Metrology tool

Abstract

An optical metrology system for measuring a periodic target. The system comprises first and second emission paths configured to emit first and second illumination beams. The system comprises first and second collection paths configured to collect first and second reflected beams formed by the first and second illumination beams. The system comprises a detector optically coupled to the first and second collection paths and configured to measure intensities of the first and second reflected beams. The system comprises an actuation system configured to generate relative movement between the periodic target and the first and second illumination beams. The system comprises a processor configured to determine alignment information at least partly based on a phase of the measured intensities of the first and second reflected beams. The system maybe at least partly implemented in the form of a photonic integrated circuit.

Description

METROLOGY TOOLCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24201357.1 which was filed on 19 September 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a new metrology system or metrology tool. The present invention also relates to a new photonic integrated circuit for use in a metrology tool and for emitting at least two illumination beams and collecting at least two reflected beams.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).

[0004] To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features 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.

[0005] Low-ki lithography may be used to process features with dimensions smaller than the classical resolution limit of a lithographic apparatus. In such a process, the resolution formula may be expressed as CD = ki / Z / NA. where X 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 forcontrolling a stability of the lithographic apparatus may be used to improve reproduction of the pattern at low kl.

[0006] Known metrology systems for determining overlay information involve the use of diffraction-based measurements of a periodic target. Diffraction-based measurements are suitable for high-performance overlay measurements of relatively complex device layers comprising relatively small features that require high accuracy measurements (e.g. on the sub-nanometer scale). However, many devices include less complex device layers comprising relatively large features that require less accurate overlay measurements. On such layers, image-based overlay measurements may be preferable to diffraction-based overlay measurements due to, for example, increased measurement speed and / or reduced target requirements. In image-based overlay measurements, a target is imaged in an aerial view such that a relative position of two features of the target can be seen in the image. Common targets include, for example, box-in-box patterns and / or line-and-space patterns. A line-and-space pattern may, for example, comprise pairs of gratings having a relatively large pitch (e.g. a pitch of about 1 pm or more, such as about 1.5 pm), such that the grating lines can be resolved using a medium -aperture microscope (e.g. having a numerical aperture of between about 0.3 and about 0.7). More advanced image-based overlay metrology systems utilize image analysis algorithms that allow sub-pixel accuracy measurements of overlay.

[0007] It may be desirable to provide a new metrology system, and components therefor, for determining one or more parameters of a periodic target (for example on a wafer) that at least partially addresses one or more problems associated with known arrangements, whether such problems are identified herein or otherwise.SUMMARY

[0008] According to a first aspect of the present disclosure there is provided an optical metrology system for measuring a periodic target. The optical metrology system comprises first and second emission paths configured to emit first and second illumination beams. The optical metrology system comprises first and second collection paths configured to collect first and second reflected beams formed by the first and second illumination beams. The optical metrology system comprises a detector optically coupled to the first and second collection paths and configured to measure intensities of the first and second reflected beams. The optical metrology system comprises an actuation system configured to generate relative movement between the periodic target and the first and second illumination beams. The optical metrology system comprises a processor configured to determine alignment information at least partly based on a phase of the measured intensities of the first and second reflected beams.

[0009] The optical metrology system of the present disclosure advantageously provides alignment information such as, for example, overlay information, in a simple, compact and inexpensive way compared to known diffraction-based metrology systems.

[0010] The first and second emission paths and the first and second collection paths may form part of a photonic integrated circuit. The photonic integrated circuit may have an area of about 200X400 jim2.

[0011] The first and second emission paths may be substantially the same. The first and second collection paths may be substantially the same.

[0012] The first and second emission and collection path may comprise waveguides. The waveguides may comprise Aluminum Oxide or Silicon Nitride.

[0013] The first and second reflected beams may comprise specular reflections. The first and second reflected beams may consist of specular reflections.

[0014] The detector may comprise a photodiode.

[0015] The periodic structure may comprise a grating pattern. The periodic structure may comprise a plurality of grating patterns. The periodic structure may be formed on a substrate. The substrate may be held by a substrate support.

[0016] The actuation system may be configured to move the optical metrology system and / or the substrate support.

[0017] The relative movement may comprise a scanning motion such that the first and second illumination beams scan across the periodic structure in a scanning direction. The scanning direction may be arranged with respect to the periodic structure such that the intensities of the first and second reflected beams vary during the scanning motion. The intensities of the first and second reflected beams may vary periodically during the scanning motion.

[0018] The actuation system may comprise a piezoelectric element, e.g. an XYZ piezoelectric element.

[0019] The optical metrology system may comprise a plurality of first and second emission paths. The optical metrology system may comprise a plurality of first and second collection paths. A first pair of the plurality of first and second emission paths may be configured to provide first and second illumination beams having a first characteristic. A second pair of the plurality of first and second emission paths may be configured to provide first and second illumination beams having a second characteristic that is different to the first characteristic.

[0020] The first and second characteristics of the pairs of first and second illumination beams may comprise different wavelengths. The first and second characteristics may comprise different polarizations. The first and second characteristics may comprise different cross-sectional areas. The first and second characteristics may comprise different cross-sectional shapes. The first and second characteristics may comprise different spacings between the respective first and second illumination beams. The first and second characteristics may comprise different off-axis illumination angles. The first and second characteristics may comprise different focal lengths.

[0021] The pluralities of first and second emission and collection paths may form part of a plurality of photonic integrated circuits.

[0022] The plurality of photonic integrated circuits may be formed on a single substrate.

[0023] The optical metrology system may comprise a beam steering system configured to adjust a position of at least one of the first and second illumination beams.

[0024] The beam steering system may be configured to adjust positions of both the first and second illumination beams together and / or independently.

