Measurement method and device for determining complex-valued fields

Abstract

A method of measuring a structure, and an associated metrology apparatus and computer program are disclosed. The method comprises: obtaining an amplitude distribution of scattered radiation associated with a measurement of a first structure on a first substrate; and obtaining a reference phase distribution associated with a reference measurement of at least one reference structure on a reference substrate. The at least one reference structure is a structure that is different from the first structure but is nominally identical with respect to at least a plurality of key parameters. The method further comprises: determining a complex-valued field describing the first structure from the amplitude distribution and the reference phase distribution.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the priority of European application No. 19194973.4 filed on September 2, 2019, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to a metrology method and apparatus for determining properties of structures on a substrate. Background Art

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

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

[0006] Low k1 lithography can be used to process features with sizes 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 the radiation used, 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 it is the half pitch), and k1 is an empirical resolution factor. In general, the smaller the k1, the more difficult it is to reproduce on the substrate a pattern of a shape and size similar 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. These steps include, for example (but not limited to), optimization of NA, custom 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") of the design layout), or other methods generally defined as "resolution enhancement technology" (RET). Alternatively, a strict control loop for controlling the stability of the lithographic apparatus can be used to improve the reproduction of patterns at low k1.

[0007] During lithography, it is desirable to frequently measure the resulting structure, 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 used to refer to such tools may be metrology equipment or inspection equipment.

[0008] The metrology device can use the computationally acquired phase to apply aberration correction to an image captured by the metrology device. One described method for phase calculation uses multiple different images, such as multiple images of the same target under different focusing conditions. This typically requires mechanical movement of the sample (e.g., target) and multiple image captures with a high time cost. Summary of the invention

[0009] It is desirable to reduce acquisition time and increase throughput when performing phase recovery in metrology applications.

[0010] Embodiments of the invention are disclosed in the claims and in the detailed description.

[0011] In a first aspect of the present invention, a method for measuring a structure is provided, comprising: obtaining an amplitude distribution of scattered radiation associated with a measurement of a first structure on a first substrate; obtaining a reference phase distribution associated with a reference measurement of at least one reference structure on a reference substrate, the at least one reference structure being a structure different from the first structure; the at least one reference structure and the first structure being nominally identical in at least a plurality of key parameters; and determining a complex-valued field for describing the first structure based on the amplitude distribution and the reference phase distribution.

[0012] Further aspects of the invention comprise a computer program comprising program instructions operable to perform the method of the first aspect, a non-transitory computer program carrier comprising said computer program, a processing device operable to run said computer program, a metrology device operable to perform the method of the first aspect, and a lithography unit comprising such a metrology device. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0014] - Figure 1 depicting a schematic overview of a lithographic apparatus;

[0015] - Figure 2 depicts a schematic overview of a lithography cell;

[0016] - Figure 3 depicts a schematic representation of overall lithography showing the collaboration between three key technologies used to optimize semiconductor manufacturing;

[0017] - Figure 4 is a schematic illustration of a scatterometry device;

[0018] - Figure 5 The figure shows a measurement arrangement which can be used when performing a method according to an embodiment of the invention;

[0019] - Figure 6 is a flowchart describing a method according to an embodiment of the present invention; and

[0020] - Figure 7 is a block diagram illustrating a computer system that can assist in implementing a method according to an embodiment of the present invention. DETAILED DESCRIPTION

[0021] In this document, the terms "radiation" and "beam" are used to cover all types of electromagnetic radiation, including ultraviolet radiation (e.g., having a wavelength of 365nm, 248nm, 193nm, 157nm or 126nm) and EUV (extreme ultraviolet radiation, e.g., having a wavelength in the range of about 5nm to 100nm).

[0022] The terms "reticle", "mask" or "patterning device" as used herein may be broadly interpreted to refer to a general patterning device that can be used to impart a patterned cross-section to an incident beam that corresponds to the pattern to be produced in a target portion of a substrate. The term "light valve" may also be used in this context. 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.

[0023] Figure 1 A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA comprises an illumination system (also referred to as an illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation, or EUV radiation), a mask support (e.g., a mask stage) MT configured to support a patterning device (e.g., a 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., a wafer stage) WT configured to hold a substrate (e.g., a wafer coated with resist) 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., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0024] In operation, the illumination system IL receives a radiation beam from a radiation source SO, for example via a beam delivery system BD. The illumination system IL may include various types of optical components for directing, shaping and / or controlling the radiation, such as refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical components, or any combination thereof. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in a cross-section of the radiation beam B at the plane of the patterning device MA.

[0025] The term "projection system" PS as used herein should be broadly interpreted as covering various types of projection systems suitable for the exposure radiation used and / or for other factors such as the use of immersion liquid or the use of a vacuum, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems or any combination thereof. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0026] 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, so as to fill the space between the projection system PS and the substrate W, which is also known as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0027] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also known as a "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or a step of 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 being used for exposing a pattern on the other substrate W.

[0028] In addition to the substrate support WT, the lithographic apparatus LA may also 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 parts of the lithographic apparatus, for example a part of the projection system PS or a part of the system 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.

