Radiation coupling system

Abstract

A radiation coupling system comprising a cladding provided within a substrate, wherein an array of strips is provided within the cladding, wherein the strips extend into the cladding from a face of the cladding, wherein the strips have a refractive index which is greater than the refractive index of the cladding, and wherein the strips are configured to together provide an optically coupled structure which acts as a waveguide; wherein the radiation coupling system further comprises a plurality of waveguides which are coupled to the array of strips.

Description

RADIATION COUPLING SYSTEMCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24216198.2 which was filed on 28 November 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a radiation coupling system. The radiation coupling system may form part of a radiation source system. The radiation source system may form part of a metrology tool or an exposure apparatus (e.g. a lithographic apparatus).BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern (also often referred to as “design layout” or “design”) at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer). A lithographic apparatus is an example of an exposure apparatus. Other examples of exposure apparatus are an actinic inspection apparatus such as an actinic mask inspection apparatus.

[0004] Metrology tools, such as a scatterometer, topography measurement system, or position measurement system are used to measure properties of substrates. The measurements may be performed after exposure of a substrate. The measurements may be used to adjust operation of the lithographic apparatus that exposed the substrate to improve pattern reproduction. Adjustments may include modifying the illumination scheme and / or, adjusting the projection optics.

[0005] The properties of the radiation used by a metrology tool may affect the type and quality of measurements that may be performed. For some applications, it may be advantageous to use multiple radiation wavelengths to measure a substrate. Multiple different wavelengths may be able to propagate, irradiate, and scatter off a metrology target with no or minimal interference with other wavelengths. Therefore different wavelengths may for example be used to obtain more metrology data. The different wavelengths may for example be provided as a sequence of different wavelengths (i.e. a series of wavelengths). Different radiation wavelengths may also be able to interrogate and discover different properties of a metrology target.

[0006] A broadband radiation source may provide a broadband radiation beam. A radiation coupling system may be used to receive the broadband radiation beam and separate the radiation into a plurality of waveguides. The separated radiation may then be used to illuminate a substrate for metrology measurements or other measurements.

[0007] It may be difficult to couple a broadband radiation beam into a plurality of waveguides.SUMMARY

[0008] According to a first aspect of the invention there is provided a radiation coupling system comprising a cladding provided within a substrate, wherein an array of strips is provided within the cladding, wherein the strips extend into the cladding from a face of the cladding, wherein the strips have a refractive index which is greater than the refractive index of the cladding, and wherein the strips are configured to together provide an optically coupled structure which acts as a waveguide; wherein the radiation coupling system further comprises a plurality of waveguides which are coupled to the array of strips.

[0009] Advantageously, the radiation coupling system may provide effective coupling of radiation into the plurality of waveguides whilst avoiding generating a very high intensity of radiation that could cause damage or undesirable nonlinear effects.

[0010] The strips may be configured such that radiation is weakly confined to the cross-sections of the strips.

[0011] Each strip may be not capable in isolation of acting as a waveguide for radiation having a wavelength of at least 600 nm.

[0012] The array of strips may be a two-dimensional array.

[0013] Outer edges of outermost strips of the array of strips may collectively define a perimeter, and an average refractive index of the area within the perimeter may be greater than a refractive index of the area outside of the perimeter.

[0014] The average refractive index of the area within the perimeter may be up to 0.1 more than the refractive index of the area outside of the perimeter.

[0015] The area within the perimeter may be at least 3 pm2. The area within the perimeter may be at least 10 pm2

[0016] Adjacent strips may be separated by no more than 2 pm. Adjacent strips may be separated by at least 0.4 pm.

[0017] At least some of the strips may taper outwardly to form the waveguides.

[0018] The outward tapers of the strips may be adiabatic.

[0019] The outward tapers of the strips may be staggered.

[0020] The outward tapers of the strips may be configured to provide wavelength selective coupling of radiation into the waveguides.

[0021] At least some of the waveguides may include bends which increase separation between adjacent waveguides.

[0022] The bends may be adiabatic.

[0023] Some of the strips may taper inwardly to a zero cross-sectional area.

[0024] The inward tapers may be adiabatic.

[0025] According to a second aspect of the invention there is provided a radiation source comprising an emitter and the radiation coupling system of the first aspect, wherein the emitter is configured to provide radiation having a wavelength range between 400nm and 900nm, and wherein the strips and the cladding are configured such that less than 50% of the radiation propagating from the face and into the cladding is confined within the strips. The strips and the cladding may be configured such that less than 30% of the radiation propagating from the face and into the cladding is confined within the strips. The strips and the cladding may be configured such that less than 10% of the radiation propagating from the face and into the cladding is confined within the strips.

[0026] According to a third aspect of the invention there is provided a radiation coupling system comprising a cladding provided within a semiconductor substrate, within which an array of waveguides is provided, wherein the waveguides extend into the cladding from a face of the cladding, and wherein the waveguides have a refractive index which is greater than the refractive index of the cladding, wherein the radiation coupling system further comprises an array of dichroic mirrors associated with the array of waveguides, each dichroic mirror being configured to direct radiation into a different waveguide.

[0027] According to a fourth aspect of the invention there is provided a measurement system comprising a radiation source, a radiation coupling system of any preceding aspect and a controller.

[0028] According to a fifth aspect of the invention there is provided an exposure apparatus comprising a radiation source and further comprising the radiation coupling system of any of preceding aspect.

[0029] Features of different aspects of the invention may be combined together.BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 depicts a schematic overview of a lithographic apparatus;Figure 2 depicts a schematic overview of a lithographic cell;Figure 3 depicts a schematic representation of holistic lithography, representing a cooperation between three key technologies to optimize semiconductor manufacturing;Figure 4 depicts a schematic representation of a scatterometer which may include a radiation source according to an embodiment of the invention;Figure 5 depicts a schematic representation of a dark field scatterometer which may include a radiation source according to an embodiment of the invention;Figure 6 depicts an alignment sensor which may include a radiation source according to an embodiment of the invention;Figure 7 depicts a schematic representation of a measurement system according to an embodiment of the invention;Figure 8 depicts a schematic representation of a radiation coupling system according to an embodiment of the invention;Figure 9 depicts coupling of radiation to the radiation coupling system of Figure 8 determined using mathematical modelling;Figure 10 depicts a schematic representation of a radiation coupling system according to a further embodiment of the invention;Figure 11 depicts a schematic representation of a radiation coupling system according to a still further embodiment of the invention;Figure 12 depicts a schematic representation of a radiation coupling system according to a still further embodiment of the invention;Figure 13 depicts a schematic representation of a radiation coupling system according to a still further embodiment of the invention;Figure 14 depicts coupling of radiation within a radiation coupling system according to an embodiment of the invention, determined using mathematical modelling; andFigure 15 depicts a schematic representation of a radiation coupling system according to a still further embodiment of the invention.DETAILED DESCRIPTION

[0031] To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The lithographic apparatus LA is an example of an exposure apparatus. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0032] Low-ki lithography may be used to process features with dimensions smaller than the classical resolution limit of a lithographic apparatus. In such process, the resolution formula may be expressed as CD = kjxX / NA, where I is the wavelength of radiation employed, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the “critical dimension” (generally the smallest feature size printed, but in this case half-pitch) 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 may be applied to the lithographic projection apparatus and / or design layout. These include, for example, but not limited to, optimization of NA, customized illumination schemes, use of phase shifting patterning devices, various optimization 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 for controlling a stability of the lithographic apparatus may be used to improve reproduction of the pattern at low kl .

[0033] In the present document, the terms “radiation” and “beam” are used 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).

[0034] The term “reticle”, “mask” or “patterning device” as employed in this text may be broadly interpreted as referring to a generic patterning device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Besides the classic mask (transmissive or reflective, binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include a programmable mirror array and a programmable LCD array.

[0035] Figure 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation or EUV radiation), a mask support (e.g., 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 certain parameters, a substrate support (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support in accordance with certain parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0036] 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 may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof, for directing, shaping, and / or controlling radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross section at a plane of the patterning device MA.

[0037] The term “projection system” PS used herein should be broadly interpreted as 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 may be considered as synonymous with the more general term “projection system” PS.

[0038] The lithographic apparatus LA may be of a type wherein at least a portion of the substrate may be 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 - which is also referred to as immersion lithography.More information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0039] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also named “dual stage”). In such “multiple stage” machine, the substrate supports WT may be used in parallel, and / or steps in preparation of a subsequent exposure of the substrate W may be carried out on the substrate W located on one of the substrate support WT while another substrate W on the other substrate support WT is being used for exposing a pattern on the other substrate W.

[0040] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement stage. The measurement stage is arranged to hold a sensor and / or a cleaning device. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold multiple sensors. The cleaning device may be arranged to clean part of the lithographic apparatus, for example a part of the projection system PS or a part of a system that provides the immersion liquid. The measurement stage may move beneath the projection system PS when the substrate support WT is away from the projection system PS.

