Non-centrosymmetric fringe-based metrology targets and methods

Abstract

There is provided a method of metrology comprising: obtaining calibration data from at least one first target comprising at least a first cluster of sub-targets and a second cluster of sub-targets, wherein said calibration data comprises a set of measurements of said at least one first target in a first orientation and a set of measurements of said at least one first target in a second orientation; and calibrating a model based on said calibration data such that the calibrated model is operable to: predict, from measurement data of at least one second target comprising at least a first cluster of sub-targets, predicted measurement data as would be obtained from a second cluster of sub-targets; or relate measurement data of at least one second target comprising at least a first cluster of sub-targets to corrected measurement data of the at least one second target.

Description

NON-CENTROSYMMETRIC FRINGE-BASED METROLOGY TARGETS AND METHODS CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 23218849.0 which was filed on 20 December 2023 and which is incorporated herein in its entirety by reference. FIELD

[0002] The present invention relates to methods and apparatus for metrology usable, for example, in the manufacture of devices by lithographic techniques and to methods of manufacturing devices using lithographic techniques. BACKGROUND

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay, a measure of the accuracy of alignment of two layers in a device. Overlay may be described in terms of the degree of misalignment between the two layers, for example reference to a measured overlay of 1nm may describe a situation where two layers are misaligned by 1nm.

[0004] Recently, various forms of scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation – e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle – to obtain a “spectrum” from which a property of interest of the target can be determined. Determination of the property of interest may be performed by various techniques: e.g., reconstruction of the target by iterative approaches such as rigorous coupled wave analysis or finite element methods; library searches; and principal component analysis.

[0005] The targets used by conventional scatterometers are relatively large, e.g., 40μm by 40μm, gratings and the measurement beam generates a spot that is smaller than the grating (i.e., the grating is underfilled). This simplifies mathematical reconstruction of the target as it can be regarded as infinite. ConfidentialHowever, in order to reduce the size of the targets, e.g., to 10μm by 10μm or less, e.g., so they can be positioned in amongst product features, rather than in the scribe lane, metrology has been proposed in which the grating is made smaller than the measurement spot (i.e., the grating is overfilled). Typically, such targets are measured using dark field scatterometry in which the zeroth order of diffraction (corresponding to a specular reflection) is blocked, and only higher orders processed. Examples of dark field metrology can be found in international patent applications WO 2009 / 078708 and WO 2009 / 106279 which documents are hereby incorporated by reference in their entirety. Further developments of the technique have been described in patent publications US20110027704A, US20110043791A and US20120242940A. The contents of all these applications are also incorporated herein by reference. Diffraction-based overlay using dark-field detection of the diffraction orders enables overlay measurements on smaller targets. These targets can be smaller than the illumination spot and may be surrounded by product structures on a wafer. Targets can comprise multiple gratings which can be measured in one image.

[0006] In the known metrology technique, overlay measurement results are obtained by measuring the target twice under certain conditions, while either rotating the target or changing the illumination mode or imaging mode to obtain separately the -1st and the +1st diffraction order intensities. The intensity asymmetry, a comparison of these diffraction order intensities, for a given target provides a measurement of target asymmetry, that is asymmetry in the target. This asymmetry in the target can be used as an indicator of overlay (undesired misalignment of two layers). Another known method measures a phase difference between dark-field images of two different types of sub-target.

[0007] Metrology measurements may be prone to optical or lens aberrations. To alleviate this, metrology targets may be designed to be centrosymmetric, wherein a target is rotationally symmetric around a center of origin which may allow for aberration effects to be separated from an overlay measurement. Centrosymmetric metrology targets may be larger than non-centrosymmetric targets. Typically, centrosymmetric targets are located in a scribe lane of a wafer. As an area of a wafer used for generating devices increases (e.g., to keep up with industry demands to increase device density), the area available for locating targets in a scribe lane may decrease. In some cases, this may lead to targets being located in a device area. Thus, there is a desire to use smaller metrology targets.

[0008] Accordingly, it may be preferable to use non-centrosymmetric metrology targets which may be smaller than centrosymmetric targets. Thus, it is desirable to improve methods relating to non- centrosymmetric metrology targets to mitigate optical or lens aberrations and thus extract metrology measurement signals from a reduced wafer area available for metrology marks. SUMMARY

[0009] In a first aspect, there is provided a method of metrology comprising: obtaining calibration data from at least one first target comprising at least a first cluster of sub-targets and a second cluster of sub- targets, wherein said calibration data comprises a set of measurements of said at least one first target in Confidentiala first orientation and a set of measurements of said at least one first target in a second orientation; and calibrating a model based on said calibration data such that the calibrated model is operable to: predict, from measurement data of at least one second target comprising at least a first cluster of sub-targets, predicted measurement data as would be obtained from a second cluster of sub-targets; or relate measurement data of at least one second target comprising at least a first cluster of sub-targets to corrected measurement data of the at least one second target.

[0010] The above and other aspects of the invention will be understood from a consideration of the examples described below. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 depicts a lithographic apparatus according to an embodiment of the invention; Figure 2 depicts a lithographic cell or cluster according to an embodiment of the invention; Figure 3(a) comprises a schematic diagram of a dark field scatterometer for use in measuring targets according to embodiments of the invention using a first pair of illumination apertures; Figure 3(b) illustrates a detail of diffraction spectrum of a target grating for a given direction of illumination; Figure 3(c) illustrates a second pair of illumination apertures providing further illumination modes in using the scatterometer for diffraction based overlay measurements; and Figure 3(d) illustrates a third pair of illumination apertures combining the first and second pair of apertures; Figure 4 depicts a known form of multiple grating target and an outline of a measurement spot on a substrate; Figure 5 depicts a cluster of metrology targets in accordance with some aspects of the present disclosure. Figure 6 depicts a rotationally symmetric metrology target in accordance with some aspects of the present disclosure. Figures 7(a) and 7(b) depict exemplary target layouts. Figure 8 depicts an exemplary wafer layout. Figures 9(a-e) depict schematic illustrations of exemplary field measurement layouts. Figure 10 depicts a method of metrology in accordance with some embodiments. Figure 11 depicts a method of metrology in accordance with some embodiments. Figure 12 depicts a method of metrology in accordance with some embodiments. Figure 13 depicts a method of metrology in accordance with some embodiments. DETAILED DESCRIPTION

[0012] Before describing embodiments of the invention in detail, it is instructive to present an example environment in which embodiments of the present invention may be implemented. Confidential

[0013] Figure 1 schematically depicts a lithographic apparatus LA. The apparatus includes an illumination optical system (illuminator) IL configured to condition a radiation beam B (e.g., UV radiation or DUV radiation), a patterning device support or support structure (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 in accordance with certain parameters; a substrate table (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 in accordance with certain parameters; and a projection optical 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., including one or more dies) of the substrate W.