[0029] In operation, a radiation beam B is incident on a patterning device (e.g. a mask) MA held on a mask support MT and is patterned by a pattern (design layout) present on the patterning device MA. Having traversed the mask MA, the radiation beam B passes through a projection system PS which focuses the beam onto a target portion C of the substrate W. With the aid of a second positioner PW and a position 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 in a focused and aligned position. Similarly, the first positioner PM and possibly a further position sensor (possibly another position sensor not present) are provided. Figure 1 The patterning device MA may be accurately positioned relative to the path of the radiation beam B using the mask alignment marks M1, M2 and the substrate alignment marks P1, P2. Although the substrate alignment marks P1, P2 occupy dedicated target portions as illustrated, the marks may be located in spaces between target portions. When the substrate alignment marks P1, P2 are located between target portions C, they are referred to as scribe line alignment marks.

[0030] like Figure 2 As shown in FIG. 1 , the lithographic apparatus LA may form part of a lithocell LC (sometimes also referred to as a lithocell or litho cluster), which typically also includes equipment for performing pre-exposure and post-exposure processes on a substrate W. Conventionally, these equipment 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 transport device or robot RO picks up the substrate W from the input / output ports I / O1, I / O2, moves the substrate W between the different process devices, and transfers the substrate W to a feed station LB of the lithographic apparatus LA. The devices in the lithocell, which are also generally referred to as tracks or coating and developing systems, are typically under the control of a track control unit TCU, which itself may be controlled by a management control system SCS, which may also control the lithographic apparatus LA, for example, via a litho control unit LACU.

[0031] In order to correctly and consistently expose the substrate W exposed by the lithographic apparatus LA, it is desirable to inspect the substrate to measure properties of the patterned structure, 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 the lithography cell LC. In particular, in the case of inspection before other substrates W of the same batch or lot are yet to be exposed or processed, if errors are detected, adjustments may be made, for example, to the exposure of subsequent substrates or to other processing steps to be performed on the substrate W.

[0032] The inspection apparatus, which may also be referred to as a metrology apparatus, is used to determine properties of a substrate W, and in particular, to determine how properties of different substrates W vary 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 separate device. The inspection apparatus may measure properties of a latent image (the image in the resist layer after exposure), or properties of a semi-latent image (the image in the resist layer after a post-exposure bake step PEB), or properties of a developed resist image (wherein exposed or unexposed parts of the resist have been removed), or even properties of an etched image (after a pattern transfer step such as etching).

[0033] Typically, the patterning process in the lithographic apparatus LA is one of the most critical steps in the process, which requires a high accuracy in the dimensioning and placement of structures on the substrate W. In order to ensure this high accuracy, three systems can be combined in Figure 3 . One of these systems is the lithography apparatus LA, which is (in practice) connected to a metrology tool MET (a second system) and to 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 remains within the process window. The process window defines the range of process parameters (e.g., dose, focus, overlay or overlay accuracy) within which a particular manufacturing process produces a defined result (e.g., a functional semiconductor device) - typically the process parameters in the lithography process or patterning process are allowed to vary within the range.

[0034] The computer system CL can use (part of) the design layout to be patterned to predict which resolution enhancement techniques to use and perform computer lithography simulations and calculations to determine which mask layouts and lithography equipment settings achieve the maximum overall process window (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., at Figure 3 0 in the figure).

[0035] 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, for example, possible drifts in the calibration state of the lithographic apparatus LA (in Figure 3 SC3 is depicted by multiple arrows in the third scale SC3).

[0036] During lithography, it is desirable to make frequent measurements of the structures produced, 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 often rely on the supply of dedicated metrology targets, such as underfilled targets (targets in the form of simple gratings or overlapping gratings in different layers, which are large enough that the measurement beam produces a spot smaller than the grating) or overfilled targets (so that the illumination spot partially or completely contains the target). In addition, the use of metrology tools (e.g., angle-resolved scatterometers that illuminate underfilled targets such as gratings) allows the use of so-called reconstruction methods, in which 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 simulated results with the measured results. The parameters of the model are adjusted until the simulated interaction produces a diffraction pattern similar to the diffraction pattern observed from a real target.

[0037] A scatterometer is a multifunctional instrument that allows to measure parameters of a lithographic process by having a sensor in the pupil or a plane conjugated to the pupil of the objective of the scatterometer, which measurement is usually referred to as pupil-based measurement, or by having a sensor in the image plane or a plane conjugated to the image plane, in which case the measurement is usually referred to as image-based or field-based measurement. Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP1,628,164A, which are incorporated herein by reference in their entirety. The aforementioned scatterometers can measure multiple targets from multiple gratings in one image using light from soft x-rays and visible to near IR wave ranges.

[0038] exist Figure 4 A measurement device such as a scatterometer is depicted in . The measurement device 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 wavelength λ). From such data, the structure or profile 8 which produced 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 parameters are assumed based on knowledge of the process used to manufacture the structure, leaving only a few parameters of the structure to be determined from the scatterometry data. Such a scatterometer can be configured as a normal incidence scatterometer or an oblique incidence scatterometer.

[0039] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, a reconstruction method can be applied to the measured signal to reconstruct or calculate the properties of the grating. Such a reconstruction can be produced, for example, by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measured results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to the diffraction pattern observed from a real target.

