Metrology target and related metrology method

Abstract

A substrate including at least one target is disclosed. The target comprises a plurality of sub-targets, the plurality of sub-targets at least comprise a first sub-target and a second sub-target, and each of the plurality of sub-targets comprises at least one sub-segment periodic structure with a first region and a second region which are repeated, wherein at least one of the first region or the second region includes sub-segment regions formed by periodic sub-features. The first sub-target comprises a first sub-segment characteristic of its sub-segment region and the second sub-target comprises a second sub-segment characteristic of its sub-segment region, the first sub-segment characteristic and the second sub-segment characteristic differing in at least one sub-segment parameter.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP application 22184952.4, filed on July 14, 2022, and the entire contents of which are incorporated herein by reference. Background Art

[0003] The present invention relates to a metrology apparatus and method which can be used to perform metrology when manufacturing a device, for example by photolithography. Technical Field

[0004] A lithographic apparatus is a machine that applies a desired pattern to a substrate (usually to a target portion of the substrate). A lithographic apparatus can be used, for example, to manufacture an integrated circuit (IC). In this case, a pattern forming device (or alternatively referred to as a mask or reticle) can be used to generate a circuit pattern to be formed on an individual layer of the IC. The pattern can be transferred to a target portion (e.g., including a portion of a die, a die, or several dies) on a substrate (e.g., a silicon wafer). The transfer of the pattern is usually performed via imaging onto a layer of radiation-sensitive material (resist) disposed on the substrate. In general, a single substrate will contain a network of adjacent target portions that are patterned in sequence.

[0005] During lithography, it is often desirable to make measurements of the resulting structures, e.g., for process control and verification. Various tools are known for making such measurements, including scanning electron microscopes, which are often used to measure critical dimensions (CD), and specialized tools for measuring overlay (the accuracy of alignment of two layers in a device). In recent years, various forms of scatterometers have been developed for use in the field of lithography. These devices direct a radiation beam onto a target and measure one or more properties of the scattered radiation: for example, intensity at a single reflection angle as a function of wavelength; intensity at one or more wavelengths as a function of reflection angle; or polarization as a function of reflection angle, to obtain a diffraction "spectrum" from which properties of interest of the target can be determined.

[0006] Examples of known scatterometers include angle-resolved scatterometers of the type described in US2006033921A1 and US2010201963A1. The target used by such scatterometers is a relatively large (e.g., 40 μm×40 μm) grating, and the measurement beam generates a spot smaller than the grating (i.e., the grating is underfilled). Examples of dark field imaging metrology can be found in international patent applications US20100328655A1 and US2011069292A1, which are hereby incorporated by reference in their entirety. Further developments of the technology are described in disclosed patent publications US20110027704A, US20110043791A, US2011102753A1, US20120044470A, US20120123581A, US20130258310A, US20130271740A and WO2013178422A1. These targets can be smaller than the illumination spot and can be surrounded by product structures on the 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.

[0007] An important parameter of interest that can be monitored is overlay, which is a measure of the misalignment between patterns in different layers (e.g., zero overlay indicates perfect alignment). Overlay can be monitored by measuring an overlay target that is designed to have an overlay-related asymmetry; this asymmetry can be measured by the metrology tool and the overlay inferred. The metrology target can include a pair of periodic structures or gratings, one grating for each related layer. When the metrology tool measures the structural asymmetry, any non-overlay-related asymmetry (such as asymmetry in individual gratings) will appear as an overlay measurement error. One type of grating asymmetry inherent to sub-segmented targets (where individual features or the space itself of the target grating is segmented) is called CD imbalance, where the CD of the first one or more features of each target feature is less than the nominal feature CD, and the CD of the last one or more features of each target feature is greater than the nominal feature CD.

[0008] It would be desirable to be able to correct this CD imbalance. Summary of the invention

[0009] In a first aspect, the present invention includes a substrate comprising at least one target, the target comprising a plurality of sub-targets, the plurality of sub-targets comprising at least a first sub-target and a second sub-target, each of the plurality of sub-targets comprising at least one sub-segment periodic structure having a repeated first region and a second region, wherein at least one of the first region or the second region comprises a sub-segment region formed by periodic sub-features; wherein the first sub-target comprises a first sub-segment characteristic of its sub-segment region, the second sub-target comprises a second sub-segment characteristic of its sub-segment region, and the first sub-segment characteristic and the second sub-segment characteristic differ in at least one sub-segment parameter.

[0010] In a second aspect, the present invention includes a method for measuring a parameter of interest, comprising: obtaining first measurement data from at least a first sub-target of a target, the at least first sub-target including a first sub-segment characteristic; determining a first parameter of interest value from the first measurement data; obtaining second measurement data from at least a second sub-target of the target, the at least second sub-target including a second sub-segment characteristic, the first sub-segment characteristic and the second sub-segment characteristic differing in at least one sub-segment parameter; determining a second parameter of interest value from the second measurement data; and determining a corrected parameter of interest value from the first parameter of interest value and the second parameter of interest value.