[0014] The illumination optical system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

[0015] The patterning device support holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The patterning device support can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The patterning device support may be a frame or a table, for example, which may be fixed or movable as required. The patterning device support may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.”

[0016] The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so-called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0017] The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam, which is reflected by the mirror matrix. Confidential

[0018] As here depicted, the apparatus is of a transmissive type (e.g., employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g., employing a programmable mirror array of a type as referred to above, or employing a reflective mask).

[0019] The lithographic apparatus may also 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 and the substrate. An immersion liquid may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. The term “immersion” as used herein does not mean that a structure, such as a substrate, must be submerged in liquid, but rather only means that liquid is located between the projection system and the substrate during exposure.

[0020] Referring to Figure 1, the illuminator IL receives a radiation beam from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD including, for example, suitable directing mirrors and / or a beam expander. In other cases, the source may be an integral part of the lithographic apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

[0021] The illuminator IL may include an adjuster AD for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and / or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may include various other components, such as an integrator IN and a condenser CO. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross section.

[0022] The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the patterning device support (e.g., mask table MT), and is patterned by the patterning device. Having traversed the patterning device (e.g., mask) MA, the radiation beam B passes through the projection optical system PS, which focuses the beam onto a target portion C of the substrate W, thereby projecting an image of the pattern on the target portion C. With the aid of the second positioner PW and position sensor IF (e.g., an interferometric device, linear encoder, 2-D encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor (which is not explicitly depicted in Figure 1) can be used to accurately position the patterning device (e.g., mask) MA with respect to the path of the radiation beam B, e.g., after mechanical retrieval from a mask library, or during a scan. Confidential

[0023] Patterning device (e.g., mask) MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the patterning device (e.g., mask) MA, the mask alignment marks may be located between the dies. Small alignment markers may also be included within dies, in amongst the device features, in which case it is desirable that the markers be as small as possible and not require any different imaging or process conditions than adjacent features. The alignment system, which detects the alignment markers is described further below.

[0024] Lithographic apparatus LA in this example is of a so-called dual stage type which has two substrate tables WTa, WTb and two stations – an exposure station and a measurement station – between which the substrate tables can be exchanged. While one substrate on one substrate table is being exposed at the exposure station, another substrate can be loaded onto the other substrate table at the measurement station and various preparatory steps carried out. The preparatory steps may include mapping the surface control of the substrate using a level sensor LS and measuring the position of alignment markers on the substrate using an alignment sensor AS. This enables a substantial increase in the throughput of the apparatus.

[0025] The depicted apparatus can be used in a variety of modes, including for example a step mode or a scan mode. The construction and operation of lithographic apparatus is well known to those skilled in the art and need not be described further for an understanding of the present invention.

[0026] As shown in Figure 2, the lithographic apparatus LA forms part of a lithographic system, referred to as a lithographic cell LC or a lithocell or cluster. The lithographic cell LC may also include apparatus to perform pre- and post-exposure processes on a substrate. Conventionally these include spin coaters SC to deposit resist layers, developers DE to develop exposed resist, chill plates CH and bake plates BK. A substrate handler, or robot, RO picks up substrates from input / output ports I / O1, I / O2, moves them between the different process apparatus and delivers then to the loading bay LB of the lithographic apparatus. These devices, which are often collectively referred to as the track, are under the control of a track control unit TCU which is itself controlled by the supervisory control system SCS, which also controls the lithographic apparatus via lithography control unit LACU. Thus, the different apparatus can be operated to maximize throughput and processing efficiency.

[0027] A metrology apparatus suitable for use in embodiments of the invention is shown in Figure 3(a). A target T and diffracted rays of measurement radiation used to illuminate the target are illustrated in more detail in Figure 3(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 Confidential15 by an optical system comprising lenses 12, 14 and objective lens 16. These lenses 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 lenses 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. The illumination 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, such as those which enable simultaneous illumination and detection from two opposing directions in combination with optical wedges to separate the resultant images. 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.

[0028] As shown in Figure 3(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 3(a) and 3(b) are shown somewhat off axis, purely to enable them to be more easily distinguished in the diagram.

[0029] 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 3(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 Confidentialapplied using aperture plate 13S the -1 diffracted rays (labeled -1(S)) are the ones which enter the lens 16.

[0030] 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.

[0031] 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 zeroth order 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.

[0032] The particular forms of aperture plate 13 and field stop 21 shown in Figure 3 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 3) can be used in measurements, instead of or in addition to the first order beams.

[0033] 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 3(c) and (d). The use of these, and numerous other variations and applications of the apparatus are described in prior published applications, mentioned above.

[0034] It is known to use an apparatus such as illustrated in Figure 3(a) to measure overlay. To achieve this, overlay targets may be printed on a wafer in two layers. One method of overlay metrology (sometimes referred to as micro-diffraction based overlay (μDBO) infers overlay from an asymmetry imbalance in complementary diffraction orders from targets comprising a respective grating in each layer. Typically, in μDBO the gratings have the same single pitch in each layer, though there may be an imposed bias between the two targets.

[0035] Figure 4 is an example target on a wafer W which may be measured using an alternative known diffraction based overlay (DBO) metrology method. Such a metrology method may be overfilled such Confidentialthat the full target is captured within the measurement spot 31. In this DBO metrology method, the measured asymmetry signal may be a phase difference asymmetry from a pair of complementary sub- targets (a first type sub-target or “M pad” and a second type sub-target or “W pad”). In the example target shown, there are two such pairs of sub-targets, a first pair 32, 34 oriented in a first direction of the substrate plane and a second pair 33, 35 oriented in a second direction of the substrate plane. Each sub-target comprises overlaid periodic structures or gratings in respective layers for which an overlay value is to be measured. In contrast to the more common µDBO target (where the pitches are the same per layer), a sub-target such as illustrated in Figure 4 has gratings of different pitches in each of the two layers.

