Scanning electron microscope (SEM) backscattered electron (BSE) focusing target and method

Abstract

There is provided a method for evaluating a scanning electron microscope (SEM) system, including accessing SEM images of two or more sets of overlay targets, where each set of overlay targets includes an embedded feature at an embedded depth and a top feature, where in at least one of the two or more sets of overlay targets, the embedded feature is at an embedded depth, and the top feature is at a top depth. The top features are recessed, each of the recesses has a corresponding recess depth, and the recess depths of the top features of the two or more overlay target sets are different; and determining a beam tilt angle of the SEM system based on the SEM images of the two or more sets of overlay targets.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. application 63 / 411,863, filed on September 30, 2022, which is incorporated herein by reference in its entirety. Technical Field

[0003] The present invention generally relates to scanning electron microscope (SEM) system characterization and evaluation and a method for SEM performance monitoring based on SEM target structures. Background Art

[0004] During the manufacturing process of integrated circuits (ICs), unfinished or completed circuit components are inspected to ensure that they are manufactured according to the design and are free of defects. Inspection systems using optical microscopes or charged particle (e.g., electron) beam microscopes (such as scanning electron microscopes (SEMs)) can be employed. As the physical size of IC components continues to shrink and their structures continue to become more complex, the accuracy and throughput of defect detection and inspection become more important. The overall image quality depends on, among other things, a combination of high secondary electron and backscattered electron signal detection efficiency. Backscattered electrons have higher emission energy to escape from deeper layers of the sample, so their detection may be desirable for imaging of complex structures (such as buried layers, nodes, high aspect ratio trenches, or holes) of 3D NAND devices. For applications such as overlay metrology, it may be desirable to simultaneously obtain high-quality imaging and efficient surface information collection from secondary electrons and obtain buried layer information from backscattered electrons, which emphasizes the need to use multiple electron detectors in the SEM. The ability to monitor and detect IC non-idealities may be limited by the image quality of the inspection system, including the alignment or calibration of the SEM system.

[0005] In the context of semiconductor manufacturing, there is a need to monitor and qualify SEM system alignment and calibration. Summary of the invention

[0006] Embodiments of the present disclosure provide systems and methods for designing SEM targets and processing SEM images thereof to evaluate and identify SEM systems, such as metrology and inspection. Embodiments provide features and SEM target designs that can be used to determine beam tilt angles, resolution, contrast, overlay accuracy, and other metrics of SEM systems. The SEM system can be adjusted (e.g., aligned) based on the determined metrics so that the SEM image quality can be maintained (e.g., above a threshold) or improved.

[0007] According to an embodiment, a method for evaluating or identifying a scanning electron microscope (SEM) system is provided, comprising: accessing SEM images of two or more sets of overlay targets, wherein each overlay target includes a buried feature and a top feature, the buried feature is at a buried depth, wherein, in at least one of the two or more sets of overlay targets, the top feature is recessed, each of the recesses has a corresponding recess depth, wherein the recess depths of the top features of the two or more sets of overlay targets are different; and determining a beam tilt angle of the SEM system based on the SEM images of the two or more sets of overlay targets.

[0008] In an embodiment, determining the beam tilt angle comprises determining the beam tilt angle based on a setup-acquire relationship for overlay determined ("acquire") for a number of sets of overlay targets with programmed variation ("setup") overlay.

[0009] In an embodiment, determining the beam tilt angle based on the setup-acquire relationship of the overlay includes: obtaining a measurement value of at least one of the depression depth, the difference between the depression depths, or a combination thereof; determining an intercept value based on a fit of the setup-acquire relationship of the overlay of two or more sets of overlay targets; and determining the beam tilt angle based on the intercept value and the measurement value of at least one of the depression depth, the difference between the depression depths, or a combination thereof.

[0010] According to another embodiment, a method for evaluating a scanning electron microscope (SEM) system is provided, comprising: accessing an SEM image of a plurality of cells comprising various patterns, wherein each cell within the plurality of cells comprises a pattern having a specific pitch, wherein the pattern comprises buried features at a specific buried depth, and wherein the SEM image comprises images of the plurality of cells within a single field of view (FOV); and determining, for at least one cell within the single FOV, a relationship between its pitch and at least one of contrast, resolution, or a combination thereof.

[0011] According to another embodiment, a method for evaluating or identifying a scanning electron microscope (SEM) system is provided, comprising: accessing SEM images of a plurality of cells comprising various patterns, wherein each cell within the plurality of cells comprises a pattern having a specific pitch, wherein the pattern comprises buried features and top features separated by a buried depth, wherein the buried features and the top features are separated by an overlay offset in a direction perpendicular to the buried depth, wherein the specific pitch varies between at least some of the plurality of cells, and wherein the SEM image comprises an image of the plurality of cells within a single field of view (FOV); and determining a relationship between pitch and overlay accuracy based on the SEM images of the plurality of cells having corresponding pitches.

[0012] According to another embodiment, a measurement structure for evaluating or qualifying a scanning electron microscope (SEM) is provided. The structure includes: a plurality of regions on a wafer, each region including a buried feature, the buried feature having a measurable structural characteristic, wherein the buried feature is buried below a top feature, the buried feature has a specific buried depth, wherein at least some of the top features are recessed, the top features have a corresponding recess depth, wherein a first set of regions includes a first set of engraving targets and a second set of regions includes a second set of engraving targets, wherein the recess depth of the first set of engraving targets is different from the recess depth of the second set of engraving targets, and wherein the first set of engraving targets and the second set of engraving targets are adjacent to each other.

[0013] According to another embodiment, a SEM target for evaluating a SEM system is provided, wherein the SEM target includes a plurality of regions on a wafer, each region containing a buried feature, the buried feature having a measurable structural characteristic, wherein the buried feature is buried below a top feature, the buried feature having a corresponding buried depth, wherein at least some of the top features are recessed, the top features having a corresponding recess depth, wherein a first set of regions includes a first set of programming overlay targets and a second set of regions includes a second set of programming overlay targets, wherein the recess depth of the first set of programming overlay targets is different from the recess depth of the second set of programming overlay targets, and wherein the first overlay target and the second overlay target are adjacent to each other.

[0014] According to another embodiment, a method for making the measurement structure of any other embodiment is provided.

[0015] According to another embodiment, one or more non-transitory machine-readable media having instructions thereon are provided that, when executed by a processor, are configured to fabricate the measurement structure of any other embodiment. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which corresponding reference characters indicate corresponding parts, and in which:

[0017] Figure 1 is a schematic diagram illustrating an exemplary electron beam inspection (EBI) system according to an embodiment.

[0018] Figure 2 is a schematic diagram of an exemplary electron beam tool according to an embodiment.

[0019] Figure 3 Depicted is a schematic representation of overall lithography showing the collaboration between three techniques to optimize semiconductor manufacturing in accordance with an embodiment.

[0020] Figure 4 Depicted is a schematic representation of an example SEM target in accordance with an embodiment.

[0021] Figures 5A to 5B Depicted are views of buried features in an example SEM target in accordance with an embodiment.

[0022] Figures 6A to 6C An example graph depicting the relationship between contrast and landing energy of buried features at various burial depths in accordance with an embodiment.

[0023] Figure 7 Depicted is an example graph of a measured relationship between contrast and landing energy for a SEM target including buried features in accordance with an embodiment.

[0024] Figure 8 A method based on SEM evaluation of a SEM target including buried features of various pitches according to an embodiment is illustrated.

[0025] Figures 9A to 9B Depicted are views of overlay features in an example SEM target in accordance with an embodiment.

[0026] Fig.10 Depicted are example overlay relationships in an example SEM target according to an embodiment.

[0027] Figures 11A to 11C An example graph depicting the relationship between accuracy and landing energy for a SEM target including overlaid features in accordance with an embodiment.

[0028] Figures 12A to 12B Depicted are views of tilt angle calibration features in an example SEM target in accordance with an embodiment.

[0029] Figures 13A to 13B Depicted is a view of a tilt angle calibration feature with a recess in an example SEM target in accordance with an embodiment.

[0030] Figures 14A to 14B Depicted is a tilt angle calibration feature of a SEM target according to an embodiment.

[0031] Fig.15 Depicted is an example graph of a setup-acquire relationship between overlays acquired using a tilt angle calibration feature of a SEM objective in accordance with an embodiment.

[0032] Fig.16 A method of SEM target generation according to an embodiment is illustrated.

[0033] Fig.17 is a block diagram of an example computer system according to an embodiment of the present disclosure. DETAILED DESCRIPTION

[0034] The embodiments of the present disclosure are described in detail with reference to the accompanying drawings, which are provided as illustrative examples of the present disclosure to enable those skilled in the art to practice the present disclosure. It is noteworthy that the following figures and examples are not intended to limit the scope of the present disclosure to a single embodiment, but other embodiments are possible by exchanging some or all of the described or illustrated elements. Moreover, in the case where certain elements of the present disclosure can be partially or completely implemented using known components, only those parts of such known components necessary for understanding the present disclosure will be described, and the detailed description of other parts of such known components will be omitted to avoid confusing the present disclosure. The embodiments described as being implemented in software should not be limited thereto, but may include embodiments implemented in hardware or a combination of software and hardware, and vice versa, which is obvious to those skilled in the art, unless otherwise specified herein. In this specification, embodiments showing singular components should not be considered as limiting; on the contrary, the present disclosure is intended to cover other embodiments including multiple identical components, and vice versa, unless otherwise explicitly stated herein. Moreover, the applicant does not intend to give uncommon or special meanings to any term in the specification or claims unless explicitly stated as such. Further, the present disclosure covers the current and future known equivalents of the known components referenced herein by illustration.

[0035] Although specific reference may be made herein to the manufacture of ICs, it should be clearly understood that the description herein has many other possible applications. For example, it may be used in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid crystal display panels, thin film magnetic heads, etc. Those skilled in the art will appreciate that in the context of such alternative applications, any use of the terms "wafer" or "die" herein should be considered interchangeable with the more general terms "substrate" and "target portion", respectively.

[0036] The patterning device may include or may form one or more patterns.The patterns may be generated based on a pattern or design layout using a CAD (Computer Aided Design) program, a process often referred to as EDA (Electronic Design Automation).