[0025] The beam steering system may be configured to adjust a spacing between the first and second illumination beams. The beam steering system may be configured to adjust the spacing between the first and second illumination beams to account for a spacing between first and second patterns that form part of the periodic structure. For example, the beam steering system may be configured to change the spacing between the first and second illumination beams to substantially match a spacing between two gratings of a periodic structure formed on a substrate.

[0026] The beam steering system may be configured to direct the first illumination beam at a first pattern and direct the second illumination beam at a second pattern. The first and second patterns may be on different layers of a substrate.

[0027] Different periodic structures having different spacings may be used by different users and / or across different substrate and / or integrated circuit designs. The beam steering system advantageously allows the optical metrology system to adapt to use with different periodic structures.

[0028] The optical metrology system may comprise a beam shaping system configured to adjust a shape of at least one of the first and second illumination beams.

[0029] The beam shaping system may be configured to adjust shapes of both the first and second illumination beams together and / or independently.

[0030] The beam steering system and the beam shaping system may be the same system. The beam steering system and / or the beam shaping system may comprise an optical phased array.

[0031] At least one of the first and second emission paths may be configured such that at least one of the first and second illumination beams has a cross-sectional profile having a first extent along a first axis and a second extent along a second axis orthogonal to the first axis. The first extent may be greater than the second extent. The second axis may be substantially parallel to a direction of relative movement generated by the actuation system.

[0032] Providing an illumination beam that is relatively narrow in the scanning direction and relatively wide in a non-scanning direction advantageously reduces a sensitivity of the optical metrology system to target size.

[0033] At least one of the first and second collection paths may comprise a collection area that is elongate along a direction substantially parallel to the second axis.

[0034] Providing an elongate capture area advantageously reduces a sensitivity of the optical metrology system to non-target structures present nearby a relatively small periodic target.

[0035] The optical metrology system may comprise a lock-in amplifier configured to receive signals indicative of the intensities of the first and second reflected beams from the detector. Theprocessor may be configured to receive a phase output from the lock-in amplifier. The processor may be configured to determine the alignment information at least partly based on the phase output.

[0036] The processor may be configured to perform the following normalization function on the measured intensities of the first and second reflected beams before comparing the intensities of the first and second reflected beams:where / (t) is a function of the measured intensity of one of the first and second reflected beams over time t, ( >□) indicates time -averaging and | > | indicates a norm function for discrete and real intensity measurement functions: \I | =

[0037] The alignment information may comprise overlay information.

[0038] The processor may be configured to determine the overlay information at least partly based on the following equation:where OV is the overlay information, p is a pitch of at least part of the periodic target from which the first and second reflected beams reflect, (t) is a function of the normalized measured intensity of the first reflected beam over time t, I2(t) is a function of the normalized measured intensity of the second reflected beam over time t, ) indicates a time-averaged product of the two functions, and e is an offset due to sensor imperfections.

[0039] The pitch of the periodic structure may be known in advance. For example, the pitch of the periodic structure may be about 1.5 pm or more. For example, the pitch of the periodic structure may be about 2 pm or less. Alternatively, the pitch of the periodic structure may be determined, e.g. by determining a dominant Fourier component of the intensity functions ^(t) and 12(f).

[0040] The periodic target may comprise a first pattern and a second pattern arranged substantially orthogonally to the first pattern. The actuation system may be configured to generate relative movement between the periodic target and the first and second illumination beams such that the first and second illumination beams are incident upon at least part of the first pattern and at least part of the second pattern.

[0041] According to another aspect of the present disclosure, there is provided a method of optically measuring a periodic target. The method comprises emitting first and second illumination beams at the periodic target. The method comprises measuring intensities of first and second reflected beams formed by the first and second illumination beams. The method comprises generating relativemovement between the periodic target and the first and second illumination beams. The method comprises determining alignment information at least partly based on a phase of the measured intensities.

[0042] According to another aspect of the present disclosure, there is provided a photonic integrated circuit. The photonic integrated circuit comprises first and second emission waveguides configured to receive electromagnetic radiation. The photonic integrated circuit comprises an emission coupler optically coupled to the first and second emission waveguides and configured to emit first and second illumination beams. The photonic integrated circuit comprises a collection coupler configured to collect first and second reflected beams formed by the first and second illumination beams. The photonic integrated circuit comprises first and second collection waveguides optically coupled to the collection coupler and configured to receive the first and second reflected beams.

[0043] Any of the above listed aspects and features of the present disclosure may be combined in any way.BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Embodiments 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 schematically depicts an optical metrology system for measuring a periodic target in accordance with the present disclosure;Figure 5 schematically depicts a cross-sectional view from the side of an optical metrology system in accordance with the present disclosure;Figure 6 schematically depicts a top-down view of a periodic target that may be measured by an optical metrology system according to the present disclosure;Figure 7 schematically depicts an optical metrology system comprising a plurality of first and second emission and collection paths in accordance with the present disclosure; and,Figure 8 schematically depicts an example of a portion of an optical metrology system comprising a beam steering mechanism in accordance with the present disclosure.DETAILED DESCRIPTION

[0045] 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).

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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 carriedout 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.

[0052] 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.

[0053] 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.

[0054] 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 by a supervisory control system SCS, which may also control the lithographic apparatus LA, e.g. via lithography control unit LACU.

[0055] 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 apparatus (not shown) may be included in the lithocell LC. If errorsare 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.

[0056] An inspection apparatus or inspection tool, which may also be referred to as a metrology system or 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).

[0057] 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 Figure 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.

