Substrate comprising a target arrangement and associated at least one patterning device, lithographic method and metrology method

Abstract

A substrate and associated patterning device are disclosed. The substrate comprises at least one target arrangement suitable for metrology of a lithographic process, the target arrangement comprising at least one pair of similar target areas, the at least one pair of similar target areas being arranged such that the target arrangement is centrosymmetric, or at least the target areas for measurement in a single direction together are centrosymmetric. A metrology method for measuring the substrate is also disclosed. A metrology method is also disclosed, comprising measuring such a target arrangement and determining a value of a parameter of interest from the scattered radiation, while correcting for a deformation of the metrology apparatus used.

Description

[0001] Cross Reference to Related Applications

[0002] This application claims priority to European application 20173476.1 filed on May 7, 2020 and European application 20182160.0 filed on June 25, 2020, the contents of which are incorporated by reference herein in their entirety. TECHNICAL FIELD

[0003] The present invention relates to a target arrangement for metrology of a lithographic process, and to a method for measuring a parameter of a lithographic process. BACKGROUND

[0004] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. The 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) can 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) of the substrate (e.g., a silicon wafer). Typically, the pattern is transferred by imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. Typically, a single substrate will contain a network of adjacent target portions each to be imaged with an individual pattern. 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 alignment accuracy of two layers in a device. Overlay can be described in terms of the degree of misalignment between the two layers, e.g., a measurement of overlay of 1 nm can describe a situation where the two layers are misaligned by 1 nm.

[0005] 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— for example, intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of angle of reflection; or polarization as a function of angle of reflection— to obtain a “spectrum” that can be used to determine a property of interest of the target. The determination of the property of interest can be performed by various techniques: reconstruction of the target, e.g., by iterative methods such as rigorous coupled wave analysis or finite element method; library search; and principal component analysis.

[0006] The targets used by conventional scatterometers are relatively large gratings, for example 40 pm by 40 pm, and the measurement beam produces a spot that is smaller than the grating (i.e. the grating is underfilled). This situation simplifies the mathematical reconstruction of the target because the target can be considered to be infinite. However, to reduce the size of the target, for example to 10 pm by 10 pm or smaller, for example so that it can be positioned in a product feature rather than a scribe line, it has been proposed to measure the grating to be smaller than the measurement spot (i.e. the grating is overfilled). These targets are typically measured using dark- field scatterometry, where the zeroth diffraction order (corresponding to the specular reflection) is blocked and only the higher orders are processed. Examples of dark-field metrology can be found in international patent applications WO 2009 / 078708 and WO 2009 / 106279, the contents of whose files are hereby incorporated by reference. Further developments of the technique have been described in patent publications US20110027704A, US20110043791A and US20120242970A. Modifications to the apparatus to improve throughput are described in US2010201963A1 and US2011102753A1. 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 can be surrounded by product structures on the wafer. The targets can comprise multiple gratings that can be measured in one image.

[0007] In known metrology techniques, an overlay measurement is obtained by measuring an overlay target twice under certain conditions, while rotating the overlay target or changing the illumination mode or imaging mode to separately obtain -1storder intensity and +1storder intensity. The asymmetry in intensity with respect to a given overlay target (comparison of these diffraction order intensities) provides a measurement of the asymmetry in the target. This asymmetry in the overlay target can be used as an indicator of overlay (unwanted misalignment of two layers).

[0008] Known methods of metrology measurement can be adversely affected by the metrology apparatus, for example, defects in the optical column can induce distortions in the images formed which can adversely affect the metrology obtained. SUMMARY

[0009] It would be desirable to be able to perform metrology of a lithographic process with improved accuracy.

[0010] In a first aspect, the present invention provides a substrate comprising at least one target arrangement suitable for metrology of a lithographic process, the target arrangement comprising at least one pair of similar target regions, the at least one pair of similar target regions being arranged such that the target arrangement is centrosymmetric, or at least the target regions for measurement in a single direction together are centrosymmetric.

[0011] In a second aspect, the present invention provides at least one patterning device comprising one or more target features configured to pattern a beam to form at least one target arrangement suitable for metrology of a lithographic process, the target arrangement comprising at least one pair of similar target regions arranged such that the target arrangement is centrosymmetric, or at least the target regions together are centrosymmetric for measurements in a single direction.

[0012] In a third aspect, the present invention provides a lithographic method comprising: obtaining at least one patterning device comprising one or more target features configured to pattern a beam to form at least one target arrangement suitable for metrology of a lithographic process, the target arrangement comprising at least one pair of similar target regions arranged such that the target arrangement is centrosymmetric, or at least the target regions together are centrosymmetric for measurements in a single direction; and forming the at least one target arrangement on a substrate using the at least one patterning device.

[0013] In a fourth aspect, the present invention provides a metrology method comprising: a) obtaining a substrate comprising at least one target arrangement suitable for metrology of a lithographic process, the target arrangement comprising at least one pair of similar target regions arranged such that the target arrangement is centrosymmetric, or at least the target regions together are centrosymmetric for measurements in a single direction; b) illuminating the target arrangement with measurement illumination and capturing resulting scattered radiation from the target arrangement; and c) determining a value of a parameter of interest from the scattered radiation while correcting for a deformation of a metrology apparatus used to perform at least step b).

[0014] Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. It is noted that the present invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will readily occur to those skilled in the art, based on the teachings herein. BRIEF DESCRIPTION OF DRAWINGS

[0015] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings:

[0016] Figure 1 depicting a lithographic apparatus according to embodiments of the invention;

[0017] Figure 2 depicting a lithographic apparatus according to embodiments of the invention;

[0018] Figure 3comprising: (a) a schematic of a dark field scatterometer for measuring a target using a first pair of illumination apertures; (b) details of a diffraction spectrum of a target grating for a given illumination direction; (c) a second pair of illumination apertures providing a further illumination mode when using the scatterometer for diffraction based overlay measurements; and (d) a third pair of illumination apertures combining the first pair of apertures with the second pair of apertures;

[0019] Figure 4 A known form of multi-grating target depicted on a substrate, and a measurement spot profile;

[0020] Figure 5 A known form of multi-grating target depicted on a substrate, and a measurement spot profile; Figure 3 An image of a target obtained in a scatterometer of the kind Figure 4 An image of a target obtained in a scatterometer of the kind

[0021] Figure 6 (a) to Figure 6 (h) depicts an example of a target arrangement according to an embodiment of the application;

[0022] Figure 7 (a) and Figure 7 (b) depicts how a central symmetric target region can be extracted from a non- central symmetric arrangement according to an embodiment of the application; and

[0023] Figure 8 (a) and Figure (b) illustrate two measurement configurations of a target arrangement according to an embodiment of the application. DETAILED DESCRIPTION

[0024] Before the embodiments of the application are described in detail, it is instructive to present an example environment that can be used in implementing an embodiment of the application.

[0025] Figure 1 A lithographic apparatus LA is schematically depicted. The apparatus includes an illumination optical system (illuminator) IL configured to condition a radiation beam B (e.g., UV 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.

[0026] The illumination optical system can include various types of optical or non-optical components for directing, shaping, or controlling the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic, or other types of components, or any combination thereof.

[0027] 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 can be, for example, a frame or table, which can be fixed or moveable as required. The patterning device support can ensure that the patterning device is at a desired location, for example, relative to the projection system. Any use of the terms “reticle” or “mask” herein can be considered synonymous with the more general term “patterning device”.