[0036] More specifically a target such as illustrated in Figure 4 comprises an arrangement of two different types of sub-targets (e.g., per direction): an “M pad” or “M sub-grating” 34, 35 which comprises a bottom grating having a smaller pitch ^^than that of a top grating ^^, and a “W pad” or “W sub-grating” 32, 33 which has these gratings reversed (i.e., it may have the same pitches as the M pad but with the larger pitch ^^in the top layer, although strictly speaking the two sub-targets do not need to have the same pitches). This is shown in the cross-section detail of one of these pairs 32, 34. In this manner, the target bias changes continuously along each target. The overlay signal is encoded in the resultant imaged Moiré patterns or intensity fringes (e.g., from dark field images of diffracted radiation from the sub-targets). A phase can be measured from each image from the position of the fringes within a target region of interest. Note that the actual arrangement of these sub-targets may vary from shown.

[0037] In such a method, an asymmetry signal ^ may be defined as the phase difference between adiffraction order from an “M pad” and a corresponding diffraction order from a “W pad”, e.g., ^ =^^^ − ^^^^^ (e.g., sometimes referred to as the normal images) or ^^^ − ^^^^^ (e.g., sometimesreferred to as the complementary images), where ^^^ − ^^^^^, ^^^ − ^^^^^ are the measuredphase difference between the “M pad” and “W pad” of the +1 diffraction order and -1 diffraction order respectively (using other diffraction orders are possible). Optionally a sum may be taken over bothdiffraction orders of a complementary diffraction order pair: e.g., ^ = ^^^ − ^^^^^ + ^^^ − ^^^^^.As such, it can be appreciated that the concepts described herein to different types ofasymmetry signal. This measurement principle is described in et al, Novel diffraction-based overlay metrology utilizing phase-based overlay for improved robustness, Proc. SPIE 11611, Metrology, Inspection, and Process Control for Semiconductor Manufacturing XXXV, 1161126 (22 February 2021), which is incorporated herein by reference.

[0038] Conventionally for such targets, the overlay ^^ can be extracted from the phase differences between the M pad and W pad target images, e.g., Fourier plane images from a +1 diffraction order and -1 diffraction order respectively, according to the following equation: Confidential^^ = 1^^^^4^ ^ + ^^^^ − ^^^^^ + ^^^ − ^^^^^^ ^1^^ ^^although, as already mentioned, the phase signal in the square bracket may also be only one of^^^ − ^^^^^ or ^^^ − ^^^^^. The term in the square brackets may be known as ^ ^^^^^.

[0039] Although there are several benefits, a phased-based application may be sensitive to sensor non- idealities that may have an adverse effect on accuracy of a parameter of interest such as overlay and machine-to-machine (M2M) matching performance.

[0040] To alleviate these issues, targets are typically designed to be centrosymmetric (i.e., rotationally symmetric), wherein each target comprises repetitions of a cluster of sub-targets (e.g., where a cluster is an M pad and W pad pair, or optionally an M pad and W pad per direction of the substrate plane) with a first cluster rotated 180 degrees to a second cluster. In this way, each target may comprise a 180- degree rotationally symmetric target that may be used to correct for the impact of sensor non-idealities on measurement performance. In this way, the parameter of interest (e.g., overlay) may be isolated from sensor effects, thus improving M2M matching performance.

[0041] A drawback of the centrosymmetric design is that each target is now twice the size than necessary to determine overlay without taking into account of the sensor non-idealities. This uses valuable reticle and substrate area (or “real estate”). For example, centrosymmetric targets are typically located in a scribe lane of a wafer. As an area on a wafer used to generate devices increases, the scribe lane area may decrease and become too small to accommodate metrology targets. As a result, metrology targets may have to be placed in a device area. Furthermore, because the number of required target pads for a centrosymmetric is doubled with respect to a non-centrosymmetric target, twice the number of edge exclusions (i.e., an area of the image of the target considered unusable) are required in the processing of the resulting dark-field images. Therefore, there is a limit on how much the size of a centrosymmetric target may be reduced by, whilst retaining enough signal for accurately determining overlay from measurements of the target.

[0042] This issue coupled with size constraints that result from a desire in industry to increase device density on a wafer (i.e., there is less “real-estate” available to allocate for metrology targets) may render a typical centrosymmetric target design unusable in some cases.

[0043] Accordingly, there is a desire to improve implementation of non-centrosymmetric fringe-based metrology targets to negate sensor non-idealities such that target sizes may be reduced whilst maintaining high overlay performance.

[0044] In accordance with some aspects of the present disclosure, it may be possible to measure at least one first target or centrosymmetric target comprising one or more non-centrosymmetric target clusters in order to calibrate for sensor aberrations, or sensor errors, and determine a true overlay of said one or more second targets or non-centrosymmetric targets (e.g., single clusters). Subsequently a correction model may be trained based on a determined sensor error. The trained correction model may Confidentialbe applied to measurements of further non-centrosymmetric targets of the same type. Advantageously, non-centrosymmetric targets may be used across a wafer to determine a corrected overlay, whilst the larger centrosymmetric target may serve only to calibrate for a sensor error, thus reducing the area required on a wafer for targets, allowing size reduction without information loss.

[0045] Figure 5 shows a second target 301 in accordance with some embodiments, which may in some examples be a non-centrosymmetric target. Second target 301 may comprise a single cluster of sub- targets 302 (i.e., cluster of metrology sub-targets or features). The single cluster of sub-targets 302 may comprise a set of complementary sub-targets (a first type sub-target or “M pad” and a second type sub- target or “W pad”). In the example target shown, there are two such pairs of sub-targets, a first pair Mx, Wx oriented in a first direction of the substrate plane and a second pair My, Wy oriented in a second direction of the substrate plane. In other examples not shown, the X and Y directions are not collocated, and may be located separately. As explained previously, an overlay may be determined from a phase difference (i.e., a differential measurement) between a diffraction order of a complementary pair of the first type of sub-target and the second type of sub-target. In the example, this may be represented as^^ = ^^^ − ^^^, wherein ^^ is a measured phase delta. Owing to sensor aberrations, or total sensorerror term Err, the measured phase delta may not be a true phase delta. A true phase delta ^^truemay be represented by ^^true= ^^ – Err.

[0046] Figure 6 shows a first target 401 in accordance with some embodiments, which may be a rotationally symmetric (i.e., centrosymmetric) target. First target 401 may comprise a first cluster of sub-targets (C1) 402 that may be similar to the single cluster of sub-targets 302 comprised in second target 301 and a second cluster of sub-targets (C2) 403 that may be similar to the cluster of sub-targets 302 comprised in the second target 301 rotated through 180 degrees (e.g., with respect to a sensor frame of reference). Said first target 401, also known as a calibration target, may in some cases be used for calibration in order to account for sensor induced errors (e.g., sensor aberrations) by determining calibration data relating to at least one of said first target 401. Each said cluster of sub-targets 402403 may comprise two pairs of sub-targets oriented in different, mutually orthogonal directions.