[0037] Now refer to Figure 1 , Figure 1 An exemplary electron beam inspection (EBI) system 100 consistent with embodiments of the present disclosure is illustrated. Figure 1 As shown, the EBI system 100 includes a main chamber 110, a load lock chamber 120, an electron beam tool 140, and an equipment front end module (EFEM) 130. The electron beam tool 140 is located in the main chamber 110. The exemplary EBI system 100 can be a single beam or multi-beam system. Although the description and drawings relate to electron beams, it is to be understood that the embodiments are not used to limit the present disclosure to specific charged particles.

[0038] The EFEM 130 includes a first loading port 130a and a second loading port 130b. The EFEM 130 may include additional loading ports (multiple). The first loading port 130a and the second loading port 130b receive wafer front opening pods (FOUPs) containing wafers (e.g., one or more semiconductor wafers made of (multiple) other materials) or samples (wafers and samples are collectively referred to as "wafers" hereinafter) to be inspected. One or more robotic arms (not shown) in the EFEM 130 transport the wafers to the load lock chamber 120.

[0039] The load lock chamber 120 is connected to a load / lock vacuum pump system (not shown), which removes gas molecules in the load lock chamber 120 to reach a first pressure lower than atmospheric pressure. After reaching the first pressure, one or more robotic arms (not shown) transport the wafer from the load lock chamber 120 to the main chamber 110. The main chamber 110 is connected to a main chamber vacuum pump system (not shown), which removes gas molecules in the main chamber 110 to reach a second pressure lower than the first pressure. After reaching the second pressure, the wafer is inspected by an electron beam tool 140. In some embodiments, the electron beam tool 140 may include a single beam inspection tool.

[0040] The controller 150 may be electronically connected to the electron beam tool 140 and may also be electronically connected to other components. The controller 150 may be a computer configured to perform various controls of the EBI system 100. The controller 150 may also include processing circuitry configured to perform various signal and image processing functions. Although the controller 150 may be in Figure 1 1 as being external to the structure including the main chamber 110, the load lock chamber 120, and the EFEM 130, but it is understood that the controller 150 may be part of the structure.

[0041] Figure 2 A schematic diagram of an exemplary imaging system 200 is illustrated in accordance with an embodiment of the present disclosure. Figure 2 The electron beam tool 140 can be configured for use with the EBI system 100. The electron beam tool 140 can be a single beam device or a multi-beam device. Figure 2As shown, the electron beam tool 140 includes a motorized sample stage 201 and a wafer holder 202 supported by the motorized sample stage 202 to hold a wafer 203 to be inspected. The electron beam tool 140 also includes an objective lens assembly 204, an electron detector 206 (including electron sensor surfaces 206a and 206b), an objective lens aperture 208, a focusing lens 210, a beam limiting aperture 212, a gun hole 214, an anode 216, and a cathode 218. In some embodiments, the objective lens assembly 204 may include a modified swing retarding immersion objective (SORIL) including a pole piece 204a, a control electrode 204b, a deflector 204c, and an excitation coil 204d. The electron beam tool 140 may additionally include an energy dispersive X-ray spectrometer (EDS) detector (not shown) to characterize the material on the wafer 203.

[0042] A primary electron beam 220 is emitted from the cathode 218 by applying a voltage between the anode 216 and the cathode 218. The primary electron beam 220 passes through the gun hole 214 and the beam limiting aperture 212, both of which can determine the size of the electron beam entering the condenser lens 210, which resides below the beam limiting aperture 214. The condenser lens 210 focuses the primary electron beam 220 before the beam enters the objective lens aperture 208 to set the size of the electron beam before entering the objective lens assembly 204. The deflector 204c deflects the primary electron beam 220 to facilitate beam scanning on the wafer. For example, during the scanning process, the deflector 204c can be controlled to deflect the primary electron beam 220 to different locations on the top surface of the wafer 203 in sequence at different time points to provide data for image reconstruction of different portions of the wafer 203. Moreover, the deflector 204c can be controlled to deflect the primary electron beam 220 to different sides of the wafer 203 at specific locations at different time points, providing data for the stereoscopic image reconstruction of the wafer structure at that location. Further, in some embodiments, the anode 216 and the cathode 218 can be configured to generate multiple primary electron beams 220, and the electron beam tool 140 can include multiple deflectors 204c to simultaneously project multiple primary electron beams 220 to different parts / sides of the wafer, providing data for image reconstruction of different parts of the wafer 203.

[0043] The excitation coil 204d and the pole piece 204a generate a magnetic field that starts at one end of the pole piece 204a and ends at the other end of the pole piece 204a. A portion of the wafer 203 scanned by the primary electron beam 220 may be immersed in the magnetic field and may be charged, which in turn generates an electric field. The electric field reduces the energy of the primary electron beam 220 that strikes the surface of the wafer 203 before colliding with the wafer 203. The control electrode 204b, which is electrically isolated from the pole piece 204a, controls the electric field on the wafer 203 to prevent micro-arching of the wafer 203 and ensure proper beam focusing.

[0044] Upon receiving the primary electron beam 220, a secondary electron beam 222 may be emitted from a portion of the wafer 203. The secondary electron beam 222 may form a beam spot on the sensor surfaces 206a and 206b of the electron detector 206. The electron detector 206 may generate a signal (e.g., voltage, current, etc.) representing the intensity of the beam spot and provide the signal to the image processing system 250. The intensity of the secondary electron beam 222 and the beam spot generated thereby may vary according to the external or internal structure of the wafer 203. Moreover, as discussed above, the primary electron beam 220 may be projected onto different locations of the top surface of the wafer or onto different sides of the wafer at a specific location to generate secondary electron beams 222 (and the beam spot generated thereby) of different intensities. Therefore, by mapping the intensity of the beam spot with the location of the wafer 203, the processing system may reconstruct an image reflecting the internal or surface structure of the wafer 203.

[0045] The imaging system 200 can be used to inspect the wafer 203 on the sample stage 201 and includes the electron beam tool 140, as discussed above. The imaging system 200 can also include an image processing system 250, which includes an image acquisition device 260, a storage device 270 and a controller 150. The image acquisition device 260 can include one or more processors. For example, the image acquisition device 260 can include a computer, a server, a mainframe, a terminal, a personal computer, any kind of mobile computing device, etc. or a combination thereof. The image acquisition device 260 can be connected to the detector 206 of the electron beam tool 140 through a medium such as an electrical conductor, a fiber optic cable, a portable storage medium, IR, Bluetooth, the Internet, a wireless network, a radio station or a combination thereof. The image acquisition device 260 can receive a signal from the detector 206 and can construct an image. The image acquisition device 260 can therefore acquire an image of the wafer 203. The image acquisition device 260 can also perform various post-processing functions, such as generating a profile, superimposing indicators on the acquired image, etc. The image acquisition device 260 can be configured to perform adjustments such as brightness and contrast of the acquired image. The storage device 270 may be a storage medium such as a hard disk, a cloud storage device, a random access memory (RAM), other types of computer readable memory, etc. The storage device 270 may be coupled to the image acquirer 260 and may be used to save the scanned original image data as a raw image and a post-processed image. The image acquirer 260 and the storage device 270 may be connected to the controller 150. In some embodiments, the image acquirer 260, the storage device 270, and the controller 150 may be integrated together into a control unit.

[0046] In some embodiments, the image acquirer 260 can acquire one or more images of the sample based on the imaging signal received from the detector 206. The imaging signal can correspond to a scanning operation for performing charged particle imaging. The acquired image can be a single image including multiple imaging areas. The single image can be stored in the storage device 270. The single image can be an original image that can be divided into multiple segments. Each segment in the segment can include an imaging area that contains a feature of the wafer 203.

[0047] Figure 3 A schematic representation of overall lithography is depicted, showing the collaboration between the three technologies to optimize semiconductor manufacturing. Typically, the patterning process in the lithography apparatus LA is one of the most critical steps in processing, requiring the dimensioning and placement of structures on the substrate W with high accuracy ( Figure 1 ). To ensure this high accuracy, the three systems (in this example) can be combined in a so-called “holistic” control environment, such as Figure 3 Schematically depicted. One of these systems is a lithographic apparatus LA, which is (virtually) connected to a metrology apparatus (e.g., a metrology tool) MT (a second system) and a computer system CL (a third system). The "holistic" environment can be configured 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 lithographic apparatus LA stays within the process window. The process window defines a 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) - typically within which process parameters of a lithographic process or patterning process are allowed to vary.

[0048] 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 2 Typically, 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 MT) to predict whether defects may be present (e.g. due to suboptimal processing). Figure 2 ) is depicted by an arrow pointing to “0” in the second scale SC2.

[0049] The metrology equipment (tool) MT can provide input to the computer system CL to achieve accurate simulation and prediction, and can provide feedback to the lithographic apparatus LA to identify possible drift, for example in the calibration state of the lithographic apparatus LA (in Figure 3 SC3 is depicted by multiple arrows in the third scale SC3).

[0050] During the lithography process, it is desirable to make frequent measurements of the created structures, for example for process control and verification. Different types of metrology tools MT for making such measurements are known, including scanning electron microscopes or various forms of optical metrology tools, image-based or scatterometry-based metrology tools. Image analysis of images obtained from optical metrology tools and scanning electron microscopes can be used to measure various dimensions (e.g. CD, overlay, edge placement error (EPE) etc.) and detect defects in the structure. In some cases, features of one layer of a structure may obscure features of another layer or the same layer of the structure in the image. For example, in a scanning electron microscope (SEM) image, this may be the case where one layer is physically located above another layer, or when one layer is electronically rich and therefore brighter than another layer. In the case where features are partially obscured in the image, the position of the image can be determined based on template matching.

[0051] Figure 4 A schematic representation 700 of an example SEM target including multiple regions is depicted. The schematic representation 700 is depicted relative to an x-axis 702 and a y-axis 704, which represent directions in a plane substantially parallel to a fabrication surface (e.g., a wafer surface). In this document, a general x-axis and a general y-axis are provided to help describe elements in a plane parallel to the fabrication surface, while a general z-axis is used to describe directions substantially perpendicular to the fabrication surface (e.g., a fabrication direction). The axes are presented only for ease of description, where fabrication need not be performed in a plane, on a substantially flat surface, substantially perpendicular to a surface, etc. The fabrication process is not limited to those fabrication processes described by these axes, which are provided only for ease of description. These axes can be represented by polar coordinates, cylindrical coordinates, different orientations of Cartesian coordinates, etc.

