Method of mitigating crosstalk in metrology images

Abstract

A method of determining an orthogonally normalized structure of interest reference image is disclosed for application to a measurement image of the structure of interest to correct for the effects of crosstalk from at least one lossy structure. The method includes determining a structure of interest reference image based on knowledge of the structure of interest; determining at least one impairment structure reference image based on knowledge of the at least one impairment structure; and orthogonally normalizing the reference image of the structure of interest to the at least one impairment reference image to obtain the orthogonally normalized reference image of the structure of interest.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP application 22187892.9, filed on July 29, 2022, and incorporated herein by reference in its entirety. Technical Field

[0003] The present invention relates to a metrology method and a device which can be used, for example, to determine properties of structures on a substrate. Background Art

[0004] A lithographic apparatus is a machine configured to apply a desired pattern onto a substrate. A lithographic apparatus may be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may project a pattern (also often referred to as a "design layout" or "design"), for example at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).

[0005] To project a pattern onto a substrate, a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of a feature that can be formed on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. A lithographic apparatus using extreme ultraviolet (EUV) radiation with a wavelength in the range of 4 to 20 nm (e.g., 6.7 nm or 13.5 nm) can be used to form smaller features on a substrate than a lithographic apparatus using radiation with a wavelength of, for example, 193 nm.

[0006] Low k1 lithography can be used to process features with dimensions smaller than the classical resolution limit of the lithographic apparatus. In such a process, the resolution formula can be expressed as CD = k1 × λ / NA, where λ is the wavelength of radiation employed, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the "critical dimension" (usually the smallest feature size printed, but in this case the half pitch), and k1 is an empirical resolution factor. In general, the smaller the k1, the more difficult it is to reproduce a pattern on a substrate that is similar in shape and size to that planned by the circuit designer to achieve specific electrical functionality and performance. To overcome these difficulties, complex fine-tuning steps can be applied to the lithographic projection apparatus and / or the design layout. For example, these include, but are not limited to, optimization of the NA, customized illumination schemes, use of phase-shifted patterning devices, various optimizations of the design layout such as optical proximity correction (OPC, sometimes also referred to as "optical and process correction") in the design layout, or other methods generally defined as "resolution enhancement techniques" (RET). Alternatively, a tight control loop for controlling the stability of the lithographic apparatus can be used to improve the reproduction of patterns at low k1.

[0007] During the lithography process, it is desirable to make frequent measurements of the structures created, for example for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes or various forms of metrology equipment, such as scatterometers. A general term to refer to such tools may be metrology equipment or inspection equipment.

[0008] In known metrology techniques, the overlap measurement result is obtained by measuring the overlap target twice under specific conditions, while rotating the overlap target or changing the illumination mode or imaging mode to obtain a single -1 st and +1 st Diffraction order intensities (or diffraction efficiencies, a related parameter). For a given overlay target, intensity asymmetry (or diffraction efficiency asymmetry) (i.e., comparison of these diffraction order intensities) provides a measure of target asymmetry (i.e., asymmetry in the target). This asymmetry in an overlay target can be used as an indicator of overlay (unwanted misalignment of the two layers).

[0009] The measurement of a parameter of interest, such as overlay, is usually based on acquiring a measurement image of a target, where the imaged target is subject to crosstalk from adjacent structures. This crosstalk affects the inferred value of the parameter of interest.

[0010] It is desirable to mitigate this crosstalk. Summary of the invention

[0011] Embodiments of the invention are disclosed in the claims and the detailed description.

[0012] In a first aspect of the present invention, a method for determining an orthogonally normalized reference image of a structure of interest is provided, wherein the orthogonally normalized reference image of the structure of interest is used to be applied to a measurement image of the structure of interest to correct the influence of crosstalk from at least one impairment structure, the method comprising: determining the reference image of the structure of interest based on knowledge of the structure of interest; determining at least one impairment structure reference image based on knowledge of at least one impairment structure; and orthogonally normalizing the reference image of the structure of interest to at least one impairment reference image to obtain an orthogonally normalized reference image of the structure of interest.

[0013] According to a second aspect of the present invention, a method for determining measurement parameters related to a structure of interest is provided, the method comprising: obtaining an orthogonally normalized structure of interest reference image related to the structure of interest, the orthogonally normalized structure of interest reference image being orthogonally normalized relative to at least one damaged structure reference image related to at least one damaged structure; obtaining at least one measurement image, the at least one measurement image comprising an image of the structure of interest affected by the damaged structure; and determining measurement parameters by combining each of the at least one measurement image and the orthogonally normalized structure of interest reference image, so that the determined measurement parameters are corrected for the influence of the damaged structure.

[0014] According to a third aspect of the invention there is provided a computer program comprising program instructions operable, when run on a suitable apparatus, to perform the method of the first aspect.

[0015] The invention further provides a processing arrangement and a measurement device comprising the computer program of the second aspect.

[0016] These and other aspects and advantages of the apparatus and methods disclosed herein will be appreciated by considering the following description and accompanying drawings of exemplary embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0018] Figure 1 depicts a schematic overview of a lithography apparatus;

[0019] Figure 2 A schematic overview of a lithography unit is depicted;

[0020] Figure 3 Depicted is a schematic representation of overall lithography showing the collaboration between three key technologies to optimize semiconductor manufacturing;

[0021] Figure 4 is a schematic illustration of a scatterometry device;

[0022] Figure 5 comprising (a) a schematic diagram of a dark field scatterometer for measuring a target according to an embodiment of the present invention using a first pair of illumination apertures, (b) details of a diffraction spectrum of a target grating given an illumination direction, (c) a second pair of illumination apertures providing a further illumination mode when using the scatterometer for diffraction-based overlay measurements, (d) a third pair of illumination apertures combining the first pair of apertures and the second pair of apertures; and (e) other examples of illumination apertures that may be used for other metrology applications; and

[0023] Figure 6The profile of the measurement spot on the known form of the multi-grating target and the substrate is depicted;

[0024] Figure 7 Depicted in Figure 5 (a) obtained from the scatterometer Figure 6 An image of the target;

[0025] Figure 8 schematically depicts a dark-field digital holographic microscope (df-DHM) operating in a parallel acquisition scheme according to an embodiment;

[0026] Fig. 9 including an image representation of the target and surrounding or surrounding detrimental structures;

[0027] Fig.10 Includes image representation of target pad and individual destructive structures;

[0028] Fig.11 A block diagram of a computer system for controlling a method according to an embodiment is depicted. DETAILED DESCRIPTION

[0029] In this document, the terms "radiation" and "beam" are used to cover all types of electromagnetic radiation, including ultraviolet radiation (e.g., wavelengths of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultraviolet radiation, e.g., wavelengths in the range of about 5 to 100 nm).

[0030] The terms "reticle", "mask" or "patterning device" as used herein may be broadly interpreted as referring to a general patterning device that can be used to impart a patterned cross-section to an incoming radiation beam, corresponding to the pattern to be created in a target portion of a substrate. In this context, the term "light valve" may also be used. In addition to classical masks (transmissive or reflective, binary, phase-shifting, hybrid, etc.), other examples of such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0031] Figure 1A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA comprises an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g. UV radiation, DUV radiation or EUV radiation), a mask support (e.g. mask table) MT configured to support a patterning device (e.g. mask) MA and connected to a first positioner PM (configured to accurately position the patterning device MA according to certain parameters), a substrate support (e.g. wafer stage) WT configured to hold a substrate (e.g. resist coated wafer) W and connected to a second positioner PW (configured to accurately position the substrate support according to certain parameters), and a projection system (e.g. refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0032] In operation, the illumination system IL receives a radiation beam from a radiation source SO (e.g., via a beam delivery system BD). The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical components, or any combination thereof, for directing, shaping and / or controlling the radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0033] The term "projection system" PS as used herein should be broadly interpreted as covering various types of projection systems, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems or any combination thereof, as appropriate with respect to the exposure radiation used and / or other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0034] The lithographic apparatus LA 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, such as water, to fill the space between the projection system PS and the substrate W, which is also known as immersion lithography. More information on immersion technology is given in US6952253, which is incorporated herein by reference.