[0040] 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 such 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.

[0041] In a third embodiment, the scatterometer MT is an ellipsometry scatterometer. An ellipsometry scatterometer allows the parameters of a lithography process to be determined by measuring the scattered radiation for each polarization state. Such a metrology device emits polarized light (such as linear, circular or elliptical) by using, for example, an appropriate polarization filter in the illumination section of the metrology device. A source suitable for the metrology device may also provide polarized radiation. Various embodiments of existing ellipsometry scatterometers are described in US 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.

[0042] 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 the reflection spectrum and / or an asymmetry in the detection configuration (the asymmetry being related to the extent of the overlap). The two (typically overlapping) grating structures may be applied in two different layers (not necessarily consecutive layers) and the two grating structures may be formed at approximately the same position on the wafer. The scatterometer may have a symmetrical detection configuration as described, for example, in the commonly owned patent application EP1,628,164A so that any asymmetry can be clearly identified. This provides a straightforward way to measure misalignment in the gratings. Further examples of the measurement of the overlay error between two layers containing a periodic structure as a target via the asymmetry of the periodic structure may be found in PCT Patent Application Publication No. WO 2011 / 012624 or US Patent Application No. US 20160161863, which are incorporated herein by reference in their entirety.

[0043] Other parameters of interest may be focus and dose. Focus and dose may be determined simultaneously by scatterometry as described in US patent application US2011-0249244, which is incorporated herein by reference in its entirety (or alternatively by scanning electron microscopy). A single structure with a unique combination of critical dimension and sidewall angle measurements for each point in a focus energy matrix (FEM, also referred to as a focus exposure matrix) may be used. If these unique combinations of critical dimensions and sidewall angles are available, focus and dose may be uniquely determined from these measurements.

[0044] The metrology target may be a composite grating set formed primarily in resist by a lithography process and also after, for example, an etching process. Typically, the pitch and line width of the structures in the gratings rely heavily on the measurement optics (particularly the NA of the optics) to be able to capture the diffraction orders from the metrology target. As indicated earlier, the diffraction signal may be used to determine the phase shift between two layers (also referred to as "overlap" or "overlay accuracy") or may be used to reconstruct at least a portion of the original grating as produced by the lithography process. Such a reconstruction may be used to provide guidance on the quality of the lithography process and may be used to control at least a portion of the lithography process. The target may have smaller subsections configured to mimic the size of a functional portion of a design layout in the target. Due to such subsections, the target will behave more similarly to the functional portion of the design layout, so that the overall process parameter measurement more preferably resembles the functional portion of the design layout. The target may be measured in an underfill mode or in an overfill mode. In the underfill mode, the measurement beam produces a spot that is smaller than the overall target. In the overfill mode, the measurement beam produces a spot that is larger than the overall target. In such an overfill mode, it is also possible to measure different targets simultaneously and thus to determine different process parameters simultaneously.

[0045] The overall quality of the measurement of a lithography parameter using a specific target is at least partially determined by the measurement option used to measure such lithography parameters. The term "substrate measurement option" may include one or more parameters of the measurement itself, one or more parameters of one or more patterns measured, or both. For example, if the measurement used in the substrate measurement option is a diffraction-based optical measurement, one or more of the measured parameters 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, etc. One of the criteria for selecting a measurement option may, for example, be the sensitivity of one of the measurement parameters to process changes. More examples are described in US patent application US2016-0161863, which is incorporated herein by reference in its entirety, and published US patent application US 2016 / 0370717A1.

[0046] A metrology apparatus using computational imaging / phase retrieval methods has been described in US patent publication US2019 / 0107781, which is incorporated herein by reference. Such a metrology apparatus can use relatively simple sensor optics that have ordinary or even relatively common aberration performance. Thus, the sensor optics can be allowed to have aberrations, and therefore the sensor optics produce relatively aberrated images. Of course, unless something is done to compensate for the effects of these optical aberrations, simply allowing larger aberrations in the sensor optics will have an unacceptable impact on image quality. Therefore, computational imaging techniques are used to compensate for the negative impact of the relaxation of aberration performance in the sensor optics.

[0047] In this method, the intensity and phase of the target are obtained from one or more intensity measurements of the target. Phase recovery can use previous information of the measurement target (for example, to be included in the loss function that forms the starting point to derive / design the phase recovery algorithm). Alternatively, or in combination with the previous information method, diversity measurement can be performed. In order to achieve diversity, the imaging system is slightly changed between measurements. An example of diversity measurement is cross-focus stepping, that is, by obtaining measurement results at different focus positions. Alternative methods for introducing diversity include, for example, using different illumination wavelengths or different wavelength ranges, adjusting the illumination, or changing the incident angle of illumination on the target between measurements. Phase recovery itself can be based on the aforementioned US2019 / 0107781 or the content described in patent application EP17199764 (also incorporated herein by reference). This describes determining the corresponding phase recovery based on the intensity measurement, so that the interaction of the target with the illumination radiation is described based on the electric field or complex-valued field of the target ("complex" here means that there are both amplitude and phase information). The intensity measurement can have a lower quality than the intensity measurement used in conventional measurement, and can therefore be defocused as described. The described interaction may include a representation of the electric and / or magnetic fields directly above the target. In such an embodiment, the illuminated target electric and / or magnetic field image is modeled as an equivalent source description by means of infinitesimal current and / or magnetic current dipoles on a surface (e.g., two-dimensional) in a plane parallel to the target. Such a plane may, for example, be a plane directly above the target, e.g., a plane in focus according to the Rayleigh criterion, but the location of the model plane is not important: once the amplitude and phase at one plane are known, the amplitude and phase can be propagated computationally to any other plane (focus plane, defocus plane, or even pupil plane). Alternatively, the description may include a composite transfer of the target or its two-dimensional equivalent.

