Measurement target and related measurement method

Abstract

A substrate including at least one target is disclosed. The target includes a plurality of sub-targets, the plurality of sub-targets including at least a first sub-target and a second sub-target, each of the plurality of sub-targets including at least one sub-segmented periodic structure having a repetition of a first region and a second region, at least one of the first region or the second region including a sub-segmented region formed of periodic sub-features. The first sub-target includes first sub-segmentation characteristics of its sub-segmented region, the second sub-target includes second sub-segmentation characteristics of its sub-segmented region, and the first sub-segmentation characteristics and the second sub-segmentation characteristics differ with respect to at least one sub-segmentation parameter.

Description

Technical Field

[0001] [Cross - Reference to Related Applications] This application claims the benefit of European Patent Application No. 22184952.4, filed on July 14, 2022, the entire content of which is incorporated herein by reference.

[0002] The present invention relates to a measuring device and method useful for performing measurements, for example, in the manufacture of devices by lithography techniques.

Background Art

[0003] A lithography apparatus is a machine that applies a desired pattern onto a target portion of a substrate. A lithography apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that case, a patterning device, also referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., part of a die, one die or several dies) on a substrate (e.g., a silicon wafer). The transfer of the pattern typically occurs via imaging onto a layer of radiation - sensitive material (resist) provided on the substrate. In general, a single substrate will include a network of adjacent target portions that are patterned successively.

[0004] In a lithography process, for example for process control and verification, it is often required to measure the structures being formed. Various tools are known for performing such measurements, including scanning electron microscopes often used for measuring critical dimensions (CD), and tools specialized for measuring overlay, which is the alignment accuracy between two layers of a device. Recently, various forms of scatterometers have been developed for use in the lithography field. This apparatus directs a beam of radiation onto a target and measures one or more characteristics of the scattered radiation (e.g., intensity as a function of wavelength at a single reflection angle, intensity as a function of reflection angle at one or more wavelengths, or polarization as a function of reflection angle) to obtain a diffraction "spectrum". The characteristics of the target of interest can be determined from this "spectrum".

[0005] Examples of known scatterometers include angular scatterometers of the type described in US2006033921A1 and US2010201963A1. The targets used in such scatterometers are relatively large, e.g., a 40 μm × 40 μm grid, and the measurement beam generates a spot smaller than the grid (i.e., the grid is underfilled). Examples of dark field imaging measurements can be found in international patent applications US20100328655A1 and US2011069292A1, the entire contents of which are incorporated herein by reference. Another development of this technology is described in patent application publications US20110027704A, US20110043791A, US2011102753A1, US20120044470A, US20120123581A, US20130258310A, US20130271740A and WO2013178422A1. These targets may be smaller than the illumination spot and may be surrounded by product structures on the wafer. Composite grid targets can be used to measure multiple grids in one image. The contents of all these applications are also incorporated herein by reference.

Summary of the Invention

Problems to be Solved by the Invention

[0006] An important parameter that can be monitored is the overlay, which is a measure of the misalignment between patterns of different layers (e.g., an overlay of zero indicates perfect alignment). The overlay can be monitored by measuring an overlay target designed to have an asymmetry that depends on the overlay. This asymmetry can be measured by a metrology tool, and the overlay can be inferred. The metrology target is composed of a periodic structure or a pair of gratings, with one grating for each associated layer. Since the metrology tool measures the asymmetry of the structure, asymmetries that do not depend on the overlay, such as the asymmetry of individual gratings, appear as overlay measurement errors. One type of grating asymmetry inherent in a segmented target (where individual features or spaces of the target grating are segmented) is known as CD imbalance. Here, the CD of the first one or more features of each target feature is smaller than the nominal feature CD, and the CD of the last one or more features of each target feature is larger than the nominal feature CD.

[0007] It is desirable to be able to correct this CD imbalance.

Means for Solving the Problem

[0008] A first aspect of the present invention is a substrate including at least one target, the target including a plurality of sub-targets, the plurality of sub-targets including at least one first sub-target and a second sub-target, each of the plurality of sub-targets including at least one sub-segmented periodic structure having a repetition of a first region and a second region, at least one of the first region or the second region including a sub-segmented region formed of periodic sub-features, the first sub-target including a first sub-segmentation characteristic with respect to its sub-segmented region, the second sub-target including a second sub-segmentation characteristic with respect to its sub-segmented region, the first sub-segmentation characteristic and the second sub-segmentation characteristic being different with respect to at least one sub-segmentation parameter, the substrate.

[0009] A second aspect of the present invention is a method of 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-segmentation characteristic, determining a first value of the parameter of interest 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-segmentation characteristic, the first sub-segmentation characteristic and the second sub-segmentation characteristic being different with respect to at least one sub-segmentation parameter, determining a second value of the parameter of interest from the second measurement data, and determining a value of the corrected parameter of interest from the first value of the parameter of interest and the second value of the parameter of interest.