[0035] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also referred to as a "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or steps for preparing a substrate W for subsequent exposure may be performed on a substrate W located on one of the substrate supports WT, while another substrate W on another substrate support WT is used to expose a pattern on another substrate W.

[0036] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement platform. The measurement platform is arranged to hold sensors and / or cleaning means. The sensors may be arranged to measure properties of the projection system PS or properties of the radiation beam B. The measurement platform may hold a plurality of sensors. The cleaning means may be arranged to clean a part of the lithographic apparatus, e.g. a part of the projection system PS or a part of a system for providing immersion liquid. The measurement platform may be moved under the projection system PS when the substrate support WT is away from the projection system PS.

[0037] In operation, a radiation beam B is incident on a patterning device (e.g. a mask) MA and is patterned by a pattern (design layout) present on the patterning device MA, which is held on a mask support MT. After traversing the mask MA, the radiation beam B passes through a projection system PS which focuses the beam onto a target portion C of a substrate W. With the aid of a second positioner PW and a position measurement system IF, the substrate support WT can be accurately moved, for example in order to position different target portions C in the path of the radiation beam B at focused and aligned positions. Similarly, the first positioner PM and possibly another position sensor (not shown) are also provided. Figure 1 The patterning device MA (depicted explicitly in FIG. 1 ) can be used to accurately position the patterning device MA relative to the path of the radiation beam B. The patterning device MA and the substrate W can be aligned using the mask alignment marks M1, M2 and the substrate alignment marks P1, P2. Although the illustrated substrate alignment marks P1, P2 occupy dedicated target portions, they can be located in the space between target portions. When the substrate alignment marks P1, P2 are located between target portions C, these are referred to as scribe line alignment marks.

[0038] like Figure 2 As shown, the lithography apparatus LA may form part of a lithography cell LC, which is sometimes also referred to as a lithography cell or (lithography) cluster, which typically also includes equipment for performing pre- and post-exposure processes on a substrate W. 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, for example, for regulating the temperature of the substrate W (e.g. for regulating the solvent in the resist layer). A substrate handler or robot RO picks up the substrate W from the input / output ports I / O1, I / O2, moves them between the different process equipment, and delivers the substrate W to a feed station LB of the lithography apparatus LA. The devices in the lithography cell (often also collectively referred to as tracks) are typically controlled by a track control unit TCU, which itself may be controlled by a supervisory control system SCS, which may also control the lithography apparatus LA, for example via a lithography control unit LACU.

[0039] In order for substrates W exposed by lithographic apparatus LA to be correctly and consistently exposed, it is desirable to inspect the substrates to measure properties of the patterned structures, such as overlay errors between subsequent layers, line thickness, critical dimensions (CD), etc. For this purpose, an inspection tool (not shown) may be included in lithography cell LC. If errors are detected, for example, adjustments may be made to the exposure of subsequent substrates or other processing steps to be performed on substrate W, particularly if the inspection is done before other substrates W of the same batch or lot are still to be exposed or processed.

[0040] An inspection apparatus (which may also be referred to as a metrology apparatus) is used to determine properties of a substrate W, in particular how properties vary from one substrate W to another, or how properties associated with different layers of the same substrate W vary from layer to layer. The inspection apparatus may alternatively be configured to identify defects on the substrate W, and may for example be part of a lithography cell LC, or may be integrated into a lithography apparatus LA, or may even be a stand-alone device. The inspection apparatus may measure properties on a latent image (the image in the resist layer after exposure) or a semi-latent image (the image in the resist layer after a post-exposure bake step PEB), or a developed resist image (where exposed or unexposed parts of the resist have been removed) or even an etched image (after a pattern transfer step such as etching).

[0041] Typically, the patterning process in the lithography apparatus LA is one of the most critical steps in the process, which requires high accuracy in dimensioning and placement of structures on the substrate W. To ensure this high accuracy, the three systems can be combined into a so-called "holistic" control environment, such as Figure 3 Schematically depicted in . One of these systems is the lithography apparatus LA, which is (in practice) connected to a metrology tool MET (a second system) and a computer system CL (a third system). The key to this "holistic" environment is to optimize the cooperation between these three systems to enhance the overall process window, and to provide a tight control loop to ensure that the patterning performed by the lithography apparatus LA stays within the process window. The process window defines the range of process parameters (e.g. dose, focus, overlay) within which a specific manufacturing process will produce a defined result (e.g. a functional semiconductor device), and within which process parameters of a lithography process or patterning process are typically allowed to vary.

[0042] The computer system CL can use (a portion of) the design layout to be patterned to predict which resolution enhancement techniques to use, and perform computational lithography simulations and calculations to determine which mask layouts and lithography equipment settings achieve the maximum overall process window for the patterning process (in Figure 3Typically, the resolution enhancement technique is arranged to match the patterning possibilities of the lithographic apparatus LA. The computer system CL may also be used to detect where within the process window the lithographic apparatus LA is currently operating (e.g. using input from a metrology tool MET) to predict whether defects may be present due to, for example, suboptimal processing (e.g., in the process window in which the lithographic apparatus LA is currently operating). Figure 3 ) is depicted by an arrow pointing to “0” in the second scale SC2.

[0043] The metrology tool MET may provide input to the computer system CL to enable accurate simulations and predictions, and may provide feedback to the lithographic apparatus LA to identify possible drifts, e.g. in the calibration state of the lithographic apparatus LA (in Figure 3 ) as depicted by multiple arrows in a third scale SC3.

[0044] During a lithographic process, it is desirable to make frequent measurements of the structures created, for example for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes or various forms of metrology equipment, such as scatterometers. Examples of known scatterometers typically rely on the provision of dedicated metrology targets, such as an underfilled target (a target in the form of a single grating or overlapping gratings in different layers, which is large enough for the measurement beam to generate a spot smaller than the grating) or an overfilled target (where the illumination spot partially or completely encompasses the target). Further, the use of a metrology tool, such as an angle-resolved scatterometer that illuminates an underfilled target such as a grating, allows the use of so-called reconstruction methods, where the properties of the grating can be calculated by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measurement results. The parameters of the model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from a real target.

[0045] Scatterometers are versatile instruments that allow measuring parameters of a lithography process by placing a sensor in the pupil of the scatterometer objective or in a plane conjugate to the pupil (these measurements are usually referred to as pupil-based measurements) or by placing a sensor in the image plane or in a plane conjugate to the image plane (in this case, these measurements are usually referred to as image-based or field-based measurements). Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP1,628,164A, which are all incorporated herein by reference. In one image, the above scatterometer can measure multiple targets from multiple gratings using light from soft x-rays and visible to the near IR wave range.

[0046] Measurement equipment such as scatterometers are used in Figure 4 . It comprises a broadband (white light) radiation projector 2 which projects radiation 5 onto a substrate W. The reflected or scattered radiation 10 is passed to a spectrometer detector 4 which measures the spectrum 6 of the specularly reflected radiation 10 (i.e. the intensity I as a function of the wavelength λ). From this data the structure or profile 8 which gave rise to the detected spectrum can be reconstructed by a processing unit PU, for example by rigorous coupled wave analysis and nonlinear regression or by comparison with a library of simulated spectra. Typically, for the reconstruction the general form of the structure is known and some of the parameters are set by knowledge of the structure manufacturing process, leaving only a few parameters of the structure to be determined by scatterometry data. Such a scatterometer can be configured as a normal incidence scatterometer or an oblique incidence scatterometer.