[0041] In operation, the radiation beam B is incident on the patterning device, e.g. mask, MA which is held on the mask support MT, and is patterned by the pattern (design layout) present on patterning device MA. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and a position measurement system IF, the substrate support WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B at a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in Figure 1) may be used to accurately position the patterning device MA with respect to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks Pl, P2 as illustrated occupy dedicated target portions, they may be located in spaces between target portions. Substrate alignment marks Pl, P2 are known as scribe-lane alignment marks when these are located between the target portions C.

[0042] As shown in Figure 2 the lithographic apparatus LA may form part of a lithographic cell LC, also sometimes referred to as a lithocell or (litho)cluster, which often also includes apparatus to perform pre- and post-exposure processes on a substrate W. Conventionally these include spin coaters SC to deposit resist layers, developers DE to develop exposed resist, chill plates CH and bake plates BK, e.g. for conditioning the temperature of substrates W e.g. for conditioning solvents in the resist layers. A substrate handler, or robot, RO picks up substrates W from input / output ports I / Ol, I / O2, moves them between the different process apparatus and delivers the substrates W to the loading bay LB of the lithographic apparatus LA. The devices in the lithocell, which are often also collectively referred to as the track, are typically under the control of a track control unit TCU that in itself may be controlled bya supervisory control system SCS, which may also control the lithographic apparatus LA, e.g. via lithography control unit LACU.

[0043] In order for the substrates W exposed by the lithographic apparatus LA to be exposed correctly and consistently, it is desirable to inspect substrates to measure properties of patterned structures, such as overlay errors between subsequent layers, line thicknesses, critical dimensions (CD), etc. For this purpose, inspection tools (not shown) may be included in the lithocell LC. If errors are detected, adjustments, for example, may be made to exposures of subsequent substrates or to other processing steps that are to be performed on the substrates W, especially if the inspection is done before other substrates W of the same batch or lot are still to be exposed or processed.

[0044] An inspection apparatus, which may also be referred to as a metrology tool, is used to determine properties of the substrates W, and in particular, how properties of different substrates W vary or how properties associated with different layers of the same substrate W vary from layer to layer. The inspection apparatus may alternatively be constructed to identify defects on the substrate W and may, for example, be part of the lithocell LC, or may be integrated into the lithographic apparatus LA, or may even be a stand-alone device. The inspection apparatus may measure the properties on a latent image (image in a resist layer after the exposure), or on a semi-latent image (image in a resist layer after a post-exposure bake step PEB), or on a developed resist image (in which the exposed or unexposed parts of the resist have been removed), or even on an etched image (after a pattern transfer step such as etching).

[0045] Typically the patterning process in a lithographic apparatus LA is one of the most critical steps in the processing which requires high accuracy of dimensioning and placement of structures on the substrate W. To ensure this high accuracy, three systems may be combined in a so called “holistic” control environment as schematically depicted in Fig. 3. One of these systems is the lithographic apparatus LA which is (virtually) connected to a metrology tool MT (a second system) and to a computer system CL (a third system). The key of such “holistic” environment is to optimize the cooperation between these three systems to enhance the overall process window and provide tight control loops to ensure that the patterning performed by the lithographic apparatus LA stays within a process window. The process window defines a range of process parameters (e.g. dose, focus, overlay) within which a specific manufacturing process yields a defined result (e.g. a functional semiconductor device) - typically within which the process parameters in the lithographic process or patterning process are allowed to vary.

[0046] The computer system CL may use (part of) the design layout to be patterned to predict which resolution enhancement techniques to use and to perform computational lithography simulations and calculations to determine which mask layout and lithographic apparatus settings achieve the largest overall process window of the patterning process (depicted in Fig. 3 by the double arrow in the first scale SCI). Typically, the resolution enhancement techniques are arranged to match the patterning possibilities of the lithographic apparatus LA. The computer system CL may also be used to detectwhere within the process window the lithographic apparatus LA is currently operating (e.g. using input from the metrology tool MT) to predict whether defects may be present due to e.g. sub-optimal processing (depicted in Fig. 3 by the arrow pointing “0” in the second scale SC2).

[0047] The metrology tool MT may provide input to the computer system CL to enable accurate simulations and predictions, and may provide feedback to the lithographic apparatus LA to identify possible drifts, e.g. in a calibration status of the lithographic apparatus LA (depicted in Fig. 3 by the multiple arrows in the third scale SC3).

[0048] In lithographic processes, it is desirable to make frequently measurements of the structures created, e.g., for process control and verification. Tools to make such measurement are typically called metrology tools MT. Different types of metrology tools MT for making such measurements are known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. Scatterometers are versatile instruments which allow measurements of the parameters of a lithographic process by having a sensor in the pupil or a conjugate plane with the pupil of the objective of the scatterometer, measurements usually referred as pupil based measurements, or by having the sensor in the image plane or a plane conjugate with the image plane, in which case the measurements are usually referred as image or field based measurements. Such scatterometers and the associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP 1,628, 164A, incorporated herein by reference in their entirety. Aforementioned scatterometers may measure gratings using light from soft x-ray and visible to near-IR wavelength range.

[0049] In a first embodiment, the scatterometer MT is an angular resolved scatterometer. In such a scatterometer reconstruction methods may be applied to the measured signal to reconstruct or calculate properties of the grating. Such reconstruction may, for example, result from simulating interaction of scattered radiation with a mathematical model of the target structure and comparing the simulation results with those of a measurement. Parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from the real target.

[0050] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such spectroscopic scatterometer MT, the radiation emitted by a radiation source is directed onto the target and the reflected or scattered radiation from the target is directed to a spectrometer detector, which measures a spectrum (i.e. a measurement of intensity as a function of wavelength) of the specular reflected radiation. From this data, the structure or profile of the target giving rise to the detected spectrum may be reconstructed, e.g. by Rigorous Coupled Wave Analysis and nonlinear regression or by comparison with a library of simulated spectra.

[0051] In a third embodiment, the scatterometer MT is a ellipsometric scatterometer. The ellipsometric scatterometer allows for determining parameters of a lithographic process by measuring scattered radiation for each polarization state. Such metrology apparatus emits polarized light (such as linear, circular, or elliptic) by using, for example, appropriate polarization filters in the illumination section ofthe metrology apparatus. A source suitable for the metrology apparatus may provide polarized radiation as well. Various embodiments of existing ellipsometric scatterometers are described in US 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 incorporated herein by reference in their entirety.

[0052] 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 asymmetry in the reflected spectrum and / or the detection configuration, the asymmetry being related to the extent of the overlay. The two (typically overlapping) grating structures may be applied in two different layers (not necessarily consecutive layers), and may be formed substantially at the same position on the wafer. The scatterometer may have a symmetrical detection configuration as described e.g. in co-owned patent application EP 1,628, 164A, such that any asymmetry is clearly distinguishable. This provides a straightforward way to measure misalignment in gratings. Further examples for measuring overlay error between the two layers containing periodic structures as target is measured through asymmetry of the periodic structures may be found in PCT patent application publication no. WO 2011 / 012624 or US patent application US 20160161863, incorporated herein by reference in its entirety.

[0053] Other parameters of interest may be focus and dose. Focus and dose may be determined simultaneously by scatterometry (or alternatively by scanning electron microscopy) as described in US patent application US2011-0249244, incorporated herein by reference in its entirety. A single structure may be used which has a unique combination of critical dimension and sidewall angle measurements for each point in a focus energy matrix (FEM - also referred to as Focus Exposure Matrix). If these unique combinations of critical dimension and sidewall angle are available, the focus and dose values may be uniquely determined from these measurements.

[0054] A metrology target may be an ensemble of composite gratings, formed by a lithographic process, mostly in resist, but also after etch process for example. Typically the pitch and line-width of the structures in the gratings strongly depend on the measurement optics (in particular the NA of the optics) to be able to capture diffraction orders coming from the metrology targets. As indicated earlier, the diffracted signal may be used to determine shifts between two layers (also referred to ‘overlay’) or may be used to reconstruct at least part of the original grating as produced by the lithographic process. This reconstruction may be used to provide guidance of the quality of the lithographic process and may be used to control at least part of the lithographic process. Targets may have smaller sub-segmentation which are configured to mimic dimensions of the functional part of the design layout in a target. Due to this sub-segmentation, the targets will behave more similar to the functional part of the design layout such that the overall process parameter measurements resembles the functional part of the design layout better. The targets may be measured in an underfilled mode or in an overfilled mode. In the underfilled mode, the measurement beam generates a spot that is smaller than the overall target. In the overfilled mode, the measurement beam generates a spot that is larger than the overall target. In such overfilledmode, it may also be possible to measure different targets simultaneously, thus determining different processing parameters at the same time.

[0055] Overall measurement quality of a lithographic parameter using a specific target is at least partially determined by the measurement recipe used to measure this lithographic parameter. The term “substrate measurement recipe” may include one or more parameters of the measurement itself, one or more parameters of the one or more patterns measured, or both. For example, if the measurement used in a substrate measurement recipe is a diffraction-based optical measurement, one or more of the parameters of the measurement may include the wavelength of the radiation, the polarization of the radiation, the incident angle of radiation relative to the substrate, the orientation of radiation relative to a pattern on the substrate, etc. One of the criteria to select a measurement recipe may, for example, be a sensitivity of one of the measurement parameters to processing variations. More examples are described in US patent application US2016-0161863 and published US patent application US 2016 / 0370717Alincorporated herein by reference in its entirety.