[0011] Other features and advantages of the present invention and the structure and operation of various embodiments of the present invention are described in detail below with reference to the accompanying drawings. Note that the present invention is not limited to the specific embodiments described herein. These embodiments proposed herein are for illustrative purposes only. Based on the teachings contained herein, other embodiments will be apparent to those skilled in the relevant art (s). BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which corresponding reference numerals indicate corresponding parts, and in which:

[0013] Figure 1 The photolithographic apparatus is depicted;

[0014] Figure 2 depicts a lithography cell or cluster in which the inspection apparatus according to the invention may be used;

[0015] Figure 3 schematically illustrates an inspection apparatus adapted to perform angle-resolved scatterometry and dark-field imaging inspection methods;

[0016] Figure 4 illustrates the phenomenon of CD imbalance in a sub-segmented grating; and

[0017] Figure 5is a schematic diagram of an overlay target according to an embodiment. DETAILED DESCRIPTION

[0018] Before describing the various embodiments of the present invention in detail, it is instructive to present an example environment in which the various embodiments of the present invention may be implemented.

[0019] Figure 1 A lithographic apparatus LA is schematically depicted. The apparatus comprises 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., mask table) MT configured to support a patterning device (e.g., mask) MA and connected to a first positioner PM configured to accurately position the patterning device according to certain parameters; two substrate tables (e.g., wafer stages) WTa and WTb, each configured to hold a substrate (e.g., a wafer coated with resist) W and each connected to a second positioner PW configured to accurately position the substrate according to 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 the patterning device MA onto a target portion C (e.g., comprising 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 the position of the patterning device and substrate and the positions of features thereon.

[0020] The illumination system may include various types of optical components for directing, shaping, or controlling radiation, such as refractive components, reflective components, magnetic components, electromagnetic components, electrostatic components, or other types of optical components, or any combination thereof.

[0021] 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 the patterning device is held in a vacuum environment. The patterning device support can take many forms; the patterning device support can ensure that the patterning device is in a desired position, for example, relative to the projection system.

[0022] The term "patterning device" as used herein should be broadly interpreted as referring to any apparatus that can be used to impart a pattern to a cross-section of a radiation beam so as to create a pattern in a target portion of a substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so-called assist features. Typically, the pattern imparted to the radiation beam corresponds to a specific functional layer in a device being created in the target portion (such as an integrated circuit).

[0023] As depicted here, the device is of the transmissive type (e.g., employing a transmissive patterning device). Alternatively, the device may be of the reflective type (e.g., employing a programmable mirror array of the type mentioned above or employing a reflective mask). Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Any use of the term "reticle" or "mask" herein may be considered synonymous with the more general term "patterning device". The term "patterning device" may also be interpreted to refer to a device that stores pattern information in digital form for use in controlling such a programmable patterning device.

[0024] The term "projection system" as used herein should be broadly interpreted to cover any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic, and electrostatic optical systems, or any combination thereof, depending on the exposure radiation being used or 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 synonymous with the more general term "projection system".

[0025] The lithographic apparatus may also be of a type in which at least a portion of the substrate may be covered by a relatively high refractive index liquid (e.g., water) so as to fill the space between the projection system and the substrate. Immersion liquid may also be applied to other spaces in the lithographic apparatus, for example, the space between the mask and the projection system. It is well known in the art that immersion techniques are used to increase the numerical aperture of the projection system.

[0026] 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 this case, the source is not considered to form part of the lithographic apparatus, and the radiation beam is transferred from the source SO to the illuminator IL by means of a beam delivery system BD comprising, for example, suitable directional mirrors and / or a beam expander. In other cases, the source may be an integral part of the lithographic apparatus, for example, when the source is a mercury lamp. If desired, the source SO and the illuminator IL together with the beam delivery system BD are referred to as a radiation system.

[0027] The illuminator IL may, for example, comprise an adjuster AD for adjusting the angular intensity distribution of the radiation beam, an integrator IN and a condenser CO. The illuminator may be used to condition the radiation beam to have a desired uniformity and intensity distribution in its cross-section.

[0028] The radiation beam B is incident on a patterning device MA held on a patterning device support MT and is patterned by the patterning device. After passing through the patterning device (e.g. a mask) MA, the radiation beam B passes through a projection system PS which focuses the beam onto a target portion C of the substrate W. With the aid of a second positioner PW and a position sensor IF (e.g. an interferometric device, a linear encoder, a 2D encoder or a capacitive sensor), the substrate table WTa or WTb can be accurately moved, for example, in order to position a different target portion C in the path of the radiation beam B. Similarly, a first positioner PM and a further position sensor (which is located in the substrate) are used to position the substrate table WTa or WTb. Figure 1 ) can be used to accurately position the patterning device (e.g., reticle / mask) MA relative to the path of the radiation beam B, for example after mechanical retrieval from a mask library or during scanning.

[0029] Mask alignment marks M1, M2 and substrate alignment marks P1, P2 can be used to align the pattern forming device (e.g., reticle / mask) MA and substrate W. Although the substrate alignment marks occupy dedicated target portions as shown, they can be located in the spaces between target portions (these are called scribe alignment marks). Similarly, in the case 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 marks can also be included within the die in the device features, in which case it can be desired that the marks be as small as possible and without any imaging or processing conditions that are different from adjacent features. An alignment system for detecting alignment marks is also described below.