[0052] Schematic representation 700 includes an overview of a SEM target 716, which includes a plurality of regions 714 (e.g., cells). SEM target 716 is depicted as a linear array of a plurality of regions 714, but may consist of a collection of regions organized along non-perpendicular axes (e.g., as a collection of parallelograms). SEM target 716 is depicted as including a plurality of regions 714 separated by distance 712. SEM target 716 may include a plurality of regions 714 separated, interspersed, etc. by various distances, which regions 714 may or may not have linear edges. SEM target 716 may be symmetrical in size (e.g., square) along each of its major dimensions, or may be rectangular, circular, elliptical, etc. SEM target 716 may fit within a single field of view of a SEM system used to acquire SEM images. SEM target 716 may have a size 710, which may be, for example, 20 μm. SEM target 716 may have a size 710 corresponding to the SEM FOV—e.g., 16 μm, 40 μm, etc. The size 710 may be the same or different for the major dimensions (e.g., the depicted x-axis 702 and y-axis 704). For example, the distance 712 separating the regions 714 may be 0.2 μm, while the size of the regions 714 may be 2.0 μm. These dimensions are provided as examples, and the SEM target 716, the distance 712, and the regions 714 may have larger or smaller dimensions depending on the application, including, for example, eight times as large as described or as small as the minimum feature size of the process layer. The regions 714 are depicted as being surrounded by a boundary, but may or may not have boundary features (e.g., linear fabrication features such as trenches or berms, fabricated directional patterns, etc.). The boundary features may be used to orient or navigate to a portion of the SEM image of the SEM target 716, and the boundary features may vary based on the location within the SEM target 716, or may not vary.

[0053] The regions 714 of the SEM target 716 can be substantially similar, e.g., can contain similar features, including similar features of different sizes. The regions 714 of the SEM target 716 can be different, e.g., can contain different features, including features that can be used for different purposes (such as aligned along different axes). For example, the regions 714 can contain different types of features (e.g., buried features, top features, recessed features, etc.), features of different orientations (e.g., oriented along the x-axis 702, oriented along the y-axis 704, oriented in a linear array, staggered, etc.), features of different measurable structural characteristics (e.g., pitch, feature separation, buried depth, lateral offset, etc.), etc. The regions 714 can contain a mixture of features that are substantially similar in some respects but different in other respects. For example, the regions 714 can contain repeated sets of features that have different pitches in different regions 714. The regions 714 can contain various features, such as features for measuring beam tilt angles (e.g., telecentricity angles) (e.g., tilt angle calibration features), features for measuring resolution, contrast, or a combination of resolution and contrast, features for measuring overlay accuracy, etc. The region 714 of the SEM target 716 may be further divided into a plurality of regions, or may include undivided regions.

[0054] Also shown is a detailed view 740 of an example of region 714, where the detailed view 740 is at a different, larger scale than the SEM target 716 in the schematic representation 700. The detailed view 740 depicts a region 714 that can be further divided into a plurality of regions 724 (e.g., cells). The term region is used to refer to sub-units of the SEM target 716 and sub-units of the region 714, where such terminology is used merely for ease of description, but different terms for the constituent units may be used, such as region and sub-region, segment and region, etc. The region 714 is depicted as a linear array of regions 724, but may consist of a collection of regions organized along a non-perpendicular axis (e.g., as a collection of parallelograms). The region 714 is depicted as including a plurality of regions 724 separated by distances 722. The region 714 may include a plurality of regions 724 separated by various distances, interspersed, etc., and these regions 724 may or may not have linear edges. Region 714 may be symmetrical in size (e.g., square) along each of its major dimensions, or may be rectangular, circular, elliptical, etc. When used to acquire an SEM image, region 714 may fit within a single field of view of the SEM system. Region 714 may have a size 720, which may be 2 μm, for example. Size 720 may be the same or different for major dimensions (e.g., the x-axis 702 and y-axis 704 depicted). For example, the distance 722 separating region 724 may be 0.1 μm, while the size of region 724 may be 0.6 μm. These dimensions are provided as examples, and region 724, distance 722, and region 724 may have larger or smaller sizes depending on the application, such as eight times as large as described or as small as the minimum feature size of the process layer. Region 724 is depicted as being surrounded by a boundary, but may or may not have boundary features. Boundary features may be used to orient or navigate to a portion of the SEM image of the SEM target 716. The boundary of region 724 may include a portion of the boundary of region 714 , or may be in addition to the boundary of region 714 .

[0055] In some embodiments, sub-regions 724 of region 714 may be substantially similar. In some embodiments, sub-regions 724 may be different. For example, region 724 may include features of different types (e.g., buried features, top features, recessed features, offset features, etc.), features of different orientations (e.g., oriented along x-axis 702, oriented along y-axis 704, oriented in a linear array, staggered, etc.), features of different measurable structural characteristics (e.g., pitch, feature separation, buried depth, lateral offset, etc.), etc. In some embodiments, region 724 may include a mixture of features that are substantially similar in some respects but different in other respects. For example, region 724 may include repeated sets of features that have different pitches in different regions 724. In another example, some regions 724 may include repeated sets of features that have different orientations (e.g., of the long axis of a rectangular feature) in different regions 724. Regions 724 may include various features together, which may include features for measuring beam tilt angles, features for measuring resolution, contrast, or a combination of resolution and contrast, features for measuring overlay accuracy, etc.

[0056] Also depicted are example relationships between regions 724 of region 714, where Roman characters A to I represent placement locations of respective regions 724. In some embodiments, region 724 may have various measurable structural features. In some embodiments, region 724 having the smallest measurable structural feature among various measurable structural features (e.g., the smallest pitch among various pitches) may be placed at the center of region 714 (e.g., in region 724A). Region 724 having a measurable structural feature greater than the smallest measurable structural feature may be placed around the center of region 714 (e.g., in regions 724B to 724I). In some embodiments, measurable structural features may vary between many regions 724. In an example, the size of the measurable structural features of region 724 may increase in a predetermined pattern, such as from 724A to 724B to 724C to 724E to 724D to 724F to 724G to 724G. The relationship between regions depicted by Roman characters A to I is provided as an example, and other relationships and orientations may be used.

[0057] Also shown are detailed views 734a-734c of multiple instances of region 724, where detailed views 734a-734c correspond to some possible orientations of region 724. Detailed views 734a-734c are depicted at a different, larger scale than SEM target 716, and at a different, larger scale than detailed view 740 of region 714. Detailed view 734a depicts a one-dimensional (e.g., line) pattern. Detailed view 734b depicts a two-dimensional (e.g., contact hole pattern) where the two-dimensional features are staggered. Detailed view 734c depicts a two-dimensional (e.g., contact hole pattern) where the two-dimensional features are arranged but not staggered. Detailed views 734a-734c depict features that may correspond to buried features, top features, recessed features, etc. Detailed views 734a-734c do not explicitly depict features of different types, depths, sizes, etc. within a single region 724; however, region 724 may contain features of multiple types, depths, sizes, etc., such as overlaid features, recessed and topped features, features with different orientations, etc. Detailed views 734a-734c are provided as examples, and other feature arrangements may be used.

[0058] Region 724 is depicted as a straight line, but may be composed of regions with non-perpendicular axes (e.g., as a collection of parallelograms). Region 724 may be symmetrical in size along each major dimension (e.g., square), or may be rectangular, circular, elliptical, etc. Region 724 may fit within a single field of view of a SEM system used to acquire the SEM image. Region 724 may have a size 730, which may be, for example, 0.6 μm. Size 730 may be the same or different for major dimensions (e.g., the depicted x-axis 702 and y-axis 704). Features of region 724 may have a variety of sizes, including sizes that may be the same or different for major dimensions of region 724. The number of features of region 724 may correspond to the feature size of region 724. For example, for a one-dimensional feature (e.g., a line) with a line width of 60 nm, a region 724 with a size 730 of 0.6 μm may contain ten features (e.g., with a total size of 0.6 μm in one direction, for a 60 nm feature, it includes space for 10 total features). In another example, the size of the two-dimensional features (e.g., contact holes) can be 45nm, such as in an arranged but not staggered pattern. In another example, the size of the two-dimensional features (e.g., contact holes) can be 30nm, such as in a staggered pattern. The size and shape of the features of region 724 can correspond to the features of a production device using a SEM device. For example, the features with the smallest measurable structural features may correspond to the CD, orientation, dimensions, etc. of the production features. These sizes are provided as examples, and the features of region 724 and region 724 can have larger or smaller sizes depending on the application, such as including eight times as large as described or as small as the minimum feature size of the process layer. Region 724 is depicted as being surrounded by a boundary, but may or may not have boundary features (e.g., linear fabrication features, such as trenches or berms, fabricated directional patterns, etc.). Boundary features can be used to orient or navigate to a portion of the SEM image of the SEM target 716, which boundary features may vary based on the position within the SEM target 716, or may not vary.

[0059] Regions 724 of SEM target 716 may be substantially similar—e.g., may contain similar features, including similar features of different sizes. Regions 724 of SEM target 716 may be different, e.g., may contain different features, which may be used for different purposes (such as alignment along different axes). For example, regions 724 may contain different types of features (e.g., buried features, top features, recessed features, etc.), features of different orientations (e.g., oriented along x-axis 702, oriented along y-axis 704, oriented in a linear array, staggered, etc.), features of different measurable structural characteristics (e.g., pitch, feature separation, buried depth, lateral offset, etc.), etc. Regions 724 may contain a mixture of features that are substantially similar in some respects but different in other respects. For example, regions 724 may contain repeated sets of features that have different pitches in different regions 724. Regions 724 may contain various features, such as features for measuring beam tilt angles (e.g., tilt angle calibration features), features for measuring resolution, contrast, or a combination of resolution and contrast, features for measuring overlay accuracy, etc.

