Metrology method and associated metrology and exposure apparatuses

Abstract

Disclosed is a method of metrology, comprising: obtaining first metrology data relating to a respective measurement of one or more marks on a substrate; obtaining intra-mark position dependent metrology data relating to a plurality of intra-mark locations within each of said one or more marks; determining, from said intra-mark position dependent metrology data, intra-mark variation metric data describing an intra-mark variation for each of said one or more marks; and determining corrected metrology data from said first metrology data and said intra-mark variation metric data.

Description

METROLOGY METHOD AND ASSOCIATED METROLOGY AND EXPOSURE APPARATUSES CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 688,061 which was filed on 28 August2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to methods and apparatus usable, for example, in the manufacture of devices by lithographic techniques, and to methods of manufacturing devices using lithographic techniques. The invention relates more particularly to metrology sensors and lithography apparatuses having such a metrology sensor. BACKGROUND ART

[0003] An exposure apparatus or 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 a die, one die, 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, asingle substrate will contain a network of adjacent target portions that are successively patterned. Thesetarget portions are commonly referred to as “fields”.

[0004] In the manufacture of complex devices, typically many lithographic patterning steps are performed, thereby forming functional features in successive layers on the substrate. A critical aspect of performance of the lithographic apparatus is therefore the ability to place the applied pattern correctly and accurately in relation to features laid down (by the same apparatus or a different lithographic apparatus) in previous layers. For this purpose, the substrate is provided with one or more sets of alignment marks. Each mark is a structure whose position can be measured at a later time using a position sensor, typically an optical position sensor or alignment sensor. The lithographic apparatus includes one or more alignment sensors by which positions of marks on a substrate can be measured accurately. Different types of marks and different types of alignment sensors are known from different manufacturers and different products of the same manufacturer.

[0005] In other applications, metrology sensors are used for measuring exposed structures on a substrate (either in resist and / or after etch). A fast and non-invasive form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. Examples of known scatterometers Company Secretinclude angle-resolved scatterometers of the type described in US2006033921A1 and US2010201963A1. In addition to measurement of feature shapes by reconstruction, diffraction based overlay can be measured using such apparatus, as described in published patent application US2006066855A1. Diffraction-based overlay metrology using dark-field imaging of the diffraction orders enables overlay measurements on smaller targets. Examples of dark field imaging 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 published patent publications US20110027704A, US20110043791A, US2011102753A1, US20120044470A, US20120123581A, US20130258310A, US20130271740A and WO2013178422A1. These targets can be smaller than the illumination spot and may be surrounded by product structures on a wafer. Multiple gratings can be measured in one image, using a composite grating target. The contents of all these applications are also incorporated herein by reference.

[0006] In some metrology applications, such as in some scatterometers or alignment sensors, imperfections in metrology targets can result in a variation in a measured value from that target. It would be desirable to improve one or more aspects of such metrology. SUMMARY OF THE INVENTION

[0007] The invention in a first aspect provides a method of metrology, comprising: obtaining first metrology data relating to a respective measurement of one or more marks on a substrate; obtaining intra-mark position dependent metrology data relating to a plurality of intra-mark locations within each of said one or more marks; determining, from said intra-mark position dependent metrology data, intra- mark variation metric data describing an intra-mark variation for each of said one or more marks; anddetermining corrected metrology data from said first metrology data and said intra-mark variationmetric data.

[0008] The invention in a second aspect provides a method of determining a performance indicator, comprising: obtaining intra-mark position dependent metrology data relating to a plurality of intra-mark locations within one or more marks on a substrate; determining, from said intra-mark position dependent metrology data, intra-mark variation metric data describing an intra-mark variation for each of said one or more marks; and determining, from said intra-mark variation metric data, at least one performance indicator.

[0009] Also disclosed is a computer program, alignment sensor and a lithographic apparatus being operable to perform the method of the first aspect.

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

[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; Figure 2 illustrates schematically measurement and exposure processes in the apparatus of Figure 1; Figure 3 is a schematic illustration of an alignment sensor adaptable according to an embodiment of the invention; Figure 4 conceptually illustrates the diffracted fields from measurement of an alignment mark; and Figure 5 is a flowchart describing a method according to concepts described herein. DETAILED DESCRIPTION OF EMBODIMENTS

[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.

[0013] Figure 1 schematically depicts a lithographic apparatus LA. The apparatus includes an illumination 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; two substrate tables (e.g., a wafer table) WTa and WTb each constructed to hold a substrate (e.g., a resist coated wafer) W and each connected to a second positioner PW configured to accurately position the substrate 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., including one or more dies) of the substrate W. A reference frame RF connects the various components, and serves as a reference for setting and measuring positions of the patterning device and substrate and of features on them.

[0014] The illumination 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 MT 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 MT may be a frame or a table, for example, whichmay be fixed or movable as required. The patterning device support may ensure that the patterningdevice is at a desired position, for example with respect to the projection system.

[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 Company Secretbeam 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] As here depicted, the apparatus is of a transmissive type (e.g., employing a transmissive patterning device). 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). Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.” The term “patterning device” can also be interpreted as referring to a device storing in digital form pattern information for use in controlling such a programmable patterning device.

[0018] The term “projection system” used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, 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”.

[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.

[0020] In operation, 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 casesthe source may be an integral part of the lithographic apparatus, for example when the source is amercury 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 for example include an adjuster AD for adjusting the angular intensity distribution of the radiation beam, an integrator IN and a condenser CO. The illuminator may be usedto condition the radiation beam, to have a desired uniformity and intensity distribution in its crosssection.

[0022] The radiation beam B is incident on the patterning device MA, which is held on the patterning device support 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 system PS, which focuses the beam Company Secretonto a target portion C of the substrate W. 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 WTa or WTb 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.

[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 marks 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] The depicted apparatus could be used in a variety of modes. In a scan mode, the patterning device support (e.g., mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e., a single dynamic exposure). The speed and direction of the substrate table WT relative to the patterning device support (e.g., mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (inthe non-scanning direction) of the target portion in a single dynamic exposure, whereas the length ofthe scanning motion determines the height (in the scanning direction) of the target portion. Other types of lithographic apparatus and modes of operation are possible, as is well-known in the art. For example, a step mode is known. In so-called “maskless” lithography, a programmable patterning device is held stationary but with a changing pattern, and the substrate table WT is moved or scanned.

