifically referred to herein in the context of a lithographic apparatus, embodiments of the present invention may be used in other apparatuses. Embodiments of the present invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus for measuring or processing an object such as a wafer (or other substrate) or a mask (or other patterning device). These apparatuses are sometimes generally referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0162] Although the above has specifically referred to the use of embodiments of the present invention in the context of optical lithography, the present invention is not limited to optical lithography and can be used in other applications, such as imprint lithography, if the context permits.

[0163] Particular reference has been made to "measurement apparatus / tools / systems" or "inspection apparatus / tools / systems", but these terms may refer to the same or similar types of tools, apparatus, or systems. For example, an inspection apparatus or measurement apparatus including an embodiment of the present invention can be used to determine the characteristics of structures on a substrate or a wafer. For example, an inspection apparatus or measurement apparatus including an embodiment of the present invention can be used to detect defects in a substrate or defects in structures on a substrate or a wafer. In such embodiments, the target characteristics of the structures on the substrate may be related to defects in the structures, the absence of specific parts of the structures, or the presence of unwanted structures on the substrate or the wafer.

[0164] Although specific embodiments of the present invention have been described above, it will be understood that the present invention may be practiced in ways other than those described. The above description is intended to be illustrative rather than limiting. Thus, it will be apparent to those skilled in the art that modifications can be made to the described invention without departing from the scope of the claims set forth below.

Claims

Claim 1 A broadband radiation generating device, comprising a hollow core fiber at least partially disposed within a housing, the housing comprising: a main body; a heat transport region configured to transport heat from the hollow core fiber to the main body of the housing, the heat transport region comprising a first medium disposed between the hollow core fiber and the main body of the housing; in at least one radial direction from the hollow core fiber, the heat transport region further comprises a second medium disposed between the first medium and the main body of the housing over a portion of the length of the housing, the second medium comprising a solid and / or a liquid; a radial distance between the hollow core fiber and the second medium is 2 mm or less over the portion; the second medium forms a slit over at least a portion of the length of the housing, the slit extending in a direction perpendicular to the longitudinal axis of the hollow core fiber, the slit being wider than the diameter of the hollow core fiber, characterized by the device. Claim 2 The device according to claim 1, wherein the hollow core fiber is capable of freely moving within the first medium in at least one direction perpendicular to the longitudinal axis of the hollow core fiber. Claim 3 The device according to claim 1 or 2, wherein the radial distance between the hollow core fiber and the second medium is a distance in at least one radial direction. Claim 4 The device according to any one of claims 1 to 3, further comprising a cooling channel, the cooling channel being external to the main body, the cooling channel being configured to cool the main body during use. Claim 5 The device according to claim 1, wherein the width of the slit is 2 mm or less, preferably the width of the slit is 1 mm or less. Claim 6 The device according to any one of claims 1 to 5, wherein a portion of the length of the housing surrounds a portion of the hollow core fiber, the portion of the hollow core fiber being a portion where broadband radiation generation occurs. Claim 7 The device according to any one of claims 1 to 6, wherein a portion of the length of the housing is at least 1 cm in length. Claim 8 The device according to any one of claims 1 to 7, wherein the hollow core fiber is configured to hang vertically during use. Claim 9 The device according to any one of claims 1 to 8, wherein the second medium further comprises one or more internal cooling channels extending in a direction parallel to the longitudinal axis of the hollow core fiber. Claim 10 The device according to any one of claims 1 to 9, wherein the first medium comprises a gas and / or a liquid.

11. The device according to any one of claims 1 to 10, wherein the second medium comprises a solid, and the solid comprises a metal.

12. The device according to any one of claims 1 to 11, wherein in at least one radial direction, the radial distance between the hollow core fiber and the second medium is 200 μm, preferably the radial distance is 100 μm.

13. The device according to any one of claims 1 to 12, wherein in at least one radial direction, the distance between the second medium and at least a part of the length of the hollow core fiber is zero.

14. The device according to any one of claims 1 to 13, wherein the radial distance between the hollow core fiber and the second medium is 1 mm or less over the part.

15. A method for generating broadband radiation, comprising: radiating input radiation from an input radiation source; receiving the input radiation by the device according to any one of claims 1 to 14; using the device to generate an output including broadband radiation. ​

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