[0048] Phase recovery can include: modeling the effect of the interaction between the illuminating radiation and the target on the diffracted radiation to obtain a modeled intensity pattern; and optimizing the phase and amplitude of the electric field / complex-valued field within the model so as to minimize the difference between the modeled intensity pattern and the detected intensity pattern. More specifically, during measurement acquisition, an image (e.g., an image of the target) is captured on a detector (at a detection plane) and its intensity is measured. A phase recovery algorithm is used to determine the amplitude and phase of the electric field at a plane, for example, parallel to the target (e.g., directly above the target). The phase recovery algorithm uses a forward model of the sensor (e.g., taking into account aberrations) to computationally image the target to obtain modeled values ​​of the intensity and phase of the field at the detection plane. No target model is required. The difference between the modeled intensity value and the detected intensity value is minimized in terms of phase and amplitude (e.g., iteratively), and the corresponding modeled phase value obtained is regarded as the recovered phase. Specific methods for using complex-valued fields in metrology applications are described in PCT application PCT / EP2019 / 052658, which is also incorporated herein by reference.

[0049] Performing measurements continuously at multiple focus levels takes a lot of time. Therefore, obtaining progressively defocused images is slow, resulting in slow measurement speed and low throughput. A simple calculation proves this to be the case. Assuming that 5 defocused images are acquired for each combination of 4 directions and 5 (sequentially captured) wavelengths, and each image takes 1ms to capture, then it takes about 100ms to measure each target. This does not include the time spent moving the stage and switching wavelengths. In addition, the phase retrieval calculation (which is typically iterative) itself can be computationally intensive and take a long time to converge to a solution.

[0050] Therefore, it is proposed to perform an iterative phase recovery for reference measurements on one or several reference targets, and the resulting reference phase distribution is used to determine the complex-valued field in a production setting. Such an approach can be considered a "phase transplantation" technique, where only one aberration image is obtained, and instead of performing phase recovery for each individual target measurement, it is proposed to "transplant" or infer the target phase distribution from an earlier reference phase measurement of one or more reference targets. The reference targets should be nominally similar or identical to each other and to the targets to be measured in the production phase, at least in terms of key parameters (e.g. such as pitch, grating size, grating orientation, composition), and measured under substantially the same measurement / illumination conditions. In this way, the reference phase measurement can include the generation of a pre-computed and stored reference phase distribution or phase template. Such a pre-computed phase template can be modified and appended to the measured target amplitude to determine a "semi-synthetic" complex-valued field distribution; this can then be corrected for aberrations from the optical system (e.g. based on knowledge of the sensor aberrations). Overlay or other parameters of interest can then be measured from the corrected image.

[0051] The reference phase distribution or phase template can be obtained by pre-performing phase retrieval on at least one reference target based on more conventional phase retrieval techniques, such as defocus stepping (and / or multiple captures with another parameter varied). Many suitable phase retrieval techniques are described in WO2019 / 068459, which is incorporated herein by reference in its entirety (particularly in the section entitled "Example phase retrieval algorithm"). The phase template used can be based on a single phase retrieval that has been pre-performed and stored, or alternatively an average phase distribution recovered from several similar targets. In an embodiment, more than one nominally similar target (in terms of at least many key parameters) is measured and the results are averaged to determine the phase template. Nominally similar targets can be targets that are designed to be identical (at least in terms of the key parameters), ignoring unplanned or unexpected variations; for example, variations due to mirror effects or unexpected overlay errors (as opposed to intentional biases). Since each die is typically nominally identical, nominally identical targets should be exposed at many locations on the substrate. It is therefore suggested to measure only a few of these targets and average several of these targets in this phase template creation step. For example, the number of targets selected for this step may be less than 50, less than 20, less than 10 or less than 6.

[0052] The phase template can then be used for use in the determination of all complex-valued fields of a target, e.g. in a production setting, where throughput optimization is important. During a measurement of a parameter of interest (e.g. an overlay measurement), for example a single intensity image (e.g. of a single diffraction order) is captured for given measurement conditions. The corresponding amplitude distribution at the detector plane (i.e. a spatial description of the field amplitude associated with the target) can be obtained directly, e.g. by a square root operation on the captured intensity distribution. The phase template or reference phase distribution can be combined (e.g. aligned and added) with the amplitude distribution in order to determine the complex-valued field distribution (e.g. a spatial description of the complex-valued field) of the target.

