Apparatus for broadband radiation generation

Abstract

An apparatus for broadband radiation generation includes an optical fiber disposed in an enclosure, the enclosure including: a body; and a heat transfer region configured to transfer heat from the optical fiber to the body, the heat transfer region including a first medium disposed between the optical fiber and the body; wherein in at least one radial direction from the optical fiber, the heat transfer region further comprises a second medium disposed on a portion of the enclosure between the first medium and the body, the second medium comprising a solid and / or a liquid; and wherein a radial distance between the optical fiber and the second medium is less than or equal to 2 mm, and the second medium forms a slit on the portion of the enclosure, the slit extending in a direction perpendicular to a longitudinal axis of the optical fiber, the slit being wider than a diameter of the optical fiber.

Description

Technical Field

[0001] The invention relates to a device for broadband radiation generation, comprising a hollow core optical fiber at least partially arranged in an enclosure. Background Art

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

[0003] To project a pattern on a substrate, a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features that can be formed on a substrate. Typical wavelengths currently used are 365nm (i-line), 248nm, 193nm, and 13.5nm. A lithographic apparatus using extreme ultraviolet (EUV) radiation with a wavelength in the range of 4nm-20nm (e.g., 6.7nm or 13.5nm) can be used to form smaller features on a substrate than a lithographic apparatus using radiation with a wavelength of, for example, 193nm.

[0004] Low k1 lithography can be used to process features with dimensions smaller than the classical resolution limit of the lithographic apparatus. In such a process, the resolution formula can be expressed as CD=k1×λ / NA, where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the "critical dimension" (usually the minimum feature size printed, but in this case it is the half pitch), and k1 is an empirical resolution factor. In general, the smaller the k1, the more difficult it is to reproduce on a substrate a pattern similar in shape and size to that planned by the circuit designer to achieve specific electrical functions and performance. To overcome these difficulties, complex fine-tuning steps can be applied to the lithographic projection apparatus and / or the design layout. These include, for example, but are not limited to, optimization of NA, customized illumination schemes, use of phase-shifted patterning equipment, various optimizations of the design layout (such as optical proximity correction (OPC, sometimes also called "optical and process correction") in the design layout), or other methods generally defined as "resolution enhancement technology" (RET). Alternatively, a tight control loop for controlling the stability of the lithographic apparatus can be used to improve the reproduction of patterns at low k1.

[0005] In the field of lithography, many measurement systems may be used both within and outside the lithographic apparatus. Typically, such a measurement system may use a radiation source to illuminate a target with radiation, and a detection system operable to measure at least one property of a portion of the incident radiation scattered from the target. An example of a measurement system external to the lithographic apparatus is an inspection device or metrology device, which may be used to determine the properties of a pattern previously projected onto a substrate by the lithographic apparatus. For example, such an external inspection device may include a scatterometer. Examples of measurement systems that may be provided within the lithographic apparatus include: a topography measurement system (also called a level sensor); a position measurement system (e.g., an interferometric measurement device) for determining the position of a mask or wafer stage; and an alignment sensor for determining the position of an alignment mark. These measurement devices may use electromagnetic radiation for measurement.

[0006] Different types of radiation can be used to interrogate different types of properties of the pattern. Some measurement systems may use a broadband radiation source. Such a broadband radiation source may be a supercontinuum source and may include an optical fiber with a nonlinear medium through which a pulsed pump radiation beam is propagated to broaden the spectrum of the radiation.

[0007] One challenge associated with the realization of such a supercontinuum source is to extend the lifetime of the fiber by mitigating existing failure mechanisms. Damage (such as glass deposition) at the fiber input and output ends has been observed during use. In addition, in cases where hollow-core photonic crystal fibers include antiresonant elements or capillaries surrounding the core to confine radiation propagating through the core, these elements have been found to exhibit permanent mechanical deformation. Both of these effects can have an adverse effect on performance, as they can lead to a reduction in the output power of the supercontinuum source and ultimately to changes in the output spectrum.

[0008] It may be desirable to provide alternative apparatus (e.g., optical fiber) and methods for use in an apparatus for receiving input radiation and broadening the frequency range of the input radiation to provide (broadband) output radiation which at least partially addresses one or more problems associated with the prior art, whether noted herein or otherwise. Summary of the invention

[0009] According to a first aspect of the present invention, there is provided a device for broadband radiation generation, the device comprising a hollow-core optical fiber at least partially disposed in a shell, the shell comprising: a body; and a heat transfer region configured to transfer heat from the hollow-core optical fiber to the body of the shell, the heat transfer region comprising a first medium disposed between the hollow-core optical fiber and the body of the shell; wherein in at least one radial direction from the hollow-core optical fiber, the heat transfer region further comprises a second medium disposed on a portion of the length of the shell and located between the first medium and the body of the shell, the second medium comprising a solid and / or a liquid; and wherein a radial distance between the hollow-core optical fiber (e.g., a surface of the hollow-core optical fiber) and the second medium (e.g., a surface of the second medium) on the portion is less than or equal to 2 mm, and wherein the second medium forms a slit on at least a portion of the length of the shell, the slit extending in a direction perpendicular to the longitudinal axis of the hollow-core optical fiber, and the slit is wider than the diameter of the hollow-core optical fiber.

[0010] A hollow core optical fiber (also referred to as an "optical fiber") may be a hollow core photonic crystal fiber (HC-PCF). In some examples, the HC-PCF may include a hollow core extending along the axial direction of 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 sheath region surrounding the inner cladding region.

[0011] It should be understood that, as used herein, an inner cladding region refers to a region of an optical fiber that is used to guide radiation propagating through the hollow core of the optical fiber (i.e., confine such radiation within the hollow core). In particular, the inner cladding region is arranged to confine radiation propagating through the optical fiber primarily within the hollow core and guide the radiation along the optical fiber.

[0012] The wall portions of the multiple capillaries facing the hollow core can be used as antiresonance elements. It should be understood that the antiresonance element refers to an element arranged to confine radiation in the hollow core primarily by antiresonance. Optical fibers including antiresonance elements or structures are known in the art as antiresonance fibers, tubular fibers, single-ring fibers, negative curvature fibers, or suppressed coupling fibers. Various different designs of such optical fibers are known in the art. In particular, the term antiresonance element is not intended to include an element arranged to confine radiation in the hollow core primarily by creating a photonic band gap in the cladding region (e.g., a Kagome photonic crystal fiber).

[0013] Typically, each capillary wall portion at least partially defines a hollow core and separates the hollow core from a cavity. For example, each capillary may include a cavity separated from the hollow core by a wall. It will be appreciated that the wall portion may act as an anti-resonant Fabry-Perot resonator for radiation propagating through the hollow core (and may be incident on the wall at a grazing angle of incidence). The thickness of the wall portion may be suitable to ensure that reflection back to the hollow core is generally enhanced, while transmission into the cavity is generally suppressed.

[0014] The optical fiber may be suitable for use in a device for receiving input radiation and widening the frequency range of the input radiation to provide output radiation. An optical fiber that uses an antiresonant element to guide radiation may have a wider transmission window (i.e., have a larger transmission bandwidth) than a photonic bandgap fiber. Therefore, advantageously, such an optical fiber may be more suitable for use in a device for receiving input radiation and widening the frequency range of the input radiation to provide output radiation (e.g., a supercontinuum source).

[0015] Additionally or alternatively, optical fibers may be suitable for beam delivery applications (without extending the frequency range of the radiation).

[0016] The optical fiber may be suitable for use as an optical fiber for transmitting light from a light source to a sensor.

[0017] An enclosure (which may alternatively be referred to as a "reservoir," "housing," "container," or "plenum") may be used to control, regulate, and / or monitor optical fibers and / or materials (e.g., solids, liquids, and / or gases) contained therein. As used herein, the "body" of an enclosure generally refers to an edge region (e.g., outer wall(s)) of the enclosure to which heat may be transferred via a heat transfer region. Heat transferred to the body of the enclosure may then be transferred out of the enclosure via a cooling circuit and / or one or more cooling channels.

[0018] Without wishing to be bound by theory, failure of broadband radiation (eg, supercontinuum) generating systems caused by fiber damage may be at least partially affected by fiber material temperature, and thus it may be desirable to improve thermal management of the optical fiber and / or nonlinear media.

[0019] In known systems, the medium that facilitates heat transfer between the optical fiber and the body of the surrounding housing is typically a gas, typically a gas that forms a nonlinear 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 that advantageously provides improved thermal conductivity compared to a gas, thereby better transferring heat away from the optical 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 optical fiber to the body of the surrounding shell.

[0021] Ideally, the optical fiber should be fixed in direct contact with the second medium to provide optimal thermal contact between the optical fiber and the second medium. However, fixing the optical fiber to a solid second medium can cause stress inside the optical fiber due to the different thermal expansion coefficients of the optical fiber and the solid medium, ultimately reducing the performance of the optical fiber. Therefore, in the area of ​​the second medium, the optical fiber is usually allowed to move freely due to thermal expansion (e.g., thermal expansion of the shell and / or the optical fiber) instead, and may only partially contact the second medium at any given time, or not directly contact at all. Generally, the optical fiber should not be attached to the second medium. The inventors have found that direct contact of the optical fiber with the second medium is not required to achieve sufficient heat transfer between the optical fiber and the second medium. Preferably, at any given point on the shell portion including the second medium, the radial distance between the optical fiber (e.g., the surface of the optical fiber) and the second medium (e.g., the surface of the second medium) is less than or equal to 1 mm to provide sufficient heat transfer between the optical fiber and the second medium.

[0022] Heat transfer over a non-zero distance between the optical fiber and the second medium is facilitated by a first medium, which may include a gas and / or a liquid. For example, the first medium may include a gas configured to generate broadband radiation (i.e., a nonlinear medium, a working medium, or a working gas). In some examples, the first medium may include helium, which may advantageously improve heat transfer between the optical fiber and the second medium compared to a first medium consisting solely of a nonlinear medium. In some examples, the first medium may include a liquid. Advantageously, the liquid first medium provides improved heat transfer between the optical fiber and the second medium (compared to a gaseous first medium) while also allowing free movement of the optical fiber (e.g., compared to a solid first medium).