[0056] A metrology apparatus, such as a scatterometer SMI, is depicted in figure 4. It comprises a broadband (white light) radiation projector 2 which projects radiation onto a substrate 6. The reflected or scattered radiation is passed to a spectrometer detector 4, which measures a spectrum 10 (i.e. a measurement of intensity Ini as a function of wavelength X) of the specular reflected radiation. From this data, the structure or profile giving rise to the detected spectrum may be reconstructed by processing unit PU, e.g. by Rigorous Coupled Wave Analysis and nonlinear regression or by comparison with a library of simulated spectra as shown at the bottom of Figure 4. In general, for the reconstruction, the general form of the structure is known and some parameters are assumed from knowledge of the process by which the structure was made, leaving only a few parameters of the structure to be determined from the scatterometry data. Such a scatterometer may be configured as a normal-incidence scatterometer or an oblique-incidence scatterometer.

[0057] In lithographic processes, it is desirable to make frequently measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes or various forms of metrology apparatuses, such as scatterometers. Examples of known scatterometers often rely on provision of dedicated metrology targets, such as underfilled targets (a target, in the form of a simple grating or overlapping gratings in different layers, that is large enough that a measurement beam generates a spot that is smaller than the grating) or overfilled targets (whereby the illumination spot partially or completely contains the target). Further, the use of metrology tools, for example an angular resolved scatterometter illuminating an underfilled target, such as a grating, allows the use of so-called reconstruction methods where the properties of the grating can be calculated by simulating interaction of scattered radiation with a mathematical model of the target structure and comparing the simulation results with those of a measurement. Parameters of the model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from the real target.

[0058] Scatterometers are versatile instruments which allow measurements of the parameters of a lithographic process by having a sensor in the pupil or a conjugate plane with the pupil of the objective of the scatterometer, measurements usually referred as pupil based measurements, or by having the sensor in the image plane or a plane conjugate with the image plane, in which case the measurements are usually referred as image or field based measurements. Such scatterometers and the associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP1,628,164A, incorporated herein by reference in their entirety. Aforementioned scatterometers can measure in one image multiple targets from multiple gratings using light from soft x-ray and visible to near-IR wave range.

[0059] A topography measurement system, level sensor or height sensor, and which may be integrated in the lithographic apparatus, is arranged to measure a topography of a top surface of a substrate (or wafer). A map of the topography of the substrate, also referred to as height map, may be generated from these measurements indicating a height of the substrate as a function of the position on the substrate. This height map may subsequently be used to correct the position of the substrate during transfer of the pattern on the substrate, in order to provide an aerial image of the patterning device in a properly focus position on the substrate. It will be understood that “height” in this context refers to a dimension broadly out of the plane to the substrate (also referred to as Z-axis). Typically, the level or height sensor performs measurements at a fixed location (relative to its own optical system) and a relative movement between the substrate and the optical system of the level or height sensor results in height measurements at locations across the substrate.

[0060] Figure 5(a) presents an embodiment of a metrology apparatus and, more specifically, a dark field scatterometer. A target T and diffracted rays of measurement radiation used to illuminate the target are illustrated in more detail in Figure 5(b). The metrology apparatus illustrated is of a type known as a dark field metrology apparatus. The metrology apparatus may be a stand-alone device or incorporated in either the lithographic apparatus LA, e.g., at the measurement station, or the lithographic cell LC. An optical axis, which has several branches throughout the apparatus, is represented by a dotted line O. In this apparatus, light emitted by source 11 (e.g., a xenon lamp) is directed onto substrate W via a beam splitter 15 by an optical system comprising lens 12, 14 and objective lens 16. These lens are arranged in a double sequence of a 4F arrangement. A different lens arrangement can be used, provided that it still provides a substrate image onto a detector, and simultaneously allows for access of an intermediate pupil-plane for spatial-frequency filtering. Therefore, the angular range at which the radiation is incident on the substrate can be selected by defining a spatial intensity distribution in a plane that presents the spatial spectrum of the substrate plane, here referred to as a (conjugate) pupil plane. In particular, this can be done by inserting an aperture plate 13 of suitable form between lens 12 and 14, in a plane which is a back-projected image of the objective lens pupil plane. In the example illustrated, aperture plate 13 has different forms, labeled 13N and 13S, allowing different illumination modes to be selected. Theillumination system in the present examples forms an off-axis illumination mode. In the first illumination mode, aperture plate 13N provides off-axis from a direction designated, for the sake of description only, as ‘north’ . In a second illumination mode, aperture plate 13S is used to provide similar illumination, but from an opposite direction, labeled ‘south’. Other modes of illumination are possible by using different apertures. The rest of the pupil plane is desirably dark as any unnecessary light outside the desired illumination mode will interfere with the desired measurement signals.

[0061] As shown in Figure 5(b), target T is placed with substrate W normal to the optical axis O of objective lens 16. The substrate W may be supported by a support (not shown). A ray of measurement radiation I impinging on target T from an angle off the axis O gives rise to a zeroth order ray (solid line 0) and two first order rays (dot-chain line +1 and double dot-chain line -1). It should be remembered that with an overfilled small target, these rays are just one of many parallel rays covering the area of the substrate including metrology target T and other features. Since the aperture in plate 13 has a finite width (necessary to admit a useful quantity of light, the incident rays I will in fact occupy a range of angles, and the diffracted rays 0 and +1 / -1 will be spread out somewhat. According to the point spread function of a small target, each order +1 and -1 will be further spread over a range of angles, not a single ideal ray as shown. Note that the grating pitches of the targets and the illumination angles can be designed or adjusted so that the first order rays entering the objective lens are closely aligned with the central optical axis. The rays illustrated in Figure 5(a) and 3(b) are shown somewhat off axis, purely to enable them to be more easily distinguished in the diagram.

[0062] At least the 0 and +1 orders diffracted by the target T on substrate W are collected by objective lens 16 and directed back through beam splitter 15. Returning to Figure 5(a), both the first and second illumination modes are illustrated, by designating diametrically opposite apertures labeled as north (N) and south (S). When the incident ray I of measurement radiation is from the north side of the optical axis, that is when the first illumination mode is applied using aperture plate 13N, the +1 diffracted rays, which are labeled +1(N), enter the objective lens 16. In contrast, when the second illumination mode is applied using aperture plate 13S the -1 diffracted rays (labeled 1(S)) are the ones which enter the lens 16.

[0063] A second beam splitter 17 divides the diffracted beams into two measurement branches. In a first measurement branch, optical system 18 forms a diffraction spectrum (pupil plane image) of the target on first sensor 19 (e.g. a CCD or CMOS sensor) using the zeroth and first order diffractive beams. Each diffraction order hits a different point on the sensor, so that image processing can compare and contrast orders. The pupil plane image captured by sensor 19 can be used for focusing the metrology apparatus and / or normalizing intensity measurements of the first order beam. The pupil plane image can also be used for many measurement purposes such as reconstruction.

[0064] In the second measurement branch, optical system 20, 22 forms an image of the target T on sensor 23 (e.g. a CCD or CMOS sensor). In the second measurement branch, an aperture stop 21 is provided in a plane that is conjugate to the pupil-plane. Aperture stop 21 functions to block the zerothorder diffracted beam so that the image of the target formed on sensor 23 is formed only from the -1 or +1 first order beam. The images captured by sensors 19 and 23 are output to processor PU which processes the image, the function of which will depend on the particular type of measurements being performed. Note that the term ‘image’ is used here in a broad sense. An image of the grating lines as such will not be formed, if only one of the -1 and +1 orders is present.

[0065] The particular forms of aperture plate 13 and field stop 21 shown in Figure 5 are purely examples. In another embodiment of the invention, on-axis illumination of the targets is used and an aperture stop with an off-axis aperture is used to pass substantially only one first order of diffracted light to the sensor. In yet other embodiments, 2nd, 3rd and higher order beams (not shown in Figure 5) can be used in measurements, instead of or in addition to the first order beams.

[0066] In order to make the measurement radiation adaptable to these different types of measurement, the aperture plate 13 may comprise a number of aperture patterns formed around a disc, which rotates to bring a desired pattern into place. Note that aperture plate 13N or 13S can only be used to measure gratings oriented in one direction (X or Y depending on the set-up). For measurement of an orthogonal grating, rotation of the target through 90° and 270° might be implemented. Different aperture plates are shown in Figures 5(c) and (d). The use of these, and numerous other variations and applications of the apparatus are described in prior published applications, mentioned above.

[0067] Figure 6 is a schematic block diagram of an embodiment of a known alignment sensor AS, such as is described, for example, in US6961116, and which is incorporated by reference. Radiation source RSO provides a beam RB of radiation of one or more wavelengths, which is diverted by diverting lens onto a mark, such as mark AM located on substrate W, as an illumination spot SP. In this example the diverting lens comprises a spot mirror SM and an objective lens OL. The illumination spot SP, by which the mark AM is illuminated, may be slightly smaller in diameter than the width of the mark itself.