[0025] Combinations and / or variations on the above described modes of use or entirely different modes of use may also be employed.

[0026] Lithographic apparatus LA is of a so-called dual stage type which has two substrate tables WTa,WTb and two stations – an exposure station EXP and a measurement station MEA – between which thesubstrate 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. This enables a substantial increase in the throughput of the apparatus. The preparatory steps may include mapping the surface height contours of the substrate using a level sensor LS and measuring the position of alignment markers on the substrate using an alignment sensor AS. If the position sensor IF is not capable of measuring the position of the Company Secretsubstrate table while it is at the measurement station as well as at the exposure station, a second position sensor may be provided to enable the positions of the substrate table to be tracked at both stations, relative to reference frame RF. Other arrangements are known and usable instead of the dual-stage arrangement shown. For example, other lithographic apparatuses are known in which a substrate table and a measurement table are provided. These are docked together when performing preparatory measurements, and then undocked while the substrate table undergoes exposure.

[0027] Figure 2 illustrates the steps to expose target portions (e.g. dies) on a substrate W in the dual stage apparatus of Figure 1. On the left hand side within a dotted box are steps performed at ameasurement station MEA, while the right hand side shows steps performed at the exposure stationEXP. From time to time, one of the substrate tables WTa, WTb will be at the exposure station, while the other is at the measurement station, as described above. For the purposes of this description, it is assumed that a substrate W has already been loaded into the exposure station. At step 200, a new substrate W’ is loaded to the apparatus by a mechanism not shown. These two substrates are processed in parallel in order to increase the throughput of the lithographic apparatus.

[0028] Referring initially to the newly-loaded substrate W’, this may be a previously unprocessed substrate, prepared with a new photo resist for first time exposure in the apparatus. In general, however, the lithography process described will be merely one step in a series of exposure and processing steps, so that substrate W’ has been through this apparatus and / or other lithography apparatuses, several times already, and may have subsequent processes to undergo as well. Particularly for the problem of improving overlay performance, the task is to ensure that new patterns are applied in exactly the correct position on a substrate that has already been subjected to one or more cycles of patterning and processing. These processing steps progressively introduce distortions in the substrate that must bemeasured and corrected for, to achieve satisfactory overlay performance.

[0029] The previous and / or subsequent patterning step may be performed in other lithography apparatuses, as just mentioned, and may even be performed in different types of lithography apparatus. For example, some layers in the device manufacturing process which are very demanding in parameters such as resolution and overlay may be performed in a more advanced lithography tool than other layers that are less demanding. Therefore some layers may be exposed in an immersion type lithography tool, while others are exposed in a ‘dry’ tool. Some layers may be exposed in a tool working at DUV wavelengths, while others are exposed using EUV wavelength radiation.

[0030] At 202, alignment measurements using the substrate marks P1 etc. and image sensors (not shown) are used to measure and record alignment of the substrate relative to substrate table WTa / WTb.In addition, several alignment marks across the substrate W’ will be measured using alignment sensorAS. These measurements are used in one embodiment to establish a “wafer grid”, which maps very accurately the distribution of marks across the substrate, including any distortion relative to a nominal rectangular grid. Company Secret

[0031] At step 204, a map of wafer height (Z) against X-Y position is measured also using the level sensor LS. Conventionally, the height map is used only to achieve accurate focusing of the exposed pattern. It may be used for other purposes in addition.

[0032] When substrate W’ was loaded, recipe data 206 were received, defining the exposures to be performed, and also properties of the wafer and the patterns previously made and to be made upon it. To these recipe data are added the measurements of wafer position, wafer grid and height map that were made at 202, 204, so that a complete set of recipe and measurement data 208 can be passed to the exposure station EXP. The measurements of alignment data for example comprise X and Y positions of alignment targets formed in a fixed or nominally fixed relationship to the product patterns that arethe product of the lithographic process. These alignment data, taken just before exposure, are used togenerate an alignment model with parameters that fit the model to the data. These parameters and the alignment model will be used during the exposure operation to correct positions of patterns applied in the current lithographic step. The model in use interpolates positional deviations between the measured positions. A conventional alignment model might comprise four, five or six parameters, together defining translation, rotation and scaling of the ‘ideal’ grid, in different dimensions. Advanced modelsare known that use more parameters.

[0033] At 210, wafers W’ and W are swapped, so that the measured substrate W’ becomes the substrate W entering the exposure station EXP. In the example apparatus of Figure 1, this swapping is performed by exchanging the supports WTa and WTb within the apparatus, so that the substrates W, W’ remain accurately clamped and positioned on those supports, to preserve relative alignment between the substrate tables and substrates themselves. Accordingly, once the tables have been swapped, determining the relative position between projection system PS and substrate table WTb (formerly WTa) is all that is necessary to make use of the measurement information 202, 204 for the substrate W (formerly W’) in control of the exposure steps. At step 212, reticle alignment is performed using the mask alignment marks M1, M2. In steps 214, 216, 218, scanning motions and radiation pulses are applied at successive target locations across the substrate W, in order to complete the exposure of a number of patterns.

[0034] By using the alignment data and height map obtained at the measuring station in the performance of the exposure steps, these patterns are accurately aligned with respect to the desiredlocations, and, in particular, with respect to features previously laid down on the same substrate. Theexposed substrate, now labeled W” is unloaded from the apparatus at step 220, to undergo etching or other processes, in accordance with the exposed pattern.

[0035] The skilled person will know that the above description is a simplified overview of a number of very detailed steps involved in one example of a real manufacturing situation. For example ratherthan measuring alignment in a single pass, often there will be separate phases of coarse and finemeasurement, using the same or different marks. The coarse and / or fine alignment measurement steps can be performed before or after the height measurement, or interleaved. Company Secret

[0036] In the manufacture of complex devices, typically many lithographic patterning steps are performed, thereby forming functional features in successive layers on the substrate. A critical aspect of performance of the lithographic apparatus is therefore the ability to place the applied pattern correctly and accurately in relation to features laid down in previous layers (by the same apparatus or a different lithographic apparatus). For this purpose, the substrate is provided with one or more sets of marks. Each mark is a structure whose position can be measured at a later time using a position sensor, typically an optical position sensor. The position sensor may be referred to as “alignment sensor” and marks may be referred to as “alignment marks”.