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

[0024] Typically, the thermal expansion coefficient of the enclosure is much greater than that of the hollow core fiber. When the device is in a horizontal orientation, the fiber is typically clamped in place in the enclosure at the opposite end. Therefore, when the device is in the "off" or "cold" state, the fiber can be arranged to bend in a direction perpendicular to the longitudinal axis of the fiber to compensate for thermal expansion of the enclosure, which may cause the fiber to straighten when the device reaches operating temperature. In this case, the composition of the heat transfer area needs to facilitate this thermal expansion by allowing the fiber to move in this direction.

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

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

[0027] In some examples, the apparatus further comprises a cooling passage located outside the body, the cooling passage being configured to cool the body in use. For example, the cooling passage may comprise a passage configured to allow a fluid (e.g., water) to flow through the exterior of the enclosure as part of a cooling circuit.

[0028] In some examples, in the slot, the optical fiber can move freely in the slot in a direction perpendicular to the longitudinal axis of the optical fiber (e.g., due to thermal expansion of the surrounding shell), while the optical fiber maintains sufficiently close contact with the second medium (i.e., the optical fiber is almost completely surrounded by the second medium) to achieve good heat transfer from the optical fiber to the outside. The slot can be filled with the first medium.

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

[0030] In some examples, a portion of the length of the enclosure surrounds a portion of the hollow-core fiber, where broadband radiation generation occurs (i.e., a portion or region of the hollow-core fiber configured for broadband radiation generation). The portion or region of the hollow-core fiber where broadband radiation generation occurs is typically where the highest temperature is reached in the fiber, so including a second medium in this region can provide more effective thermal management than any other portion of the fiber.

[0031] In some examples, the portion of the length of the enclosure is at least 1 cm long.

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

[0033] In some examples, the portion of the length of the enclosure is at least 2% long of the total length of the optical fiber.

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

[0035] In some examples, the heat transport region includes the second medium located only over a portion of the length of the enclosure surrounding a portion of the hollow core fiber where broadband radiation generation occurs (ie, the second medium may not be present in any other portion of the enclosure).

[0036] In some examples, the hollow core fiber is configured to be suspended vertically in use. For example, the enclosure can be arranged in a vertical orientation so that the optical fiber is suspended under the action of gravity. In this orientation, the optical fiber can be maintained at the uppermost end so that the optical fiber is generally positioned very close to the second medium (in some cases, zero spatial separation in at least one radial direction). In some examples, the vertically suspended optical fiber can at least partially rest on the second medium. However, vertical suspension can advantageously prevent the optical fiber from being trapped by the second medium (particularly when the second medium includes a solid material), such as when the second medium surrounds the optical fiber on all sides (i.e., in all radial directions) or surrounds more than half of the optical fiber.

[0037] In some examples, the hollow core fiber and the enclosure are arranged horizontally, such as with the fiber clamped at opposite 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 optical fiber. For example, where the second medium includes a solid, the internal cooling channel may include a hollow hole passing through the second medium, the hollow hole being configured to allow a cooling fluid (e.g., water) to flow through the hole, for example as part of a cooling circuit. Alternatively or additionally, where the second medium includes a liquid, the internal cooling channel may include a conduit passing through the second medium. The conduit 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 device includes one or more internal cooling channels and a cooling channel external to the body, as described herein.

[0039] In some examples, the first medium includes a gas. In some examples, the first medium may include a gas configured to generate broadband radiation (ie, a nonlinear medium). In some examples, the first medium may include a gas mixture. In some examples, the first medium may include helium.

[0040] In some instances, the first medium comprises a liquid (e.g., water, liquid gallium and / or another liquid metal, carbon disulfide (CS2), and / or tetrachloroethylene (C2Cl4)). Advantageously, the liquid first medium provides improved heat transfer between the optical fiber and the second medium (compared to a gaseous first medium) while also allowing the optical fiber to move freely (e.g., due to thermal expansion of the enclosure and / or the optical fiber). In some examples, both the first medium and the second medium comprise a liquid such that the heat transmission medium is mostly or entirely liquid in at least one radial direction between the optical fiber and the body of the enclosure.

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

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

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

[0044] According to a second aspect of the invention there is provided a method of generating broadband radiation, the method comprising: emitting input radiation from an input radiation source; receiving the input radiation by an apparatus according to the present description; and generating an output comprising broadband radiation using the apparatus. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0046] - Figure 1 A schematic diagram of a photolithography apparatus is depicted;

[0047] - Figure 2 A schematic diagram of a lithography unit is depicted;

[0048] - Figure 3 A schematic diagram depicting the overall lithography, showing the integration of the three key technologies

[0049] to optimize semiconductor manufacturing;

[0050] - Figure 4 A schematic diagram of a scatterometry measurement tool is depicted;

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

[0052] - Figure 6 A schematic diagram of an alignment sensor metrology tool is depicted;

[0053] - Figure 7 is a schematic cross-sectional view of an optical fiber in a transverse (xy) plane (i.e., perpendicular to the fiber axis);

[0054] - Figure 8 yes Figure 7 A schematic cross-sectional view of the optical fiber shown in a plane including the axis of the optical fiber (i.e., the yz plane);

[0055] - Fig. 9 An apparatus for broadband radiation generation is depicted;

[0056] - Fig.10 Depicts the surrounding shell Fig. 9 An apparatus for generating broadband radiation of the type shown;

[0057] - Fig.11 Depicted is a schematic diagram of a radiation source for providing broadband output radiation, the radiation source comprising Fig.10 The device for broadband radiation generation shown;

[0058] - Fig.12 shows a cross section of a device for broadband radiation generation including a heat transport region according to the present invention;

[0059] - Fig.13 shows a numerical simulation of an optical fiber disposed in a gas medium, the gas medium being comprised by a solid medium, wherein the distance between the optical fiber and the solid medium is varied;

[0060] - Fig.14 Shows Fig.13 Results of numerical simulations are shown;

[0061] - Fig.15 shows a cross section of a device for broadband radiation generation according to the invention comprising a heat transport region comprising a first medium and a second medium;

[0062] - Fig.16 shows a cross section of a device for broadband radiation generation according to the present invention, the device comprising a heat transport region comprising a first medium and a second medium, wherein the second medium forms a slot extending in a direction perpendicular to the longitudinal axis of the optical fiber;

[0063] - Fig.17 A cross section of a device for broadband radiation generation including cooling channels located outside the body of the enclosure is shown;

[0064] - Fig.18 shows a cross section of a device for broadband radiation generation including one or more internal cooling channels;

[0065] - Fig.19 shows a cross section of a device for broadband radiation generation according to the invention, wherein the second medium extends only over a portion of the length of the enclosure in a direction parallel to the longitudinal axis of the optical fiber;

[0066] - Fig. 20 shows a cross section of a device for broadband radiation generation including a heat transport region according to the present invention, the device being arranged in a vertical orientation; and

[0067] - Fig.21 A method of generating broadband radiation according to the invention is schematically illustrated. DETAILED DESCRIPTION

[0068] In this document, the terms "radiation" and "beam" are used to cover all types of electromagnetic radiation, including ultraviolet radiation (e.g., wavelengths of 365nm, 248nm, 193nm, 157nm or 126nm) and EUV (extreme ultraviolet radiation, e.g., wavelengths in the range of about 5nm-100nm).

[0069] The terms "reticle", "mask" or "patterning device" as used herein may be broadly interpreted as referring to a general patterning device that can be used to impart to an incident radiation beam a patterned cross-section corresponding to the pattern to be created in a target portion of a substrate. The term "light valve" may also be used in this context. In addition to classical masks (transmissive or reflective, binary, phase-shifting, hybrid, etc.), other examples of such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0070] Figure 1 A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA comprises an illumination system (also referred to as 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 positioner PM configured to precisely position the patterning device MA according to certain parameters, a substrate support (e.g., a wafer stage) WT constructed to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to precisely position the substrate support according to certain parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0071] In operation, the illumination system IL receives a radiation beam from a radiation source SO, for example via a beam delivery system BD. The illumination system IL may include various types of optical elements for directing, shaping and / or controlling the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical elements, or any combination thereof. The illuminator IL may be used to condition the radiation beam B so that it has a desired spatial and angular intensity distribution in a cross-section at the plane of the patterning device MA.

[0072] The term "projection system" PS as used herein should be broadly interpreted as including various types of projection systems, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, depending on the exposure radiation used and / or other factors, such as the use of immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein may be considered a synonym for the more general term "projection system" PS.

[0073] The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index (e.g. water) to fill the space between the projection system PS and the substrate W, which is also referred to as immersion lithography. More information on immersion technology is given in US6952253, which is incorporated herein by reference.

[0074] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also referred to as "dual-stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or preparation steps for subsequent exposure of a substrate W may be performed on a substrate W located on one of the substrate supports WT, while another substrate W on another substrate support WT is used to expose a pattern on another substrate W.

[0075] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement stage. The measurement stage is arranged to hold sensors and / or cleaning equipment. The sensors may be arranged to measure characteristics of the projection system PS or characteristics of the radiation beam B. The measurement stage may hold a plurality of sensors. The cleaning equipment may be arranged to clean a part of the lithographic apparatus, for example a part of the projection system PS or a part of the system for providing immersion liquid. The measurement stage may be moved under the projection system PS when the substrate support WT is away from the projection system PS.