[0068] Radiation diffracted by the mark AM is collimated (in this example via the objective lens OL) into an information-carrying beam IB. The term “diffracted” is intended to include zero-order diffraction from the mark (which may be referred to as reflection). A self-referencing interferometer SRI, e.g. of the type disclosed in US6961116 mentioned above, interferes the beam IB with itself after which the beam is received by a photodetector PD. Additional optics (not shown) may be included to provide separate beams in case more than one wavelength is created by the radiation source RSO. The photodetector may be a single element, or it may comprise a number of pixels, if desired. The photodetector may comprise a sensor array.

[0069] The diverting lens, which in this example comprises the spot mirror SM, may also serve to block zero order radiation reflected from the mark, so that the information-carrying beam IB comprises only higher order diffracted radiation from the mark AM (this is not essential to the measurement, but improves signal to noise ratios).

[0070] Intensity signals SI are supplied to a processing unit PU. By a combination of optical processing in the block SRI and computational processing in the unit PU, values for X- and Y-position on the substrate relative to a reference frame are output.

[0071] A single measurement of the type illustrated only fixes the position of the mark within a certain range corresponding to one pitch of the mark. Coarser measurement techniques are used in conjunction with this to identify which period of a sine wave is the one containing the marked position. The same process at coarser and / or finer levels may be repeated at different wavelengths for increased accuracy and / or for robust detection of the mark irrespective of the materials from which the mark is made, and materials on and / or below which the mark is provided. The wavelengths may be multiplexed and demultiplexed optically so as to be processed simultaneously, and / or they may be multiplexed by time division or frequency division.

[0072] In this example, the alignment sensor and spot SP remain stationary, while it is the substrate W that moves. The alignment sensor can thus be mounted rigidly and accurately to a reference frame, while effectively scanning the mark AM in a direction opposite to the direction of movement of substrate W. The substrate W is controlled in this movement by its mounting on a substrate support and a substrate positioning system controlling the movement of the substrate support. A substrate support position sensor (e.g. an interferometer) measures the position of the substrate support (not shown). In an embodiment, one or more (alignment) marks are provided on the substrate support. A measurement of the position of the marks provided on the substrate support allows the position of the substrate support as determined by the position sensor to be calibrated (e.g. relative to a frame to which the alignment system is connected). A measurement of the position of the alignment marks provided on the substrate allows the position of the substrate relative to the substrate support to be determined.

[0073] Metrology and / or inspection tools (also referred to as measurement tools) such as the ones described above often use radiation to obtain measurement data. Depending on the measurement target and the properties to be measured, different types of radiation may be used. One differing property of radiation is the wavelength(s) used to obtain a measurement, as different wavelengths may provide different information about a measurement target. Some measurement tools may use broadband radiation, such as supercontinuum radiation, either to measure using broadband radiation, or to be able to tune and select the measurement wavelength(s) to be used. Depending on the range of output wavelengths and properties of the broadband source, difference methods may be used to obtain the broadband radiation. In some implementations for generating broadband radiation, nonlinear effects may be used to broaden narrow wavelength range input radiation (also referred to as pump radiation). Different known setups and methods exist to achieve nonlinear broadening. Often these methods rely on the confinement of the pump radiation to achieve high intensities needed to experience significant nonlinear effects.

[0074] Figure 7 schematically depicts a measurement system 30 according to an embodiment of the invention. The measurement system 30 may for example be a metrology tool, a level sensor or analignment sensor (e.g. as described above). The measurement system 30 comprises a source 32. The source 32 may be configured to emit broadband radiation, e.g. radiation having a wavelength range from 400 nm to 900 nm, e.g. radiation having a wavelength range from 350 nm to 1600 nm, e.g. radiation having a wavelength range from 532 nm to 980 nm. In general the broadband radiation may have a wavelength range of at least 400 nm.

[0075] A radiation beam 34 output from the source 32 passes into a radiation coupling system 36. The radiation coupling system is according to an embodiment of the invention and is described further below. Optionally, optics 38 may be located between the source 32 and the radiation coupling system 36. The optics 38 may comprise an optical fiber and / or lenses.

[0076] A plurality of radiation beams 40 are output from the radiation coupling system 36. The plurality of radiation beams 40 may have different wavelengths. The radiation coupling system 36 may be operative to select one of the plurality of radiation beams 40 and block the other radiation beams. In Figure 7 a first radiation beam 40 is shown as a solid line, indicating that that beam has been selected, and the other radiation beams are shown as dashed lines, indicating that those beams have been blocked. The radiation coupling system 36 may output each of the plurality of radiation beams 40 in turn. The radiation beams 40 may have different wavelengths. In Figure 7 three radiation beams 40 are depicted. However, a larger number of radiation beams may be provided by the radiation coupling system 36, e.g. five radiation beams or more.

[0077] The plurality of radiation beams 40 output from the radiation coupling system 36 are incident upon a substrate 42 (or other item which is to be measured, e.g. a mask). A detection system 44 detects radiation which has been reflected by the substrate 42. Optics 46 may optionally be located between the radiation coupling system 36 and the substrate 42. The optics may comprise an optical fiber and / or lenses. The optics may be as described further above for a metrology tool, a level sensor or an alignment sensor (or other measurement system). Optics (not depicted) may also be provided between the substrate 42 and the detection system 44.

[0078] A controller 48 may be connected to the radiation coupling system 36. The controller 48 may also be connected to the detection system 44. The controller may control operation of the radiation coupling system 36 so that the radiation coupling system outputs each of the plurality of radiation beams 40 in turn. In this way, radiation beams 40 having different wavelengths may be used by the measurement system. The radiation coupling system 36 may be a photonic integrated circuit which includes photonic switches that can be used to selectively direct radiation from different waveguides to an output.

[0079] Cartesian coordinate are depicted in Figure 7 and some of the other Figures. The Cartesian coordinates are provided to assist with explanation of embodiments of the invention. They are not intended to imply that elements of the invention must have a particular orientation.

[0080] A front face 50 of the radiation coupling system 36 is depicted in Figure 8. The radiation coupling system 36 comprises an array of dielectric strips 52a-k (only some of which are labelled)provided in a cladding dielectric 54. The cladding dielectric 54 may be larger in cross-section than is depicted in Figure 8. The dielectric strips 52a-k extend into the cladding dielectric 54 in the Y-direction. The cladding dielectric 54 may be provided in a silicon substrate (or other semiconductor substrate). Although eleven dielectric strips 52a-k are provided in the depicted embodiment, a different number of dielectric strips may be provided.

[0081] The dielectric strips 52a-k have a refractive index which is greater than the refractive index of the cladding dielectric 54. The dielectric strips 52a-k are configured to provide an optically coupled structure which acts as a waveguide.

[0082] The dielectric strips 52a-k are provided as an array. In the embodiment depicted in Figure 8, the array of dielectric strips 52a-k is a 2-dimensional array. The dielectric strips 52a-k are provided as three rows. A middle row comprises five dielectric strips 52a-e. Upper and lower rows each comprise three dielectric strips 52f-k.

[0083] In an example, each dielectric strip 52 may have a width w of 170 nm and may have a thickness t of 50 nm. In an example, separation between adjacent dielectric strips 52a-k in a row (in this example X-direction separation) may be 1 pm. In an example, separation between adjacent dielectric strips 52a- k in different rows (in this example Y-direction separation) may be 1.4 pm.

[0084] The dimensions of the dielectric strips and the separation between adjacent dielectric strips 52a- k may be selected based in part upon wavelengths of radiation that will be coupled to the radiation coupling system 36. In one example, the radiation coupling system may be configured to couple radiation having wavelengths in the range 400nm to lOOOnm. A dielectric strip according to an embodiment suitable for the wavelength range 400nm to lOOOnm may have a width w of at least 20 nm and may have a width of up to 500nm. The dielectric strip may have a thickness of at least 20 nm and may have a thickness of up to 200 nm. In an embodiment suitable for the wavelength range 400nm to lOOOnm, separation between adjacent dielectric strips in a row may be at least 0.4 pm and may be up to 1 pm. In an embodiment suitable for the wavelength range 400nm to lOOOnm, separation between adjacent dielectric strips in different rows may be at least 0.4 pm and may be up to 1 pm. In general, the minimum dimensions may be dictated by lithographic fabrication techniques used to form the dielectric strips. In general, large dimensions and larger separations may be provided for embodiments configured to handle longer wavelengths.

[0085] There may be a relationship between the cross-sectional size of the dielectric strips and the separation between the dielectric strips. Increasing the cross-sectional size will mean that a larger proportion of radiation will be confined within the dielectric strips, and this in turn means that the dielectric strips should be closer together in order to avoid a reduction of the optical coupling between dielectric strips.

[0086] A dashed line 56 depicts a perimeter which is defined by outer edges of outermost dielectric strips 52a,e,f-k of the array of dielectric strips. An average refractive index of an area within the perimeter 56 is greater than a refractive index of an area of the cladding dielectric 54 outside of theperimeter. The average refractive index of the area within the perimeter 56 may for example be at least 0.01 more than the refractive index of the area of cladding dielectric 54 outside of the perimeter. The average refractive index of the area within the perimeter 56 may for example be up to 0.1 more than the refractive index of the area of cladding dielectric 54 outside of the perimeter.