[0047] In some cases, said determining calibration data may comprise determining a total sensor error term, or error term Err from differences of a first-type sub-target of the first cluster 402 of each said first target 401 and a corresponding first-type sub-target of the second cluster 403 of each said first target 401, and from differences of a second-type sub-target of the first cluster 402 of each said first target 401 and a corresponding second-type sub-target of the second cluster 403 of each said first target 401.

[0048] For example, determining calibration data may comprise measuring phase data comprising a phase difference between pairs of corresponding sub-targets in the first cluster 402 and the second cluster 403, known henceforth as a cluster delta. That is, said phase data may comprise phase difference data describing a phase difference between at least one pair of sub-targets in each said cluster of sub- Confidentialtargets 402403, said at least one pair of sub-targets comprising a first type sub-target and a second type sub-target.

[0049] In an example, a first cluster delta dCL, may be determined by subtracting a phase measurement of a first type sub-target Mx of the first cluster of sub-targets 402 (^^^^^ from a phase measurement ofa first type sub-target Mx of the second cluster of sub-targets 403 ^^M^^^ (i.e., a differentialmeasurement) at wafer rotation 0 degrees (WR0), i.e., a first orientation, which may include a sensor error associated with a measurement area at wafer rotation 0°, err1 and err2 i.e.: ^^^^ ! = ^^^^ − ^M^^ + "##^ − "##^ (2)

[0050] Subsequently, a second cluster delta may be determined for the same sub-targets at wafer rotation 180 degrees (WR180), i.e., a second orientation. At 180 degrees rotation a phase associated with the first and second cluster of sub-targets 402403 may change sign (relative to the first orientation, i.e., wafer rotation 0 degrees), and the second cluster delta at 180 degree rotation may be determined by: ^^^^ ^$! = −^^^^ + ^M^^ + "##^ − "##^ (3)

[0051] The sensor error "##^ − "##^ may be estimated by summing the cluster deltas and dividing by2 (i.e., when summing, the opposite sign phase deltas may cancel, leaving the sensor error that has been measured twice (i.e., at 0 and 180 degrees rotations)). After 180 degrees rotation, the first type sub- target Mx of first cluster of sub-targets 402 may occupy an area of a measurement spot that the first type sub-target Mxof second cluster of sub-targets 403 occupied at wafer rotation 0 degrees and vice versa. Thus, it may be ensured that the sensor error corresponding to a particular measurement area of a measurement spot are observed when taking measurements.

[0052] The cluster delta determination may be repeated for sub-targets of a second type (i.e., W pad). In this way, phase measurements may be taken that include sensor errors related to a measurement area of the spot corresponding to the W pads at 0 and 180 degrees rotation.

[0053] The sum of the sensor error for each of the measured type of sub-targets may be equal to anestimated total sensor error term, or error term Err, i.e.: Err = ^"##^ − "##^^^ + ^"##^ − "##^^^ where^"##^ − "##^^^ is the sensor error from the sub-targets of a first type and ^"##^ − "##^^^ is the sensorerror of the sub-targets of a second type.

[0054] In some examples, said first type sub-target may comprise a first layer periodic structure having a first pitch p1 and a second layer periodic structure having a second pitch p2, and said second type sub- target comprises a first layer periodic structure having the second pitch and a second layer periodic structure having the first pitch. Confidential

[0055] In some examples, normal and complementary measurements of the first target 401 may be measured, i.e., such that the normal and complementary measurements may comprise a complementary diffraction order pair, e.g., (^^^^)+1 and (^^^^^-1. That is, said calibration data may comprise a set of measurements from positive diffraction orders of a complimentary pair of diffraction orders of scattered radiation from said at least one first target 401 and a set of measurements from negative diffraction orders of a complimentary pair of diffraction orders of scattered radiation from said at least one first target 401.

[0056] Advantageously, a learning model (or correction model) may be obtained in a calibration step. Said correction model may be determined from a true phase delta (i.e., without a sensor error) and calibration data which may, for example, be determined based on a measured phase delta on a training set of targets. In accordance with the above disclosure, said calibration data may comprise a set of measurements of at least one of said first target 401 in a first orientation and a set of measurements of said first target 401 in a second orientation, said first and second orientation differing by 180 degrees.

[0057] Subsequently, the correction model may be used to correct measurements of targets corresponding to the second target 301 located elsewhere on a wafer. That is, measurement data from one or more second target 301 may be corrected using said model so as to obtain corrected measurement data. Said measurement data and corrected measurement data may comprise phase data or parameter of interest data derived therefrom. Said second target 301 may comprise a single cluster of sub-targets 302 similar to the first cluster of sub-targets 402 (or second cluster of sub-targets 403) comprised in the first (i.e., calibration) target 401. In this way, the correction model that has been trained on the first target 401 may be applied to other targets that comprise a cluster of sub-targets similar to the first cluster of sub-targets 402 and / or second cluster of sub-targets 403. Thus, the need for exclusive use of centrosymmetric targets, that may be twice the size of non-centrosymmetric targets, can be avoided, saving space on a reticle and wafer.

[0058] Once an estimated error term Err is determined, it may be used to determine a true phase delta for the first cluster of sub-targets 402: ^^C1, true=^ ^^^^– Err). In some examples, the corrected (i.e., true) phase delta for the first cluster of sub-targets 402 and the measured phase delta for the first cluster of sub-targets 402 may be used to determine the correction model or canonical correlation model by finding parameters a, b, avgA and avgB to satisfy: %∗ 'Δ^^^,)*+, − -. / 01 = - ∗ ^Δ^^^ − -. / ^^ (4)

[0059] It will be may an average for sub-targets in the first cluster of sub-targets 402 at 0 and 180 degrees rotation, wherein avgA and avgB are the mean values of Δ^^^and Δ^^^,)*+,respectively as calculated from target measurements over a wafer during Confidentialthe correction model training. Thus, the bracketed terms in Eq.4 may be considered as average removed delta phases.

[0060] In some examples, the correction model may take into account cluster-dependent aberrations. For example, target location information may be included in the right-hand side of Equation (4) with Δ^^^, (e.g., using a circular basis function).

[0061] Although the present example describes a correction model as a canonical correlation model, this is only an example, and other methods and / or correction models are envisioned. One such alternative, for example, may be a partial least squares regression-based correction model.