[0076] In operation, a radiation beam B is incident on a patterning device (e.g., mask) MA held on a mask support MT and is patterned by a pattern (design layout) present on the patterning device MA. After passing through the mask MA, the radiation beam B passes through a projection system PS, which focuses the beam onto a target portion C of the substrate W. With the help of a second positioner PW and a position measurement system IF, the substrate support WT can be precisely moved, for example, to position different target portions C in a focused and aligned position in the path of the radiation beam B. Similarly, the first positioner PM and possibly another position sensor ( Figure 1 The patterning device MA (not explicitly shown) can be used to accurately position the patterning device MA relative to the path of the radiation beam B. The patterning device MA and the substrate W can be aligned using the mask alignment marks M1, M2 and the substrate alignment marks P1, P2. Although the substrate alignment marks P1, P2 as shown occupy dedicated target portions, they can be located in the space between target portions. When the substrate alignment marks P1, P2 are located between the target portions C, they are called scribe lane alignment marks.

[0077] like Figure 2As shown, the lithography apparatus LA may form part of a lithography cell LC, sometimes also referred to as a lithography cell or (lithography) cluster, which typically also includes devices for performing pre-exposure and post-exposure processing on a substrate W. Conventionally, these devices include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a cooling plate CH and a baking plate BK, which is used, for example, to regulate the temperature of the substrate W, for example, for regulating a solvent in the resist layer. A substrate handler or robot RO picks up substrates W from input / output ports I / O1, I / O2, moves them between different processing devices, and transports the substrates W to a loading area LB of the lithography apparatus LA. The devices in the lithography cell (also often collectively referred to as tracks) are typically controlled by a track control unit TCU, which itself may be controlled by a monitoring system SCS, which may also control the lithography apparatus LA, for example via the lithography control unit LACU.

[0078] In order for the substrate W exposed by the lithography apparatus LA to be correctly and consistently exposed, the substrate needs to be inspected to measure characteristics of the patterned structure, such as overlay errors between subsequent layers, line thickness, critical dimensions (CD), etc. For this purpose, an inspection tool (not shown) may be included in the lithography cell LC. If an error is detected, the exposure of subsequent substrates or other processing steps to be performed on the substrate W may be adjusted, particularly if the inspection is completed before other substrates W of the same batch or lot still need to be exposed or processed.

[0079] The inspection device (which may also be referred to as a metrology device) is used to determine characteristics of the substrate W, in particular how the properties of different substrates W vary, or how the properties associated with different layers of the same substrate W vary from layer to layer. The inspection device may alternatively be configured to identify defects on the substrate W, and may for example be part of the lithography cell LC, or may be integrated into the lithography apparatus LA, or may even be a stand-alone device. The inspection device may measure characteristics on a latent image (the image in the resist layer after exposure), a semi-latent image (the image in the resist layer after a post-exposure bake step PEB), a developed resist image (wherein exposed or unexposed parts of the resist have been removed), or even an etched image (after a pattern transfer step such as etching).

[0080] Typically, the patterning process in the lithography apparatus LA is one of the most critical steps in the process, which requires high accuracy in determining the size and placement of structures on the substrate W. To ensure this high accuracy, three systems can be combined in a so-called "holistic" control environment, such as Figure 3As shown. One of these systems is a lithography apparatus LA, which is (virtually) connected to a metrology tool MT (a second system) and a computer system CL (a third system). The key to this "holistic" environment is to optimize the collaboration between these three systems to enhance the overall process window, and to provide a tight control loop to ensure that the patterning performed by the lithography apparatus LA remains within the process window. The process window defines the range of process parameters (e.g., dose, focus, overlay) within which a particular manufacturing process will produce a defined result (e.g., a functional semiconductor device), and process parameters in the lithography process or patterning process are usually allowed to vary within this range.

[0081] The computer system CL can use (a portion of) the design layout to be patterned to predict which resolution enhancement technology to use, and perform computational lithography simulations and calculations to determine which mask layout and lithography apparatus settings achieve the maximum overall process window (in Figure 3 Typically, the resolution enhancement technique is arranged to match the patterning possibilities of the lithographic apparatus LA. The computer system CL may also be used to detect the current operating position of the lithographic apparatus LA within the process window (e.g. using input from the metrology tool MT) to predict whether defects (e.g. due to a suboptimal process) may exist. Figure 3 (as indicated by the arrow pointing to “0” in the second scale SC2).

[0082] The metrology tool MT may provide input to the computer system CL to enable accurate simulations and predictions, and may provide feedback to the lithographic apparatus LA to identify possible drifts, e.g. in the calibration state of the lithographic apparatus LA (e.g. Figure 3 Different types of metrology tools MT for measuring one or more characteristics associated with the lithographic apparatus and / or the substrate to be patterned will now be described.

[0083] In the lithography process, it is necessary to frequently measure the created structures, for example for process control and verification. The tool for performing such measurements is generally referred to as a measurement tool MT. Different types of measurement tools MT for performing such measurements are known, including scanning electron microscopes or various forms of scatterometer measurement tools MT. A scatterometer is a multifunctional instrument that can measure parameters of a lithography process by installing a sensor in the pupil of the objective lens of the scatterometer or in a plane conjugated with the pupil (generally referred to as pupil-based measurement), or by installing a sensor in an image plane or in a plane conjugated with the image plane (in this case, measurement is generally referred to as image-based or field-based measurement). Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP1,628,164A, the entire contents of which are incorporated herein by reference. The scatterometers described above can measure gratings using light from soft x-rays and visible to near-infrared wavelengths.

[0084] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, reconstruction methods can be applied to the measured signals to reconstruct or calculate the properties of the grating. For example, such a reconstruction can be achieved by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measured results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from a real target.

[0085] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, radiation emitted by a radiation source is directed onto a target, and reflected or scattered radiation from the target is directed to a spectrometer detector which measures the spectrum of the specularly reflected radiation (i.e. a measure of the variation of intensity with wavelength). From these data, the structure or profile of the target which gave rise to the detected spectrum can be reconstructed, for example by rigorous coupled wave analysis and nonlinear regression, or by comparison with a library of simulated spectra.

[0086] In a third embodiment, the scatterometer MT is an ellipsometer. An ellipsometer allows the parameters of a lithography process to be determined by measuring the scattered radiation in each polarization state. Such a measurement device emits polarized light (such as linear, circular or elliptically polarized light) by using an appropriate polarization filter in the irradiation portion of the measurement device. A source suitable for the measurement device can also provide polarized radiation. Various embodiments of existing ellipsometers are described in U.S. patent applications 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.

[0087] In one embodiment of the scatterometer MT, the scatterometer MT is suitable for measuring the overlay of two misaligned gratings or periodic structures by measuring the reflection spectrum and / or an asymmetry in the detection configuration, the asymmetry being related to the degree of overlay. The two (usually overlapping) grating structures can be applied to two different layers (not necessarily continuous layers) and can be formed at substantially the same position on the wafer. The scatterometer can have a symmetrical detection configuration, such as described in co-owned patent application EP1,628,164A, so that any asymmetry can be clearly distinguished. This provides a method for directly measuring grating misalignment. Other examples of measuring the overlay error between two layers including a periodic structure as a target by means of the asymmetry of the periodic structure can be found in PCT patent application publication number WO 2011 / 012624 or in US patent application US20160161863, the entire contents of which are incorporated herein by reference.

[0088] Other parameters of interest may be focus and dose. Focus and dose may be determined simultaneously by scatterometry (or scanning electron microscopy), as described in U.S. Patent Application US2011-0249244, the entire contents of which are incorporated herein by reference. A single structure may be used that has a unique combination of critical dimension and sidewall angle measurements for each point in the focus energy matrix (FEM, also called focus exposure matrix). If these unique combinations of critical dimension and sidewall angle are available, focus and dose values ​​may be uniquely determined from these measurements.

[0089] The metrology target can be a collection of composite gratings formed by a lithography process, mainly in resist, but also for example after an etching process. Typically, the pitch and line width of the structures in the gratings strongly rely on the measurement optics (particularly the NA of the optics) in order to be able to capture the diffraction orders from the metrology target. As indicated before, the diffraction signal can be used to determine the offset between two layers (also called "overlay") and can also be used to reconstruct at least a portion of the original grating produced by the lithography process. This reconstruction can be used to provide guidance on the quality of the lithography process and can be used to control at least a portion of the lithography process. The target can have smaller subdivisions that are configured to mimic the size of a functional portion of the design layout in the target. Due to this subdivision, the behavior of the target will be more similar to the functional portion of the design layout, so that the overall process parameter measurement better resembles the functional portion of the design layout. The target can be measured in an unfilled mode or in an overfilled mode. In the unfilled mode, the spot generated by the measurement beam is smaller than the overall target. In the overfilled mode, the spot generated by the measurement beam is larger than the overall target. In this overfilled mode, different targets can also be measured simultaneously, thereby determining different process parameters simultaneously.

[0090] The overall measurement quality of a lithography parameter using a specific target is at least partially determined by the measurement recipe used to measure the lithography parameter. The term "substrate measurement recipe" may include one or more parameters of the measurement itself, one or more parameters of the one or more patterns measured, or both. For example, if the measurement used in the substrate measurement recipe is a diffraction-based optical measurement, the one or more parameters measured may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, and the like. For example, one of the criteria for selecting a measurement recipe may be the sensitivity of one of the measurement parameters to process variations. More examples are described in U.S. patent application US2016-0161863 and published U.S. patent application US2016 / 0370717A1, the entire contents of which are incorporated herein by reference.

[0091] Figure 4 A measurement device, such as a scatterometer SM1, is depicted in FIG. 1 . It comprises a broadband (white light) radiation projector 2, which projects radiation onto a substrate 6. The reflected or scattered radiation is passed to a spectrometer detector 4, which measures the spectrum 10 of the specularly reflected radiation (i.e., a measurement of the intensity In1 as a function of wavelength λ). From these data, a processing unit PU can reconstruct the structure or profile that gave rise to the detected spectrum, for example by rigorous coupled wave analysis and nonlinear regression, or with Figure 4A library of simulated spectra is shown at the bottom for comparison. Typically, for the reconstruction, the general form of the structure is known and some parameters are assumed based on knowledge of the process by which the structure was made, leaving only a few parameters of the structure to be determined from the scatterometry data. Such a scatterometer can be configured as either a normal-incidence scatterometer or an oblique-incidence scatterometer.