[0058] 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 Figure 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 detect where within the process window the lithographic apparatus LA is currently operating (e.g. using input from the metrology tool MT) to predict whether defects may be present due to e.g. sub-optimal processing (depicted in Figure 3 by the arrow pointing “0” in the second scale SC2).

[0059] 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 identifypossible drifts, e.g. in a calibration status of the lithographic apparatus LA (depicted in Figure 3 by the multiple arrows in the third scale SC3).

[0060] In lithographic processes, it is desirable to make frequent measurements of the structures created, e.g., for process control and verification. Tools to make such measurements 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 EP1,628,164A, incorporated herein by reference. Aforementioned scatterometers may measure gratings using light from soft x-ray and visible to near-IR wavelength range.

[0061] 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 EP1,628,164A, 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 targets 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.

[0062] 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.

[0063] 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 overfilled mode, it may also be possible to measure different targets simultaneously, thus determining different processing parameters at the same time.

[0064] 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 / 0370717A1 incorporated herein by reference in its entirety.

[0065] 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.

[0066] Figure 4 schematically depicts an optical metrology system 100 for measuring a periodic target 110 in accordance with the present disclosure. In the example of Fig. 4, the periodic target 110 comprises a first grating 111 and a second grating 112. The optical metrology system 100 comprises first and second emission paths 121, 122 configured to emit first and second illumination beams 131, 132. In the example of Fig. 4, only the circumferences of the first and second illumination beams 131, 132 are visible such that the portions of the first and second gratings 111, 112 that are illuminated by the first and second illumination beams 131, 132 remain visible in Figure 4. In practice, the first and second illumination beams 131, 132 may be filled. In the example of Fig. 4, the optical metrology system 100 comprises an electromagnetic radiation source 170. The electromagnetic radiation source 170 may be a separate device coupled via optical fibers or may be incorporated with other components of the optical metrology system on a photonic integrated circuit as described in more detail below. The first and second emission paths 121, 122 are optically coupled to the electromagnetic radiation source 170. The first and second illumination beams 131, 132 comprise electromagnetic radiation generated by the electromagnetic radiation source 170. The first and second illumination beams 131, 132 are emitted simultaneously.

[0067] The optical metrology system 100 comprises first and second collection paths 141, 142 configured to collect first and second reflected beams (not shown in Fig. 4) formed by the first and second illumination beams 131, 132. The optical metrology system 100 comprises a detector 150 optically coupled to the first and second collection paths 141, 142 and configured to measure intensities of the first and second reflected beams. The detector 150 may, for example, comprise a photodiode. The optical metrology system 100 comprises an actuation system (not shown in Fig. 4) configured to generate relative movement between the periodic target 110 and the first and second illumination beams 131, 132. A direction across which the first and second illumination beams 131, 132 scan across the first and second gratings 111, 112 of the periodic structure 110 is shown by arrow 195 in Fig. 4. The optical metrology system 100 comprises a processor 160 configured to determine alignment information at least partly based on a phase of the measured intensities of the first and second reflected beams.

[0068] In the example of Fig. 4, the first and second emission paths 121, 122 and the first and second collection paths 141, 142 form part of a photonic integrated circuit. The photonic integrated circuit may be formed of materials such as, for example, SiN and / or A1O. Fig. 4 shows a cross-sectional top-down view from a plane of the photonic integrated circuit. A separation between the photonic integrated circuit and the periodic structure along the Z axis may, for example, be about 100 pm or more. The separation between the photonic integrated circuit and the periodic structure along the Z axis may, for example, be about 1mm or less The photonic integrated circuit comprises first and second emission waveguides 181, 182 configured to receive electromagnetic radiation (e.g. from the electromagnetic radiation source 170). The photonic integrated circuit comprises an emission coupler optically coupled to the first and second emission waveguides 181, 182 and configured to emit the first and second illumination beams 131, 132. In the example of Fig. 4, the emission coupler comprises firstand second emission grating couplers 183, 184. The first emission grating coupler 183 is optically coupled to the first emission waveguide 181 and the second emission grating coupler 184 is optically coupled to the second emission waveguide 182. The first emission grating coupler 183 is configured to emit the first illumination beam 131 and the second emission grating coupler 184 is configured to emit the second illumination beam 132. One of the first and second emission grating couplers 183, 184 may be stacked on top of the other of the first and second emission grating couplers 183, 184. The combination of the first and second emission grating couplers 183, 184 may be referred to as a bisplitter emission grating coupler. In an alternative embodiment, the emission coupler comprises a single grating coupler configured to emit the first and second illumination beams 131, 132. However, due to less available scope for tailoring a single grating coupler to two illumination beams, this may come at the cost of a loss of transmission efficiency.

[0069] The photonic integrated circuit comprises a collection coupler configured to collect first and second reflected beams formed by the first and second illumination beams 131, 132. The photonic integrated circuit comprises first and second collection waveguides 191, 192 optically coupled to the collection coupler and configured to receive the first and second reflected beams. In the example of Fig. 4, the collection coupler comprises first and second collection grating couplers 193, 194. The first collection grating coupler 193 is optically coupled to the first collection waveguide 191 and the second collection grating coupler 194 is optically coupled to the second collection waveguide 192. The first collection grating coupler 193 is configured to collect the first reflected beam and the second collection grating coupler 194 is configured to collect the second reflected beam. One of the first and second collection grating couplers 193, 194 may be stacked on top of the other of the first and second collection grating couplers 193, 194. The combination of the first and second collection grating couplers 193, 194 may be referred to as a bisplitter collection grating coupler.