[0010] Further features and advantages of the present invention will be described in detail below with reference to the accompanying drawings, together with the structure and operation of various embodiments of the present invention. It should be noted that the present invention is not limited to the specific embodiments described herein. Such embodiments are shown herein for illustrative purposes only. Additional embodiments will be apparent to those skilled in the art based on the teachings contained herein.

Brief Description of the Drawings

[0011] Here, by way of mere example, embodiments of the present invention will be described with reference to the accompanying schematic diagrams. In these drawings, the same reference numerals denote corresponding parts.

Figure 1

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Embodiments for Carrying Out the Invention

[0012] Before describing embodiments of the present invention in detail, it is helpful to present an exemplary environment in which embodiments of the present invention can be implemented.

[0013] FIG. 1 is a diagram schematically showing a lithographic apparatus LA. The apparatus comprises an illumination 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 positioning device PM configured to accurately position the patterning device in accordance with certain parameters; two substrate tables (e.g., wafer tables) WTa and WTb each constructed to hold a substrate (e.g., a resist-coated wafer) W and each connected to a second positioning device PW configured to accurately position the substrate in accordance with certain parameters; and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g., including one or more dies) of the substrate W. A reference frame RF connects the various components and serves as a reference for setting and measuring the positions of the patterning device and the substrate and features thereon.

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

[0015] The patterning device support holds the patterning device in a manner depending on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions such as whether the patterning device is held in a vacuum environment. The patterning device support can take many forms. The patterning device support can, for example, ensure that the patterning device is in a desired position with respect to the projection system.

[0016] The term "patterning device" as used in this document should be broadly construed to refer to any device that can be used to pattern a cross-section of a radiation beam to create a pattern in a target portion of a substrate. It should be noted that the pattern imparted to the radiation beam need not exactly match the desired pattern in the target portion of the substrate, for example, when the pattern includes phase-shift features or so-called assist features. In general, the pattern imparted to the radiation beam will correspond to a particular functional layer within the device being created in the target portion, such as an integrated circuit.

[0017] As illustrated, the apparatus is transmissive (e.g., using a transmissive patterning device). Alternatively, the apparatus may be reflective (e.g., using a programmable mirror array as described above or a reflective mask). Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Any use of the terms "reticle" or "mask" in this document may be considered synonymous with the more general term "patterning device". The term "patterning device" may also be construed to refer to a device that stores pattern information in digital form for use in controlling, for example, a programmable patterning device.

[0018] The term "projection system" as used in this document should be broadly construed to encompass any form of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic, and electrostatic optical systems or any combination thereof, provided that it is appropriate for the exposure radiation used, the use of immersion liquid, or the use of a vacuum, etc. Any use of the term "projection lens" in this document may be considered synonymous with the more general term "projection system".

[0019] The lithographic apparatus may be of a type in which at least a portion of a substrate is covered by a liquid having a relatively high refractive index (e.g. water), and the space between the projection system and the substrate is filled. The immersion liquid may also be applied to other spaces in the lithographic apparatus, such as between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems.

[0020] During operation, the illumination system IL receives a radiation beam from a radiation source SO. The radiation source and the lithographic apparatus may be separate, for example when the radiation source is an excimer laser. In such a case, the radiation source is not considered to form part of the lithographic apparatus, and the radiation beam passes from the radiation source SO towards the illuminator IL, for example with the aid of a beam delivery system BD including suitable directing mirrors and / or a beam expander. In another case, for example when the radiation source is a mercury lamp, the radiation source may be an integral part of the lithographic apparatus. The radiation source SO and the illuminator IL may, if desired, together with the beam delivery system BD be referred to as a radiation system.

[0021] The illuminator IL may include, for example, 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 adjust the radiation beam so as to have a desired uniformity and intensity distribution in its cross-section.

[0022] 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 that focuses the beam on a target portion C of the substrate W. With the aid of a second positioning device PW and a position sensor IF (e.g., an interferometer device, a linear encoder, or a capacitance sensor), the substrate table WTa or WTb can be accurately moved, for example, so that different target portions C are located on the path of the radiation beam B. Similarly, using a first positioning device PM and another position sensor (not explicitly shown in FIG. 1), the patterning device (e.g., a reticle / mask) MA can be accurately positioned with respect to the path of the radiation beam B, for example, after a mechanical search from a mask library or during a scan.

[0023] The patterning device (e.g., a reticle / mask) MA and the substrate W can be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks are depicted as occupying dedicated target portions, they may be arranged in the spaces between the target portions (known as scribe line alignment marks). Similarly, if two or more dies are provided on the patterning device (e.g., a mask) MA, the mask alignment marks may be arranged between the dies. Small alignment marks may be included between the device features inside the die, in which case it is desirable that the markers be as small as possible and not require any imaging or process conditions that are different from those of adjacent features. An alignment system for detecting the alignment markers is described separately below.