[0092] In photolithography processes, it is frequently necessary to measure the structures created, for example for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes or various forms of metrology devices, such as scatterometers. Examples of known scatterometers typically rely on the provision of dedicated metrology targets, such as an unfilled target (a target in the form of a simple grating or overlapping gratings in different layers, which is large enough to allow the measurement beam to generate a spot smaller than the grating) or an overfilled target (i.e. the irradiation spot partially or completely includes the target). In addition, the use of a metrology tool (e.g. an angle-resolved scatterometer that irradiates an unfilled target such as a grating) allows the use of so-called reconstruction methods, in which the properties of the grating can be calculated by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measurement results. The parameters of the model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from a real target.

[0093] A scatterometer is a versatile instrument that allows to measure parameters of a lithography process by mounting a sensor in the pupil of the objective of the scatterometer or in a plane conjugate to the pupil, the measurement usually being referred to as pupil-based measurement, or by mounting the sensor in the image plane or in a plane conjugate to the image plane, in which case the measurement is usually referred to as image-based or field-based measurement. Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP1,628,164A, the entire contents of which are incorporated herein by reference. The above scatterometer can measure multiple targets from multiple gratings in one image using light from soft x-rays and visible to near infrared wavelength range.

[0094] A topography measurement system, a level sensor or a height sensor, which can be integrated in a lithography apparatus, is arranged to measure the topography of the top surface of a substrate (or wafer). A topography map (also called a height map) of the substrate can be generated based on these measurements, indicating the height of the substrate as a function of position on the substrate. The height map can then be used to correct the position of the substrate during transfer of a pattern on the substrate so as to provide an aerial image of the patterning device at an appropriate focus position on the substrate. It will be understood that in this context, "height" refers to a dimension that is substantially beyond the plane of the substrate (also called the Z-axis). Typically, the level or height sensor performs measurements at a fixed position (relative to its own optical system), and the relative movement between the substrate and the optical system of the level or height sensor results in height measurements at different locations on the substrate.

[0095] Examples of level or height sensors LS known in the art are Figure 5 As schematically shown in Figure 5 Only the operating principle is shown. In this example, the level sensor comprises an optical system comprising a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO, which provides a radiation beam LSB applied by a projection grating PGR of the projection unit LSP. For example, the radiation source LSO may be a narrowband or broadband radiation source, such as a supercontinuum light source, polarized or unpolarized, pulsed or continuous, such as a polarized or unpolarized laser beam. The radiation source LSO may include a plurality of radiation sources with 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, but may additionally or alternatively include UV and / or IR radiation and any wavelength range suitable for reflection from the substrate surface.

[0096] The projection grating PGR is a periodic grating including a periodic structure that generates a radiation beam BE1 with a periodically varying intensity. The radiation beam BE1 with a periodically varying intensity is directed toward a measurement position MLO on a substrate W, with an incident angle ANG of between 0 and 90 degrees, typically between 70 and 80 degrees, relative to an axis (Z axis) perpendicular to the incident substrate surface. At the measurement position MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and directed toward a detection unit LSD.

[0097] In order to determine the height level at the measuring position MLO, the level sensor further comprises a detection system comprising a detection grating DGR, a detector DET and a processing unit (not shown) for processing an output signal of the detector DET. The detection grating DGR may be identical to the projection grating PGR. The detector DET generates a detector output signal which is indicative of the received light, for example indicating the intensity of the received light, such as a photodetector, or representing the spatial distribution of the received intensity, such as a camera. The detector DET may comprise any combination of one or more detector types.

[0098] By means of triangulation techniques the height level of the measurement position MLO can be determined. The detected height level is usually related to the signal intensity measured by the detector DET, which has a periodicity that depends, inter alia, on the design of the projection grating PGR and the (tilted) angle of incidence ANG.

[0099] The projection unit LSP and / or the detection unit LSD may comprise further optical elements such as lenses and / or mirrors (not shown) along the path of the patterned radiation beam between the projection grating PGR and the detection grating DGR.

[0100] In one embodiment, the detection grating DGR can be omitted and the detector DET can be placed where the detection grating DGR is located. This configuration provides a more direct detection of the image of the projection grating PGR.

[0101] In order to effectively cover the surface of the substrate W, the level sensor LS may be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating a measurement area MLO or spot array covering a larger measurement range.

[0102] Various general types of height sensors are disclosed, for example, in US7265364 and US7646471, both of which are incorporated herein by reference. A height sensor using UV radiation rather than visible or infrared radiation is disclosed in US2010233600A1, which is incorporated herein by reference. In WO2016102127A1, which is incorporated by reference, a compact height sensor is described that uses a multi-element detector to detect and identify the position of a grating image without the need for a detection grating.

[0103] The position measurement system PMS may comprise any type of sensor suitable for determining the position of the substrate support WT. The position measurement system PMS may comprise any type of sensor suitable for determining the position of the mask support MT. The sensor may be an optical sensor, such as an interferometer or an encoder. The position measurement system PMS may comprise a combined system of an interferometer and an encoder. The sensor may be another type of sensor, such as a magnetic sensor, a capacitive sensor or an inductive sensor. The position measurement system PMS may determine the position relative to a reference, such as a 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 a velocity or an acceleration.

[0104] The position measurement system PMS may include an encoder system. For example, an encoder system is known from US patent application US2007 / 0058173A1 filed on September 7, 2006, which is incorporated herein by reference. The encoder system includes an encoder head, a grating and a sensor. The encoder system can receive a primary radiation beam and a secondary radiation beam. Both the primary radiation beam and the secondary radiation beam originate from the same radiation beam, i.e., the original radiation beam. At least one of the primary radiation beam and the secondary radiation beam is generated by diffracting the original radiation beam by the grating. If both the primary radiation beam and the secondary radiation beam are generated by diffracting the original radiation beam by the grating, the primary radiation beam needs to have a different diffraction order from the secondary radiation beam. For example, the different diffraction orders are +1, -1, +2 and -2. 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 may be arranged on the substrate structure WT. The other of the encoder head and the grating may be arranged on the metrology frame MF or the base frame BF. For example, a plurality of encoder heads are arranged on the metrology frame MF, and the grating is arranged on the top surface of the substrate support WT. In another example, the grating is arranged on the bottom surface of the substrate support WT, and the encoder head is arranged below the substrate support WT.

[0105] The position measurement system PMS may include an interferometer system. Interferometer systems are known from, for example, U.S. Pat. No. 6,020,964 filed on July 13, 1998, which is incorporated herein by reference. The interferometer system may include a beam splitter, a reflector, a reference reflector, and a sensor. The radiation beam is split into a reference beam and a measurement beam by the beam splitter. The measurement beam propagates to the reflector and is reflected back to the beam splitter by the reflector. The reference beam propagates to the reference reflector and is reflected back to the beam splitter by the reference reflector. At the beam splitter, the measurement beam and the reference beam are combined into a combined radiation beam. The combined radiation beam is incident on the sensor. The sensor determines the phase or frequency of the combined radiation beam. The sensor generates a signal based on the phase or frequency. The signal represents the displacement of the reflector. In one embodiment, the reflector is connected to the substrate support WT. The reference reflector can be connected to the measurement frame MF. In one embodiment, the measurement beam and the reference beam are combined into a combined radiation beam by an additional optical element instead of a beam splitter.

[0106] When manufacturing complex devices, many lithographic patterning steps are typically performed to form functional features in successive layers on a substrate. Therefore, a key aspect of the performance of a lithographic apparatus is the ability to correctly and accurately place the applied pattern relative to features placed in previous layers (by the same apparatus or a different lithographic apparatus). To this end, one or more sets of marks are provided on the substrate. Each mark is a structure whose position can be measured later using a position sensor, typically an optical position sensor. The position sensor may be referred to as an "alignment sensor" and the mark may be referred to as an "alignment mark". The mark may also be referred to as a measurement target.

[0107] The lithographic apparatus may include one or more (e.g., multiple) alignment sensors, by which the positions of alignment marks provided on the substrate can be accurately measured. The alignment (or position) sensor can obtain position information from alignment marks formed on the substrate using optical phenomena such as diffraction and interference. An example of an alignment sensor currently used in a lithographic apparatus is based on a self-referencing interferometer described in US6961116. Various enhancements and modifications of position sensors have been developed, such as disclosed in US2015261097A1. All of these disclosed contents are incorporated herein by reference.

[0108] The mark or alignment mark may comprise a series of strips formed on or in a layer provided on the substrate, or comprise strips formed (directly) in the substrate. These strips may be regularly spaced and act as grating lines, so that the mark may be considered as a diffraction grating with a known spatial period (pitch). Depending on the direction of these grating lines, a mark may be designed to allow measurement of a position along the X-axis or along the Y-axis (whose direction is substantially perpendicular to the X-axis). Marks comprising strips arranged at +45 degrees and / or -45 degrees relative to the X-axis and the Y-axis allow combined X and Y measurements to be performed using the techniques described in US2009 / 195768A, which is incorporated herein by reference.

[0109] The alignment sensor optically scans each mark with a radiation spot to acquire a periodically varying signal, such as a sine wave. The phase of this signal is analyzed to determine the position of the mark and thus the position of the substrate relative to the alignment sensor, which in turn is fixed relative to the reference frame of the lithographic apparatus. So-called coarse and fine marks associated with different (coarse and fine) mark sizes may be provided so that the alignment sensor can distinguish between different periods of the periodic signal, as well as the exact position (phase) within a period. Marks of different pitches may also be used for this purpose.

[0110] Measuring the position of the marks may also provide information about deformation of the substrate on which the marks are provided, for example in the form of a wafer grid. Deformation of the substrate may occur, for example, due to electrostatic clamping of the substrate to the substrate stage and / or heating of the substrate when the substrate is exposed to radiation.

[0111] Figure 6 is a schematic block diagram of an embodiment of a known alignment sensor AS, such as described in US6961116, which is incorporated herein by reference. A radiation source RSO provides a radiation beam RB of one or more wavelengths, which is steered by an optical device onto a mark, such as a mark AM located on a substrate W, as an illumination spot SP. In this example, the steering optics include a spot reflector SM and an objective lens OL. The diameter of the illumination spot SP illuminating the mark AM may be slightly smaller than the width of the mark itself.