[0062] A phase delta on the application target 301 may be measured to provide measurement data, and using the previously determined parameters a, b, avgA and avgB, the true phase delta, i.e., corrected measurement data, for the application target 301 may be obtained (i.e., from the linear solution of equation (4)). A parameter of interest may be determined from the corrected measurement data. In some examples the parameter of interest may be overlay. That is, in some examples the true phase delta may be used to determine a true overlay value.

[0063] Thus, measurement data from one or more application targets (i.e., one or more second targets 301), comprising a single cluster of sub-targets 302 may be obtained, a correction model relating to the first target 401 may be obtained, and corrected measurement data may be obtained using said correction model. In other words, said correction model may relate, in terms of an error term, the corrected measurement data to said measurement data as measured, wherein a correction step may comprise applying the model to the measurement data to obtain said corrected measurement data.

[0064] Advantageously, an overlay of a first target 301 used in calibration may not be required to determine the true overlay on a second target 301 in an application phase. This may allow the first target 401 used for calibration and the second target 301 in the application phase to have two distinct overlays (e.g., owing to processing issues).

[0065] Although the above disclosure describes a canonical correlation model trained on a measured phase delta and an error corrected (i.e., true) phase delta of a cluster of sub-targets, it will be appreciated that other algorithmic solutions may be applicable. For example, in some cases a model (e.g., a canonical correlation model) may be trained on a phase delta of a first cluster of sub targets 402 and a phase delta of a second cluster of sub-targets 403.

[0066] In some cases, total sensor error term Err may be computed as described above, and Gaussian kernel interpolation weights may be determined in order to extrapolate the sensor error to the location of a second target 301 (i.e., a non-centrosymmetric target).

[0067] In some cases, it may be possible to predict, based on training data (e.g., with a prediction algorithm) measurements of application target sub-targets at a second orientation. In this case, it may not be necessary to perform measurements of application target sub-targets at the second orientation which may improve throughput. In some examples the first and second orientation may differ by 180 degrees. Confidential

[0068] Figures 7(a) and 7(b) show non-limiting examples of centrosymmetric and non- centrosymmetric target configurations. In contrast to the square / rectangle examples set out in Figure 5 and Figure 6 (i.e., a square centrosymmetric target 401 and rectangular non-centrosymmetric target 301), a target configuration may comprise a rectangular centrosymmetric target 701 and a square non- centrosymmetric target 702, or in an alternative target configuration both a centrosymmetric 703 and a non-centrosymmetric 704 target may be square. In some cases, such as described in US patent application US20160140267A1 (incorporated herein by reference), targets and / or target configurations may be determined by a modelling process that takes into account lithographic processes. For example, individual steps of a lithography process may be modeled into a single process sequence that may simulate physical substrate processing. The process sequence may drive the physical creation of a target or target configuration.

[0069] Figure 8 is a schematic illustration of a purely exemplary wafer layout according to an embodiment. The wafer layout shown here comprises a plurality of device areas 801 separated by scribe lanes 802.

[0070] Figure 9(a-e) are schematic illustrations of purely exemplary field measurement layouts. In particular, the number of elements shown in each field measurement layout and their positions is purely exemplary.

[0071] Figure 9(a) illustrates a typical field measurement layout comprising a plurality of second targets or single cluster targets 301 and a plurality of first, or calibration targets 401 in scribe lane 803. In this example, there are no targets in device area 802. Figure 9(b) illustrates another typical field measurement layout wherein the scribe lane 803 is reduced in size compared to the arrangement of Figure 9(a), thereby increasing the device area 803. In this example, a plurality of second targets or single cluster targets 301 are comprised in the device area, and a plurality of first targets 401 are comprised in the scribe lane 803. Figure 9(c) illustrates another typical field measurement layout wherein the scribe lane 803 is further reduced in size with respect to Figures 9(a) and 9(b), such that it can now only accommodate the second target size; e.g. a plurality of second targets or single cluster targets 301 in the scribe lane 803. This may lead to first target 401 being comprised in the device area, which may reduce the area available for devices.

[0072] Figures 9(d) and 9(e) are schematic illustrations of purely exemplary field measurement layouts in accordance with some embodiments. Figure 9(d) illustrates a layout wherein the scribe lane 803 is reduced in size compared to the foregoing examples such that neither second targets or single cluster targets 301 or first targets 401 can be accommodated in said scribe lane 803. In this case, both single cluster targets 301 and first target 401 are accommodated in the device area 802. By employing the methods described herein, it may be possible to calibrate for optical or lens aberrations that may affect measurement of single cluster targets 301 using a first target 401 (i.e., taking into account first and second clusters 402403) that may be applied to second targets 301, thus reducing the need for larger first targets 401 that may take up more space in device area 802. Confidential

[0073] Figure 9(e) illustrates a layout wherein there are fewer than one calibration target 401 per device area (i.e., die), i.e., a calibration target may be comprised in every n device areas, wherein n may be greater than 1 as required by any particular use case.

[0074] With reference to Figure 10, the present embodiment may be summarized in the following steps: in a calibration phase 1001, capturing 1002 calibration data for a first cluster of sub-targets Δ^^^402 and a second cluster of sub-targets Δ^^^403 comprised in a first target 401 (i.e., centrosymmetric target) at first and second wafer rotations; determining 1003 an error term Err based on the calibration data; determining 1004 corrected calibration data Δ^^^ )*+,for the first cluster of sub-targets 402; and training 1005 a correction model based on the calibration data Δ^^^and corrected calibration data Δ^^^ )*+,. In an application phase 1006, the method may comprise capturing 1007 measurement data Δ^^^,233from a first cluster of sub-targets comprised in a second target 301 (i.e., non-centrosymmetric target 301) and predicting 1008, based on the measurement data and the correction model, corrected data Δ^^^,233,)*+,. A corrected overlay value may be determined 1009 from the corrected measurement data.

[0075] With reference to Figure 11, another embodiment may comprise the following steps: in a training phase 1101, determining 1102 an error term Err. As in the previous embodiment, in this case the error term Err may be determined from calibration data determined from a first target 401 comprising a first cluster of sub-targets 402, and a second cluster of sub-targets 403 in accordance with aspects of the present disclosure. Spatial interpolation weights, e.g., via Gaussian Kernel interpolation or any other suitable spatial interpolation method, may be determined 1103 based on a position of the first cluster of sub-targets 402 on a wafer and the determined error term Err. In an application phase 1105, the determined spatial interpolation weights may be used to predict 1106 the error term Err at a location of a second target 301 on the wafer by extrapolating the error term Err using the determined spatial interpolation weights to the location of the second target 301. Measurement data may be determined for the second target 301 Δ^^^,233which may be corrected using the extrapolated error term Err. Corrected measurement data (e.g., Δ^^^,233 )*+,) may be determined 1107 by subtracting the extrapolated error term Err from the measurement data Δ^^^,233. Subsequently, a corrected overlayvalue may be determined 1108 based on the corrected measurement data.