[0030] The depicted device can be used in a variety of modes. In scanning mode, the pattern forming device support (e.g., mask table) MT and the substrate table WT are scanned synchronously while the pattern imparted to the radiation beam is projected onto the target portion C (i.e., a single dynamic exposure). The speed and direction of the substrate table WT relative to the pattern forming device support (e.g., mask table) MT can be determined by the (reduced) magnification and image reversal characteristics of the projection system PS. In scanning mode, the maximum size of the exposure field limits the width of the target portion (in the non-scanning direction) in a single dynamic exposure, while the length of the scanning movement determines the height of the target portion (in the scanning direction). As is well known in the art, other types of lithographic devices and operating modes are also possible. For example, stepping modes are known. In so-called "maskless" lithography, the programmable pattern forming device remains stationary, but the pattern is constantly changing, and the substrate table WT is moved or scanned.

[0031] Combinations and / or variations of the above-described modes of use or entirely different modes of use may also be employed.

[0032] The lithography apparatus LA is of a so-called dual stage type, having two substrate tables WTa, WTb and two stations (exposure station EXP and measurement station MEA), between which the substrate table can be exchanged. While a substrate on one substrate table is being exposed at the exposure station, another substrate can be loaded onto another substrate table at the measurement station and various preparatory steps are implemented. This enables a significant increase in the throughput of the apparatus. The preparatory steps may include mapping the surface height profile of the substrate using a horizontal sensor LS, and measuring the position of the alignment mark on the substrate using an alignment sensor AS. If the position sensor IF cannot measure the position of the substrate table when the substrate table is located at the measurement station and at the exposure station, a second position sensor may be provided to enable the position of the substrate table to be tracked at the two stations relative to the reference frame RF. Other arrangements are known and can be used to replace the dual stage arrangement shown. For example, other lithography apparatuses in which a substrate table and a measurement table are provided are known. When performing preparatory measurements, they are docked together and then undocked while the substrate table is exposed.

[0033] like Figure 2 As shown, the lithography apparatus LA forms part of a lithography cell LC (sometimes also referred to as a lithography cell or cluster), which also includes devices for performing pre-exposure processes and post-exposure processes on a substrate. Traditionally, these devices include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH and a baking plate BK. A substrate gripper or robot RO picks up substrates from input / output ports I / O1, I / O2, moves the substrates between different processing devices, and then delivers them to a feed station LB of the lithography apparatus. These devices, which are often collectively referred to as tracks, are under the control of a track control unit TCU, which itself is controlled by a supervisory control system SCS, which also controls the lithography apparatus via a lithography control unit LACU. Therefore, different devices can be operated to maximize production output and processing efficiency.

[0034] In order to ensure that the substrates exposed by the lithography apparatus are correctly and consistently exposed, it may be desirable to inspect the exposed substrates to measure properties or parameters of interest, such as overlay between subsequent layers, line thickness, critical dimension (CD), etc. Thus, the manufacturing facility in which the lithography unit LC is located also includes a measurement system MET, which receives some or all of the substrates W that have been processed in the lithography unit. The measurement results are provided directly or indirectly to the supervisory control system SCS. If an error is detected, the exposure of subsequent substrates can be adjusted, especially if the inspection can be performed quickly and quickly enough so that other substrates of the same batch still need to be exposed. In addition, the substrates that have been exposed can be stripped and reworked to improve the yield or discarded to avoid further processing of substrates that are known to be defective. In the case where only some target portions on the substrate are defective, further exposure can be performed only on those good target portions.

[0035] In the measurement system MET, the inspection device is used to determine the properties of the substrate, in particular, how the properties of different substrates or different layers of the same substrate vary from layer to layer. The inspection device can be integrated into the lithography device LA or the lithography unit LC, or can be a stand-alone device. In order to enable the fastest measurement, it is desirable that the inspection device measures the properties in the exposed resist layer immediately after exposure. However, the contrast of the latent image in the resist is very low (i.e., the difference in refractive index between the part of the resist that has been exposed to radiation and the part that has not been exposed to radiation is very small), and not all inspection devices are sensitive enough to make useful measurements of the latent image. Therefore, the measurement can be made after the post-exposure baking step (PEB), which is usually the first step implemented 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 called a semi-latent image. It is also possible to measure the developed resist image (at this time, 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 possibilities for reworking defective substrates but can still provide useful information.