[0087] Dielectric strips 52b-d may which do not form part of the perimeter 56 may be distributed within the perimeter.

[0088] The electric field of radiation in a given dielectric strip 52a-k extends to adjacent dielectric strips. This provides optical coupling between the dielectric strips. The dielectric strips 52a-k provide an optically coupled structure which acts as a waveguide for the radiation beam 34 received from the source 32.

[0089] The area within the perimeter 56 may for example be at least 3 pm2. The area within the perimeter 56 may for example be at least 10 pm2.

[0090] The embodiment, the radiation coupling system 36 may be formed from a combination of silicon nitride and silicon dioxide (SiN / SiC ). In an embodiment, the radiation coupling system 36 may be formed from aluminium oxide / silicon dioxide (AlO / SiCF) or aluminum nitride / silicon nitride (AlN / SiN). SiN / SiOz may be capable of supporting an effective waveguide mode for radiation having a wavelength in the range 500nm-1600nm. AlO / SiOz or AlN / SiN may be capable of supporting an effective waveguide mode for radiation having a wavelength in the range 350nm-1600nm. Other dielectrics may be used.

[0091] The refractive index of a dielectric strip 52a-k may for example be 2 for radiation at a wavelength of 600nm. The refractive index of the cladding dielectric 54 may for example be 1.45 for radiation at a wavelength of 600nm. A difference between the refractive index of the dielectric strip 52a-k and the refractive index of the cladding dielectric 54 may for example be at least 0.15. This difference may apply for radiation at 1.6pm for a dielectric strip formed from A1O surrounded by cladding formed from SiOz. A difference between the refractive index of the dielectric strip 52a-k and the refractive index of the cladding dielectric 54 may for example be up to 4. This difference may apply for radiation at 400nm for a dielectric strip formed from Si surrounded by cladding formed from SiOz.

[0092] The dielectric strips 52a-k in the cladding dielectric 54 may have a cut-off wavelength which is shorter than the shortest wavelength of the radiation beam 34. In this context the term cut-off wavelength may be interpreted as meaning a wavelength above which no light can be guided by the strip For example, the radiation beam may have a wavelength of at least 400nm, and the dielectric strips 52a-k may have a cut-off wavelength which is below 400nm. In general, the dielectric strips 52a-k may be configured such that they do not act as a single mode dielectric waveguide for radiation of the incident radiation beam 34. Instead, the dielectric strips 52a-k collectively support an effective guided mode for radiation of the incident radiation beam 34.

[0093] The dielectric strips 52a-k may have a cut-off wavelength which is shorter than a majority of wavelengths of the radiation beam 34. For example, the radiation beam may have a wavelength rangeof 400nm to 900nm, and the dielectric strips 52a-k may have a cut-off wavelength which is 600nm. The dielectric strips 52a-k do not act as a single mode dielectric waveguide for radiation with a wavelength above 600nm. Instead, the dielectric strips 52a-k collectively support an effective guided mode for the radiation with a wavelength above 600nm. The dielectric strips 52a-k may act as a single mode dielectric waveguide for radiation with a wavelengths below 600nm.

[0094] In an embodiment, each dielectric strip 52a-k is not capable in isolation of acting as a waveguide for radiation having a wavelength of at least 600 nm. This may be achieved by selection of the cross- sectional dimensions of the dielectric strips 52a-k. In an embodiment, each dielectric strip 52a-k is not capable in isolation of acting as a waveguide for radiation having a wavelength of at least 400 nm. This may be achieved by selection of the cross-sectional dimensions of the dielectric strips 52a-k.

[0095] The dielectric strips 52a-k may be configured such that radiation is weakly confined to the dielectric strip cross-sections. For example, up to 10% of the radiation beam 34 may be confined to the cross-sections of the dielectric strips 52a-k. Advantageously, the remaining 90% or more of the radiation beam 34 extends into the cladding dielectric 54 and provides optical coupling between dielectric strips 52a-k. In an embodiment, up to 30% of the radiation beam 34 may be confined to the cross-sections of the dielectric strips 52a-k. This may be less advantageous because it provides weaker optical coupling between the dielectric strips 52a-k. In an embodiment, up to 50% of the radiation beam 34 may be confined to the cross-sections of the dielectric strips 52a-k. This may be less advantageous because it provides even weaker optical coupling between the dielectric strips 52a-k.

[0096] One or more of the above properties of the dielectric strips may apply for other embodiments.

[0097] Figure 9 depicts waveguiding of incident radiation provided by the radiation coupling system 36 depicted in Figure 8 and with the dimensions mentioned above. The radiation coupling system 36 is formed from SiN / SiCF- The images in Figure 9 were obtained using modelling software. A lens with a numerical aperture of 0.13 is used to focus the radiation at the radiation coupling system 36 in the model. As mentioned further above, the radiation beam 34 incident at the radiation coupling system 36 may be a broadband radiation beam. Figure 9 depicts coupling of example wavelengths which may be present in the broadband radiation beam. The example wavelengths are 500 nm, 700 nm, 900 nm and 1000 nm.

[0098] Figure 9A depicts radiation with a wavelength of 500 nm incident at the face 50 of the radiation coupling system 36. The depicted area of the face 50 is 20 pm x 20 pm. As depicted, the radiation beam 34 with a wavelength of 500 nm has a full width half maximum (FWHM) of roughly 2.5 pm. The radiation beam 34 is aligned with a central strip 52c of the radiation coupling system 36 (see Figure 8).

[0099] Figure 9B depicts an effective waveguide mode provided by the radiation coupling system 36 for the 500 nm wavelength radiation beam 34. The effective waveguide mode is distributed between dielectric strips and centered on the central strip 52c (see Figure 8). A coupling efficiency of 82% is achieved according to the modelling software. In practice the coupling efficiency may be a little different.

[0100] Figure 9C depicts the incident radiation beam 34 at 700 nm wavelength. As depicted, the beam has an FWHM of around 5 pm. This is bigger than the FWHM of the 500 nm beam. This is because at longer wavelengths a focused beam has a larger cross-sectional area.

[0101] Figure 9D depicts an effective waveguide mode provided by the radiation coupling system 36 for the 700 nm wavelength radiation beam 34. As may be seen, the effective waveguide mode is distributed between dielectric strips and centered on the central strip 52c (see Figure 8). The effective waveguide mode is larger than for 500 nm radiation. A coupling efficiency of 88% is achieved.

[0102] Figure 9E depicts the incident radiation beam 34 at 900 nm wavelength. As depicted, the beam has an FWHM of around 7 pm. This is bigger than the FWHM of the 700 nm beam. Figure 9F depicts an effective waveguide mode provided by the radiation coupling system 36 for the 900 nm wavelength radiation beam 34. As may be seen, the effective waveguide mode is distributed between dielectric strips and centered on the central strip 52c (see Figure 8). The effective waveguide mode is larger than for 700 nm radiation. A coupling efficiency of 90% is achieved.

[0103] Figure 9G depicts the incident radiation beam 34 at 1000 nm wavelength. As depicted, the beam has an FWHM of around 8 pm. This is bigger than the FWHM of the 900 nm beam. Figure 9H depicts an effective waveguide mode provided by the radiation coupling system 36 for the 1000 nm wavelength radiation beam 34. As may be seen, the effective waveguide mode is distributed between dielectric strips and centered on the central strip 52c (see Figure 8). The effective waveguide mode is larger than for 900 nm radiation. A coupling efficiency of 91% is achieved.

[0104] Figure 9 indicates the efficiency of coupling of radiation into the effective waveguide mode formed by the optically coupled dielectric strips 52a-k. The efficiency ranges from 82% to 91%. The efficiency is lower at shorter wavelengths because at shorter wavelengths the refractive index of SiN (the material used in the optical modelling) increases at a faster rate than the refractive index of SiCF. As a result, the radiation becomes increasingly concentrated in the SiN strips 52a-k, deteriorating the overlap of radiation between adjacent strips and the optical coupling between adjacent strips. Consequently, the overlap between the waveguided mode as depicted in Figure 9B and the incident radiation beam as depicted in Figure 9A (for 500 nm radiation) is not as good as the overlap between for example the guided mode depicted in Figure 9G and the incident radiation depicted in Figure 9H (for lOOOnm radiation).

[0105] Coupling efficiency for longer wavelengths may start to reduce because the area of the radiation beam at the input face of the radiation coupling system is larger than the array of dielectric strips. This may be addressed by providing more dielectric strips so that a larger area is covered.

[0106] As may be understood from the example depicted in Figure 9, the area of the effective waveguide mode of radiation in the radiation coupling system 36 may be considerably bigger than would be the case if the radiation was confined in a conventional waveguide mode (i.e. a waveguide formed from a single area of higher refractive index dielectric surrounded by an area of lower refractive index dielectric). The area of the effective waveguide mode provided by the optically coupled dielectricstrips 52a-k may generally correspond with the area of a waveguide mode of an optical fibre. This may facilitate efficient coupling of the radiation beam 34 from an optical fibre into the radiation coupling system 36 (compared with for example coupling into a single waveguide). This may also apply for other embodiments.