[0076] With reference to Figure 12, another embodiment may comprise the following steps: in a calibration phase 1201, capturing 1202 calibration data Δ^^^of a first cluster of sub-targets 402 of a first target 401 (i.e., centrosymmetric target 401) to provide first calibration data, capturing 1203 calibration data Δ^^^of a second cluster of sub-targets 403 of the first target 401 to provide second calibration data, and training 1204 a training model (e.g., a canonical correlation model) based on the first calibration data and the second calibration data. In examples where the training model is a canonical correlation model the training model may take the form: Confidential% ∗ 'Δ^^^^ ,4^ − -. / 01 = - ∗ 'Δ^^^^ ,4^ − -. / ^1 (5)wherein WR or , normal orcomplimentary measurements, and -. / ^ and -. / 0 are the wafer average of Δ^^^ and Δ^^^respectively.

[0077] In an application phase 1205 the method may comprise: capturing 1206 measurement data of a first cluster of sub-targets of a second target 301 (i.e., non-centrosymmetric target 301) Δ^^^,233andpredicting 1207, based on said training model, predicted measurement data Δ5^^^, , as would be obtainedfrom a second cluster of sub-targets 403 of the second target 301. The method may further comprise determining 1208 a correction factor 6 based on the predicted measurement data and the measurement data of the first cluster of sub-targets the second target 301. The correction factor for normal and complimentary measurements may be determined according to the following equations: 7^ ^ ^4 = ^ ∗ 8Δ^ ^^ ,4 − Δ^5^^^ ,49 (6)

[0078] The correction factor , may absolute phases ^^^ from saidpredicted measurement data to the following relations.^:;,<=*^ ,4 = ^:;^ ,4 − 0.5 ∗ 7;4 ^8^ ^:;, <=*^ ,^ = ^:;^ ,^ − 0.5 ∗ 7;^ ^9^^^;,<=* = ^^; + 0.5 ∗ 7; ^10^ ^^;,<=* = ^^; + 0.5 ∗ 7;^ ,4 ^ ,4 4 ^ ,^ ^ ,^ ^ ^11^error may the first cluster of the second target 301 and the determined absolute phases ^^^in accordance with aspects described previously (i.e., using cluster deltas). Subsequently, the determined error term Err may be used to correct 1211 an overlay measurement of the second target 301.

[0080] With reference to Figure 13, another embodiment may be realized based on the assumption that the overlay is the same for a first cluster of sub targets 402 and a second cluster of sub-targets 403. Such an embodiment may comprise a training phase 1301130213031304 and an application phase 1305 130613071308 having steps identical to steps 1201 to 1208 in the previous embodiment. However, rather than determining the absolute phases of ^^^, in the present embodiment the absolute phases ^^^of the first cluster of sub-targets in the second target 301 are determined 1309. A corrected overlay may be determined 1310 from the absolute phases ^^^of the second target 301. Confidential

[0081] While each target cluster is shown to comprise targets for measurement of overlay in both directions of the substrate plane, this is optional and targets comprising sub-targets for measuring overlay in only as single substrate plane direction are also envisaged within the scope of this disclosure.

[0082] The concepts herein also relate to a substrate comprising at least one second target 301 (e.g., single cluster target 302) and at least one first target 401 (e.g., centrosymmetric or calibration target). In an embodiment, each exposure field of the substrate may comprise fewer than 5, fewer than 4, fewer than 3 or a single calibration target, with all other overlay targets per field being single cluster targets. In some examples, the substrate may comprise fewer than one calibration target per exposure field, i.e., a calibration target may be comprised in every n exposure fields, wherein n is greater than 1 as required by any particular use case. The concepts herein also relate to a set of reticles (e.g., one reticle per layer of the target) comprising target features for forming said substrate, e.g., each reticle comprising at least one second target forming features for forming said at least one second target 301 and at least one first target 401 forming features for forming said at least one first target 401.

[0083] In the embodiments described, the second targets or single cluster targets 301 may be measured at both 0 and 180 degrees orientation. However, other methods have been described for estimating one of these measurements from the other. As such, the method may comprise measuring each second target 301 in a first orientation only, and estimating measurements of said at least one second target 301 in a second orientation from said measurements of said at least one second target 301 in a first orientation.

[0084] In an embodiment there may be provided a computer program comprising program instructions operable to perform any of the methods described herein when run on a suitable apparatus.

[0085] In some embodiments, there may be provided a non-transient computer program carrier comprising said computer program.

[0086] In some embodiments there may be provided a processing arrangement that may comprise a non-transient computer program carrier comprising a computer program comprising program instructions operable to perform any of the methods described herein, when run on a suitable apparatus; and a processor operable to run the computer program comprised on said non-transient computer program carrier.

[0087] In some embodiments there may be provided a metrology apparatus, e.g., a scatterometer, that may comprise: a support for a substrate; an optical system for illuminating said structure with measurement radiation; a detector for detecting the measurement radiation scattered by the structure; and said processing arrangement.

[0088] In some embodiments there may be provided a substrate comprising: one or more second targets 301, each second target comprising a single cluster of sub-targets 302; and at least one first target 401 comprising at least a first cluster of sub-targets 402 and a second cluster of sub-targets 403, the first cluster of sub-targets 402 being similar to said second cluster of sub-targets 403 rotated through 180 degrees, such that said first target 401 is centrosymmetric, said first cluster of sub-targets 402 being further similar to each said one or more second targets 301. Confidential

[0089] Each said cluster of sub-targets 302402403 may comprise at least one pair of sub-targets, said at least one pair of sub-targets comprising a first type sub-target and a second type sub-target.

[0090] Said first type sub-target may comprise a first layer periodic structure having a first pitch p1 and a second layer periodic structure having a second pitch p2, and said second type sub-target comprises a first layer periodic structure having the second pitch and a second layer periodic structure having the first pitch.

[0091] Each said cluster may comprise two of said pairs of sub-targets, each said pair oriented in different, mutually orthogonal, directions.

[0092] The substrate may comprise fewer than 3 said first targets 401 per each exposure field, and a plurality of said second targets.