[0047] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, reconstruction methods can be applied to the measurement signal to reconstruct or calculate the properties of the grating. For example, such a reconstruction can be obtained by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measurement results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from a real target.

[0048] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, radiation emitted by a radiation source is directed onto a target and reflected or scattered radiation from the target is directed to a spectrometer detector which measures the spectrum of the specularly reflected radiation (i.e. measures the intensity as a function of wavelength). From this data, the structure or profile of the target producing the detected spectrum can be reconstructed, for example by rigorous coupled wave analysis and nonlinear regression or by comparison with a library of simulated spectra.

[0049] In a third embodiment, the scatterometer MT is an ellipsometer. An ellipsometer allows the parameters of a lithography process to be determined by measuring the scattered radiation for each polarization state. Such a measurement device emits polarized light (such as linear, circular or elliptical) by using, for example, an appropriate polarization filter in the illumination section of the measurement device. A source suitable for the measurement device may also provide polarized radiation. Various embodiments of existing ellipsometers are described in U.S. patent applications 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,449, 12 / 920,968, 12 / 922,587, 13 / 000,229, 13 / 033,135, 13 / 533,110 and 13 / 891,410, which are incorporated herein by reference in their entirety.

[0050] In one embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the overlap of two misaligned gratings or periodic structures by measuring a reflection spectrum and / or an asymmetry in the detection configuration, the asymmetry being related to the degree of the overlap. The two (usually overlapping) grating structures may be applied in two different layers (not necessarily consecutive layers) and may be formed at substantially the same position on the wafer. The scatterometer may have a symmetrical detection configuration such as described in co-owned patent application EP1,628,164A, so that any asymmetry is clearly distinguishable. This provides a direct way of measuring misalignment in the grating. Other examples for measuring the overlap error between two layers containing a periodic structure when the target passes through the asymmetry measurement of the periodic structure can be found in PCT patent application publication number WO 2011 / 012624 or US patent application US 20160161863, which are incorporated herein by reference in their entirety. Other parameters of interest may be focus and dose. Focus and dose may be determined simultaneously by scatterometry (or alternatively by scanning electron microscopy), as described in US Patent Application US2011-0249244, which is incorporated herein by reference in its entirety.

[0051] The target can be measured in underfill mode or in overfill mode. In underfill mode, the measuring beam generates a spot that is smaller than the overall target. In overfill mode, the measuring beam generates a spot that is larger than the overall target. In this overfill mode, it is also possible to measure different targets simultaneously and thus determine different process parameters simultaneously.

[0052] The overall measurement quality of a lithography parameter using a specific target is determined at least in part by the measurement profile used to measure the lithography parameter. The term "substrate measurement profile" may include one or more parameters of the measurement itself, one or more parameters of one or more patterns being measured, or both. For example, if the measurement used in the substrate measurement profile is a diffraction-based optical measurement, the one or more parameters measured may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, and the like. One of the criteria for selecting a measurement profile may be, for example, the sensitivity of one of the measurement parameters to process variations. More examples are described in U.S. patent application US2016 / 0161863 and published U.S. patent application US 2016 / 0370717A1, which are incorporated herein by reference in their entirety.

[0053] Figure 5 (a) presents an embodiment of a measurement device, more specifically an embodiment of a dark field scatterometer. The target T and the diffracted rays of the measurement radiation used to illuminate the target are Figure 5(b) is illustrated in more detail. The illustrated measurement device is a type known as a dark field measurement device. The measurement device can be a stand-alone device or incorporated into the lithography device LA, for example at the measurement station, or incorporated into the lithography unit LC. The optical axis with several branches in the entire device is represented by the dotted line O. In this device, the light emitted by the source 11 (for example a xenon lamp) is guided to the 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 of a 4F arrangement. Different lens arrangements can be used, provided that they still provide the substrate image to the detector while allowing access to the intermediate pupil plane for spatial frequency filtering. Therefore, the angular range of radiation incident on the substrate can be selected by defining the spatial intensity distribution in the plane (here referred to as the (conjugate) pupil plane) that presents the spatial spectrum of the substrate plane. In particular, this can be accomplished by inserting an aperture plate 13 of a suitable form between lenses 12 and 14 in the plane that is the back-projected image of the objective lens pupil plane. In the illustrated example, the aperture plate 13 has different forms, labeled 13N and 13S, allowing different illumination modes to be selected. The illumination system in this example forms an off-axis illumination mode. In a first illumination mode, the aperture plate 13N provides off-axis to a direction designated as 'North', for illustration only. In a second illumination mode, the aperture plate 13S is used to provide similar illumination, but from the opposite direction, labeled 'South'. Other illumination modes are possible by using different apertures. The rest of the pupil plane is preferably dark, as any unwanted light beyond the desired illumination mode will interfere with the desired measurement signal.

[0054] like Figure 5 As shown in (b), the target T is placed with the substrate along the normal to the optical axis O of the objective lens 16. The substrate W may be supported by a support (not shown). A ray of measurement radiation I striking the target T from an angle offset from the axis O produces a zero-order ray (solid line 0) and two first-order rays (dash-dotted line +1 and dash-dotted line -1). It should be remembered that with an overfilled small target, these rays are only one of many parallel rays covering the substrate area (including the measurement target T and other features). Since the aperture in the plate 13 has a finite width (necessary to allow an effective amount of light to enter), the incident ray I will in fact occupy a certain angle range, and the diffracted rays 0 and +1 / -1 will be slightly spread out. Depending on the point spread function of the small target, each order +1 and -1 will be further spread out to a certain angle range, rather than the single ideal ray shown. It should be noted that the grating pitch and illumination angle of the target can be designed or adjusted so that the first-order rays entering the objective are closely aligned with the central optical axis. Figure 5 The rays illustrated in (a) and 3(b) are shown slightly off-axis purely to make them easier to distinguish in the drawings.

[0055] At least the 0th and +1st orders diffracted by the target T on the substrate W are collected by the objective lens 16 and directed back through the beam splitter 15. Return to Figure 5 (a), the first illumination mode and the second illumination mode are each illustrated by designating diametrically opposed apertures labeled as north (N) and south (S). When the incident ray I of the measurement radiation comes from the north side of the optical axis, that is, when the first illumination mode is applied using the aperture plate 13N, the +1 diffracted ray labeled as +1 (N) enters the objective lens 16. In contrast, when the second illumination mode is applied using the aperture plate 13S, the -1 diffracted ray (labeled as 1 (S)) is the ray that enters the lens 16.

[0056] The second beam splitter 17 divides the diffracted beam into two measurement branches. In the first measurement branch, the optical system 18 uses the zero-order and first-order diffracted beams to form a diffraction spectrum (pupil plane image) of the target on a first sensor 19 (e.g., a CCD or CMOS sensor). Each diffraction order hits a different point on the sensor, so that image processing can compare and contrast multiple orders. The pupil plane image captured by the sensor 19 can be used to focus the metrology device and / or normalize the intensity measurement of the first-order beam. The pupil plane image can also be used for many measurement purposes, such as reconstruction.