[0053] Figure 5 The measurement arrangement according to an embodiment is shown. The arrangement is largely similar to the arrangement described in the aforementioned documents US2019 / 0107781 and PCT / EP2019 / 05265, which utilizes computational phase retrieval techniques to correct for simpler lens designs with phase aberrations. Figure 5 , a highly simplified schematic diagram shows a measurement beam MB of radiation (e.g. from a source not shown) directed onto a target T on a substrate S. Higher order diffracted radiation DR is captured by an objective lens OL and directed to a detector D. No zero order radiation ZR is captured and therefore the captured image is a dark field image.

[0054] Figure 6 6 is a flow chart describing the proposed method. In an initial reference phase 600, one or more reference targets are selected 605 from a test wafer. In an embodiment, several different (although nominally identical) reference targets may be selected for averaging any phase errors. In another embodiment, a target on a benchmark may be selected. The reference target should have the same key parameters and properties as the target to be formed for metrology purposes and to which the phase measurement will be transplanted. A reference measurement step 610 includes measuring the reference target and recovering the phase using a phase recovery technique, for example using standard methods (such as defocus stepping). This measurement step is performed at one or more desired wavelengths and illumination conditions corresponding to those desired wavelengths and illumination conditions to be used in the production phase 625. The (e.g., averaged) reference phase distribution obtained from the measurement step 610 is stored as a phase template 615. Optionally, an equivalent reference amplitude distribution or amplitude template 620 may be determined and stored. Such an amplitude template 620 may include the square root of the detected intensity distribution obtained in the measurement step 610, and may also include the average amplitude distribution of the selected reference targets (when more than one). The amplitude template 620 and the phase template 615 may be combined into a reference complex field template for use.

[0055] In a production phase 625, an overlay measurement is performed 630, during which a single intensity image of the target is measured (at least for given measurement conditions, such as wavelength, polarization and / or measurement direction). From the intensity image, an amplitude distribution 635 is obtained (e.g. by a square root operation).

[0056] In practice, in a production setting, the target from which the amplitude template 635 is derived and the reference target from which the reference complex field template or reference phase template is generated may not necessarily be located at the same focus position and / or in the same planar position relative to the image sensor. Therefore, in an optional alignment step 640, the amplitude distribution 635 is aligned with the amplitude template 620 with sub-pixel accuracy or precision in, for example, 3 dimensions to obtain a registered amplitude template 645. In order to align (focus) in the z direction, step 640 may include digitally defocusing the reference complex field or reference amplitude field and comparing the intensity at different focus offsets with the measured image. The defocus value is fixed at a value corresponding to the best match to the image (e.g., according to an image correlation metric). The defocused reference field can then be offset in the x and y directions and the image aligned with sub-pixel accuracy. These steps help ensure that the targets measured at different locations are properly aligned with the phase template (e.g., in all 3 dimensions) at the next step 650.

[0057] The complex valued field determination step 650 may include combining the phase template 615 with the registered amplitude template 645 to determine the target's complex valued field 655. However, where step 640 is omitted, the phase template 615 may be applied directly to the amplitude distribution 635 to determine the complex valued field 655.

[0058] Optional step 660 can include performing an adjustment for the overlap-induced global phase offset. The global phase offset is an overlap-dependent phase offset over the entire field. Under normal circumstances, it is not necessary to know the global phase offset to reconstruct the image because the global phase offset carries any information related to the shape (phase change) within the image. However, in the case where the aberrations are so severe that the images overlap (e.g., the overlap of the positive and negative bias pad images overlaps each other), the global phase offset can be processed as a variable that can be mathematically tuned (e.g., before or during the next step) to maximize the quality of the corrected image. However, such severe aberrations are not typical, so this step may not be necessary.

[0059] At step 665, lens aberrations (e.g., aberrations due to imperfections in the sensor optics) in the complex-valued field 655 are corrected. This may include Fourier transforming the complex-valued field 655 to obtain a description of the corresponding pupil plane. The lens aberration phase distribution 670 may then be conjugated and multiplied with the Fourier transformed complex-valued field. Optionally, the defocus aberration may be digitally adjusted to maximize the image quality in the corrected image or corrected complex-valued field description 675. Note that the defocus aberration is an aberration caused by the wafer platform not being at the corrected focus of the lens. The defocus aberration is controllable and is a user-induced aberration. The defocus aberration is different from the lens aberration phase distribution, which is an inherent aberration in an imperfect lens, is fixed and is not user-controllable.

[0060] The target being measured on the wafer should be nominally identical to the reference target in at least key parameters, including pitch, grating size, and grating orientation. They should also be measured under the same illumination conditions (e.g., wavelength, illumination angle, polarization, illumination distribution, etc.). Preferably, a narrow illumination numerical aperture NA can be used, and a single diffraction order is collected so that the 1st order scattering from the (e.g., μDBO) target can be accurately approximated as an isolated square hole. Although the optical system has aberrations, the aberrations of the lens should not be too high so as not to cause diffraction of the product structure and overlap with the target image.