[0028] The term “patterning device” as used herein should be interpreted in its broadest sense as a reference to any device that can be used to impart a pattern in a beam of radiation for the purpose of generating a pattern in a target portion of a substrate. It should be noted that, for example, the pattern imparted to the beam of radiation can not exactly correspond to the desired pattern in the target portion of the substrate, if, for example, the pattern includes a shift pattern or a so-called assist feature. Typically, the pattern imparted to the beam of radiation will correspond to a particular functional layer in a device being patterned, such as an integrated circuit.

[0029] The patterning device can 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. Examples of programmable mirror arrays use matrices of small mirrors, each of which can be individually tilted so as to steer the incoming beam of radiation into different directions. The tilted mirrors impart a pattern in the beam of radiation which is reflected by the matrix of mirrors.

[0030] As depicted here, the apparatus is of the transmissive type (e.g. using a transmissive mask). Alternatively, the apparatus can be of the reflective type (e.g. using a programmable mirror array of the type mentioned above, or using a reflective mask).

[0031] The lithographic apparatus can also be of a type that includes a liquid supply system to supply a liquid to a space between the projection system and the substrate. One example of such a liquid is water. Another example of such a liquid is ultraviolet- transparent salt aqueous solution. The space between the projection system and the substrate can be filled with the liquid. The liquid can act as a coolant to cool the projection system and / or the substrate. The liquid can also act as a medium through which the projection system can project the patterned radiation onto the substrate. The liquid can also act as a medium through which the projection system can project the patterned radiation onto the substrate. The term “immersion” as used herein does not mean that a structure, such as the substrate, must be submerged in or under the liquid, but merely means that the liquid is present at the space between the projection system and the substrate during exposure.

[0032] Reference Figure 1 The illuminator IL receives a radiation beam from a radiation source SO. The source and the lithographic apparatus can 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 comprising, for example, suitable directing mirrors and / or a beam expander. In other cases the source can 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, can be referred to as a radiation system.

[0033] The illuminator IL can 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 can include various other components, such as an integrator IN and a condenser CO. The illuminator can be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.

[0034] The radiation beam B is incident on the patterning device (e.g., mask) MA held on the patterning device support (e.g., mask table) MT, and is patterned by said 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, thus transferring the pattern to the target portion C. The substrate table WT is accurately positionable by means of the second positioner PW and position sensor IF (e.g., an interferometer, a linear encoder, a 2D encoder, or a capacitance sensor), for example, to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor (not explicitly depicted in FIG. 1) can be used to accurately position the patterning device (e.g., mask) MA in the path of the radiation beam B, e.g., after being acquired from a magazine of patterning devices, or during a scan. Figure 1 The lithographic apparatus can also be of a type that includes a liquid supply system to supply a liquid to a space between the projection system and the substrate. One example of such a liquid is water. Another example of such a liquid is ultraviolet- transparent salt aqueous solution. The space between the projection system and the substrate can be filled with the liquid. The liquid can act as a coolant to cool the projection system and / or the substrate. The liquid can also act as a medium through which the projection system can project the patterned radiation onto the substrate. The liquid can also act as a medium through which the projection system can project the patterned radiation onto the substrate. The term “immersion” as used herein does not mean that a structure, such as the substrate, must be submerged in or under the liquid, but merely means that the liquid is present at the space between the projection system and the substrate during exposure.

[0035] The pattern forming device (e.g., mask) MA and the substrate W can be aligned using the mask alignment marks Ml, M2 and the substrate alignment marks PI, P2. While the substrate alignment marks as illustrated occupy dedicated target portions, the substrate alignment marks can be located in spaces between target portions (these marks are referred to as scribe-lane alignment marks). Similarly, in cases where more than one die is provided on the pattern forming device (e.g., mask) MA, the mask alignment marks can be located between the dies. Small alignment markers can also be included within the die among the device features, in which case it is desirable to make the markers as small as possible and without requiring any imaging or process conditions different from the adjacent features. Alignment systems that detect the alignment markers are further described below.

[0036] The lithography apparatus LA in this example is of the so-called twin- stage type, having 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 table is exposed at the exposure station, another substrate can be loaded onto the other substrate table at the measurement station and various preparatory steps can be performed. The preparatory steps can include surface control mapping of the substrate using a level sensor LS, and measurement of the position of alignment markers on the substrate using an alignment sensor AS. This arrangement enables a substantial increase in throughput of the apparatus.

[0037] The depicted apparatus can be used in both of a number of modes including, for example, a step-and-repeat mode or a scan mode. The construction and operation of lithography apparatuses are generally well known to those skilled in the art and do not need to be described further for understanding of the present application.

[0038] As Figure 2 As shown in FIG. 1, the lithography apparatus LA forms part of a lithography system, which is referred to as a lithography cell LC or litho cell or litho cluster. The lithography cell LC can also include apparatuses for performing pre-exposure and post-exposure processes on the substrate. Typically, these apparatuses include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH, and a bake plate BK. A substrate handling device or robot RO picks substrates from input / output ports I / Ol, I / O2, moves the substrates between the different process apparatuses, and then passes the substrates to a load bay LB of the lithography apparatus. These devices, often collectively referred to as a track or track coat develop system, are controlled by a track or track coat develop system control unit TCU, which itself is controlled by a management control system SCS, which also controls the lithography apparatus via a lithography control unit LACU. Thus, the different apparatuses can be operated to maximize throughput and processing efficiency.

[0039] In order to expose substrates correctly and consistently by a lithographic apparatus, it is desirable to inspect the exposed substrates to measure properties such as overlay error between subsequent layers, line thickness, critical dimension (CD), etc. Therefore, a manufacturing facility in which the lithographic cell LC is located also includes a metrology system MET which receives some or all of the substrates W that have been processed in the lithographic cell. The metrology results are provided, directly or indirectly, to a supervisory control system SCS. If errors are detected, adjustments can be made to the exposure of subsequent substrates, especially if the inspection can be done sufficiently quickly and rapidly that other substrates of the same batch are still to be exposed. Furthermore, substrates that have been exposed can be stripped and reworked to improve yield or discarded, thereby avoiding performing further processing on substrates that are known to be defective. In the case that only some target portions of a substrate are defective, further exposure can be performed only on those target portions that are good.

[0040] Within the metrology system MET, inspection apparatuses are used to determine properties of the substrates and, in particular, to determine how properties of different substrates or different layers of the same substrate vary from layer to layer. The inspection apparatuses can be integrated into the lithographic apparatus LA or lithographic cell LC, or can be separate devices. To achieve the fastest measurements, it is desirable to measure the properties in the exposed resist layer immediately after exposure. However, the latent image in the resist has very low contrast - there is only a very small difference in refractive index between the parts of the resist that have been exposed to radiation and the parts that have not been exposed to radiation - and not all inspection apparatuses have sufficient sensitivity to make useful measurements of the latent image. Therefore, measurements can be taken after a post-exposure bake step (PEB), which is typically the first step performed on the exposed substrate and increases the contrast between the exposed and unexposed parts of the resist. At this stage, the image in the resist can be referred to as a semi-latent image. It is also possible to make measurements of the developed resist image - at which point the exposed or unexposed parts of the resist have been removed - or after a pattern transfer step such as etching. The latter possibility limits the possibility of reworking defective substrates, but can still provide useful information.