[0107] Because the effective waveguide mode of radiation in the radiation coupling system 36 is relatively large, the radiation beam 34 does not need to be focused as tightly as would be the case for coupling into a conventional optical waveguide. This means that the peak intensity of the radiation beam 34 is reduced and the potential for damage to the radiation coupling system 36 is reduced (compared with the potential for damage to a conventional waveguide). In addition, undesirable nonlinear effects (e.g. two-photon absorption) may be reduced. The relatively large area of the effective waveguide mode (compared with a conventional waveguide) may allow for more efficient coupling of broadband radiation (compared with coupling to a conventional waveguide). This may also apply for other embodiments.

[0108] Figure 10 schematically depicts a radiation coupling system 36 according to an embodiment of the invention. Figure 10A depicts an entrance face 50 of the radiation coupling system, and corresponds with Figure 8. Thus, eleven dielectric strips 52a-k are provided as a 2-dimensional array in a dielectric cladding . To avoid over-complicating Figure 10A labels for each dielectric strip 52a-k are not included, but the dielectric strips are referred to here using the labels depicted in Figure 8. The array of dielectric strips consists of a central row of five strips 52a-e, an upper row of three strips 52f-h, and a lower row of three strips 52i-k. Three planes A-C are depicted using dashed lines in Figure 10A. Figure 10B is a cross section of the radiation coupling system 36 taken at line B of Figure 10A. Figure 10C is a cross section of the radiation coupling system 36 at taken at line A (or equivalently line C because the cross section at line C is identical to the cross section at line A).

[0109] The dielectric strips 52a-k may be incapable in isolation of acting as a waveguide for incident radiation (e.g. radiation having a wavelength of at least 600 nanometres). Specifically, the cross- sectional area of each radiation strip 52 may be smaller than the cross-sectional area needed to support a propagating waveguide mode. Instead, the dielectric strips 52a-k provide an optically coupled structure which acts as a waveguide for the radiation.

[0110] As depicted in Figure 10B, the dielectric strips 52a-e of the middle row have an initial portion with a constant cross-sectional area as a function of depth (in the Y-direction) into the radiation coupling system 36. Beyond the initial portion, the dielectric strips 52a-e taper outwardly as a function of depth into the radiation coupling system 36. Although Figure 10B depicts only an outward taper in the X- direction, an outward taper in the Z-direction is also present. The cross-sectional area of each dielectric strip 52a-e increases to an area which is sufficiently large to support a waveguide mode for example for radiation having a wavelength of at least 600nm (e.g. a single mode waveguide for radiation having a wavelength of at least 600nm). Thus, a transition from a plurality of dielectric strips 52a-e which cannotindividually support a waveguide mode to a plurality of waveguides 60a-e is provided. The area of each waveguide 60a-e may be substantially constant once the outward taper has finished.

[0111] The outward tapers of the dielectric strips 52a-e may be staggered, for example as depicted in Figure 10B. Staggering the outward tapers allows for wavelength selectivity of the radiation that is coupled to each waveguide 60a-e by controlling dispersion of the dielectric strips 52a-e via control of their cross-sectional areas. Dispersion of a waveguide is the effective index of that waveguide as a function of wavelength. In an example, the right hand dielectric strip 52e and the effective waveguide formed by the other four dielectric strips 52a-d may be considered. The dispersion of the right hand dielectric strip 52e decreases monotonically (with respect to wavelength) with a first slope. The dispersion of the waveguide formed by the other four dielectric strips 52a-d decreases monotonically (with respect to wavelength) with a second slope. The dispersion of the right hand dielectric strip 52e increases along the outward taper because the cross-section of the dielectric strip is increasing. The dispersion increases to a maximum when the taper stops, i.e. the dispersion is at a maximum where the right hand waveguide 60e with constant cross-sectional area begins. The slope of the dispersion of the waveguide 60e crosses the slope of the dispersion of the effective waveguide of the four dielectric strips 52a-d at a given wavelength. At this cross-over wavelength, the power of radiation will be split equally between the right hand waveguide 60e and the effective waveguide formed by the four dielectric strips 52a-d. At other wavelengths the radiation will preferentially couple to the waveguide 60e or effective waveguide 52a-d depending on which of these has the highest effective refractive index for that wavelength. In this example, shorter wavelengths may be coupled to the waveguide 60e and longer wavelengths may remain in the effective waveguide formed by the dielectric strips 52a-d. The crossover wavelength increases along the taper as the cross-sectional area of the taper increases. The maximum cross-over wavelength occurs at the end of the taper. This maximum cross-over wavelength determines the cross-over wavelength of radiation coupled into the waveguide 60e (i.e. the maximum wavelength of radiation coupled into the waveguide).

[0112] The same coupling effect may be used for the staggered outward tapers of the other four dielectric strips 52a-d. In each case, the taper 52a-d may end at a waveguide with a different cross- sectional area, thereby providing preferential coupling for different wavelengths. In an example the staggered tapers may each provide coupling at a longer wavelength than the previous taper.

[0113] The outward taper of each dielectric strip 52a-e may be adiabatic. In this document adiabatic tapering of a dielectric strip may refer to tapering which extends for example over a distance of hundreds of microns. Such a relatively long taper provides lower loss of radiation (and thus higher efficiency) than would be the case for a shorter taper. Outward tapers provided in other embodiments may be adiabatic. In general, a rate of change of a taper’s width vs its length may be less than 1 nm per micron. This may provide an adiabatic taper.

[0114] Four of the waveguides 60a, c-e include a bend 64a, c-e. The bends 64a, c-e increase separation between adjacent waveguides 60a-e. This allows easier coupling of waveguided modes out of thewaveguides 60a-e. Bends provided in other embodiments may be adiabatic. In general, a rate of change for the waveguide position in x versus its position in y may be less than 1 nm per micron. This may provide an adiabatic bend.

[0115] As depicted in Figure 10C, the dielectric strips 52f-k of the upper and lower rows have an initial portion with a constant cross-sectional area as a function of depth (in the Y-direction) into the radiation coupling system 36. Beyond the initial portion, the dielectric strips 52f-k of the upper and lower rows taper inwardly as a function of depth into the radiation coupling system 36. The inward tapering continues to a zero cross-sectional area (i.e. until the dielectric strips have tapered to nothing). The inward taper of each dielectric strip 52f-k may be adiabatic.

[0116] The inwardly tapering dielectric strips 52f-k are optically coupled to the outwardly tapering dielectric strips 52a-e. As a result of the tapering and the optical coupling, radiation is coupled from the dielectric strips of the upper and lower rows 52f-k to the dielectric strips of the middle row 52a-e and into the semiconductor waveguides 60a-e.

[0117] Thus, radiation which is incident at a face 50 of the radiation coupling system 36 enters the radiation coupling system 36 as an effective guided mode collectively supported by the optically coupled dielectric strips 52a-k. As the radiation propagates into the radiation coupling system 36, the radiation is coupled into the dielectric waveguides 60a-e.

[0118] The coupling of the incident radiation beam 34 into radiation coupling system 36 may be achieved with a broadband incident radiation beam having a FWHM diameter of for example at least 2pm. (e.g. at a wavelength of 500mn). For longer wavelengths the FWHM diameter may be considerably larger than this (e.g. at least 5pm). The array of dielectric strips advantageously is able to receive and support radiation as an effective waveguide without requiring tighter focusing of the radiation beam 34 (which could otherwise cause damage to the device’s facet due to high radiation intensity). Furthermore, embodiments of the invention are able to couple radiation supported by the dielectric strips 52a-k to a plurality of dielectric waveguides 60a-e, each of them carrying a portion of the optical spectrum contained in the incident radiation beam. The radiation propagating in the waveguides 60a-e may then be used as desired. For example, the radiation coupling system 36 may be a photonic integrated chip and may be used to divide the colour spectrum of the incident radiation beam into the waveguides 60a-e and use them subsequently to provide the optical signal for a measurement (e.g. of a lithographic substrata or mask).

[0119] A radiation coupling system 36 according to an alternative embodiment of the invention is depicted in Figure 11. Figure 11A corresponds with Figure 10A and depicts the input face 50 of the radiation coupling system 36. The input face has 11 dielectric strips 72a-k arranged as three rows A-C. A middle row B has five dielectric strips 72a-e, and upper and lower rows A, C each have three dielectric strips 72f-k.

[0120] Figure 1 IB depicts a cross section of the radiation coupling system at line B. As may be seen, the dielectric strips 72a-e have an initial portion with a constant cross-sectional area as a function ofdepth (in the Y-direction) into the radiation coupling system 36. Three of the dielectric strips 72a, c,e then taper outwardly to form three waveguides 70a-c (which are capable of supporting propagating radiation as a single propagating mode). Although the outward taper is depicted only in the X-direction, outward tapering may also occur in the Y-direction. The taper may be adiabatic. Two dielectric strips 72b, d which are located between outwardly tapering dielectric strips 72a, c,e taper inwardly to a zero cross-sectional area (i.e. until the dielectric strips have tapered to nothing). The effect of the inward and outer tapers is to couple radiation from the inwardly tapering dielectric strips 72b, d to the outwardly tapering dielectric strips 72a, c,e. In this way radiation is coupled to the three waveguides 70a-c.