[0070] Figure 5 schematically depicts a cross-sectional view from the side of an optical metrology system 100 in accordance with the present disclosure. The first emission grating coupler 183 receives light from the first emission waveguide (not shown in Fig. 5) and is configured to emit the first illumination beam 131 towards the first grating 111 of the periodic structure. The first illumination beam 131 is emitted off-axis, e.g. at an oblique angle relative to the periodic target. The first emission grating coupler 183 focuses the first illumination beam 131 to form a spot on the first grating 111. The periodic structure may be formed on a substrate W such as, for example, the lithographic wafer of Fig. 1. The first illumination beam 131 at least partly reflects from the first grating 111 to form the first reflected beam 301. The first collection grating coupler 193 is configured to collect the first reflected beam 301 and couple the first reflected beam 301 into a first collection waveguide (not shown in Fig. 5). The first emission grating 183 and the first collection grating 193 form part of a photonic integrated circuit 105. An actuation system generates relative movement between the first grating 111 and the first illumination beam 131 such that the first illumination beam 131 scans across the first grating 111 in the direction indicated by arrow 295. Alternatively, the actuation system may generate relative movementbetween the first grating 111 and the first illumination beam 131 such that the first illumination beam 131 scans across the first grating 111 in a direction having a first component indicated by arrow 295 and a second component in a direction orthogonal to the arrow 295, e.g. as shown in the example 45° scanning motion of Fig. 6). The actuation system may, for example, one or more motion stages similar to the first and second positioners PM, PW of Fig. 1. Alternatively, the actuation system may comprise one or more piezoelectric elements upon which the photonic integrated circuits are attached (e.g. glued). The reflected beam 301 intensity is measured by a detector (not shown) which may form part of the photonic integrated circuit 105 whilst the first illumination beam 131 is scanned across the first grating 111 (e.g. by moving the photonic integrated circuit 105 and / or the substrate W). The light reaches the detector by being coupled into a collection waveguide of the photonic integrated circuit 105 by a collection coupler 193, the collection waveguide being optically coupled to the detector. Comparing the signals generated by the first and second reflected beams reveals alignment information such as overlay information.

[0071] Referring again to Fig. 4, as the first and second illumination beams 131, 132 are scanned across the first and second gratings 111, 112, substantially periodic signals (t) and I2(t) are generated by the detector. The exact form of each signal at least partly depends on the optical properties of the periodic structure 110, the geometry of the collection coupler and the scan speed of the measurement scan. The following provides an example of how overlay information may be determined from the signals. A first step may involve normalizing each signal to a zero mean and unit amplitude. For example, the processor may be configured to perform the following normalization function on the measured intensities of the first and second reflected beams before comparing the intensities of the first and second reflected beams (Equation 1):where / (t) is a function of the measured intensity of one of the first and second reflected beams over time t, (□) indicates time -averaging and | | indicates a norm function for discrete and real intensity measurement functions whereby: | / | =Overlay information may then be determined from an inner product of the normalised signals (t) and I2(t). For example, the processor may be configured to determine the overlay information at least partly based on the following (Equation 2):where OV is the overlay information, p is a pitch of at least part of the periodic target from which the first and second reflected beams reflect, (t) is a function of the normalised measured intensity of thefirst reflected beam over time t, I2(t) is a function of the normalized measured intensity of the second reflected beam over time t, ) indicates a time-averaged product of the two functions, and e is an offset due to detector imperfections.

[0072] The pitch p of the periodic target (e.g. of the gratings 111, 112) may be known in advance. Alternatively, the pitch p of the periodic target may be estimated by determining a dominant Fourier component of the normalized signals / i(t) and I2t . The offset e may be calibrated out through measurement. For example, the periodic structure may be measured once along one scan direction and measured again along a second scan direction that is opposite to the first scan direction, and / or by measuring the inverse gratings of a multi -grating periodic target (such as that shown in Fig. 7).

[0073] Instead of recording the intensity signals ^(t) and I2(t) and determining overlay information in a subsequent step, the intensity signals (t) and I2(t) may be fed into a lock-in amplifier directly. A phase output of the lock-in amplifier may encode overlay up to a constant. That is, the optical metrology system may comprise a lock-in amplifier configured to receive signals indicative of the intensities of the first and second reflected beams from the detector. The processor may be configured to receive a phase output from the lock-in amplifier and determine the alignment (e.g. overlay) information at least partly based on the phase output of the lock-in amplifier.

[0074] Figure 6 schematically depicts a top-down view of a periodic target 600 that may be measured by an optical metrology system according to the present disclosure. The periodic target 600 may correspond to a standard image-based overlay target. However, compared to the relative arrangement between the periodic target and the illumination beam shown in Fig. 5, the periodic target 600 of Fig. 6 has been rotated relative to the illumination beams such that the illumination beams scan across the periodic target in a direction that is oriented at about 45° relative to gratings of the periodic target 600. This advantageously allows overlay to be measured along both the X and Y axes in a single scan, thereby saving travel and switching time as discussed in greater detail below.

[0075] In the example of Fig. 6, the periodic target 600 comprises a first and second patterns 610, 620 that are arranged substantially parallel to each other, and may be referred to as “X” gratings. The periodic structure 600 further comprises third and fourth patterns 630, 640 that are arranged substantially parallel to each other, but substantially orthogonally with respect to the first and second patterns 610, 620. The third and fourth patterns 630, 640 may be referred to as “Y” gratings due to be orthogonally arranged relative to the “X” gratings. Each of the patterns 610-640 comprises two gratings . A first set of the gratings 611, 621, 631, 642 is formed in a first layer of substrate and a second set of the gratings 612, 622, 632, 642 is formed in a second layer of the substrate that is different to the first layer. For example, the first layer may be on top of the second layer. The first and second layers may be consecutive layers or may be separated by intermediate layers. First and second illumination beams 131, 132 are shown scanning across the periodic target 600 in a scanning direction indicated by arrows. The paths 651, 652 followed by the illumination beams 131, 132 as they scan across theperiodic structure 600 are also shown. As can be seen, the actuation system is configured to generate relative movement between the periodic target 600 and the first and second illumination beams 131, 132 such that the first and second illumination beams 131, 132 are incident upon at least part of one pattern 610 and at least part of an orthogonally arranged pattern 630, 640. That is, the paths 651, 652 cover parts of both the “X” and ‘ Y” gratings, so overlay information along both orthogonal directions may be extracted from the intensity signals provided by the detector.