[0041] Figure 3 A metrology apparatus is shown in (a). Figure 3(b) illustrates in more detail the target T and the diffracted rays of the measurement radiation used to illuminate it. The illustrated metrology apparatus is of a type known as a dark-field metrology apparatus. The metrology apparatus depicted in this manner is merely exemplary, to provide an explanation of dark-field metrology. The metrology apparatus can be a standalone device or incorporated into a lithography apparatus LA, for example, at a measurement station or into a lithography cell LC. The optical axis, which has several branches throughout the apparatus, is indicated by a dashed line O. In such an apparatus, light emitted by a source 11 (e.g., a xenon lamp) is directed onto a substrate W via a beam splitter 15 by an optical system comprising lenses 12, 14 and an objective lens 16. These lenses are arranged in a dual sequence of 4F arrangements. Different lens arrangements can be used, provided they still provide an image of the substrate to the detector and simultaneously allow access to an intermediate pupil plane for spatial frequency filtering. Thus, the angular range of radiation incident on the substrate can be selected by defining a spatial intensity distribution in a plane representing the spatial spectrum of the substrate plane (herein referred to as the (conjugate) pupil plane). In particular, this can be achieved by inserting an aperture plate 13 of suitable form between the lens 12 and the lens 14 in a plane which is a back-projected image of the objective pupil plane. In the illustrated example, the aperture plate 13 has different forms, designated 13N and 13S, thereby allowing different illumination modes to be selected. The illumination system in this example forms off-axis illumination modes. In a first illumination mode, the aperture plate 13N provides off-axis illumination from a direction which is designated "North" for descriptive purposes only. In a second illumination mode, the aperture plate 13S is used to provide similar illumination, but from the opposite direction, designated "South". Other illumination modes are possible by using different holes or apertures. The remainder of the pupil plane is desirably dark, since any unwanted light outside the desired illumination mode will interfere with the desired measurement signal.

[0042] like Figure 3(b) shows that the target T is placed such that the substrate W is perpendicular to the optical axis O of the objective 16. The substrate W can be supported by a support (not shown in the figure). A measurement radiation ray I, which is illuminated onto the target T at an angle to the axis O, gives rise to a zeroth order ray (solid line 0) and two first order rays (dotted line +1 and double dotted line -1). It should be borne in mind that in the case of using overfilled small targets, these rays are merely one of many parallel rays covering the area of the substrate including the metrology target T and other features. Since the aperture in the plate 13 has a finite width (necessary to receive a useful amount of light), the incident ray I will in reality occupy an angular range, and the diffracted rays 0 and +1 / -1 will be slightly spread out. Each order +1 and -1 will be further spread over an angular range according to the point spread function of the small target, rather than a single ideal ray as shown. It should be noted that the grating pitch of the target and the angle of illumination can be designed or adjusted such that the first order rays entering the objective are closely aligned with the central optical axis. Figure 3 (a) and Figure 3 The rays illustrated in (b) are shown slightly off-axis to merely enable them to be more easily distinguished in the illustration.

[0043] At least the 0th and +1st orders diffracted by the target T on the substrate W are collected by the objective 16 and directed back through the beam splitter 15. Returning to Figure 3 (a), both the first and second illumination modes are illustrated by indicating completely opposite apertures labelled North (N) and South (S). When the incident ray I of measurement radiation comes from the North side of the optical axis (i.e. when the first illumination mode is applied using aperture plate 13N), the +1st diffracted ray, labelled +1(N), enters the objective 16. In contrast, when the second illumination mode is applied using aperture plate 13S, the -1st diffracted ray, labelled -1(S), is the diffracted ray that enters the lens 16.

[0044] The second beam splitter 17 divides the diffracted beam into two measurement branches. In the first measurement branch, the optical system 18 uses the zeroth and first order diffracted beams to form a diffraction spectrum (pupil plane image) of the target on a first sensor 19, such as a CCD or CMOS sensor. Different points on the sensor in each diffracted order enable image processing to compare and contrast the orders. The pupil plane image captured by the sensor 19 can be used for focusing the metrology apparatus and / or normalising the intensity measurement of the first order beam. The pupil plane image can also be used for many measurement purposes such as reconstruction.

[0045] In the second measurement branch, the optical system 20, 22 forms an image of the target T on a sensor 23, e.g. a CCD or CMOS sensor. In the second measurement branch, an aperture stop 21 is provided in a plane conjugate to the pupil plane. The aperture stop 21 is used to block the zeroth order diffraction beam, so that the image of the target formed on the sensor 23 is formed from only the -1 or +1 order beams. The images captured by the sensors 19 and 23 are output to a processor PU which processes the images, the function of which will depend on the particular type of measurement being performed. It should be noted that the term "image" is used here in a broad sense. Thus, if there is only one of the -1 and +1 orders, an image of the grating lines will not be formed.

[0046] Figure 3 The specific form of the aperture plate 13 and field stop 21 shown in Fig. 1 is merely an example. In another embodiment of the application, on-axis illumination of the target is used, and an aperture stop with an off-axis aperture is used to pass substantially only one first order diffracted light to the sensor. In other examples, two quadrant apertures can be used. This can enable simultaneous detection of the positive and negative orders, as described in US2010201963A1, mentioned above. Embodiments with a wedge (segmented prism or other suitable element) in the detection branch, as described in US2011102753A1, mentioned above, can be used to separate the orders for spatial imaging in a single image. In yet other embodiments, second order, third order and higher order beams are used in the measurement in addition to or instead of first order beams Figure 3 (not shown in Fig. 1). In yet other embodiments, a segmented prism can be used instead of the aperture stop 21, enabling simultaneous capture of both +1 and -1 orders at spatially separated sites on the image sensor 23.

[0047] In order to enable the measurement radiation to be adapted to these different types of measurement, the aperture plate 13 can comprise a plurality of aperture patterns formed around a disc, which is rotated to bring the desired pattern into position. It should be noted that the aperture plate 13N or 13S can be used only for measuring gratings oriented in one direction (X or Y, depending on the setting). In order to measure orthogonal gratings, a target rotation of up to 90° and 270° can be implemented. Figure 3 (c) and Figure 4 Different aperture plates are shown in (d). The use of these aperture plates and many other variations and applications of the apparatus are described in the previously published applications mentioned above.

[0048] Figure 5Depicts an overlapping target or composite overlapping target formed on a substrate according to known practice. The overlapping target in this example includes four sub-targets (e.g., gratings) 32 to 35, which are closely positioned so that they will all be within a measurement spot 31 formed by a beam of measurement radiation of a metrology device. The four sub-overlapping targets are therefore all illuminated simultaneously and imaged simultaneously on the sensor 23. In an example dedicated to overlapping measurement, the sub-targets 32 to 35 are themselves composite structures formed by overlapping gratings patterned in different layers of a semiconductor device formed on a substrate W. The sub-targets 32 to 35 can have overlapping offsets that are biased differently to facilitate measurement of overlap between layers having different portions of the composite sub-targets. The sub-targets 32 to 35 can also differ in their orientation (as shown) so as to diffract the incident radiation in the X and Y directions. In one example, sub-targets 32 and 34 are X-direction sub-targets having offsets of +d and -d, respectively. Gratings 33 and 35 are Y-direction sub-targets having offsets of +d and -d, respectively. Individual images of these sub-targets can be identified in the image captured by the sensor 23. This is just one example of an overlapping target. An overlapping target may include more or less than four sub-targets.