[0024] The apparatus shown can be used in a variety of modes. In the scan mode, while the patterning device support (e.g., mask table) MT and the substrate table WT are scanned synchronously, the pattern applied to the radiation beam is projected onto the target portion C (i.e., single dynamic exposure). The speed and direction of the substrate table WT relative to the patterning device support (e.g., mask table) MT can be determined by the magnification (reduction) characteristics and the image inversion characteristics of the projection system PS. In the scan mode, the maximum size of the exposure field limits the width (in the non-scan direction) of the target portion in a single dynamic exposure, while the length of the scan operation determines the height (in the scan direction) of the target portion. As is well known in the art, other types of lithographic apparatus and operating modes are also possible. For example, the step mode is known. In so-called “maskless” lithography, the programmable patterning device is held stationary but with a change in pattern, while the substrate table WT is moved or scanned.

[0025] The above-described usage modes may be combined, and / or modifications may be made to the above-described usage modes, or completely different usage modes may be used.

[0026] The lithography apparatus LA is of the so-called dual-stage type having two substrate tables WTa, WTb and two stations, and the substrate table can be exchanged between the exposure station EXP and the measurement station MEA. While one substrate on one substrate table is being exposed at the exposure station, another substrate can be loaded onto the other substrate table at the measurement station and various preparation steps are carried out. This enables a substantial increase in the throughput of the apparatus. The preparation steps may include mapping the contour of the surface of the substrate using the level sensor LS and measuring the position of the alignment markers on the substrate using the alignment sensor AS. If the position sensor IF cannot measure the position of the substrate table while it is at the measurement station or the exposure station, a second position sensor may be provided to enable tracking of the position of the substrate table at both stations with respect to the reference frame RF. Other configurations are known and available instead of the dual-stage configuration shown. For example, other lithography apparatuses are known in which a substrate table and a measurement table are provided. These are docked together when preliminary measurements are carried out, while being separated when the substrate table is being exposed.

[0027] As shown in FIG. 2, the lithography apparatus LA, often also referred to as a litho cell or cluster, forms part of a lithography cell LC which also includes apparatus for performing pre - and post - exposure processes on a substrate. Conventionally, these include a spin coater SC for depositing a resist layer, a developing device DE for developing the exposed resist, a cooling plate CH and a baking plate BK. A substrate handler or robot RO removes the substrate from the input / output ports I / O1, I / O2, moves the substrate between different process devices, and transports it to the loading bay LB of the lithography apparatus. These devices, often collectively referred to as a track, are under the control of a track control unit TCU. The TCU itself is controlled by a supervisory control system SCS which also controls the lithography apparatus via a lithography control unit LACU. Thus, the different devices can be operated so as to maximize throughput and process efficiency.

[0028] In order to accurately and consistently expose the substrates exposed by the lithography apparatus, it is desirable to inspect the exposed substrates to measure important characteristics or parameters such as overlay, line width, critical dimension (CD), etc. between subsequent layers. Thus, the manufacturing equipment disposed within the litho cell LC also includes a metrology system MET which receives some or all of the substrates W processed within the litho cell. The metrology results are provided directly or indirectly to the supervisory control system SCS. If an error is detected, adjustments may be made to the exposure of subsequent substrates, especially if the inspection can be performed quickly and at high speed to the extent that other substrates of the same batch are still being exposed. Also, substrates that have already been exposed may be stripped and re - processed or discarded to improve yield, thereby avoiding further processes being performed on substrates known to be defective. If only some of the target portions of a substrate are defective, further exposure can be performed only on the good target portions.

[0029] In metrology system MET, the inspection apparatus is used to determine the characteristics of a substrate, in particular to determine how the characteristics of different substrates or different layers of the same substrate differ from layer to layer. The inspection apparatus may be integrated into a lithography apparatus LA or a lithocell LC, or may be a stand-alone apparatus. In order to enable the fastest measurements, it is desirable for the inspection apparatus to measure the characteristics in the exposed resist immediately after exposure. However, the latent image in the resist has a very low contrast and there is only a very small refractive index difference between the resist portions exposed by radiation and those not, and not all inspection apparatuses have sufficient sensitivity to perform an effective measurement of the latent image. Therefore, the measurement may be performed after the post-exposure bake step (PEB). The PEB is typically the first step performed on the exposed substrate and increases the contrast between the exposed and unexposed portions of the resist. At this stage, the image in the resist may be referred to as a semi-latent image. It is also possible to perform a measurement of the developed image (at this point, either the exposed or unexposed portion of the resist has been removed), or after a pattern transfer step such as etching. The latter possibility limits the possibility of reworking substrate defects but may provide further useful information.