[0093] The substrate may comprise only one said first target 401 per each exposure field, and a plurality of said second targets.

[0094] In some embodiments there may be provided a set of reticles, each reticle comprising target features for forming: one or more second targets 301, each second target comprising a single cluster of sub-targets 302; and at least one first target 401 comprising at least a first cluster of sub-targets 402 and a second cluster of sub-targets 403, the first cluster of sub-targets 402 being similar to said second cluster of sub-targets 403 rotated through 180 degrees, such that said first target 401 is centrosymmetric, said first cluster of sub-targets 402 being further similar to each said one or more second targets 301.

[0095] Each said cluster of sub-targets 302402403 may comprise at least one pair of sub-targets, said at least one pair of sub-targets comprising a first type sub-target and a second type sub-target.

[0096] Said first type sub-target may comprise a first layer periodic structure having a first pitch and a second layer periodic structure having a second pitch, and said second type sub-target comprises a first layer periodic structure having the second pitch and a second layer periodic structure having the first pitch.

[0097] Each said cluster of sub-targets 302402403 may comprise two of said pairs of sub-targets, each said pair oriented in different, mutually orthogonal, directions.

[0098] The set of reticles may comprise target features for fewer than 3 said first targets 401 and a plurality of said second targets 301.

[0099] The set of reticles may comprise target features for only one said first target 401 and a plurality of said second targets 301.

[0100] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described.

[0101] 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 may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist Confidentialsupplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured.

[0102] Further embodiments are presented in below numbered clauses: 1. A method of metrology comprising: obtaining calibration data from at least one first target comprising at least a first cluster of sub-targets and a second cluster of sub-targets, wherein said calibration data comprises a set of measurements of said at least one first target in a first orientation and a set of measurements of said at least one first target in a second orientation; and calibrating a model based on said calibration data such that the calibrated model is operable to: predict, from measurement data of at least one second target comprising at least a first cluster of sub- targets, predicted measurement data as would be obtained from a second cluster of sub-targets; or relate measurement data of at least one second target comprising at least a first cluster of sub-targets to corrected measurement data of the at least one second target. 2. A method according to clause 1, wherein said calibration data comprises a set of measurements from positive diffraction orders of a complementary pair of diffraction orders of scattered radiation from said at least one first target and a set of measurements from negative diffraction orders of the complementary pair of diffraction orders of the scattered radiation from said at least one first target. 3. A method according to clause 1 or 2, further comprising: obtaining said measurement data from one or more second targets, each second target comprising a single cluster of sub-targets similar to the first cluster of sub-targets comprised in the first target; and correcting said measurement data using said model so as to obtain said corrected measurement data. 4. A method according to clause 3, wherein said model relates, in terms of an error term, the corrected measurement data to said measurement data as measured, and said correcting step comprises applying the model to the measurement data to obtain said corrected measurement data. 5. A method according to clause 4, comprising, from said calibration data: determining said error term from differences of a first-type sub-target of the first cluster of each said first target and a corresponding first-type sub-target of the second cluster of each said first target, and from differences of a second-type sub-target of the first cluster of each said first target and a corresponding second-type sub-target of the second cluster of each said first target; and determining said model based on said error term. 6. A method according to any of clauses 1 or 2, wherein said model relates calibration data from the first cluster of sub-targets to calibration data from the second cluster of sub-targets. 7. A method according to clause 6, further comprising: predicting, using said measurement data and said model, predicted measurement data as would be obtained from said second cluster of sub-targets; determining a correction factor from the measurement data and predicted measurement data; and Confidentialdetermining corrected measurement data from said correction factor and said measurement data. 8. A method according to clause 1 or 2, comprising: determining an error term based on the calibration data; determining spatial interpolation weights based on said error term; and extrapolating and / or interpolating the determined error term to a respective location of each said one or more second targets based on the spatial interpolation weights and the measurement data; and obtaining corrected measurement data based on the measurement data and the extrapolated and / or interpolated error term. 9. A method according to any preceding clause, wherein said measurement data and corrected measurement data comprise phase data or parameter of interest data derived therefrom. 10. A method according to clause 9, wherein said phase data comprises phase difference data describing a phase difference between at least one pair of sub-targets in each said cluster of sub-targets, said at least one pair of sub-targets comprising a first type sub-target and a second type sub-target. 11. A method according to clause 10, wherein said first type sub-target comprises a first layer periodic structure having a first pitch and a second layer periodic structure having a second pitch, and said second type sub-target comprises a first layer periodic structure having the second pitch and a second layer periodic structure having the first pitch. 12. A method according to clause 10 or 11, wherein each said cluster comprises two of said pairs of sub-targets, each said pair oriented in different, mutually orthogonal, directions. 13. A method according to any preceding clause, wherein said measurement data is obtained from measurements of said at least one first target in a first orientation and in a second orientation, said first and second orientation differing by 180 degrees, wherein said first target is centrosymmetric. 14. A method according to any of clauses 1 to 12, wherein said measurement data is obtained from measurements of said at least one second target in a first orientation only, said measurement data further comprising estimated measurement data, and the method comprises: estimating measurements of said at least one second target in a second orientation from said measurements of said at least one second target in a first orientation to obtain said estimated measurement data. 15. A method according to any preceding clause, comprising determining a parameter of interest from the corrected measurement data. 16. A method according to clause 15, wherein the parameter of interest is overlay. 17. A method according to any preceding clause, comprising measuring said at least one second target to obtain said measurement data. 18. A method according to any preceding clause, comprising measuring said at least one first target to obtain said calibration data. 19. A computer program comprising program instructions operable to perform the method of any preceding clause, when run on a suitable apparatus. Confidential20. A non-transient computer program carrier comprising the computer program of clause 19. 21. A processing arrangement comprising: a non-transient computer program carrier comprising a computer program comprising program instructions operable to perform the method of any of clauses 1 to 18, when run on a suitable apparatus; and a processor operable to run the computer program comprised on said non-transient computer program carrier. 22. A metrology apparatus comprising: a support for a substrate; an optical system for illuminating said structure with measurement radiation; a detector for detecting the measurement radiation scattered by the structure; and the processing arrangement of clause 21. 23. A substrate comprising: one or more second targets, each second target comprising a single cluster of sub-targets; and at least one first target comprising at least a first cluster of sub-targets and a second cluster of sub- targets, the first cluster of sub-targets being similar to said second cluster of sub-targets rotated through 180 degrees, such that said first target is centrosymmetric, said first cluster of sub-targets being further similar to each said one or more second targets. 24. A substrate according to clause 23, wherein each said cluster of sub-targets comprises at least one pair of sub-targets, said at least one pair of sub-targets comprising a first type sub-target and a second type sub-target. 25. A substrate according to clause 24, wherein said first type sub-target comprises a first layer periodic structure having a first pitch and a second layer periodic structure having a second pitch, and said second type sub-target comprises a first layer periodic structure having the second pitch and a second layer periodic structure having the first pitch. 26. A substrate according to clause 24 or 25, wherein each said cluster comprises two of said pairs of sub-targets, each said pair oriented in different, mutually orthogonal, directions. 27. A substrate according to any of clauses 23 to 26, comprising fewer than 3 said first targets per each exposure field, and a plurality of said second targets. 28. A substrate according to any of clauses 23 to 26, comprising only one said first target per each exposure field, and a plurality of said second targets. 29. A set of reticles, each reticle comprising target features for forming: one or more second targets, each second target comprising a single cluster of sub-targets; and at least one first target comprising at least a first cluster of sub-targets and a second cluster of sub- targets, the first cluster of sub-targets being similar to said second cluster of sub-targets rotated through 180 degrees, such that said first target is centrosymmetric, said first cluster of sub-targets being further similar to each said one or more second targets. Confidential30. A set of reticles according to clause 29, wherein each said cluster of sub-targets comprises at least one pair of sub-targets, said at least one pair of sub-targets comprising a first type sub-target and a second type sub-target. 31. A set of reticles according to clause 30, wherein said first type sub-target comprises a first layer periodic structure having a first pitch and a second layer periodic structure having a second pitch, and said second type sub-target comprises a first layer periodic structure having the second pitch and a second layer periodic structure having the first pitch. 32. A set of reticles according to clause 30 or 31, wherein each said cluster comprises two of said pairs of sub-targets, each said pair oriented in different, mutually orthogonal, directions. 33. A set of reticles according to any of clauses 29 to 32, comprising target features for fewer than 3 said first targets and a plurality of said second targets. 34. A set of reticles according to any of clauses 29 to 32, comprising target features for only one said first target and a plurality of said second targets.