[0076] Relative to Fig. 5, overlay information determined using the scan of Fig. 6 is now represented along a diagonal direction. In order to reconstruct the overlay information along the original axes X and Y, the following equations may be used (Equations 3 and 4):where OVx is overlay information along the X axis, OVY is overlay information along the Y axis, OVpiis Equation 2 when applied to a first pitch of the periodic structure, OVP2 is Equation 2 when applied to a second pitch of the periodic structure, and 6 is a relative orientation between the illumination beams and the periodic structure (which, in the example of Fig. 6, is about 45°).

[0077] Figure 7 schematically depicts an optical metrology system 400 comprising a plurality of first and second emission and collection paths in accordance with the present disclosure. The optical metrology system 400 comprises a first pair of first and second emission paths 121, 122, a second pair of first and second emission paths 221, 222 and athird pair of first and second emission paths 321, 322. The optical metrology system 400 may comprise a greater or lesser number of pairs of first and second emission paths. The first pair of first and second emission paths 121, 122 is configured to emit a first pair of first and second illumination beams 131, 132 at a first periodic target 110. The first periodic target 110 comprises a first grating 111 across which the first illumination beam 131 scans and a second grating 112 across which the second illumination beam 132 scans. The second pair of first and second emission paths 221, 222 is configured to emit a second pair of first and second illumination beams 231, 232 at a second periodic target 210. The second periodic target 210 comprises a first grating 211 across which the first illumination beam 231 scans and a second grating 212 across which the second illumination beam 232 scans. The third pair of first and second emission paths 321, 322 is configured to emit a third pair of first and second illumination beams 331, 332 at a third periodic target 310. The third periodic target 310 comprises a first grating 311 across which the first illumination beam 331 scans and a second grating 312 across which the second illumination beam 332 scans.

[0078] The optical metrology system 400 comprises a first pair of first and second collection paths 141, 142, a second pair of first and second collection paths 241, 242 and a third pair of first and second collection paths 341, 342. The optical metrology system 400 may comprise a greater or lesser number of pairs of first and second collection paths. The first pair of first and second collection paths 141, 142 is configured to collect a first pair of first and second reflected beams (not shown) formed by the first pair of first and second illumination beams 131, 132. The second pair of first and second collection paths 241, 242 is configured to collect a second pair of first and second reflected beams (not shown) formed by the second pair of first and second illumination beams 231, 232. The third pair of first and second collection paths 341 , 342 is configured to collect a third pair of first and second reflected beams (not shown) formed by the third pair of first and second illumination beams 331, 332.

[0079] The pluralities of first and second emission and collection paths may form part of a plurality of photonic integrated circuits. In the example of Fig. 7, the first pairs of emission and collection paths 121, 122, 141, 142 form part of a first photonic integrated circuit, the second pairs of emission and collection paths 221, 222, 241, 242 form part of a second photonic integrated circuit and the third pairs of emission and collection paths 321, 322, 341, 342 form part of a third photonic integrated circuit. The plurality of photonic integrated circuits are formed on a single substrate 440.

[0080] The first, second and third periodic targets 110, 210, 310 are located on a substrate W, such as the one shown in Fig. 1. The optical metrology system 400 comprises one or more detectors (not shown) optically coupled to the collection paths 141, 142, 242, 242, 341, 342 and configured to measure intensities of the pairs of first and second reflected beams. The optical metrology system 400 comprises an actuation system (not shown) configured to generate relative movement between the periodic targets 110, 210, 310 and the pairs of first and second illumination beams 131, 132, 231, 232, 331, 332. In the example of Fig. 7, the actuation system causes the optical metrology system 400 to scan across the substrate W in the direction of arrow 410. The optical metrology system 400 comprises a processor (not shown) configured to determine alignment information at least partly based on a phase of the measured intensities of each pair of first and second reflected beams. As the optical metrology system 400 comprises a plurality of first and second emission and collection paths, the optical metrology system 400 can simultaneously determine alignment information from different regions of the substrate W, thereby increasing a speed with which alignment characteristics of the periodic targets 110, 210, 310 may be measured. For example, the optical metrology system 400 may determine an overlay of the substrate W at multiple different regions simultaneously during a single measurement scan, thereby improving a throughput of the optical metrology system 400 compared to known systems.

[0081] An optical metrology system that is only able to produce two illumination beams (such as that shown in Fig. 4) having a fixed separation has the downside that only a limited range of periodic targets having a similar separation between gratings may be measured. However, due to the compact nature of photonic integrated circuits, multiple photonic integrated circuits comprising illumination beams having different characteristics, such as different illumination beam separations, may beincorporated into a single optical metrology system. For example, a photonic integrated circuit of the present disclosure may occupy an area of about 200x200 pm to operate at a working distance of about 100 pm. To increase a versatility of the optical metrology system by enabling the measurement of periodic targets comprising different grating separations, a plurality of these photonic integrated circuits, each having a different associated illumination beam separation, may be incorporated into a single optical metrology system (e.g. as a scan head). Due to the compact nature of the photonic integrated circuits, multiple photonic integrated circuits may be positioned over a single wafer comprising a plurality of periodic targets. The photonic integrate circuits may be arranged to form a single composite system or multiple independent systems (each containing one or more photonic integrated circuits). Such a sensor head may comprise many photonic integrated circuits formed on a single substrate. Practically, for wafer metrology uses, a size of such a substrate may be limited by a loss of focus that may occur due to substrate warpage. For lithographic uses, a convenient sensor head may be just large enough to measure overlay at several positions in one scanner exposure field having a size of, for example, about 23 X 33 mm2. In another example, such a sensor head may have an area that is about the same size as an area of a substrate that is to be examined by the sensor head, thereby allowing approximately all fields of the substrate to be measured simultaneously by the sensor head.