[0030] A measuring device suitable for use in an embodiment of the present invention is shown in Fig. 3(a). It should be noted that this is only an example of a suitable measuring device. Another suitable measuring device can use EUV radiation as disclosed in, for example, WO2017 / 186483A1. Many other types of measuring devices are known and can be used similarly to implement the concepts disclosed herein. The target structure T and the diffracted rays of the measurement radiation used to irradiate the target structure are shown in more detail in Fig. 3(b). The measuring device shown is of a type known as a dark-field measuring device. The measuring device may be a stand-alone device or may be incorporated into either a lithography apparatus LA (e.g., at a measurement station) or a lithography cell LC. The optical axis having a plurality of branches through the device is indicated by the dashed line O. In this device, the light output by a light 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 configured in a double sequence in a 4F arrangement. Different lens arrangements can also be used if an image of the substrate is to be given on the detector, and at the same time, this lens arrangement enables the use of an intermediate pupil plane for spatial frequency filtering. Thus, the angular range at the position where the radiation is incident on the substrate represents the spatial spectrum in the substrate plane and can be selected by defining the spatial intensity distribution in the plane, herein referred to as the (conjugate) pupil plane. Specifically, this can be achieved by inserting an aperture plate 13 of a suitable shape in the plane between lenses 12 and 14 and which is the inverse projection image of the objective lens pupil plane. In the example shown, the aperture plates 13 denoted by reference numerals 13N, 13S have different shapes and enable the selection of different illumination modes. The illumination system of this example forms an off-axis illumination mode. In the first illumination mode, the aperture plate 13N provides off-axis illumination from a direction designated "north" for illustrative purposes only. In the second illumination mode, the aperture plate 13S is used to provide illumination from the opposite direction, which is similar but named "south". By using different apertures, other illumination modes are also possible.As a specific alternative, the aperture plate 13Q has a four-segmented illumination aperture, and illumination enters from two diagonal quadrants, while the other two quadrants are blocked (the illuminated and blocked quadrants can be interchanged with those shown). Such an arrangement can be used for simultaneous imaging of the +1st and -1st diffraction orders, and is further described in the aforementioned US2010201963A1. Since any unwanted light outside the desired illumination mode interferes with the desired measurement signal, it is desirable that the rest of the pupil plane be dark.

[0031] As shown in FIG. 3(b), the target structure T is arranged on the substrate W so as to be orthogonal to the optical axis O of the objective lens 16. The substrate W may be supported by a support (not shown). The light rays of the measurement radiation I incident on the target structure T from an angle offset from the axis O generate a zero-order ray (solid line 0) and two first-order rays (dash-dotted line +1 and double dash-dotted line -1), which are hereinafter referred to as a pair of complementary diffraction orders. It should be noted that the pair of complementary diffraction orders may be any higher-order pair, such as a pair of +2, -2, etc., and is not limited to the first-order complementary pair. In the case of a small protruding target structure, it must not be forgotten that these light rays are just one of a number of parallel light rays covering the area of the substrate including the measurement target structure T and other features. Since the aperture of the plate 13 has a finite width (necessary to admit an effective amount of light), in practice the incident light ray I occupies a certain angular range, and the diffracted rays 0 and +1 / -1 will spread somewhat. According to the point spread function of the small target, each of the +1 and -1 orders will spread further over a certain angular range and will not be a single ideal ray as shown. Note that the grating pitch of the target structure and the illumination angle can be designed or adjusted so that the first-order rays are aligned near the central optical axis and enter the objective lens. The light rays shown in FIGS. 3(a) and 3(b) are shown somewhat off-axis in the drawing so that they can be easily distinguishable in the drawing.

[0032] 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 to return through the beam splitter 15. Returning to Fig. 3(a), both the first and second illumination modes are shown by designating radially opposite apertures marked with north (N) and south (S) signs. When the incident light ray I of the measurement radiation is from the north side of the optical axis, that is, when the first illumination mode is applied using the aperture plate 13N, the +1st order diffracted ray marked with the sign of +1(N) is incident on the objective lens 16. Conversely, when the second illumination mode is applied using the aperture plate 13S, the -1st order diffracted ray marked with the sign of (-1(S)) is incident on the objective lens 16.

[0033] The second beam splitter 17 divides the diffracted beam into two measurement paths. In the first measurement path, the optical system 18 forms a diffraction spectrum (pupil plane image) of the target structure on the first sensor 19 (e.g., a CCD or CMOS sensor) using the 0th and 1st order diffracted beams. Since each diffraction order hits different points on the sensor, image processing can compare and contrast the orders. The pupil plane image captured by the sensor 19 can be used for focusing adjustment of the measuring device and / or normalization of the intensity measurement of the 1st order diffracted beam. The pupil plane image can also be used for many measurement purposes such as reconstruction.

[0034] In the second measurement path, the optical systems 20, 22 form an image of the target T on the sensor 23 (e.g., a CCD or CMOS sensor). In the second measurement path, an aperture stop 21a is provided in the plane 21 conjugate to the pupil plane. The aperture stop 21a functions to block the zero-order diffraction beam so that the image of the target formed on the sensor 23 is formed only from the -1 or +1 order beams. The measurement tool can also use a wedge structure 21b that separates the first-order diffracted light and the zero-order diffracted light instead of a simple aperture stop 21a. Thereby, the first-order diffracted light and the zero-order diffracted light are imaged separately (apart) on the detector 23. Such an arrangement is described in the aforementioned US2011102753A1 and can be used to simultaneously acquire the +1 and -1 order diffracted lights in one or both directions of the substrate surface (e.g., in combination with a quadruple-segmented illumination aperture 13Q).