[0061] While the above description has referred to overlay as a parameter of interest, the concepts are generally more useful and relevant for other lithography metrology applications on a target (e.g., measuring a characteristic or parameter of interest such as overlay, focus, dose, or one or more geometric parameter values ​​such as critical dimension, sidewall angle, edge placement error, or local critical dimension uniformity). Overlay can include measures of misalignment of structures in different layers such as in a double patterning process and / or measures of intra-layer overlay (i.e., measures of misalignment of structures in a single layer).

[0062] Figure 71 is a block diagram illustrating a computer system 1600 that can assist in implementing the methods and processes disclosed herein. The computer system 1600 includes a bus 1602 or other communication mechanism for communicating information, and a processor 1604 (or multiple processors 1604 and 1605) connected to the bus 1602 for processing information. The computer system 1600 also includes a main memory 1606, such as a random access memory (RAM) or other dynamic storage device, connected to the bus 1602 for storing information and instructions to be executed by the processor 1604. The main memory 1606 can also be used to store temporary variables or other intermediate information during the execution of instructions to be executed by the processor 1604. The computer system 1600 also includes a read-only memory (ROM) 1608 or other static storage device connected to the bus 1602 for storing static information and instructions for the processor 1604. A storage device 1610, such as a magnetic disk or optical disk, is provided and connected to the bus 1602 for storing information and instructions.

[0063] The computer system 1600 can be connected to a display 1612 for displaying information to a computer user via a bus 1602, such as a cathode ray tube (CRT) or a flat panel display or a touch panel display. An input device 1614 including alphanumeric keys and other keys is connected to the bus 1602 for communicating information and command selections to the processor 1604. Another type of user input device is a cursor control member 1616, such as a mouse, a trackball or a cursor direction key, for communicating direction information and command selections to the processor 1604 and for controlling the movement of the cursor on the display 1612. Such an 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 allow the device to specify a position in a plane. A touch panel (screen) display can also be used as an input device.

[0064] As described herein, one or more of the methods may be performed by the computer system 1600 in response to the processor 1604 executing one or more sequences of one or more instructions contained in the main memory 1606. These instructions may be read from another computer-readable medium (such as storage device 1610) into the main memory 1606. The execution of the sequence of instructions contained in the main memory 1606 causes the processor 1604 to perform the process steps described herein. One or more processors in a multi-processing arrangement may also be used to execute the sequence of instructions contained in the main memory 1606. In alternative embodiments, hard-wired circuits may be used instead of or in combination with software instructions. Therefore, the description herein is not limited to any specific combination of hardware circuitry and software.

[0065] As used herein, the term "computer-readable medium" refers to any medium that participates in providing instructions to the processor 1604 for execution. Such a medium can be in many forms, including but not limited to non-volatile media, volatile media, and transmission media. Non-volatile media include, for example, optical disks or disks, such as storage devices 1610. Volatile media include dynamic memory, such as main memory 1606. Transmission media include coaxial cables, copper wires, and optical fibers, including wires containing bus 1602. Transmission media can also take the form of sound waves or light waves, such as, sound waves or light waves generated during radio frequency (RF) and infrared (IR) data communications. Common forms of computer-readable media include, for example, floppy disks, flexible disks, hard disks, magnetic tapes, any other magnetic media, CD-ROMs, DVDs, any other optical media, punch cards, paper tapes, any other physical media with hole patterns, RAMs, PROMs and EPROMs, FLASH-EPROMs, any other memory chips or cassettes, carriers as described below, or any other media that can be read by a computer.

[0066] Various forms of computer-readable media may be involved when one or more sequences of one or more instructions are loaded to the processor 1604 for execution. For example, initially the instructions may be carried on a disk of a remote computer. The remote computer may load the instructions into its dynamic memory and send the instructions via a telephone line using a modem. The modem local to the computer system 1600 may receive data on the telephone line and convert the data into an infrared signal using an infrared transmitter. An infrared detector connected to the bus 1602 may receive the data carried in the infrared signal and place the data on the bus 1602. The bus 1602 carries the data to the main memory 1606, from which the processor 1604 obtains and executes the instructions. The instructions received by the main memory 1606 may optionally be stored on the storage device 1610 before or after being executed by the processor 1604.

[0067] Computer system 1600 also preferably includes a communication interface 1618 connected to bus 1602. Communication interface 1618 provides a bidirectional data communication connection connected to a network link 1620 connected to a local area network 1622. For example, communication interface 1618 can be an integrated services digital network (ISDN) card or a modem to provide a data communication connection with a telephone line of a corresponding type. As another example, communication interface 1618 can be a local area network (LAN) card to provide a data communication connection with a compatible LAN. Wireless links can also be implemented. In any such embodiment, communication interface 1618 sends and receives an electrical signal, an electromagnetic signal, or an optical signal carrying a digital data stream representing various types of information.

[0068] The network link 1620 typically provides data communication with other data devices via one or more networks. For example, the network link 1620 may provide a connection to a host computer 1624 through a local area network 1622 or a connection to data equipment operated by an Internet Service Provider (ISP) 1626. The ISP 1626 in turn provides data communication services via a global packet data communication network (now commonly referred to as the "Internet") 1628. Both the local area network 1622 and the Internet 1628 use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on the network link 1620 and through the communication interface 1618, which carry the digital data to and from the computer system 1600, are exemplary forms of carrier waves that transport the information.