[0060] Figures 5A to 5B Depicted is a view of buried features in an example SEM target. Figure 5A A cross-sectional view of an example SEM target is depicted, depicted relative to a z-axis 806 and an x-axis 802. As with other common axes described herein, common axes are provided for ease of description. The example SEM target may include a substrate 810 (which may be a bottom layer, a bulk layer, etc., such as silicon on insulator), a buried feature layer 814, a fill layer 816, and a top layer 818. The interfaces between the layers may include one or more adhesion layers or interstitial layers, such as an adhesion layer 812 depicted between the substrate 810 and the buried feature layer 814.

[0061] The substrate 810 may be etched or otherwise patterned, such as during a process of creating features in the buried feature layer 814. In some embodiments, the substrate 810 may be unpatterned. The buried feature layer 814 may include features having measurable structural characteristics (e.g., one-dimensional features, two-dimensional features, etc.). The buried feature layer 814 may include features having a pitch 820, wherein for ease of description, the pitch 820 is depicted along the x-axis 802. The pitch 820 may be symmetrical, wherein the features of the buried feature layer 814 may have substantially similar feature sizes 820b and feature spacings 820a. In some embodiments, the pitch 820 may be asymmetrical, wherein the features of the buried feature layer 814 may have different feature sizes 820b and feature spacings 820a. The buried feature layer 814 may be located at a buried depth 830. The buried depth 830 may correspond to a depth of a SEM, such as an electron penetration depth. The buried depth 830 can be selected before the SEM target is made based on the corresponding depth of the features of the production wafer (e.g., selected during the design of the manufacturing process of the SEM target). The buried depth 830 can be selected before the SEM target is made, such as based on the depth corresponding to the landing energy (LE) of the SEM used to measure the production wafer, where the electron penetration depth can vary based on the LE. Based on the buried depth 830, a corresponding landing energy (LE) can be selected for SEM imaging of the SEM target, where the electron penetration depth is a function of the LE. The buried depth 830 can include the depth of the top layer 818, or can measure the distance between the top of the buried feature layer 814 and the top of the fill layer 816. In some embodiments, the top layer 818 can be omitted or can be substantially included in the fill layer 816. The substrate 810 can be grounded, including grounded via a back electrode or metal layer, or otherwise provided with an electron source.

[0062] The materials making up substrate 810, buried feature layer 814, fill layer 816, and top layer 818 may have different SEM characteristics. For example, substrate 810 and buried feature layer 814 may have contrasting SEM imaging characteristics, such as backscatter coefficient η, secondary electron yield δ, etc. Substrate 810 and buried feature layer 814 may have contrasting brightness, such as due to contrasting material types or conductivities (e.g., metals versus semiconductors), atomic numbers (e.g., titanium versus silicon), etc. Fill layer 816 and top layer 818 may be less sensitive or less detectable in SEM imaging (including in BSE responses).

[0063] Figure 5B Depicts a substrate plan view of an example SEM target (e.g. Figure 5A802 and y-axis 804. As with other common axes described herein, the common axes are provided for ease of description. Dashed rectangle 850 surrounds feature 852 of an example SEM target. Dashed rectangle 850 may surround an area corresponding to an area of ​​a SEM target, such as Figure 4 The dotted rectangle 850 is depicted as a polygonal pattern. The feature 852 may correspond to a buried feature layer (e.g. Figure 5A The feature 852 is depicted as a rectangle, but may include other features, such as contact holes, lines, irregular features, etc.

[0064] The dashed rectangle 850 may correspond to an SEM image of an example SEM target. The SEM image may contain information related to a buried feature layer (e.g., Figure 5A The SEM image may contain regions (e.g., pixel segments) corresponding to signals (such as feature 852) from the substrate (e.g., buried feature layer 814). Figure 5A The SEM image may include areas (e.g., pixel segments) that correspond to signals from other layers of the example SEM target (e.g., fill layer 816, top layer 818, etc.), which may be patterned or unpatterned. The contrast level of the SEM image may be determined based on the intensity of the individual regions of the SEM image. For example, the ratio of contrast to noise may be determined. In another example, the contrast variation of each pixel may be determined. The contrast level may be determined based on the difference between the area of ​​the SEM image corresponding to the buried feature (e.g., feature 852) and the area of ​​the SEM image that does not correspond to the buried feature. An enlargement of the SEM image corresponding to the example SEM target is shown in rectangle 854. In the illustrative example, feature 852 appears as a black pixel in the SEM image, while the area of ​​the example SEM target other than feature 852 corresponds to a white pixel in the SEM image. These pixel values ​​are provided only for ease of description, and the pixel contrast may be lower (e.g., dark gray and light gray) or even opposite (e.g., feature 852 is brighter than the background). Contrast can be measured as the difference between the pixel values ​​of feature 852 and other regions and / or the rate of change between pixels corresponding to feature 852 and other regions. Rectangle 854 shows an example rate of change between black pixels of feature 852 and white pixels of other regions. The contrast of multiple features or regions can be determined, such as multiple regions of the same or different SEM targets (such as Figure 4The contrast may be determined for a plurality of feature sets, wherein at least some of the features or feature sets have different measurable structural characteristics. In some cases, the contrast of one or more regions may not be determined, such as if the SEM image is not sharp enough to distinguish between buried features or between buried features and areas outside of buried features. The contrast may be a function of measurable structural characteristics, such as pitch, burial depth, feature size, etc. The contrast may be a function of SEM parameters, such as LE, beam spot size, detector type, etc.

[0065] Figures 6A to 6C Example graph depicting the relationship between contrast and landing energy for buried features at various burial depths. Fig. 6A A graph 900 is depicted that depicts contrast (as contrast-to-noise ratio along axis 904) as a function of LE (in kiloelectronvolts (keV) along axis 902). Graph 900 illustrates a feature set associated with a SEM target, such as Figure 5A and 5B The graph 900 shows a line 910 corresponding to a SEM target having a pitch of 30 nm and a buried depth of 30 nm. Figure 5A and 5B The dotted line 912 corresponds to the SEM target having a pitch of 45 nm and a buried depth of 30 nm. Figure 5A and 5B 914) corresponding to the dotted line 914 depicted) for an SEM target having a pitch of 60 nm and a buried depth of 30 nm. Each of the regions having a feature set (e.g., corresponding to line 910, dashed line 912, and dashed line 914) can be contained within a single field of view (FOV) of the SEM image. The FOV can be an area over which the SEM system can acquire images without significant drift. The FOV may contain features, regions, feature sets, etc., and it can be assumed that the SEM conditions are substantially similar. An SEM image of a single FOV can be acquired in a single operation (including raster scanning) without stitching multiple images together or significantly resetting the SEM beam optics. For each feature set in the feature set of the SEM target, the contrast exhibits a maximum at an optimal LE, where the optimal LE varies based on the pitch. For line 910, the optimal LE is approximately 22 keV; for dashed line 912, the optimal LE is approximately 16 keV; and for dashed line 914, the optimal LE is approximately 13 keV. For all LEs depicted, larger pitches have higher overall contrast, but may exhibit crossover in some instances.

[0066] Figure 6BA graph 920 is depicted that depicts contrast (as contrast-to-noise ratio along axis 924) as a function of LE (in keV along axis 922). Graph 920 shows the relationship between the characteristics of the SEM target (such as Figure 5A and 5B The graph 920 shows a line 930 corresponding to a SEM target having a pitch of 30 nm and a buried depth of 60 nm. Figure 5A and 5B The dotted line 932 corresponds to the SEM target having a pitch of 45 nm and a buried depth of 60 nm. Figure 5A and 5B 934 ) corresponding to the dotted line 934 depicted), the pitch of the SEM target is 60nm and the buried depth is 60nm. Each of the regions having a feature set (e.g., corresponding to line 930, dash-dotted line 932, and dashed line 934) can be contained within a single FOV of the SEM image. For each feature set in the feature set of the SEM target, the contrast exhibits a maximum value at an optimal LE, where the optimal LE varies based on the pitch. For line 930, the optimal LE is approximately 30keV; for dash-dotted line 932, the optimal LE is approximately 20keV; and for dashed line 934, the optimal LE is approximately 16keV. For each pitch (e.g., the pitch of line 930 is 30nm, the pitch of dash-dotted line 932 is 45nm, and the pitch of dashed line 934 is 60nm), the optimal LE is approximately 16keV. Fig. 6A Compared to the 30nm burial depth of graph 900, the optimal LE of the 60nm burial depth of graph 920 is larger. Fig. 6A Each pitch of graph 920 (e.g., line 930 has a pitch of 30 nm, dotted line 932 has a pitch of 45 nm, and dashed line 934 has a pitch of 60 nm) has less contrast than the 30 nm burial depth of graph 900. The overall contrast is higher for larger pitches for all LEs depicted, but may exhibit crossover in some instances.

[0067] Figure 6C A graph 940 is depicted which depicts contrast (as contrast-to-noise ratio along axis 944) as a function of LE (in keV along axis 942). Graph 940 shows the relationship between the characteristics of the SEM target (such as Figure 5A and 5B 950 corresponding to a SEM target having a pitch of 30 nm and a buried depth of 120 nm. Graph 940 shows a characteristic set of SEM targets such as Figure 5A and 5BThe dotted line 952 corresponds to the SEM target having a pitch of 45 nm and a buried depth of 120 nm. Figure 5A and 5B 954 corresponding to the dotted line 954 depicted), the SEM target has a pitch of 60nm and a buried depth of 120nm. Each of the regions having a feature set (e.g., corresponding to line 950, dash-dotted line 952, and dashed line 954) can be contained within a single FOV of the SEM image. For each feature set in the feature set of the SEM target, the contrast exhibits a maximum value at an optimal LE within the depicted LE range, where the optimal LE varies based on the pitch. For line 950, the optimal LE is approximately 50keV (e.g., the maximum LE depicted); for dash-dotted line 952, the optimal LE is approximately 35keV; and for dashed line 954, the optimal LE is approximately 26keV. For each pitch (e.g., the pitch of line 950 is 30nm, the pitch of dash-dotted line 952 is 45nm, and the pitch of dashed line 954 is 60nm), the optimal LE is approximately 26keV. Figure 6B Compared to the 60nm burial depth of graph 920, the optimal LE of the 120nm burial depth of graph 940 is larger. Figure 6B Each pitch of graph 940 (e.g., line 950 has a pitch of 30 nm, dotted line 952 has a pitch of 45 nm, and dashed line 954 has a pitch of 60 nm) has a smaller contrast ratio for a 120 nm burial depth than for a 60 nm burial depth for graph 920. For all LEs depicted, the overall contrast is higher for larger pitches, but may exhibit crossover in some instances.