[0037] A lithographic apparatus may include one or more (e.g. a plurality of) alignment sensors by which positions of alignment marks provided on a substrate can be measured accurately. Alignment (or position) sensors may use optical phenomena such as diffraction and interference to obtain position information from alignment marks formed on the substrate. An example of an alignment sensor used in current lithographic apparatus is based on a self-referencing interferometer as described in US6961116. Various enhancements and modifications of the position sensor have been developed, for example as disclosed in US2015261097A1. The contents of all of these publications are incorporated herein by reference.

[0038] A mark, or alignment mark, may comprise a series of bars formed on or in a layer provided on the substrate or formed (directly) in the substrate. The bars may be regularly spaced and act as grating lines so that the mark can be regarded as a diffraction grating with a well-known spatial period (pitch). Depending on the orientation of these grating lines, a mark may be designed to allow measurement of a position along the X axis, or along the Y axis (which is oriented substantially perpendicular to the X axis). A mark comprising bars that are arranged at +45 degrees and / or -45 degrees with respect to boththe X- and Y-axes allows for a combined X- and Y- measurement using techniques as described inUS2009 / 195768A, which is incorporated by reference.

[0039] The alignment sensor scans each mark optically with a spot of radiation to obtain a periodically varying signal, such as a sine wave. The phase of this signal is analyzed, to determine the position of the mark and, hence, of the substrate relative to the alignment sensor, which, in turn, is fixated relative to a reference frame of a lithographic apparatus. So-called coarse and fine marks may be provided, related to different (coarse and fine) mark dimensions, so that the alignment sensor can distinguish between different cycles of the periodic signal, as well as the exact position (phase) within a cycle. Marks of different pitches may also be used for this purpose.

[0040] Measuring the position of the marks may also provide information on a deformation of the substrate on which the marks are provided, for example in the form of a wafer grid. Deformation of the substrate may occur by, for example, electrostatic clamping of the substrate to the substrate table and / or heating of the substrate when the substrate is exposed to radiation.

[0041] Figure 3 is a schematic block diagram of an embodiment of a known alignment sensor AS. Radiation source RSO provides a beam RB of radiation of one or more wavelengths, which is diverted Company Secretby diverting optics onto a mark, such as mark AM located on substrate W, as an illumination spot SP. In this example the diverting optics comprises a spot mirror SM and an objective lens OL. The illumination spot SP, by which the mark AM is illuminated, may be slightly smaller in diameter than the width of the mark itself.

[0042] Radiation diffracted by the mark AM is collimated (in this example via the objective lens OL) into an information-carrying beam IB. The term “diffracted” is intended to include complementary higher diffracted orders; e.g.,: +1 and -1 diffracted orders (labelled +1, -1) and optionally zero-orderdiffraction from the mark (which may be referred to as reflection). A self-referencing interferometerSRI, e.g. of the type disclosed in US6961116 mentioned above, interferes the beam IB with itself after which the beam is received by a photodetector PD. Additional optics (not shown) may be included to provide separate beams in case more than one wavelength is created by the radiation source RSO. The photodetector may be a single element, or it may comprise a number of pixels, if desired. The photodetector may comprise a sensor array.

[0043] The diverting optics, which in this example comprises the spot mirror SM, may also serve to block zero order radiation reflected from the mark, so that the information-carrying beam IB comprises only higher order diffracted radiation from the mark AM (this is not essential to the measurement, but improves signal to noise ratios).

[0044] SRI Intensity signals SSI are supplied to a processing unit PU. By a combination of optical processing in the self-referencing interferometer SRI and computational processing in the unit PU,values for X- and Y-position on the substrate relative to a reference frame are output.

[0045] A single measurement of the type illustrated only fixes the position of the mark within a certain range corresponding to one pitch of the mark. Coarser measurement techniques are used in conjunction with this to identify which period of a sine wave is the one containing the marked position. The same process at coarser and / or finer levels are repeated at different wavelengths for increased accuracy and / or for robust detection of the mark irrespective of the materials from which the mark is made, and materials on and / or below which the mark is provided. Improvements in performing and processing such multiple wavelength measurements are disclosed below.

[0046] In the context of wafer alignment, the following approaches are in use or have been proposed to correct the mark position for mark asymmetry (asymmetry in the alignment mark which results in aposition error or offset): OCW (Optimal Color Weighing- described in more detail in US publicationUS2019 / 0094721 A1 which is incorporated herein by reference), OCIW (Optimal Color and IntensityWeighing - described in more detail in PCT publication WO 2017032534 A2) and WAMM (WaferAlignment Model Mapping - described in more detail in PCT publications WO 2019001871 A1 andWO 2017060054 A1). In each of these cases, training to reference data is needed or desired. This means that these corrections can only be accurately performed if sufficient training data is available and if the process variations in the training data are representative for the variations in the wafers that need to be corrected. This reference data may be measured by a reference sensor, e.g., hindsight overlay data. Company Secret

[0047] To address this, it has previously been proposed to use a spectral dispersion model(s) and / or spectral dispersion equation(s) and / or spectral dispersion approximation(s), such as the Forouhi- Bloomer dispersion equation(s) / approximation(s), a Lorentz oscillator based dispersion model, one or more Kramers-Kronig dispersion relations and / or one or more Plemelj dispersion relations, to convert measured intensity asymmetry information (typically measured for a limited number of wavelengths) in a position correction for mark asymmetry. Such methods are disclosed in WO2021 / 122016, which isincorporated herein by reference. Such methods may comprise mathematically calculating the intensityasymmetry value into a phase offset value corresponding to mark asymmetry (e.g., calculating the phase offset from intensity asymmetry using physics principles). This may be implemented on an alignment sensor (but alternatively also on an overlay sensor, a focus sensor and / or a leveling sensor) having a functionality to measure the intensity of the positive diffraction order and of the negative diffraction order. As such, the intensity measurements in this context relate to the diffracted orders +1, -1 (and notSRI intensity signals SSI of Figure 3). This dispersion model(s) and / or dispersion equation(s) and / ordispersion approximation(s) based approach results in a physics first principles based approach, the benefit of which is that it avoids (or at least partially) the need for a training to reference data obtained by a sensor other than the alignment sensor itself.