[0035] The captured images of the sensors 19 and 23 are output to a processing unit PU that processes the images. The function of the PU will depend on the specific form of the measurement to be performed. Note that the term "image" as used in this document is in a broad sense. If only one of the -1 and +1 orders exists, the image of the grating lines itself will not be formed.

[0036] The positional error can be caused by an overlay error (often called "overlay"). Overlay is the error in placing the first feature at the first exposure with respect to the second feature at the second exposure. The lithographic apparatus minimizes the overlay by accurately aligning each substrate with respect to a reference before pattern formation, for example, by measuring the position of the alignment marks on the substrate using alignment sensors, and also by using feedback correction in the exposure process based on overlay measurements on the measurement target after exposure using an appropriate measurement tool as shown in FIG. 3(a).

[0037] One of the known measurement methods that can be performed using the measurement tool as shown in Fig. 3(a) is known as diffraction-based overlay (DBO) or micro-diffraction-based overlay (μDBO). In such μDBO technology, the imaging branch (detector 23) of the measurement tool is used. Each main "image" used for inferring the parameter of interest is formed only from higher (e.g., first-order) diffraction orders, so instead of a decomposed pattern, regions with specific intensity levels are displayed. The zero-order (specular reflection) is usually blocked or redirected elsewhere (e.g., to another part of the detector for monitoring purposes). It is not directly used in μDBO measurements. The asymmetry of a structure such as a μDBO target can be determined from the intensity asymmetry or intensity difference between complementary diffraction order pairs (usually complementary first-order diffraction order pairs, i.e., +1st order and -1st order as shown in Fig. 3(b), but technically +2 / -2 order or higher orders can also be used). Thus, a first intensity value is obtained from the image formed from the +1st order, a second intensity value is obtained from the -1st order, and the difference between these intensity values is used to calculate the asymmetry of the structure. The overlay target may be composed of two superimposed periodic structures or gratings in different layers. If there is an overlay (i.e., a shift between the layers), a shift between the two gratings occurs and appears as an asymmetry of the entire target (an intentional offset may be provided between the two superimposed gratings to correct for a specific asymmetry of the target). Thus, the asymmetry of this target, i.e., the overlay, can be measured using the method described above.

[0038] An alternative overlay measurement method is called continuous DBO or cDBO, and the measured asymmetry signal can be the asymmetry of the phase difference from a pair of complementary sub-targets ("M pad" and "W pad") rather than the intensity asymmetry described above. More specifically, in cDBO, the asymmetry signal is defined as the phase difference between the diffraction order from the "M pad" and the corresponding diffraction order from the "W pad", and optionally averaged over the diffraction orders of both complementary diffraction order pairs. For example, A=(φ M -φW ) +1 +(φ M -φ W ) -1 where (φ M -φ W ) +1 and (φ M -φ W ) -1 are the measured phase differences between the "M pad" and the "W pad" at the +1st diffraction order and the -1st diffraction order, respectively. Thus, it can be seen that the concepts described here can be applied to different types of asymmetric signals. The principle of cDBO is described in "New Diffraction-Based Overlay Metrology Utilizing Phase-Based Overlay to Improve Robustness" by Matsunobu et al., Proc. SPIE 11611, Metrology, Inspection, and Process Control for Semiconductor Manufacturing XXXV, 1161126 (February 22, 2021), which is incorporated herein by reference.

[0039] Similar to the μDBO target, the cDBO target is composed of a periodic structure or grating overlaid on each layer that measures the overlay value. While the pitch of the grating is the same in two layers like the μDBO target, the cDBO target is composed of sub-targets with different grating pitches in two layers. More specifically, a typical cDBO target is composed of the arrangement of two different types of sub-targets (for example, for each direction). That is, the "M pad" or "M sub-grating" is composed of a lower grating with a smaller pitch than the upper grating, and the "W pad" or "W sub-grating" has these gratings inverted (that is, having the same pitch as the M pad, but the pitch of the upper layer is larger).

[0040] In overlay measurement, it is assumed that the grating used for the target is exactly symmetric. Under this assumption, the actual overlay is proportional to the asymmetry of the measured intensity. In reality, there are many processes that can cause the grating of the overlay target to become asymmetric, such as chemical mechanical polishing (CMP), etching, deposition, etc.

[0041] To enable measurement using current optical measurement techniques (e.g., scatterometers as described above), the measurable pitch or main pitch of the target grating may be orders of magnitude larger than the pitch and / or CD or product feature (actual functional device feature). As a result of wafer processing steps such as polishing, deposition, etching, etc., it is often desirable to segment the target grating with features like the product to avoid damage to the target grating and / or contamination of the device area. Such features like the product are composed of features with a size and / or resolution similar to the product features (e.g., having a critical dimension (CD) of the same order as the CD of the product features). Such a target grating may be called a sub-segmentation target grating composed of sub-segmentation of either the lines (first region) and / or spaces (second region) of the target grating. The periodic structure or grating defined by these first and second regions can form a pitch resolvable by the measurement tool, e.g., a pitch resolvable by a scatterometer as described above. For example, each line or each space may be segmented to form a periodic (1D) array of multiple features with periodic sub-features, e.g., a CD and / or pitch of the size of the product.

[0042] Figure 4 shows a particular type of unwanted grid asymmetry addressed by the concepts described herein, which may also be referred to as CD imbalance. CD imbalance particularly applies to segmented targets, for example, targets formed from one or more segmented grids. The figure shows the lower grid BG of a target (e.g., a μDBO target or a cDBO target) that includes a series of features (first region FR) and / or spaces (second region SR). Each feature and / or first region FR (which may also be the second region) is divided into sub-features to form segmented marks SF. The details of a single segmented feature are shown in the drawing.

[0043] Note that segmentation can be in the feature / first region only, the space / second region only, or both the line / first region and the space / second region. In the latter case, the segmentation can be such that the "space" includes a periodic sub-structure that includes a periodic arrangement of multiple features. Each has a smaller CD than the multiple features (which also include a periodic sub-structure) contained within the "line" of the target, and thus has different optical properties. Note that the segmentation is not necessarily 1D periodic. Other examples of segmentation include contact arrays, alternating contacts, slanted lines, or other segmented structures. The actual form of the segmentation is not important.

[0044] Each sub-feature is designed to have the same width or CD (i.e., the sub-feature width in the periodic direction of the grid labeled X). All sub-features except the inner sub-feature ISF, i.e., the two outermost sub-features OSF1, OSF2 (i.e., the first sub-feature OSF1 and the last sub-feature OSF2), typically have substantially the same CD. However, the two (or more) outermost sub-features can have different CDs. More specifically, the CD of the first sub-feature of the feature is smaller than that of the inner sub-feature by the CD difference ΔCD taken from the first edge from the outermost side of the feature, and the CD of the last sub-feature of the grid BG is larger than that of the inner sub-feature by the same CD difference ΔCD added to the second edge from the outermost side of the feature. This is as if the outer edge portion of the first sub-feature OSF1 is taken out from the first sub-feature and added to the outer edge of the last sub-feature OSF2. More features than the two outermost features are affected in this way, but the most affected are the outermost features. The number of affected features is not important for the concept disclosed here. This CD difference is caused by optical / processing disturbances. Ideally, all CDs are the same.

[0045] This type of asymmetry causes overlay measurement errors (errors in the measurement of the overlay). The inventor quantified this error as follows. Referring to FIG. 4 again, the left end or the first sub-feature has edges at positions x 0,0 and x 0,1 , and the right end or the last sub-feature has edges at positions x N-1,0 and x N-1,1 . Here,

Number

[0046] When calculating the optical center of gravity CoG of the modeled feature, the following results. [Number] When the number N of sub - features is sufficiently large, this can be approximated as follows. [Number] Therefore, the influence of CD imbalance on the overlay error increases as the sub - features are more isolated.

[0047] To address this CD imbalance, a target layout and a method of measuring an overlay using such a target layout have been proposed. With this target layout and method, an overlay value with the influence of the CD imbalance of the target grid corrected can be measured.

[0048] The proposed target concept includes a plurality of sub - targets. The plurality of sub - targets includes at least a first sub - target and a second sub - target. Each of the plurality of sub - targets includes at least one sub - segmented periodic structure having repetitions of a first region and a second region. The first region or the second region includes periodic sub - features. The first sub - target includes first sub - segmentation characteristics. The second sub - target includes second sub - segmentation characteristics. The first sub - segmentation characteristics and the second sub - segmentation characteristics are different with respect to at least one sub - segmentation parameter.

[0049] The proposed target concept includes at least a first pad type or sub - target type including at least one (e.g., bottom) periodic structure or lattice having a first sub - segmentation characteristic, and a second pad type or sub - target type including at least one (e.g., bottom) periodic structure or lattice having a second sub - segmentation characteristic. The first sub - segmentation characteristic includes a first number N1 of sub - features (e.g., per line or space) per first region or second region, and a first sub - feature CD or width CD1 of the sub - feature, and the second sub - segmentation characteristic includes a second number N2 of sub - features per first region or second region, and a second sub - feature CD or width CD2 of the sub - feature. Here, N1≠N2 and / or CD1≠CD2. Therefore, it should be understood that only one of the number of sub - features or the sub - feature CD needs to be different between the sub - target types.

[0050] In one embodiment, the target can include a first pair of sub - targets each including one of these two sub - target types (e.g., the first target and the second target), and a second pair of sub - targets each including one of these two sub - target types (e.g., a third target having a first sub - segmentation characteristic and a fourth target having a second sub - segmentation characteristic).

[0051] For example, in an embodiment of μDBO, as is well known in μDBO metrology, each of these pairs of sub - targets can have a different bias (strictly speaking, only one pair needs a bias such that there is a bias difference between the two pairs). In a common configuration, the bias of each pair can be equal in magnitude and opposite in direction.

[0052] In one embodiment, the target can 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 can include a pair of the first sub-targets and a pair of the second sub-targets (e.g., a total of eight sub-targets) for each measurement direction.

[0053] FIG. 5 is a schematic diagram of a target according to the foregoing example. Instead of having two pads or sub-targets for each measurement direction as used in μDBO, the target here includes four pads or sub-targets (a total of eight sub-targets) for each measurement direction. For each direction, each of the pair of the first sub-targets and the pair of the second sub-targets includes a first sub-target type having first sub-segmentation characteristics N1, CD1 and a second sub-target type having second sub-segmentation characteristics N2, CD2, where N1≠N2 and / or CD1≠CD2. For each direction, the first pair of sub-targets can have a first bias (e.g., +d), and the second pair of sub-targets can have a second bias (e.g., -d).

[0054] Using the above formula for the center of gravity, the following can be found.

Equation

Equation

[0055] Measured overlay value OV measured1 , OV measured2 can each be obtained using a standard μDBO intensity asymmetry measurement method. For example, OV measured1 is measured from the intensity asymmetry measurement from the first sub-target of the first sub-target type, and OV measured2 is measured from the second sub-target of the second sub-target type.

[0056] Alternatively, assuming a target as shown in FIG. 5, OV measured1 is measured from the first pair of intensity asymmetry measurements from the first sub-target type from the first pair of sub-targets and from the first sub-target type from the second pair of sub-targets, and OV measured2 is measured from the second sub-target type from the first pair of sub-targets and from the second sub-target type from the second pair of sub-targets. Considering the different biases between the first pair of sub-targets and the second pair of sub-targets, before performing the proposed correction of CD imbalance, other target asymmetry corrections can be performed when obtaining the overlay values OV measured1 , OV measured2 . These initial corrections (obtaining the overlay values OV measured1 , OV measured2 ) may potentially (partially) correct the influence of CD imbalance as well, but it can be assumed that the target asymmetry correction captures the CD imbalance in the same way (with the same efficiency) for both sub-targets. Therefore, it is necessary to cancel these when determining OV true according to the above formula to prevent double correction.

[0057] The assumptions underlying these concepts, for example, that ΔCD is the same for both sub-target types and CoG is a proxy for the measured overlay, have been shown to be sufficiently correct.

[0058] It can be understood that the same basic concepts can be equally applied to cDBO measurements and cDBO target types. In such an approach, a first sub-target pair including two M sub-targets (optionally, for each direction) and a second sub-target pair including two W sub-targets can be used. The first sub-target pair can include a first M sub-target type having a first sub-segmentation characteristic for at least one grid and a second M sub-target type having a second sub-segmentation characteristic for at least one grid. Similarly, the second sub-target pair can include a first W sub-target type having a first sub-segmentation characteristic for at least one grid and a second W sub-target type having a second sub-segmentation characteristic for at least one grid. In such a case, the position of the delta fringes (measured phase difference) is directly related to the centroid delta between different sub-target types, but with moiré amplification.

[0059] Further embodiments according to the present invention are described in the numbered paragraphs below: 1. A substrate including at least one target, wherein the target includes a plurality of sub-targets, the plurality of sub-targets includes at least a first sub-target and a second sub-target, each of the plurality of sub-targets includes at least one sub-segmented periodic structure having a repetition of a first region and a second region, at least one of the first region or the second region includes a sub-segmented region formed of periodic sub-features, the first sub-target includes a first sub-segmentation characteristic for its sub-segmented region, the second sub-target includes a second sub-segmentation characteristic for its sub-segmented region, and the first sub-segmentation characteristic and the second sub-segmentation characteristic are different with respect to at least one sub-segmentation parameter, the substrate. 2. The substrate according to claim 1, wherein the at least one sub-segmentation parameter includes one or both of an intended sub-feature width of the sub-feature and the number of sub-features per segmented area. 3. The substrate according to claim 1 or 2, wherein the plurality of sub-targets include at least the repetition of the first sub-target and the second sub-target for each measurement direction in the substrate plane of the substrate. 4. The substrate according to claim 1, 2 or 3, wherein each of the sub-targets includes a pair of periodic structures each having a respective periodic structure in each of two layers, and each pair of periodic structures includes the at least one segmented periodic structure. 5. The substrate according to claim 4, wherein the at least one segmented periodic structure includes a bottom periodic structure of each sub-target. 6. The plurality of targets a pair of first sub-targets including the first sub-target and the second sub-target, a pair of second targets including a third sub-target having the first sub-segmentation characteristic and a fourth sub-target having the second sub-segmentation characteristic, The substrate according to claim 4 or 5, comprising: 7. The pair of periodic structures within each sub-target have the same pitch, There is an intentional offset difference in the pair of periodic structures between the pair of first sub-targets and the pair of second sub-targets. The substrate according to claim 6. 8. The pair of first sub-targets each have a first intentional offset, the pair of second sub-targets each have a second offset, and the first offset and the second offset are equal in magnitude and opposite in direction. The substrate according to claim 6 or 7. 9. The pair of periodic structures within each sub-target have different pitches, and the order of the lattice is reversed in the target layer between the pair of first sub-targets and the pair of second sub-targets. The substrate according to claim 6. 10. For each measurement direction in the substrate plane of the substrate, the plurality of sub-targets include at least repetitions of the pair of the first sub-targets and the pair of the second sub-targets, and the substrate according to any one of items 6 to 9.

[0060] As used herein, the terms "radiation" and "beam" encompass any kind of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of 365 nm, 248 nm, 193 nm, 157 nm or 126 nm, or in the vicinity thereof) and extreme ultraviolet (EUV) radiation (e.g., having a wavelength in the range of 5 - 20 nm), and also include particle beams such as ion beams and electron beams.

[0061] The term "lens" may be referred to as any one or combination of any kind of optical element including refractive, reflective, magnetic, electromagnetic and electrostatic optical elements when the context permits.

[0062] The term "target" should not be construed to mean only a dedicated target formed for a specific purpose of measurement. The term "target" should be understood to include other structures including product structures having characteristics suitable for measurement applications.

[0063] The foregoing description of specific embodiments will clearly disclose the general nature of the present invention, such that others can, without undue experimentation and without departing from the general concept of the present invention, readily modify and / or adapt such specific embodiments to various applications by applying knowledge within the scope of those skilled in the art. Therefore, such adaptations and modifications are intended to be within the meaning and scope of the disclosed embodiments based on the teachings and suggestions presented herein. It will be understood that the expressions or terms herein are for purposes of illustration by way of example and not limitation, as such terms or expressions will be interpreted by those skilled in the art in light of the teachings and guidance.

[0064] The scope of the present invention should not be limited to 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 including at least one target, wherein the target includes a plurality of sub-targets, the plurality of sub-targets include at least one first sub-target and a second sub-target, and each of the plurality of sub-targets includes at least one sub-segmented periodic structure having a repetition of a first region and a second region, and at least one of the first region or the second region includes a sub-segmented region formed of periodic sub-features. The first sub-target includes first sub-segmentation characteristics with respect to its sub-segmented region, the second sub-target includes second sub-segmentation characteristics with respect to its sub-segmented region, and the first sub-segmentation characteristics and the second sub-segmentation characteristics are different with respect to at least one sub-segmentation parameter, the substrate.

2. The substrate according to claim 1, wherein the at least one sub-segmentation parameter includes one or both of an intended sub-feature width of the sub-feature and the number of sub-features per sub-segmented region.

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

4. The substrate according to claim 1, 2 or 3, wherein each sub-target includes a pair of periodic structures each having its own periodic structure in each of two layers, and each pair of periodic structures includes the at least one sub-segmented periodic structure.

5. The substrate according to claim 4, wherein the at least one sub-segmented periodic structure includes a bottom periodic structure of each sub-target.

6. The plurality of targets includes a pair of first sub-targets including the first sub-target and the second sub-target, a pair of second targets including a third sub-target having the first sub-segmentation characteristics and a fourth sub-target having the second sub-segmentation characteristics, The substrate according to claim 4 or 5, including.

7. The pair of periodic structures within each sub-target has the same pitch. The substrate according to claim 6, wherein there is an intentional offset difference in the pair of periodic structures between the pair of the first subtargets and the pair of the second subtargets.

8. A method for measuring a parameter of interest, comprising: acquiring first measurement data from at least a first subtarget of a target, wherein the at least first subtarget includes first sub-segmentation characteristics; determining a first value of the parameter of interest from the first measurement data; acquiring second measurement data from at least a second subtarget of the target, wherein the at least second subtarget includes second sub-segmentation characteristics, and the first sub-segmentation characteristics and the second sub-segmentation characteristics are different with respect to at least one sub-segmentation parameter; determining a second value of the parameter of interest from the second measurement data; determining a value of the corrected parameter of interest from the first value and the second value of the parameter of interest. A method comprising the above steps.

9. The method according to claim 8, wherein the at least one sub-segmentation parameter includes one or both of an intended sub-feature width of the sub-feature and a number of sub-features per segmented region.

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

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

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

13. A measuring apparatus comprising the processing apparatus according to claim 12.

14. A computer program comprising program instructions operable to execute the method according to any one of claims 8 to 11 when executed by a suitable apparatus.

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

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