[0068] For a SEM system, a relationship between LE and contrast of a measurable structural feature can be determined. This relationship can be determined by modeling. This relationship can be determined by performing baseline measurements on the SEM system. This relationship can be determined by a mixture of experimental and modeling approaches. Then, by comparing the contrast measurements of one or more LEs with the relationship between LE and contrast of the measurable structural feature, a performance metric for the SEM system can be determined. Performance metrics can be determined for different types of features. For example, performance metrics can be determined for one-dimensional features, two-dimensional features, two-dimensional features in various orientations, and the like. Performance metrics can be used to measure the performance of SEM alignment. Performance metrics can indicate beam spot size, beam alignment, LE accuracy, and the like.

[0069] Figure 7 Depicted is an example graph of the measured relationship between contrast and landing energy for a SEM target including buried features. Figure 7A graph 1000 is depicted that depicts contrast (as contrast-to-noise ratio along axis 1004) as a function of LE (in keV along axis 1002). Graph 1000 illustrates a feature set associated with a SEM target, such as Figure 5A and 5B 1010). The contrast of the SEM target is measured at multiple LEs, such as at 13keV, 18keV, 20keV, 25keV, 27.5keV, and 30keV. For the measured structure, the contrast exhibits a maximum at the optimal LE within the depicted LE range. For line 1010, the optimal LE is about 25keV. In some embodiments, the measurement relationship may include a measurement relationship for a feature set having various measurable structural characteristics. The measurement relationship may be determined based on the selected LE, where each LE may correspond to a SEM image acquired from the SEM system. The measurement relationship may be determined for multiple features, regions, feature sets, etc. of the SEM target contained within a single FOV of the SEM system.

[0070] Figure 8 A method for SEM evaluation based on SEM targets including buried features of various pitches is illustrated. Each of these operations is described in detail below. The operations of method 1100 presented below are intended to be illustrative. In some embodiments, method 1100 may be accomplished with one or more additional operations not described and / or without one or more operations discussed. Additionally, the operations of method 1100 may be performed in a manner similar to those described above. Figure 8 The order illustrated in and described below is not intended to be limiting. In some embodiments, one or more portions of method 1100 may be implemented in one or more processing devices (e.g., one or more processors) (e.g., by simulation, modeling, etc.). The one or more processing devices may include one or more devices that perform some or all of the operations of method 1100 in response to instructions electronically stored on an electronic storage medium. For example, the one or more processing devices may include one or more devices configured by hardware, firmware, and / or software to be specifically designed to perform one or more operations of method 1100.

[0071] In operation 1102, a SEM image is acquired. The SEM image may correspond to a plurality of features, a set of features, an area, etc. The SEM image may correspond to a single FOV. The SEM image may be all or a portion of a SEM target. The SEM image is acquired at a given LE, which is acquired together with the SEM image.

[0072] In operation 1104, a region of the SEM image corresponding to a given pitch is identified. The region of the SEM image may be selected based on position in the SEM image (e.g., from left to right, from center outward), based on pitch (e.g., from small pitch to largest pitch, etc.). Conversely, pitch may be another one or more measurable structural features. The region of the SEM image corresponding to the pitch may be a region (e.g., Figure 4 region 724), feature sets (such as Figure 5B The region of the SEM image corresponding to the pitch may include some or all of the SEM image. The SEM image may contain regions that do not correspond to the pitch or other features used for contrast and / or resolution determination.

[0073] In operation 1106, contrast and / or resolution are determined for a region of the SEM image corresponding to a given pitch. The contrast and / or resolution may be determined based on pixel values ​​of the SEM image. The contrast and / or resolution may be determined as an absolute value, a relative value, etc. The contrast and / or resolution may be determined as a ratio, a derivative, etc.

[0074] In operation 1108, it is determined whether other areas of the SEM image (corresponding to the pitch) remain to be identified. Conversely, it may be determined that other pitches remain for identification. If other areas of the SEM image remain to be identified, the process continues to operation 1106, where another area of ​​the SEM image is identified. If other areas of the SEM image have not yet been identified, the process continues to operation 1110, where the SEM performance indicators are determined.

[0075] In operation 1110, a performance indicator is determined based on the determined contrast and / or resolution of the SEM image area. The performance indicator can be qualitative (e.g., the SEM alignment is good), can be relative (e.g., the SEM alignment is better than previously determined), can be quantitative (e.g., the SEM alignment is 90% of optimal alignment), etc. The performance indicator can be determined for multiple pitches. The performance indicator can instead be determined based on multiple pitches.

[0076] In operation 1112, the SEM system may be adjusted based on the performance indicator. The SEM system may be further aligned based on the performance indicator indicating that the alignment is not optimal. The SEM system may not be further aligned, such as based on the performance indicator indicating that the alignment exceeds a threshold. The SEM system may be adjusted by other means, such as by adjusting the LE, which is not an alignment operation.

[0077] As described above, method 1100 (and / or other methods and systems described herein) is configured to perform SEM evaluation based on a SEM target that includes buried features of various pitches.

[0078] Figures 9A to 9B Depicted is a view of overlaid features in an example SEM target. Fig.9A A cross-sectional view of a portion of an example SEM target is depicted, depicted relative to a z-axis 1206 and an x-axis 1202. As with other common axes described herein, common axes are provided for ease of description. The example SEM target may include a substrate 1210 (which may be a bottom layer, a bulk layer, etc., such as silicon on insulator), a buried feature layer 1214, a fill layer 1216, and a top feature layer 1218. The interfaces between the layers may include one or more adhesion layers or interstitial layers, such as adhesion layer 1212 depicted between substrate 1210 and buried feature layer 1214.

[0079] The substrate 1210 may be etched or otherwise patterned, such as during a process of creating features in the buried feature layer 1214 and / or creating features in the top feature layer 1218. In some embodiments, the substrate 1210 may be unpatterned. The buried feature layer 1214 may include features having measurable structural characteristics (e.g., one-dimensional features, two-dimensional features, etc.). The buried feature layer 1214 may include features having a pitch 1220, wherein for ease of description, the pitch 1220 is depicted along the x-axis 1202. The pitch 1220 may be symmetrical, wherein the features of the buried feature layer 1214 may have substantially similar feature sizes 1220b and feature spacings 1220a. In some embodiments, the pitch 1220 may be asymmetrical, wherein the features of the buried feature layer 1214 may have different feature sizes 1220b and feature spacings 1220a. The buried feature layer 1214 may be located at a buried depth 1230.

[0080] The top feature layer 1218 may include features having measurable structural features (e.g., one-dimensional features, two-dimensional features, etc.). The features of the top feature layer 1218 may include features having the same or different measurable structural features as the buried feature layer 1214. The top feature layer 1218 may include features having a pitch 1240, wherein for ease of description, the pitch 1240 is depicted along the x-axis 1202. The pitch 1240 may be symmetrical, wherein the features of the top feature layer 1218 may have substantially similar feature sizes 1240b and feature spacings 1240a. In some embodiments, the pitch 1240 may be asymmetrical, wherein the features of the top feature layer 1218 may have different feature sizes 1240b and feature spacings 1240a. The pitch 1240 may be the same as or different from the pitch 1220. Fig.9A (and Fig. 9B) illustrates an example where pitch 1240 is equal to pitch 1220. Feature size 1240b may be the same as or different from feature size 1220b. Feature spacing 1240a may be the same as or different from feature spacing 1220a. Within a region, pitch 1220 and pitch 1240 may be substantially equal (feature spacing 1240a and feature spacing 1240a and feature size 1220b and feature size 1220b may be different) such that buried feature layer 1214 and top feature layer 1218 have substantially the same lateral offset within the region (as depicted in the x-direction).

[0081] The buried depth 1230 may correspond to a depth of the SEM, such as an electron penetration depth. The buried depth 1230 may be determined based on a corresponding depth of a feature of a production wafer. The buried depth 1230 may be selected based on a landing energy (LE) of the SEM, such as an LE used for production wafer imaging, where the electron penetration depth is a function of the LE. The buried depth 1230 may include the depth of the top feature layer 1218 (e.g., as depicted, from the surface of the SEM target), or may measure the distance between the top of the buried feature layer 1214 and the bottom of the top feature layer 1218. In some embodiments, additional top or capping layers may be fabricated over the top feature layer 1218, where the depth of the additional layer(s) may or may not be included in the determination of the buried depth 1230. The substrate 1210 may be grounded, including via a back electrode or metal layer, or otherwise provided with an electron source.

[0082] The materials constituting substrate 1210, buried feature layer 1214, fill layer 1216, and top feature layer 1218 may have different SEM characteristics. For example, substrate 1210, buried feature layer 1214, and top feature layer 1218 may have contrasting SEM imaging characteristics, such as backscatter coefficient η, secondary electron yield δ, etc. Substrate 1210, buried feature layer 1214, and top feature layer 1218 may have contrasting brightness, such as due to contrasting material types or conductivities (e.g., metals versus semiconductors), atomic numbers (e.g., titanium versus silicon), etc.

[0083] Fig. 9B Depicts a substrate plan view of an example SEM target (e.g. Fig.9A 1202 and y-axis 1204. As with other common axes described herein, the common axes are provided for ease of description. Dashed rectangle 1250 surrounds buried features 1252 and top features 1254 of at least a portion of an example SEM target. Dashed rectangle 1250 may surround an area corresponding to an area of ​​a SEM target, such as Figure 4 The area 724. The dashed rectangle 1250 is depicted as a polygonal pattern.