[0069] The computer system 1600 can send messages and receive data including program code through the network, network link 1620 and communication interface 1618. In the Internet example, the server 1630 may transmit the requested program code for the application via the Internet 1628, ISP 1626, local area network 1622 and communication interface 1618. For example, one such downloaded application can provide one or more of the technologies described herein. The received code can be executed by the processor 1604 when it is received, and / or stored in the storage device 1610 or other non-volatile storage for later execution. In this way, the computer system 1600 can obtain application code in the form of a carrier wave.

[0070] Further embodiments are disclosed in the following numbered aspects:

[0071] 1. A method for measuring a structure, comprising:

[0072] obtaining an amplitude distribution of scattered radiation associated with a measurement of a first structure on a first substrate;

[0073] obtaining a reference phase distribution associated with a reference measurement of at least one reference structure on a reference substrate, the at least one reference structure being a different structure than the first structure; the at least one reference structure being nominally identical to the first structure with respect to at least a plurality of key parameters; and

[0074] A complex-valued field for describing the first structure is determined based on the amplitude distribution and a reference phase distribution.

[0075] 2. The method according to clause 1, wherein the measurement of the first structure and the reference measurement are performed under the same measurement conditions.

[0076] 3. A method according to clause 1 or 2, wherein the at least one reference structure is measured at a time prior to the measurement of the first structure to determine the reference phase distribution.

[0077] 4. A method according to aspect 3, wherein the reference measurement is performed in a reference phase, and the reference phase distribution obtained from the reference measurement is used in a plurality of subsequent measurements performed on a plurality of structures other than the first structure to determine a complex-valued field for each of the plurality of structures, the plurality of structures being nominally identical to the at least one reference structure in terms of at least the plurality of key parameters.

[0078] 5. A method according to any preceding aspect, wherein the structure comprises a periodic grating structure and the key parameters comprise grating pitch, grating size and grating orientation.

[0079] 6. The method according to any preceding aspect, wherein the reference phase distribution is obtained from reference measurements performed on a plurality of reference structures, wherein a phase distribution is determined for each of the reference structures and the phase distributions are averaged to determine the reference phase distribution.

[0080] 7. A method according to any preceding aspect, comprising:

[0081] aligning the amplitude distribution with a reference amplitude distribution associated with the reference measurement performed on the at least one reference structure to obtain a registered amplitude template; and

[0082] The reference distribution is applied to the registered amplitude template to determine the complex-valued field.

[0083] 8. A method according to any preceding aspect, comprising performing an adjustment for an overlap induced global phase offset.

[0084] 9. The method of clause 8, wherein said performing an adjustment of the overlap-induced global phase offset comprises optimizing an image quality described by the complex-valued field as a function of the global phase offset.

[0085] 10. A method according to any preceding aspect, wherein said determining a complex-valued field comprises performing a correction of said complex-valued field to correct for aberrations in sensor optics used to obtain said amplitude distribution.

[0086] 11. A method according to clause 10, wherein said performing correction of said complex-valued field comprises the following steps:

[0087] performing Fourier transformation on the complex-valued field to obtain a Fourier-transformed complex-valued field;

[0088] obtaining a conjugate lens aberration phase distribution associated with the aberration in the sensor optics; and

[0089] The conjugate lens aberration phase distribution is multiplied by the Fourier transformed complex valued field.

[0090] 12. The method of clause 11, wherein performing correction of the complex-valued field further comprises:

[0091] Defocus aberrations are digitally adjusted to maximize the image quality described by the corrected complex-valued field.

[0092] 13. The method according to any preceding aspect, comprising performing the measuring of the first structure by:

[0093] irradiating said first structure with measuring radiation;

[0094] capturing scattered radiation from the measurement radiation on a detector, the detector being operable to detect an intensity distribution of the scattered radiation; and

[0095] The amplitude distribution is determined based on the intensity distribution.

[0096] 14. The method of clause 13, wherein capturing scattered radiation comprises capturing a single diffraction order.

[0097] 15. A method according to any preceding aspect, comprising:

[0098] measuring the at least one reference structure;

[0099] Phase recovery is performed to obtain the reference phase distribution.

[0100] 16. A method according to clause 15, wherein said measuring said at least one reference structure comprises performing repeated acquisitions with at least one parameter varied to provide sufficient information for said phase recovery.

[0101] 17. A computer program comprising program instructions operable to perform the method according to any one of clauses 1 to 16 when run on a suitable device.

[0102] 18. A non-transitory computer program carrier comprising a computer program according to clause 17.

[0103] 19. A processing device operable to run a computer program according to clause 17.

[0104] 20. A metrology device operable to perform the method according to any one of aspects 1 to 16.

[0105] 21. A lithography unit comprising:

[0106] The metrology apparatus of clause 20, the metrology apparatus being configured to perform measurements from which corrections for a lithography process are determined; and

[0107] A lithographic apparatus is operable to perform the lithographic process in accordance with the correction.