[0049] Figure 3 Shown in the use of Figure 3 (d) In the case of orifice plate 13NW or 13SE Figure 4 Used in devices Figure 4 Figure 4 is an example of an image that can be formed on and detected by the sensor 23 due to overlapping targets. Although the pupil plane image sensor 19 cannot distinguish between different individual sub-targets 32 to 35, the image sensor 23 can perform such a distinction. The shaded area 40 represents the field of the image on the sensor, within which the illumination spot 31 on the substrate is imaged into the corresponding circular area 41. Within such a circular area, the rectangular areas 42 to 45 represent the images of the small overlapping target sub-targets 32 to 35. If the overlapping target is located in the product area, product features in the periphery of the image field may also be visible. The image processor and controller PU use pattern recognition to process these images to identify the individual images 42 to 45 of the sub-targets 32 to 35. In this way, the images do not have to be aligned very accurately at specific locations within the sensor frame, which greatly improves the throughput of the measurement equipment as a whole.

[0050] Once individual images of an overlapping target have been identified, the intensities of those individual images can be measured, for example, by averaging or summing the intensity values ​​of selected pixels within the identified regions. The intensities and / or other properties of the images can be compared to one another. These results can be combined to measure different parameters of the lithography process. Overlay performance is an important example of such a parameter.

[0051] For example, the overlay error (i.e. the undesired and unintentional overlay misalignment) between two layers within the sub-targets 32 to 35 is measured using a method such as described in the above-mentioned application US20110027704A. This method can be referred to as micro-diffraction based overlay (μDBO). This measurement can be made by overlay target asymmetry, as revealed by comparing its intensity in the +1st and -1st order dark field images (other corresponding higher orders can be compared, for example +2nd and -2nd orders) to obtain a measure of intensity asymmetry.

[0052] In known methods using a multi-grating target such as illustrated in Figure 4 in which the overlay OV can be determined from the following equation:

[0053]

[0054] wherein

[0055] I+1 is the +1st diffraction order (e.g. intensity value) from the positive bias target;

[0056] I-1 is the -1st diffraction order from the positive bias target;

[0057] I+1 is the +1st diffraction order from the negative bias target;

[0058] I-1 is the -1st diffraction order from the negative bias target;

[0059] e.g. asymmetry in the +1st and -1st intensity from the positive bias target; and

[0060] e.g. asymmetry in the +1st and -1st intensity from the negative bias target.

[0061] Equation 1 can be re-formulated in terms of a sensitivity coefficient K, which is a stack-dependent parameter with specific properties independent of overlay (perfect target is assumed):

[0062] A +d +A -d = K OV (Equation 2)

[0063] wherein:

[0064]

[0065] Equation 2 is a simple linear equation, based on the assumption of small bias values and overlay errors (compared to the pitch of the gratings forming the sub-targets). However, the dependence of asymmetry on overlay error and bias has roughly a sinusoidal form over a wider range, and a sinusoidal model can also be used instead of the linear model of equation 2.

[0066] The known method using four different sub-targets requires the presence of a border (not shown in Figure 5 and Figure 6 ) around each sub-target, so that the sub-targets are unique in the image 40. This means that due to edge effects, a certain part of the area that is patterned is not usable. In addition, the use of only two specific offsets enforces the assumption of linearity described above, which can lead to inaccuracies when the true relationship is non-linear.

[0067] The known method of metrology of a lithographic process can be adversely affected by the metrology apparatus, for example, defects of the optical column can induce a distortion of the formed image, which can adversely affect the resulting metrology. In addition, such defects are specific to a particular metrology apparatus, so differences between different metrology apparatuses can adversely affect the metrology process in an environment comprising multiple metrology apparatuses. More specifically, this distortion is very constant for a given combination of target, sensor and measurement profile, and will therefore not show up in any fingerprint measured across a wafer; it will give a constant distortion offset. However, since this distortion offset is tool dependent, it will immediately cause unacceptably high matching problems.

[0068] It would therefore be desirable to be able to perform metrology of a lithographic process with improved accuracy.

[0069] Disclosed herein is a target arrangement suitable for metrology of a lithographic process, the target arrangement comprising at least one pair of similar target regions arranged in a centrosymmetric manner. The centrosymmetric arrangement can be such that the at least one pair of similar target regions (at least for measurement in a single direction) will appear identical to the metrology apparatus when rotated through 180 degrees around a centrosymmetric point; i.e. the two similar target regions of each pair of target regions (or at least the target regions for measurement in a single direction) are effectively swapped. Centrosymmetry means that if the target arrangement is described by a set of points plotted in x and y (parallel to the substrate plane), then each point x, y has an indistinguishable counterpart -x, -y, where the origin defines the centrosymmetric point.

[0070] The targets or target areas within a target arrangement can be targets suitable for measuring overlay, focus, dose, or physical parameters such as tilt, sidewall angle, critical dimension, and other dimensions of interest of a device structure present on a lithographic wafer. The targets within a target arrangement can be targets suitable for image-based metrology (IBO), diffraction-based metrology (DBO), or other forms of metrology. The targets described herein can be suitable for digital holographic metrology. Such a target arrangement can be present on a substrate (e.g., as printed) or on one or more patterning devices (e.g., two patterning devices used to form two layers of a target arrangement when the parameter of interest is overlay).

[0071] The target arrangement can comprise at least a first pair of similar target areas and a second pair of similar target areas. The target areas in each pair can be similar in size (length and width), pitch, line width, and line / space ratio of each of its constituent periodic structures (where a target area has more than one constituent periodic structure). As such, similar in this context means substantially the same in size and form (e.g., except for processing and other unintended differences / variations). A "centrally symmetric pair of target areas" or "a pair of target areas" in this context can comprise two corresponding and similar areas that replace each other within a measurement spot when the substrate / wafer stage is rotated through 180 degrees around a center point of symmetry.

[0072] In a particular example (e.g., for overlay metrology), the target arrangement can comprise a first pair of similar target areas of a first type and a second pair of target areas of a second type. In an embodiment, the two types of target areas each can comprise two constituent periodic structures (one per layer) having a first pitch and a second pitch, respectively, with the order of the periodic structures swapped between the two types; i.e., a target area of the first type can have structures with the first pitch in its top layer and structures with the second pitch in its bottom layer, with the order of the structures reversed for a target area of the second type.

[0073] In an embodiment, the target arrangement can comprise at least a pair of target areas per measurement direction (e.g., to perform metrology in two orthogonal directions). In an embodiment, the target arrangement can comprise a first pair of target areas and a second pair of target areas per measurement direction (e.g., as described above). The central symmetry of a two-direction target arrangement can be related to the entire target arrangement (i.e., there is a single center point of symmetry at the center of the target arrangement), or only to the target areas of a single direction (i.e., there are two center points of symmetry, each at the center of the target areas of a single direction).

[0074] One, some or all of the target regions in a target region can each comprise an entire target (e.g. a single target), and / or one, some or all of the target regions of a target area can comprise a portion of a larger (single) target. For example, if a pair of target regions (e.g. regions of interest) can be identified for the target arrangement that are centrosymmetric, then a target arrangement that is not centrosymmetric per se can still be used. Furthermore, a single target can be divided into a plurality of target regions, i.e. it can comprise a pair of target regions, e.g. if centered on (i.e. centered around) a centrosymmetric point.