[0084] The buried features 1252 may correspond to a buried feature layer (eg, Fig.9A The top feature 1254 may correspond to a top feature layer (e.g., Fig.9A 1204). The buried features 1252 and the top features 1254 are depicted as rectangles, but may include other features, such as contact holes, lines, irregular features, etc. The buried features 1252 and the top features 1254 may be features of the same or different shapes. The buried features 1252 and the top features 1254 may be features of the same or different sizes in one or more dimensions. The buried features 1252 and the top features 1254 may be offset in one or more lateral directions (e.g., along the x-axis 1202, along the y-axis 1204, etc.). The buried features 1252 and the top features 1254 may be offset by a first lateral offset 1262, as depicted along the x-axis 1202. The buried features 1252 and the top features 1254 may be offset by a second lateral offset 1264, as depicted along the y-axis 1204. The buried feature 1252 and the top feature 1254 may have a negligible lateral offset, for example, the first lateral offset 1262 and the second lateral offset 1264 may be substantially zero. Because the buried feature 1252 and the top feature 1254 may be different in size and shape, the buried feature 1252 and the top feature 1254 may not be completely overlaid even if the first lateral offset 1262 and the second lateral offset 1264 are substantially zero. For example, when the first lateral offset 1262 and the second lateral offset 1264 are less than half the size of the smallest of the buried feature 1252 and the top feature 1254, the buried feature 1252 and the top feature 1254 may partially overlap. For example, when the first lateral offset 1262 and the second lateral offset 1264 are greater than the size of the largest of the buried feature 1252 and the top feature 1254, the buried feature 1252 and the top feature may be non-overlapping.

[0085] Dashed rectangle 1250 may correspond to a SEM image of at least a portion of an example SEM target. The SEM image may include regions (e.g., pixel segments) corresponding to signals from buried feature 1252. The SEM image may include regions (e.g., pixel segments) corresponding to signals from top feature 1254. The SEM image may include regions that include signals corresponding to other layers of the example SEM target (e.g., fill layer 1216, bonding layer 1212, etc.), which may be patterned or unpatterned. A measure of overlay may be determined based on the SEM image. The measure of overlay may be determined based on template matching or other image processing methods. The measure of overlay may measure a feature of the SEM image corresponding to at least one of the first lateral offset 1262, the second lateral offset 1264, or a combination thereof. The measure of overlay may depend on contrast, for example because template matching accuracy may depend on contrast levels. Determination of the measure of overlay may depend on contrast levels.

[0086] A measure of overlay accuracy can be determined based on a measure of overlay and knowledge of at least one of a first lateral offset 1262, a second lateral offset 1264, or a combination thereof. The first lateral offset 1262 can be known based on fabrication details (such as a design layout). The second lateral offset 1264 can be known based on fabrication details (such as a design layout). The measure of overlay accuracy can be determined based on a signal-to-noise ratio (SNR) of intensity levels in various regions of the SEM image. The measure of overlay accuracy can be determined based on a distribution of overlay measurements on repeatable cells (including all repeatable cells, substantially all repeatable cells, some repeatable cells, etc.). A repeatable cell can include one or more buried features 1252 and top features 1254. The measure of overlay accuracy can be determined based on a standard deviation of overlay measurements on multiple repeatable cells. The measure of overlay accuracy can be determined as an absolute value (e.g., in nm), a percentage, etc. It can be, for example, relative to multiple regions (such as Figure 4 A measure of overlay accuracy is determined based on a plurality of measurable structural features corresponding to a pitch in region 724).

[0087] The measure of overlay accuracy may be determined for a plurality of features, feature sets, regions, etc. of a SEM target, wherein at least some of the feature sets have different measurable structural features. In some cases, it may not be possible to determine a measure of overlay accuracy for one or more feature sets of a SEM target, such as if the SEM image is not sharp enough to distinguish buried features from top features. The measure of overlay accuracy may be a function of measurable structural features such as pitch, buried depth, feature size, etc. Contrast may be a function of SEM parameters such as LE, beam spot size, detector type, etc.

[0088] Fig.10Depicted are example overlay relationships in example SEM targets. Fig.10 A substrate plan view of an area of ​​at least a portion of an example SEM target is depicted, depicted relative to an x-axis 1302 and a y-axis 1304. As with other common axes described herein, the common axes are provided for ease of description. Fig.10 Regions 1330, 1340, and 1350 are depicted, which include regions of example SEM targets (e.g., such as Figure 4 Regions 1330, 1340, and 1350 include buried features 1310 and top features 1320. The buried features 1310 of region 1330 are coaxial with the top features 1320 of region 1330. The buried features 1310 of region 1340 are laterally offset in the x-direction and the y-direction relative to the top features of region 1340. The buried features 1310 and the top features 1320 of region 1330 partially overlap. The buried features 1310 of region 1350 are laterally offset in the x-direction and the y-direction relative to the top features of region 1350, wherein the offset in the y-direction is greater than the offset in the x-direction. The buried features 1310 and the top features 1320 of region 1350 do not overlap. Example SEM targets may include regions of various features having different measurable structural characteristics, such as regions 1330, 1340, and 1350. Regions 1330, 1340, and 1350 are provided as illustrative examples where regions of the SEM target may vary in feature size, feature shape, one or more lateral offsets, and so forth.

[0089] Figures 11A to 11C An example graph depicting the relationship between accuracy and landing energy for a SEM target including overlaid features in accordance with an embodiment. Fig.11A A graph 1400 is depicted that depicts overlay accuracy (along axis 1404) as a function of LE (in keV along axis 1402). Graph 1400 shows the relationship between the overlay accuracy and the LE of the SEM target. Fig.9A and 9B The graph 1400 shows a line 1410 corresponding to a SEM target having a pitch of 30 nm and a buried depth of 30 nm. Fig.9A and 9B The dotted line 1412 corresponds to the SEM target having a pitch of 45 nm and a buried depth of 30 nm. Fig.9A and 9B1414) corresponding to the dotted line 1414 depicted), the pitch of the SEM target is 60nm and the buried depth is 30nm. Each of the feature sets (e.g., corresponding to line 1410, dotted line 1412, and dotted line 1414) can be contained within a single FOV of the SEM image. For each of the regions of the SEM target, the overlay accuracy exhibits a minimum at a minimum LE, where the minimum LE varies based on the pitch. For line 1410, the minimum LE is approximately 24keV; for dotted line 1412, the minimum LE is approximately 15keV; and for dotted line 1414, the minimum LE is approximately 12keV. Overlay accuracy is a function of pitch and LE, where overlay accuracy is greater for larger pitches at higher LEs and lower for larger pitches at low LEs.

[0090] Fig. 11B A graph 1420 is depicted that depicts overlay accuracy (along axis 924) as a function of LE (in keV along axis 1422). Graph 1420 shows the relationship between the overlay accuracy and the LE of the SEM target. Fig.9A and 9B 1430 corresponding to the SEM target having a pitch of 30 nm and a buried depth of 60 nm. Fig.9A and 9B The dotted line 1432 corresponds to the SEM target having a pitch of 45 nm and a buried depth of 60 nm. Fig.9A and 9B 1434 corresponding to the dotted line 1434 depicted), the SEM target has a pitch of 60nm and a buried depth of 60nm. Each of the feature sets (e.g., corresponding to line 1430, dash-dotted line 1432, and dashed line 1434) can be contained within a single FOV of the SEM image. For each of the features, regions, feature sets, etc. of the SEM target, the contrast exhibits a minimum at a minimum LE, where the minimum LE varies based on the pitch. For line 1430, the minimum LE is approximately 30keV; for dash-dotted line 1432, the minimum LE is approximately 22keV; and for dashed line 1434, the minimum LE is approximately 16keV. Overlay accuracy is a function of pitch and LE, where overlay accuracy is greater for larger pitches at higher LEs and lower for larger pitches at low LEs. Fig.11A Compared to graph 1400 , points of interest (eg, the points where line 1430 intersects dash-dot line 1432 , the points where line 1430 intersects dashed line 1434 , and the points where dash-dot line 1432 intersects dashed line 1434 ) appear at a larger LE for graph 1420 .

[0091] Fig. 11C A graph 1440 is depicted that depicts overlay accuracy (along axis 944) as a function of LE (in keV along axis 1442). Graph 1440 shows the relationship between the overlay accuracy and the LE (in keV along axis 1442) and the SEM target. Fig.9A and 9B 1450 corresponding to a SEM target having a pitch of 30 nm and a buried depth of 120 nm. Graph 1440 shows a characteristic set of SEM targets such as Fig.9A and 9B The dotted line 1452 corresponds to the SEM target having a pitch of 45 nm and a buried depth of 120 nm. Fig.9A and 9B 1454 corresponding to the dotted line 1454 depicted), the pitch of the SEM target is 60nm and the burial depth is 120nm. Each feature set in the feature set (e.g., corresponding to line 1450, dash-dotted line 1452, and dashed line 1454) can be contained within a single FOV of the SEM image of the SEM target. For each feature set in the feature set, the overlay accuracy exhibits a minimum at the minimum LE within the depicted LE range, where the minimum LE varies based on the pitch. For line 1450, the minimum LE is approximately 50keV (e.g., the maximum LE depicted); for dash-dotted line 1452, the minimum LE is approximately 35keV; and for dashed line 1454, the minimum LE is approximately 26keV. Contrast and accuracy may be inversely correlated. With Fig. 11B Compared to graph 1420 , points of interest (eg, points where line 1430 intersects dash-dot line 1432 , points where line 1430 intersects dashed line 1434 , and points where dash-dot line 1432 intersects dashed line 1434 ) appear at a larger LE in graph 1440 .

[0092] For a SEM system, a relationship between the LE and overlay accuracy of a measurable structural feature can be determined. This relationship can be determined by modeling. This relationship can be determined by performing a baseline measurement on the SEM system. This relationship can be determined by a mixture of experimental and modeling approaches. Then, by comparing the measured values ​​of the overlay accuracy of one or more LEs with the relationship between the LE and overlay accuracy of the measurable structural feature, a performance indicator of the SEM system can be determined. Performance indicators can be determined for different types of features. For example, performance indicators can be determined for one-dimensional features, two-dimensional features, two-dimensional features in various orientations, and the like. Performance indicators can be used to measure the performance of SEM alignment. Performance indicators can indicate beam spot size, beam alignment, LE accuracy, and the like.