[0036] Figure 3 A metrology device suitable for use with embodiments of the present invention is shown in (a). Note that this is only one example of a suitable metrology device. Alternative suitable metrology devices may use EUV radiation, such as, for example, EUV radiation disclosed in WO2017 / 186483A1. Many other types of metrology devices are known and may likewise be used to implement the concepts disclosed herein. Figure 3(b) illustrates in more detail the target structure T and the diffracted rays of the measurement radiation used to illuminate the target structure. The illustrated measurement device is of a type known as a dark field measurement device. The measurement device may be a stand-alone device, or incorporated into a lithography device LA (e.g., a lithography device LA at a measurement station) or a lithography cell LC. An optical axis having several branches in the entire device is represented by a dotted line O. In the device, light emitted by a source 11 (e.g., a xenon lamp) is directed onto a substrate W via a beam splitter 15 through an optical system comprising lenses 12, 14 and an objective lens 16. These lenses are arranged in a double sequence of a 4F arrangement. Different lens arrangements may be used, as long as it still provides a substrate image onto the detector and at the same time allows access to an intermediate pupil plane for spatial frequency filtering. Thus, the angular range of radiation incident on the substrate may be selected by defining a spatial intensity distribution in a plane presenting the spatial spectrum of the substrate plane (here referred to as the (conjugate) pupil plane). In particular, this can be achieved by inserting an aperture plate 13 of appropriate form between lenses 12 and 14 in the plane of the back-projected image of the objective pupil plane. In the illustrated example, the aperture plate 13 has different forms, marked as 13N and 13S, allowing different illumination modes to be selected. The illumination system in this example forms an off-axis illumination mode. In a first illumination mode, the aperture plate 13N provides off-axis illumination from a direction, which is marked as 'North' for descriptive purposes only. In a second illumination mode, the aperture plate 13S is used to provide similar illumination from the opposite direction, which is marked as 'South'. Other illumination modes can also be achieved using different apertures. A specific alternative aperture plate 13Q includes a quartered illumination aperture, in which illumination is permitted via two diagonally opposite quadrants, and the other two quadrants are blocked (the illuminated and blocked quadrants can be swapped with those shown). This arrangement can be used for simultaneous imaging of +1 and -1 diffraction orders, and is also described in the aforementioned US2010201963A1. Ideally, the rest of the pupil plane is dark, since any unwanted light outside the desired illumination pattern may interfere with the desired measurement signal.

[0037] like Figure 3(b) shows that the substrate W of the target structure T is placed perpendicular to the optical axis O of the objective lens 16. The substrate W can be supported by a support (not shown). The measuring radiation light I impinges on the target structure T from an angle offset from the axis O to produce a zero-order ray (solid line 0) and two first-order rays (dotted chain line +1 and double-dotted chain line -1), which are referred to as complementary diffraction order pairs below. It should be noted that the complementary diffraction order pairs can be any higher order pairs; for example, +2, -2 pairs, etc., and are not limited to first-order complementary pairs. It should be remembered that for overfilled small target structures, these rays are only one of many parallel rays covering the area of ​​the substrate including the measurement target structure T and other features. Due to the limited width of the aperture in the plate 13 (which must accommodate a useful amount of light), the incident light ray I actually occupies a certain angle range, and the diffracted rays 0 and +1 / -1 are slightly spread out. According to the point spread function of the small target, each order +1 and -1 is further spread out over a certain angle range, rather than a single ideal ray as shown. Note that the illumination angle and the grating pitch of the target structure can be designed or adjusted so that the first-order light entering the objective is closely aligned with the central optical axis. Figure 3 A and Figure 3 The rays illustrated in B are shown somewhat off-axis purely to make them easier to distinguish in the figure.

[0038] At least the 0th and +1st orders diffracted by the target structure T on the substrate W are collected by the objective lens 16 and directed back through the beam splitter 15. Return to Figure 3 (a), both the first illumination mode and the second illumination mode are described by specifying the apertures along the opposite diameters (which are marked as north (N) and south (S)). When the incident ray I of the measurement radiation comes from the north side of the optical axis, that is, when the first illumination mode is applied using the aperture plate 13N, the +1 diffracted ray marked as +1 (N) enters the objective lens 16. In contrast, when the second illumination mode is applied using the aperture plate 13S, the -1 diffracted ray (marked as 1 (S)) is the diffracted ray that enters the lens 16.

[0039] A second beam splitter 17 splits the diffracted beam into two measurement branches. In the first measurement branch, an optical system 18 forms a diffraction spectrum (pupil plane image) of the target structure on a first sensor 19 (e.g., a CCD sensor or a CMOS sensor) using the zero-order diffracted beam and the first-order diffracted beam. Each diffraction order hits a different point on the sensor so that image processing can compare and contrast multiple orders. The pupil plane image captured by the sensor 19 can be used to focus the metrology device and / or normalize the intensity measurement of the first-order beam. The pupil plane image can also be used for many measurement purposes, such as reconstruction.

[0040] In the second measurement branch, the optical system 20, 22 forms an image of the target structure T on a sensor 23 (e.g., a CCD sensor or a CMOS sensor). In the second measurement branch, an aperture stop 21 is provided in a plane 21 conjugate to the pupil plane. The function of the aperture stop 21 is to block the zeroth order diffraction beam so that the image of the target formed on the sensor is formed only by the -1st order beam or the +1st order beam. Instead of a simple aperture stop 21a, the metrology tool can use a wedge arrangement 21b that separates the first order from the zeroth order so that they can be imaged separately (separated) on the detector 23. Such an arrangement is described in the aforementioned US2011102753A1 and can be used (e.g., in combination with a quartered illumination aperture 13Q) to simultaneously acquire the +1st and -1st diffraction orders in one or two directions in the substrate plane.