[0112] The radiation diffracted by the mark AM is collimated (in this case via an objective lens OL) into a beam IB carrying the information. The term "diffraction" is intended to include the zeroth order diffraction (which may be referred to as reflection) from the mark. A self-referencing interferometer SRI, for example of the type disclosed in the above-mentioned US6961116, causes the beam IB to interfere with itself, after which the beam is received by a photodetector PD. In the case where the radiation source RSO produces more than one wavelength, additional optics (not shown) may be included to provide separate beams. The photodetector may be a single element or, if desired, may comprise a plurality of pixels. The photodetector may comprise an array of sensors.

[0113] The steering optics comprising a spot mirror SM in this example may also be used to block zeroth order radiation reflected from the mark so that the information carrying beam IB comprises only higher order diffracted radiation from the mark AM (this is not essential for the measurement but may improve the signal to noise ratio).

[0114] The intensity signal SI is provided to the processing unit PU. By a combination of optical processing in block SRI and computational processing in unit PU, the values ​​of the X and Y position on the substrate relative to the reference frame are output.

[0115] 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. Coarser measurement techniques are used in conjunction with this to identify which period of the sine wave includes the position of the mark. The same process at coarser and / or finer levels can be repeated at different wavelengths to increase accuracy and / or robustly detect the mark, regardless of what material the mark is made of and the material on and / or below which the mark is placed. The wavelengths can be optically multiplexed and demultiplexed for simultaneous processing, and / or they can be multiplexed by time or frequency division.

[0116] In this example, the alignment sensor and the spot SP remain stationary, while the substrate W moves. Thus, the alignment sensor can be rigidly and accurately mounted to the reference frame, while effectively scanning the mark AM in a direction opposite to the direction of movement of the substrate W. The substrate W is controlled in this movement by mounting it on a substrate support and by a substrate positioning system that controls the movement of the substrate support. A substrate support position sensor (e.g., an interferometer) measures the position of a substrate support (not shown). In one embodiment, one or more (alignment) marks are provided on the substrate support. Measurement of the position of the marks provided on the substrate support allows calibration of the position of the substrate support determined by the position sensor (e.g., relative to a frame to which the alignment system is connected). By measuring the position of the alignment marks provided on the substrate, the position of the substrate relative to the substrate support can be determined.

[0117] A measurement tool MT, such as a scatterometer, a topography measurement system or a position measurement system as described above, may perform measurements using radiation originating from a radiation source. The characteristics of the radiation used by the measurement tool may affect the type and quality of the measurements that can be performed. For some applications, it may be advantageous to use multiple radiation frequencies to measure a substrate, for example broadband radiation may be used. Multiple different frequencies may be able to propagate, illuminate and scatter the measurement target without causing interference or minimal interference to other frequencies. Thus, for example, different frequencies may be used to acquire more measurement data simultaneously. Different radiation frequencies may also be able to interrogate and discover different characteristics of the measurement target. Broadband radiation may be used for a measurement system MT, such as a level sensor, an alignment mark measurement system, a scattering measurement tool or an inspection tool. The broadband radiation source may be a supercontinuum source.

[0118] It may be difficult to generate high quality broadband radiation, such as supercontinuum radiation. One method for generating broadband radiation may be to broaden high power narrowband or single frequency input radiation, for example by exploiting nonlinear, higher order effects. The input radiation (which may be generated using a laser) may be referred to as pump radiation. In order to obtain high power radiation for the broadening effect, the radiation may be confined to a small region, thereby achieving strong local high intensity radiation. In these regions, the radiation may interact with the broadening structures and / or materials forming the nonlinear medium, thereby producing broadband output radiation. In the high intensity radiation region, different materials and / or structures may be used to achieve and / or improve radiation broadening by providing a suitable nonlinear medium.

[0119] In some implementations, as shown below Figures 9 to 11 As further discussed, methods and apparatus for broadening input radiation may use an optical fiber to confine the input radiation, and broaden the input radiation to output broadband radiation. The optical fiber may be a hollow core fiber, and may include an internal structure to achieve efficient guidance and confinement of radiation in the optical fiber. The optical fiber may be a hollow core photonic crystal fiber (HC-PCF), which is particularly suitable for strong radiation confinement, mainly within the hollow core of the optical fiber, to achieve high radiation intensity. The hollow core of the optical fiber may be filled with a gas as a broadening medium for broadening the input radiation. This optical fiber and gas arrangement may be used to create a supercontinuum radiation source. The radiation input to the optical fiber may be electromagnetic radiation, such as radiation in one or more of the infrared, visible, ultraviolet, and extreme ultraviolet spectra. The output radiation may consist of or include broadband radiation, which may be referred to herein as white light.

[0120] Embodiments of the present invention relate to new designs of optical fibers for such broadband radiation sources, and broadband radiation sources comprising the novel optical fibers. The novel optical fiber is a hollow core photonic crystal fiber (HC-PCF). In particular, the novel optical fiber is a hollow core photonic crystal fiber that includes an antiresonant structure for limiting radiation. Such optical fibers including antiresonant structures are known in the art as antiresonant fibers, tubular fibers, single ring fibers, negative curvature fibers, or suppressed coupling fibers. Various different designs of such optical fibers are known in the art.

[0121] It should be understood that an antiresonant element refers to an element arranged to confine radiation within a hollow core primarily by antiresonance. In particular, the term antiresonant element is not intended to include an element arranged to confine radiation within a hollow core primarily by creating a photonic bandgap in a cladding portion (e.g., a Kagome photonic crystal fiber). Pure photonic bandgap fibers provide very low losses within a very limited bandwidth. Optical fibers that use antiresonant elements to guide radiation can have a wider transmission window (i.e., have a larger transmission bandwidth) than photonic bandgap fibers. Therefore, advantageously, such optical fibers can be more suitable for use in devices (e.g., supercontinuum sources) for receiving input radiation and widening the frequency range of the input radiation to provide output radiation.

[0122] Figure 7 and Figure 8 is a schematic cross-sectional view of an example of a hollow core optical fiber (ie, hollow core optical fiber) 100 in two mutually perpendicular planes, namely an xy plane and a yz plane.

[0123] The optical fiber 100 includes an elongated body that is longer in one dimension than the other two dimensions of the optical fiber 100. The longer dimension may be referred to as the axial direction and may define an axis 101, also referred to as the longitudinal axis, of the optical fiber 100. The other two dimensions define what may be referred to as transverse planes. Figure 7 A cross section of the optical fiber 100 is shown in a transverse plane (ie perpendicular to the axis 101), which plane is labeled the xy plane. Figure 8 A cross section of the optical fiber 100 is shown in a plane including the axis 101 , in particular the xz plane. The transverse cross section of the optical fiber 100 may be substantially constant along the optical fiber axis 101 .

[0124] It should be understood that the optical fiber 100 has a certain degree of flexibility, and therefore the direction of the axis 101 is generally not consistent along the length of the optical fiber 100. Terms such as the optical axis 101, transverse cross section, etc. will be understood to refer to the local optical axis 101, local transverse cross section, etc. In addition, when components are described as cylindrical or tubular, these terms will be interpreted to include shapes that may have deformed when the optical fiber 100 is bent.

[0125] The optical fiber 100 may have any length, and it will be appreciated that the length of the optical fiber 100 may depend on the application (e.g., the amount of spectral broadening desired in applications within a supercontinuum radiation source). The length of the optical fiber 100 may be between 1 cm and 10 m, for example, the length of the optical fiber 100 may be between 10 cm and 100 cm.

[0126] 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 confine radiation propagating through the optical fiber 100 to be primarily located within the hollow core 102 and to guide the radiation along the optical fiber 100. The hollow core 102 of the optical fiber 100 may be substantially disposed in a central region of the optical fiber 100, such that the axis 101 of the optical fiber 100 may also define the axis of the hollow core 102 of the optical fiber 100.

[0127] The inner cladding region includes a plurality of capillaries 104, such as tubular capillaries, surrounding the hollow core 102. In particular, Figure 7 and Figure 8 In the example shown, the inner cladding region includes a single ring of six tubular capillaries 104 .

[0128] The capillary 104 may also be referred to as a tube. The cross-section of the capillary 104 may be circular, or may have other shapes. Each capillary 104 includes a generally cylindrical wall portion 105 that at least partially defines the hollow core 102 of the optical fiber 100 and separates the hollow core 102 from the cavity 106. Each capillary wall portion 105 facing the hollow core serves as an anti-resonant element for guiding radiation propagating through the optical fiber 100. It should be understood that the wall portion 105 may serve as an anti-reflection Fabry-Perot resonator for radiation propagating through the hollow core 102 (and the radiation may be incident on the wall portion 105 at a grazing angle of incidence). The thickness 160 of the wall portion 105 may be suitable to ensure that reflections returning to the hollow core 102 are generally enhanced, while transmission into 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.

[0129] It should be understood that, as used herein, the term inner cladding region is intended to refer to the region of the optical fiber 100 that is used to guide radiation propagating through the optical fiber 100 (i.e., the capillary 104 that confines the radiation within the hollow core 102). The radiation may be confined in the form of a transverse mode to propagate along the optical fiber axis 101.

[0130] The sheath region 110 is generally tubular and supports the capillaries 104 of the inner cladding region. The capillaries 104 are evenly distributed around the inner surface of the sheath region 110. Six capillaries 104 can be described as being arranged in a symmetrical arrangement around the hollow core 102. In an embodiment including six capillaries 104, the capillaries 104 can be described as being arranged in a generally hexagonal form.

[0131] The capillaries 104 are arranged so that each capillary does not contact any other capillary 104. Each capillary 104 contacts the jacket region 110 and is spaced apart from adjacent capillaries 104 in an annular structure. This arrangement can be beneficial because it can increase the transmission bandwidth of the optical fiber 100 (e.g., relative to an arrangement in which the capillaries contact each other). Alternatively, in some embodiments, each capillary 104 can contact adjacent capillaries 104 in an annular structure.