[0048] Asymmetry in an alignment mark results in a phase offset which is additional to the position information encoded in the phase data measured by the alignment sensor. The intensity asymmetry is resultant only from the mark asymmetry. Therefore, the method proposed herein may use a dispersion model per diffraction order (e.g., +1 and -1 diffraction order) to describe the measured intensity asymmetry (or related parameter such as amplitude asymmetry), but alternatively diffractions orders pairs might also be combined, e.g., summed or subtracted. By fitting the dispersion models to the measured intensity data (e.g., for a number of illumination conditions such as wavelengths / polarizations or combinations thereof), an estimate for a phase offset (per wavelength or common to all wavelengths) which is equivalent to the intensity / amplitude asymmetry can be determined. This phase offset can be used to determine a corresponding position offset or correction. Measurement of intensity asymmetry at different illumination conditions may be performed in parallel or sequentially.

[0049] Note that any mention of determining a phase offset value or phase offset can be read as determining a position correction as the terms are synonymous.

[0050] Determining a generic dispersion model may comprise modeling the change in refractive index (or similar parameter) against wavelength / polarization (or propagation direction in the material) for each target using the measurement data from each wavelength and polarization combination. The model may then comprise a model of the target as a transmission function.

[0051] Figure 4 illustrates a measurement of an alignment mark AM. An example stack ST comprises one or more layers L, each having respective properties such as refractive indices and thicknesses, and the alignment mark AM beneath these layers L. The stack is measured using alignment illuminationdescribed by a (frequency ^ dependent) electric field ^^^^(^), and the captured diffraction orders areCompany Secretrespectively described by electric fields ^ି(^),^ା(^). In each case, the frequency ^ describes theangular frequency ^ = 2^^ / ^ (where c is the speed of light and λ is measurement wavelength). Thesefield variables are (typically) complex value variables, which comprise both amplitude and phase information on the electric (scalar) field. For an optional (full) vectorial electric field treatment, each (orthogonal) electric field component may be processed individually, in turn. In addition, these individual problems may optionally be coupled, so as to exploit commonalities (e.g., a common dispersion model parameter).

[0052] In a paper, “A fast Fourier transform implementation of the Kramers-Kronig relations: Application to anomalous and left handed propagation” Lucas et al AIP Advances 2, 032144 (2012)(incorporated herein by reference), the Kramers-Kronig relations have been derived from simplecausality considerations. Taking this teaching and applying it to complex fields coming from analignment mark, a phase offset ^ା(^) − ^ି(^), and therefore alignment position deviation (APD)Δ(^), of an alignment mark can be expressed solely in terms of the measured intensities:^ ^ ∗ା(^) − ^ି(^) = ଶ (^^(H்(log(^ା^^ା^ ) − log(^ ∗ି^^ି^ )))^Δ(^) =8^^ (^^(H்(log(^ା^) − log(^ି^)))where H்is the Hilbert transform: see sections II.A and II.B from the aforementioned Lucas article. Note the Im() operator is not essential, it is added to suppress numerical noise.

[0053] This formula was used in patent application WO2021 / 122016 (incorporated herein byreference) to compute an expected alignment error based on intensity asymmetries measured on that specific alignment mark for a number of different illumination settings. This document describes using a Kramers-Kronig based dispersion relationship to transform measured intensity asymmetry information into phase / position information and vice versa. Such methods make use of a dispersion transform relation that exists between the measured pupil intensity asymmetry and the position error due to mark deformation (target deformation or structure deformation). This enables correction of the measured alignment mark position for any mark deformation, via transforming the measured intensity asymmetry into a position correction.

[0054] As the dispersion transforms can be stated in terms of Fourier transforms, these dispersion relations are linear (as is the Fourier transform). As such, it was proposed in patent application WO2023 / 036521 (incorporated herein by reference) to exploit only (or mainly) the linearity of the dispersion relations, without making (mostly) use of the dispersion transform relations themselves. This allows the computation of the unknown wafer deformations (e.g., of typically one whole wafer; i.e., a wafer-level Kramers-Kronig based algorithm) to be posed (for example) as a rank minimization problem. In order to exploit the linearity, the methodology proposed therein requires measurements of multiple alignment marks located on a single wafer or more wafers. Company Secret

[0055] The intensity asymmetry measurements indicate the number of dispersion eigenvectors required to decompose the mark deformation induced dispersion across the wafer. An incorrectly calculated wafer deformation will likely increase this number of dispersion eigenvectors. This increase can be even better observed when both polarizations are included and / or when more than one target type is measured per wafer location (multi-target).

[0056] Some of the basic physics behind the concepts disclosed in WO2023036521A1 and / orWO2021 / 122016 will now be described. Referring back to Figure 4, the diffracted (complex) fields

[0057] The alignment sensor may measure the diffraction order phase difference ^ା(^) − ^ି(^),which yields the measured alignment position deviation (APD):where P is the mark pitch.

[0058] This measured position deviation is a combination of wafer deformation Δ^^, which is mechanical and therefore not wavelength dependent, and mark deformationwhich is wavelength dependent: Δ(^) = sgnIt is the wafer deformation that is the parameter of interest, while the mark deformation is a nuisance parameter. Consequently, an aim of the proposed method is to determine the wafer deformation by itself, without the mark deformation impact.