[0103] The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g., having a wavelength in the range of 1-100 nm), as well as particle beams, such as ion beams or electron beams.

[0104] The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components. Reflective components are likely to be used in an apparatus operating in the UV and / or EUV ranges.

[0105] The breadth and scope of the present invention should not be limited by any of the above- described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. Confidential

Claims

CLAIMS 1. A method of metrology comprising: obtaining calibration data from at least one first target comprising at least a first cluster of sub-targets and a second cluster of sub-targets, wherein said calibration data comprises a set of measurements of said at least one first target in a first orientation and a set of measurements of said at least one first target in a second orientation; and calibrating a model based on said calibration data such that the calibrated model is operable to: predict, from measurement data of at least one second target comprising at least a first cluster of sub- targets, predicted measurement data as would be obtained from a second cluster of sub-targets; or relate measurement data of at least one second target comprising at least a first cluster of sub-targets to corrected measurement data of the at least one second target.

2. A method as claimed in claim 1, wherein said calibration data comprises a set of measurements from positive diffraction orders of a complementary pair of diffraction orders of scattered radiation from said at least one first target and a set of measurements from negative diffraction orders of the complementary pair of diffraction orders of the scattered radiation from said at least one first target.

3. A method as claimed in claim 1 or 2, further comprising: obtaining said measurement data from one or more second targets, each second target comprising a single cluster of sub-targets similar to the first cluster of sub-targets comprised in the first target; and correcting said measurement data using said model so as to obtain said corrected measurement data.

4. A method as claimed in claim 3, wherein said model relates, in terms of an error term, the corrected measurement data to said measurement data as measured, and said correcting step comprises applying the model to the measurement data to obtain said corrected measurement data.

5. A method as claimed in claim 4, comprising, from said calibration data: determining said error term from differences of a first-type sub-target of the first cluster of each said first target and a corresponding first-type sub-target of the second cluster of each said first target, and from differences of a second-type sub-target of the first cluster of each said first target and a corresponding second-type sub-target of the second cluster of each said first target; and determining said model based on said error term.

6. A method as claimed in any of claims 1 or 2, wherein said model relates calibration data from the first cluster of sub-targets to calibration data from the second cluster of sub-targets. Confidential7. A method as claimed in claim 6, further comprising: predicting, using said measurement data and said model, predicted measurement data as would be obtained from said second cluster of sub-targets; determining a correction factor from the measurement data and predicted measurement data; and determining corrected measurement data from said correction factor and said measurement data.

8. A method as claimed in claim 1 or 2, comprising: determining an error term based on the calibration data; determining spatial interpolation weights based on said error term; and extrapolating and / or interpolating the determined error term to a respective location of each said one or more second targets based on the spatial interpolation weights and the measurement data; and obtaining corrected measurement data based on the measurement data and the extrapolated and / or interpolated error term.

9. A method as claimed in any preceding claim, wherein said measurement data and corrected measurement data comprise phase data or parameter of interest data derived therefrom.

10. A method as claimed in claim 9, wherein said phase data comprises phase difference data describing a phase difference between at least one pair of sub-targets in each said cluster of sub-targets, said at least one pair of sub-targets comprising a first type sub-target and a second type sub-target.

11. A method as claimed in claim 10, wherein said first type sub-target comprises a first layer periodic structure having a first pitch and a second layer periodic structure having a second pitch, and said second type sub-target comprises a first layer periodic structure having the second pitch and a second layer periodic structure having the first pitch.

12. A method as claimed in claim 10 or 11, wherein each said cluster comprises two of said pairs of sub-targets, each said pair oriented in different, mutually orthogonal, directions.

13. A method as claimed in any preceding claim, wherein said measurement data is obtained from measurements of said at least one first target in a first orientation and in a second orientation, said first and second orientation differing by 180 degrees, wherein said first target is centrosymmetric.

14. A method as claimed in any of claims 1 to 12, wherein said measurement data is obtained from measurements of said at least one second target in a first orientation only, said measurement data further comprising estimated measurement data, and the method comprises: Confidentialestimating measurements of said at least one second target in a second orientation from said measurements of said at least one second target in a first orientation to obtain said estimated measurement data.

15. A processing arrangement comprising: a non-transient computer program carrier comprising a computer program comprising program instructions operable to perform the method of any of claims 1 to 14, when run on a suitable apparatus; and a processor operable to run the computer program comprised on said non-transient computer program carrier. Confidential

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