[0075] The presence of defects in a metrology apparatus manifests as a distortion in the measurements performed by the apparatus. This distortion is characteristic of a particular metrology apparatus and thus will be different when the same metrology target is measured on different metrology apparatuses. In order to make the metrology steps accurate when performing metrology on different metrology apparatuses, it is desirable to decouple the metrology of the lithographic process from the particular contribution of each metrology apparatus. This decoupling can be achieved by producing a transformation in the measurement of a target, for example, with a known change in the measured parameter (i.e. the measured parameter). For example, measuring the metrology parameter in the case of a platform rotation WR0 (e.g. in a first orientation) and measuring the same parameter in the case of a platform rotation of WR180 (e.g. in a second orientation that is 180 degrees relative to the first orientation) induces a change in the measured overlay, for example, by changing the sign of the measured overlay, while the contribution based on the distortion remains unchanged. The parameter of interest can be extracted as the change in phase between the measurements. In other words, in the case of the two orientations, substantially the same target arrangement is measured and thus the measurements are affected by exactly the same distortion from the tool. However, since the wafer is rotated between the multiple acquisitions, the overlay has a changed sign. Thus, in the case of measuring substantially the same (except that the overlay has a changed sign), the overlay can be acquired as the (e.g. phase) difference between the measurements.

[0076] This mechanism is particularly suitable for metrology targets in which the phase of the measured signal (i.e. the measured signal) is used to determine the parameter of interest of the metrology process from the metrology step.

[0077] Figure 6 Various target arrangements are described such that a parameter of interest is measured while at the same time mitigating (e.g. eliminating) the particular distortion of the metrology apparatus used. The notation “Ax” describes a target that belongs to type A and is suitable for measuring a parameter of the lithographic process in direction x. Similarly, “Bx” describes a target of type B that is suitable for measuring a parameter in direction x. “Ay” and “By” are used to describe targets of type A and B for measuring a parameter in direction y. Each of the pads in the form of a rectangle or square is a metrology target, e.g. a target comprising superposed gratings, in which the pitch of the top grating is different from the pitch of the bottom grating. In the case of the target of type A, the pitch of the top grating is different from the pitch of the bottom grating. In the case of the target of type B, the pitch of the top grating is the same as the pitch of the bottom grating.Figure 6 In the marking of Figure 2A, the A-type targets have a top grating pitch that is smaller than the pitch of the bottom grating, and the B-type targets have a top grating pitch that is larger than the pitch of the bottom grating.

[0078] Figure 6 (a) and Figure 6 (b) shows an arrangement in which each target region is a single square or rectangular target, with both targets arranged around a common center of symmetry point. In Figure 6 (c) and Figure 6 (d) the arrangement, two of the plurality of central target regions are formed by a single target (labeled Bx), with the center of symmetry point at the center of such target so as to define a pair of target regions on either side of the point line. Such an arrangement shows that the size of the target regions can be different for different pairs of target regions. Figure 6 (e) shows a rectangular target arrangement, while Figure 6 (f) shows a similar arrangement, but in which the target regions are divided into two groups. Figure 6 (g) shows an arrangement in which the target arrangement is grouped according to the measurement direction and in which only these direction groups are centrosymmetric around the respective center of symmetry points CSPx, CSPy, while the entire target arrangement is not centrosymmetric. Figure 6 (h) shows a second arrangement in which the target arrangement is grouped according to the measurement direction, and in which each group has an odd number of targets (e.g., 3 targets), so that two of the central target regions are formed by a single target in each direction.

[0079] In all arrangements of Figure 2A, Figure 6 the target arrangement as defined on the substrate has the desired centrosymmetry (although only in Figure 6 (g) and Figure 7 each direction in the examples of (h)). However, any target arrangement suitable for metrology of a lithographic process can be used, as long as it comprises at least two target regions positioned within the target arrangement so that the measured property (e.g., region of interest) of the at least two target regions is centrosymmetric or has symmetry after rotation by 180 degrees. The target arrangement can not be arranged in the required centrosymmetric manner. However, assuming that the corresponding similarly centrosymmetric target regions or regions of interest (ROIs) can be identified, the decoupling of the measured parameters from the undesired metrology contribution specific to the tool can be performed.

[0080] Figure 7 Figure 2B illustrates such an embodiment. Figure 7(b) is an example of a target arrangement that is not inherently centrosymmetric. However, by careful selection of ROIs, a corresponding pair of centrosymmetric pairs of target regions or ROIs (of the same size and type) can be identified. For example, while targets Ax and Ax' have different shapes / sizes, they have a corresponding pair of centrosymmetric pairs of target regions that can be selected as regions of interest ROIs. In this way, Figure 7 The non-centrosymmetric target arrangement of (b) can be measured in substantially the same way as the centrosymmetric target arrangement of (a) and any target arrangement (comprising at least two targets) that enables the placement of centrosymmetric ROIs to be within the scope of the present disclosure. Figure 6 (a) Figure 7 The centrosymmetric target arrangement of (a) and any target arrangement (comprising at least two targets) that enables the placement of centrosymmetric ROIs to be within the scope of the present disclosure is measured in substantially the same way.

[0081] The metrology method for measuring a parameter of the lithographic process can comprise measuring at least two target regions of a metrology target arrangement by illuminating the corresponding targets with radiation, detecting radiation scattered by the targets and determining a property in the measurement of the targets, wherein the property has symmetry after rotation. The property measured can be a region of interest (ROI) in a measured image of the target arrangement. The method comprises an algorithm for selecting the ROI such that the selected ROI is symmetric (i.e. centrosymmetric) after rotation through 180 degrees.

[0082] For example, such an algorithm can identify similar regions within an image of the target arrangement (which can be a pupil / Fourier plane image or an image plane image) that have approximately similar image signatures (intensity and / or phase patterns) and identify one or more pairs of target regions (at least in each direction) within these similar regions such that they have a common size and are centrosymmetric around a centrosymmetric point. The centrosymmetric point can be defined by the algorithm in determining the ROI, or it can be a predetermined centrosymmetric point (e.g. as noted as CSP on (b)). Figure 8

[0083] An advantage of the method is that the measurements necessary to determine a parameter of the lithographic process are decoupled from the obstructive contribution of the metrology apparatus and from possible misplacement of the positions of the targets within the target arrangement.

[0084] In embodiments, the pitch of each periodic structure forming a target in the target arrangement can or can not be resolvable by optical radiation.

[0085] There are a number of methods of measuring a target arrangement as disclosed herein. In each case, the targets can be measured in each direction separately or simultaneously.

[0086] In Figure 8 ​In the embodiment illustrated in the middle, only one set of target regions is measured per acquisition, wherein a set of target regions comprises only one target region in each pair of similar target regions. This allows the target regions to be larger, since there is no need for the entire target arrangement to fit within the measurement spot. Figure 8 (a) shows the target arrangement at orientation WR0 for a first acquisition, and Figure 8 (b) shows the same target arrangement at WR180 for a second acquisition. Note that in both cases, a substantially similar arrangement of target regions is captured in the measurement spot MS (except that the overlap will have a changed orientation). To better illustrate this, each target region in each pair of similar target regions is identified separately (i.e. Ax1, Ax2 etc. for the first type target regions in the x-direction), so that it can be seen that it is Figure 8 the target region labelled "1" in the measurement spot MS of (a) and Figure 3 the target region labelled "2" in the measurement spot MS of (b).

[0087] Note that the reference to different orientations does not necessarily mean that the substrate / target arrangement is physically rotated. Instead, the orientations are relative to the illumination direction, and the illumination can be changed, e.g. by illuminating the target arrangement from fully opposite (i.e. diametrically opposite) directions, rather than rotating the substrate. This can be done, for example, by using Figure 3 the fully opposite holes 13N, 13S (or 13E, 13W) illustrated in (a) (or Figure 8 (d)). Similarly, any reference to rotation equally refers to a change in the illumination direction / illumination profile. It is also known that these fully opposite pairs of holes (or other simultaneous bidirectional illumination profiles) can be used simultaneously, with the resulting images optically separated and captured simultaneously, so the terms first acquisition and second acquisition do not necessarily imply or suggest a temporal order, these acquisitions can be taken simultaneously.