[0041] The image captured by sensors 19 and 23 is output to a processor PU which processes the image, the function of which will depend on the particular type of measurement being performed. Note that the term 'image' is used here in a broad sense. Thus, if only one of the -1 and +1 orders is present, no image of the raster lines will be formed.

[0042] Position errors may occur due to overlay errors (often referred to as "overlay"). Overlay is the error in placing a first feature during a first exposure relative to a second feature during a second exposure. Lithographic apparatus minimizes overlay by accurately aligning each substrate to a reference prior to patterning, for example, by using alignment sensors to measure the positions of alignment marks on the substrate; and by using feedback corrections to the exposure process, for example, based on the use of suitable metrology tools such as Figure 3 (a) to measure the overlay on the metrology target after exposure.

[0043] You can use measurement tools such as Figure 3 A known metrology method performed by a metrology tool (such as the metrology tool illustrated in (a)) is known as diffraction-based overlay (DBO) or micro-diffraction-based overlay (μDBO). This μDBO technique uses the imaging branch of the metrology tool (detector 23). Each of the main “images” used to derive the parameter of interest is formed only by the corresponding higher (e.g. first) diffraction order, so that a resolved pattern will now be shown rather than areas of certain intensity levels. The zeroth order (specularly reflected radiation) is typically blocked or diverted elsewhere (e.g. to another part of the detector for monitoring purposes); it is not directly used for μDBO measurements. Asymmetries in structures such as μDBO targets can be detected by complementary diffraction order pairs (typically first complementary diffraction order pairs, i.e., Figure 3(b) may be determined by an intensity asymmetry or intensity difference between the +1 and -1 orders (shown in (b), although +2 / -2 or higher orders may technically be used). Thus, a first intensity value may be obtained from an image formed by the +1 order, and a second intensity value may be obtained from the -1 order; wherein the difference in these intensity values ​​is used to calculate the asymmetry of the structure. The overlay target may comprise two overlaid periodic structures or gratings in different layers, such that any overlay (i.e., misalignment between the layers) will result in a misalignment of the two gratings, which manifests as an asymmetry of the target as a whole (a deliberate offset may be provided between the two overlaid gratings to correct for certain target asymmetries). Thus, the techniques just described may be used to measure this target asymmetry and hence the overlay.

[0044] An alternative overlay metrology approach is referred to as continuous DBO or cDBO, where the measured asymmetry signal can be the phase difference asymmetry from a complementary sub-target pair ("M pad" and "W pad"), rather than the intensity asymmetry just described. More specifically, in cDBO, the asymmetry signal A can be defined as the phase difference between a diffraction order from "M pad" and the corresponding diffraction order from "W pad", optionally averaged over the two diffraction orders of the complementary diffraction order pair: For example, A = (φ M -φ W ) +1 +(φ M -φ W ) -1 , where (φ M -φ W ) +1 、(φ M -φ W ) -1 The measured phase difference between the "M pad" and the "W pad" of the +1 diffraction order and the -1 diffraction order, respectively. As such, it should be appreciated that the concepts described herein can be applied to different types of asymmetry signals. The principles of cDBO are described in Matsunobu et al., "Novel diffraction-based overlay metrology utilizing phase-based overlay for improved robustness," Proc. SPIE 11611, Metrology, Inspection, and Process Control for Semiconductor Manufacturing XXXV, 1161126 (Feb. 22, 2021), which is incorporated herein by reference.

[0045] Like the μDBO target, the cDBO target includes an overlaid periodic structure or grating in the corresponding layer whose overlay values ​​are to be measured. Instead of having gratings with the same pitch in the two layers as in the μDBO target, the cDBO target includes sub-targets, each sub-target having gratings with different pitches in the two layers. More specifically, a typical cDBO target includes an arrangement of two different types of sub-targets (e.g., for each direction): an "M pad" or "M sub-grating", which includes a bottom grating with a smaller pitch than the top grating; and a "W pad" or "W sub-grating", which inverts these gratings (i.e., it has the same pitch as the M pad, but has a larger pitch in the top layer).

[0046] Overlay metrology assumes that the grating used in the target is strictly symmetric. Under this assumption, the true overlay is proportional to the measured intensity asymmetry. In reality, there are many processes that make the grating of the overlay target asymmetric, for example, chemical mechanical polishing (CMP), etching, deposition, etc.

[0047] For measurement using current optical metrology techniques (e.g., a scatterometer such as the one described above), the measurable pitch or principal pitch of the target grating may be several orders of magnitude larger than the pitch and / or CD or product features (actual functional device features). It is often desirable to segment the target grating using product-like features to avoid damage to the target grating and / or contamination of device areas due to wafer processing steps such as polishing, deposition, or etching. Such product-like features may include features of similar size and / or resolution as product features (e.g., having a CD of the same order of magnitude as the critical dimension (CD) of the product features). Such a target grating may be referred to as a sub-segmented target grating, which includes sub-segments of the lines (first regions) and / or spaces (second regions) of the target grating. The periodic structure or grating defined by these first and second regions may form a metrology tool resolvable pitch, for example, resolvable by a scatterometer such as the one described above. For example, each row or each space may be segmented to include periodic sub-features, such as a periodic (1D) array of multiple features having a product amplitude CD and / or pitch.