[0059] In addition to measuring the phase difference ^ା(^) − ^ି(^), an alignment sensor maycomprise alignment channels to measure the intensity ^ା(^), ^ି(^) of each of the (respectively positiveand negative) diffraction orders individually, or sum of the intensities of two or more positive andnegative diffraction orders. Intensity channels are sensitive for mark deformationandinsensitive for wafer deformation Δ^^as wafer deformation (i.e., a grating position shift) impacts the diffraction order phase only. Company Secret

[0060] The Fourier transform Fି^{^±(^)}(^) is causal and it can be mathematically proven thatFି^(^) is causal. Based on the measured signals ^ା(^) − ^ି(^), ^ା(^), ^ି(^), acoupling function ^(^) may be constructed having a causal Fourier transform Fି^{^(^)}(^):sgn(^) ∙ Δ^^)where ^(^) is the measured asymmetry indicator (where n denotes the diffraction order, e.g., n=1 forthe first order diffraction, and P is the period of the alignment mark):^^(^) =8^^ ∙ log

[0061] The units of Δெ^(^) and ^(^) are equal, and ^(−^) = ^(^), which follows from timereversal symmetry. A causal signal satisfies ^(^) = ^(^) g(t), where the impulse response ^(^) =Fourier transforming this yields the convolution:where H{^} denotes the Hilbert transform.

[0062] Inserting the coupling function ^(^) = ^(^) + ^ Plemelj dispersionrelations:

[0063] These dispersion relations translate the measured asymmetry indicator ^(^) into the unknownmark deformation Δெ^(^) and vice versa. The calculated mark deformation Δெ^(^) can be subtractedfrom the measured alignment data (e.g., per wavelength), i.e., subtracted from the alignment position deviation (APD).

[0064] For example, rewriting the first of these dispersion relations into its Kramers-Kronig integralform yields:Company Secret

[0065] However, there is a drawback in the methods disclosed in these methods. While the physics behind the concepts described therein is sound, there are practical limitations when applying the concepts to alignment metrology in a real-world setting. In particular, there are only a limited number of wavelengths (e.g., 12 discrete wavelengths in the visible range) available for a measurement. The Hilbert transform relation described by Equation (6a) can therefore in practice only be numerically evaluated using the (e.g., 12) measured wavelengths, leading to an unacceptable inaccuracy. Recipe training using overlay reference data may be employed to address this inaccuracy, e.g., training of theOCIW (Optimal Color and Intensity Weighting) weights. However, this requires the collection ofoverlay reference data and an additional training step.

[0066] To address these issues, a method of metrology is proposed which comprises: obtaining first metrology data (e.g., alignment data or APD data) relating to a respective measurement of one or more marks on a substrate; obtaining intra-mark position dependent metrology data (e.g., intra-mark position dependent intensity data from a pair of complementary diffraction orders diffracted from the mark) relating to a plurality of intra-mark locations within each of said one or more marks; determining, from said intra-mark position dependent metrology data, intra-mark variation metric data describing an intra- mark variation for each of said one or more marks; and determining corrected metrology data (corrected alignment data or corrected APD data) from said first metrology data and said intra-mark variation metric data.

[0067] To implement this, a new spatial dispersion relation is proposed, which can be derived fromfirst principles, that describes an intra-mark variation metric Δூெ^(^) in terms of asymmetry indicator^(^):

[0068] Asymmetry indicator ^(^) is as defined by Equation 4, but with a spatial dependence, as theintensities on which it is based are measured at a plurality of different spatial locations (different ^); i.e., different intra-mark scan sample positions ^: ^^(^) =8^^ ∙ log

[0069] Of course, the diffraction order intensities ^ା(^, ^), ^ି(^, ^) and therefore the asymmetryindicator ^(^, ^) are also dependent on the acquisition setting; however the measurements of differentacquisition settings can be treated individually for the methods proposed herein. An acquisition setting may comprise a polarization and wavelength combination of the illumination used to measure the mark. Each mark may be measured with different acquisition settings, which may comprise measuring with Company Secretvarious different wavelengths only, various different polarizations only or different combinations where both wavelength and polarization are varied.

[0070] It can be appreciated that the spatial coordinate ^ is used arbitrarily for convenience on this 1-dimensional example. The 1-dimensional examples described herein are equally applicable to the ^direction as will be apparent to the skilled person. Additionally, 2-dimensional (^,^) and 3-dimensional(^, ^, ^) will also be described.

[0071] The spatial dispersion relation described by Equation (8) can be numerically evaluated using all measured intra-mark samples available to the FFT (Fast Fourier Transform) phase fit. These samples typically number hundreds, e.g., a typical number of measured intra-mark samples is between 200 and 900, between 300 and 500 or between 350 and 400. This leads to an improved accuracy compared to the accuracy of the spectral dispersion relation described by Equation (6a), which can in practice only be computed using 12 measured wavelengths. As an FFT may be used for the phase fit, it may be attractive / beneficial to use the FFT form:where F{^} denotes the Fourier transform, Fି^{^} its inverse and ^௫ is the spatial frequencycoordinate corresponding with intra-mark scan sample position ^ (the corresponding spatial frequencycoordinate ^௬ is used for the y-direction ^).

[0072] A pupil stop of the metrology apparatus (e.g., the alignment sensor) may be used to provide the necessary masking of the electric field in the pupil, so that spatial dispersion relation (8) applies. Pupil masking is the analogy of temporal causality, and as spectral dispersion relation (6a) originates from temporal causality, spatial dispersion relation (8) originates from pupil masking. As such, themeasurements may be made (and later computed) as a function of intra-mark position ^ and not angularfrequency ^ as before. Therefore, it is now possible to measure only one wavelength and still make useof spatial dispersion relation (8).

[0073] The spatial dispersion relation described by Equation (8) (and its FFT form (10)) assumes that|^(^)| ≪ ^, to ensure convergence. It can be further recognized that, as the Hilbert transform of aconstant is zero, a constant intensity asymmetry ^(^) is interpreted by spatial dispersion relation (8)and its FFT form (10) as a zero intra-mark position variation (Δூெ^ = 0). This is why spatial dispersionrelation (8) and its FFT form (10) do not yield mark deformation in the manner of spectral dispersion relation (6a), but instead yields intra-mark variation.