[0088] In another embodiment, the entire target arrangement can be measured at two orientations. Alternatively, the entire target arrangement can be measured at only a single orientation (and with a single acquisition). This is possible, for example, if there is no significant gradient of deformation (i.e. the deformation is the same regardless of where the target arrangement is located within the measurement spot). Such a method can comprise measuring the entire target arrangement at only a single orientation (e.g. WR0); and determining the overlap of one of the first type target regions and one of the second type target regions (e.g. Ax1 to Bx1 or Ay1 to By1 in the figure) with another of the first type target regions and another of the second type target regions (e.g. Ax2 to Bx2 or Ay2 to By2 in the figure) from the single acquisition. Figure 8 Figure 8 ​The overlap can be averaged (e.g. per direction) by taking the difference in overlap (Ax2 to Bx2 or Ay2 to By2 in the figures of FIG. 1 ). This is because the target area will experience opposite deformations which will cancel out.

[0089] In an embodiment, it is proposed to measure the deformation offset for less than 3 target arrangements (e.g., only a single target arrangement (using (e.g.) any of the methods described above)) and apply this offset to all other target arrangements on the wafer. This scenario works if the offset is very stable. This means that only one centrosymmetric target arrangement needs to be printed (e.g., once per field, since the mask repeats between different fields), and all other target arrangements can be made smaller (e.g., half the size). This concept can also be extended and include measuring multiple target arrangements on the wafer in the case of both WR0 and WR180, and training a model (e.g., a regression model or a machine learning model, such as a neural network) to predict the deformation offset for all other target arrangements. This can also be trained for 1 centrosymmetric target per field, leaving all other targets non-centrosymmetric. It should also be noted that centrosymmetric calibration can be included in the model training, thereby improving throughput.

[0090] In an embodiment, each pad (eg Figure 8 The lines of the top and bottom gratings of (a) Ax1 and Bx1) are arranged in such a way that at least one line of the top grating of Ax1 is superimposed on or on top of a line from the bottom grating of Ax1 when imaged from above. This arrangement is known in the art as "line-on-line". Due to the different spacing of the top and bottom gratings of each pad, line-on-line refers to a single line for the purposes described throughout the various embodiments of this invention, or to pairs of lines if the relationship between the spacing and the size of the pads allows. In another embodiment, adjacent pads (e.g. Figure 4 The lines Ax1 and Bx1, or By2 and Ay2, or Ay1 and By1, or Bx2 and Ax2 of (a) are shifted relative to each other by a distance given by a multiple of the pitch of the stripes formed on the image sensor divided by 8.

[0091] In an embodiment, the Figure 5 and Figure 6 The target formation described in Figure 7 、 Figure 8 and Figure 6 In an embodiment, Figure 6 The pad Ax of (a) may have the same pitch of the top and bottom gratings and be shifted relative to each other by a bias of +d, and Figure 4The spacers Bx of (a) can have the same pitch between the top grating and the bottom grating and be shifted relative to each other by an offset -d, where the + and - signs indicate the direction of the offset (as for example from Figure 4 "positive" and "negative" offsets as understood). Similar offsets can be used for the remaining spacers in the target arrangement. Such a target formed in this way is a Figure 4 center-symmetric variant of the target described in the background. Such an improved center-symmetric target can be more resilient to illumination distortions (i.e. intensity non-uniformities).

[0092] The illumination optics of a metrology tool can cause "uniform" distortions, which can not be favorable for accurately measuring the value of the metrology of interest. The center-symmetric layout removes such effects due to its geometry. In embodiments, the "uniform" distortion suppression can be achieved by re-centering each pair of spacers as described in Figure 6 , Figure 7 , Figure 8 or Figure 3 each pair of spacers as described in the background.

[0093] In an overlay embodiment, as already described, the target arrangement can comprise (e.g. per direction): a pair of A-type targets having a grating with a first pitch pi on top of a grating with a second pitch p2; and a pair of B-type targets having the grating pitches swapped such that the second pitch p2 grating is on top of the first pitch pi grating. In such an embodiment, the overlay OV can be extracted from the phase difference between the A-type and B-type target images in the normal and complementary images respectively (e.g. Fourier plane images from the +1 and -1 diffraction orders respectively) according to the following equation extracting the overlay OV:

[0094]

[0095] where the phase can be measured from each image as the distance between the fringes (e.g. intensity fringes) of the target region.

[0096] It will be appreciated that all the specific arrangements shown are examples only, and that there are a near infinite number of possible target arrangements that fall within the scope of the present disclosure. For example, the target arrangements can include only target areas for measurement in only a single direction. Spacing can also be added between the pads to combat cross-talk and / or parallax issues. The target arrangements illustrated have been those designed for measurement of overlay. However, the concepts herein also apply to target arrangements designed for measurement of another parameter of interest. For example, focus arrangements (e.g., formed with areas having focus sensitive asymmetry) can also benefit as the asymmetry depending on focus will also change the sign in the measurement and thus decouple from the deformation bias depending on the metrology tool.

[0097] In association with the physical grating structures of the targets implemented on the substrate and the patterning device, embodiments can include a computer program containing one or more sequences of machine-readable instructions describing a method of measuring a target on a substrate and / or analyzing a measurement to obtain information about a lithographic process. The computer program can be, for example, stored on a data storage medium (e.g. a semiconductor memory, a magnetic or optical disk) and / or implemented in the hardware of a device, such as the apparatus of Figure 2 The units PU in the apparatus of Figure 3 The control unit LACU of the apparatus of Such a computer program can also be provided on a data storage medium (e.g. a semiconductor memory, a magnetic or optical disk) on which the computer program is stored in a way that enables it to be read by a device such as the apparatus of ​ In case an existing metrology apparatus of the type shown is already in production and / or in use, the present invention can be implemented by providing an updated computer program product for causing a processor to perform the steps necessary to calculate the overlay error.

[0098] The program can optionally be arranged to control an optical system, a substrate support, etc. to perform the steps necessary to calculate the overlay error for measuring asymmetry on a suitable plurality of targets.

[0099] Thus, a target arrangement suitable for metrology of a lithographic process is disclosed, the target arrangement comprising at least two targets positioned within the target arrangement such that the target arrangement has symmetry after rotation. The at least two targets can be positioned within the target arrangement such that a measured property of the at least two targets has symmetry after rotation. A method for measuring a parameter of a lithographic process is also disclosed, comprising measuring at least two targets of a metrology target arrangement by illuminating the targets with radiation and detecting radiation scattered by the targets and determining a property in the measurement of the targets, wherein the property has symmetry after rotation.

[0100] Further embodiments of the present invention are described in the following numbered clauses:

[0101] 1. A substrate comprising at least one target arrangement adapted for metrology of a lithographic process, the target arrangement comprising at least one pair of similar target areas, the at least one pair of similar target areas being arranged such that the target arrangement is centrosymmetric, or at least the target areas for measurements in a single direction together are centrosymmetric.

[0102] 2. The substrate according to aspect 1, wherein the target arrangement is such that the at least one pair of similar target areas, or at least the target areas for measurements in a single direction, will appear identical to the metrology apparatus when rotated through 180 degrees around a point of centrosymmetry.