[0048] Figure 4The diagram illustrates a specific type of undesired grating asymmetry addressed by the concepts described herein, which is sometimes referred to as CD imbalance. CD imbalance is particularly applicable to sub-segmented targets, for example, targets formed by one or more sub-segmented gratings. The figure shows a bottom grating BG of a target (e.g., a μDBO target or a cDBO target) comprising a series of features (first regions FR) and / or spaces (second regions SR). Each of the features and / or first regions FR (although it may be a second region) is segmented into sub-features to form sub-segmented marks SF. Details of a single segmented feature are shown in the figure.

[0049] It should be appreciated that the segmentation can be only features / first regions, only spaces / second regions, or both lines / first regions and spaces / second regions. In the latter case, the segmentation can be such that the "space" includes a periodic substructure that includes a periodic array of multiple features, each feature having a smaller CD than the multiple features included in the "line" of the target (which also includes the periodic substructure), and therefore having different optical properties. Note that the subsegments do not have to be 1D periodic. Other examples of subsegments include contact arrays, staggered relief, tilted lines, or any other subsegment structures. The actual form of the subsegments is not important.

[0050] Each of the subfeatures is designed to have the same width or CD (i.e., the width of the subfeature in the periodic direction of the grating marked as X). The inner subfeatures ISF (i.e., all subfeatures except the outermost two subfeatures OSF1, OSF2 (i.e., the first subfeature OSF1 and the last subfeature OSF2)) typically have substantially the same CD. However, the two (or more) outermost subfeatures may have different CDs; more specifically, the CD of the first subfeature of a feature is smaller than that of the inner subfeature by a CD difference ΔCD taken from the first outermost edge of the feature, and the CD of the last subfeature of the grating BG is larger than that of the inner subfeature by this same CD difference ΔCD added to the second outermost edge of the feature. This is as if the outer edge portion of the first subfeature OSF1 was taken from the first subfeature and added to the outer edge of the last subfeature OSF2. More features than the two outermost features may be affected in this manner; however, the outermost features are affected the most. The number of affected features is not critical to the concepts disclosed herein. This CD difference exists due to optical / processing interference. Ideally, all CDs would be identical.

[0051] This type of asymmetry leads to an overlay measurement error (error in the overlay measurement). The inventors quantify this error as follows. Figure 4 , the leftmost or first sub-feature is at position x 0，0 and x0，1 has an edge at position x and the rightmost or last child feature is at position x N-1，0 and x N-1，1 has edges where:

[0052]

[0053] where CD is the (expected) CD, N is the number of sub-features included in each feature (region), and p s is the sub-segment pitch (the pitch of the sub-features).

[0054] Calculating the optical center of gravity CoG of the modeled feature yields:

[0055]

[0056] For a sufficiently high number of sub-features N, this can be approximated as:

[0057]

[0058] Therefore, for more isolated sub-features, the impact of CD imbalance on overlay error becomes larger.

[0059] In order to solve this CD imbalance, a target arrangement and a method of measuring overlay using this target arrangement are proposed. The target arrangement and the method enable overlay values ​​to be measured which have been corrected for the effects of CD imbalance in the target grating.

[0060] The proposed target concept includes multiple sub-targets, which include at least a first sub-target and a second sub-target, each of the multiple sub-targets includes at least one sub-segment periodic structure having a repeated first region and a second region, wherein the first region or the second region includes a periodic sub-feature; wherein the first sub-target includes a first sub-segment characteristic, the second sub-target includes a second sub-segment characteristic, and the first sub-segment characteristic and the second sub-segment characteristic differ in at least one sub-segment parameter.

[0061] The proposed target concept includes a target having at least one first pad type or sub-target type and a second pad type or sub-target type, the first pad type or sub-target type including at least one (e.g., bottom) periodic structure or grating having a first sub-segment characteristic, the second pad type or sub-target type including at least one (e.g., bottom) periodic structure or grating having a second sub-segment characteristic. The first sub-segment characteristic may include a first number N of sub-features per first region or second region (e.g., per line or space) 1 and the first sub-feature CD or sub-feature width CD 1, and the second sub-segment characteristic may include a second number N of sub-features of each first region or second region 2 and the second sub-feature CD or sub-feature width CD 2 , where N 1 ≠N 2 and / or CD 1 ≠CD 2 As such, it should be appreciated that only one of the number of sub-features or sub-feature CDs need differ between sub-target types.

[0062] In one embodiment, the target may include a first sub-target pair and a second sub-target pair, wherein the first sub-target pair includes each of the two sub-target types (e.g., the first target and the second target), and the second sub-target pair includes each of the two sub-target types (e.g., a third target having the first sub-segment characteristics and a fourth target having the second sub-segment characteristics).

[0063] For example, in a μDBO embodiment, as is well known in μDBO measurements, each of these sub-target pairs may have a different bias (strictly speaking, only one pair needs to be biased so that there is a bias difference between the two pairs). In a typical arrangement, the respective biases of each pair may be equal in magnitude but opposite in direction.