[0093] Figures 12A to 12B Depicted is a view of the tilt angle calibration feature in an example SEM objective. Fig. 12A A cross-sectional view of at least a portion of an example SEM target is depicted, depicted relative to a z-axis 1506 and an x-axis 1502. As with other common axes described herein, common axes are provided for ease of description. The example SEM target may include a substrate 1510 (which may be a bottom layer, a bulk layer, etc., such as silicon on insulator), a buried feature layer 1514, a fill layer 1516, and a top feature layer 1518. The interfaces between the layers may include one or more adhesion layers or interstitial layers, such as adhesion layer 1512 depicted between substrate 1510 and buried feature layer 1514.

[0094] The substrate 1510 may be etched or otherwise patterned, such as during a process of creating features in the buried feature layer 1514 and / or creating features in the top feature layer 1518. In some embodiments, the substrate 1510 may be unpatterned. The buried feature layer 1514 may include features having measurable structural characteristics (e.g., one-dimensional features, two-dimensional features, etc.). The buried feature layer 1514 may include features having a pitch 1520, wherein for ease of description, the pitch 1520 is depicted along the x-axis 1502. The pitch 1520 may be symmetrical, wherein the features of the buried feature layer 1514 may have substantially similar feature sizes 1520b and feature spacings 1520a. In some embodiments, the pitch 1520 may be asymmetrical, wherein the features of the buried feature layer 1514 may have different feature sizes 1520b and feature spacings 1520a. The buried feature layer 1514 may be located at a buried depth 1530.

[0095] The top feature layer 1518 may include features having measurable structural features (e.g., one-dimensional features, two-dimensional features, etc.). The features of the top feature layer 1518 may include features having the same or different measurable structural features as the buried feature layer 1514. The top feature layer 1518 may include features having a pitch 1540, wherein for ease of description, the pitch 1540 is depicted along the x-axis 1502. The pitch 1540 may be symmetrical, wherein the features of the top feature layer 1518 may have substantially similar feature sizes 1540b and feature spacings 1540a. In some embodiments, the pitch 1540 may be asymmetrical, wherein the features of the top feature layer 1518 may have different feature sizes 1540b and feature spacings 1540a. The pitch 1540 may be the same or different from the pitch 1520. The feature size 1540b may be the same or different from the feature size 1520b. The feature spacing 1540a may be the same or different from the feature spacing 1520a. Within a region, pitch 1520 and pitch 1540 can be substantially equal (feature spacing 1540a and feature spacing 1520a and feature size 1520b and feature size 1520b can be different) so that the buried feature layer 1514 and the top feature layer 1518 have substantially the same lateral offset within the region (as depicted in the x-direction).

[0096] The buried depth 1530 may correspond to the depth of the SEM, such as the electron penetration depth. The buried depth 1530 may be selected based on the corresponding depth of the features of the production wafer. The buried depth 1530 may be selected based on the landing energy (LE) of the SEM, such as the LE used for production wafer imaging, where the electron penetration depth is a function of the LE. The buried depth 1530 may include the depth of the top feature layer 1518 (e.g., as depicted, starting from the surface of the SEM target), or may measure the distance between the top of the buried feature layer 1514 and the bottom of the top feature layer 1518. In some embodiments, additional top or capping layers may be made above the top feature layer 1518, where the depth of the additional layer(s) may or may not be included in the determination of the buried depth 1530. In some embodiments, the top feature layer 1518 may be a layer created by filling an etched recess. The top feature layer 1518 may then be planarized, such as via chemical mechanical polishing, to be substantially flush with the surface of the fill layer 1516. The substrate 1510 may be grounded, including via a back electrode or metal layer, or otherwise provided with an electron source.

[0097] The materials making up substrate 1510, buried feature layer 1514, fill layer 1516, and top feature layer 1518 may have different SEM characteristics. For example, substrate 1510, buried feature layer 1514, and top feature layer 1518 may have contrasting SEM imaging characteristics, such as backscatter coefficient η, secondary electron yield δ, etc. Substrate 1510, buried feature layer 1514, and top feature layer 1518 may have contrasting brightness, such as due to contrasting material types or conductivities (e.g., metals versus semiconductors), atomic numbers (e.g., titanium versus silicon), etc.

[0098] Fig. 12B A substrate plan view depicting tilt angle calibration features in an example SEM target (eg Fig. 12A 1502 and y-axis 1504. As with other common axes described herein, the common axes are provided for ease of description. Dashed rectangle 1550 surrounds buried features 1552 and top features 1554 of an example SEM target. Dashed rectangle 1550 may surround an area corresponding to an area of ​​a SEM target, such as Figure 4 The area 724. The dashed rectangle 1550 is depicted as a polygonal pattern.

[0099] The buried features 1552 may correspond to a buried feature layer (eg, Fig. 12A The top feature 1554 may correspond to a top feature layer (e.g., Fig. 12A1502). The buried features 1552 and the top features 1554 are depicted as rectangles, but may include other features, such as contact holes, lines, irregular features, etc. The buried features 1552 and the top features 1554 may be features of the same or different shapes. The buried features 1552 and the top features 1554 may be features of the same or different sizes in one or more dimensions. The buried features 1552 and the top features 1554 may be offset in one or more lateral directions (e.g., along the x-axis 1502, along the y-axis 1504, etc.). The buried features 1552 and the top features 1554 may be offset by a first lateral offset 1562, as depicted along the x-axis 1502. Along the y-axis 1504, the buried features 1552 and the top features 1554 may also or instead be offset by a second lateral offset. In the illustrative example, a negligible second lateral offset is depicted. The buried feature 1552 and the top feature 1554 may have a negligible lateral offset, for example, the first lateral offset 1562 and the second lateral offset may be substantially zero. Because the size and shape of the buried feature 1552 and the top feature 1554 may be different, the buried feature 1552 and the top feature 1554 may not completely overlap even if the first lateral offset 1562 and the second lateral offset are substantially zero. For example, when the first lateral offset 1562 and the second lateral offset are less than half the size of the smallest of the buried feature 1552 and the top feature 1554, the buried feature 1552 and the top feature 1554 may partially overlap. For example, when the first lateral offset 1562 and the second lateral offset are greater than the size of the largest of the buried feature 1552 and the top feature 1554, the buried feature 1552 and the top feature may not overlap.

[0100] Dashed rectangle 1550 may correspond to a SEM image of an example SEM target. The SEM image may include regions (e.g., pixel segments) corresponding to signals from buried features 1552. The SEM image may include regions (e.g., pixel segments) corresponding to signals from top features 1554. The SEM image may include regions that include signals corresponding to other layers of the example SEM target (e.g., fill layer 1516, bonding layer 1512, etc.), which may be patterned or unpatterned. A measure of overlay may be determined based on the SEM image. The measure of overlay may be determined based on template matching or other image processing methods. The measure of overlay may measure a feature of the SEM image corresponding to at least one of the first lateral offset 1562, the second lateral offset, or a combination thereof. The measure of overlay may depend on contrast, for example, because template matching accuracy may depend on contrast levels, the determination of the measure of overlay may depend on contrast levels.

[0101] The set-get overlay relationship can be determined based on a measure of the overlay and knowledge of the first lateral offset 1562 and the second lateral offset. The first lateral offset 1562 can be known based on fabrication details such as the design layout. The second lateral offset can be known based on fabrication details such as the design layout. The set-get overlay relationship can be determined based on multiple measurements of overlay of SEM images with different lateral offsets.

[0102] For each lateral direction, a setup-acquisition overlay relationship can be determined based on a fitted first order polynomial function. The setup-acquisition overlay relationship can be plotted (e.g., as a scatter plot) for multiple values ​​of known overlay (e.g., setup overlay) and multiple measured values ​​of overlay (e.g., acquisition overlay). A set of features of the SEM target (e.g., with different lateral offsets) can be plotted. Fig. 12A and 12B The slope and / or offset can be determined by the set-get overlay relationship of the feature set of the image. Figure 4 A plurality of set-and-get overlay relationships are determined based on a plurality of measurable structural features corresponding to pitches in region 724).

[0103] Figures 13A to 13B Depicted is a view of the tilt angle calibration feature in an example SEM objective. Fig.13A and 13B Corresponding to Fig. 12A and 12B Similar features to those depicted, except Fig.13A and Fig. 13B The top feature is concave. Fig.13A A cross-sectional view of at least a portion of an example SEM target is depicted, depicted relative to a z-axis 1606 and an x-axis 1602. As with other common axes described herein, common axes are provided for ease of description. The example SEM target may include a substrate 1610 (which may be a bottom layer, a bulk layer, etc., such as silicon on insulator), a buried feature layer 1614, a fill layer 1616, a recessed top feature layer 1618, and a recessed layer 1619. The interfaces between the layers may include one or more adhesion layers or interstitial layers, such as an adhesion layer 1612 depicted between the substrate 1610 and the buried feature layer 1614.

[0104] Substrate 1610 may be etched or otherwise patterned, such as during a process of creating features in buried feature layer 1614, creating features in recessed top feature layer 1618, and / or creating features in recessed layer 1619. In some embodiments, substrate 1610 may be unpatterned. Buried feature layer 1614 may include features having measurable structural characteristics (e.g., one-dimensional features, two-dimensional features, etc.). Buried feature layer 1614 may include features having a pitch 1620, wherein for ease of description, pitch 1620 is depicted along x-axis 1602. Pitch 1620 may be symmetrical, wherein the features of buried feature layer 1614 may have substantially similar feature sizes 1620b and feature spacings 1620a. In some embodiments, pitch 1620 may be asymmetrical, wherein the features of buried feature layer 1614 may have different feature sizes 1620b and feature spacings 1620a. Buried feature layer 1614 may be located at a buried depth 1630.