[0103] 3. The substrate according to aspect 1 or 2, wherein the at least one pair of similar target areas comprises at least a first pair of similar target areas and a second pair of similar target areas.

[0104] 4. The substrate according to aspect 3, wherein the first pair of similar target areas comprises a first type of target area and the second pair of similar target areas comprises a second type of target area.

[0105] 5. The substrate according to aspect 4, wherein the first type of target area comprises a periodic structure with a first pitch in a first layer and a periodic structure with a second pitch in a second layer, and the second type of target area comprises a periodic structure with the second pitch in the first layer and a periodic structure with the first pitch in the second layer.

[0106] 6. The substrate according to any preceding aspect, wherein the at least one pair of target areas comprises at least one pair of target areas for each measurement direction for performing metrology in two orthogonal directions.

[0107] 7. The substrate according to any preceding aspect, wherein the target arrangement has a common point of centrosymmetry for the target areas of each measurement direction.

[0108] 8. The substrate according to any preceding aspect, wherein the target arrangement is centrosymmetric.

[0109] 9. The substrate according to any of aspects 1 to 6, wherein the target arrangement has a separate point of centrosymmetry for the target areas of each measurement direction.

[0110] 10. The substrate according to aspect 9, wherein the target arrangement is centrosymmetric for each measurement direction.

[0111] 11. The substrate according to any preceding aspect, wherein each of the target areas comprises a separate target structure.

[0112] 12. The substrate according to any one of aspects 1 to 10, wherein at least some of the target areas comprise a portion of a larger target structure.

[0113] 13. The substrate according to any preceding aspect, wherein the target arrangement is adapted to measure one or more of: overlay, focus, dose, and a physical parameter related to a device structure present on the substrate.

[0114] 14. The substrate according to any preceding aspect, wherein the target arrangement is adapted for image-based metrology and / or for diffraction-based metrology.

[0115] 15. At least one patterning device comprising one or more target features configured to pattern a beam to form at least one target arrangement adapted for metrology of a lithographic process, the target arrangement comprising at least one pair of similar target areas arranged such that the target arrangement is centrosymmetric, or at least the target areas for measurement in a single direction together are centrosymmetric.

[0116] 16. The at least one patterning device according to aspect 15, wherein the one or more target features are configured such that the target arrangement is formed such that the at least one pair of similar target areas, or at least the target areas for measurement in a single direction, will appear identical to the metrology apparatus when rotated through 180 degrees about a centre point of symmetry.

[0117] 17. The at least one patterning device according to aspect 15 or 16, wherein the one or more target features are configured such that the at least one pair of similar target areas comprises at least a first pair of similar target areas and a second pair of similar target areas.

[0118] 18. The at least one patterning device according to aspect 17, wherein the one or more target features are configured such that the first pair of similar target areas comprises a first type of target area and the second pair of similar target areas comprises a second type of target area.

[0119] 19. The at least one patterning device according to aspect 18, wherein:

[0120] the at least one patterning device comprises: a first patterning device comprising one or more target features for patterning a beam to form a structure in a first layer; and a second patterning device comprising one or more target features for patterning a beam to form a structure in a second layer; and

[0121] The one or more target features of the first and second patterning device are such that the target regions of the first type comprise a periodic structure with a first pitch in the first layer and a periodic structure with a second pitch in the second layer, and the target regions of the second type comprise a periodic structure with the second pitch in the first layer and a periodic structure with the first pitch in the second layer.

[0122] 20. The at least one patterning device according to any of aspects 15 to 19, wherein the one or more target features are configured such that the at least one pair of target regions comprises at least one pair of target regions per measurement direction for performing metrology in two orthogonal directions.

[0123] 21. The at least one patterning device according to any of aspects 15 to 20, wherein the one or more target features are configured such that the target arrangement has a common center of symmetry for the target regions per measurement direction.

[0124] 22. The at least one patterning device according to any of aspects 15 to 21, wherein the one or more target features are configured such that the target arrangement is center- symmetric.

[0125] 23. The at least one patterning device according to any of aspects 15 to 20, wherein the one or more target features are configured such that the target arrangement has a separate center of symmetry for the target regions per measurement direction.

[0126] 24. The at least one patterning device according to aspect 23, wherein the one or more target features are configured such that the target arrangement is center-symmetric per measurement direction.

[0127] 25. The at least one patterning device according to any of aspects 15 to 24, wherein the one or more target features are configured such that each of the target regions is defined as a separate target structure.

[0128] 26. The at least one patterning device according to any of aspects 15 to 25, wherein the one or more target features are configured such that the target arrangement is adapted for measuring one or more of: overlay, focus, dose, and a physical parameter related to a device structure present on the at least one patterning device.

[0129] 27. The at least one patterning device according to any of aspects 15 to 26, wherein the one or more target features are configured such that the target arrangement is suitable for image-based metrology and / or for diffraction-based metrology.

[0130] 28. The at least one patterning device according to any of aspects 15 to 27, comprising a plurality of the target features, each target feature for forming a target arrangement or constituent layer thereof, wherein only one target feature or a suitable subset of the plurality of target features is configured to form a target arrangement or constituent layer thereof with the at least one pair of similar target regions.

[0131] 29. The at least one patterning device according to aspect 28, wherein the remaining target features are each configured to form a target arrangement or constituent layer thereof with only one target region of the at least one pair of similar target regions.

[0132] 30. A lithographic method, comprising:

[0133] obtaining at least one patterning device comprising one or more target features configured for patterning a beam to form at least one target arrangement suitable for metrology of a lithographic process, the target arrangement comprising at least one pair of similar target regions arranged such that the target arrangement is centrosymmetric, or at least the target regions for measurement in a single direction together are centrosymmetric; and

[0134] forming at least one target arrangement on a substrate using the at least one patterning device.

[0135] 31. The lithographic method according to aspect 30, wherein the at least one patterning device comprises the at least one patterning device according to any of aspects 16 to 29.

[0136] 32. A metrology method, comprising:

[0137] a) obtaining a substrate comprising at least one target arrangement suitable for metrology of a lithographic process, the target arrangement comprising at least one pair of similar target regions arranged such that the target arrangement is centrosymmetric, or at least the target regions for measurement in a single direction together are centrosymmetric;

[0138] b) illuminating the target arrangement with measurement illumination and capturing resulting scattered radiation from the target arrangement; and

[0139] c) determining a value of a parameter of interest from the scattered radiation while correcting for a deformation of a metrology apparatus used to perform at least step b).

[0140] 33. The metrology method according to clause 32, comprising:

[0141] performing step b) in a first acquisition in which the substrate is in a first orientation and in a second acquisition in which the substrate is in a second orientation that is 180 degrees relative to the first orientation about a center of symmetry of the target arrangement with respect to the illumination direction; and

[0142] wherein step c) comprises using measurement data from the first acquisition and the second acquisition to cancel the effect of the deformation on the parameter of interest.

[0143] 34. The metrology method according to clause 33, comprising measuring only one set of target regions per acquisition, wherein a set of target regions comprises only one target region in each pair of similar target regions.

[0144] 35. The metrology method according to clause 33, wherein the complete target arrangement is measured at each acquisition.

[0145] 36. The metrology method according to any of clauses 32 to 35, wherein the at least one pair of similar target regions comprises at least a first pair of similar target regions and a second pair of similar target regions; and

[0146] the first pair of similar target regions comprises a first type of target region and the second pair of similar target regions comprises a second type of target region.