[0057] In the second measurement branch, the optical system 20, 22 forms an image of the target T on a sensor 23 (e.g. a CCD or CMOS sensor). In the second measurement branch, an aperture stop 21 is provided in a plane conjugate to the pupil plane. The aperture stop 21 is used to block the zeroth order diffraction beam so that the target image formed on the sensor 23 is formed only by the -1 or +1 first order beam. The image captured by the sensors 19 and 23 is output to a processor PU that processes the image, the function of which will depend on the specific type of measurement being performed. Note that the term 'image' is used here in a broad sense. If only one of the -1 and +1 orders is present, such a raster line image will not be formed.

[0058] Figure 5 The particular forms of aperture plate 13 and field stop 21 shown are examples only. In another embodiment of the invention, axial illumination of the target is used and an aperture stop with an off-axis aperture is used to pass substantially only one first order diffracted light to the sensor. In other embodiments, 2nd, 3rd and higher order beams ( Figure 5 ) can be used for measurement instead of or as an addition to the first-order beam.

[0059] In order to adapt the measurement radiation to these different types of measurements, the aperture plate 13 may include multiple aperture patterns formed around a disk that rotates to place the desired pattern in place. Note that aperture plates 13N or 13S can only be used to measure gratings oriented in one direction (X or Y, depending on the setup). For measurements of orthogonal gratings, target rotations of 90° and 270° can be achieved. Different aperture plates such as Figure 5 The use of these aperture plates and many other variations and applications of this apparatus are described in the previously published applications referenced above.

[0060] Figure 6 An overlapping target or composite overlapping target formed on a substrate according to known practices is depicted. The overlapping target in this example includes four sub-targets (e.g., gratings) 32 to 35, which are closely positioned together so that they are all within a measurement spot 31 formed by a measurement radiation illumination beam of a measurement device. Therefore, the four sub-overlapping targets are all illuminated simultaneously and imaged simultaneously on the sensor 23. In an example dedicated to measuring overlap, the sub-targets 32 to 35 are themselves composite structures formed by overlapping gratings patterned in different layers of a semiconductor device formed on a substrate W. The sub-targets 32 to 35 can have overlapping offsets with different biases to facilitate measuring the overlap between layers forming different parts of the composite sub-target. Reference will be made below to Figure 7 Explain what is meant by overlapping bias. As shown, the orientation of sub-targets 32 to 35 may also be different so as to diffract incoming radiation in the X and Y directions. In one example, sub-targets 32 and 34 are X-direction sub-targets with biases of +d, -d, respectively. Sub-targets 33 and 35 are Y-direction sub-targets with offsets of +d and -d, respectively. Individual images of these sub-targets can be identified in the image captured by sensor 23. This is just one example of overlapping targets. Overlapping targets can include more or less than 4 sub-targets.

[0061] Figure 7 Shown in Figure 5 (a) Used in equipment Figure 6 Overlapping targets, using Figure 5(d) An example of an image that can be formed on and detected by the sensor 23 using the aperture plate 13NW or 13SE of FIG. 1 . Although the pupil plane image sensor 19 cannot distinguish between different individual sub-targets 32 to 35, the image sensor 23 can do so. The shaded area 40 represents the image field on the sensor within which the illumination spot 31 on the substrate is imaged into the corresponding circular area 41. Within this, the rectangular areas 42 to 45 represent images of small overlapping target sub-targets 32 to 35. If the overlapping targets are located in the product area, product features may also be visible at the periphery of this image field. The image processor and controller PU use pattern recognition to process these images to identify the individual images 42 to 45 of the sub-targets 32 to 35. In this way, the images do not have to be aligned very accurately at specific locations within the sensor frame, which greatly improves the throughput of the entire measurement device.

[0062] Once the individual images of the overlapping target are identified, the intensity of these individual images can be measured, for example by averaging or summing the intensity values ​​of selected pixels within the identified area. The intensity and / or other properties of the images can be compared to each other. These results can be combined to measure different parameters of the lithography process. Overlay performance is an important example of such a parameter.

[0063] Figure 8 An imaging branch of a dark field digital holographic microscope (df-DHM) 800 according to an embodiment is schematically illustrated. The dark field digital holographic microscope (df-DHM) comprises an imaging branch and an illumination branch. In this embodiment, a metrology target 860 comprising a structure on a substrate 850 is illuminated by two radiation illumination beams, namely a first radiation illumination beam 810 and a second radiation illumination beam 820. In an embodiment, the two illumination beams 810, 820 may illuminate the metrology target 860 simultaneously.

[0064] In an embodiment, the first illumination beam 810 may be incident on the measurement target 860 at a first angle of incidence along a first direction relative to the optical axis OA. The second illumination beam 820 may be incident on the measurement target 860 at a second angle of incidence along a second direction relative to the optical axis OA. The first angle of incidence of the first illumination beam 810 and the second angle of incidence of the second illumination beam 820 may be substantially the same. The angle of incidence of each illumination beam may be, for example, in the range of 70 degrees to 90 degrees, in the range of 50 degrees to 90 degrees, in the range of 30 degrees to 90 degrees, in the range of 10 degrees to 90 degrees. The illumination of the measurement target 860 may cause radiation to be scattered from the target. In an embodiment, the first illumination beam 810 may be incident on the measurement target 860 at a first azimuth corresponding to the first direction. The second illumination beam 820 may be incident on the measurement target 860 at a second azimuth corresponding to the second direction. The first azimuth angle of the first illumination beam 810 and the second azimuth angle of the second illumination beam 820 may be different; for example, opposite angles that are 180 degrees apart.

[0065] Depending on the structure of the measurement target 860, the scattered radiation may include reflected radiation, diffracted radiation, or transmitted radiation. In this embodiment, the measurement target may be an overlapping target based on diffraction; and each illumination beam may correspond to a scattered beam including at least one non-zero diffraction order. Each scattered beam carries information of the illuminated measurement target. For example, the first illumination beam 810 may correspond to a first scattered beam 811 including a positive first diffraction order +1 DF; the second illumination beam 820 may correspond to a second scattered beam 821 including a negative first diffraction order -1 DF. The zero diffraction order and other undesirable diffraction orders may be blocked by a beam blocking element (not shown) or configured to fall completely outside the NA of the objective lens 870. Therefore, the df-DHM may be operated in a dark field mode. Note that in some embodiments, one or more optical elements (e.g., a lens combination) may be used to achieve the same optical effect of the objective lens 870.

[0066] Both scattered beams 811, 821 may be collected by an objective lens 870 and subsequently refocused onto an image sensor 880. The objective lens 870 may include a plurality of lenses, and / or the df-DHM 800 may include a lens system having two or more lenses, such as an objective lens and an imaging lens, similar to Fig. 9An exemplary df-DHG of , thereby defining a pupil plane of the objective lens between the two lenses and an image plane at the focus of the imaging lens. In this embodiment, a portion 812 of the first scattered beam 811 and a portion 822 of the second scattered beam 821 are simultaneously incident on a common position of the image sensor 880. At the same time, two reference radiation beams (i.e., a first reference beam 830 and a second reference beam 840) are incident on the same position of the image sensor 880. The four beams can be grouped into two pairs of scattered radiation and reference radiation. For example, a first scattered reference beam pair can include a portion 812 of the first scattered beam 811 and a first reference beam 830. Similarly, a portion 822 of the second scattered reference beam pair can include a second scattered beam 821 and a second reference beam 840. The two scattered reference beam pairs can then form two interference patterns (holographic images) that at least partially overlap in the spatial domain.

[0067] In an embodiment, to separate two at least partially spatially overlapping interference patterns (e.g., in the spatial frequency domain), the first reference beam 830 may have a first angle of incidence relative to the optical axis OA, and the second reference beam 840 may have a second angle of incidence relative to the optical axis OA; the first angle of incidence and the second angle of incidence are different. Alternatively or in addition, the first reference beam 830 may have a first azimuth angle relative to the optical axis OA, and the second reference beam 840 may have a second azimuth angle relative to the optical axis OA; the first azimuth angle and the second azimuth angle are different.