[0074] It is proposed that the intra-mark variation metric Δூெ^(^), e.g., as determined via spatialdispersion relation (8) or its FFT form (10), may be subtracted from the measured aligned position (i.e., subtracted from the APD) thereby removing the intra-mark variation. Company Secret

[0075] The subtraction may be done per acquisition setting (e.g., per wavelength and / or per polarization or per combination thereof). This means that the proposed subtraction of the intra-mark variation may additionally be combined with existing spectral weighting algorithms such as OCW (Optimal Color Weighting), WQ-weighting or MCLR (Multi Color Lowest Residual). This can result in a stabilization of spectral weighting algorithms, as the removal of intra-mark variation partially removes mark-to-mark, wafer-to-wafer and / or lot-to-lot variations (i.e., the contribution from intra- mark variation).

[0076] This subtraction may use the same signal processing for the intensity data measured via the “intensity channels” (used to measure the asymmetry parameter and therefore the intra-mark variation metric) and the alignment data / APD or phase data measured by the “phase channels”. This may comprise, for example, applying any apodization applied in the FFT phase fit to the processing of the intensity channels. This may be done to prevent any influence on the accuracy of spatial dispersion relation (8) and / or its FFT form (10), which are linear dispersion relations (as the Hilbert transform is a linear transform).

[0077] To improve the aligned position ODCOM (Optical Density Center Of Mass) position to the overlay POI (Point Of Interest) position, non-linear weighting terms may be included in the spectral weighting algorithm, such as a weighting according to one of the aforementioned spectral weighting algorithms. For example, the following framework may be used in which function g() is optimized forperformance and may include non-linear terms, so as to point to the POI and not ODCOM.^ = ^ற ∙ ^ + ^൫^ − ^ற ∙ ^, ^൯ (11)where ^ are the measured positions (per color and polarization) weighted with weights ^ to result inposition ^. Here function ^() makes use of all information not yet used by the linear color weighting.

[0078] The above description provides a one-dimensional example, and of course, is equally applicable to the other substrate plane dimension (e.g., y) to determine intra-mark variation metric Δூெ^(^). A two-dimensional extension can be envisaged, which can be applied, for example, to a camera based wafer alignment sensor. Such a two-dimensional extension may comprise, in essence, a one- dimensional processing of a one-dimensional cross-section of the two-dimensional data, e.g., performed per dimension. As such, the dispersion relation in this 2D case is still a 1D dispersion relation determined in accordance with Equations (8) and (9).

[0079] The proposed method is compatible with measurement methods employing an angled scan of the mark grating (i.e., an oblique scan direction at an angle not aligned with either the x axis or y axis).In this approach, the mark period P used in determining the intra-mark variation metric may be anapparent grating period set equal to the apparent (i.e., not physical) period of the target grating (i.e., the apparent period in the x (or y) direction as appropriate). This apparent period can be determined by Company Secretsimple trigonometry. As such, Equation (9) will be dependent on x and y (as the intensities will be so dependent) and the period may be the apparent period. The apparent period, for example, may scale with the cosine of the angle between the cross-section direction and the grating physical period direction. It is also possible, as an alternative, to omit the scaling factor based on the mark pitch, as it applies equally to mapping the phase to position and intensity to asymmetry indicator ^.

[0080] Additionally, a three-dimensional dispersion relation can be constructed by combining spectral dispersion relation (6a) and spatial dispersion relation (8). The result, in FFT form, is: Δି^ூெ^(^, ^,^) = −Im ^F ^sgn(^) ∙ sgn(^௫) ∙ sgn൫^௬൯ ∙ F {^(^, ^,^)}^^ (12)where t denotes the time coordinate corresponding with angular frequency ^. Here, the intensities andtherefore ^ is now also dependent on wavelength. Again the pitch may be the apparent pitch, or thescaling factor omitted.

[0081] Figure 5 is a flowchart describing an alignment method using the concepts described. At step500, an alignment measurement is made to obtain alignment data Δ (e.g., first metrology data, positiondata or APD data). Optionally, the alignment data may be obtained using multiple wavelengths and / or polarizations to obtain alignment data Δ(^), as has been described. This step may be performed based on the phase channels of a metrology apparatus, e.g., in accordance with Equation (2) above.

[0082] At step 510, intra-mark position dependent metrology data or intensity data ^ା(^), ^ି(^) (e.g.,optionally obtained using multiple wavelengths and / or polarizations ^ା(^, ^), ^ି(^, ^)) may bemeasured via intensity channels of the metrology apparatus for various intra-mark scan sample positionsor intra-mark positions ^ per alignment mark.

[0083] The intensity data ^ା(^), ^ି (^) may be used to determine intra-mark variation metric Δூெ^(^).For example, this intra-mark variation metric Δூெ^(^) can be calculated using any of the equations (8),(10) or (12); i.e., to calculate 1D intra-mark variation metric Δூெ^(^) / Δூெ^(^), 2D intra-mark variationmetric Δூெ^(^, ^) or 3D intra-mark variation metric Δூெ^(^, ^, ^). For brevity, Δூெ^(^) will be usedin the foregoing steps, although it can be appreciated that the other examples may be used.

[0084] At step 520, an aligned position error Δூெ^ may be calculated from the intra-mark variationmetric Δூெ^(^). For example, this may be calculated according to:where ^^ the coordinates where the signal samples were acquired and ^ is an apodization function,chosen such that analogy with phase channel signal processing is preserved (as has been described above). Company Secret

[0085] In calculating intra-mark variation metric from asymmetry indicator , for example using Equation (8), the noise correction procedures may be employed so as to be analogous with phase channel signal processing. This noise correcting procedures may for example address laser intensity noise, photon / electron shot noise and electronics noise.

[0086] At step 530, A corrected aligned position Δ^^^^(corrected APD data), with intra-mark variation removed, may be determined according to: Δ^^^^ = Δ − Δூெ^ (14)

[0087] This method may be performed per alignment mark for all alignment marks on a substrate.