[0132] The six capillaries 104 of the inner cladding region are arranged in an annular structure around the hollow core 102. The inner surface of the annular 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 can be defined as the smallest dimension between opposing capillaries, as shown by arrows 114) can be between 5 μm and 100 μm. In some embodiments, the diameter 114 of the hollow core 102 can be between 5 μm and 50 μm. In some embodiments, the diameter 114 of the hollow core 102 can 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 complexity and nonlinear characteristics of the hollow core optical fiber 100.

[0133] exist Figure 7 and Figure 8 In the embodiment shown, the inner cladding region includes a single ring arrangement of capillaries 104 (facing the hollow core wall portion 105, which acts as an anti-resonance element). Therefore, a line in any radial direction from the center of the hollow core 102 to the outside of the optical fiber 100 passes through no more than one capillary 104.

[0134] It should be understood that other embodiments may be provided with different anti-resonance element arrangements. These may include arrangements with multiple anti-resonance element rings and arrangements with nested anti-resonance elements. Figure 7 and Figure 8 The illustrated embodiment comprises a ring of six capillaries 104 having a wall portion 105, but in other embodiments one or more rings comprising any number of antiresonant elements (e.g., 4, 5, 6, 7, 8, 9, 10, 11 or 12 capillaries) may be provided in the inner cladding region.

[0135] exist Figure 7 and Figure 8 In the illustrated embodiment, the inner cladding region comprises a circular cross-section. However, it should be understood that other embodiments may provide an inner cladding region having a non-circular cross-sectional shape. For example, in one embodiment of the present invention, the inner cladding region may have a hexagonal cross-section. The hexagonal cross-section may advantageously facilitate easier placement of the capillaries 104 in a symmetrical arrangement. For example, six capillaries 104 may each be placed at the vertices of the hexagonal cross-section to provide an arrangement of the capillaries 104 having hexagonal symmetry.

[0136] The optical fiber 100 may be referred to as a hollow core photonic crystal fiber (HC-PCF). Typically, such a hollow core photonic crystal fiber includes an inner cladding region (which may include antiresonant elements, for example) for guiding radiation within the optical fiber and a jacket region. The jacket region is typically a jacket or tube of material that supports the inner cladding region.

[0137] Fig. 9 The overall arrangement of an apparatus 120 for receiving input radiation 122 and broadening the frequency range of the input radiation 122 to provide broadband output radiation 124 is schematically shown. The apparatus 120 comprises an optical fiber 100 having a hollow core 102 for guiding radiation propagating through the optical fiber 100. It should be appreciated that to aid in the clarity of the figure, Fig. 9 Only the hollow core 102 of the optical fiber 100 is shown (and the cladding and support portions are not distinguished). The device 120 also includes a gas 126 disposed within the hollow core 102, wherein the gas includes a working component capable of broadening the frequency range of the received input radiation 126 to provide broadband output radiation 124.

[0138] The working component of the gas 126 may be an inert gas. The working component may include one or more of argon, krypton, neon, helium, and xenon. As an alternative or in addition to the inert gas, the working component may include a molecular gas (e.g., H2, N2, O2, CH4, SF6).

[0139] In one implementation, the gas 126 may be disposed within the hollow core 102 at least during the period of receiving the input radiation 122 to produce the broadband output radiation 124. It should be understood that the gas 126 may be completely or partially absent from the hollow core 102 when the apparatus 120 is not receiving the input radiation 122 for producing the broadband output radiation. Generally, the apparatus 120 includes a means for providing the gas 126 within the hollow core 102 of the optical fiber 100. Such means for providing the gas 126 within the hollow core 102 of the optical fiber 100 may include a reservoir, as now referred to Fig.10 discussed.

[0140] Fig.10 The embodiment also includes a casing 128. Fig. 9 1. The optical fiber 100 is disposed within an enclosure 128. The enclosure 128 may also be referred to as a reservoir, a housing, or a container. The enclosure 128 includes a heat transfer region 127. The heat transfer region 127 may include a gas, such as the same gas 126 as within the hollow core 102 of the optical fiber 100, and / or helium. As described herein, the heat transfer region 127 may also include a second medium, which may include a solid and / or a liquid, for improving heat conduction between the optical fiber 100 and the body or exterior of the enclosure 128.

[0141] The enclosure 128 may include one or more features known in the art for controlling, regulating and / or monitoring the composition of any gas within the enclosure 128. The reservoir may include a first transparent window 130. In use, the optical fiber 100 is disposed within the enclosure 128 such that the first transparent window 130 is located near the input end of the optical fiber 100. The first transparent window 130 may form a portion of a wall of the enclosure 128. The first transparent window 130 is transparent to at least the frequencies of the received input radiation so that the received output radiation 122 (or at least a substantial portion thereof) may be coupled into the optical fiber 100 located within the enclosure 128. The enclosure 128 may include a second transparent window 132 that forms a portion of a wall of the enclosure 128. In use, when the optical fiber 100 is disposed within the enclosure 128, the second transparent window 132 is located near the output end of the optical fiber 100. The second transparent window 132 is transparent to at least the frequencies of the broadband output radiation 124 of the device 120.

[0142] Alternatively, in another embodiment, the two opposite ends of the optical fiber 100 can be placed in different enclosures. The optical fiber 100 can include a first end configured to receive input radiation 122 and a second end for outputting broadband output radiation 124. The first end can be placed in a first enclosure including a heat transfer region 127. The second end can be placed in a second enclosure, wherein the second enclosure can also include a heat transfer region 127. The functions of the enclosures can be as described above. Fig.10 The first enclosure may include a first transparent window configured to be transparent to the input radiation 122. The second enclosure may include a second transparent window configured to be transparent to the broadband output broadband radiation 124. The first enclosure and the second enclosure may also include a sealable opening for allowing the optical fiber 100 to be partially placed inside the enclosure and partially placed outside the enclosure, thereby sealing the gas inside the enclosure. The optical fiber 100 may also include an intermediate portion that is not included in the enclosure. For relatively long embodiments of the optical fiber 100 (for example, when the length is greater than 1m), this arrangement using two separate enclosures may be particularly convenient. It should be understood that for this arrangement using two separate enclosures, the two enclosures (which may include one or more features known in the art for controlling, regulating and / or monitoring the composition of any gas in the two enclosures) can be considered to provide a means for providing a gas 126 in the hollow core 102 of the optical fiber 100.

[0143] In this case, a window may be transparent to a frequency if at least 50%, 75%, 85%, 90%, 95% or 99% of incident radiation of that frequency on the window is transmitted through the window.

[0144] Both the first transparent window 130 and the second transparent window 132 can form a gas-tight seal within the wall of the enclosure 128, so that the gas 126 and any gas or liquid of the heat transfer area 127 can be contained within the enclosure 128. It should 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.

[0145] In order to achieve frequency broadening, high intensity radiation may be required. An advantage of having a hollow core fiber 100 is that it can achieve high intensity radiation by strong spatial confinement of the radiation propagating through the fiber 100, thereby achieving a high local radiation intensity. In addition, the hollow core design (e.g., compared to a solid core design) can produce a higher quality transmission mode (e.g., having a greater proportion of single-mode transmission). The radiation intensity within the fiber 100 can be high, for example due to a high received input radiation intensity and / or due to the strong spatial confinement of the radiation within the fiber 100.

[0146] An advantage of using a hollow core optical fiber 100 may be that a majority of the radiation guided within the optical fiber 100 is confined within the hollow core 102. Therefore, a majority of the interactions of the radiation within the optical fiber 100 are interactions with the gas 126 provided within the hollow core 102 of the optical fiber 100. As a result, the broadening effect of the working components of the gas 126 on the radiation may be increased.

[0147] The received input radiation 122 may be electromagnetic radiation. The input radiation 122 may be received as pulsed radiation. For example, the input radiation 122 may include ultrafast pulses. When the radiation interacts with the gas 126, the mechanism of spectral broadening may be, for example, one or more of four wave mixing, modulation instability, working gas ionization, Raman effect, Kerr nonlinearity, soliton formation, or soliton fission. In particular, spectral broadening may be achieved by one or both of soliton formation or soliton fission.

[0148] The input radiation 122 may be coherent radiation. The input radiation 122 may be collimated radiation, which may have the advantage of facilitating and increasing the efficiency of coupling the input radiation 122 into the optical fiber 100. The input radiation 122 may include a single frequency or a narrow range of frequencies. The input radiation 122 may be generated by a laser. Similarly, the output radiation 124 may be collimated and / or may be coherent.

[0149] 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. Continuous radiation can be advantageous for use in many applications, such as in measurement applications. For example, a continuous frequency range can be used to query a large number of characteristics. For example, a continuous frequency range can be used to determine and / or eliminate the frequency dependence of a measured characteristic. The supercontinuum output radiation 124 can include, for example, electromagnetic radiation having a wavelength range of 100nm-4000nm. The frequency range of the broadband output radiation 124 can be, for example, 400nm-900nm, 500nm-900nm, or 200nm-2000nm. The supercontinuum output radiation 124 can include white light.

[0150] Fig.11 A radiation source 134 is depicted for providing broadband output radiation. The radiation source 134 includes a Fig.10 The device 120 described. The radiation source 34 also includes an input radiation source 136 configured to provide the input radiation 122 to the device 120. The device 120 can receive the input radiation 122 from the input radiation source 136 and broaden it to provide the output radiation 124.

[0151] The input radiation 122 provided by the input radiation source 136 may be pulsed. The input radiation 122 may include one or more electromagnetic radiations having a frequency 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 in the order of 1 kHz to 100 MHz. The pulse energy may be in the order of 0.1 μJ to 100 μJ, for example, 1 μJ-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 hundred W. For example, the average power of the input radiation 122 may be 20 W-50 W.