[0082] Referring to Fig. 7, the pairs of emission paths may be configured to provide illumination beams having different characteristics to each other. For example, the first pair of the plurality of first and second emission paths 121, 122 may be configured to provide the first pair of first and second illumination beams 131, 132 having a first characteristic, whilst the second pair of the plurality of first and second emission paths 221, 222 is configured to provide the second pair of first and second illumination beams 231, 232 having a second characteristic that is different to the first characteristic. In the example of Fig. 7, the first pair of the plurality of first and second emission paths 121, 122 is configured to provide substantially circular illumination beams 131, 132 having a first separation 421 at the first periodic target 110. The first separation 421 corresponds to a separation between the first and second gratings 111, 112 of the first periodic target 110. The second periodic target 210 comprises first and second gratings 211, 212 having a greater separation than the gratings 111, 112 of the first periodic target 110. As such, the first pair of illumination beams 131, 132 may not be suitable for measuring the second periodic target 210. However, the second pair of the plurality of first and second emission paths 221, 222 is configured to provide substantially circular illumination beams 231, 232 having a second separation 422 at the second periodic target 210. The second separation 422 is greater than the first separation 421, and corresponds to the separation between the gratings 211, 212 of the second periodic target 210. As such, by using emission paths that are configured to provide illumination beams having different characteristics to each other, different periodic targets having different characteristics may still be measured by the optical metrology system 400, thereby improving a versatility of the optical metrology system 400 compared to known systems.

[0083] As another example, the third pair of the plurality of first and second emission paths 321, 322 may be configured to provide the third pair of first and second illumination beams 331, 332 having a third characteristic that is again different to either the first or second pairs of illumination beams 131, 132, 231, 232. In the example of Fig. 7, the third pair of the plurality of first and second emission paths 321, 322 is configured to provide substantially elliptical illumination beams 331, 332 at the third periodic target 310. This is in contrast to the substantially circular beams 131, 132, 231, 232 provided by the first and second pairs of emission paths 121, 122, 221 , 222. Each of the third pair of illumination beams 331, 332 has a cross-sectional profile having a first extent 431 along a first axis Y and a second extent 432 along a second axis X orthogonal to the first axis Y. The first extent 431 is greater than the second extent 432. The second axis X is substantially parallel to a direction of relative movement 410 generated by the actuation system. At least one of the collection paths 141, 142, 241, 242, 341, 342 may comprise a collection area that is elongate along a direction substantially parallel to the second axis X. This arrangement assists in alleviating the potential downside of illuminating non-target structures on the substrate. The detector generates a signal from a substrate region that is illuminated and reflects the illumination beams to form the reflected beams. Designing a circuit that has either an illumination or collection area that is small over both axes may be challenging, so it is likely that non-target structures will be illuminated and light reflected from non-target structures will be collected. It may be simpler to design the circuit such that the illumination area is narrow along one axis and wide along the other orthogonal axis, and design the optical metrology system such that the collection area is respectively wide and narrow along these same axes, such that the overlap area is small in two dimensions. As can be seen in the example of Fig. 7, each collection path 141, 142, 241, 242, 341, 342 has a dimension along the X axis that is greater than a dimension along the Y axis. For example, a grating coupler of each collection path 141, 142, 241, 242, 341, 342 may have a dimension along the X axis that is greater than a dimension along the Y axis.

[0084] The differing characteristics of the pairs of illumination beams may correspond to features other than beam separation and / or shape. For example, the illumination beams may comprise different wavelengths of electromagnetic radiation. For example, the optical metrology system may be configured to emit a plurality of illumination beams having wavelengths across the range of about 500 nm to about 900 run, e.g. for measuring periodic targets having different pitches. If desired, wider wavelength ranges may be used up to a wavelength-dependent transparency of the material used to form the optical metrology system. For example, A1O photonic integrated circuits may convey wavelengths of about 200nm or more and about 2000 nm or less. As another example, SiN photonic integrated circuits may convey wavelengths of about 400 nm or more and about 2300nm or less. For example, about twelve wavelengths or less may be used across these ranges. As an alternative or additional example, the first and second characteristics may comprise different polarizations. For example, two different pairs of illumination beams may comprise orthogonally polarized light. Different wavelengthsand / or polarizations may allow different intensity measurements to be read out in parallel, e.g. across twelve different wavelengths and two different polarizations.

[0085] Any of the optical metrology systems may comprise a beam steering system configured to adjust a position of at least one of the first and second illumination beams and / or a beam shaping system configured to adjust a shape of at least one of the first and second illumination beams. The beam steering system and the beam shaping system may be the same system. For example, the beam steering system and / or the beam shaping system may comprise an optical phased array. For example, the optical phased array may comprise a two dimensional array of coherent emitters having controllable output phase by, for example, variation of a waveguide length of each of the emitters. Phase tuners such as, for example, thermo-optic or electro-optic phase tuners may be used to provide beam steering and / or beam shaping.