[0088] As an alternative to, or an addition to the above correction, the intra-mark variation metricΔூெ^(^) may be used to determine one or more KPIs (Key Performance Indicators) for intra-markvariation. An example of a (mathematical norm based) intra-mark variation KPI may comprisedetermining the maximum absolute intra-mark position deviation max|Δூெ^(^)| over all intra-markscan sample positions ^. Another example of such a KPI may comprise an root-mean-square sum ofΔூெ^(^) over all intra-mark sample positions ^. Alternatively or in addition, the KPI determinationmay comprise determining a curvature of intra-mark variation metric in terms of an appropriate low order model (e.g., low order polynomial fit or limiting to only low spatial frequencies), determining similarity or dissimilarity of the intra-mark variation metric data between each of said one or moremarks within a given wafer or across wafers (e.g., by using an appropriate variance minimizationmethod such as a principal component analysis), and / or evaluating information content of the intra- mark variation metric data between each of the marks within a given wafer or across wafers in terms of an effective rank or norm.

[0089] Such a KPI may be used for process monitoring and / or for recipe creation (e.g., wavelength, polarization and / or mark selection). Note that as the mark position will only be affected by the even component of the intra-mark variation and not by the odd component, a method may comprise using only the even component of the intra-mark variation in, for example, determining the KPI.

[0090] The above description has described the concepts in term of removal of the intra-mark variation from phase channel measurements. As a possible extension / variation, the intra-mark variation may be removed from the intensity channel data, e.g., via the alternative spatial dispersion relation: ^ெை^(^) = −H{Δ(^)}, (15)where Δ(^) describes the measured aligned positions (intra-mark position dependent metrology data inthis example) and ^ெை^(^) describes the modelled intensity asymmetry (for correcting the measuredintensity asymmetry, or first metrology data in this example). Company Secret

[0091] This provides both phase channel data and intensity channel data, each of which having been corrected to remove the intra-mark variation. This provides an opportunity to combine this extendedremoval of intra-mark variation with existing spectral weighting algorithms that also make use ofintensity channels, such as OCIW, leading to a stabilization of OCIW, as the removal of intra-mark variation reduces mark-to-mark, wafer-to-wafer and / or lot-to-lot variations.

[0092] The proposed method provides a first principles physics-based position correction based on an intra-mark variation metric describing signal variation (e.g., asymmetry metric variation) which varies spatially with position within the alignment mark.

[0093] The term “mark” may describe any mark or target formed on a substrate, including those specifically formed for the purposes of metrology (whether position / alignment metrology or otherwise) or any other structure used for the purposes of metrology (including actual product structure). The terms mark and target may be used synonymously and interchangeably.

[0094] The methods described herein may be used for other forms of metrology such as overlay metrology, e.g., to remove the effects of inter-mark (or inter-target) variation in overlay metrology.

[0095] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described.

[0096] While the above concepts are described in terms of wafer alignment, they are applicable to other metrology aspects such as, for example: 1) overlay metrology, 2) focus metrology and 3) wafer levelling.

[0097] 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 supplied 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.

[0098] 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.

[0099] 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.

[0100] Aspects of the invention are set out in the clauses below. Company Secret1. A metrology method, comprising: obtaining first metrology data relating to a respectivemeasurement of one or more marks on a substrate; obtaining intra-mark position dependent metrology data relating to a plurality of intra-mark locations within each of said one or more marks; determining, from said intra-mark position dependent metrology data, intra-mark variation metric data describing an intra-mark variation for each of said one or more marks; and determining corrected metrology data from said first metrology data and said intra-mark variation metric data.2. A method according to clause 1, wherein said intra-mark position dependent metrology datacomprises intensity data describing an intensity of at least one diffraction order diffracted from a plurality of intra-mark locations within each of said one or more marks.3. A method according to clause 1 or 2, further comprising, for each said one or more mark:determining an asymmetry metric from a comparison and / or ratio of each diffraction order of a pair of complementary higher diffracted orders diffracted from said mark; and determining the intra-mark variation metric data from each said asymmetry metric.4. A method according to clause 3, wherein the step of determining said intra-mark variation metricdata comprises applying a transform to said asymmetry metric.5. A method according to clause 4, wherein said transform is a Hilbert transform or an equivalent FastFourier transform.6. A method according to any preceding clause, comprising: determining an aligned position error foreach of the one or more marks from said intra-mark variation metric data; and correcting each said respective measurement of the one or more marks by applying the corresponding aligned position error to each said respective measurement.7. A method according to any preceding clause, wherein said first metrology data and intra-markposition dependent metrology data are obtained for a plurality of acquisition settings; and said method is performed individually for said plurality of acquisition settings.8. A method according to clause 7, comprising determining a weighting per said acquisition settingfor said first metrology data; and determining said corrected metrology data from said first metrology data as weighted by said weighting.9. A method according to any preceding clause, wherein said one or more marks comprise alignmentmarks.10. A method according to any preceding clause, wherein said first metrology data comprises positionor alignment data and said corrected metrology data comprise corrected position or alignment data.11. A method according to clause 10, wherein said position or alignment data is determined from phasemeasurements of said one or more marks.12. A method according to any of clauses 1 to 8, wherein said first metrology data comprises positionor alignment data and said corrected metrology data comprise corrected position or alignment data13. A method according to clause 12, wherein said one or more marks comprise overlay targets.Company Secret14. A method according to any preceding clause, comprising determining at least one performanceindicator from said intra-mark variation metric data.15. A method according to clause 14, comprising determining said at least one performance indicatorfrom a maximum absolute intra-mark variation over said plurality of intra-mark locations for each of said one or more marks.16. A method according to clause 14 or 15, comprising determining said at least one performanceindicator from a root-mean-square sum of said intra-mark variation data over said plurality of intra- mark locations for each of said one or more marks.17. A method according to clause 14, 15 or 16 comprising determining a measurement recipe based onsaid at least one performance indicator.18. A method according to any preceding clause, wherein said first metrology data comprises intensitydata describing an intensity of at least one diffraction order; and said intra-mark position dependent metrology data comprises phase, position or alignment data.19. A method according to any preceding clause, wherein said first metrology data comprises twodimensional metrology data, said intra-mark position dependent metrology data comprises two dimensional intra-mark position dependent metrology data, and said method comprises determining, from said two dimensional intra-mark position dependent metrology data, two dimensional intra-mark variation metric data.20. A method according to any preceding clause, wherein said first metrology data comprises threedimensional metrology data, said intra-mark position dependent metrology data comprises three dimensional intra-mark position dependent metrology data, and said method comprises determining, from said three dimensional intra-mark position dependent metrology data, three dimensional intra- mark variation metric data, wherein said third dimension relates to an acquisition setting.21. A method according to any preceding clause, comprising determining a correction for a subsequentlithographic exposure based on the corrected metrology data.22. A method according to clause 21, comprising performing said subsequent lithographic exposure onone or more subsequent substrates.23. A method of determining a performance indicator, comprising: obtaining intra-mark positiondependent metrology data relating to a plurality of intra-mark locations within one or more marks on a substrate; determining, from said intra-mark position dependent metrology data, intra-mark variation metric data describing an intra-mark variation for each of said one or more marks; and determining, from said intra-mark variation metric data, at least one performance indicator.24. A method according to clause 23, comprising determining said at least one performance indicatorfrom a maximum absolute intra-mark variation over said plurality of intra-mark locations for each of said one or more marks. Company Secret25. A method according to clause 23 or 24, comprising determining said at least one performanceindicator from a root-mean-square sum of said intra-mark variation metric data over said plurality of intra-mark locations for each of said one or more marks.26. A method according to clause 23, 24 or 25, comprising determining said at least one performanceindicator from one or more of: determining a curvature of intra-mark variation metric data in terms of a low order model, determining similarity or dissimilarity of the intra-mark variation metric data between each of said one or more marks within a given substrate or across substrates; and / or evaluating information content of the intra-mark variation metric data between each of the one or more marks within a given wafer or across wafers in terms of an effective rank or norm.27. A method according to any of clauses 23 to 26, wherein said intra-mark position dependentmetrology data comprises intensity data describing an intensity of at least one diffraction order diffracted from a plurality of intra-mark locations within each of said one or more marks.28. A method according to any of clauses 23 to 27, further comprising, for each said one or more mark:determining an asymmetry metric from a comparison and / or ratio of each diffraction order of a pair of complementary higher diffracted orders diffracted from said mark; and determining the intra-mark variation metric data from each said asymmetry metric.29. A method according to clause 28, wherein the step of determining said intra-mark variation metricdata comprises applying a transform to said asymmetry metric.30. A method according to clause 29, wherein said transform is a Hilbert transform or an equivalentFast Fourier transform.31. A computer program comprising program instructions operable to perform the method of any ofclauses 1 to 30, when run on a suitable apparatus.32. A non-transient computer program carrier comprising the computer program of clause 31.33. A processing system comprising a processor and a storage device comprising the computer programof clause 31.34. An alignment sensor comprising a processing system as claimed in clause 33.35. An exposure apparatus comprising: a patterning device support for supporting a patterning device;a substrate support for supporting a substrate; and the alignment sensor of clause 34.