[0121] Each of the waveguides 70a-c has a different cross-sectional area. The cross-sectional area provides a wavelength cut off for a waveguide mode provided by each waveguide 70a-c. The first waveguide 70a has a maximum wavelength which is smaller than the maximum wavelength of the second waveguide 70b. The second waveguide 70b has a maximum wavelength which is smaller than the maximum wavelength of the third waveguide 70c. Other arrangements of waveguides with different cross-sectional areas may be provided. The waveguide 70a with the smallest cross-sectional area will preferentially have coupled to it radiation of shorter wavelengths than the radiation coupled to the other waveguides 70b, c. The waveguide 70c with the largest cross-sectional area will preferentially have coupled to it radiation of longer wavelengths than the radiation coupled to the other waveguides 70a, b. The coupling of radiation to the waveguides 70a-c may be determined by the dispersion of the waveguides (as explained further above in connection with Figure 10).

[0122] Two of the waveguides 70a, c include bends which move the waveguides 70a-c apart from each other. The bends may be adiabatic.

[0123] Figure 11C depicts in cross section the radiation coupling system at cross-sectional line A and cross-sectional line C (the structure is the same at both lines). The dielectric strips 72f-k taper to a zero cross-sectional area. Radiation from these dielectric strips 72f-k is coupled to the waveguides 70a-c.

[0124] In general, the radiation coupling system 36 may couple an incident radiation beam 38 into a plurality of dielectric waveguides. For example, coupling may be into at least three dielectric waveguides. For example, coupling may be into five or more dielectric waveguides. If coupling were to only a single dielectric waveguide then the intensity of radiation within that waveguide could cause damage to the dielectric and could lead to undesirable nonlinear effects. Coupling to a plurality of dielectric waveguides (e.g. three or more, e.g. five or more) advantageously means that the intensity of radiation in each dielectric waveguide is lower. This reduces the likelihood of damage and of nonlinear effects.

[0125] It may be particularly important to reduce a likelihood of the radiation beam causing damage and nonlinear effects when the radiation beam 34 is pulsed. The radiation beam 34 may comprise pulses as short as 250fs (or even shorter than this). Such short pulses can cause very high peak radiation intensities. Embodiments of the invention may be particularly useful in avoiding damage due to high intensity radiation and also avoiding undesirable nonlinear optical effects for such short pulses.Embodiments of the invention may achieve this by receiving as an input at the radiation coupling system 36 a radiation beam 34 which is not tightly focused to a single dielectric waveguide. The radiation beam 34 incident at the radiation coupling system 36 may for example have an FWHM of at least 2 microns. Embodiments of the invention may couple the radiation beam 34 to a plurality of dielectric waveguides, the plurality of dielectric waveguides acting to spread the intensity of radiation and thereby reduce the peak intensity.

[0126] Figure 12 depicts a radiation coupling system according to a further alternative embodiment of the invention. Referring to Figure 12A, the radiation coupling system 36 has an input face 50. In this embodiment the array of dielectric strips is one dimensional array consisting of five dielectric strips 82a-e provided in cladding 54. The dielectric strips are arranged along a line A. Where an embodiment of this type is used, an incident radiation beam may be provided with an oval cross section rather than a circular cross section to facilitate coupling of the radiation beam to the radiation coupling system 36. This may for example be achieved using a cylindrical lens.

[0127] Figure 12B depicts the radiation coupling system 36 in cross section at the line A. The dielectric strips 82a-e have an initial portion with a constant cross-sectional area as a function of depth (in the Y- direction) into the radiation coupling system 36. Beyond the initial portion, the dielectric strips 82a-e taper outwardly as a function of depth into the radiation coupling system 36. Although Figure 12B depicts only an outward taper in the X-direction, an outward taper in the Z-direction is also present. The tapers are staggered. The cross-sectional area of each dielectric strip 82a-e increases to an area which is sufficiently large to support a waveguide mode for example for radiation having a wavelength of at least 600nm (e.g. a single mode waveguide for radiation having a wavelength of at least 600nm). Thus, a transition from a plurality of dielectric strips 82a-e which cannot individually support a waveguide mode to a plurality of waveguides 80a-e is provided. The area of each waveguide 80a-e may be substantially constant once the outward taper has finished.

[0128] The staggered tapers may be arranged to provide wavelength selective coupling into each waveguide 80a-e. The tapers may be adiabatic.

[0129] Four of the waveguides 80a, c-e include bends 84a, c-e. The bends 84a, c-e may be adiabatic. The adiabatic bends 84a, c-e increase separation between adjacent waveguides 80a-e so that they become uncoupled. This also allows easier coupling of waveguided modes out of the waveguides 80a- e.

[0130] In the embodiment of Figure 12, there are no inwardly tapering dielectric strips (for example of the type present in the embodiment of Figure 10).

[0131] Figure 13 schematically depicts a radiation coupling system 36 according to a further alternative embodiment of the invention. The radiation coupling system 36 is viewed in cross section from above. The radiation coupling system 36 may have an input face with multiple rows of dielectric strips (e.g. of the type depicted in Figure 10), or may have an input face with a single row of dielectric strips (e.g. of the type depicted in Figure 12). Five dielectric strips 92a-e initially have a constant cross-section andthen taper outwardly to form five dielectric waveguides 90a-e. The tapers are staggered and are configured to provide wavelength selective radiation coupling.

[0132] Four of the dielectric waveguides 90a, c-e are provided with a bend 94a, c-e, the bends acting to separate the waveguides from each other, and to provide some wavelength filtering. Each dielectric waveguide 90a-e is provided with a Bragg reflector 96a-e.

[0133] Each Bragg reflector 96a-e consists of a periodic pattern of grooves deposited or etched in the dielectric waveguide. The Bragg reflectors 96a-e can be configured to reject optical radiation over bandwidths in the order of 100 nm, and may provide extinction ratios of ~40 dB. Multiple Bragg reflectors can be implemented in series to cover bandwidths larger than 100 nm. The Bragg reflectors 90a-e are provided after the bends 90a, c-e and provide additional wavelength filtering.

[0134] In the embodiment depicted in Figure 13, each Bragg reflector 96a-e acts to limit the wavelength (X) of radiation which is emitted from a respective dielectric waveguide 90a-e. Thus, the Bragg reflectors 96a-c may improve wavelength selectivity provided by the radiation coupling system 36. The radiation coupling system of Figure 13 may for example provide the following output:• 1stwaveguide 90a wavelength band: 900 nm < X < 1 pm; Bragg reflector band: X < 900 nm• 2ndwaveguide 90b wavelength band: 800 nm < X < 900 nm; Bragg reflector band: X > 900 nm, X < 800 nm• 3rdwaveguide 90c wavelength band: 700 nm < X < 800 nm; Bragg reflector band: X > 800 nm, X < 700 nm• 4thwaveguide 90d wavelength band: 500 nm < X < 600 nm; Bragg reflector band: X > 600 nm, X < 500 nm• 5thwaveguide 90e wavelength band: 400 nm < X < 500 nm; Bragg reflector band: X > 500 nm

[0135] Bragg reflectors may be particularly useful for example if strong suppression of unwanted wavelengths is desired. For example, it may be desirable for some metrology tools to use radiation in which undesired wavelengths are suppressed by at least 30 dB. Bragg reflectors may be provided in dielectric waveguides of a radiation coupling system of an embodiment in order to achieve this. More than one Bragg reflector per dielectric waveguide may be provided. Embodiments of the invention may use other wavelength-selective elements such as micro-ring resonator filters, Fabry-Perot cavities, cascaded Mach-Zehnder interferometers, or cascaded optical couplers.

[0136] Figure 14 depicts a radiation coupling system 36 providing wavelength-selective coupling of radiation from an array of dielectric strips 102a-e into a dielectric waveguide 100a. Figure 14 was obtained using mathematical modelling. The radiation coupling system used in the model is formed from SiN and SiCF.

[0137] The dielectric waveguide 100a is a single mode waveguide with a cut-off wavelength of 550nm. The dielectric waveguide 100a is provided adjacent to the dielectric strips 102a-e and is opticallycoupled to the dielectric strips. Radiation with a wavelength < 550 nm is decoupled from the effective waveguide formed by the dielectric strips 102a-e into the dielectric waveguide 100a. Radiation with a wavelength > 550 nm remains in the effective waveguide formed by the dielectric strips 102a-e.

[0138] Figure 14A depicts radiation with a wavelength of 500 nm, and Figure 14B depicts radiation with a wavelength of 600 nm. It can be seen that the radiation with a wavelength of 500 nm is coupled into the dielectric waveguide 100a whereas radiation with a wavelength of 600 nm remains in the effective waveguide formed by the dielectric strips 102a-e. The wavelength selectivity is around 80%. That is, at 500 nm 80% of the radiation is coupled into the dielectric waveguide 100a and 20% remains in the effective waveguide formed by the dielectric strips 102a-e. At 600 nm 80% of the radiation remains in the effective waveguide formed by the dielectric strips 102a-e and 20% is coupled into the dielectric waveguide 100a. This ratio may be changed for instance by changing the dispersion of the effective waveguide formed by the dielectric strips, their spacing, and / or by changing the dispersion of the transition into dielectric waveguide 100a.