[0086] Figure 8 schematically depicts an example of a portion of an optical metrology system comprising a beam steering mechanism in accordance with the present disclosure. The beam steering mechanism comprises an optical phased system comprising two grating couplers 801, 802. The grating couplers 801, 802 may form part of a photonic integrated circuit 810. A first grating coupler 801 has a first phase t^and a second grating 802 coupler has a second phase <p2. A difference between the first and second phases (p -(p2may be controlled using, for example, thermo-optic or electro-optic tuners. By varying the phase difference (p -(p2, the illumination profile of the emitted illumination beam 820 will shift across a periodic structure 840 of a substrate W along the horizontal axis as shown by arrow 830. More complex beam steering and / or shaping systems (e.g. comprising a two-dimensional array of grating couplers), may be used.

[0087] In general, optical phased arrays modify an intensity profile of the illumination beams projected onto a target (e.g. a periodic structure) via the relative phases (or intensities) of the individual array elements. In order to perform beam steering, the phases of the individual array elements are adjusted such that constructive interference occurs at a given target position. This may lead to a maximum intensity at the target location. Similarly, individual array elements may be adjusted to shape a cross-sectional profile of the illumination beams, for instance in order to avoid a spurious detection signal from a reflective non-target structure.

[0088] The optical metrology system according to the present disclosure may be formed by bulk optical components (e.g. lenses, photonic integrated circuitry, or some hybrid combination of the two. It will be appreciated that photonic integrated circuitry is generally preferred due to its compact and lightweight nature.

[0089] 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. For example, 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. For example, the inspection apparatus or metrology apparatus that comprises an embodiment of theinvention 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.

[0090] 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, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.

[0091] 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 apparatus, or any apparatus 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.

[0092] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.

[0093] 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 clauses set out below.1. An optical metrology system for measuring a periodic target comprising: first and second emission paths configured to emit first and second illumination beams; first and second collection paths configured to collect first and second reflected beams formed by the first and second illumination beams; a detector optically coupled to the first and second collection paths and configured to measure intensities of the first and second reflected beams; an actuation system configured to generate relative movement between the periodic target and the first and second illumination beams; and, a processor configured to determine alignment information at least partly based on a phase of the measured intensities of the first and second reflected beams.2. The optical metrology system of clause 1, configured such that the first and second illumination beams are emitted simultaneously.3. The optical metrology system of any preceding clause, comprising an electromagnetic radiation source, wherein the first and second emission paths are optically coupled to the electromagneticradiation source, and wherein the first and second illumination beams comprise electromagnetic radiation generated by the electromagnetic radiation source.4. The optical metrology system of any preceding clause, wherein the electromagnetic radiation comprises a wavelength of about 500 nm or more.5. The optical metrology system of any preceding clause, wherein the electromagnetic radiation comprises a wavelength of about 900 nm or less.6. The optical metrology system of any preceding clause, wherein the electromagnetic radiation comprises a plurality of wavelengths.7. The optical metrology system of any preceding clause, wherein the electromagnetic radiation is polarized.8. The optical metrology system of clause 7, wherein the electromagnetic radiation comprises a plurality of polarizations.9. The optical metrology system of any preceding clause, wherein the first and second emission paths and the first and second collection paths form part of a photonic integrated circuit.10. The optical metrology system of clause 9, wherein the photonic integrated circuit occupies an area of about 400mm2or less11. The optical metrology system of clause 9 or clause 10, wherein the photonic integrated circuit occupies an area of about 10000pm2or more.12. The optical metrology system of any of clauses 9 to 11, wherein the photonic integrated circuit comprises a radiation source optically coupled to the first and second emission waveguides and is configured to generate the electromagnetic radiation.13. The optical metrology system of any of clauses 9 to 11, wherein the photonic integrated circuit comprises an optical fiber optically coupled to the first and second emission waveguides and configured to receive electromagnetic radiation from an external radiation source.14. The optical metrology system of any preceding clause, wherein the first and second emission waveguides comprise Aluminum Oxide and / or Silicon Nitride.15. The optical metrology system of any preceding clause, wherein the first and second emission paths are configured to emit the first and second illumination beams such that they are off-axis.16. The optical metrology system of clause 15, wherein the first and second emission paths are configured to emit the first and second illumination beams at an off-axis angle of about 45°.17. The optical metrology system of any preceding clause, comprising an emission coupler configured to emit the first and second illumination beams.18. The optical metrology system of clause 17, wherein the emission coupler consists of a single grating coupler.19. The optical metrology system of clause 17, wherein the emission coupler comprises: a first emission grating coupler configured to emit the first illumination beam; and, a second emission grating coupler configured to emit the second illumination beam.20. The optical metrology system of any preceding clause, wherein the first emission path is configured to focus the first illumination beam to a first focal length, and wherein the second emission path is configured to focus the second illumination beam to a second focal length, wherein the first and second focal lengths are different such that different layers of a target of interest may be probed by the first and second illumination beams.21. The optical metrology system of any preceding clause, wherein the first and second emission paths are configured such that the first and second illumination beams are spaced apart by about 2 pm or more at the periodic target.22. The optical metrology system of any preceding clause, wherein the first and second emission paths are configured such that the first and second illumination beams are spaced apart by about 20 pm or less at the periodic target.23. The optical metrology system of any preceding clause, wherein the first emission path is configured such that the first illumination beam has a first cross-sectional area at the first focal length, and the second emission path is configured such that the second illumination beam has a second cross- sectional area at a second focal length, wherein the first and second cross-sectional areas are different.24. The optical metrology system of any preceding clause, wherein the first emission path is configured such that the first illumination beam has a first cross-sectional shape, and the second emission path is configured such that the second illumination beam has a second cross-sectional shape, wherein the first and second cross-sectional shapes are different.25. The optical metrology system of any preceding clause, wherein at least one of the first and second emission paths is configured such that at least one of the first and second illumination beams has a substantially circular cross-sectional shape.26. The optical metrology system of any preceding clause, wherein at least one of first and second emission paths is configured such that at least one of the first and second illumination beams has a diameter of about 2 pm or more.27. The optical metrology system of any preceding clause, wherein at least one of the first and second emission paths is configured such that at least one of the first and second illumination beams has a cross-sectional shape having a first extent along a first axis and a second extent along a second axis orthogonal to the first axis, wherein the first and second extents are different.28. The optical metrology system of any preceding clause, wherein the first emission path is configured to emit a first range of wavelengths, and the second emission path is configured to emit a second range of wavelengths, wherein the first and second ranges of wavelengths are different.29. The optical metrology system of any preceding clause, wherein the first collection path comprises a first collection grating coupler configured to collect the first reflected beam, and wherein the second collection path comprises a second collection grating coupler configured to collect the second reflected beam.30. The optical metrology system of any preceding clause, wherein the first collection path is configured to collect the first reflected beam from a first focal length, and the second collection path is configured to collect the second illumination beam from a second focal length, wherein the first and second focal lengths are different. 31. The optical metrology system of any preceding clause, wherein the first collection path is configured to collect a first range of wavelengths, and the second collection path is configured to collect a second range of wavelengths, wherein the first and second ranges of wavelengths are different.32. The optical metrology system of any preceding clause, wherein the detector is configured to detect the intensity of the first reflected beam independently of the intensity of the second reflected beam.