[0101] 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. Company Secret

Claims

CLAIMS1. A metrology method, comprising:obtaining first metrology data relating to a respective measurement of one or more marks on a substrate; obtaining intra-mark position dependent metrology data relating to a plurality of intra-mark locations within each of said one or more marks; determining, from said intra-mark position dependent metrology data, intra-mark variation metric data describing an intra-mark variation for each of said one or more marks; and determining corrected metrology data from said first metrology data and said intra-mark variation metric data.

2. A method as claimed in claim 1, wherein said intra-mark position dependent metrology datacomprises intensity data describing an intensity of at least one diffraction order diffracted from a plurality of intra-mark locations within each of said one or more marks.

3. A method as claimed in claim 1 or 2, further comprising, for each said one or more mark:determining an asymmetry metric from a comparison and / or ratio of each diffraction order of a pair of complementary higher diffracted orders diffracted from said mark; and determining the intra-mark variation metric data from each said asymmetry metric.

4. A method as claimed in claim 3, wherein the step of determining said intra-mark variationmetric data comprises applying a transform to said asymmetry metric.

5. A method as claimed in claim 4, wherein said transform is a Hilbert transform or anequivalent Fast Fourier transform.

6. A method as claimed in any preceding claim, comprising:determining an aligned position error for each of the one or more marks from said intra-mark variation metric data; and correcting each said respective measurement of the one or more marks by applying the corresponding aligned position error to each said respective measurement.

7. A method as claimed in any preceding claim, wherein said first metrology data and intra-markposition dependent metrology data are obtained for a plurality of acquisition settings; and said method is performed individually for said plurality of acquisition settings. Company Secret8. A method as claimed in claim 7, comprising determining a weighting per said acquisitionsetting for said first metrology data; and determining said corrected metrology data from said first metrology data as weighted by saidweighting.

9. A method as claimed in any preceding claim, wherein said first metrology data comprisesposition or alignment data and said corrected metrology data comprise corrected position or alignment data.

10. A method of determining a performance indicator, comprising:obtaining intra-mark position dependent metrology data relating to a plurality of intra-mark locations within one or more marks on a substrate; determining, from said intra-mark position dependent metrology data, intra-mark variation metric data describing an intra-mark variation for each of said one or more marks; and determining, from said intra-mark variation metric data, at least one performance indicator.

11. A method as claimed in claim 10, comprising determining said at least one performanceindicator from a maximum absolute intra-mark variation over said plurality of intra-mark locations foreach of said one or more marks.

12. A method as claimed in claim 10 or 11, comprising determining said at least one performanceindicator from a root-mean-square sum of said intra-mark variation metric data over said plurality of intra-mark locations for each of said one or more marks.

13. A method as claimed in claim 10, 11 or 12, comprising determining said at least oneperformance indicator from one or more of: determining a curvature of intra-mark variation metric data in terms of a low order model, determining similarity or dissimilarity of the intra-mark variation metric data between each of said one or more marks within a given substrate or across substrates; and / or evaluating information content of the intra-mark variation metric data between each of the one or more marks within a given wafer or across wafers in terms of an effective rank or norm.

14. A computer program comprising program instructions operable to perform the method of anyof claims 1 to 13, when run on a suitable apparatus.

15. A processing system comprising a processor and a storage device comprising the computerprogram of claim 14. Company Secret

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