[0068] You can use Figure 5 (a) or Figure 8 One known metrology method performed by the illustrated metrology tool is known as diffraction-based overlay (DBO) or micro-diffraction-based overlay (µDBO). This µDBO technique uses the imaging branch of the metrology tool (the branch through the detector 23) and is based on a pair of complementary diffraction order pairs (usually a pair of complementary first diffraction orders, i.e., the first diffraction order may include the +1 order and the second diffraction order may include the -1 order, such as Figure 5 The asymmetry in the structure is determined by the intensity or diffraction efficiency asymmetry or the intensity or diffraction efficiency difference between the first and second diffraction orders (illustrated in (b)). Therefore, in this context, the terms "first" and "second" do not refer to the diffraction orders, but are simply used to distinguish two diffraction orders of a complementary pair, noting that the first and second diffraction orders can be +2 and -2 diffraction orders or higher complementary diffraction order pairs. The zeroth order (specular radiation) is usually blocked or diverted elsewhere (e.g. to another part of the detector for monitoring purposes); it is not used for µDBO measurements. The main "image" for the inference of the parameters of interest is formed only by the higher (e.g. first) diffraction orders. Depending on the target design, the asymmetry of the structure can be used to infer parameters of interest, such as overlay or focus.

[0069] Overlay is just one parameter of interest that can be measured using this technique. Another parameter of interest could be the exposure focus. This exposure focus can be measured using diffraction-based focus (DBF) or micro-diffraction-based focus (µDBF) metrology. The measurement principle for µDBO and µDBF is essentially the same (target asymmetry is measured via the intensity asymmetry of complementary diffraction orders). The main difference between the two techniques is in the targets: µDBO targets have an overlay-related asymmetry and µDBF targets have an exposure focus-based asymmetry. Similarly, other target types and metrology techniques can be used, such as the continuous diffraction-based overlay (cDBO) target and the corresponding cDBO measurement technique. More generally, the underlying concept applies to any metrology application where spatially resolved images are acquired.

[0070] In the absence of any target defects (asymmetries in one or both gratings constituting the composite target), an overlay target requires only one pad or subtarget (the two terms are used synonymously) in each measurement direction, conventionally referred to as an X pad for measuring the parameter of interest (such as overlay in the X direction) and a Y pad for measuring the parameter of interest (such as overlay in the Y direction), where X and Y are two perpendicular directions in the substrate plane. However, targets often have asymmetric defects, and these defects are usually addressed by providing two pads in each direction in each target to correct for the offset caused by the unwanted asymmetry.

[0071] Examples of such objectives have been described above, and Figure 6 As shown, each pair of pads in each direction has an intentional bias of +d and -d. It should be appreciated that applying biases of equal magnitude and opposite direction is convenient but not required. It is relevant to this µDBO measurement that the two pads have a bias difference (e.g. one pad may be unbiased).

[0072] Specifically, the overlap OV can be calculated as:

[0073]

[0074] where d is the target bias amplitude, and ΔI +d and ΔI -d is the difference in measured diffraction efficiency or intensity between the positive and negative diffraction orders for a pad with positive and negative bias, respectively. Diffraction efficiency and intensity are related measurement parameters and can be used interchangeably.

[0075] By finding the target location and integrating a specific region of interest (ROI) within the camera image, the captured µDBO camera image (such as Figure 7The intensity or diffraction efficiency is measured in 3D images of the optical spectrum (see Figure 1). These regions are usually rectangular (for convenience) and can be integrated according to a binary template; this is computationally very efficient but does not take into account known optical aberrations or impairment structures.

[0076] Overlay and other parameters of interest measured using the method described are very susceptible to crosstalk between different pads and / or between pads and adjacent detrimental structures (e.g., product structures). This is a problem that scatterometer tools such as Figure 5 (a) is often done using partially coherent measurement radiation) and digital holographic microscopy tools such as Figure 8 In the latter case, coherent radiation is known to produce coherent imaging artifacts ('ringing'), which further increases the likelihood of crosstalk.

[0077] In either case, calculating the diffraction efficiency or intensity by straightforward rectangular window integration means that the measurement will suffer from large errors or fail altogether when there are nearby destructive structures that may appear bright (i.e., high diffraction efficiency) in the captured image.

[0078] Fig. 9 An image is shown comprising a target (such as a typical µDBO target) comprising four pads (or sub-targets) P1, P2, P3, P4 surrounded by the area of ​​a destructive structure N1, N2, N3. As can be seen, parts of the destructive structure are captured within the measurement spot MS. By the specific example measurement of the intensity or diffraction efficiency of a single pad, for example P1 may be subject to crosstalk from the other pads P2, P3, P4 and the destructive structure (particularly the destructive structures N1, N2).

[0079] Most crosstalk issues can be circumvented by widening the keep-out zone around the target to ensure that no structures are in view (captured within the measurement spot) from which crosstalk may occur. However, this does not address crosstalk from other pads of the target and is undesirable in any case as it increases the metrology footprint.

[0080] Therefore, it is proposed to find the diffraction efficiency or other measurement parameter by determining the overlap between the captured image and a calculated or calibrated reference image which has been orthogonally normalized for the expected crosstalk sources. In theory, this can correct for all crosstalk, although there may be practical limitations which may limit its effectiveness. However, even in cases where it is not practically possible to correct for all crosstalk, the proposed method will provide significant improvements and crosstalk suppression.

[0081] Therefore, a method for determining an orthogonally normalized reference image of a structure of interest is disclosed herein, and the orthogonally normalized reference image of the structure of interest is used to be applied to a measured image of the structure of interest to correct the influence of crosstalk from at least one impairment structure. The method includes determining the reference image of the structure of interest based on knowledge of the structure of interest; determining at least one impairment structure reference image based on knowledge of at least one impairment structure; and orthogonally normalizing the reference image of the structure of interest to the at least one impairment reference image to obtain an orthogonally normalized reference image of the structure of interest.

[0082] A method for determining measurement parameters related to a structure of interest is also disclosed. The method includes obtaining an orthogonally normalized structure of interest reference image related to the structure of interest, the orthogonally normalized structure of interest reference image has been orthogonally normalized relative to at least one impaired structure reference image related to at least one impaired structure; obtaining at least one measurement image, the at least one measurement image including an image of the structure of interest affected by the impaired structure; and determining measurement parameters by combining each of the at least one measurement image and the orthogonally normalized structure of interest reference image, so that the determined measurement parameters are corrected for the influence of the impaired structure.

[0083] Fig.10 An image comprising an overlapping pad A and detriment B is illustrated. This exemplary arrangement will be used to describe the proposed concepts. The example will be particularly relevant to the incoherent imaging regime; the applicability of these concepts to other coherence regimes will be described later. An image I of a pad (structure of interest) A and detriment structure B can be written as:

[0084]

[0085] Where A and B represent (e.g. normalized) intensity images obtained for an independent pad and an independent impairment, respectively. Either of these intensity images A, B can be used as a reference image for calculation and / or calibration; for example, a structure of interest reference image A and an impairment reference image B. The pre-factors α and β correspond to the diffraction efficiencies (or other measurement parameters) of the two pads.

[0086] More specifically, a reference image A of the structure of interest can be simulated / estimated based on the knowledge of the parameters of the structure of interest and the knowledge of the imaging optics. This can be achieved, for example, by applying (multiple) known (4D) point spread functions of the imaging optics to the target / structure design. In a similar manner, an impairment reference image B of the suspected crosstalk / impairment structure can be determined.