Claims

CLAIMS1. An optical metrology system for measuring a periodic target comprising: first and second emission paths configured to emit first and second illumination beams; first and second collection paths configured to collect first and second reflected beams formed by the first and second illumination beams; a detector optically coupled to the first and second collection paths and configured to measure intensities of the first and second reflected beams; an actuation system configured to generate relative movement between the periodic target and the first and second illumination beams; and, a processor configured to determine alignment information at least partly based on a phase of the measured intensities of the first and second reflected beams.

2. The optical metrology system of claim 1, wherein the first and second emission paths and the first and second collection paths form part of a photonic integrated circuit.

3. The optical metrology system of claim 1 or claim 2, comprising: a plurality of first and second emission paths; and, a plurality of first and second collection paths, wherein a first pair of the plurality of first and second emission paths is configured to provide first and second illumination beams having a first characteristic, and a second pair of the plurality of first and second emission paths is configured to provide first and second illumination beams having a second characteristic that is different to the first characteristic.

4. The optical metrology system of claim 3, wherein the pluralities of first and second emission and collection paths form part of a plurality of photonic integrated circuits.

5. The optical metrology system of claim 4, wherein the plurality of photonic integrated circuits are formed on a single substrate.

6. The optical metrology system of any preceding claim, comprising a beam steering system configured to adjust a position of at least one of the first and second illumination beams.

7. The optical metrology system of any preceding claim, comprising a beam shaping system configured to adjust a shape of at least one of the first and second illumination beams.

8. The optical metrology system of any preceding claim, wherein at least one of the first and second emission paths is configured such that at least one of the first and second illumination beams has a cross-sectional profile having a first extent along a first axis and a second extent along a second axis orthogonal to the first axis, wherein the first extent is greater than the second extent, and wherein the second axis is substantially parallel to a direction of relative movement generated by the actuation system.

9. The optical metrology system of claim 8, wherein at least one of the first and second collection paths comprises a collection area that is elongate along a direction substantially parallel to the second axis.

10. The optical metrology system of any preceding claim, comprising a lock-in amplifier configured to receive signals indicative of the intensities of the first and second reflected beams from the detector, wherein the processor is configured to: receive a phase output from the lock-in amplifier; and, determine the alignment information at least partly based on the phase output.

11. The optical metrology system of any preceding claim, wherein the processor is configured to perform the following normalization function on the measured intensities of the first and second reflected beams before comparing the intensities of the first and second reflected beams:where / (t) is a function of the measured intensity of one of the first and second reflected beams over time t, (□) indicates time-averaging and 11" | indicates a norm function for discrete and real intensity measurement functions: | / | =12. The optical metrology system of claim 11, wherein the alignment information comprises overlay information, and wherein the processor is configured to determine the overlay information at least partly based on the following equation:where OV is the overlay information, p is a pitch of at least part of the periodic target from which the first and second reflected beams reflect, (t) is a function of the normalized measured intensity of the first reflected beam over time t, I2(t) is a function of the normalized measured intensity of the second reflected beam over time t. ) indicates a time-averaged product of the two functions, and 6 is an offset due to sensor imperfections.

13. The optical metrology system of any preceding claim, wherein the periodic target comprises a first pattern and a second pattern arranged substantially orthogonally to the first pattern, wherein the actuation system is configured to generate relative movement between the periodic target and the first and second illumination beams such that the first and second illumination beams are incident upon at least part of the first pattern and at least part of the second pattern.

14. A method of optically measuring a periodic target comprising: emitting first and second illumination beams at the periodic target; measuring intensities of first and second reflected beams formed by the first and second illumination beams; generating relative movement between the periodic target and the first and second illumination beams; and, determining alignment information at least partly based on a phase of the measured intensities.

15. A photonic integrated circuit comprising : first and second emission waveguides configured to receive electromagnetic radiation; an emission coupler optically coupled to the first and second emission waveguides and configured to emit first and second illumination beams; a collection coupler configured to collect first and second reflected beams formed by the first and second illumination beams; and, first and second collection waveguides optically coupled to the collection coupler and configured to receive the first and second reflected beams.

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