[0147] 37. The metrology method according to clause 36, wherein the first type of target region comprises a periodic structure with a first pitch in a first layer and a periodic structure with a second pitch in a second layer, and the second type of target region comprises a periodic structure with the second pitch in the first layer and a periodic structure with the first pitch in the second layer.

[0148] 38. The metrology method according to clause 36 or 37, comprising measuring the complete target arrangement only in a single orientation; and wherein step c) comprises:

[0149] averaging a first difference and a second difference to cancel the effect of the deformation on the parameter of interest, wherein the first difference comprises a difference in values of the parameter of interest obtained from a first target region in the first pair of similar target regions and a first target region in the second pair of similar target regions, and the second difference comprises a difference in values of the parameter of interest obtained from a second target region in the first pair of similar target regions and a second target region in the second pair of similar target regions.

[0150] 39. The metrology method of any of aspects 32 to 38, comprising identifying a corresponding plurality of pairs of similar regions of interest that are centrosymmetrically similar from an image of the target arrangement obtained from the collected scattered radiation.

[0151] 40. The metrology method of aspect 39, wherein the regions of interest define the target regions.

[0152] 41. The metrology method of any of aspects 32 to 40, wherein the substrate comprises a plurality of target arrangements, and only a proper subset of target arrangements on the substrate comprises the at least one pair of similar target regions arranged centrosymmetrically; and the method comprises:

[0153] determining a deformation bias describing the deformation only from measurements of the proper subset of target arrangements; and

[0154] correcting measurements from at least some of the plurality of target arrangements not in the proper subset with the deformation bias.

[0155] 42. The metrology method of aspect 41, wherein the proper subset of target arrangements comprises fewer than three target arrangements per exposure field, and at least some of the remaining target arrangements comprise the remaining target arrangements in the exposure field.

[0156] 43. The metrology method of any of aspects 32 to 40, wherein the substrate comprises a plurality of target arrangements, and only a proper subset of target arrangements on the substrate comprises the at least one pair of similar target regions arranged centrosymmetrically; and the method comprises:

[0157] obtaining a trained model operable to predict a deformation bias describing the deformation; and

[0158] correcting measurements from at least some of the plurality of target arrangements not in the proper subset with the deformation bias.

[0159] 44. The metrology method of aspect 43, comprising an initial step of training the model by measuring the plurality of target arrangements in a first acquisition in which the substrate is in a first orientation and in a second acquisition in which the substrate is in a second orientation, the second orientation comprising a 180 degree rotation.

[0160] 45. The metrology method of any of aspects 32 to 44, wherein the at least one pair of target regions comprises at least one pair of target regions per measurement direction for performing metrology in two orthogonal directions.

[0161] 46. The metrology method of any of clauses 32-45, wherein the parameter of interest comprises one or more of: overlay, focus, dose, and a physical parameter related to a device structure present on the substrate.

[0162] 47. A computer program comprising program instructions operable to cause a metrology apparatus to perform the method of any of clauses 32-45 when run on a suitable device.

[0163] 48. A non-transitory computer program carrier comprising the computer program of clause 47.

[0164] 49. A processing arrangement comprising:

[0165] the non-transitory computer program carrier of clause 47; and

[0166] a processor operable to run the computer program.

[0167] 50. A metrology apparatus comprising the processing arrangement of clause 49.

[0168] 51. A lithographic apparatus comprising:

[0169] a patterning device support to support a patterning device;

[0170] a substrate support to support a substrate;

[0171] wherein the lithographic apparatus is operable to perform the method of clause 30 or 31.

[0172] While the above can specifically refer to the use of embodiments of the application in the context of optical lithography, it will be appreciated that embodiments of the application can be used in other applications (e.g. imprint lithography) and are not limited to optical lithography where the context allows. In imprint lithography, topography in a patterning device defines a pattern that is created on a substrate. The topography of the patterning device can be pressed into a resist layer that is supplied to the substrate, where the resist is solidified by the application of electromagnetic radiation, heat, pressure, or a combination thereof. After the resist is solidified, the patterning device is moved out of the resist, leaving the pattern therein.

[0173] 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 5-20 nm), as well as particle beams, such as ion beams or electron beams.

[0174] The term "lens" may refer to any one or combination of various types of elements, including refractive, reflective, magnetic, electromagnetic, and electrostatic elements, as the context allows.

[0175] The foregoing description of specific embodiments will therefore fully reveal the general nature of the invention: others can readily modify and / or adapt such specific embodiments for various applications by applying knowledge understood by those skilled in the art without departing from the general concept of the invention, without undue experimentation. Therefore, based on the teachings and guidance presented herein, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments. It should be understood that the phraseology or terminology herein is for purposes of description (for example) and not limitation, so that the terms or phrases of this specification will be interpreted by those skilled in the art in accordance with the teachings and guidance.

[0176] Thus, 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.

Claims

1. A substrate comprising at least one target arrangement suitable for metrology of a lithographic process, the target arrangement comprising at least a first set of target areas and a second set of target areas, the first set of target areas being configured for measurements along a first direction and the second set of target areas being configured for measurements along a second direction different from the first direction, in, the first set of target areas includes at least a first target type and a second target type, the first target type and the second target type each being configured for measurement along the first direction, and the second target type having a spacing value or direction different from that of the first target type, and Wherein, for each group in the first group of target areas and the second group of target areas, the target arrangement also includes a corresponding similar group of target areas, and the corresponding similar group of target areas are positioned to be symmetrical about a point located within the area defined by the target arrangement relative to the corresponding first group of target areas and the second group of target areas.

2. The substrate according to claim 1, wherein The targets are arranged such that the at least one pair of similar target areas, or at least target areas for measurement in a single direction, will appear identical to the metrology device when rotated through 180 degrees about a central point of symmetry.

3. The substrate according to claim 1 or 2, wherein The at least one pair of similar target areas includes at least a first pair of similar target areas and a second pair of similar target areas.

4. The substrate according to claim 3, wherein The first pair of similar target areas comprises a first type of target area, and the second pair of similar target areas comprises a second type of target area.

5. The substrate according to claim 4, wherein The first type of target area includes a periodic structure with a first spacing in the first layer and a periodic structure with a second spacing in the second layer, and the second type of target area includes a periodic structure with the second spacing in the first layer and a periodic structure with the first spacing in the second layer.

6. A substrate according to any preceding claim, wherein The at least one pair of target areas includes at least one pair of target areas for each measurement direction for performing measurements in two orthogonal directions.

7. A substrate according to any preceding claim, wherein The target arrangement has a common central point of symmetry for the target area for each measuring direction.

8. A substrate according to any preceding claim, wherein The target arrangement is centrosymmetric.

9. The substrate according to any one of claims 1 to 6, wherein The target arrangement has a separate central point of symmetry for the target area in each measuring direction.

10. The substrate according to claim 9, wherein The target arrangement is centrally symmetrical in each measurement direction.

11. A substrate according to any preceding claim, wherein Each of the target regions includes a separate target structure.

12. The substrate according to any one of claims 1 to 10, wherein At least some of the target regions comprise portions of a larger target structure.

13. A substrate according to any preceding claim, wherein The target arrangement is suitable for measuring one or more of: overlay, focus, dose, and a physical parameter associated with a device structure present on the substrate.

14. A substrate according to any preceding claim, wherein The target arrangement is suitable for image-based metrology and / or for diffraction-based metrology.

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