[0105] The recessed top feature layer 1618 may include features having measurable structural features (e.g., one-dimensional features, two-dimensional features, etc.). The features of the recessed top feature layer 1618 may include features having the same or different measurable structural features as the buried feature layer 1614. The recessed top feature layer 1618 may include features having a pitch 1640, wherein for ease of description, the pitch 1640 is depicted along the x-axis 1602. The pitch 1640 may be symmetrical, wherein the features of the recessed top feature layer 1618 may have substantially similar feature sizes 1640b and feature spacings 1640a. In some embodiments, the pitch 1640 may be asymmetrical, wherein the features of the recessed top feature layer 1618 may have different feature sizes 1640b and feature spacings 1640a. The pitch 1640 may be the same or different from the pitch 1620. The feature size 1640b may be the same or different from the feature size 1620b. The feature spacing 1640a may be the same or different from the feature spacing 1620a. Within a region, pitch 1620 and pitch 1640 can be substantially equal (feature spacing 1620a and feature spacing 1640a and feature size 1620b and feature size 1640b can be different) so that the buried feature layer 1614 and the recessed top feature layer 1618 have substantially the same lateral offset within the region (as depicted in the x-direction).

[0106] The recessed top feature layer 1618 may be recessed to a recessed depth 1634 corresponding to the recessed layer 1619. The recessed layer 1619 may correspond to a void space. The recessed layer 1619 may be created by etching, such as by selectively etching the recessed top feature layer 1618. The recessed top feature layer 1618 may have a fill depth 1632. The fill depth 1632 may be created by filling the void features to generate the recessed top feature layer 1618, which may then be etched to generate the recessed layer 1619. The recessed layer 1619 may be created by etching the recessed top feature layer 1618. The buried depth 1630 may or may not substantially correspond to the sum of the recessed depth 1634 and the fill depth 1632. The recessed layer 1619 may constitute a majority of the buried depth 1630. For example, the recessed depth 1634 may be more than half of the buried depth 1630. Fig. 12A and 12B The feature set may be Fig.13A and 13B The feature set is basically similar, except Fig.13A There is a recessed layer 1619 having a recessed depth 1634.

[0107] The buried depth 1630 may correspond to the depth of a SEM, such as an electron penetration depth. The buried depth 1630 may be the same as the buried depth of a feature set having a negligible recess depth 1634.

[0108] In some embodiments, the tilt angle calibration features having substantially similar burial depths but different recessed depths may be adjacent to each other. In some embodiments, the tilt angle calibration features having substantially similar burial depths but different recessed depths may be located in substantially adjacent areas, including interspersed areas. The tilt angle calibration features having substantially similar burial depths but different recessed depths may include features such as Figures 12A to 12B and Figures 13A to 13B The relationship depicted may be characterized. Tilt angle calibration features having substantially similar burial depths but different recess depths may be substantially similar, where the only substantial difference between the features may be the recess top feature (e.g., the presence of a recess layer). Different recess depths may allow for determination of different set-acquire relationships (e.g., corresponding to different recess depths), where beam tilt angles may be determined based on the different set-acquire relationships. Larger recess depths may be more sensitive to beam tilt angles, and therefore allow for higher beam tilt angle determination sensitivity.

[0109] The buried depth 1630 may be selected based on a corresponding depth of features of a production wafer. For SEM beam tilt angle calibration, the buried depth 1630 may be selected based on a depth corresponding to a landing energy (LE) of the SEM, where the electron penetration depth may vary based on the LE. The buried depth 1630 may include the depth of the recessed top feature layer 1618 (e.g., as depicted), or the distance between the top of the buried feature layer 1614 and the bottom of the recessed top feature layer 1618 may be measured. In some embodiments, an additional top or cap layer may be fabricated over the recessed top feature layer 1618, where the depth of the additional layer(s) may or may not be included in the determination of the buried depth 1630. The additional layer may be formed over the fill layer 1616, rather than over the recess above the features of the recessed top feature layer 1618. The buried depth 1630 may be related to the depth of the buried depth 1630. Fig. 12A The burial depth 1530 is substantially the same.

[0110] The materials constituting the substrate 1610, the buried feature layer 1614, the fill layer 1616, and the recessed top feature layer 1618 may be different from those constituting Fig. 12A The materials of substrate 1510, buried feature layer 1514, fill layer 1516, and top feature layer 1518 are substantially the same. Recessed layer 1619 may be a void layer or may be a fill layer, wherein recessed layer 1619 may be relatively transparent to SEM imaging.

[0111] Fig. 13B A substrate plan view depicting tilt angle calibration features in an example SEM target (eg Fig.13A 1602 and y-axis 1604. As with other common axes described herein, the common axes are provided for ease of description. Dashed rectangle 1650 surrounds buried features 1652 and top features 1654 of an example SEM target. Dashed rectangle 1650 may surround an area corresponding to an area of ​​a SEM target, such as Figure 4 The dashed rectangle 1650 is depicted as having asymmetric dimensions (eg, having a major axis along the x-axis 1602 and a minor axis along the y-axis 1604), but example SEM targets may include regions with or without symmetric dimensions.

[0112] The buried feature 1652 may correspond to a buried feature layer (eg, Fig.13A The top feature 1654 may correspond to a top feature layer (e.g., Fig.13A1602). The buried features 1652 and the top features 1654 are depicted as rectangles, but may include other features, such as contact holes, lines, irregular features, etc. The buried features 1652 and the top features 1654 may be features of the same or different shapes. The buried features 1652 and the top features 1654 may be features of the same or different sizes in one or more dimensions. The buried features 1652 and the top features 1654 may be offset in one or more lateral directions (e.g., along the x-axis 1602, along the y-axis 1604, etc.). The buried features 1652 and the top features 1654 may be offset by a first lateral offset 1662, as depicted along the x-axis 1602. Along the y-axis 1604, the buried features 1652 and the top features 1654 may also or instead be offset by a second lateral offset. In the illustrative example, a negligible second lateral offset is depicted. The buried feature 1652 and the top feature 1654 may have a negligible lateral offset—for example, the first lateral offset 1662 and the second lateral offset may be substantially zero. Because the buried feature 1652 and the top feature 1654 may be different in size and shape, the buried feature 1652 and the top feature 1654 may not completely overlap even if the first lateral offset 1662 and the second lateral offset are substantially zero. For example, when the first lateral offset 1662 and the second lateral offset are less than half the size of the smallest of the buried feature 1652 and the top feature 1654, the buried feature 1652 and the top feature 1654 may partially overlap. For example, when the first lateral offset 1662 and the second lateral offset are greater than the size of the largest of the buried feature 1652 and the top feature 1654, the buried feature 1652 and the top feature may not overlap.

[0113] Dashed rectangle 1650 may correspond to a SEM image of an example SEM target. The SEM image may include regions (e.g., pixel segments) corresponding to signals from buried features 1652. The SEM image may include regions (e.g., pixel segments) corresponding to signals from top features 1654. The SEM image may include regions that include signals corresponding to other layers of the example SEM target (e.g., fill layer 1616, adhesive layer 1612, etc.), which may be patterned or unpatterned. A measure of overlay may be determined based on the SEM image. The measure of overlay may be determined based on template matching or other image processing methods. The measure of overlay may measure a feature of the SEM image corresponding to at least one of the first lateral offset 1662, the second lateral offset, or a combination thereof. The measure of overlay may depend on contrast, for example because template matching accuracy may depend on contrast levels. Determination of the measure of overlay may depend on contrast levels. The measure of overlay may be affected by beam tilt angles, which may interact with recessed spaces to change the measure of overlay that was originally expected.

[0114] The set-get overlay relationship can be determined based on the measurement of overlay and the knowledge of the first lateral offset 1662 and the second lateral offset. The first lateral offset 1662 can be known based on manufacturing details (such as design layout). The second lateral offset can be known based on manufacturing details (such as design layout). The set-get overlay relationship can be determined based on the intensity of various regions of the SEM image. The set-get overlay relationship can be determined based on multiple measurements of overlay of regions with different lateral offsets. The beam tilt angle may affect the set-get overlay relationship. The set-get overlay relationship may also be affected by the depth of the recess of the top feature. By determining the set-get overlay relationship for different recess depths and determining the relationship between them, the beam tilt angle can be determined.

[0115] For each lateral direction, a setup-acquisition overlay relationship may be determined based on a fitted first order polynomial function, etc. The setup-acquisition overlay relationship may be plotted (e.g., as a scatter plot) for multiple values ​​of known overlay (e.g., setup overlay) and multiple measured values ​​of overlay (e.g., acquisition overlay). A set of features may be plotted for a given feature set of an SEM target (e.g., features with different lateral offsets). Fig.13A and 13B The slope and / or offset can be determined from the set-acquire overlay relationship of the measurement structure of the Figure 4 A plurality of set-and-get overlay relationships are determined based on a plurality of measurable structural features corresponding to pitches in region 724).

[0116] Figures 14A to 14B Tilt angle calibration features of the SEM target are depicted. Fig.14A Depicted are cross-sectional views and substrate plan views of tilt angle calibration features with various configurations and beam tilt angles. Fig.14A A cross-sectional view 1700 and a substrate plan view 1708 (e.g., corresponding to an SEM image) depict a tilt angle calibration feature having negligible recess depth, negligible lateral offset, and negligible beam tilt angle 1702. In the substrate plan view 1708, the shaded circles representing top layer features and the white circles representing buried layer features (at a buried depth 1704) are concentric about a shared center.

[0117] Fig.14AA cross-sectional view 1720 and a substrate plan view 1728 of a tilt angle calibration feature having a negligible recess depth and negligible lateral offset (e.g., substantially similar to the tilt angle calibration feature depicted in the cross-sectional view 1700 and the substrate plan view 1708), having a non-zero beam tilt angle (i.e., beam tilt angle 1722) are depicted. In the substrate plan view 1728, the shaded circles representing top layer features are laterally centered (along the x-axis 1772) on their corresponding top layer features. However, in the substrate plan view 1728, the white circles representing buried layer features are laterally offset (along the x-axis 1772) from their corresponding buried layer features by a distance 1724. The distance 1724 may vary with the depth (e.g., of the buried feature) and the beam tilt angle 1722.

[0118] Fig.14A A cross-sectional view 1730 and a substrate plan view 1738 of a tilt angle calibration feature are depicted, wherein the recess depth of the selective recess is non-zero and the lateral offset is negligible at a non-zero beam tilt angle (i.e., beam tilt angle 1722). In substrate plan view 1738, the shaded circles representing top features are offset laterally (along x-axis 1772) from their corresponding top features by a distance 1739. