Metrology methods and related apparatus

Abstract

A metrology method is disclosed that includes measuring at least one ambient observable parameter related to ambient signal contributions to a metrology signal, including contributions to the metrology signal that are not attributable to the at least one target being measured, and determining a correction from the ambient signal observable parameter, the correction being used to correct first measurement data, the first measurement data related to a measurement of one or more targets using measurement radiation that forms a measurement spot on one or more of the one or more targets that is larger than one of the targets.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Patent Application No. 63 / 122,641, filed December 8, 2020, which is incorporated herein by reference in its entirety.

[0002] The present invention relates to metrology apparatus and methods that can be used, for example, to perform metrology in the manufacture of devices by lithographic techniques. [Background technology]

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A patterning device, alternatively called a mask or reticle, may be used to generate the circuit pattern that will be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of one or several dies) on the substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. In lithographic processes, it is frequently desirable to perform measurements of the structures produced, for example for process control and verification. A variety of tools are known for making such measurements, including scanning electron microscopes, which are often used to measure critical dimensions (CD), and dedicated tools for measuring overlay, the accuracy of alignment of two layers within a device.

[0004]

[0004] In recent years, various forms of scatterometers have been developed for use in lithography. These devices direct a beam of radiation onto a target and, by measuring one or more properties of the scattered radiation (e.g., intensity at a single angle of reflection as a function of wavelength, intensity at one or more wavelengths as a function of angle of reflection, or polarization as a function of angle of reflection), obtain a diffraction "spectrum" from which properties of interest of the target can be determined.

[0005]

[0005] Examples of known scatterometers include angle-resolved scatterometers of the type described in U.S. Patent Application Nos. 2006033921A1 and 2010201963A1. The targets used by such scatterometers are relatively large, e.g., 40 μm×40 μm, and the target and measurement beam produce spots smaller than the grating (i.e., the target is underfilled). Examples of dark-field imaging metrology can be found in International U.S. Patent Application Nos. 20100328655A1 and 2011069292A1, which are incorporated herein by reference in their entireties. Further developments of this technology are described in published patent publications, U.S. Patent No. 20110027704A, U.S. Patent No. 20110043791A, U.S. Patent Application No. 2011102753A1, U.S. Patent No. 20120044470A, U.S. Patent No. 20120123581A, U.S. Patent No. 20130258310A, U.S. Patent No. 20130271740A, and International Publication No. 2013178422A1. These targets may be smaller than the illumination spot (i.e., the target is overfilled) and may be surrounded by product structures on the wafer. Multiple grating targets can be used to measure multiple gratings in a single image. The contents of all of these applications are also incorporated herein by reference.

[0006]

[0006] Overfilled metrology techniques can result in capturing other structures within the measurement spot, which can lead to crosstalk (contributions from adjacent structures in the measurement signal). This result applies to overfilled overlay / focus metrology and overfilled alignment as well.

[0007]

[0007] It is therefore desirable to improve the accuracy of metrology of overfilled targets. Summary of the Invention

[0008]

[0008] In a first aspect, the present invention provides a metrology method, the method comprising measuring at least one ambient observable parameter related to ambient signal contributions to a metrology signal, including contributions to the metrology signal not attributable to at least one target being measured, determining a correction from the ambient signal observable parameter, obtaining first measurement data, the first measurement data related to measurement of one or more targets using measurement radiation that forms a measurement spot on one or more of the one or more targets that is larger than one of the targets, and applying the correction to the first measurement data.

[0009]

[0009] In a second aspect, the present invention provides a metrology apparatus comprising a support for a substrate having at least one of a target and a product structure thereon, an optical system for measuring each target, a processor, and a computer program carrier containing a computer program operable to enable the processor to control the metrology apparatus to perform the method of the first aspect.

[0010]

[0010] The present invention further provides a computer program product including machine-readable instructions for causing a processor to perform the method of the first aspect, an associated metrology apparatus, a lithography system, and a method for manufacturing a device.

[0011]

[0011] Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. It should be noted that the present invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to those skilled in the art based on the teachings contained herein. [Brief explanation of the drawings]

[0012]

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

[0013] [Figure 1] 1 depicts a lithographic apparatus; [Figure 2] 1 shows a lithographic cell or cluster in which an inspection apparatus according to the present invention can be used. [Figure 3a] 1 illustrates schematically an inspection apparatus adapted to perform angle resolved scatterometry and dark field imaging inspection methods; [Figure 3b] 1 illustrates schematically an inspection apparatus adapted to perform angle resolved scatterometry and dark field imaging inspection methods; [Figure 4] 1 is a schematic diagram of an alignment sensor adaptable in accordance with one embodiment of the present invention; [Figure 5] FIG. 1 is a schematic diagram of an alternative metrology device adaptable in accordance with an embodiment of the present invention. [Figure 6a] 2 shows a pupil image of the input radiation. [Figure 6b] 6 shows a pupil image of an off-axis illumination beam illustrating the operating principle of the metrology device of FIG. 5. [Figure 6c] 6 shows a pupil image of an off-axis illumination beam illustrating another principle of operation of the metrology device of FIG. 5 . [Figure 7]10A and 10B illustrate schematic diagrams of overfilled measurements made on a metrology target, affected by surrounding structures. [Figure 8] 1 is a diagram illustrating a schematic representation of a measurement of an invisible target structure, where the invisible target structure and said method based on metrology of the invisible target structure are according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0014] Before describing embodiments of the present invention in detail, it is beneficial to present an example environment in which embodiments of the present invention may be implemented.

[0015] 1 schematically depicts a lithographic apparatus LA. The apparatus includes an illumination system (illuminator) IL configured to condition a radiation beam B (e.g., UV or DUV radiation), a patterning device support or support structure (e.g., mask table) MT constructed 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 tables) WTa and WTb each constructed to hold a substrate (e.g., a resist-coated wafer) W and each connected to a second positioner PW configured to accurately position the substrate 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 of the substrate W (e.g., comprising one or more dies). 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 of features thereon.

[0016]

[0015] The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping or controlling radiation.

[0017] The patterning device support holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The patterning device support may take many forms. The patterning device support may ensure that the patterning device is at a desired position, for example with respect to the projection system.

[0018]

[0017] As used herein, the term "patterning device" should be interpreted broadly to refer to any device that can be used to impart a radiation beam having a pattern in its cross-section 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 particular functional layer in a device being created in the target portion, such as an integrated circuit.

[0019] As depicted here, the apparatus is of a transmissive type (e.g., employs a transmissive patterning device). Alternatively, the apparatus may be of a reflective type (e.g., employs a programmable mirror array of a type as referred to above, or employs a reflective mask). Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Any use of the terms "reticle" or "mask" herein may be considered synonymous with the more general term "patterning device". The term "patterning device" may also be interpreted as referring to a device that digitally stores pattern information for use in controlling such a programmable patterning device.

[0020]

[0019] The term "projection system" as used herein should be interpreted broadly as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, that are appropriate for the exposure radiation 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 can be considered as synonymous with the more general term "projection system".

[0021]

[0020] The lithographic apparatus may be of a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the projection system and the substrate. Immersion liquids may also be provided in other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems.

[0022]

[0021] In operation, the illuminator IL receives a radiation beam from the radiation source SO. The radiation source and the lithographic apparatus may be separate entities, for example when the radiation source is an excimer laser. In such cases, the radiation source is not considered to form part of the lithographic apparatus, and the radiation beam is passed from the radiation source SO to the illuminator IL using a beam delivery system BD, which may include, for example, suitable directing mirrors and / or beam expanders. In other cases, the radiation source may be an integral part of the lithographic apparatus, for example when the radiation source is a mercury lamp. The radiation source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

[0023] 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 that it has a desired uniformity and intensity distribution in its cross-section.

[0024]

[0023] The radiation beam B is incident on the patterning device MA, which is held on the patterning device support MT, and is patterned by the patterning device. After traversing the patterning device (e.g. 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. Using 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 to position a different target portion C in the path of the radiation beam B. Similarly, the patterning device (e.g. reticle / mask) MA can be accurately positioned with respect to the path of the radiation beam B using the first positioner PM and another position sensor (not explicitly depicted in Figure 1), for example after a machine lookup in a mask library or during a scan.

[0025]

[0024] The patterning device (e.g. reticle / mask) MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the patterning device (e.g. mask) MA, the mask alignment marks may be located between the dies. Small alignment marks may also be comprised within a die, even among device features, in which case it is desirable for the marker to be as small as possible and not require different imaging or process conditions than adjacent features. Alignment systems for detecting alignment markers are described further below.

[0026] The depicted apparatus can be used in various modes. In scan mode, the patterning device support (e.g. mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT relative to the patterning device support (e.g. mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scan direction) of the target portion in a single dynamic exposure, while the length of the scanning movement determines the height (in the scan direction) of the target portion. Other types of lithographic apparatus and modes of operation are possible, as are known in the art. A step mode is known, for example. In so-called "maskless" lithography, a programmable patterning device is held stationary but the pattern is changed, and the substrate table WT is moved or scanned.

[0027] Combinations and / or variations on the above described modes of use or entirely different modes of use may also be employed.

[0028] The lithographic apparatus LA is of a so-called dual-stage type, which has two substrate tables WTa, WTb and two stations (an exposure station EXP and a measurement station MEA) between which the substrate tables can be swapped. 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 can be performed. This can significantly increase the throughput of the apparatus. The preparation steps can include mapping the surface height contour of the substrate using a level sensor LS and measuring the position of an alignment marker on the substrate using an alignment sensor AS. If the position sensor IF cannot measure the position of the substrate table while it is in the measurement station and while it is in the exposure station, a second position sensor can be provided to track the position of the substrate table relative to the reference frame RF at both stations. Other configurations are known and can be used instead of the dual-stage configuration shown. For example, other lithographic apparatuses are known that are provided with a substrate table and a measurement table. They are docked together when the preliminary measurements are carried out and undocked while the substrate table is exposed.

[0029] As shown in FIG. 2, the lithography apparatus LA forms part of a lithographic cell LC, sometimes called a lithocell or cluster, which also includes apparatus for performing pre-exposure and post-exposure processing on the substrate. Conventionally, these include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH, and a bake plate BK. A substrate handler or robot RO picks up substrates from input / output ports I / O1, I / O2, moves them between the different process tools, and then delivers them 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, which is itself controlled by a supervisory control system SCS (which also controls the lithography apparatus via a lithography control unit LACU). Thus, by operating the different tools, throughput and processing efficiency can be maximized.

[0030] To ensure that substrates exposed by a lithography apparatus are consistently and correctly exposed, it is desirable to inspect the exposed substrates to measure characteristics such as resulting layer-to-layer overlay error, line thickness, critical dimension (CD), and the like. Accordingly, the manufacturing facility in which the lithocell LC is located also includes a metrology system MET, which receives some or all of the substrates W processed in the lithocell. Metrology results are provided directly or indirectly to a monitoring and control system SCS. If an error is detected, adjustments to the exposure of subsequent substrates may be made, particularly if inspection can be performed quickly enough to still allow other substrates in the same batch to be exposed. Also, already exposed substrates may be stripped and reworked or discarded to improve yield, thereby avoiding further processing of substrates known to be defective. If only some target portions of a substrate are defective, further exposures may be performed only on good target portions.

[0031] Within a metrology system MET, an inspection tool is used to determine the properties of the substrate, particularly to see how the properties of different substrates or different layers of the same substrate vary from layer to layer. The inspection tool may be integrated with the lithography tool LA or the lithocell LC, or it may be a stand-alone device. To enable the most rapid measurements, it is desirable for the inspection tool to measure the properties of the exposed resist layer immediately after exposure. However, the latent image in the resist has very low contrast (there is only a very small difference in refractive index between the exposed and unexposed resist parts), and not all inspection tools are sensitive enough to make valid measurements of the latent image. Therefore, measurements may be made after a post-exposure bake step (PEB), which is customarily the first step performed on an exposed substrate to increase the contrast between the exposed and unexposed parts of the resist. At this stage, the image in the resist is sometimes called a semi-latent image. It is also possible to make measurements of the developed resist image once the exposed or unexposed parts of the resist have been removed, or after a pattern transfer step such as etching. The latter possibility limits the possibility of reworking defective substrates, but may still provide useful information.

[0032]

[0031] One example of a metrology device suitable for metrology in the context of lithography monitoring is a scatterometer. Scatterometers can include dark-field scatterometers (where the zeroth order is blocked before the detector so that only diffracted higher orders are captured) and bright-field scatterometers, which also capture the zeroth order. Some scatterometers are capable of both dark-field and bright-field metrology. A known type of dark-field scatterometry technique compares the intensity of each pair of complementary higher diffraction orders (e.g., comparing the intensity of the +1st and -1st orders, respectively) to determine asymmetry in the measured target (the magnitude of the intensity difference scale associated with the asymmetry). The asymmetry of the target can then be used to investigate various parameters of interest, such as the focus setting or overlay when the target was formed.

[0033] A metrology apparatus suitable for use in embodiments of the present invention is shown in FIG. 3(a). Note that this is just one example of a suitable metrology apparatus. An alternative suitable metrology apparatus may use EUV radiation, for example, as disclosed in WO 2017 / 186483 A1. A target structure T and diffracted rays of measurement radiation used to illuminate the target structure are shown in more detail in FIG. 3(b). The illustrated metrology apparatus is of a type known as a dark-field metrology apparatus. The metrology apparatus may be a stand-alone device or may be integrated into a lithography apparatus LA, for example, at a measurement station, or into a lithographic cell LC. The dotted line O represents an optical axis with several branches throughout the apparatus. In this apparatus, light emitted 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 including lenses 12, 14, and an objective lens 16. These lenses are arranged in a double sequence in a 4F configuration. A different lens arrangement can be used, as long as it still provides an image of the substrate on the detector while simultaneously allowing access to an intermediate pupil plane for spatial frequency filtering. Therefore, the angular range at which radiation is incident on the substrate can be selected by defining a spatial intensity distribution in a plane that represents the spatial spectrum of the substrate plane, referred to herein as the (conjugate) pupil plane. Specifically, this can be done by inserting an aperture plate 13 of appropriate shape between lenses 12 and 14 in a plane that is a back-projected image of the objective lens pupil plane. In the illustrated example, aperture plate 13 has different shapes (labeled 13N and 13S), allowing different illumination modes to be selected. The illumination system in this example forms an off-axis illumination mode. In the first illumination mode, aperture plate 13N provides off-axis illumination from a direction designated "north" for illustrative purposes only. In the second illumination mode, aperture plate 13S is used to provide similar illumination from the opposite direction, labeled "south." Using different apertures, other illumination modes are possible. The remainder of the pupil plane is desirably dark, as any unwanted light outside the desired illumination mode will interfere with the desired measurement signal.

[0034] As shown in FIG. 3(b), the target structure T is positioned such that the substrate W is perpendicular to the optical axis O of the objective lens 16. The substrate W may be supported by a support (not shown). A measurement radiation beam I impinging on the target structure T from an angle off axis O generates a zeroth order ray (solid line 0) and two first order rays (dashed-dotted lines +1 and dashed-dotted lines −1), which are hereinafter referred to as complementary diffraction order pairs. Note that the complementary diffraction order pairs may be any higher order pairs, such as +2 and −2 pairs, and are not limited to first order complementary pairs. Note that in the case of overfilled small target structures, these rays are only one of many parallel rays covering the area of ​​the substrate including the metrology target structure T and other features. Because the aperture of the plate 13 has a finite width (necessary to accept a useful amount of light), the incident beam I actually occupies a certain angular range, and the diffracted beams 0 and +1 / −1 are slightly diverged. According to the point spread function of the small target, each +1 and -1 order is spread out over a range of angles (rather than a single ideal ray as shown). Note that the grating pitch and illumination angle of the target structure can be designed or adjusted so that the first order ray entering the objective lens is precisely aligned with the central optical axis. The rays illustrated in Figures 3(a) and 3(b) are shown slightly off-axis simply to make them easier to distinguish in the figures.

[0035] 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. Returning to FIG. 3( a), both the first and second illumination modes are illustrated by designating diametrically opposed apertures labeled north (N) and south (S). When the incident ray I of measurement radiation is from the north side of the optical axis, i.e., when the first illumination mode is applied using aperture plate 13N, the +1 diffracted ray (labeled +1(N)) enters the objective lens 16. In contrast, when the second illumination mode is applied using aperture plate 13S, the −1 diffracted ray (labeled −1(S)) is the diffracted ray that enters the lens 16.

[0036] A second beam splitter 17 splits the diffracted beam into two measurement branches. In the first measurement branch, an optical system 18 uses the zeroth and first diffraction order beams to form a diffraction spectrum (pupil plane image or angle-resolved image) of the target structure on a first sensor 19 (e.g., a CCD or CMOS sensor). Each diffraction order impinges on a different point on the sensor so that image processing can compare and contrast the orders. The pupil plane image captured by sensor 19 can be used to focus a 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 or metrology based on asymmetry of the pupil plane image.

[0037] In the second measurement branch, the optical system 20, 22 forms an image of the target T on a sensor 23 (e.g., a CCD or CMOS sensor). In the second measurement branch, an aperture stop 21 is provided in a plane conjugate with the pupil plane. The aperture stop 21 functions to block the 0th order diffracted beam so that the image of the target formed on the sensor 23 is formed only from the −1st or +1st order beam. The images captured by the sensors 19 and 23 are output to a processor PU (whose functionality depends on the specific type of measurement being performed), which processes the images. Note that the term “image” is used in a broad sense here. Therefore, if only one of the −1st and +1st orders is present, no image of the grating lines is formed.

[0038] Another type of metrology device is an alignment sensor. A lithographic apparatus may include one or more (e.g., multiple) alignment sensors that can accurately measure the position of alignment marks provided on a substrate. Alignment (or position) sensors may use optical phenomena such as diffraction and interference to derive position information from alignment marks formed on the substrate. One example of an alignment sensor used in current lithographic apparatuses is based on a self-referencing interferometer, such as that described in U.S. Patent No. 6,961,116. Various extensions and modifications of position sensors have been developed, for example, as disclosed in U.S. Patent Application No. 2015261097A1. The contents of all these publications are incorporated herein by reference.

[0039]

[0038] A mark or alignment mark (more generally, a type of target) may comprise a series of bars formed on or in a layer provided on a substrate, or (directly) on the substrate. The bars are regularly spaced and act as grating lines, so that the mark may be considered a diffraction grating with a known spatial period (pitch). Depending on the orientation of these grating lines, the mark may be designed to allow measurement of position along the X axis or along the Y axis (oriented substantially perpendicular to the X axis). When the mark comprises bars arranged at +45 degrees and / or -45 degrees to both the X and Y axes, combined X and Y measurements are possible using techniques such as those described in US 2009 / 195768A, which is incorporated by reference.

[0040]

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

[0041]

[0040] Measuring the position of the marks can also provide information about deformations of the substrate on which the marks are provided, for example in the form of a wafer grid, which may be caused, for example, by electrostatic clamping of the substrate to the substrate table and / or by heating of the substrate when exposing it to radiation.

[0042] 4 is a schematic block diagram of an embodiment of a known alignment sensor AS. A radiation source RSO provides a beam RB of radiation of one or more wavelengths, which is redirected by redirecting optics as an illumination spot SP onto a mark, such as a mark AM arranged on a substrate W. In this example, the redirecting optics comprises a spot mirror SM and an objective lens OL. The illumination spot SP illuminating the mark AM may have a diameter that is slightly smaller than the width of the mark itself.

[0043]

[0042] Radiation diffracted by the mark AM is collimated (in this example via an objective lens OL) into an information-carrying beam IB. The term "diffracted" is intended to include zeroth order diffraction from the mark (which is sometimes called reflection). A self-referencing interferometer SRI, for example of the type disclosed in the above-mentioned U.S. Pat. No. 6,961,116, causes the beam IB to interfere with itself, and the beam is then received by a photodetector PD. If more than one wavelength is produced by the radiation source RSO, additional optics (not shown) may be included to provide separate beams. The photodetector may be a single element or may include several pixels, if desired. The photodetector may include a sensor array.

[0044]

[0043] The redirecting optical system (which in this example includes a spot mirror SM) can also act to block the zeroth order radiation reflected from the mark, so that the information-carrying beam IB contains only higher order diffracted radiation from the mark AM (this is not essential for the measurement, but improves the signal-to-noise ratio).

[0045]

[0044] The intensity signal SI is fed to a processing unit PU. The combination of optical processing in block SRI and computational processing in unit PU outputs values ​​for the X and Y positions on the substrate relative to a reference frame.

[0046] A single measurement of the type shown merely fixes the position of the mark within a certain range corresponding to one pitch of the mark. Along with this, coarser measurement techniques are used to identify which period of the sine wave contains the marked position. The same process is repeated at coarser and / or finer levels with different wavelengths to detect the mark with greater precision and / or robustness, regardless of the material the mark is made from and the material on which it is located above and / or below. Improvements in making and processing such multi-wavelength measurements are disclosed below.

[0047]

[0046] More recently, another specific type of metrology sensor, having both alignment and product / process monitoring metrology applications, is described in EP 18195488.4 and EP 19150245.9, which are incorporated herein by reference. This describes a metrology device with optimized coherence. More specifically, this metrology device is configured to generate multiple spatially incoherent measurement illumination beams, each of which (or both beams of a measurement pair of which, each measurement pair corresponds to a measurement direction) has corresponding regions in their cross-sections, and the phase relationship between the beams in these regions is known, i.e., there is mutual spatial coherence for the corresponding regions.

[0048]

[0047] Such a metrology device can measure small-pitch targets with acceptable (minimal) interference artifacts (speckle) and can also operate in dark-field mode. Such a metrology device can be used as a position sensor or alignment sensor to measure the position of a substrate (e.g., measuring the position of a periodic structure or an alignment mark relative to a fixed reference position). However, the metrology device can also be used to measure overlay (e.g., measuring the relative position of periodic structures of different layers, or even of the same layer in the case of stitching marks). The metrology device can also measure asymmetries in periodic structures and therefore can be used to measure any parameter based on target asymmetry measurements (e.g., overlay using diffraction-based overlay (DBO) techniques, or focus using diffraction-based focus (DBF) techniques, etc.).

[0049] FIG. 5 shows a possible implementation of such a metrology device. The metrology device essentially operates as a standard microscope with a novel illumination mode. The metrology device 300 includes an optical module 305 that contains the main components of the device. An illumination source 310 (which may be located external to the module 305 and optically coupled to the module 305 by a multimode fiber 315) provides a spatially incoherent radiation beam 320 to the optical module 305. An optical component 317 delivers this spatially incoherent radiation beam 320 to a coherent off-axis illumination generator 325. This component is particularly important to the concepts herein and will be described in further detail. The coherent off-axis illumination generator 325 generates multiple (e.g., four) off-axis beams 330 from the spatially incoherent radiation beam 320. The properties of these off-axis beams 330 are described in further detail below. The zeroth order of the illumination generator may be blocked by an illumination zeroth order blocking element 375. This zeroth order is present only in some of the example coherent off-axis illumination generators described herein (e.g., phase grating-based illumination generators) and may therefore be omitted if no such zeroth order illumination is generated. The off-axis beam 330 is delivered (through optical component 335 and) a spot mirror 340 to an (e.g., high NA) objective lens 345. The objective lens focuses the off-axis beam 330 onto a sample (e.g., a periodic structure / alignment mark) disposed on a substrate 350, where the beam scatters and diffracts. The scattered higher diffraction orders 355+, 355− (e.g., the +1st and −1st orders, respectively) propagate back through the spot mirror 340 and are focused by optical component 360 onto a sensor or camera 365, where they interfere to form an interference pattern. A processor 380 running appropriate software can then process the images of the interference pattern captured by the camera 365 .

[0050] The zeroth order diffracted (specularly reflected) radiation is blocked at an appropriate location in the detection branch, for example by a spot mirror 340 and / or a separate detection zeroth order blocking element. Note that there is a zeroth order reflection for each off-axis illumination beam, i.e., in the current embodiment, there are four of these zeroth order reflections in total. Exemplary aperture profiles suitable for blocking the four zeroth order reflections are shown in Figures 4(b) and (c) and are labeled 422. The metrology device therefore operated as a "dark field" metrology device.

[0051]

[0050] The main concept of the proposed metrology device is to induce spatial coherence in the measurement illumination only where necessary. More specifically, spatial coherence is induced between corresponding pairs of pupil points in each of the off-axis beams 330. More specifically, a set of pupil points includes a corresponding single pupil point in each of the off-axis beams, and the set of pupil points are spatially coherent with each other, but each pupil point is incoherent with respect to all other pupil points in the same beam. Optimizing the coherence of the measurement illumination in this way makes it possible to perform dark-field off-axis illumination on targets with small pitches, with minimal speckle artifacts, since each off-axis beam 330 is spatially incoherent.

[0052]

[0051] Figure 6 shows three pupil images to illustrate this concept. Figure 6(a) shows a first pupil image related to pupil plane P1 of Figure 5, and Figures 6(b) and 6(c) respectively show second pupil images related to pupil plane P2 of Figure 5. Figure 6(a) shows (a cross-section of) a spatially incoherent radiation beam 320, and Figures 6(b) and 6(c) show (a cross-section of) an off-axis beam 330 generated by a coherent off-axis illumination generator 325 in two different embodiments. In each case, the extent of the outer circle 395 corresponds to the maximum detection NA of the microscope objective, which may be, purely by way of example, 0.95 NA.

[0053]

[0052] Triangles 400 in each pupil indicate pairs of pupil points that are spatially coherent with respect to one another. Similarly, crosses 405 indicate other pairs of pupil points that are spatially coherent with respect to one another. The triangles are spatially incoherent with respect to the crosses and all other pupil points corresponding to the beam propagation. The general principle (in the example shown in Figure 6(b)) is that each pair of pupil points that are spatially coherent with respect to one another (each pair of coherent points) has the same spacing within the illumination pupil P2 as all other pairs of coherent points. Thus, in this embodiment, each pair of coherent points is a translation within the pupil of all other pairs of coherent points.

[0054] In FIG. 6(b), the spacing between each pupil point in the first set of coherent points represented by triangles 400 must be equal to the spacing between each pupil point in the set of coherent points represented by crosses 405. "Spacing" in this context is directional; that is, the set of crosses (second set of points) cannot be rotated relative to the set of triangles (first set of points). Therefore, each off-axis beam 330 contains its own incoherent radiation. However, both off-axis beams 330 contain identical beams with corresponding sets of points that have a known phase relationship (spatial coherence) in their cross sections. Note that the points in each set of points do not need to be equally spaced (e.g., the spacing between the four triangles 405 in this example does not need to be equal). Therefore, the off-axis beams 330 do not need to be symmetrically positioned within the pupil.

[0055]

[0054] Figure 6(c) extends this basic concept to show that mutual spatial coherence can only be achieved between beams corresponding to a single measurement direction, where beam 330X corresponds to a first direction (X direction) and beam 330Y corresponds to a second direction (Y direction). In this example, the squares and + symbols indicate pairs of pupil points that correspond to, but are not necessarily spatially coherent with, the pairs of pupil points represented by the triangles and crosses, respectively. However, the crosses are mutually spatially coherent, just like the + symbols, and the crosses are geometric translations of the pupils of the + symbols. Thus, in Figure 6(c), the off-axis beams are only pairwise coherent.

[0056] In this embodiment, off-axis beams are considered separately by direction, for example, X direction 330X and Y direction 330Y. The beam pair 330X that generates the captured X-direction diffraction order needs to be coherent only with each other (just as point pair 400X is coherent with each other, as is point pair 405X). Similarly, the beam pair 330Y that generates the captured Y-direction diffraction order needs to be coherent only with each other (just as point pair 400Y is coherent with each other, as is point pair 405Y). However, there is no need for coherence between point pairs 400X and 400Y or between point pairs 405X and 405Y. Therefore, the off-axis beam pairs corresponding to each considered measurement direction include coherent point pairs. As before, for each pair of beams corresponding to a measurement direction, each pair of coherent points is a geometric translation in the pupil of all other pairs of coherent points.

[0057]

[0056] Overfill metrology techniques, in which the metrology target is overfilled (i.e., the target is smaller than the measurement spot), allow the metrology target to be smaller, thereby saving space and allowing more metrology targets to be accommodated and / or allowing metrology targets to be placed within the product area or in other strategic locations.

[0058] Current diffraction-based measurements on overfilled targets, including both post-exposure measurements (e.g., overlay or focus measurements) or pre-exposure measurements (e.g., alignment), are susceptible to crosstalk from a number of sources. These sources include, for example, residual sensor or camera ghosts or artifacts, and information from neighboring features (e.g., product structures, dummy structures, and / or other metrology targets). This crosstalk contributes to the measurement signal and causes errors (i.e., the crosstalk contribution is not related to the parameter of interest).

[0059] FIG. 7 shows two X-direction sub-targets ST X+ , ST X- , and two Y-direction sub-targets ST Y+ , ST Y- A specific example is presented to illustrate the problem of crosstalk associated with the measurement of metrology targets (e.g., overlay targets) that include four sub-targets. Measurement of such targets may be performed using a measurement spot MS large enough to measure all four sub-targets simultaneously. However, the measurement signal associated with the target in one direction (e.g., two X-direction sub-targets ST) may be distorted. X+ , ST X-The measurement of the target (measurement of the target) may be affected by (e.g., include a contribution due to) radiation scattered from the surrounding structure SS. The surrounding structure SS in this context may include background dummy patterns and / or adjacent product structures during the measurement of the target. The surrounding structure SS in this context may also include other overlay pads / sub-targets or alignment marks (e.g., two Y-direction sub-targets ST when considering an X-direction target). Y+ , ST Y ) This crosstalk can be problematic for both pre-exposure metrology (alignment) and post-exposure metrology (e.g., overlay, focus, etc.).

[0060] Most current correction methods assume that there are no dummy / target structures. However, in reality, there may be an intensity landscape around the metrology target, which generates an asymmetric contribution (in overfilled measurements) at the detector / camera.

[0061]

[0060] In the context of alignment, the influence of surrounding structures is considered to be one of the biggest problems when performing wafer alignment for smaller alignment targets (or alignment marks), such as, for example, 10 μm x 10 μm marks (or, more generally, marks / targets smaller than 40 μm, 30 μm, 20 μm, or 15 μm in one or both directions in the substrate plane). For example, some of the radiation coming from surrounding structures on the wafer will scatter from edges (e.g., edges of the pupil stop in a metrology tool) or from high frequency defects in the optical system (e.g., scratch-dig). This radiation will eventually enter the region of interest and lead to errors in the alignment signal. Also, surrounding structures may affect processing effects of the marks (e.g., asymmetries and / or layer thicknesses), for example due to polishing steps, which may also lead to errors in the alignment signal. Both of these effects are expected to have an impact on the absolute alignment accuracy and on the accuracy variations from wafer to wafer.

[0062]

[0061] This specification proposes to quantify and correct crosstalk by calibrating and removing the crosstalk contribution to the metrology signal. Such correction may be based on determining the ambient signal contribution to the measurement, where the ambient signal contribution may represent a contribution to the metrology signal that originates from something external to the target of interest, for example, which may propagate spurious radiation back to the metrology sensor that should only measure the metrology signal from the target. In this context, the metrology signal may include radiation scattered from the actual target or its sub-targets (and / or regions of interest within the target).

[0063]

[0062] A first embodiment involves calibrating the amount of radiation from surrounding structures that bleeds into, and thus contributes to, the actual metrology signal. Following calibration of this leakage radiation from adjacent features, a mathematical correction amount can be determined and applied to the measurement signal to correct for this unwanted contribution. The calibration may be performed via physical measurements (calibration measurements) using a metrology tool, which may include offline measurements (e.g., not during manufacturing). In one example, the calibration may be based on calibration measurements on special "invisible targets" that are designed to be invisible to the metrology sensor. Invisible targets and related calibrations are described below.

[0064] In another embodiment, such calibration may involve, for example, measuring the swing curves of the target and surrounding structures, respectively, and then comparing the swing curves. The swing curves may represent the change in the measured parameter values ​​(e.g., any observable parameter, such as intensity, intensity imbalance, phase, stack sensitivity, or any other relevant parameter) under the lighting conditions used to obtain the measured parameter values. This comparison may use statistical techniques (e.g., component analysis, such as principal component analysis, independent component analysis, and / or singular value decomposition).

[0065] In one embodiment, such a method may include comparing the swing curve (e.g., asymmetry as a function of wavelength) of the target with the swing curves of the surrounding structures. If the wavelength dependences of the target and the surrounding structures are significantly different (e.g., due to structural differences), they can be separated using known statistical techniques (PCA, ICA, etc.) to obtain their respective fingerprints. Based on these statistical techniques, the influence of the surrounding structures can be removed by removing the fingerprints related to the surrounding structures.

[0066]

[0065] The calibration measurements may include target measurement data including target observable parameter values ​​related to respective regions of interest (ROIs) of the target measurements (e.g., one or more first ROIs related to the target and surrounding observable parameter data from one or more second ROIs related to the surroundings (which may include adjacent targets or sub-targets)). Alternatively, measurements may be taken separately for the target and surroundings. Also, observable parameters for surrounding structures may be measured using a tool other than that used to measure the target.

[0067] As a specific example, a correction amount may be determined to correct measurements for a typical composite target including one or more respective sub-targets or pads in each of the X and Y directions (e.g., two directions in the substrate plane). Contributions from targets in other directions may affect the measurement signal from the sub-target being measured. For example, to determine a parameter such as overlay in the X direction, a signal from, for example, an X target sub-pad may be obtained, which may include a signal contribution from the Y sub-target. To determine the contribution from the Y target in the X target signal, comparing the swing curves may include comparing a first swing curve from the X target with a second swing curve from the Y target.

[0068]

[0067] It should be noted that ambient signal contributions may arise, at least in part, from asymmetric sensors, and at least some of the methods disclosed herein can also correct for such asymmetric sensor contributions. Thus, if such methods are based on calibration, they are tool dependent.

[0069] In one embodiment, the proposed method may include the following two steps: 1. Measuring any suitable ambient observable parameters of the surrounding structures (eg, visible on the camera due to overfill of the mark). 2. Correcting the first measurement data based on the observable parameters. For example, correcting the measurement data may involve multiplying the observable parameters by one or more constants or coefficients (or, more generally, the corrected measurement data may be a function of the observable parameters). Such a function may, for example, convert the observable parameters into a correction quantity, which cancels out the contribution of the ambient signal. Such a method may include, for example, determining a correction relationship (e.g., a function or coefficient) during a calibration phase or other phase. In the context of alignment, a specific example is determining a correction relationship (e.g., a function or coefficient) for the aligned position APD. corrected APD corrected =APD measured It may be calculated as + constant * observable parameter.

[0070] Such a scheme is similar to the optimal color (and / or intensity) weighting (OCW) scheme as described in US publication US Patent Application No. 2019 / 0094721 A1, which is incorporated herein by reference. The main difference is that the observable parameters are those measured for the surrounding structures, rather than the aligned positions or intensity imbalances in different colors (i.e., related to the target itself).

[0071]

[0070] The observable measured for the surrounding structure may be, or be related to, the magnitude of, for example, one or more of the following: · Signal strength or (e.g., average) intensity over one or more regions of interest (ROIs) on the camera / detector corresponding to surrounding structures. The amplitude of the interference pattern (e.g., this is a quantity determined by a fitting algorithm in an optimized coherence metrology tool, such as that shown in Figure 5), e.g., within one or more regions of interest (ROIs) on the camera / detector corresponding to surrounding structures. Aligned position (a standard quantity determined by the fitting algorithm mentioned above, i.e. essentially the phase difference between the +1 and -1 orders (and / or higher orders). This may be measured directly by fringe positions in one or more regions of interest (ROI) on the camera / detector corresponding to the surrounding structures). Asymmetry (measuring the asymmetry of the surrounding structures can be of particular interest if there is a correlation between the asymmetry of the surrounding structures and the grating asymmetry of the mark / target). Asymmetry is a standard quantity for dark-field metrology devices such as those shown in Figure 4, and can also be measured by metrology devices such as those shown in Figure 5, which have a detection branch that detects intensity imbalances in parallel with the standard interference pattern). ·Strength imbalance. Fringe visibility (the more asymmetric the lattice of the surrounding structure, the lower the fringe visibility). The difference between aligned positions (or more generally, measurements) for different colors (if alignment measurements in multiple colors are available, the grating asymmetry can be estimated. This can also replace the intensity imbalance if an intensity imbalance measurement is not available).

[0072] In such an embodiment, an important consideration relates to how to determine the constants in the correction. An alignment embodiment may, for example, include performing wafer alignment without correction, exposing the wafer, and measuring the overlay on the exposed wafer in a calibration stage. From the overlay measurements, the constants (and / or which observable parameters to use) can be optimized so that corrections (e.g., corresponding functions or coefficients / constants) can be determined that, if applied during wafer alignment (i.e., to the alignment data acquired in the first step), would have improved overlay performance (i.e., minimized overlay error). Such a method is similar to the method used to determine weights in current OCW methods.

[0073] Another embodiment may involve using a shadow mode during the wafer fabrication process, which continuously monitors whether updating correction constants and / or observable parameters improves overlay (assuming a feedback signal such as overlay is available) or other performance parameters indicative of the quality of the lithography process. Such an approach may be possible without a feedback signal if the relationship between the observables at the surrounding structures and the amount of correction required (e.g., of the alignment position) is understood / known / modeled. This may be based, for example, on a perfectly accurate sensor (and stack) model. However, this is difficult to achieve. One way to mitigate this may be, for example, to measure how much light scatters from the surrounding structures to the target / mark (region of interest) and simulate / model the effect of this on the aligned position or other parameter of interest.

[0074] In many of the described embodiments, multiple different observable parameters (and corresponding correction constants) may be used simultaneously. This may be necessary and / or may result in improved performance, for example, when several independent process variations occur in the surrounding structure that require correction (since the number of measurements / observables needs to be at least equal to the number of variables to be corrected).

[0075] The methods described herein can provide measurement corrections (e.g., aligned position corrections) for each position within the target / mark. Such embodiments may impose weighting on correction factors as a function of position relative to surrounding structures, e.g., smaller correction factors may be imposed the further away from the surrounding structures. This can be particularly beneficial when using optimized coherence tools such as those shown in FIG. 5 or other metrology tools that can obtain measurements as a function of target / mark position (e.g., measuring aligned position or local APD for each location within the mark), thereby allowing for corrections for, e.g., local mark deformations, etc.

[0076]

[0075] The above method can be combined with OC(I)W (Optimal Color and Intensity Weighting).

[0077] Although the methods described above in the context of alignment have been described with respect to measurements using image-based tools (e.g., the optimized coherence image-based tool of FIG. 5), these methods are also applicable to more conventional (e.g., SRI-based) alignment sensors, such as those shown in FIG. 4, for measuring small marks. Such methods may be based on the assumption that it is not possible to completely underfill a small mark. Alternatively, or in addition, it is possible to use an intentionally longer scan length over the mark so that surrounding structures are captured in the measurement (e.g., because that is useful information).

[0078]

[0077] Although the above description describes the application of a linear correction model, higher-order correction models can also be used. The correction model can also be a machine learning model (and therefore suitably trained), such as a neural network.

[0079] A second method uses a special target, referred to herein as an invisible target (which does not provide a signal to the metrology tool, i.e., is invisible to the metrology tool). Using such a target allows the ambient signal contribution to be measured directly, rather than inferring it from another observable parameter. For example, the invisible target may be placed in the vicinity of the metrology target (e.g., within the area where the parameter of interest is being measured). The region of interest on the camera image corresponding to the actual invisible target should contain no signal (e.g., intensity), and therefore, the signal detected in this region can be considered the ambient signal contribution. In one embodiment, this ambient signal contribution, measured directly by measuring the invisible target, may be simply subtracted from the metrology signal from the metrology target. This also directly corrects for residual calibration errors.

[0080] An invisible target may, for example, include a grating with a period that does not produce propagating diffraction orders that can be captured by the metrology tool. Only the zeroth order is produced, or at least propagates as far as the collection optics. Thus, radiation from this target is not absorbed, but is reflected back to the zeroth order of the illumination tool, where it is blocked (e.g., when using the tool in dark-field mode; for example, the metrology tools of Figures 3 and 5 can operate in dark-field mode). The "higher" diffraction orders fade away and therefore do not propagate to the collection optics / detector and are not "seen" by the metrology tool. In this way, the dummy target is invisible. Alternatively, an invisible target may include reflective areas or other features that are not visible to the metrology target (e.g., that scatter / reflect only radiation propagating in the zeroth order).

[0081]

[0080] Figure 8 shows an invisible target in the (e.g., overlay) metrology context of Figure 7. Such an invisible target includes one or more invisible regions NV having the characteristics described above. When such a target is measured, the signal detected in the region corresponding to the invisible region NV is due to the surrounding structures SS (and possibly sensor asymmetry). This signal may therefore be subtracted from the target measurement.

[0082]

[0081] The invisible target may include a similar or identical form (e.g., contour / shape) to a corresponding metrology target (e.g., a metrology target for which corrections are to be determined). In this way, the configuration of the surrounding structures relative to the target can be best represented.

[0083]

[0082] Although invisible targets can be measured during manufacturing as described above to directly measure the ambient signal contribution to the target measurement, measuring such targets in this manner is not always desirable (e.g., there is a throughput penalty associated with the additional measurement of the invisible target). Therefore, another embodiment includes measuring the invisible target only at calibration time to determine a correction factor or function, and applying the correction factor or function to measurements of the ambient structures (e.g., measurements of observable parameters) (e.g., during the manufacturing process). Such measurements of the ambient structures can be determined from the same image as the target image, i.e., thereby incurring no throughput penalty. Of course, the ambient structures may optionally be measured separately.

[0084] By determining a function that relates the measured background asymmetry to the asymmetry (or intensity / phase) of an invisible target, it is not necessary to measure the invisible target after calibration. This function can be determined once (e.g., per stack / illumination condition) in a single calibration using the invisible target. Then, only the surrounding structures need to be measured, and the measurements transformed using the determined relationship to determine the intensity and / or phase contribution in the actual target measurement due to parasitic leakage from the surrounding structures. This contribution can then be subtracted from the measured intensity and / or phase to obtain the correct value (i.e., without the ambient signal contribution / parasitic leakage term). Such a method can account for differences in the stack between the calibration site and the measurement site (e.g., different backgrounds on the wafer).

[0085] Calibration may involve measuring an invisible target and determining the ambient signal contribution (e.g., once) for each nominal stack and for each recipe setting used. In each case, corresponding measurements of one or more observable parameters of the surrounding structures are made (from the same image or otherwise). The relationship between the ambient signal contribution and one or more observable parameters can then be determined (e.g., for each nominal stack / measurement recipe combination). As a simple example, if the intensity in the ROI corresponding to the ambient location is 20 times the intensity in the ROI corresponding to the invisible target, the determined function may simply be a factor / scale of 0.05. As before, more complex or higher-order functions / models may be determined. Alternatively, a machine learning / neural network model may be trained to learn this relationship during the calibration phase. This calibration scheme assumes that the redistribution of light inside the sensor is independent of the stack.

[0086]

[0085] Accordingly, disclosed herein is a substrate comprising at least one invisible target that is invisible to a metrology tool. The invisible target has a period that does not produce propagating diffraction orders that can be captured by the metrology tool. Also disclosed is a reticle that includes reticle features configured to form such a substrate when exposed in a lithography process.

[0087] Instead of using an invisible target, the effect of surrounding structures can be quantified by calibrating an overfilled measurement with an underfilled measurement that includes only the target structure inside the measurement spot (and is therefore not affected by the surrounding structures). Such a method may include the following steps: Measuring the target using the underfilled spot. Measuring the same target with the overfilled spot. Calculating the difference between the underfilled and overfilled measurements, the difference being a measure of the influence of the surroundings on the overfilled measurement. Correlating this difference with measurements (intensity / asymmetry) of surrounding structures (e.g. similar to the calibration method described above in relation to invisible targets). Correcting for the effects of asymmetry in future measurements by measuring the background (surrounding structures) and applying relationships estimated from the functional behavior as determined in the previous steps.

[0088] In a further embodiment, a further calibration method consists of measuring the intensity of light reflected from a single pad of the four-pad configuration of FIG. 7. Such a measurement provides information about the amount of light intensity available outside the physical boundaries of the pad. The calibration further subtracts the scaled intensity from the intensity actually measured on the pad with surrounding pads. In a further embodiment, the intensity values ​​acquired at each pixel on the detection camera (multiple pixels forming a measured image of the target configuration) are quality assessed, i.e., by analyzing whether the asymmetry follows a linear behavior. Pixels with non-linear behavior are distinguished and rejected and / or labeled. Furthermore, the measured intensity values ​​of distinguished pixels are used to correct the measurements of non-distinguished pixels for the above-mentioned crosstalk effects, for example by subtraction.

[0089] The methods described herein can find application in any type of metrology for overfilled targets. Accordingly, such targets can be smaller. For example, the targets can be smaller than 40 μm, 30 μm, 20 μm, 15 μm, or 10 μm in one or both directions in the substrate plane.

[0090]

[0089] As used herein, the terms "radiation" and "beam" encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having wavelengths at or near 365, 355, 248, 193, 157, or 126 nm) and extreme ultraviolet (EUV) radiation (e.g., having wavelengths in the range of 5 to 20 nm), as well as particle beams such as ion beams or electron beams.

[0091]

[0090] The term "lens", where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

[0092] The term target should not be construed to mean only a dedicated target formed for the specific purpose of metrology. The term target should be understood to encompass other structures, including product structures, that have properties suitable for metrology applications. The term target encompasses targets used for alignment, conventionally referred to as alignment marks or marks. Such alignment targets or marks may also include actual product structures or dedicated alignment targets suitable for use in alignment.

[0093]

[0092] Further embodiments of the present invention are described in the following numbered clauses. 1. A metrology method comprising: measuring at least one ambient observable parameter related to ambient signal contributions to the metrology signal, including contributions to the metrology signal that are not attributable to the at least one target being measured; determining a correction from the ambient signal observable parameters; acquiring first measurement data related to a measurement of one or more targets using measurement radiation that forms a measurement spot on one or more of the one or more targets that is larger than one of the targets; and applying the correction to the first measurement data. 2. The method of clause 1, wherein the ambient signal contribution includes a contribution due to ambient structures captured within the measurement spot when measuring the target. 3. At least one ambient signal observable parameter is signal strength or strength indicators corresponding to surrounding structures; the amplitude of the interference pattern corresponding to the surrounding structure; aligned positions and / or fringe positions corresponding to surrounding structures; asymmetry corresponding to surrounding structures, corresponding strength imbalances in surrounding structures; fringe visibility corresponding to surrounding structures; 3. The method of claim 1 or 2, comprising one or more of: a difference between the aligned positions of different colors corresponding to surrounding structures. 4. The steps of measuring at least one ambient observable parameter and determining a correction are performed in an initial calibration phase, the calibration phase further comprising: 4. The method of any one of clauses 1 to 3, comprising determining, in a calibration stage, the correction as at least one correction relationship between at least one ambient observable parameter and an ambient signal contribution. 5. The method of clause 4, wherein determining at least one correction relationship includes determining the correction relationship for each of a plurality of different nominal stack and / or measurement radiation illumination conditions. 6. The method of clause 4 or 5, comprising obtaining calibration measurement data, including calibration target data and corresponding calibration ambient observable parameter data. 7. The method of clause 6, wherein the calibration target data relates to an invisible target that is not visible to the metrology tool that measures the invisible target. 8. The method of clause 7, wherein the invisible target has a period that does not produce propagating diffraction orders that can be captured by the metrology tool. 9. The calibration target data describes metrology signal values ​​of a target region of interest corresponding to an invisible target within a measured image of the invisible target; 9. The method of any one of clauses 7-8, wherein determining at least one correction relationship includes determining at least one correction relationship between calibration target data and calibration ambient observable parameter data. 10. The method of clause 9, wherein calibration ambient observable parameter data is obtained from a surrounding region of interest in the measurement image, such that a corresponding set of calibration ambient observable parameter data and calibration target data is obtained from each image. 11. The calibration target data includes target swing curve data; the calibrated ambient observable parameter data includes ambient swing curve data; 7. The method of claim 6, wherein the step of determining at least one corrective relationship compares the target swing curve data and the ambient swing curve data. 12. The first measurement data includes alignment data; The step of determining at least one correction relationship includes: performing alignment measurements without correction on one or more substrates to obtain alignment data; exposing a substrate and measuring overlay on the exposed substrate to obtain overlay data; optimizing at least one correction relationship as if a corresponding correction had been applied to the alignment data that would have improved overlay performance with respect to the overlay data; 12. The method according to clause 11, comprising: 13. The method according to clause 12, wherein the optimization is carried out at least initially in a calibration phase. 14. The method of clause 12 or 13, wherein the optimization is performed in a shadow mode during the substrate manufacturing process, continuously monitoring whether updating the relationship improves overlay performance. 15. The calibration target data includes first calibration target data related to one or more calibration targets measured in overfill mode and second calibration target data related to one or more calibration targets measured in underfill mode; The method includes determining a difference between the first calibration target data and the second calibration target data; 7. The method of clause 6, wherein determining at least one correction relationship includes determining at least one correction relationship between the difference and calibration ambient observable parameter data. 16. The method of any one of clauses 4 to 15, wherein the first measurement data includes target measurement data relating to one or more targets and corresponding ambient observable parameter data relating to ambient structures in the vicinity of the one or more targets. 17. The method of clause 16, wherein at least a portion of the surrounding structure is captured within a measurement spot used to acquire the first measurement data. 18. The step of applying the correction comprises: applying the correction relationship to the ambient observable parameter data to determine a correction offset; applying a correction offset to a corresponding target measurement data in the measurement data; 18. The method according to clause 16 or 17, comprising: 19. Corresponding sets of target measurement data and ambient observable parameter data are determined from respective measurement images of each target or group of targets; the target measurement data relates to one or more target regions of interest within the measurement image; 19. A method according to any one of clauses 16 to 18, wherein the ambient observable parameter data relates to one or more ambient regions of interest within the measurement image. 20. The first subset of first measurement data relates to one or more metrology targets; a second subset of the measurement data pertaining to one or more invisible targets that are not visible to the metrology tool measuring the invisible targets; 4. The method of any one of clauses 1 to 3, wherein the correction is determined from metrology signal values ​​of an invisible target region of interest corresponding to the invisible target in a measured image of the invisible target. 21. The method of clause 20, wherein the invisible target has a period that does not produce propagating diffraction orders that can be captured by the metrology tool. 22. The first measurement data is 10. The method of any one of the preceding clauses, comprising a post-exposure measurement and one or both of a pre-exposure measurement or alignment measurement. 23. The method of clause 22, wherein the post-exposure measurements include one or both of an overlay measurement and a focus measurement. 24. A method according to any one of the preceding clauses, wherein the one or more targets are smaller than 15 μm in one or both directions in the plane of the substrate. 25. A computer program comprising processor-readable instructions which, when executed on a suitable processor-controlled device, causes the processor-controlled device to carry out a method according to any of the preceding clauses. 26. A computer program carrier containing a computer program as defined in clause 25. 27. A metrology apparatus comprising: a support for a substrate comprising one or more targets; an optical system for measuring each target; a processor; a computer program carrier according to clause 26, A metrology apparatus whereby the processor is able to control the metrology apparatus to perform the method of any one of clauses 1 to 24. 28. An illumination system configured to condition a radiation beam; a patterning device support constructed to support a patterning device, the patterning device being capable of imparting a radiation beam with a pattern in its cross-section to form a patterned radiation beam; and a substrate table constructed to hold a substrate; a projection system configured to project the patterned radiation beam onto a target portion of a substrate; - at least one metrology apparatus according to clause 27; 1. A lithography apparatus comprising: 29. A lithographic apparatus according to clause 28, wherein the at least one metrology apparatus comprises an alignment apparatus operable to perform pre-exposure metrology to perform positional metrology for positioning one or both of the patterning device support and the substrate table. 30. A lithographic apparatus according to clause 28 or 29, wherein the at least one metrology apparatus comprises a post-exposure metrology apparatus for performing post-exposure measurements on substrates exposed with structures using the lithographic apparatus.

[0094] The foregoing description of specific embodiments will make fully apparent the general nature of the present invention, such that others, by applying their knowledge in the art, can readily modify and / or adapt such specific embodiments to various applications without undue experimentation and without departing from the general concept of the present invention. Accordingly, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. The phrases or terms used herein are for purposes of illustration and description, and not of limitation, and as such, the terms or terms used herein will be interpreted by those of ordinary skill in the art in light of the teachings and guidance.

[0095]

[0094] 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. 1. A metrology method comprising: measuring at least one ambient observable parameter related to ambient signal contributions to the metrology signal, including contributions to the metrology signal that are not attributable to at least one target being measured; determining a correction from the ambient observable parameters; obtaining first measurement data, the first measurement data relating to a measurement of one or more targets using measurement radiation that forms a measurement spot on one or more of the at least one target that is larger than one of the targets; applying the correction to the first measurement data; A method comprising:

2. The method of claim 1 , wherein the ambient signal contribution includes a contribution due to ambient structures captured within the measurement spot when measuring the target.

3. The at least one ambient observable parameter is: a signal strength or strength indicator corresponding to the surrounding structure; the amplitude of an interference pattern corresponding to the surrounding structure; and registered locations and / or fringe locations corresponding to the surrounding structure; asymmetry corresponding to the surrounding structure; a strength imbalance corresponding to the surrounding structure; fringe visibility corresponding to the surrounding structure; the difference between the aligned positions of different colors corresponding to the surrounding structures; The method of claim 2 , comprising one or more of:

4. the steps of measuring at least one ambient observable parameter and determining a correction are performed in an initial calibration phase; 4. The method of claim 1, wherein the step of calibrating further comprises determining the correction as at least one correction relationship between at least one ambient observable parameter and the ambient signal contribution during the calibration step.

5. The method of claim 4 , wherein determining at least one correction relationship comprises determining a correction relationship for each of a plurality of different nominal stack and / or illumination conditions of the measurement radiation.

6. The method of claim 4 or 5, comprising obtaining calibration measurement data comprising calibration target data and corresponding calibration ambient observable parameter data.

7. 7. The method of claim 4, wherein the first measurement data comprises target measurement data relating to the one or more targets and corresponding ambient observable parameter data relating to ambient structures in the vicinity of the one or more targets.

8. the first subset of first measurement data relates to one or more metrology targets; a second subset of the measurement data relating to one or more invisible targets that are not visible to a metrology tool measuring the invisible targets; The method of any one of claims 1 to 3, wherein the correction is determined from metrology signal values ​​of an invisible target region of interest corresponding to the invisible target in a measured image of the invisible target.

9. The method of any one of claims 1 to 8, wherein the first measurement data comprises a post-exposure measurement and one or both of a pre-exposure measurement or an alignment measurement.

10. comprising processor readable instructions which, when executed on a suitable processor-controlled device, cause said processor-controlled device to carry out a method according to any one of claims 1 to 9, Computer program.

11. A computer program carrier comprising a computer program according to claim 10.

12. 1. A metrology apparatus comprising: a support for a substrate comprising said one or more targets; an optical system for measuring each target; a processor; A computer program carrier according to claim 11; whereby said processor is able to control said metrology apparatus to perform the method of any one of claims 1 to 9.

13. an illumination system configured to condition the radiation beam; a patterning device support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam; and a substrate table constructed to hold a substrate; a projection system configured to project the patterned radiation beam onto a target portion of the substrate; and At least one metrology device according to claim 12; 1. A lithography apparatus comprising:

14. 14. The lithographic apparatus of claim 13, wherein the at least one metrology apparatus comprises an alignment apparatus operable to perform pre-exposure metrology to perform positional metrology for positioning one or both of the patterning device support and the substrate table.

15. 14. The lithographic apparatus of claim 13, wherein the at least one metrology apparatus comprises a post-exposure metrology apparatus for performing post-exposure measurements on a substrate exposed with features using the lithographic apparatus.

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