[0139] Figure 15 schematically depicts a radiation coupling system according to an embodiment of the invention. The radiation coupling system 36 comprises a series of dielectric waveguides 122a-e. The dielectric waveguides 122a-e are provided in a cladding dielectric 124. The dielectric waveguides 122a- e extend into the cladding dielectric 124 from a face 126 of the dielectric or substrate. Micro-lenses 128a-e are secured to the face 126 of the cladding dielectric 124, a different micro-lens being provided above a respective dielectric waveguide 122a-e. The micro-lenses 128a-e are secured to the face 126 using epoxy. The micro-lenses 122a-e may be spaced apart from the face 126 by micro-beads 132 (see the portion of Figure 15 which depicts an enlarged view of one of the micro-lenses 128c). A series of dichroic mirrors 134 a-d is located adjacent to the dielectric waveguides 122a-e. The dichroic mirrors 134 a-d are each configured to direct a different wavelength band of radiation into a respective dielectric waveguide 122a-d. A final mirror 134e is not dichroic and is considered to direct all remaining radiation into a final dielectric waveguide 122e.

[0140] As depicted, a broadband radiation beam 34 is received by the radiation coupling system 36. A first dichroic mirror 134a reflects short wavelength radiation into the first dielectric waveguide 122a, and transmits radiation of longer wavelengths. A second dichroic mirror 134b directs a short wavelength portion of the radiation into the second dielectric waveguide 122b, and transmits radiation of longer wavelengths, etc. The final mirror 134e directs the longest wavelength radiation into the final dielectric waveguide 122e.

[0141] The radiation coupling system 36 is able to receive a broadband radiation beam 34 and couple the broadband radiation beam in a wavelength selective manner into dielectric waveguides 122a-e.

[0142] In other embodiments a different number of dielectric waveguides may be provided. In other embodiments the micro-lenses may be provided separately from the cladding dielectric 124 (e.g. not directly secured to the semiconductor substrate).

[0143] A radiation coupling system of an embodiment of the invention may be a photonic integrated circuit. The photonic integrated circuit may be configured to switch between emission of radiation from different dielectric waveguides. In this way, multiplexing may be provided. The different dielectric waveguides may be configured to emit different wavelengths of radiation.

[0144] A radiation coupling system of an embodiment of the invention may be used in an opposite direction from the depicted embodiments. That is, radiation of may be received into different dielectric waveguides, and the dielectric waveguides may couple the radiation to a single effective waveguide mode supported by dielectric strips. The radiation received into different dielectric waveguides may have different wavelengths. The radiation coupling system may be a photonic integrated circuit. The photonic integrated circuit may be configured to switch between radiation received from different dielectric waveguides. In this way, de-multiplexing may be provided.

[0145] Embodiments of the invention refer to dielectric strips and dielectric waveguides. In other embodiments the strips and / or waveguides may be formed from semiconductor. The strips may be rectangular in cross section. The strips may be provided in a rigid substrate. The substrate may for example be formed from silicon or quartz.

[0146] In the following, further features, characteristics, and exemplary technical solutions of the present disclosure will be described in terms of clauses that may be optionally claimed in any combination:1. A radiation coupling system comprising a cladding provided within a substrate, wherein an array of strips is provided within the cladding, wherein the strips extend into the cladding from a face of the cladding, wherein the strips have a refractive index which is greater than the refractive index of the cladding, and wherein the strips are configured to together provide an optically coupled structure which acts as a waveguide; wherein the radiation coupling system further comprises a plurality of waveguides which are coupled to the array of strips.2. The radiation coupling system of clause 1 , wherein the strips are configured such that radiation is weakly confined to the cross-sections of the strips.3. The radiation coupling system of clause 1 or clause 2, wherein each strip is not capable in isolation of acting as a waveguide for radiation having a wavelength of at least 600 nm.4. The radiation coupling system of any preceding clause, wherein the array of strips is a two- dimensional array.5. The radiation coupling system of clause 3, wherein outer edges of outermost strips of the array of strips collectively define a perimeter, and wherein an average refractive index of the area within the perimeter is greater than a refractive index of the area outside of the perimeter.6. The radiation coupling system of clause 4, wherein the average refractive index of the area within the perimeter is up to 0.1 more than the refractive index of the area outside of the perimeter.7. The radiation coupling system of clause 4 or clause 5, wherein the area within the perimeter is at least 3 m2.8. The radiation coupling system of any preceding clause, wherein adjacent strips are separated by no more than 2 pm.9. The radiation coupling system of any preceding clause, wherein at least some of the strips taper outwardly to form the waveguides.10. The radiation coupling system of clause 8, wherein the outward tapers of the strips are adiabatic.11. The radiation coupling system of clause 8 or clause 9, wherein the outward tapers of the strips are staggered.12. The radiation coupling system of any of clauses 8 to 10, wherein the outward tapers of the strips are configured to provide wavelength selective coupling of radiation into the waveguides.13. The radiation coupling system of any preceding clause, wherein at least some of the waveguides include bends which increase separation between adjacent waveguides.14. The radiation coupling system of clause 12, wherein the bends are adiabatic.15. The radiation coupling system of any preceding clause, wherein some of the strips taper inwardly to a zero cross-sectional area.16. The radiation coupling system of clause 14, wherein the inward tapers are adiabatic.17. A radiation source comprising an emitter and the radiation coupling system of any preceding clause, wherein the emitter is configured to provide radiation having a wavelength range between 400nm and 900nm, and wherein the strips and the cladding are configured such that less than 50% of the radiation propagating from the face and into the cladding is confined within the strips.18. A radiation coupling system comprising a cladding provided within a semiconductor substrate, within which an array of waveguides is provided, wherein the waveguides extend into the cladding from a face of the cladding, and wherein the waveguides have a refractive index which is greater than the refractive index of the cladding, wherein the radiation coupling system further comprises an array of dichroic mirrors associated with the array of waveguides, each dichroic mirror being configured to direct radiation into a different waveguide..19. A measurement system comprising a radiation source, the radiation coupling system of any preceding clause and a controller.20. An exposure apparatus comprising a radiation source and further comprising the radiation coupling system of any of clauses 1 to 18.

[0147] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.

[0148] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology tool, or anyapparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non- vacuum) conditions.

[0149] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

[0150] Although specific reference is made to “metrology apparatus / tool / system” or “inspection apparatus / tool / system”, these terms may refer to the same or similar types of tools, apparatuses or systems. E.g. the inspection or metrology apparatus that comprises an embodiment of the invention may be used to determine characteristics of structures on a substrate or on a wafer. E.g. the inspection apparatus or metrology apparatus that comprises an embodiment of the invention may be used to detect defects of a substrate or defects of structures on a substrate or on a wafer. In such an embodiment, a characteristic of interest of the structure on the substrate may relate to defects in the structure, the absence of a specific part of the structure, or the presence of an unwanted structure on the substrate or on the wafer.

Claims

CLAIMS1. A radiation coupling system comprising a cladding provided within a substrate, wherein an array of strips is provided within the cladding, wherein the strips extend into the cladding from a face of the cladding, wherein the strips have a refractive index which is greater than the refractive index of the cladding, and wherein the strips are configured to together provide an optically coupled structure which acts as a waveguide; wherein the radiation coupling system further comprises a plurality of waveguides which are coupled to the array of strips.

2. The radiation coupling system of claim 1 , wherein the strips are configured such that radiation is weakly confined to the cross-sections of the strips.

3. The radiation coupling system of claim 1 or claim 2, wherein each strip is not capable in isolation of acting as a waveguide for radiation having a wavelength of at least 600 nm.

4. The radiation coupling system of any preceding claim, wherein the array of strips is a two- dimensional array.

5. The radiation coupling system of claim 3, wherein outer edges of outermost strips of the array of strips collectively define a perimeter, and wherein an average refractive index of the area within the perimeter is greater than a refractive index of the area outside of the perimeter.

6. The radiation coupling system of claim 4, wherein the average refractive index of the area within the perimeter is up to 0.1 more than the refractive index of the area outside of the perimeter.

7. The radiation coupling system of claim 4 or claim 5, wherein the area within the perimeter is at least 3 pm2.

8. The radiation coupling system of any preceding claim, wherein adjacent strips are separated by no more than 2 pm.

9. The radiation coupling system of any preceding claim, wherein at least some of the strips taper outwardly to form the waveguides.

10. The radiation coupling system of claim 8, wherein the outward tapers of the strips are adiabatic.

11. The radiation coupling system of claim 8 or claim 9, wherein the outward tapers of the strips are staggered.

12. The radiation coupling system of any of claims 8 to 10, wherein the outward tapers of the strips are configured to provide wavelength selective coupling of radiation into the waveguides.

13. The radiation coupling system of any preceding claim, wherein at least some of the waveguides include bends which increase separation between adjacent waveguides.

14. The radiation coupling system of claim 12, wherein the bends are adiabatic.

15. The radiation coupling system of any preceding claim, wherein some of the strips taper inwardly to a zero cross-sectional area.

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