Photonic crystal fiber

Abstract

A photonic crystal fiber (10) includes a core region and a cladding region surrounding the core region. The core region and the cladding region comprise a material (12) having a first refractive index, the cladding region further comprising a plurality of microstructures (14) extending from an input end of the optical fiber along a longitudinal axis of the optical fiber to an output end of the optical fiber, the plurality of microstructures (14) having a second refractive index less than the first refractive index. The plurality of microstructures in the cladding region are arranged in a cross-sectional pattern comprising at least one ring of microstructures surrounding the core region, where a maximum range of diameters of the core region is at least 16 [mu] m, the photonic crystal fiber having a transmission bandwidth of 200 nm or greater.

Description

[0001] Cross-references to related applications

[0002] This application claims priority to European Application 23168262.6, filed on 17 April 2023, European Application 23208169.5, filed on 7 November 2023, and European Application 24150535.3, filed on 5 January 2024, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This invention relates to photonic crystal fibers. Background Technology

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

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

[0006] Low-ki lithography can be used to process features with dimensions smaller than the classical resolution limit of a lithographic apparatus. In such process, the resolution formula can be expressed as CD = kixA / 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” (generally the smallest feature size that is typically used to define resolution of the lithographic apparatus, but in this case half-pitch, i.e. half the distance between two adjacent features), and ki is an empirical resolution factor. In general, the smaller ki the more difficult it becomes to reproduce the pattern on the substrate that resembles the shape and dimensions planned by a circuit designer in order to achieve particular electrical functionality and performance. To overcome these difficulties, sophisticated fine-tuning steps can be applied to the lithography process and / or design layout. These steps can include, for example, and without limitation, optimization of NA, customized illumination schemes, use of phase- shift patterning devices, various optimizations of the design layout such as optical proximity correction (OPC, sometimes also referred to as “optical and process correction”) in the design layout, or other methods generally defined as “resolution enhancement techniques” (RET). Alternatively, tight control loops used to control stability of the lithography apparatus can be used to improve reproduction of the pattern at low ki.

[0007] In the field of lithography, a number of measurement systems can be used both within and outside the lithographic apparatus. Generally, such measurement systems can use a radiation source to irradiate 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 outside the lithographic apparatus is an inspection apparatus or a metrology apparatus, which can be used to determine properties of a pattern previously transferred to a substrate by the lithographic apparatus. Such an external inspection apparatus can for example include a scatterometer. Examples of measurement systems that can be provided within the lithographic apparatus include topography measurement systems (also referred to as level sensors), position measurement systems (e.g. interferometry devices) used to determine the position of a reticle or wafer stage, and alignment sensors used to determine the position of alignment marks. These measurement devices can use electromagnetic radiation to perform the measurements.

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

[0009] Lithographic apparatuses and / or metrology apparatuses can utilize optical fibers to deliver radiation originating from a radiation source to another component of the apparatus, such as a sensor. Among various sensors, those requiring broadband light delivery in a single spatial mode with constant phase front typically use endlessly single-mode photonic crystal fibers (ESM-PCF). Single-mode fibers typically have a very small core with a mode profile diameter of about 10 pm. For example, US 10649136 discloses microstructured delivery fibers with a core region diameter of at most about 15 pm. SUMMARY

[0010] While known photonic crystal fibers for single-mode light delivery over long distances (>1 m) have a small core diameter (diameter of at most 15 pm), the inventors of the present invention have recognized some advantages of photonic crystal fibers for single-mode light delivery with a larger core diameter.

[0011] In a first aspect of the invention, there is provided a photonic crystal fiber comprising: a core region; a cladding region surrounding the core region; wherein the core region and the cladding region comprise a material having a first refractive index, the cladding region further comprising a plurality of microstructures extending from an input end of the fiber to an output end of the fiber along a longitudinal axis of the fiber, the plurality of microstructures having a second refractive index smaller than the first refractive index; wherein the plurality of microstructures in the cladding region are arranged in a cross-sectional pattern comprising at least one ring of microstructures surrounding the core region, wherein the core region has a maximum extent of a diameter of at least 16 pm, the photonic crystal fiber having a transmission bandwidth of 200 nm or more.

[0012] The inventors have identified that a small core size leads to an increase in optical intensity, which can trigger unwanted nonlinear effects due to light-induced contamination. By having a maximum extent of a diameter of the core region of at least 16 pm, the optical intensity at the output of the fiber is reduced, since the intensity is inversely proportional to the square of the fiber core diameter, thereby avoiding these unwanted nonlinear effects and increasing the lifetime of the fiber.

[0013] In some applications, the output end of the fiber is magnified and imaged on a marker. For a given illumination area on a wafer, a small fiber core diameter means that a larger magnification is required, which leads to larger aberrations. For a given illumination area on a wafer, a diameter of the core region of at least 16 pm means that a smaller magnification is required, which can be close to 1 : 1 imaging, simplifies the optical design, generally reduces aberrations, and improves the accuracy of the measurement system.

[0014] Some measurement systems require a highly stable fiber coupling, with a relative stability of up to 10 -4Stable fiber coupling requires the incident beam to be focused onto the fiber with a positional stability much higher than the diameter of such mode profile, and thus stable at sub-micron level. A maximum extent of the diameter of the core region of at least 16 pm advantageously makes it easier to couple light into the fiber in a stable manner. Furthermore, at larger core diameter (a maximum extent of the diameter of the core region of at least 16 pm), it is easier to achieve low insertion loss without the need for precise alignment and manufacturing methods.

[0015] In implementations where the photonic crystal fiber is mechanically spliced to another fiber, a maximum extent of the diameter of the core region of at least 16 pm makes it easier to perform the mechanical splicing.

[0016] The ratio (d / A) between the diameter (d) of each of the microstructures and the pitch (A) of the microstructures can be less than 0.42, preferably less than 0.37. The ratio can be between 0.35 and 0.40. The pitch (A) is defined by the (minimum) distance between the centers of two adjacent microstructures. Note that two adjacent microstructures are two microstructures that are close (e.g. closest) to each other and without any other microstructure between the two adjacent microstructures.

[0017] The photonic crystal fiber can have a transmission loss of less than 0.5 dB / m for the fundamental mode over a transmission bandwidth, e.g. covering the entire range from 500 nm to 900 nm. The photonic crystal fiber can have a transmission loss of less than 0.1 dB / m for the fundamental mode over a transmission bandwidth, e.g. covering the range from 550 nm to 800 nm. The photonic crystal fiber can have such transmission loss when the fiber is bent with a bend diameter of at least 10 cm, preferably when bent with a bend diameter between 10 cm and 12 cm. When the photonic crystal fiber is bent, the higher order modes experience higher transmission loss, and thus the optical power carried by them leaks out of the fiber, in turn making the fiber effectively single mode. All the remaining power is now carried in the single fundamental mode. The bend can be present immediately after the light in-coupling, or immediately before the light out-coupling, depending on the actual implementation. When the fiber is straight, the transmission loss of the fundamental mode can be less than 50 dB / km between 500 nm and 900 nm.

[0018] The maximum extent of the geometric diameter of the core region can be given by 4 times the pitch (□) of the microstructures minus the difference of the diameter (d) of each of the microstructures, i.e. .

[0019] The pitch (□) of the microstructures can be less than 5 pm.

[0020] The cross-sectional pattern can comprise a plurality of microstructured rings surrounding the core region. Having a plurality of microstructured rings surrounding the core region advantageously reduces the confinement loss of the photonic crystal fiber.

[0021] The cross-sectional pattern can comprise at least six microstructured rings surrounding the core region, preferably at least seven microstructured rings surrounding the core region, preferably at least eight microstructured rings surrounding the core region.

[0022] The microstructured ring immediately adjacent to the core region can have twelve microstructures. This advantageously ensures that the photonic crystal fiber is single mode for at least one wavelength within the transmission bandwidth, while featuring a large maximum range of the core diameter of at least 16 pm. It is noted that the microstructured ring immediately adjacent to the core is the first microstructured ring seen from the center of the core.

[0023] The plurality of microstructures can comprise a medium having a second refractive index.

[0024] The medium can be air, such that the plurality of microstructures is hollow. Hollow microstructures advantageously have greater robustness to bending of the photonic crystal fiber.

[0025] Alternatively, the medium can be a solid material, for example glass, such as doped silica. Doped silica microstructures are more sensitive to bending than hollow microstructures, however, doped silica microstructures are advantageously easier to manufacture, and ends of photonic crystal fibers having doped silica microstructures are easier to polish than ends of photonic crystal fibers having hollow microstructures.

[0026] The doped silica can comprise silica doped with fluorine, wherein the molar percentage of fluorine is optionally in the range of 1% to 10%, further optionally in the range of 3% to 8%. This molar percentage of fluorine advantageously makes the fiber more robust to bending, and also reduces the confinement loss.

[0027] The material having the first refractive index can be silica. It is noted that the term "base material" is generally used for the material having the first refractive index.

[0028] The maximum range of the diameter of the core region can be less than 20 pm.

[0029] The maximum range of the diameter of the core region can be less than 50 pm, preferably less than 47 pm.

[0030] The transmission bandwidth of the delivery fiber can comprise wavelengths in the range of 200 nm to 2500 nm, preferably 400 nm to 2000 nm.

[0031] According to another aspect of the application, there is provided a lithographic apparatus comprising a photonic crystal fiber according to any of the embodiments described herein.

[0032] According to another aspect of the application, there is provided a metrology apparatus comprising a photonic crystal fiber according to any of the embodiments described herein.

[0033] The metrology apparatus can further comprise a light source and a sensor for measuring a parameter of interest of a structure on a substrate, and wherein the photonic crystal fiber is arranged to transfer light from the light source to the sensor and / or between components of the sensor.

[0034] The metrology apparatus can further comprise a light source and a sensor for measuring a parameter of interest of a structure on a substrate, and wherein the photonic crystal fiber is arranged to transfer light between components of the sensor.

[0035] According to another aspect of the application, there is provided a holographic metrology apparatus comprising a photonic crystal fiber according to any of the embodiments described herein. BRIEF DESCRIPTION OF DRAWINGS

[0036] Embodiments of the application will now be described, by way of example only, with reference to the accompanying schematic drawings in which:

[0037] - Figure 1 a schematic overview of a lithographic apparatus is depicted;

[0038] - Figure 2 a schematic overview of a lithographic cell is depicted;

[0039] - Figure 3 a schematic representation of holistic lithography is depicted, representing the cooperation between three key technologies for optimizing semiconductor manufacturing;

[0040] - Figure 4 a schematic overview of a scatterometry tool for use as a metrology device, which can comprise a photonic crystal fiber according to embodiments of the application, is depicted;

[0041] - Figure 5 a schematic overview of a level sensor metrology tool, which can comprise a photonic crystal fiber according to embodiments of the application, is depicted;

[0042] - Figure 6 a schematic overview of an alignment sensor metrology tool, which can comprise a photonic crystal fiber according to embodiments of the application, is depicted;

[0043] - Figure 7An example of a dark-field digital holographic microscope (df-DHM) that can include a photonic crystal fiber according to embodiments of the present application is schematically depicted, the df-DHM operating in a continuous acquisition scheme;

[0044] - Figure 8 A df-DHM that can include a photonic crystal fiber according to embodiments of the present application is schematically depicted, the df-DHM operating in a parallel acquisition scheme;

[0045] - Figure 9A A df-DHM that can include a photonic crystal fiber according to embodiments of the present application is schematically depicted, the df-DHM operating in a parallel acquisition scheme;

[0046] - Figure 9B A depiction Figure 9A of the k-space associated with the objective of a df-DHM illustrated in

[0047] - Figure 9C A depiction of the image spectrum in the spatial frequency domain of the diffracted orders generated after illuminating a metrology target using a df-DHM; Figure 9A

[0048] - Figure 10A A depiction of the transverse cross-section of a photonic crystal fiber and the dimensions of the photonic crystal fiber;

[0049] - Figure 10B to Figure 10D An illustration of how the diameter of the core region of an optical fiber can be measured;

[0050] - Figure 11A An illustration of example loss behavior of the fundamental mode propagating through photonic crystal fibers each having 7 microstructured rings but with different normalized inclusion diameter (d / A) values;

[0051] - Figure 11B An illustration of example loss behavior of the high order modes propagating through photonic crystal fibers each having 7 microstructured rings but with different normalized inclusion diameter (d / A) values;