[0087] The purpose of the disclosed method is to determine the structure prefactor of interest or the sub-target prefactor α from the measured image I, independent of the impairment prefactor β. By doing so, the parameters of interest can be inferred without errors due to crosstalk.

[0088] After ROI selection, a known method is to integrate a rectangular window to find a single diffraction efficiency or intensity value within the ROI. A slightly more advanced technique may include calculating the overlap between the reference image A and the measurement target image I. This yields:

[0089]

[0090] where x = A·B is the crosstalk term. For simplicity, A and B can be normalized. The effect of the crosstalk term x = A·B is to make the diffraction efficiency sensitive to the loss diffraction efficiency.

[0091] In order to solve this problem, it is proposed to calculate / determine a reference image A of a structure of interest and a corrupted reference image B, and then orthogonally normalize the reference image A of the structure of interest to the corrupted reference image B to obtain an orthogonally normalized reference image A' of the structure of interest. This can then be overlapped, i.e., combined with the measured image, to find the corrected diffraction efficiency, intensity or other measured parameter α (i.e., the measured parameter without crosstalk). For example, the orthogonal normalization can use a variation of the Gram-Schmidt process.

[0092] The orthogonally normalized structure of interest reference image A' can be determined as:

[0093] , where x = A·B

[0094] Computing the overlap between the measured image and the orthonormalized reference image (e.g. by taking their dot product) yields:

[0095]

[0096] Therefore, the measurement parameter or diffraction efficiency α can be determined independently of β and therefore without crosstalk. In fact, the knowledge of the shape and position of the impairment has been used to construct an orthogonal normalized reference image that is not affected by crosstalk from this type of impairment.

[0097] The above derivation applies to the incoherent case, where the intensities are simply added together, i.e., the imaging is linear between the target function and the measured intensity. Applying this technique for partially coherent light may still leave some residual crosstalk. However, in practice it is found that incoherence is often a reasonable approximation for partially coherent systems, and crosstalk can still be expected to be substantially reduced.

[0098] In the fully coherent case where phase information is available, such as in the DHM concept, the same technique applies without caveats: this is also a linear imaging regime and there is a linear relationship between the target function and the measured field from the sidebands in the hologram. In this case, the equation can be appropriately expressed in amplitude space:

[0099]

[0100] Where A and B are the complex amplitude images, and α and β are complex pre-factors to account for the overall phase of the template and impairments. In this case, the pre-factors α and β now represent the square root of the pad diffraction efficiency (and can therefore be squared to obtain the desired measurement parameter / diffraction efficiency), and the crosstalk term is now written as x = A·B*, where the asterisk indicates the complex conjugate. The other equations are unchanged, and the sensitivity to crosstalk from the specified source is expected to remain low.

[0101] Applying this technique relies on knowing the shape and location of the impairments. Conveniently, many impairments are large rectangles at a known distance from the pad being identified (the structure of interest). This means that the technique is broadly applicable to low coherence systems. For the fully coherent, phase-resolved case, the full pre-aberration amplitude and phase distribution of the impairment structure (related to the diffraction of the structure) should be determined or estimated. This can be found through calibration, where such a structure is available and can therefore be measured independently. The pre-aberration amplitude and phase distribution can then be used to determine the impairment structure reference image. As an alternative to known impairments, a model of the projection optical system can be used to determine the impairment structure reference image instead of calibration. It should be noted that the measured field is assumed to be corrected by the field distribution of a known illumination spot. Suboptimal choices will reduce the effectiveness of the algorithm but will not eliminate its benefits.

[0102] More generally, while any small inaccuracy in determining the distance from the structure of interest to the impairment will make the proposed technique less effective, improvements in diffraction efficiency estimates can still be expected. Since the length scale of the crosstalk is finite, only the portion of the impairment close to the structure of interest is important, so limited information may be sufficient.

[0103] In addition to the overlap calculation procedure, the reference image can be preventively orthogonally normalized at a very low cost.

[0104] The above derivation is described in terms of a single impairment; however, within theoretical and practical limits of signal level settings, the structure of interest can be orthogonally normalized to multiple impairments including, for example, other pads of the same target.

[0105] In the general case of an arbitrary number of features (which might include, for example, a structure of interest and multiple impairments), the image can be described as the sum of all contributions (where i is the number of contributions):

[0106]

[0107] With a (e.g. normalized) reference image C=[C1,C2,…,C n ] and prefactor e=[e1,e2,…,e n ]. As already described, the reference image C can be simulated, estimated or measured independently. As before, the prefactor represents a measured parameter; for example the diffraction efficiency (in the case of an intensity image) or the square root of the diffraction efficiency (in the case of an electric field image). Referring to the single impairment example given above, αA is equivalent to e1C1, and βB is equivalent to e2C2.

[0108] Taking the outer product of vector C and its complex conjugate, we can construct a matrix T = CC*, where each matrix element represents the overlap between two reference images. Using the matrix inverse T -1 Find the orthogonal normalized reference image C'=[C'1,C'2,…,C' n ]for:

[0109]

[0110] Then, we can use its corresponding orthogonal normalized reference image C' j The complex conjugate C' j *To determine any single prefactor e j (and any corrected measurement parameters) are:

[0111]

[0112] It is understood that no requirement is imposed on 4D aberration correction of the target image. Unlike rectangular window integration (and most region of interest finding techniques), the template may be aberrated due to the full 4D aberration of the imaging objective. This is only expected to degrade performance in cases where crosstalk increases dramatically, e.g. due to the proposed orthogonal normalization method neglecting some relevant signal energy of the pad of interest.

[0113] Imperfect impairment reference images can cause errors in the diffraction efficiency estimate proportional to the misalignment between the ideal reference image and the reference image used. However, since most impairments are large, rectangular, and located at a predictable distance from the target, this is unlikely to cause a significant problem.

[0114] In the case of very large crosstalk, a large overlap between the target template and the impairment template may lead to degraded performance. In this regime, the orthonormalized template is very sensitive to the (small) differences between template and impairment. This will only produce good results at extremely high signal-to-noise ratios. Of course, such scenarios are challenging for all currently known techniques, and the technique proposed here still holds promise for outperforming the currently used direct integration.

[0115] The orthogonal normalized diffraction efficiency estimate in the orthogonal normalized case greatly outperforms current integration and uncorrected correlation methods. In a representative example layout, rectangular window integration produces results that are more than 100% off for impairments with intensities 100 times greater than the pad of interest in the measured image. In contrast, the proposed orthogonal normalization technique correctly estimates the diffraction efficiency to within 0.01%. Thus, the orthogonal normalization technique allows for accurate overlay measurements where existing techniques completely fail.

[0116] Although the above description is mainly described in terms of measuring overlapping targets and overlapping parameters of interest, the proposed method is equally applicable to measuring other structures, such as focusing targets or other metrology targets. In this context, the term "target" should not necessarily mean a target formed specifically for measurement, and if used for measurement, may also include other structures, including functional product structures.

[0117] Fig.11 1 is a block diagram illustrating a computer system 1100 that can assist in implementing the methods and processes disclosed and / or claimed herein. The computer system 1100 includes a bus 1102 or other communication mechanism for communicating information and a processor 1104 (or multiple processors 1104 and 1105) coupled to the bus 1102 for processing information. The computer system 1100 also includes a main memory 1106, such as a random access memory (RAM) or other dynamic storage device, which is coupled to the bus 1102 for storing information and instructions to be executed by the processor 1104. The main memory 1106 can also be used to store temporary variables or other intermediate information during the execution of instructions to be executed by the processor 1104. The computer system 1100 also includes a read-only memory (ROM) 1108 or other static storage device, which is coupled to the bus 1102 for storing static information and instructions for the processor 1104. A storage device 1110, such as a magnetic disk or optical disk, is provided and coupled to the bus 1102 for storing information and instructions.