[0064] In one embodiment, the target may include at least one first target type and at least one second target type (e.g., an X pad and a Y pad) for each measurement direction. In one embodiment, the target may include the first sub-target pair and the second sub-target pair (e.g., a total of 8 sub-targets) for each measurement direction.

[0065] Figure 5 is a schematic diagram of a target according to the previous embodiment. Instead of having two pads or sub-targets per measurement direction as used in μDBO, the target here includes four pads or sub-targets per measurement direction (eight sub-targets in total). In each direction, each of the first sub-target pair and the second sub-target pair includes a first sub-target type N having a first sub-segment characteristic 1 , CD 1 and a second sub-target type N having a second sub-segment characteristic 2 , CD 2 , where N 1 ≠N 2 and / or CD 1 ≠CD 2 For each direction, the first sub-target pair may have a first bias (eg, +d) and the second sub-target pair may have a second bias (eg, -d).

[0066] Using the equation for the center of gravity described above, we can see that:

[0067]

[0068] Among them OV measured1 is the overlay measured using the first sub-target type, and OV measured2 is the overlay measured using the second sub-target type. Unknown is the true overlay (parameter of interest) OV true and CD imbalance ΔCD. Thus, the target design generates enough information for two equations from which the true overlay can be calculated according to the following formula:

[0069]

[0070] The measured overlay value OV can be obtained using the standard μDBO intensity asymmetry measurement measured1 , OV measured2 For example, OV may be measured based on an intensity asymmetry measure of a first sub-target of a first sub-target type. measured1 , and the OV can be measured according to the second sub-goal of the second sub-goal type measured2 .

[0071] Alternatively, assuming that Figure 5 For a target such as the one shown, the OV can be measured based on a first sub-target type from a first sub-target pair and a first intensity asymmetry measurement pair from a first sub-target type from a second sub-target pair. measured1 OV can be measured based on the second sub-goal type from the first sub-goal pair and the second sub-goal type from the second sub-goal pair. measured2 Considering the different offsets between the first sub-target pair and the second sub-target pair, before correcting the proposed CD imbalance, the overlay value OV is obtained. measured1 , OV measured2 When the other target asymmetries are corrected, these first corrections (to obtain the overlay value OV measured1 , OV measured2 ) can also potentially correct (partially) the effects of CD imbalance, but it can be assumed that the target asymmetry correction will capture the CD imbalance of both sub-targets similarly (with equal efficiency). true , these corrections should be canceled to prevent any double correction.

[0072] It has been shown that the assumptions on which these concepts are based (eg, that the ACD of both subtarget types is the same and that the CoG is a surrogate of the measured overlay) are sufficiently correct.

[0073] It should be appreciated that the same basic concepts apply equally to cDBO measurements and cDBO target types. This approach can use (e.g., optionally, in each direction) a first sub-target pair comprising two M sub-targets and a second sub-target pair comprising two W sub-targets. The first sub-target pair can include: a first M sub-target type having a first sub-segment characteristic for at least one grating in its grating; and a second M sub-target type having a second sub-segment characteristic for at least one grating in its grating. Similarly, the second sub-target pair can include: a first W sub-target type having a first sub-segment characteristic for at least one grating in its grating; and a second W sub-target type having a second sub-segment characteristic for at least one grating in its grating. In this case, the differential fringe position (measured phase difference) is now directly related to the center of gravity difference between the different sub-target types, but with Moiré magnification.

[0074] Other embodiments according to the invention are described in the following numbered clauses.

[0075] 1. A substrate comprising at least one target, the target comprising a plurality of sub-targets, the plurality of sub-targets comprising at least a first sub-target and a second sub-target, each of the plurality of sub-targets comprising at least one sub-segment periodic structure having a repeated first region and a second region, wherein at least one of the first region or the second region comprises a sub-segment region formed by periodic sub-features; and wherein the first sub-target comprises a first sub-segment characteristic of its sub-segment region, the second sub-target comprises a second sub-segment characteristic of its sub-segment region, the first sub-segment characteristic and the second sub-segment characteristic differ in at least one sub-segment parameter.

[0076] 2. The substrate of clause 1, wherein the at least one sub-segmentation parameter comprises one or both of: an expected sub-feature width of the sub-feature and a number of sub-features per sub-segmentation region.

[0077] 3. A substrate according to clause 1 or 2, wherein the plurality of sub-targets comprises at least the first sub-target and the second sub-target repeated for each measurement direction in a substrate plane of the substrate.

[0078] 4. A substrate according to clause 1, 2 or 3, wherein the sub-targets each comprise a pair of periodic structures having a corresponding periodic structure in each of the two layers, each periodic structure pair comprising the at least one sub-segmented periodic structure.

[0079] 5. A substrate according to clause 4, wherein the at least one sub-segment periodic structure comprises a bottom periodic structure in each sub-target.

[0080] 6. A substrate according to clause 4 or 5, wherein the plurality of targets comprises:

[0081] A first sub-goal pair, comprising the first sub-goal and the second sub-goal; and

[0082] The second target pair includes a third sub-target having the first sub-segment characteristic and a fourth sub-target having the second sub-segment characteristic.

[0083] 7. A substrate according to clause 6, wherein the pairs of periodic structures in each sub-target have the same pitch; and

[0084] There is an intentional offset difference in the periodic structure pair between the first sub-target pair and the second sub-target pair.

[0085] 8. A substrate according to clause 6 or 7, wherein the first sub-target pair each has a first intentional offset and the second sub-target pair each has a second offset, the first offset and the second offset being equal in magnitude and opposite in direction.

[0086] 9. The substrate of clause 6, wherein the pair of periodic structures in each sub-target has a different pitch, and wherein the order of the gratings is reversed within the target layer between the first pair of sub-targets and the second pair of sub-targets.

[0087] 10. A substrate according to any of clauses 6 to 9, wherein the plurality of sub-targets comprises at least the first sub-target pair and the second sub-target pair repeated for each measurement direction in a substrate plane of the substrate.

[0088] The terms "radiation" and "beam" as used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of approximately 365 nm, 355 nm, 248 nm, 193 nm, 157 nm, or 126 nm) and extreme ultraviolet (EUV) radiation (e.g., having a wavelength in the range of 5 nm to 20 nm), as well as particle beams (such as ion beams or electron beams).

[0089] The term "lens," where the context permits, may refer to any one or combination of various types of optical components, including refractive optical components, reflective optical components, magnetic optical components, electromagnetic optical components, and electrostatic optical components.

[0090] The term "target" should not be interpreted as meaning only dedicated targets formed for specific metrology purposes. The term "target" should be understood to encompass other structures, including product structures, having properties suitable for metrology applications.

[0091] The above description of the specific embodiments will fully reveal the general nature of the present invention, so that without departing from the general concept of the present invention, others can easily modify and / or adapt the various applications of these specific embodiments by applying the knowledge in the art without improper experimentation. Therefore, based on the teachings and guidance given herein, such adaptations and modifications are intended to be within the meaning and scope of the equivalents of the disclosed embodiments. It should be understood that the wording or terminology herein is for the purpose of describing by example rather than limiting, so that the terms or wording of this specification are to be interpreted by the technician according to teaching and guidance.

[0092] 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, the target comprising a plurality of sub-targets, the plurality of sub-targets comprising at least a first sub-target and a second sub-target, each of the plurality of sub-targets comprising at least one sub-segment periodic structure having a first region and a second region that repeat, wherein at least one of the first region or the second region comprises a sub-segment region formed by a periodic sub-feature; The first sub-target has a first sub-segment characteristic of its sub-segment area, the second sub-target has a second sub-segment characteristic of its sub-segment area, and the first sub-segment characteristic and the second sub-segment characteristic are different in at least one sub-segment parameter. 2 . The substrate of claim 1 , wherein the at least one sub-segmentation parameter comprises one or both of: an expected sub-feature width of the sub-feature and a number of sub-features per sub-segmentation region.

3. The substrate according to claim 1 or 2, wherein the plurality of sub-targets include at least the first sub-target and the second sub-target repeated for each measurement direction in a substrate plane of the substrate.

4. A substrate according to claim 1, 2 or 3, wherein the sub-targets each include a pair of periodic structures, the pair of periodic structures having corresponding periodic structures in each of the two layers, each periodic structure pair including the at least one sub-segmented periodic structure.

5. The substrate of claim 4, wherein the at least one sub-segment periodic structure comprises a bottom periodic structure in each sub-target.

6. The substrate of claim 4 or 5, wherein the plurality of targets comprises: A first sub-goal pair, comprising the first sub-goal and the second sub-goal; as well as The second target pair includes a third sub-target having the first sub-segment characteristic and a fourth sub-target having the second sub-segment characteristic.

7. The substrate of claim 6, wherein the pairs of periodic structures in each sub-target have the same pitch; and There is an intentional offset difference in the periodic structure pair between the first sub-target pair and the second sub-target pair.

8. A method for measuring a parameter of interest, comprising: obtaining first measurement data from at least a first sub-object of the object, the at least first sub-object having a first sub-segment characteristic; determining a first parameter value of interest from said first measurement data; obtaining second measurement data from at least a second sub-target of the target, the at least second sub-target having a second sub-segment characteristic, the first sub-segment characteristic and the second sub-segment characteristic differing in at least one sub-segment parameter; determining a second parameter value of interest from the second measurement data; as well as From the first parameter value of interest and the second parameter value of interest, a corrected parameter value of interest is determined. 9 . The method of claim 8 , wherein the at least one sub-segmentation parameter comprises one or both of: an expected sub-feature width of the sub-feature and a number of sub-features per sub-segmentation region.

10. A method according to claim 8 or 9, wherein the parameter of interest is overlay.

11. A method according to claim 8, 9 or 10, wherein the target comprises the at least one target of a substrate according to any one of claims 1 to 7.

12. A processing device comprising a processor and configured to execute the method according to any one of claims 8 to 11.

13. A measurement device comprising the processor according to claim 12.

14. A computer program comprising program instructions operable, when run on suitable means, to perform a method according to any one of claims 8 to 11.

15. A non-transitory computer program carrier comprising a computer program according to claim 14.

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