[0108] Although specific reference may be made herein to the use of lithographic equipment in IC manufacturing, it should be understood that the lithographic equipment described herein may have other applications. Possible other applications include manufacturing integrated optical systems, guidance and detection for magnetic domain memory, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0109] 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 for 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 an object 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 inspect a substrate for defects or defects in a structure on a substrate. In such embodiments, the characteristic of interest of a structure on a substrate may relate to a defect in the structure, the absence of a particular portion of the structure, or the presence of an undesired structure on the substrate.

[0110] Although specific reference is made to "measurement equipment / tools / systems" or "inspection equipment / tools / systems," 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 a structure on a substrate or on a wafer. For example, a detection 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 on a wafer. In such embodiments, the characteristic of interest of a structure on a substrate may relate to a defect in the structure, the absence of a particular portion of a structure, or the presence of an unwanted structure on a substrate or on a wafer.

[0111] Although the above may specifically refer 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.

[0112] While the targets or target structures described above (more generally, structures on a substrate) are metrology target structures specifically designed and formed for the purpose of measurement, 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 has been provided specifically for the measurement being performed. In addition, 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 lithography process in the target portion C. In practice, the lines and / or spaces of the overlapping gratings within the target structure may be made to include smaller structures that are similar in size to the product features.

[0113] Although specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be practiced in other ways than those described. The above description is intended to be illustrative rather than restrictive. Therefore, it will be appreciated by 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 measuring a structure, comprising: obtaining an amplitude distribution of scattered radiation associated with a measurement of a first structure on a first substrate; obtaining a reference phase distribution associated with a reference measurement of at least one reference structure on a reference substrate, the at least one reference structure being a different structure than the first structure; The first structure is designed to be identical to the at least one reference structure with respect to at least a plurality of key parameters, ignoring unplanned or undesirable variations; as well as A complex-valued field for describing the first structure is determined based on the amplitude distribution and a reference phase distribution. 2 . The method according to claim 1 , wherein the measurement of the first structure and the reference measurement are performed under the same measurement conditions.

3. A method according to claim 1 or 2, wherein the at least one reference structure is measured at a time prior to the measurement of the first structure to determine the reference phase distribution.

4. A method according to claim 3, wherein the reference measurement is performed in a reference phase and the reference phase distribution obtained from the reference measurement is used in a plurality of subsequent measurements performed on a plurality of structures other than the first structure to determine a complex-valued field for each of the plurality of structures, the plurality of structures being designed to be identical to the at least one reference structure in at least the plurality of key parameters, ignoring unplanned or undesired variations.

5. A method according to any preceding claim, wherein the structure comprises a periodic grating structure and the key parameters comprise grating pitch, grating size and grating orientation.

6. A method according to any preceding claim, wherein the reference phase distribution is obtained from reference measurements made on a plurality of reference structures, wherein a phase distribution is determined for each of the reference structures and the phase distributions are averaged to determine the reference phase distribution.

7. A method according to any preceding claim, comprising: aligning the amplitude distribution with a reference amplitude distribution associated with the reference measurement performed on the at least one reference structure to obtain a registered amplitude template; and The reference amplitude distribution is applied to the registered amplitude template to determine the complex-valued field.

8. A method according to any preceding claim, comprising performing an adjustment for an overlap induced global phase offset.

9. The method according to claim 8, wherein: The performing of adjusting the overlap-induced global phase offset includes optimizing the image quality described by the complex-valued field according to the global phase offset.

10. A method according to any preceding claim, wherein said determining a complex-valued field comprises performing a correction of the complex-valued field to correct for aberrations in sensor optics used to obtain the amplitude distribution.

11. The method according to claim 10, wherein: The performing of correction of the complex-valued field comprises the following steps: performing Fourier transformation on the complex-valued field to obtain a Fourier-transformed complex-valued field; obtaining a conjugate lens aberration phase distribution associated with the aberration in the sensor optics; and The conjugate lens aberration phase distribution is multiplied by the Fourier transformed complex valued field.

12. The method of claim 11, wherein said performing correction of said complex-valued field further comprises: Defocus aberrations are digitally adjusted to maximize the image quality described by the corrected complex-valued field.

13. A method according to any preceding claim, comprising performing measurements of the first structure by: irradiating said first structure with measuring radiation; capturing scattered radiation from the measurement radiation on a detector, the detector being operable to detect an intensity distribution of the scattered radiation; and The amplitude distribution is determined based on the intensity distribution.

14. The method according to claim 13, wherein: Said capturing the scattered radiation comprises capturing a single diffraction order.

15. A method according to any preceding claim, comprising: measuring the at least one reference structure; Phase recovery is performed to obtain the reference phase distribution.

16. The method of claim 15, wherein said measuring said at least one reference structure comprises performing repeated acquisitions with at least one parameter varied to provide sufficient information for said phase recovery.

17. A computer program product comprising program instructions operable to perform the method according to any one of claims 1 to 16 when run on a suitable device.

18. A non-transitory computer program carrier comprising the computer program product according to claim 17.

19. A processing device operable to run a computer program product according to claim 17.

20. A metrology device operable to perform a method according to any one of claims 1 to 16.

21. A lithography unit comprising: The metrology device of claim 20, the metrology device being configured to perform measurements from which corrections for a lithography process are determined; and A lithographic apparatus is operable to perform the lithographic process in accordance with the correction.

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