[0118] The computer system 1100 can be coupled to a display 1112, such as a cathode ray tube (CRT) or a flat panel or touch panel display, via the bus 1102 for displaying information to a computer user. An input device 1114 including alphanumeric keys and other keys is coupled to the bus 1102 for communicating information and command selections to the processor 1104. Another type of user input device is a cursor control 1116, such as a mouse, a trackball, or cursor direction keys, for communicating direction information and command selections to the processor 1104 and controlling cursor movement on the display 1112. The input device typically has two degrees of freedom on two axes, a first axis (e.g., x) and a second axis (e.g., y), which allows the device to specify a position in a plane. A touch panel (screen) display can also be used as an input device.

[0119] In response to processor 1104 executing one or more sequences of one or more instructions contained in main memory 1106, one or more methods described herein may be performed by computer system 1100. Such instructions may be read into main memory 1106 from another computer-readable medium such as storage device 1110. Execution of the sequence of instructions contained in main memory 1106 causes processor 1104 to perform the process steps described herein. One or more processors in a multi-processing arrangement may also be employed to execute the sequence of instructions contained in main memory 1106. In alternative embodiments, hardwired circuitry may be used in place of or in combination with software instructions. Therefore, the description herein is not limited to any specific combination of hardware circuitry and software.

[0120] The term "computer-readable medium" as used herein refers to any medium that participates in providing instructions to the processor 1104 for execution. Such media can take many forms, including but not limited to non-volatile media, volatile media, and transmission media. Non-volatile media include, for example, optical or magnetic disks, such as storage device 1110. Volatile media include dynamic memory, such as main memory 1106. Transmission media include coaxial cables, copper wires, and optical fibers, including the wires that make up bus 1102. Transmission media can also take the form of sound waves or light waves, such as those generated during radio frequency (RF) and infrared (IR) data communications. Common forms of computer-readable media include, for example, floppy disks, foldable disks, hard disks, magnetic tape, any other magnetic media, CD-ROMs, DVDs, any other optical media, punch cards, paper tape, any other physical media with a pattern of holes, RAM, PROMs and EPROMs, flash EPROMs, any other memory chips or memory cartridges, carrier waves described below, or any other media from which a computer can read.

[0121] Various forms of computer readable media may be involved in carrying one or more sequences of one or more instructions to the processor 1104 for execution. For example, the instructions may initially be carried on a disk of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to the computer system 1100 can receive the data on the telephone line and use an infrared transmitter to convert the data into an infrared signal. An infrared detector coupled to the bus 1102 can receive the data carried in the infrared signal and place the data on the bus 1102. The bus 1102 carries the data to the main memory 1106, from which the processor 1104 retrieves and executes the instructions. The instructions received by the main memory 1106 may optionally be stored on the storage device 1110 before or after execution by the processor 1104.

[0122] Preferably, the computer system 1100 also includes a communication interface 1118 coupled to the bus 1102. The communication interface 1118 provides a bidirectional data communication coupling to a network link 1120, which is connected to a local network 1122. For example, the communication interface 1118 can be an integrated services digital network (ISDN) card or a modem to provide a data communication connection with a corresponding type of telephone line. As another example, the communication interface 1118 can be a local area network (LAN) card to provide a data communication connection with a compatible LAN. A wireless link can also be implemented. In any such implementation, the communication interface 1118 sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information.

[0123] The network link 1120 typically provides data communication to other data devices through one or more networks. For example, the network link 1120 can provide a connection to a host computer 1124 or data equipment operated by an Internet Service Provider (ISP) 1126 through a local network 1122. The ISP 126, in turn, provides data communication services through a global packet data communication network, now generally referred to as the "Internet" 1128. Both the local network 1122 and the Internet 1128 use electrical, electromagnetic, or optical signals that carry digital data streams. The signals through the various networks and the signals on the network link 1120 and through the communication interface 1118 (carrying the digital data to and from the computer system 1100) are exemplary forms of carrier waves that transport the information.

[0124] The computer system 1100 can send messages and receive data, including program code, through (multiple) networks, network links 1120, and communication interfaces 1118. In the Internet example, a server 1130 can send a request code for an application program through the Internet 1128, ISP 1126, local network 1122, and communication interface 1118. For example, one such download application can provide one or more technologies described herein. The received code can be executed by the processor 1104 when it is received, and / or stored in the storage device 1110 or other non-volatile storage device for later execution. In this way, the computer system 1100 can obtain application code in the form of a carrier wave.

[0125] Other embodiments are disclosed in the subsequent numbered item lists:

[0126] 1. A method for determining an orthogonally normalized reference image of a structure of interest, the orthogonally normalized reference image of the structure of interest being used to apply to a measured image of the structure of interest to correct for the effects of crosstalk from at least one impaired structure, the method comprising:

[0127] determining a structure of interest reference image based on knowledge of the structure of interest;

[0128] determining at least one impairment structure reference image based on knowledge of at least one impairment structure; and

[0129] The structure-of-interest reference image is orthogonally normalized to at least one damaged reference image to obtain an orthogonally normalized structure-of-interest reference image.

[0130] 2. A method as defined in clause 1, wherein said step of determining a reference image of a structure of interest and said step of determining at least one reference image of a disturbed structure are both based on knowledge of the imaging optics used to obtain the measurement image.

[0131] 3. A method according to claim 2, wherein one or both of the step of determining a reference image of a structure of interest and the step of determining at least one reference image of a impairment structure comprise: applying one or more known point spread functions of an imaging optical device to the structure of interest and / or the at least one impairment structure, respectively.

[0132] 4. A method according to any of the preceding clauses, wherein one or both of the step of determining a reference image of a structure of interest and the step of determining at least one impairment structure reference image comprise: independently measuring one or both of the structure of interest and / or the at least one impairment structure.

[0133] 5. A method according to any of the preceding clauses, wherein the orthogonal normalization step comprises determining an orthogonally normalized structure reference image of interest by the product of an inverse matrix and a vector comprising the structure reference image of interest and each at least one damaged structure reference image, wherein the inverse matrix comprises the outer product of the vector and its complex conjugate.

[0134] 6. A method as defined in any of the preceding clauses, including:

[0135] obtaining a measurement image including an image of a structure of interest;

[0136] determining an overlap between the measured image and an orthogonally normalized reference image of the structure of interest; and

[0137] The measurement parameters of the structure of interest are determined from the overlap.

[0138] 7. A method as defined in clause 6, wherein determining the overlap comprises determining a dot product of the measured image and an orthogonally normalized reference image of the structure of interest.

[0139] 8. A method as defined in clause 6 or 7, wherein the measured parameter comprises amplitude, intensity or diffraction efficiency.

[0140] 9. A method as defined in clause 6, 7 or 8, comprising determining a parameter of interest from said measured parameters.

[0141] 10. A method as defined in clause 9, wherein the parameter of interest is overlap or focus.

[0142] 11. A method as defined in any preceding clause, wherein the measurement radiation used to capture the measurement image is substantially coherent.

[0143] 12. The method defined in clause 11, including:

[0144] determining or estimating a pre-aberration amplitude and phase distribution of at least one impairment structure; and

[0145] In the step of determining at least one impairment structure reference image a pre-aberration amplitude and phase distribution is used.

[0146] 13. A method as defined in any of clauses 1 to 10, wherein the measurement radiation used to capture the measurement image is incoherent or partially coherent.

[0147] 14. A method as defined in any preceding clause, wherein the structure of interest is a measurement target or a sub-target thereof.