[0152] 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 of at least 0.01 mW / nm across the wavelength band of the output radiation. The power spectral density across the wavelength band of the broadband output radiation may be at least 3 mW / nm.

[0153] The above-mentioned radiation source 134 can be provided as part of a measurement arrangement for determining a parameter of interest of a structure on a substrate. The structure on the substrate can be, for example, a photolithographic pattern applied to the substrate. The measurement arrangement can also include an illumination subsystem for illuminating the structure on the substrate. The measurement arrangement can also include a detection subsystem for detecting a portion of the radiation scattered and / or reflected by the structure. The detection subsystem can also determine the parameter of interest on the structure based on the portion of the radiation scattered and / or reflected by the structure. For example, the parameter can be overlay, alignment or leveling data of the structure on the substrate.

[0154] The above-described metrology arrangement may form part of a metrology apparatus MT.The above-described metrology arrangement may form part of an inspection apparatus.The above-described metrology arrangement may be comprised in a lithographic apparatus LA.

[0155] It should be understood that Figure 9-11 The device 120 and radiation source 134 are shown in cross-section in the yz plane.

[0156] The nonlinear white light generation process is not loss-free, since the intense pump pulses can partially ionize the working components of the gas 126. During the subsequent plasma relaxation process, the lost power is converted into heat, resulting in an increase in the temperature inside the fiber 100 and in the fiber 100 itself. This heat is transferred via the surrounding heat transfer medium 127 (usually a gas) to the (usually metal) body of the enclosure 128, where it is dissipated to a cooling loop. As an example, the power deposited in the system can be as high as several watts per meter.

[0157] Typically, the working component(s) of the gas 126 are suboptimal thermal conductors, e.g., about 10E-3 W / m K in the case of argon and 9E-3 W / m K in the case of krypton, resulting in relatively high peak temperatures within the optical fiber. Numerical simulations have shown that, with a few W / m of energy deposited in the optical fiber, the temperature within the HC-PCF can reach about 1000° C. Without wishing to be bound by theory, such high temperatures may cause damage to the optical fiber in the form of mechanical deformation of the optical fiber (e.g., capillary) and / or deposition of material at the input and / or output ends of the optical fiber 100.

[0158] Therefore, there is a need to improve the heat transfer (ie, thermal conductivity, such as by conduction, convection, and / or radiation) from the optical fiber 100 to the body of the enclosure 128 to mitigate the above-mentioned effects.

[0159] Fig.12 A cross section of a device 120 for broadband radiation generation in the xy plane (ie in a plane perpendicular to the longitudinal axis 101 of the optical fiber 100) is shown, which generally corresponds to Fig.10 and Fig.11 As described above, the optical fiber 100 is shown disposed in the housing 128. Fig.12 The optical fiber 100 is shown as being centrally (ie, coaxially) extending through the enclosure 128, but it is understood that this need not be the case and the optical fiber 100 may likewise be positioned off-center.

[0160] When the device 120 is in use (i.e., when input radiation is input to the optical fiber 100 for broadband radiation generation), the optical fiber 100 heats up as described above. Heat is generally transferred radially from the optical fiber 100 to the body 202 of the enclosure 128 via the heat transfer region 127, as shown in FIG. Fig.12 . Ideally, the optical fiber 100 would be completely surrounded by a solid material with good thermal conductivity, such as by welding the optical fiber 100 to the inner surface of the body 202 of the enclosure 128. However, the different thermal expansion coefficients of the body 202 (usually comprising a metal such as steel and / or aluminum) and the optical fiber 100 (usually comprising fused silica) will cause stress in the optical fiber 100, ultimately resulting in damage to the optical fiber 100 and / or degradation of its performance.

[0161] Instead, heat is transferred from the optical fiber 100 to the body 202 via the heat transfer region 127, which in prior art systems typically includes the same working medium (i.e., gas 126) as the working medium used to generate the broadband radiation. Recently, the inclusion of helium as a component of the working medium 126 has been shown to increase the thermal conductivity in the working medium 126, as described in WO2022 / 008164A1, the entire contents of which are incorporated herein by reference. Helium has a thermal conductivity approximately one order of magnitude higher than krypton or argon, and if mixed in sufficient proportions, such as >20% helium, can effectively increase the thermal conductivity of the working medium or working gas 126. The addition of helium to the working medium 126 can extend the life of the optical fiber by approximately 3 months, but the life of the optical fiber 100 can be further extended by further reducing the thermal resistance of the heat transfer region 127.

[0162] Fig.13 A numerical simulation of an optical fiber 100 disposed within a gas medium 204 (eg, krypton) encompassed by a solid medium 206 (particularly stainless steel) is shown. Fig.13 The right side box of corresponds to an enlarged portion of the left side box. It should be understood that the model is shown in the xy plane (i.e., a plane perpendicular to the longitudinal axis 101 of the optical fiber 100) and can generally be considered similar to Figure 9-12 The enclosure 128 of the device 120 is shown. The modeled optical fiber 100 is a HC-PCF with an outer diameter of about 180 μm. The cross section of the modeled part of the enclosure including the gas medium 204 is 15 mm×18 mm. The distance s between the optical fiber 100 and the solid medium 206 varies between 0 and 7.5 mm.

[0163] Fig.14 Shows Fig.13The numerical simulation results are shown in Figure 2. That is, Fig.14 The calculated peak temperature reached inside the optical fiber 100 is shown as a function of the power deposited in the optical fiber core at each simulated distance s. Fig.14 The lines 1402, 1404, 1406, 1408 in the graph correspond to simulated distances s of 7.5 mm, 1.0 mm, 0.2 mm, and 0 mm, respectively. Fig.14 As shown, a reduction in s to 0.2 mm results in a more than 30% reduction in fiber temperature compared to the case where the fiber is located at the center of the gas medium 204. If the distance is reduced to zero (ie, contact), the reduction can be more than 50%.

[0164] The coefficient of thermal expansion of stainless steel is approximately 30 times that of fused silica optical fiber. Typically, the optical fiber 100 is held in place within the enclosure 128 (i.e., when the device 120 is in a horizontal orientation) by being clamped or otherwise held at opposite ends. Thus, as temperature increases, the air chamber will expand more than the optical fiber, thereby causing a risk of the optical fiber being pulled out of the holder or support or breaking. Thus, when the device 120 is in an "off" or "cold" state, the optical fiber 100 is typically maintained in the initial bent arrangement to compensate for thermal expansion of the enclosure 128 as the device 120 heats to its operating temperature, after which the optical fiber may reach a substantially straightened state.

[0165] Typically, it is therefore desirable to allow the optical fiber 100 to move freely within the enclosure 120 to accommodate thermal expansion of the enclosure 120 (and / or the optical fiber 100), and thus a non-zero minimum distance between the optical fiber 100 and the solid medium 206 may be required, at least along some length of the optical fiber 100, to prevent the optical fiber from becoming trapped on the solid medium 206, thereby avoiding the risk of damage due to stress, as described above.

[0166] Fig.15 An example of an apparatus 120 according to the present invention is shown. The apparatus 120 includes a hollow core optical fiber 100 disposed within a housing 128. It should be understood that Fig.15 1 shows a cross section of the device 120 in the xy plane (i.e., in a plane perpendicular to the longitudinal axis 101 of the optical fiber 100, the optical fiber 100 extending longitudinally into the page in the z direction). A heat-transmitting medium 127 is located between the optical fiber 100 and the body 202 of the enclosure 128, and the heat-transmitting medium 127 includes a first medium 212 and a second medium 214. The first medium 212 may include a gas, such as a working component including the gas 126 disposed in the core of the optical fiber 100, and / or a gas including helium, such as a gas having a helium content greater than 20%.

[0167] The second medium 214 may include a solid, such as a metal (e.g., steel), ceramic, and / or glass. The second medium 214 is disposed between the optical fiber 100 and the body 202 of the enclosure 128 along at least one radial direction (i.e., radially from the optical fiber 100). Fig.15 In the example shown, the second medium 214 completely fills the enclosure 128 on one side of the optical fiber 100. The optical fiber 100 is spaced apart from the second medium 214 by a radial distance 250 (i.e., radially spaced from the optical fiber). The radial distance 250 can be less than or equal to 1 mm over at least a portion of the length of the optical fiber. In some examples, the radial distance 250 is less than or equal to 200 μm. In some examples, the radial distance 250 is less than or equal to 100 μm. In general, it should be understood that the radial distance 250 should be small enough to maximize heat transfer from the optical fiber 100 to the body 202 of the enclosure 128 via the second medium 214, but should be large enough to accommodate free movement of the optical fiber 100 due to thermal expansion. In some examples, the optical fiber 100 can be in direct contact with the second medium 214 over at least a portion of the length of the optical fiber 100.

[0168] In the case of a horizontal device 120, where the fiber 100 is held at opposite ends, the fiber 100 must also be able to move freely in the vertical (i.e., y) direction, e.g., to allow the fiber to move from an initially bent arrangement to a substantially straight condition when the device 120 is heated, as described above. As an example, a 100K temperature change in the fiber 100 may require approximately 1 cm of vertical movement. Fig.15 The arrangement of the second medium 214 accommodates this vertical movement because the optical fiber can move vertically freely throughout the inner diameter of the enclosure 128 (ie, within the body 202).

[0169] In some examples, one or more of the first medium 212 and the second medium 214 include a liquid, such as any of water, liquid gallium or another liquid metal, liquid nitrogen, carbon disulfide CS2, carbon tetrachloride, and / or carbon tetrachloride CCl4.

[0170] In the case where the first medium 212 comprises a liquid rather than a gas, further improvements in thermal conductivity between the optical fiber 100 and the second medium 214 may be achieved because the thermal conductivity of a liquid is generally improved relative to that of a gas.

[0171] In one example, the first medium 212 and the second medium 214 both include liquids. For example, the device 120 may be similar to Fig.12 An example is shown in which the heat transfer region 127 includes a liquid that completely surrounds the optical fiber 100.

[0172] The liquid heat transport region 127 and / or liquid first medium 212 advantageously enables free movement of the optical fiber 100 while also providing direct contact between the optical fiber 100 and the heat transport medium having a greatly improved thermal conductivity relative to the gas of prior art systems.

[0173] It will be appreciated that in the case of a liquid first medium 212 and / or second medium 214, it is generally necessary to ensure that the liquid medium is sealed relative to the gas forming the working medium 126 for broadband radiation generation. Suitable sealing means should be able to withstand the working pressure of the working medium 126, which is approximately 10 bar to 100 bar.

[0174] Fig.16 Another example of an apparatus 120 according to the present invention is shown. Fig.16 In the embodiment, the second medium 214 is arranged on the opposite side of the optical fiber 100 to form a slot in the body 202 of the enclosure 128, the slot is filled with the first medium 212, and the optical fiber 100 is located in the slot. Fig.16 The slot configuration advantageously improves heat transfer between the optical fiber 100 and the body 202 of the enclosure 128 because the optical fiber 100 is surrounded by the second medium 214 in almost all radial directions, while also allowing the optical fiber 100 to move freely in the vertical direction. The slot width d should generally be as narrow as possible to maximize the heat transfer efficiency between the optical fiber 100 and the body 202 of the enclosure 128 while ensuring that the optical fiber 100 can move freely. In one example (e.g., for an optical fiber 100 having a diameter of about 100 μm to 200 μm), d can be less than or equal to 2 mm. In another example, d can be less than or equal to 1 mm.

[0175] Fig.17 An example of a device 120 according to the present invention is shown, which also includes a cooling channel 302 outside the body 202 of the enclosure 128. For example, the enclosure 128 can be provided within the channel 302, which is configured to cool the body 202 of the enclosure 128 when the device 120 is in use by allowing a fluid such as water, helium (gas or liquid) and / or oil to flow through the body 202. The cooling channel 302 can be connected to a cooling circuit (e.g., a refrigeration system) to provide cooling of the device, such as removing heat from the body 202 after the heat is transferred from the optical fiber 100 to the body 202 of the enclosure 128 via the heat transfer region 127.

[0176] Fig.18Another example of a device 120 according to the present invention is shown, wherein the device 120 includes a second medium 214, wherein the second medium 214 includes one or more internal cooling channels 304 extending in a direction parallel to the longitudinal axis 101 of the optical fiber 100 (i.e., in the z-direction). The one or more internal cooling channels 304 may include holes passing through the solid second medium 214, and / or may include pipes extending through the liquid second medium 214. The internal cooling channels 304 may be configured to cool the body 202 of the enclosure 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 when the device 120 is in use, as Fig.17 An alternative or supplement to the type of cooling channel 302 shown. The cooling channel(s) 304 may be connected to a cooling circuit (e.g., a refrigeration system) to provide cooling of the device, such as removing heat from the body 202 after the heat is transferred from the optical fiber 100 via the heat transfer region 127 to the body 202 of the enclosure 128. Fig.18 The illustrated internal cooling passages 304 are shown as cylindrical holes or tubes passing through the second medium 214. However, it should be understood that the internal cooling passages 304 may have any shape suitable for conveying a cooling fluid along the length of the device 120. For example, one or more cooling passages 304 may have a spiral or corkscrew configuration extending through the second medium 214 in the z-direction.

[0177] It should be understood that the second medium 214 does not necessarily need to be present along the entire length of the enclosure 128. Fig.19 As shown, the second medium 214 may extend only in the z-direction over a portion of the length of the enclosure 128. The portion of the second medium 214 extending in the z-direction is Fig.19 402. It should be understood that Fig.19 Shows something like Fig.18 1 and 2. The device shown has a cross section in the yz plane. In some examples, the second medium 214 can extend a distance 402 of about 1 cm in the z direction. In some examples, the second medium 214 can extend in the z direction only over the portion of the optical fiber 100 where broadband radiation generation occurs, because this is typically the portion of the optical fiber 100 that reaches the highest operating temperature. In other words, the distance 402 can only cover the portion of the optical fiber 100 where broadband radiation occurs.

[0178] In some cases, second medium 214 may be present along most or all of the length of enclosure 128 .

[0179] Fig. 20 Another example of a device 120 according to the present invention is shown in cross section in the yz plane. It should be understood that Fig. 20 The device 120 is shown oriented perpendicular to Figure 9-11and Fig.19 100 is a schematic diagram of an example of a device 120 that is shown in FIG. 101 . In other words, the device 120 is in a vertical orientation rather than a horizontal orientation (i.e., relative to the earth), wherein the length of the optical fiber 100 extends in the y-direction rather than the z-direction. In the vertical orientation, the optical fiber 100 is suspended within the enclosure 128 (e.g., the optical fiber 100 may be clamped or held at a top end) such that the orientation of the optical fiber 100 within the enclosure 128 is determined by gravity. In this vertical orientation, the optical fiber 100 may be surrounded on all sides by the solid second medium 214 (i.e., in all radial directions), and because the optical fiber 100 does not rest against the second medium 214 under the force of gravity as it would in the horizontal orientation, the optical fiber 100 may be prevented from becoming trapped against the second medium 214. Thus, by having the optical fiber 100 surrounded on all sides by the second medium 214, the vertical orientation may promote greater thermal conductivity between the optical fiber 100 and the body 202 of the enclosure 128.

[0180] It should be appreciated that in examples where the device 120 is in a vertical orientation, heat transfer will also occur along the length of the device 120 due to convection.

[0181] Fig.21 The method for generating broadband radiation according to the present invention is schematically shown. In step 2102 of the method, input radiation is emitted from an input radiation source, for example, from Fig.11 An input radiation source 136 of the type shown and described herein may correspond to Fig.11 The input radiation 122 shown and described herein. In step 2104 of the method, the input radiation is received by the apparatus 120 described herein. In step 2106 of the method, an output comprising broadband radiation is generated. The output radiation may correspond to Fig.11 Broadband output radiation 124 is shown and described herein. The output radiation may be generated according to any of the broadband output radiation generating processes described herein.

[0182] Although the use of the lithography apparatus in IC manufacturing may be specifically mentioned herein, it should be understood that the lithography apparatus described herein may have other applications. Other possible applications include the manufacture of integrated optical systems, guidance and detection patterns of magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0183] Although specific reference may be made herein to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may also be used in other apparatuses. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object, such as a wafer (or other substrate) or a mask (or other patterning device). These apparatuses may generally be referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0184] Although specific reference may be made above to the use of embodiments of the present invention in the context of optical lithography, it will be appreciated that the present invention is not limited to optical lithography but may also be used in other applications, such as imprint lithography, where the context permits.

[0185] Although specific reference is made to a "metrology device / tool / system" or an "inspection device / tool ​​or system," these terms may refer to the same or similar types of tools, devices, or systems. For example, an inspection or metrology device including embodiments of the present invention may be used to determine characteristics of a structure on a substrate or wafer. For example, an inspection device or metrology device including embodiments of the present invention may be used to detect defects in a substrate or defects in a structure on a substrate or wafer. In such embodiments, the feature of interest in a structure on a substrate may be related to a defect in the structure, the absence of a particular portion of the structure, or the presence of an unwanted structure on the substrate or wafer.

[0186] Although specific embodiments of the present invention have been described above, it should be understood that the present invention may be practiced in a manner other than that described. The above description is intended to be illustrative rather than limiting. Therefore, it will be appreciated by those skilled in the art that modifications may be made to the present invention described without departing from the scope of the following claims.

Claims

1. An apparatus for broadband radiation generation, the apparatus comprising a hollow core optical fiber, the hollow core optical fiber being at least partially disposed in an enclosure, the enclosure comprising: ontology; as well as a heat transfer region configured to transfer heat from the hollow core optical fiber to the body of the enclosure, the heat transfer region comprising a first medium disposed between the hollow core optical fiber and the body of the enclosure; wherein in at least one radial direction from the hollow-core optical fiber, the heat transport region further comprises a second medium, the second medium being disposed on a portion of the length of the enclosure and between the first medium and the body of the enclosure, the second medium comprising a solid and / or a liquid; and wherein in said portion, the radial distance between said hollow core optical fiber and said second medium is less than or equal to 2 mm, It is characterized in that the second medium forms a slit on at least a portion of the length of the shell, the slit extends in a direction perpendicular to the longitudinal axis of the hollow-core optical fiber, and the slit is wider than the diameter of the hollow-core optical fiber.

2. The apparatus of claim 1, wherein the hollow core optical fiber is free to move within the first medium in at least one direction perpendicular to a longitudinal axis of the hollow core optical fiber.

3. The device according to claim 1 or 2, wherein the radial distance between the hollow core optical fiber and the second medium is a distance in the at least one radial direction.

4. An apparatus according to any preceding claim, further comprising a cooling channel located external to the body, the cooling channel being configured to cool the body in use. 5 . The device according to claim 1 , wherein the width of the slit is less than or equal to 2 mm, preferably, wherein the width of the slit is less than or equal to 1 mm.

6. The apparatus of any one of the preceding claims, wherein the portion of the length of the enclosure surrounds a portion of the hollow core fiber, the portion of the hollow core fiber being the portion where broadband radiation generation occurs.

7. The device of any of the preceding claims, wherein the portion of the length of the enclosure is at least 1 cm long.

8. An apparatus according to any preceding claim, wherein the hollow core optical fibre is configured to be suspended vertically in use.

9. The device according to any of the preceding claims, 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 optical fiber.

10. The device according to any of the preceding claims, wherein the first medium comprises a gas and / or a liquid.

11. The device of any of the preceding claims, wherein the second medium comprises a solid, wherein the solid comprises a metal.

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

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

14. The device according to any of the preceding claims, wherein the radial distance between the hollow core optical fiber and the second medium over the portion is less than or equal to 1 mm.

15. A method of generating broadband radiation, the method comprising: emitting input radiation from an input radiation source; receiving the input radiation by means of an apparatus according to any one of the preceding claims; as well as The apparatus is used to generate an output comprising broadband radiation.

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