[0052] - Figure 12A An illustration of example loss behavior of the fundamental mode propagating through photonic crystal fibers each having 8 microstructured rings but with different normalized inclusion diameter (d / A) values;

[0053] - Figure 12B An illustration of example loss behavior of the high order modes propagating through photonic crystal fibers each having 8 microstructured rings but with different normalized inclusion diameter (d / A) values;

[0054] - Figure 13A ​Figures illustrating example transmission loss spectra of the fundamental mode propagating through the photonic crystal fiber when the photonic crystal fiber has 7 microstructured rings and when the photonic crystal fiber has 8 microstructured rings;

[0055] - Figure 13B Figures illustrating example transmission loss spectra of the two first higher order modes propagating through the photonic crystal fiber when the photonic crystal fiber has 7 microstructured rings and when the photonic crystal fiber has 8 microstructured rings;

[0056] - Figure 14A and Figure 14B depicting a transverse cross-section of a first polarization maintaining photonic crystal fiber and a second polarization maintaining photonic crystal fiber according to a first embodiment of the present application;

[0057] - Figure 15 illustrating attenuation of the fundamental mode of the second polarization maintaining photonic crystal fiber when the fiber is straight;

[0058] - Figure 16A illustrating attenuation of the fundamental mode of the second polarization maintaining photonic crystal fiber when the fiber is bent parallel to the alignment axis of the fiber;

[0059] - Figure 16B illustrating attenuation of the fundamental mode of the second polarization maintaining photonic crystal fiber when the fiber is bent perpendicular to the alignment axis of the fiber;

[0060] - Figure 17 depicting a transverse cross-section of a polarization maintaining photonic crystal fiber according to a second embodiment of the present application;

[0061] - Figure 18 illustrating attenuation of the fundamental mode of the polarization maintaining photonic crystal fiber of the second embodiment when the fiber is straight;

[0062] - Figure 19A illustrating attenuation of the fundamental mode of the polarization maintaining photonic crystal fiber of the second embodiment when the fiber is bent parallel to the alignment axis of the fiber;

[0063] - Figure 19B illustrating attenuation of the fundamental mode of the polarization maintaining photonic crystal fiber of the second embodiment when the fiber is bent perpendicular to the alignment axis of the fiber;

[0064] - Figure 20 depicting a transverse cross-section of a polarization maintaining photonic crystal fiber according to a third embodiment of the present application;

[0065] - Figure 21 illustrating attenuation of the fundamental mode of the polarization maintaining photonic crystal fiber of the third embodiment when the fiber is straight;

[0066] - Figure 22A illustrating attenuation of the fundamental mode of the polarization maintaining photonic crystal fiber of the third embodiment when the fiber is bent parallel to the alignment axis of the fiber; and

[0067] - Figure 22B Figure illustrates the attenuation of the fundamental mode of the polarization maintaining photonic crystal fiber of the third embodiment when the fiber is bent with the fiber's alignment axis parallel to the bend. DETAILED DESCRIPTION

[0068] In the present document, the terms“radiation” and“beam” are used interchangeably to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range of about 5-100 nm).

[0069] The term“reticle”,“mask” or“patterning device” as used herein can be broadly interpreted to refer to a generic patterning device that can be used to impart a pattern to a beam of radiation, which is to be used in the fabrication of multiple devices. It should be noted that the term“reticle” or“mask” does not necessarily mean a binary, phase-shift, hybrid or multi-layer device. In general, any of the patterning devices disclosed herein can be used as a“mask” or“reticle”. In some instances, it can even be that the patterning device comprises multiple layers. Also, the term“patterning device” can include multiple devices that independently impart the desired pattern to the beam of radiation. In some instances, the patterning device can be a programmable mirror array. Consequently, the term“patterning device” should be interpreted as a general term, which can cover a wide range of devices.

[0070] Figure 1 A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA includes an illumination system, also referred to as illuminator IL, configured to condition a radiation beam B (e.g., UV, DUV, or EUV radiation), a mask support (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with a predetermined pattern, a substrate support (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support in accordance with a predetermined pattern, and a projection system (e.g., a refractive, catadioptric, or catoptric projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., including one or more dies) of the substrate W.

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

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

[0073] The lithographic apparatus LA can be of a type that includes a liquid immersion facility, in which at least a portion of the substrate W is covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system PS and the substrate W. More information on liquid immersion techniques is given in US 6952253, incorporated herein by reference.

[0074] The lithographic apparatus LA can also be of a type that includes two or more substrate supports WT (also referred to as "wafer stages"). In such "multi-stage" machines, the substrate supports WT can be used in parallel, and / or steps can be performed on a substrate W located on one of the substrate supports WT in preparation for a subsequent exposure of the substrate W, while another substrate W is being exposed on another substrate support WT.

[0075] In addition to the substrate support WT, the lithographic apparatus LA can include a measurement platform. The measurement platform is arranged to hold a sensor and / or a cleaning device. The sensor can be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement platform can hold a plurality of sensors. The cleaning device can be arranged to clean a part of the lithographic apparatus, for example a part of the projection system PS or a part of a system that provides an immersion liquid. The measurement platform can be movable under the projection system PS when the substrate support WT is away from the projection system PS.

[0076] In operation, the radiation beam B is incident on the patterning device (e.g., mask) MA held on the mask support MT, and is patterned by a pattern (design layout) present on the patterning device MA. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which projects the beam onto a target portion C of the substrate W. By means of the second positioner PW and the position measurement system IF, the substrate support WT can be accurately moved so as to position different target portions C in the path of the radiation beam B, e.g., in order to track a patterned beam that is being scanned across the target portion C. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted) can be used to accurately position the patterning device MA in the path of the radiation beam B, e.g., so as to track a patterned beam that is being scanned across the target portion C. Figure 1The mask alignment marks M1, M2 and the substrate alignment marks P1, P2 can be used to align the patterning device MA and the substrate W. While the substrate alignment marks P1, P2 as illustrated occupy dedicated target portions, they can be located in spaces between target portions. When the substrate alignment marks P1, P2 are located between target portions C, these substrate alignment marks are referred to as cross-line alignment marks. As Figure 1 As shown in the middle, a photonic crystal fiber (PCF) can be used to deliver a beam of radiation from the radiation source SO to the beam delivery system BD. A PCF according to any of the embodiments described herein can be used in a lithographic apparatus LA such as the lithographic apparatus LA depicted in the middle (e.g. to deliver a beam of radiation from the radiation source SO to the beam delivery system BD). Figure 1 As depicted in the middle, the lithographic apparatus LA) in which the PCF can be used (e.g. to deliver a beam of radiation from the radiation source SO to the beam delivery system BD).

[0077] As Figure 2 As shown in the middle, the lithographic apparatus LA can form part of a lithocell LC (sometimes also referred to as a litho cell or (litho) cluster), which also often includes apparatus for performing pre-exposure processes and post-exposure processes on the substrate W. Conventionally, these apparatus include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH and a bake plate BK, e.g. for conditioning the temperature of the substrate W, e.g. for conditioning a solvent in the resist layer. A substrate handling device or robot RO picks up the substrate W from an input / output port I / O1, I / O2, moves the substrate W between the different process apparatus and delivers the substrate W to a load deck LB of the lithographic apparatus LA. The apparatus in the lithocell, often collectively referred to as a track system, are typically under control of a track system control unit TCU, which itself can be controlled by a supervisory control system SCS, which can also control the lithographic apparatus LA, e.g. via a litho cell control unit LACU.

[0078] In order to correctly and consistently expose substrates W exposed by the lithographic apparatus LA, it is desirable to inspect the substrates to measure properties of the patterned structures, such as overlay error between subsequent layers, line thickness, critical dimension (CD), etc. To this end, an inspection tool (not shown) can be included in the lithocell LC. If errors are detected, exposure of subsequent substrates or other processing steps to be performed on the substrate W can be adjusted, e.g. if the inspection is performed before other substrates W of the same lot or batch are yet to be exposed or processed.

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

[0080] Typically the patterning process in the lithographic apparatus LA is one of the most critical steps in the process, requiring high accuracy in the sizing and placement of structures on the substrate W. To ensure such high accuracy, three systems can be combined in a so-called "holistic" control environment, as schematically depicted in Figure 3 One of these systems is the lithographic apparatus LA, which is (virtually) connected to a metrology tool MT (second system) and to a computer system CL (third system). The key of such a "holistic" environment is to optimize the cooperation between these three systems to enhance the overall process window and provide a tight control loop, ensuring that the patterning performed by the lithographic apparatus LA stays within the process window. The process window defines a range of process parameters (e.g. dose, focus, overlay) for which a certain manufacturing process results in a defined outcome (e.g. functional semiconductor devices), typically allowing variation in the process parameters in the lithography or patterning process within the defined outcome.

[0081] The computer system CL can use (parts of) the design layout to be patterned to predict which resolution enhancement techniques to use and perform computational lithography simulations and calculations to determine which mask layouts and lithographic apparatus settings achieve the largest overall process window for the patterning process (depicted in Figure 3 by the double arrow in the first scale SC1). Typically, the resolution enhancement techniques are arranged to match the patterning possibilities of the lithographic apparatus LA. The computer system CL can also be used to detect where the lithographic apparatus LA is currently operating within the process window (e.g. using input from the metrology tool MT) to predict whether there can be defects due to e.g. sub-optimal processing (depicted in Figure 3 by the arrow pointing to "0" in the second scale SC2).

[0082] The metrology tool MT can provide input to the computer system CL to enable accurate simulation and prediction, and can provide feedback to the lithography apparatus LA to identify possible drifts in the calibration status of the lithography apparatus LA (in Figure 3 depicted by the plurality of arrows in the third scale SC3).

[0083] In the lithographic process, it is desirable to frequently measure the structures produced, for example for process control and verification. The tools used for making such measurements are commonly referred to as metrology tools MT. Different types of metrology tools MT for making such measurements are well known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. Scatterometers are versatile instruments that allow measuring parameters of the lithography process by having a sensor in the pupil or in a plane conjugate to the pupil of the objective of the scatterometer (the measurement is commonly referred to as pupil-based measurement), or by having a sensor in the image plane or in a plane conjugate to the image plane, in which case the measurement is commonly referred to as image- or field-based measurement. Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP 1,628,164A, which are incorporated herein in their entirety. The aforementioned scatterometers can measure gratings using light from the soft x-ray and visible to near IR wavelength range.

[0084] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, a reconstruction method can be applied to the measured signal to reconstruct or calculate properties of the grating. Such reconstruction can for example result from simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulated results with the measured results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to the diffraction pattern observed from the 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 the target and radiation reflected or scattered from the target is directed onto a spectrometer detector, which measures the spectrum of the specular reflected radiation (i.e. a measurement of the intensity as a function of wavelength). From this data, the structure or profile of the target that produced the detected spectrum can be reconstructed, for example by rigorous coupled wave analysis and non-linear regression or by comparison with a library of simulated spectra.

[0086] In a third embodiment, the scatterometer MT is an ellipsometry scatterometer. Ellipsometry scatterometers allow for determining parameters of a lithography process by measuring the scattered radiation for each polarization state. Such a metrology apparatus emits polarized light, such as linear light, annular light or elliptical light, by using, for example, appropriate polarization filters in the illumination section of the metrology apparatus. The source adapted to the metrology apparatus can also provide polarized radiation. Multiple embodiments of existing ellipsometry scatterometers 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, which are incorporated herein by reference in their entirety.

[0087] In one embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the overlay of two misaligned gratings or periodic structures by measuring the reflectance spectrum and / or asymmetry in the detection configuration, which is related to the range of the overlay. The two (typically superimposed) grating structures can be applied in two different layers (not necessarily consecutive layers) and can be formed substantially at the same location on the wafer. The scatterometer can have a symmetric detection configuration as described, for example, in commonly owned patent application EP 1,628,164 A, such that any asymmetry is clearly distinguishable. This provides a direct way of measuring misalignment in the gratings. Further examples of measuring overlay error between two layers containing a periodic structure when measuring the target through asymmetry of the periodic structure can be found in PCT Patent Publication No. WO 2011 / 012624 or U.S. Patent Application US 20160161863, which are incorporated herein by reference in their entirety.

[0088] Other parameters of interest can be focus and dose. Focus and dose can be determined simultaneously by scatterometry as described in U.S. Patent Application US2011-0249244, which is incorporated herein by reference in its entirety, or alternatively by scanning electron microscopy. A single structure with a unique combination of critical dimension and side wall angle measurements for each point in the focus energy matrix (FEM, also known as focus exposure matrix) can be used. If a unique combination of these critical dimensions and side wall angles is available, focus and dose can be uniquely determined from these measurements.

[0089] The metrology target can be an ensemble of composite gratings formed primarily in resist by a lithographic process and also after e.g. an etching process. Typically, the pitch and line width of the structures in the grating depend largely on the measurement optics, in particular the NA of the optics, to be able to capture the diffraction orders from the metrology target. As indicated earlier, the diffraction signals can be used to determine the shift between two layers, also referred to as "overlay", or can be used to reconstruct at least part of the original grating as produced by the lithographic process. Such reconstruction can be used to provide a quality guide of the lithographic process and can be used to control at least part of the lithographic process. The target can have smaller sub-segments arranged to mimic the dimensions of the functional part of the design layout in the target. Due to such sub-segments, the target will behave more similar to the functional part of the design layout, such that the overall process parameter measurement is preferably similar to the functional part of the design layout. The target can be measured in an underfill mode or in an overfill mode. In underfill mode, the measurement beam generates a spot smaller than the overall target. In overfill mode, the measurement beam generates a spot larger than the overall target. In such overfill mode, it is also possible to measure different targets simultaneously, thus determining different process parameters simultaneously.

[0090] In Figure 4 a metrology apparatus, such as a scatterometer, is depicted. It comprises a broadband (white light) radiation projector 2 that projects radiation onto a substrate W. Reflected or scattered radiation is passed to a spectrometer detector 4 that measures a spectrum 6 of the mirror reflected radiation, i.e. a measurement of intensity as a function of wavelength. From this data, a structure or profile 8 that produced the detected spectrum can be reconstructed by a processing unit PU, e.g. by rigorous coupled wave analysis and non-linear regression or by comparison to a library of simulated spectra as shown at the bottom of Figure 3 Typically, for the reconstruction, the general form of the structure is known and some parameters are assumed from knowledge of the process that manufactured the structure, leaving only a few parameters of the structure to be determined from the scatterometry data. Such a scatterometer can be configured as a normal-incidence scatterometer or an oblique-incidence scatterometer. As shown in Figure 4 PCF can be used to deliver radiation from a broadband (white light) radiation projector 2. A PCF according to any of the embodiments described herein can be used in a metrology apparatus, such as the metrology apparatus depicted in Figure 4

[0091] ​The overall measurement quality of a lithographic parameter via a measurement of a target is at least partially determined by the measurement recipe used to measure such lithographic parameter. The term "substrate measurement recipe" can include one or more parameters of the measurement itself, one or more parameters of the one or more patterns measured, or both. For example, if the measurement in a substrate measurement recipe is a diffraction-based optical measurement, one or more of the parameters of the measurement can include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the direction of the radiation relative to the pattern on the substrate, etc. One of the criteria for selecting a measurement recipe can for example be the sensitivity of one of the measurement parameters to process variations. More examples are described in US patent application US2016-0161863 and published US patent application US 2016 / 0370717 Al, which are incorporated herein in their entirety.

[0092] Another type of metrology tool for IC manufacturing is a topography measurement system, level sensor or height sensor. Such a tool can be integrated in a lithographic apparatus for measuring the topography of the top surface of a substrate (or wafer). A topography map (also referred to as a height map) of a substrate can be generated from measurements of the substrate that indicate the height as a function of position on the substrate. This height map can then be used to correct the position of the substrate during the transfer of a pattern onto the substrate in order to provide a spatial image of the patterning device in the correct focus position on the substrate. It will be appreciated that "height" in this context refers broadly to the dimension from the plane to the substrate (also referred to as the Z-axis). Typically, a level or height sensor performs measurements at a fixed location (relative to its own optical system) and relative movement between the substrate and the optical system of the level or height sensor results in height measurements at locations across the substrate.

[0093] Figure 5 An example of a level or height sensor LS known in the art is schematically illustrated in Fig. 1, which illustrates the operating principle only. 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 providing a radiation beam LSB which is imparted with a projection grating PGR of the projection unit LSP. The radiation source LSO can be for example a narrow-band or a broadband light source, such as a supercontinuum light source, polarized or non-polarized, pulsed or continuous, such as a polarized or non-polarized laser beam. The radiation source LSO can comprise multiple radiation sources with different colors or wavelength ranges, such as multiple LEDs. The radiation source LSO of the level sensor LS is not limited to visible light radiation, but can additionally or alternatively encompass UV and / or IR radiation and any wavelength range suitable for reflection from the surface of a substrate. As Figure 5As shown in the middle, the PCF can be used to deliver radiation from a radiation source LSO. A PCF according to any of the embodiments described herein can be used in a level sensor LS (such as Figure 5 depicted in the middle.

[0094] The projection grating PGR is a periodic grating comprising a periodic structure that produces a beam of radiation BE1 with a periodically varying intensity. The beam of radiation BE1 with the periodically varying intensity is directed towards a measurement location MLO on a substrate W with an angle of incidence ANG between 0 and 90 degrees, typically between 70 and 80 degrees, with respect to an axis normal to the surface of the incident substrate (Z-axis). At the measurement location MLO, the patterned beam of radiation BE1 is reflected by the substrate W (indicated by arrow BE2) and directed towards a detection unit LSD.

[0095] For determining the height level at the measurement location 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 the output signal of the detector DET. The detection grating DGR can be identical to the projection grating PGR. The detector DET produces a detector output signal indicative of the received light, e.g. indicative of 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 can comprise any combination of one or more detector types.

[0096] By means of triangulation techniques, the height level at the measurement location MLO can be determined. The detected height level is typically related to a signal intensity as measured by the detector DET, which signal intensity has a periodicity that depends, among others, on the design of the projection grating PGR and the (inclined) angle of incidence ANG.

[0097] The projection unit LSP and / or the detection unit LSD can comprise further optical elements, such as lenses and / or mirrors, along the path (not shown) of the patterned beam of radiation between the projection grating PGR and the detection grating DGR.

[0098] In embodiments, the detection grating DGR can be omitted, and the detector DET can be placed at the position where the detection grating DGR is located. Such a configuration provides a more direct detection of the image of the projection grating PGR.

[0099] For efficiently covering the surface of the substrate W, the level sensor LS can be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating an array of measurement regions MLO or spots covering a larger measurement range.

[0100] Various height sensors of a general type are disclosed, for example, in US7265364 and US7646471, both incorporated by reference. A height sensor using UV radiation instead of visible or infrared radiation is disclosed in US2010233600A1, incorporated by reference. In WO2016102127A1, incorporated by reference, a compact height sensor is described that uses a multi-element detector to detect and discriminate the position of a grating image without the need to detect the grating.

[0101] Another type of metrology tool used in IC manufacturing is an alignment sensor. A key aspect of the performance of a lithographic apparatus can therefore be to place the applied pattern correctly and accurately with respect to features placed in a previous layer (by the same apparatus or a different lithographic apparatus). To this end, the substrate is provided with one or more sets of marks or targets. Each mark is a structure whose position can later be measured using a position sensor, typically an optical position sensor. The position sensor can be referred to as an “alignment sensor”, and the marks can be referred to as “alignment marks”.

[0102] A lithographic apparatus can include one or more (e.g. multiple) alignment sensors that can thereby accurately measure the position of alignment marks provided on a substrate. Alignment (or position) sensors can use optical phenomena such as diffraction and interference to obtain position information from alignment marks formed on a substrate. An example of an alignment sensor used in current lithographic apparatuses is based on a self-referencing interferometer as described in US6961116. Various enhancements and modifications to position sensors have been developed, for example as disclosed in US2015261097A1. The contents of all these publications are incorporated herein by reference.

[0103] Figure 6 is a schematic block diagram of an embodiment of a known alignment sensor AS such as described in, for example, US6961116 and incorporated by reference. A radiation source RSO provides a beam of radiation RB having one or more wavelengths as an illumination spot SP, which is turned onto a mark (such as a mark AM located on a substrate W) by turning optics. In this example, the turning optics comprise a point mirror SM and an objective lens OL. The diameter of the illumination spot SP illuminating the mark AM can be slightly smaller than the width of the mark itself. As Figure 6 PCF can be used to deliver the beam of radiation RB from the radiation source RSO, as shown in. A PCF according to any of the embodiments described herein can be used in an alignment sensor such as the one depicted in. Figure 6

[0104] ​Radiation diffracted by the alignment mark AM (in this example via the objective lens OL) is collimated into an information-bearing beam IB. The term "diffracted" is intended to include zeroth order diffraction from the mark (which can be referred to as reflection). A self-referencing interferometer SRI of the type disclosed in, for example, US 6961116, interferes the beam IB with itself, whereafter the beam is received by a photodetector PD. Additional optics (not shown) can be included to provide separate beams in the case that more than one wavelength is produced by the radiation source RSO. The photodetector can be a single element, or it can comprise multiple pixels as required. The photodetector can comprise a sensor array.

[0105] The turning optics, which in this example comprise a point mirror SM, can also be used to block zeroth order radiation reflected from the mark, so that the information-bearing beam IB includes only higher order diffracted radiation from the mark AM (which is not essential for the measurement, but improves the signal-to-noise ratio).

[0106] The intensity signal SI is supplied to a processing unit PU. By combining the optical processing carried out in block SRI with the computational processing carried out in unit PU, values of the X and Y positions of the substrate relative to the reference frame are output.

[0107] A single measurement of the type illustrated fixes the position of the mark only to within a certain range corresponding to one period of the mark. A coarser measurement technique is used in conjunction with this measurement to identify which period of the sinusoidal wave includes the marked position. The same process at a coarser and / or finer level can be repeated at different wavelengths for improved accuracy and / or for robust detection of the mark, regardless of the material from which the mark is made and the material disposed above and / or below the mark. The wavelengths can be optically multiplexed and / or demultiplexed for simultaneous processing, and / or the wavelengths can be multiplexed by time-division or frequency-division.

[0108] In this example, the alignment sensor and the spot SP remain stationary, while the substrate W moves. The alignment sensor can thus be robustly and accurately mounted to a reference frame, while effectively scanning the mark AM in a direction opposite to the direction of movement of the substrate W. In such movement, the substrate W is controlled by mounting the substrate W on a substrate support and by controlling the movement of the substrate support by a substrate positioning system. A substrate support position sensor (e.g. an interferometer) measures the position of the substrate support (not shown). In embodiments, one or more (alignment) marks are disposed on the substrate support. Measurement of the position of the marks disposed on the substrate support allows calibration of the position of the substrate support determined by the position sensor (e.g. relative to the frame to which the alignment system is connected). Measurement of the position of the alignment marks disposed on the substrate allows determination of the position of the substrate relative to the substrate support.

[0109] As already described above, a PCF according to any of the embodiments described herein can be implemented in a lithographic apparatus or a metrology apparatus. In addition, a PCF according to any of the embodiments described herein can be implemented in a holographic metrology apparatus. The holographic metrology apparatus can be a component of a digital holographic microscope configured to determine a parameter of interest of a structure comprising at least one first feature oriented along a first axis of a structure coordinate system and at least one second feature oriented along a second axis of the structure coordinate system.

[0110] In a diffraction-based dark-field metrology apparatus, a beam of radiation is directed onto a metrology target and one or more properties of the scattered radiation are measured in order to determine a property of interest of the target. The properties of the scattered radiation can include, for example, the intensity at a single scattering angle (e.g. as a function of wavelength) or the intensity as a function of scattering angle at one or more wavelengths.

[0111] The international patent application WO2019197117A1, which is incorporated herein by reference, discloses a method and metrology apparatus for determining a characteristic (e.g. overlay) of a structure fabricated on a substrate based on a dark-field digital holographic microscope (df-DHM). For the purposes of description, the df-DHM in Figure 7 of the international patent application WO2019197117A1 is replicated in Figure 3 . Figure 7 The disclosed df-DHM is schematically illustrated, which is particularly suitable for use in lithographic process metrology.

[0112] Figure 7 The df-DHM in comprises reference optical units 16, 18 for providing two reference illumination beams 51, 52 (reference illumination). Such two reference illumination beams 51, 52 are paired with two respective portions 41, 42 of the scattered radiation beams 31, 32 (object radiation), respectively. The two scattered-reference beam pairs are used to continuously form two interference patterns. Coherence control is provided by means of adjusting the relative optical path length difference (OPD) between the two scattered-reference beams within each beam pair. However, coherence control is not available between the two beam pairs. A PCF according to any of the embodiments described herein can be used in the holographic metrology apparatus (such as the holographic metrology apparatus depicted in Figure 7 ) to deliver the illumination beams 21, 22 and / or the reference illumination beams 51, 52.

[0113] Due to the use of a single light source and insufficient coherence control, all four beams of radiation (i.e. the first portion 41 of the scattered radiation 31, the first reference illumination 51, the second portion 42 of the scattered radiation 32 and the second reference illumination 52) are mutually coherent. If these four mutually coherent beams of radiation are allowed to reach the same location of the sensor 6 simultaneously, i.e. operating in a parallel acquisition scheme, multiple interference patterns comprising the desired information including the pattern of interest and the pattern of undesired artifacts will overlap each other. Undesired interference patterns can be formed by, for example, the interference between the portion 41 of the first scattered radiation 31 and the portion 42 of the second scattered radiation 32. Since it would be technically challenging and time consuming to completely separate the superimposed interference patterns, parallel acquisition is not practical for such an arrangement.

[0114] In Figure 7 the use of a continuous acquisition scheme in the example allows the full NA of the objective to be used for both illumination and detection. However, the system suffers from the same problem of low measurement speed due to continuous acquisition. Therefore, it is desirable to have a df-DHM that is capable of performing parallel acquisition so that high measurement speed and high design flexibility can be obtained simultaneously.

[0115] Figure 8 An imaging branch of a dark-field digital holographic microscope (df-DHM) 1000 is schematically illustrated. The dark-field digital holographic microscope (df-DHM) comprises an imaging branch and an illumination branch. In this embodiment, a metrology target 1060 comprising structures on a substrate 1050 is illuminated by two beams of illumination radiation (i.e. a first beam of illumination radiation 1010 and a second beam of illumination radiation 1020). In embodiments, such two beams of illumination 1010, 1020 can illuminate the metrology target 1060 simultaneously. A PCF according to any of the embodiments described herein can be used in a holographic metrology apparatus, such as the holographic metrology apparatus depicted in Figure 8 to deliver the first beam of illumination radiation 1010 and / or the second beam of illumination radiation 1020.

[0116] In an embodiment, a first illumination beam 1010 may be incident on the measurement target 1060 at a first incident angle relative to the optical axis OA in a first incident direction. A second illumination beam 1020 may be incident on the measurement target 1060 at a second incident angle relative to the optical axis OA in a second incident direction. The first incident angle of the first illumination beam 1010 and the second incident angle of the second illumination beam 1020 may be approximately the same. The incident angle of each illumination beam may, for example, be in the range of 70 degrees to 90 degrees, in the range of 50 degrees to 90 degrees, in the range of 30 degrees to 90 degrees, or in the range of 10 degrees to 90 degrees. Irradiation of the measurement target 1060 may result in scattered radiation from the target. In an embodiment, the first illumination beam 1010 may be incident on the measurement target 1060 at a first azimuth angle corresponding to a first direction. The second illumination beam 1020 may be incident on the measurement target 1060 at a second azimuth angle corresponding to a second direction. The first azimuth angle of the first irradiation beam 1010 and the second azimuth angle of the second irradiation beam 1020 can be different, for example, relative angles that are 180 degrees apart.

[0117] Depending on the structure of the measurement target 1060, the scattered radiation may include reflected radiation, diffracted radiation, or transmitted radiation. In this embodiment, the measurement target may be based on a diffracted overlapping target; and each illumination beam may correspond to a scattered beam including at least one non-zero diffraction order. Each scattered beam carries information about the irradiated measurement target. For example, the first illumination beam 1010 may correspond to a beam including a positive first diffraction order +1. st The first scattering beam 1011 and the second irradiation beam 1020 of the DF can correspond to including the negative first diffraction order -1. st The second scattered beam 1021 of the DF. Zero diffraction order and other undesired diffraction orders can be blocked by a beam-blocking element (not shown) or configured to fall entirely outside the NA of objective 1070. Therefore, df-DHM can operate in dark field mode. It should be noted that in some embodiments, one or more optical elements (e.g., lens combinations) can be used to achieve the same optical effect of objective 1070.

[0118] Both scattered beams 1011 and 1021 can be collected by objective lens 1070 and subsequently refocused onto image sensor 1080. Objective lens 1070 may include multiple lenses, and / or df-DHM 1000 may include lenses with two or more lenses (e.g., similar to...). Figure 7lens system of the exemplary df-DHm of the objective and imaging lens) thereby defining a pupil plane of the objective and an image plane at the focal point of the imaging lens between the two lenses. In this embodiment, portions 1012 of the first scattered beam 1011 and 1022 of the second scattered beam 1021 are simultaneously incident at a common location of the image sensor 1080. At the same time, two reference radiation beams (i.e., the first reference beam 1030 and the second reference beam 1040) are incident at the same location of the image sensor 1080. Such four beams can be grouped into two pairs of scattered radiation and reference illumination. For example, the first scattered-reference beam pair can include portions 1012 of the first scattered beam 1011 and the first reference beam 1030. Likewise, the second scattered-reference beam pair can include portions 1022 of the second scattered beam 1021 and the second reference beam 1040. These two scattered-reference beam pairs can subsequently form two interference patterns (holographic images) that are at least partially superimposed in the spatial domain. A PCF according to any of the embodiments described herein can be used in a holographic metrology apparatus (such as the holographic metrology apparatus depicted in Figure 8 to deliver the first reference beam 1030 and / or the second reference beam 1040.

[0119] In embodiments, to separate the two at least partially spatially superimposed interference patterns (e.g., in the spatial frequency domain), the first reference beam 1030 can have a first angle of incidence with respect to the optical axis OA and the second reference beam 1040 can have a second angle of incidence with respect to the optical axis OA; the first angle of incidence being different from the second angle of incidence. Alternatively or additionally, the first reference beam 1030 can have a first azimuthal angle with respect to the optical axis OA and the second reference beam 1040 can have a second azimuthal angle with respect to the optical axis OA; the first azimuthal angle being different from the second azimuthal angle.

[0120] To generate the interference pattern, the two beams of each scatter-reference beam pair should be at least partially coherent with each other to an extent sufficient to form an interference pattern. It should be noted that each scattered radiation beam can have a phase shift relative to its corresponding illumination radiation. For example, at the image plane of the image sensor 1080, such phase shift can include contributions due to the optical path length (OPD) from the metrology target 1060 to the image sensor 1080 and by the interaction with the metrology target. As described above, it is necessary to control the coherence between the first scatter-reference beam pair and the second scatter-reference beam pair such that each beam of one pair is incoherent with any beam of the other pair. In other words, interference should occur only between the beams within the same beam pair and be suppressed between different beam pairs. In this way, only the desired interference patterns (e.g., the two interference patterns formed by the corresponding scatter-reference beam pairs) are formed in superposition on the image sensor 1080, thus avoiding the problem of separating or removing the undesired interference patterns.

[0121] The metrology apparatus described above can provide an overlay measurement corresponding to a displacement of a feature along a particular axis or between two layers overlaid on each other. For example, Figure 8 The dark-field digital holographic microscope (df-DHM) 1000 includes a first illumination beam incident on the metrology target 1060 at a first azimuth angle corresponding to a first direction. The df-DHM 1000 also includes a second illumination beam 1020 incident on the metrology target 1060 at a second azimuth angle corresponding to a second direction. The first azimuth angle and the second azimuth angle can be different. For example, the relative angles can be 180 degrees apart.

[0122] The metrology apparatus, such as the df-DHM 1000, can be adapted to acquire an overlay measurement of a feature along an axis along which the illumination beams are positioned. For example, the illumination beams can be aligned along the x-axis, and can subsequently provide corresponding scattered beams corresponding to positive and negative diffraction orders. In particular, the first illumination beam 1010 can provide scattered beams including at least a positive first diffraction order +1 st DF, while the second illumination beam can provide scattered beams including at least a negative first diffraction order -1 st DF. Thus, an overlay x-component can be provided by the df-DHM in this setup. Similarly, the illumination beams can be aligned along the y-axis, and can subsequently provide corresponding scattered beams corresponding to positive and negative diffraction orders. In particular, the first illumination beam 1010 can provide scattered beams including at least a positive first diffraction order +1 st DF, while the second illumination beam can provide scattered beams including at least a negative first diffraction order -1 st DF. Thus, an overlay y-component can be provided by the df-DHM in this setup.

[0123] A limitation of metrology apparatuses such as the df-DHM 1000 is that the overlay measurement can only include an overlay component along one axis, such as the x-component OVx discussed above. To obtain other overlay components, such as the y-component OVy, additional measurements are required. For example, to obtain both the x-component OVx and the y-component OVy of the overlay for the df-DHM 1000, a rotation of the illumination beam relative to the wafer can be required. This can be achieved by, for example, a rotation of the wafer, a rotation of the illumination beam about the optical axis, or a combination thereof. A disadvantage of rotating the illumination beam relative to the wafer can be a reduction in throughput of the metrology apparatus due to the time taken to rotate the illumination beam relative to the wafer. Figure 8

[0124] Alternatively, an apparatus for obtaining both the x-component OVx and the y-component OVy of the overlay can include utilizing an additional set of illumination beams. Figure 9A A dark-field digital holographic microscope (df-DHM) 1100 is schematically illustrated, which is similar to the df-DHM 1000 of Figure 8 but includes a first set of illuminators and a second set of illuminators. Figure 11B The relative k-vectors of the illumination beams provided by the illuminators in k-space relative to the objective lens 1170 of the df-DHM 1100 are schematically illustrated.

[0125] The df-DHM 1100 can be used to measure the x-component OVx and the y-component OVy of the overlay of a structure 1160 including x-features (or first features) oriented along an x-axis (or a first axis of a coordinate system of the structure) and y-features (or second features) oriented along a y-axis (or a second axis of the coordinate system of the structure) relative to the coordinate system of the structure 1160. The structure 1160 can include a set of sub-structures. The set of sub-structures can include 2 layers, each layer including at least one x-oriented feature and at least one y-oriented feature. For example, each layer can include features including at least one x-oriented feature and at least one y-oriented feature. In some embodiments, each layer includes 2 x-oriented features and 2 y-oriented features. It will be appreciated that each layer can include any number of x-oriented features and y-oriented features. The layers can be stacked on top of each other into a stack. Although each feature has been described as being oriented along a particular axis, it will be appreciated that each feature can be oriented at least partially along the axis.

[0126] In particular, as shown in Figure 9A the first set of illuminators can include a first x-illuminator 1110 and a second x-illuminator 1120. The first x-illuminator 1110 can provide a first x-illumination beam 1111 arranged to provide a first x-diffracted order +1 st ​a first x-scattering beam 1112 of the x-DF. A second x-illuminator 1120 can provide a second x-illumination beam 1121 arranged to provide a second x-diffraction order of at least negative first x-diffraction order -1 st a second x-scattering beam 1122 of the x-DF. The second set of illuminators can include a first y-illuminator 1130 (not shown in Figure 9A ) and a second y-illuminator 1140 (not shown in Figure 9A ). The first y-illuminator 1130 can provide a first y-illumination beam (not shown) arranged to provide a first y-diffraction order of at least positive first y-diffraction order +1 st a first y-scattering beam (not shown) of the y-DF. The second y-illuminator 1140 can provide a second y-illumination beam (not shown) arranged to provide a second y-diffraction order of at least negative first y-diffraction order -1 st a second y-scattering beam (not shown) of the y-DF.

[0127] Each illumination beam can be incident on the structure 1160 of the wafer 1150 at a corresponding angle of incidence with respect to the optical axis OA. The angle of incidence of each illumination beam can be substantially the same. The angle of incidence of each illumination beam can for example be in the range of 70 degrees to 90 degrees, in the range of 50 degrees to 90 degrees, in the range of 30 degrees to 90 degrees, in the range of 10 degrees to 90 degrees.

[0128] Each illumination beam can be incident on the structure 1160 at a corresponding azimuthal angle. The relative positions of each illuminator with respect to the objective lens 1170 are illustrated in Figure 9B . Figure 9B A visual display is provided in pupil space or k-space (in the (k x , k y ) plane) of the relative positions of each illuminator with respect to the objective lens 1170. The objective lens 1170 defines a numerical aperture, labeled 1171 in Figure 9B . Radiation outside of the region defined by the numerical aperture 1171 is not captured by the objective lens 1170 and thus cannot contribute to the interference pattern captured by the image sensor 1180. Radiation inside of the region can be captured by the objective lens 1170 and thus can contribute to the interference pattern recorded by the image sensor 1180. The azimuthal angle of a beam refers to the angle between the x-axis and the incident beam or between the x-axis and the projection of the incident beam in the x-y plane. The first x-illuminator 1110 can provide the first x-illumination beam 1111 at a first azimuthal angle of 0 degrees. The second x-illuminator 1120 can provide the second x-illumination beam 1121 at a second azimuthal angle of 180 degrees. The first y-illuminator 1130 can provide the first y-illumination beam at a third azimuthal angle of 90 degrees. The second y-illuminator 1140 can provide the second y-illumination beam at a fourth azimuthal angle of 270 degrees.

[0129] Each scattered beam 1011, 1021 can be collected by objective lens 1170 and subsequently refocused onto image sensor 1180. Objective lens 1170 may include multiple lenses, and / or df-DHM 1100 may include lenses with two or more lenses (e.g., similar to...). Figure 7 An exemplary df-DHm lens system (objective and imaging lens) thereby defining the pupil plane of the objective between the two lenses and the image plane at the focal point of the imaging lens.

[0130] like Figure 9A As illustrated, a portion 1113 of the first x-scattered beam 1112 and a portion 1123 of the second x-scattered beam 1122 are simultaneously incident on a common location of the image sensor 1180. Simultaneously, two x-reference radiation beams (i.e., the first x-reference beam 1114 and the second x-reference beam 1124) are incident on the same location of the image sensor 1180. The first x-reference beam 1114 may be provided by a first reference illuminator (not shown), and the second x-reference beam may be provided by a second reference illuminator (not shown). These four beams can be grouped into two pairs of x-scattered radiation and x-reference illumination. For example, the first scattering-reference beam pair may include a portion 1113 of the first x-scattered beam 1112 and the first x-reference beam 1114. Similarly, the second scattering-reference beam pair may include a portion 1123 of the second x-scattered beam 1122 and the second x-reference beam 1124. These two scattering-reference beam pairs can subsequently form two interference patterns (holographic images) that can provide imaging information of the x-features of structure 1160. The PCF according to any of the embodiments described herein can be used in holographic measurement devices (such as...) Figure 9A The measuring device described herein may provide one or more of the following: a first x-irradiation beam 1111, a second x-irradiation beam 1121, a first x-reference beam 1114, a second x-reference beam 1124, a first y-irradiation beam, a second y-irradiation beam, a first y-reference beam, and a second y-reference beam.

[0131] Similarly, when the df-DHM 1100 utilizes the first and second y-irradiation beams, two y-reference radiation beams (i.e., the first y-reference beam (not shown) and the second y-reference beam (not shown)) can be simultaneously incident on the image sensor 1180 at the same locations as the first and second y-scattered beams, respectively. Alternatively, these four beams can be grouped into two pairs of y-scattered radiation and y-reference illumination. For example, the first scattering-reference beam pair may include a portion of the first y-scattered beam and the first y-reference beam. Similarly, the second scattering-reference beam pair may include a portion of the second y-scattered beam and the second y-reference beam. These two scattering-reference beam pairs can subsequently form two interference patterns (holographic images) that can provide imaging information of the y-features of structure 1160.

[0132] To enable parallel acquisition of all information required to determine one of the overlap components (e.g. the x-overlap component), the reference illuminators can be arranged such that each reference beam interferes only with the respective portion of the scatter-beam in its scatter-reference beam pair. Using the x-overlap component as an example, to separate the two at least partially spatially overlapping interference patterns (e.g. in the spatial frequency domain), the first x-reference beam 1114 can have a first angle of incidence with respect to the optical axis OA and the second x-reference beam 1124 can have a second angle of incidence with respect to the optical axis OA; the first angle of incidence is different from the second angle of incidence. Alternatively or additionally, the first x-reference beam 1114 can have a first azimuthal angle with respect to the optical axis OA and the second x-reference beam 1124 can have a second azimuthal angle with respect to the optical axis OA; the first azimuthal angle is different from the second azimuthal angle. In this way, each reference beam can interfere only with light from one illuminator and the direction of the interference fringes on the image sensor 1180 can be altered by choosing the angle of incidence and / or the azimuthal angle of each reference beam. Figure 9C An image spectrum in the spatial frequency domain is depicted. The image spectrum comprises a zeroth spatial frequency domain comprising a zeroth Fourier component containing base spatial spectrum or base band BB, and two (identical) higher order spatial spectra or side bands +SB1and +SB2of a first x-diffraction order +1 st and two (identical) higher order spatial spectra or side bands -SB1and -SB2of a first x-diffraction order -1 st The position of the higher order spatial spectra can be adjusted with respect to the base spectrum by, for example, changing the angle of incidence and / or the azimuthal angle of each x-reference beam.

[0133] The df-DHM 1100 can use the first set of illuminators and reference illuminators to illuminate the structure 1160 in order to provide an x-component of the overlap. Subsequently, the df-DHM can use the second set of illuminators and reference illuminators to illuminate the metrology target in order to provide a y-component of the overlap. Furthermore, if only the first set of illuminators is used, the image sensor 1180 images only features that scatter light in the x-direction, while if only the second set of illuminators is used, the image sensor 1180 images only features that scatter light in the y-direction. A drawback of the df-DHM 1100 is that four illuminators are required to provide a full overlap measurement in both directions of the substrate / target plane. Illuminators are relatively complex and expensive parts. Therefore, this requirement of four illuminators can increase the complexity and cost of the df-DHM 1100.

[0134] Reference is now made to Figure 10A and Figure 10B Photonic crystal fibers (PCFs) that can be used in the apparatus described above are described.

[0135] AsFigure 10A As shown, PCF 10 includes a core region and a surrounding coating region. Both the core region and the coating region include components with a first refractive index (n). B The substrate material 12 of the fiber. The cladding region further includes a plurality of microstructures 14 (otherwise referred to as inclusions) extending along the longitudinal axis (in the z-direction) of the fiber from the input end to the output end of the fiber. The plurality of microstructures have a second refractive index (n). inc ), where n inc <n B .

[0136] The substrate material 12 may include silica glass, such as fused silica, or other special glasses, such as soft glass, for example ZBLAN.

[0137] The plurality of microstructures 14 may include those having a second refractive index (n) inc The medium can be a solid material. The solid material can be a doped material, such as doped silicon dioxide. The doped material can be, for example, fluorine (F), germanium (Ge), and / or phosphorus (P). In embodiments where doped silicon dioxide includes fluorine-doped silicon dioxide, the molar percentage of fluorine can be, for example, in the range of 1% to 10%, such as in the range of 3% to 8%.

[0138] In other embodiments, the plurality of microstructures 14 include air, such that the plurality of microstructures are hollow bodies. In still other embodiments, the plurality of microstructures include a vacuum.

[0139] Microstructure 14 is depicted as having a circular cross-sectional shape; however, this is merely an example. Microstructure 14 can have any suitable cross-sectional shape, such as triangles, squares, polygons, such as hexagons, ellipses, etc.

[0140] like Figure 10A and Figure 10B As shown, a plurality of microstructures 14 in the encapsulated region are arranged in a cross-sectional pattern comprising at least one microstructure ring surrounding the core region. In the context of this application, the term "microstructure ring" refers to an encapsulated microstructure that is generally equidistant from the core radially and aligned in a ring configuration surrounding the core. Typically, the microstructure ring is not perfectly circular but is shaped with a number of soft angles, such as a hexagonal shape. It should be understood that this is merely an example and the microstructure ring can be arranged in a circular or elliptical shape. Preferably, all microstructures in the microstructure ring have approximately the same size and preferably are made of the same material.

[0141] Although Figure 10AThe PCF 10 is shown to have six microstructure (14) rings, but it should be understood that this is merely an example. In some embodiments, only a single microstructure ring is present. In other embodiments, multiple microstructure rings are present, such as at least six rings, such as at least seven rings, such as at least eight rings.

[0142] like Figure 10B As shown, each microstructure has a diameter (d), and the pitch Λ of the microstructures is defined as the center-to-center distance between adjacent microstructures.

[0143] exist Figure 10A and Figure 10B In the example, the microstructure ring closest to (i.e., adjacent to) the core region has twelve microstructures. In some implementations (such as the example where the microstructure ring closest to the core region has twelve microstructures), the core region has the largest extent (shown as D). core ), and minimum range (such as in Figure 10C The middle part is shown as D. core-min Furthermore, the diameter of the core region can also be defined as the average of the maximum and minimum ranges of the core region, which is referred to as D in this paper. core-avg And in Figure 10D The image is shown in the middle.

[0144] The diameter of the maximum extent of the core region (D) core )exist Figure 10A and Figure 10B The middle is shown as D core And the diameter of the covered area is Figure 10A The middle is shown as D clad The maximum geometric diameter (D) of the core region. core The diameter (D) is defined as the diameter of the circle enclosed by the first hexagonal microstructure ring closest to (i.e., adjacent to) the core region, given by the distance between microstructures at opposite vertices of the first hexagonal microstructure ring. The maximum diameter of the core region is... core The following is given:

[0145] .

[0146] That is, the diameter of the maximum extent of the core region (D) core The value is given by subtracting the difference in diameter of each microstructure from four times the pitch of the microstructure. Figure 10B As can be seen, the circle contacts the edge of the microstructure at the corner of the hexagon formed by the 12 microstructures (closest to the center of PCF 10) (see dashed line). It can also be seen that the circle does not contact the edge of the microstructure (closest to the center of PCF 10) on the side of the hexagon formed by the 12 microstructures.

[0147] For example, in one embodiment, to meet the above requirements, the microstructure ring closest to (i.e., immediately adjacent to) the core region has 12 microstructures, as Figure 10B is most clearly shown in .

[0148] Figure 10C illustrates how the measurement of the minimum extent of the core region diameter D core-min is performed. The minimum extent of the core region diameter (D core-min ) corresponds to the distance between the microstructures at opposite sides of the hexagonal microstructure ring closest to (i.e., immediately adjacent to) the core region. In Figure 9B the microstructures at the sides of the hexagonal microstructure ring that can be used in the calculation of D core-min are shown by means of a crosshatch pattern. The minimum extent of the core region diameter D core-min is given by the following formula:

[0149] .

[0150] Figure 10D illustrates how the measurement of the core region diameter D core-avg is performed when the core region has a maximum extent and a minimum extent. As Figure 10D illustrated in Figure 10D both the microstructures at the vertices and the sides of the hexagonal microstructure ring are used in the calculation of D core-avg . These microstructures are shown by means of a crosshatch pattern. The core region diameter D core-avg is given by the following formula:

[0151] .

[0152] The microstructure ring closest to (i.e., immediately adjacent to) the core region having twelve microstructures is merely an example and embodiments of the present disclosure extend to optical fibers having a different number of heat-generating microstructures in the microstructure ring closest to (i.e., immediately adjacent to) the core region. For example, the microstructure ring closest to (i.e., immediately adjacent to) the core region can have six microstructures. In such an example, the maximum extent of the core region diameter (D core ) is defined as the diameter of the circle enclosed by the first hexagonal microstructure ring, which is given by the distance between the microstructures at opposite vertices of the hexagonal microstructure ring.

[0153] The maximum extent of the core region diameter (D core ) is at least 16 pm, which corresponds to a D of 15.294 pm when using a pitch and core-avg . The maximum extent of the core region diameter (D core ) can be at least 20 pm. The core region diameter Dcore-avg may be at least 15.1 μιη, optionally at least 15.2 μιη, optionally at least 15.3 μιη, optionally at least 15.4 μιη, optionally at least 15.5 μιη, optionally at least 15.6 μιη, optionally at least 15.7 μιη, optionally at least 15.8 μιη, optionally at least 15.9 μιη, optionally at least 16 μιη. The maximum extent of the diameter of the core region can be less than 50 μιη, such as less than 47 μιη. The maximum extent of the diameter of the core region (D core ) can be less than 20 μιη, such as less than 19 μιη, optionally less than 18 μιη. The PCF 10 has a transmission bandwidth of about 200 nm or more, such as about 300 nm or more, such as about 400 nm or more, such as about 500 nm or more. In an embodiment, the transmission bandwidth is defined as the wavelength in which the transmission fiber has a transmission loss of less than 0.5 dB / m for the fundamental mode. In another embodiment, the transmission bandwidth is defined as the wavelength in which the transmission fiber has a transmission loss of less than 0.1 dB / m for the fundamental mode. Advantageously, the transmission loss is measured when the PCF 10 is bent with a bend diameter of at least 10 cm, preferably between 10 cm and 12 cm, to delocalize the higher order modes. Advantageously, the PCF 10 is single mode for at least one wavelength within the transmission bandwidth. In general, the beam quality of single mode light is much higher than that of multi-mode light, and single mode light is necessary for some applications. Preferably, the PCF 10 is single mode for at least 50% of the transmission bandwidth, such as at least 80%, such as for the entire transmission bandwidth of the PCF 10.

[0154] The transmission bandwidth of the PCF 10 can include wavelengths in the range of 200 nm to 2500 nm, such as wavelengths in the range of 400 nm to 2000 nm, such as wavelengths in the range of 400 nm to 1200 nm, such as wavelengths in the range of 400 nm to 900 nm. In an embodiment, the transmission bandwidth of the PCF 10 includes wavelengths in the range from 400 nm to 900 nm. The transmission bandwidth of the PCF 10 includes a bandwidth of at least 200 nm, such as a bandwidth of at least 300 nm, such as a bandwidth of at least 400 nm, such as the entire bandwidth (e.g., in the range from 400 nm to 900 nm).

[0155] The PCF 10 can be a polarization maintaining (PM) fiber or a non-PM fiber. A PM fiber is a fiber in which linearly polarized light can be maintained if the linearly polarized light is launched into the fiber. Advantageously, the launched polarized light maintains linear polarization during propagation along the PM transmission fiber and exits the fiber in a linearly polarized state.

[0156] Typically, the PCF 10 can have a length of at least 10 cm, for example a length of at least 3 m. In some embodiments, the PCF can have a length of at most 100 m, such as a length of between 20 m and 80 m.

[0157] A contributing factor to the optical properties of the PCF 10 is the normalized inclusion diameter (d / A), which corresponds to the ratio of the diameter (d) of each microstructure to the pitch A of the microstructure. The d / A ratio is preferably in the range of 0.35 to 0.4. The following table gives some example values of the pitch A, the diameter (d), the normalized inclusion diameter (d / A), the resulting maximum range (D core ) of the core region of the PCF 10 and the corresponding (D core-avg ):

[0158] Λ (μm) d / Λ d (μm) D core (pm)]]> D core-avg (μm)]]> 4.55 0.35 1.5925 16.6075 15.4 4.55 0.37 1.6835 16.5165 15.3 4.55 0.4 1.82 16.38 15.2 4.5 0.37 1.665 16.3 15.1 4.6 0.37 1.702 16.698 15.5 4.8 0.37 1.776 17.424 16.1 5 0.37 1.85 18.15 16.8

[0159] In some embodiments of the application, the d / A ratio is less than 0.42 (for air-clad PCFs) to ensure that the PCF 10 is single-mode for wavelengths at least in the range of 500 nm to 900 nm, such as wavelengths at least in the range of 400 nm to 900 nm.

[0160] Figure 11A Example loss behavior of the fundamental mode propagating through (bent) photonic crystal fibers in which the maximum range of the diameter (D core ) of the core region is 16.5 pm and each photonic crystal fiber has 7 microstructure rings, but with different normalized inclusion diameter (d / A) values, and Figure 11B Example loss behavior of the high-order modes propagating through (bent) photonic crystal fibers each having 7 microstructure rings. The bend diameter is 11 cm.

[0161] In Figure 11A and Figure 11B , the waveform 101 corresponds to a PCF 10 having a d / A ratio of 0.35, the waveform 102 corresponds to a PCF 10 having a d / A ratio of 0.36, the waveform 103 corresponds to a PCF 10 having a d / A ratio of 0.37, the waveform 104 corresponds to a PCF 10 having a d / A ratio of 0.38, the waveform 105 corresponds to a PCF 10 having a d / A ratio of 0.39, and the waveform 106 corresponds to a PCF 10 having a d / A ratio of 0.40.

[0162] In Figure 11B , the dashed line 200 represents the minimum loss level (10 dB / m) required for the high-order modes for the PCF 10 to be single-mode.

[0163] Figure 12A Example loss behavior is illustrated for the fundamental mode propagating through (bent) photonic crystal fibers in which the maximum extent of the core region (D core ) is 16.5 μm and each of the photonic crystal fibers has 8 microstructured rings, but with different normalized inclusion diameter (d / A) values, and Figure 12B Example loss behavior is illustrated for the high order modes propagating through (bent) photonic crystal fibers each having 8 microstructured rings. The bend diameter is 11 cm.

[0164] In Figure 12A and Figure 12B , waveform 101 corresponds to PCF 10 with a d / A ratio of 0.35, waveform 102 corresponds to PCF 10 with a d / A ratio of 0.36, waveform 103 corresponds to PCF 10 with a d / A ratio of 0.37, waveform 104 corresponds to PCF 10 with a d / A ratio of 0.38, waveform 105 corresponds to PCF 10 with a d / A ratio of 0.39, and waveform 106 corresponds to PCF 10 with a d / A ratio of 0.40.

[0165] In Figure 12B , the dashed line 200 represents the minimum loss level (10 dB / m) required for the high order modes for the PCF 10 to be single mode (i.e., to have single mode behavior).

[0166] From Figure 11A / Figure 11B and Figure 12A / Figure 12B it can be observed that for PCF 10 in which the maximum extent of the core region (D core ) is 16.5 μm, having d / A < 0.37 ensures delocalization of the high order modes such that the PCF 10 is single mode for the entire transmission bandwidth of 400 nm to 900 nm.

[0167] It will be appreciated that the PCF in which the maximum extent of the core region (D core ) is 16.5 μm is merely an example. Similarly, the transmission bandwidth of 400 nm to 900 nm is merely an example. More generally, in embodiments in which the maximum extent of the core region (D core ) is at least 16 μm and less than 50 μm, the PCF 10 is single mode for the entire transmission bandwidth when d / A < 0.42, e.g., d / A < 0.37. In some embodiments, the d / A ratio is in the range of 0.35 < d / A < 0.40, e.g., 0.35 < d / A < 0.37.

[0168] Increasing the number of microstructured (14) rings (Nr) widens the optical fiber spectral transmission band. By increasing the number of microstructured (14) rings (Nr), the high order mode loss shows very small changes. In some embodiments of the invention, the number of microstructured rings (Nr) surrounding the core region is at least 6, for example 6, 7, 8 or 9.

[0169] Figure 13A Example transmission loss waveform 201 for the fundamental mode propagating through a PCF 10 having a maximum extent of the core region of a diameter (D core ) of 16.5 μm, and example transmission loss waveform 202 for the fundamental mode propagating through a PCF 10 having a maximum extent of the core region of a diameter (D core ) of 16.5 μm when the PCF 10 has 8 microstructured rings.

[0170] Figure 13B Example transmission loss waveform 301 for a first high order mode propagating through a PCF 10 having 7 microstructured rings, and example transmission loss waveform 302 for a second high order mode propagating through a PCF 10 having 7 microstructured rings. Figure 13B Example transmission loss waveform 303 for a first high order mode propagating through a PCF 10 having 8 microstructured rings, and example transmission loss waveform 302 for a second high order mode propagating through a PCF 10 having 8 microstructured rings.

[0171] From Figure 13A and Figure 13B it can be observed that the spectral width of the fundamental mode transmission band increases by about 100 nm when the number of microstructured rings (Nr) is increased from 7 to 8 rings. The loss of the fundamental mode is less than 50 dB / km for wavelengths between 500 nm and 900 nm. At 400 nm, the loss is high and equal to 610 dB / km. For the high order modes, the expected loss is greater than 10 dB / km for wavelengths between 525 nm and 580 nm (Nr=8). Within such a range, the lowest loss is about 6.5 dB / m. Figure 13A and Figure 13B The results also apply to a bent photonic crystal fiber having a bending diameter of 11 cm.

[0172] As explained above, in embodiments of the application, the PCF 10 has a transmission bandwidth of 200 nm or more, wherein the PCF has a transmission loss of less than 0.5 dB / m (e.g. less than 0.1 dB / m) for the fundamental mode within the transmission bandwidth. Advantageously, the transmission loss is measured when the PCF 10 is bent with a bend diameter of at least 10 cm (preferably between 10 cm and 12 cm) to delocalize the higher order modes.

[0173] The bend direction can be applied along the x-axis or the y-axis. The inventors have observed that for wavelengths greater than 700 nm, the bend direction does not affect the fundamental mode loss. On the other hand, in the y-axis bend direction, for short wavelengths (e.g. wavelengths lower than or equal to 700 nm), an additional loss peak appears. For the higher order modes, the variation in the higher order mode loss is greater for the bend applied along the y-axis.

[0174] As explained above, the PCF 10 can be a polarization maintaining (PM) optical fiber. In embodiments of the application, the PM PCF 10 is achieved by incorporating a stress element into the optical fiber. The stress element is included in the cladding region and is enclosed by the host material 12 in the cladding region. The stress element extends from the input end of the optical fiber to the output end of the optical fiber along the longitudinal axis of the optical fiber (in the z-direction). The stress element induces stress in the core region (provides birefringence). In one example, the host material is silica, the microstructure 14 comprises silica doped with fluorine (e.g. with a refractive index of 12 x 10 -7 1 / K), and the stress element comprises silica doped with boron (e.g. with a refractive index of -11 x 10 -3 1 / K).

[0175] The stress element is depicted as having a hexagonal cross-sectional shape, however, this is merely an example. The stress element can have any appropriate cross-sectional shape, such as a circle, a triangle, a square, a polygon such as a hexagon, an ellipse, etc.

[0176] The normalized inclusion diameter (d / A) of the stress element corresponds to the ratio of the diameter (d) of each stress element to the pitch A of the stress element. The d / A ratio of the stress element can be, for example, 0.85.

[0177] PCF 10 comprises two sets of stress elements. Each set of stress elements comprises a plurality of stress elements, for example at least three stress elements, such as at least four stress elements, such as at least five stress elements, such as at least six stress elements, such as at least seven stress elements, such as at least eight stress elements, such as at least nine stress elements. Typically, each set of stress elements of the two sets of stress elements has the same number of stress elements to have a symmetric effect. The two sets of stress elements can be arranged mirror symmetrically around a vertical line (extending in the y-direction) through the center of the core region when viewed in a lateral cross-section of the PCF 10.

[0178] In some embodiments of the application, the two sets of stress elements have a triangular cross-sectional shape, which is illustrated in Figure 14A and Figure 14B The inventors have observed that arranging the stress elements in a triangular form helps to facilitate the leakage of higher order modes when the PCF 10 is bent.

[0179] In particular, Figure 14A and Figure 14B shows the two sets of stress elements 16, wherein the two sets of stress elements 16 each have a triangular lateral cross-sectional shape. In some examples, the apex of the triangular set of stress elements 16 points towards the core region, as illustrated in Figure 14A and Figure 14B

[0180] Figure 14A depicts a lateral cross-section of the PM PCF 10, wherein the two sets of stress elements are located entirely within the plurality of microstructure (14) rings. That is, none of the stress elements 16 in either set of stress elements is positioned outside the plurality of microstructure (14) rings. Figure 14B shows that two microstructures in the third microstructure ring have been replaced by two stress elements 16 (one stress element in each triangular set), that four microstructures in the fourth microstructure ring have been replaced by four stress elements 16 (two stress elements in each triangular set), that six microstructures in the fifth microstructure ring have been replaced by six stress elements 16 (three stress elements in each triangular set), that eight microstructures in the sixth microstructure ring have been replaced by eight stress elements 16 (four stress elements in each triangular set), and that ten microstructures in the seventh microstructure ring have been replaced by ten stress elements 16 (five stress elements in each triangular set).

[0181] Figure 14B ​A transverse cross-section of PM PCF 10 is depicted, in which each of the two sets of stress elements is partially located within a plurality of microstructure (14) rings, and each set of stress elements includes stress elements 16 positioned outside the plurality of microstructure rings. In such implementations, each of the two sets of stress elements includes stress elements 16 located further from the core region than the microstructures in the outer microstructure rings. Figure 14B A stress element 16 of one of the two sets of stress elements in region 18, located outside the plurality of microstructure rings, is shown. The inventors have observed that by adding stress elements outside the plurality of microstructure rings, the birefringence is advantageously increased.

[0182] Figure 15 A graph illustrating the attenuation of the fundamental mode of PCF 10 when the fiber is straight is shown in Figure 14B Waveform 150 corresponds to the attenuation exhibited by PCF 10 when radiation input into PCF 10 is polarized along the y-axis (fast axis), and waveform 152 corresponds to the attenuation exhibited by PCF 10 when radiation input into PCF 10 is polarized along the x-axis (slow axis). For comparison, waveform 154 corresponds to the attenuation exhibited by a non-PM fiber.

[0183] Figure 16A and Figure 16B A graph illustrating the attenuation of the fundamental mode of PCF 10 when the fiber is bent is shown in Figure 14B

[0184] Figure 16A A graph illustrating the attenuation of the fundamental mode of PCF 10 when the fiber is bent in a direction parallel to the alignment axis of the stress elements (extending in the x-direction) is shown in Figure 14B Waveform 160 corresponds to the attenuation exhibited by PCF 10 when radiation input into PCF 10 is polarized along the y-axis (fast axis), and waveform 162 corresponds to the attenuation exhibited by PCF 10 when radiation input into PCF 10 is polarized along the x-axis (slow axis).

[0185] Figure 16B A graph illustrating the attenuation of the fundamental mode of PCF 10 when the fiber is bent in a direction perpendicular to the alignment axis of the stress elements (extending in the x-direction) is shown in Figure 14B Waveform 164 corresponds to the attenuation exhibited by PCF 10 when radiation input into PCF 10 is polarized along the y-axis (fast axis), and waveform 166 corresponds to the attenuation exhibited by PCF 10 when radiation input into PCF 10 is polarized along the x-axis (slow axis).

[0186] ​It can be seen that the attenuation curve of the fundamental mode is much less perturbed when the fiber is bent in a direction parallel to the alignment axis of the stress elements. The attenuation curve of the fundamental mode with polarization parallel to the axis of the stress elements is comparable to the attenuation curve obtained when the fiber is kept straight.

[0187] In Figure 15 and Figure 16A / Figure 16B The waveforms shown in Figure 14B correspond to the PCF 10 shown in

[0188] - The microstructure comprises fluorine-doped silica; fluorine concentration: 5 mol%

[0189] - Pitch (A): 4.50 pm

[0190] - Normalized inclusion diameter of the microstructure (d / A): 0.37

[0191] - Number of rings: 7

[0192] - The stress elements comprising boron-doped silica have a refractive index: -11 x 10 -3

[0193] - Normalized inclusion diameter of the stress elements (d / A): 0.85

[0194] Although the stress elements 16 are depicted in Figure 14A and Figure 14B as having a larger cross-sectional area than the plurality of microstructures 14, this is merely an example. The stress elements 16 can have the same cross-sectional area as the plurality of microstructures 14, or a smaller cross-sectional area than the plurality of microstructures 14.

[0195] In some embodiments of the application, the two groups of stress elements have a rhombus cross-sectional shape, which is illustrated in Figure 17 Herein, a rhombus is referred to as a shape having four equal sides and no right angles (excluding a square). The inventors have observed that arranging the stress elements in a rhombus-shaped cross-sectional area allows for having a similar birefringence level as in the PCF of Figure 14B but with fewer stress elements. Figure 14B The PCF 10 of Figure 17 has a total of 56 stress elements, whereas the PCF 10 of Figure 14B has a total of 50 stress elements and achieves a similar birefringence level as in the PCF of

[0196] Figure 17 Two groups of stress elements 16 are shown, wherein the two groups of stress elements 16 each have a rhombus lateral cross-sectional shape.Figure 17 In the example, the two sets of stress elements are entirely located within the multiple microstructure (14) rings. However, it should be understood that in other examples, the diamond-shaped group of stress elements may include stress elements 16 positioned outside the multiple microstructure rings. Figure 17 As shown, the multiple microstructures 14 may each have the same diameter.

[0197] Figure 18 The diagram shows the situation when the optical fiber is straight. Figure 17 The attenuation of the fundamental harmonic mode of PCF 10 is shown in the figure. Waveform 180 corresponds to the attenuation exhibited by PCF 10 when the radiation input to PCF 10 is polarized along the y-axis (fast axis), and waveform 182 corresponds to the attenuation exhibited by PCF 10 when the radiation input to PCF 10 is polarized along the x-axis (slow axis). For comparison, waveform 184 corresponds to the attenuation exhibited by non-PM fiber.

[0198] Figure 19A and Figure 19B The diagram illustrates the bending of the optical fiber. Figure 17 The attenuation of the fundamental harmonic mode of PCF 10 is shown in the figure.

[0199] Figure 19A The diagram illustrates the bending of the optical fiber in a direction parallel to the alignment axis of the stress element (extending in the x-direction). Figure 17 The attenuation of the fundamental harmonic mode of PCF 10 is shown in the figure. Waveform 190 corresponds to the attenuation exhibited by PCF 10 when the radiation input to PCF 10 is polarized along the y-axis (fast axis), and waveform 192 corresponds to the attenuation exhibited by PCF 10 when the radiation input to PCF 10 is polarized along the x-axis (slow axis).

[0200] Figure 19B The diagram illustrates the bending of the optical fiber in a direction perpendicular to the alignment axis of the stress element (extending in the x-direction). Figure 17 The attenuation of the fundamental harmonic mode of PCF 10 is shown in the figure. Waveform 194 corresponds to the attenuation exhibited by PCF 10 when the radiation input to PCF 10 is polarized along the y-axis (fast axis), and waveform 196 corresponds to the attenuation exhibited by PCF 10 when the radiation input to PCF 10 is polarized along the x-axis (slow axis).

[0201] It can be seen that when the optical fiber is bent in a direction parallel to the alignment axis of the stress element, the attenuation curve of the fundamental harmonic mode is comparable to that obtained when the optical fiber is kept straight. However, when the optical fiber is bent in a direction perpendicular to the alignment axis of the stress element, the attenuation decreases significantly.

[0202] exist Figure 18 and Figure 19A / Figure 19B The waveform shown corresponds to the waveform in Figure 17The PCF 10 shown in

[0203] - The microstructures comprise fluorine-doped silica; fluorine concentration: 5 mol%

[0204] - Pitch (Λ): 4.50 pm

[0205] - Normalized inclusion diameter of the microstructures (d / Λ): 0.37

[0206] - Number of rings: 7

[0207] - The stress elements comprising boron-doped silica have a refractive index: -11 x 10 -3

[0208] - Normalized inclusion diameter of the stress elements (d / Λ): 0.85

[0209] Although the stress elements 16 are depicted in Figure 17 as having a larger cross-sectional area than the plurality of microstructures 14, this is merely an example. The stress elements 16 can have the same cross-sectional area as the plurality of microstructures 14, or a smaller cross-sectional area than the plurality of microstructures 14.

[0210] Figure 20 A transverse cross-section of the PM PCF 10 is depicted, wherein the two sets of stress elements 16 each have a rhombic transverse cross-sectional shape. In contrast to the PM PCF 10 shown in Figure 17 , Figure 20 The PM PCF 10 shown in L comprises two different diameters of microstructures. That is, the plurality of microstructures comprises two sets of microstructures 18 having a first diameter (d S ), and the plurality of microstructures 14 having a second diameter (d S ) that is smaller than the first diameter. That is, the remaining microstructures 14 have the second diameter (d L ). As shown in Figure 20 , the two sets of microstructures 18 having the larger diameter (d L ) can have a triangular transverse cross-sectional shape. The inventors have observed that arranging the microstructures 18 having the larger diameter (d L ) in a triangular pattern reduces the loss of the fundamental mode when the fiber is bent perpendicular to the alignment axis of the stress elements. This arrangement does not affect the evanescent field of the higher order modes.

[0211] Each set of microstructures 18 having the first diameter (d L ) comprises a plurality of microstructures 18 having the first diameter (d L ), for example at least three microstructures 18 having the first diameter (dL at least six microstructures 18, such as at least ten microstructures 18, such as at least fifteen microstructures 18, having a first diameter (d L at least six microstructures 18, such as at least ten microstructures 18, such as at least fifteen microstructures 18, having a first diameter (d L at least six microstructures 18, such as at least ten microstructures 18, such as at least fifteen microstructures 18, having a first diameter (d

[0212] While the two groups of stress elements 16 can be arranged mirror-symmetrically around a vertical line (extending in the y-direction) passing through the center of the core region when viewed in a transverse cross-section of the PCF 10, the two groups of microstructures 18 having a larger diameter (d L The two groups of microstructures 18 having a larger diameter (d

[0213] Figure 21 The attenuation of the fundamental mode of the PCF 10 is illustrated in Figure 20 when the optical fiber is straight. The waveforms 200 correspond to the attenuation exhibited by the PCF 10 when the radiation input into the PCF 10 is polarized along the y-axis (fast axis), and the waveforms 202 correspond to the attenuation exhibited by the PCF 10 when the radiation input into the PCF 10 is polarized along the x-axis (slow axis).

[0214] Figure 22A and Figure 22B The attenuation of the fundamental mode of the PCF 10 is illustrated in Figure 20 when the optical fiber is bent.

[0215] Figure 22A The attenuation of the fundamental mode of the PCF 10 is illustrated in Figure 20 when the optical fiber is bent in a direction parallel to the alignment axis of the stress elements (extending in the x-direction). The waveforms 210 correspond to the attenuation exhibited by the PCF 10 when the radiation input into the PCF 10 is polarized along the y-axis (fast axis), and the waveforms 212 correspond to the attenuation exhibited by the PCF 10 when the radiation input into the PCF 10 is polarized along the x-axis (slow axis).

[0216] Figure 22B The attenuation of the fundamental mode of the PCF 10 is illustrated in Figure 20 when the optical fiber is bent in a direction perpendicular to the alignment axis of the stress elements (extending in the x-direction). The waveforms 214 correspond to the attenuation exhibited by the PCF 10 when the radiation input into the PCF 10 is polarized along the y-axis (fast axis), and the waveforms 216 correspond to the attenuation exhibited by the PCF 10 when the radiation input into the PCF 10 is polarized along the x-axis (slow axis).

[0217] It can be seen that, in Figure 20The design of the PM PCF 10 shown in

[0218] In Figure 21 and Figure 22A / Figure 22B corresponds to the PCF 10 shown in Figure 20 with the following parameters:

[0219] - The microstructure comprises fluorine-doped silica; the fluorine concentration: 5 mol%

[0220] - Pitch (A): 4.50 pm

[0221] - Normalized inclusion diameter (d S / A) of the smaller diameter microstructures: 0.37

[0222] - Normalized inclusion diameter (d L / A) of the larger diameter microstructures: 0.70

[0223] - Number of rings: 7

[0224] - The stress elements comprising boron-doped silica have a refractive index: -11 x 10 -3

[0225] - Normalized inclusion diameter (d / A) of the stress elements: 0.85

[0226] Although the PM PCF 10 shown in Figure 14A , Figure 14B , Figure 17 and Figure 20 has seven rings of microstructures (14), it will be appreciated that this is merely an example and embodiments extend to PM PCFs 10 having at least two rings of microstructures, such as at least three rings, such as at least four rings, such as at least five rings, such as at least six rings. That is, the PM PCF 10 according to embodiments described herein has a plurality of rings of microstructures.

[0227] Although the PM PCF 10 shown in Figure 14A , Figure 14B , Figure 17 and Figure 20The stress elements in the PM PCF 10 shown are positioned starting from the third microstructure ring (and also towards the outer microstructure ring in other microstructure rings) and there are no stress elements in the first or second microstructure ring immediately adjacent to the core region. However, this is only an example, and the first and / or second microstructure rings immediately adjacent to the core region may have stress elements instead of microstructures.

[0228] In which Figure 14A , Figure 14B , Figure 17 and Figure 20 In the embodiment of PM PCF 10 shown, where the stress element is positioned starting from the third microstructure ring, this advantageously avoids the stress element affecting the guiding properties of the fundamental harmonic mode, which has been observed by the inventors, to be localized in the first two microstructure rings in the wavelength range of 400 nm to 900 nm.

[0229] exist Figure 20 In the example, the two sets of stress elements are entirely located within the multiple microstructure (14) rings. However, it should be understood that in other examples, the diamond-shaped set of stress elements may include stress elements 16 positioned outside the multiple microstructure rings.

[0230] It should be noted that, in the context of this paper, one can also understand multiple inclusions rather than multiple microstructures. In the context of this paper, if an inclusion is addressed, then a microstructure is addressed. For example, if the first microstructure ring around the core has twelve microstructures, it means that the first ring around the core has twelve inclusions. As defined herein, a microstructure / inclusion is a relatively small element in cross-sectional dimensions extending along an elongated direction of the photonic crystal fiber (e.g., from one end of the photonic crystal fiber to the other end). Microstructures / inclusions can be made of solid materials.

[0231] In the context of this paper, it has been indicated that the cladding region and the core region comprise a material having a first refractive index. It should be noted that this material is typically referred to as the substrate material. Therefore, in the context of this paper, one can understand the term "substrate material" as referring to the material having the first refractive index addressed therein.

[0232] Additional embodiments are disclosed in the following list of numbered entries:

[0233] 1. A photonic crystal fiber, comprising:

[0234] Core area;

[0235] A covering region, the covering region surrounding the core region;

[0236] wherein the core region and the cladding region comprise a material having a first refractive index, the cladding region further comprising a plurality of microstructures extending from an input end of the optical fiber to an output end of the optical fiber along a longitudinal axis of the optical fiber, the plurality of microstructures having a second refractive index smaller than the first refractive index;

[0237] wherein the plurality of microstructures in the cladding region are arranged in a cross-sectional pattern comprising at least one ring of microstructures surrounding the core region, wherein a maximum extent of the core region is at least 16 pm, the photonic crystal fiber having a transmission bandwidth of 200 nm or more.

[0238] 2. The photonic crystal fiber according to clause 1, wherein a ratio (d / A) between a diameter (d) of each of the microstructures and a pitch (A) of the microstructures is less than 0.42, preferably less than 0.37.

[0239] 3. The photonic crystal fiber according to clause 2, wherein the ratio (d / A) is between 0.35 and 0.40.

[0240] 4. The photonic crystal fiber according to any preceding clause, wherein the photonic crystal fiber has a transmission loss of less than 0.5 dB / m for a fundamental mode within the transmission bandwidth when the fiber is bent with a bend diameter of at least 10 cm.

[0241] 5. The photonic crystal fiber according to clause 4, wherein the photonic crystal fiber has the transmission loss when the fiber is bent with a bend diameter between 10 cm and 12 cm.

[0242] 6. The photonic crystal fiber according to any preceding clause, wherein the diameter (d) of the core region is given by four times a pitch (A) of the microstructures minus a difference in diameter of each of the microstructures.

[0243] 7. The photonic crystal fiber according to any preceding clause, wherein a pitch of the microstructures is less than 5 pm.

[0244] 8. The photonic crystal fiber according to any preceding clause, wherein the cross-sectional pattern comprises a plurality of rings of microstructures surrounding the core region.

[0245] 9. The photonic crystal fiber according to clause 8, wherein the cross-sectional pattern comprises at least six rings of microstructures surrounding the core region, preferably at least seven rings of microstructures surrounding the core region, preferably at least eight rings of microstructures surrounding the core region.

[0246] 10. The photonic crystal fibre according to any preceding Clause, wherein the microstructured ring immediately adjacent the core region has twelve microstructures.

[0247] 11. The photonic crystal fibre according to any preceding Clause, wherein the plurality of microstructures comprises a medium having the second refractive index.

[0248] 12. The photonic crystal fibre according to Clause 11, wherein the medium is air such that the plurality of microstructures are hollow.

[0249] 13. The photonic crystal fibre according to Clause 11, wherein the medium is a solid material.

[0250] 14. The photonic crystal fibre according to Clause 13, wherein the medium is doped silica.

[0251] 15. The photonic crystal fibre according to Clause 14, wherein the doped silica comprises silica doped with fluorine, wherein the molar percentage of fluorine is optionally in the range 1% to 10%, further optionally in the range 3% to 8%.

[0252] 16. The photonic crystal fibre according to any preceding Clause, wherein the material having the first refractive index is silica.

[0253] 17. The photonic crystal fibre according to any preceding Clause, wherein the maximum extent of the core region has a diameter less than 20 μιη.

[0254] 18. The photonic crystal fibre according to any preceding Clause, wherein the maximum extent of the core region has a diameter less than 50 μιη, preferably less than 47 μιη.

[0255] 19. The photonic crystal fibre according to any preceding Clause, wherein the transmission bandwidth of the delivery fibre comprises wavelengths in the range 200 nm to 2500 nm.

[0256] 20. The photonic crystal fibre according to any preceding Clause, wherein the plurality of microstructures in the cladding region are arranged in a cross-sectional pattern comprising a plurality of microstructured rings surrounding the core region, and the cladding region additionally comprises two sets of stress elements at least partially located within the plurality of microstructured rings, each set of stress elements comprising a plurality of stress elements extending from an input end of the fibre to an output end of the fibre along a longitudinal axis of the fibre, wherein the two sets of stress elements have a triangular cross-sectional shape.

[0257] 21. The photonic crystal fibre according to Clause 20, wherein the two sets of stress elements are located entirely within the plurality of microstructured rings.

[0258] 22. The photonic crystal fiber according to clause 20, wherein each of the two sets of stress elements comprises stress elements positioned outside the plurality of microstructured rings.

[0259] 23. The photonic crystal fiber according to any of clauses 1 to 19, wherein the plurality of microstructures in the cladding region are arranged in a cross-sectional pattern comprising a plurality of microstructured rings surrounding the core region, and the cladding region further comprises two sets of stress elements at least partially located within the plurality of microstructured rings, each of the two sets of stress elements comprising a plurality of stress elements extending from an input end of the fiber to an output end of the fiber along a longitudinal axis of the fiber, wherein the two sets of stress elements have a rhombic cross-sectional shape.

[0260] 24. The photonic crystal fiber according to clause 23, wherein each microstructure of the plurality of microstructures has the same diameter.

[0261] 25. The photonic crystal fiber according to clause 23, wherein the plurality of microstructures comprises two sets of microstructures having a first diameter, and a plurality of microstructures having a second diameter smaller than the first diameter.

[0262] 26. The photonic crystal fiber according to clause 25, wherein the two sets of microstructures having the first diameter have a triangular cross-sectional shape.

[0263] 27. The photonic crystal fiber according to clause 25 or 26, wherein the two sets of microstructures having the first diameter are positioned mirror symmetrically around the core region.

[0264] 28. The photonic crystal fiber according to any of clauses 20 to 27, wherein the two sets of stress elements are positioned mirror symmetrically around the core region.

[0265] 29. A lithographic apparatus comprising the photonic crystal fiber according to any preceding clause.

[0266] 30. A metrology apparatus comprising the photonic crystal fiber according to any of clauses 1 to 28.

[0267] 31. The metrology apparatus according to clause 30, further comprising a light source and a sensor for measuring a parameter of interest of a structure on a substrate, and wherein the photonic crystal fiber is arranged to deliver light from the light source to the sensor.

[0268] 32. The metrology apparatus of clause 30, further comprising a light source and a sensor for measuring a parameter of interest of a structure on a substrate, and wherein the photonic crystal fibre is arranged to pass light between components of the sensor.

[0269] 33. A holographic metrology apparatus comprising the photonic crystal fibre of any of clauses 1 to 28.

[0270] 34. A photonic crystal fibre comprising:

[0271] a core region;

[0272] a cladding region surrounding the core region;

[0273] wherein the core region and the cladding region comprise a material having a first refractive index, the cladding region further comprising a plurality of microstructures extending from an input end of the fibre to an output end of the fibre along a longitudinal axis of the fibre, the plurality of microstructures having a second refractive index less than the first refractive index;

[0274] wherein the plurality of microstructures in the cladding region are arranged in a cross-sectional pattern comprising at least one ring of microstructures surrounding the core region, wherein the ring of microstructures immediately adjacent the core region has twelve microstructures.

[0275] 35. The photonic crystal fibre of clause 34, wherein the core region has a maximum extent of a diameter of at least 16 μιη.

[0276] 36. The photonic crystal fibre of one of clauses 34 or 35, wherein the photonic crystal fibre has a transmission bandwidth of 200 nm or more.

[0277] It is noted that the previously defined clauses 2 to 9 and 11 to 28 can also be combined with the previously defined clauses 34, 35 or 36.

[0278] Although specific reference can be made in this text to the use of the PCF 10 in an

[0279] While embodiments of the application can be specifically referenced herein in the context of a lithographic apparatus, embodiments of the application can be used in other apparatuses. For example, the PCF 10 can form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes objects such as wafers (or other substrates) or masks (or other patterning devices). These apparatuses can generally be referred to as lithographic tools. Such lithographic tools can use vacuum conditions or ambient (non-vacuum) conditions.

[0280] While embodiments of the application have been specifically described above with reference to their use in the context of optical lithography, it will be appreciated that where circumstances permit, the application is not limited to optical lithography and the PCF 10 can be used in other applications such as imprint lithography.

[0281] While specific embodiments of the application have been described above, it will be appreciated that the application can be practiced otherwise than as described. The description is intended to be illustrative, not restrictive. Accordingly, modifications can be made by people of skill in the art without departing from the scope of the claims as expressed in the following claims section.

Claims

1. A photonic crystal fiber, comprising: Core area; A covering region, the covering region surrounding the core region; The core region and the cladding region include a material having a first refractive index. The cladding region further includes a plurality of microstructures extending from the input end of the optical fiber along the longitudinal axis of the optical fiber to the output end of the optical fiber. The plurality of microstructures have a second refractive index that is less than the first refractive index. The plurality of microstructures in the cladding region are arranged in a cross-sectional pattern comprising at least one microstructure ring surrounding the core region, wherein the diameter of the maximum extent of the core region is at least 16 μm, and the photonic crystal fiber has a transmission bandwidth of 200 nm or greater.

2. The photonic crystal fiber according to claim 1, wherein, The ratio (d / Λ) between the diameter (d) of each microstructure and the pitch (Λ) of the microstructure is less than 0.42, preferably less than 0.

37.

3. The photonic crystal fiber according to any of the preceding claims, wherein, When the optical fiber is bent with a bending diameter of at least 10 cm, the photonic crystal fiber has a transmission loss of less than 0.5 dB / m for the fundamental harmonic mode within the transmission bandwidth.

4. The photonic crystal fiber according to claim 3, wherein, The photonic crystal fiber has the transmission loss when it is bent with a bending diameter between 10 cm and 12 cm.

5. The photonic crystal fiber according to any of the preceding claims, wherein, The diameter of the maximum extent of the core region is given by four times the pitch of the microstructure minus the difference in diameter of each microstructure within the microstructure.

6. The photonic crystal fiber according to any of the preceding claims, wherein, The pitch of the microstructure is less than 5 μm.

7. The photonic crystal fiber according to any of the preceding claims, wherein, The cross-sectional pattern includes a plurality of microstructure rings surrounding the core region, and optionally, the cross-sectional pattern includes at least six microstructure rings surrounding the core region, optionally, the cross-sectional pattern includes at least seven microstructure rings surrounding the core region, optionally, the cross-sectional pattern includes at least eight microstructure rings surrounding the core region.

8. The photonic crystal fiber according to any of the preceding claims, wherein, The microstructure ring adjacent to the core region has twelve microstructures.

9. The photonic crystal fiber according to any of the preceding claims, wherein, The plurality of microstructures include a medium having the second refractive index.

10. The photonic crystal fiber according to claim 9, wherein, The medium is a solid material.

11. The photonic crystal fiber according to any of the preceding claims, wherein, The material having the first refractive index is silicon dioxide.

12. The photonic crystal fiber according to any of the preceding claims, wherein, The diameter of the maximum extent of the core region is less than 50 μm, preferably less than 47 μm.

13. A photolithography apparatus comprising a photonic crystal fiber according to any of the preceding claims.

14. A measurement device comprising a photonic crystal fiber according to any one of claims 1 to 12.

15. A holographic measurement device comprising a photonic crystal fiber according to any one of claims 1 to 12.

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