[0148] 15. A method as defined in any preceding clause, wherein said at least one destructive structure comprises a product structure and / or a sub-object other than a structure of interest.

[0149] 16. A method of determining a measurement parameter associated with a structure of interest, the method comprising:

[0150] Obtaining an orthogonally normalized structure-of-interest reference image associated with the structure of interest, wherein the orthogonally normalized structure-of-interest reference image is orthogonally normalized relative to at least one damaged structure reference image associated with at least one damaged structure;

[0151] obtaining at least one measurement image, the at least one measurement image comprising an image of the structure of interest affected by the damaged disturbing structure;

[0152] The measurement parameters are determined by combining each of the at least one measurement image and the orthogonally normalized reference image of the structure of interest, so that the determined measurement parameters are corrected for the influence of the impaired structure.

[0153] 17. A method as defined in clause 16, comprising performing the method of any one of clauses 1 to 5 to determine an orthogonally normalized structure of interest reference image.

[0154] 18. A method as defined in clause 16 or 17, wherein said combining comprises a dot product of each of said at least one measurement image with an orthogonally normalized structure of interest reference image.

[0155] 19. A method as defined in any one of clauses 16 to 18, wherein the measurement parameter comprises amplitude, intensity or diffraction efficiency.

[0156] 20. A method as defined in any of clauses 16 to 19, comprising determining a parameter of interest from said measured parameters.

[0157] 21. A method as defined in clause 20, wherein the parameter of interest is overlap or focus.

[0158] 22. A computer program comprising program instructions operable, when run on suitable equipment, to perform the method of any preceding clause.

[0159] 23. A non-transitory computer program carrier comprising the computer program of item 22.

[0160] 24. A processing arrangement comprising:

[0161] A computer program carrier comprising a computer program according to clause 23; and

[0162] A processor is operable to run on said computer program.

[0163] 25. A measurement device comprising the processing arrangement of item 24.

[0164] 26. A measuring device as defined in clause 25, comprising:

[0165] imaging optics for capturing scattered radiation from the structure of interest; and

[0166] A detector is used to detect the scattered radiation to obtain a measurement image of the structure of interest.

[0167] 27. A measuring device according to clause 25 or 26, which is a scatterometer.

[0168] 28. The measuring device according to clause 25 or 26, which is a dark field holographic microscope.

[0169] Although specific reference may be made herein to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

[0170] Although embodiments of the present invention may be specifically referenced herein in the context of inspection or measurement equipment, embodiments of the present invention may be used in other equipment. Embodiments of the present invention may form part of a mask inspection equipment, a lithography equipment, or any equipment that measures or processes objects such as a wafer (or other substrate) or a mask (or other pattern forming device). The term "measurement equipment" may also refer to an inspection equipment or an inspection system. For example, an inspection equipment including embodiments of the present invention may be used to detect defects in a substrate or defects in a structure on a substrate. In such embodiments, the characteristic of interest of a structure on a substrate may be related to a defect in the structure, the absence of a specific portion of the structure, or the presence of an unwanted structure on the substrate.

[0171] Although specific reference is made to "measurement equipment / tool / system" or "inspection equipment / tool / system", these terms may refer to the same or similar types of tools, equipment, or systems. For example, an inspection or measurement equipment including embodiments of the present invention may be used to determine characteristics of structures on a substrate or wafer. For example, an inspection equipment or measurement equipment including embodiments of the present invention may be used to detect defects in a substrate or defects in a structure on a substrate or wafer. In such embodiments, the characteristic of interest of a structure on a substrate may be related to a defect in the structure, the absence of a particular portion of the structure, or the presence of an unwanted structure on the substrate or wafer.

[0172] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention is not limited to optical lithography and may be used in other applications, such as imprint lithography, where the context permits.

[0173] While the targets or target structures described above (more generally, structures on a substrate) are metrology target structures that are specifically designed and formed for measurement purposes, in other embodiments, the properties of interest may be measured on one or more structures that are functional parts of a device formed on a substrate. Many devices have regular, grating-like structures. The terms structure, target grating, and target structure as used herein do not require that the structure be provided specifically for the measurement being performed. Further, the pitch P of the metrology target may be close to the resolution limit of the optical system of the scatterometer or may be smaller, but may be much larger than the size of a typical product feature made by a lithographic process in the target portion C. In practice, the lines and / or spaces of the overlapping gratings within the target structure may include smaller structures of similar size to the product features.

[0174] Although specific embodiments of the present invention have been described above, it is to be understood that the present invention may be practiced in a manner other than that described. The above description is intended to be illustrative, not restrictive. Therefore, it will be apparent to those skilled in the art that modifications may be made to the described invention without departing from the scope of the claims set forth below.

Claims

1. A method for determining an orthogonal normalized reference image of a structure of interest, wherein the orthogonal normalized reference image of the structure of interest is used to apply to a measured image of the structure of interest to correct for the effects of crosstalk from at least one impaired structure, the method comprising: determining a structure of interest reference image based on the knowledge of the structure of interest; determining at least one impairment structure reference image based on the knowledge of the at least one impairment structure; as well as The structure of interest reference image is orthogonally normalized to the at least one damaged reference image to obtain the orthogonally normalized structure of interest reference image.

2. The method according to claim 1, wherein the step of determining a reference image of a structure of interest and the step of determining at least one reference image of a disturbed structure are both based on knowledge of the imaging optics used to obtain the measurement image.

3. The method according to claim 2, wherein one or both of the step of determining a structure of interest reference image and the step of determining at least one impaired structure reference image comprises: One or more known point spread functions of the imaging optics are applied to the structure of interest and / or the at least one detrimental structure, respectively.

4. The method according to any preceding claim, wherein one or both of the step of determining a structure of interest reference image and the step of determining at least one impaired structure reference image comprises: One or both of the structure of interest and / or the at least one detrimental structure are measured independently.

5. A method according to any preceding claim, wherein the orthogonal normalization step comprises: The orthogonally normalized structure of interest reference image is determined by a product of an inverse matrix including an outer product of the vector and its complex conjugate and a vector including the structure of interest reference image and each at least one impaired structure of interest reference image.

6. A method according to any preceding claim, comprising: obtaining a measurement image including an image of the structure of interest; determining an overlap between the measurement image and the orthogonally normalized structure of interest reference image; as well as A measurement parameter of the structure of interest is determined from the overlap.

7. The method of claim 6, wherein determining overlap comprises: The dot product of the measurement image and the orthogonally normalized structure of interest reference image is determined.

8. The method of claim 6 or 7, wherein the measurement parameter comprises amplitude, intensity or diffraction efficiency.

9. The method according to claim 6, 7 or 8, comprising: A parameter of interest is determined from the measured parameters, and optionally, the parameter of interest is overlap or focus.

10. A method according to any preceding claim, wherein the measurement radiation used to capture the measurement image is substantially coherent.

11. The method according to claim 10, comprising: determining or estimating a pre-aberration amplitude and phase distribution of the at least one impairment structure; as well as The pre-aberration amplitude and phase distribution is used in the step of determining at least one impairment structure reference image.

12. The method according to any one of claims 1 to 9, wherein the measurement radiation used to capture the measurement image is incoherent or partially coherent.

13. A computer program comprising program instructions operable, when run on suitable equipment, to perform the method of any preceding claim.

14. A processing arrangement comprising: Computer program carrier comprising the computer program according to claim 13; as well as A processor is operable to run on said computer program.

15. A measuring device comprising: imaging optics to capture scattered radiation from the structure of interest; a detector for detecting the scattered radiation to obtain a measurement image of the structure of interest; as well as The processing arrangement of claim 14.

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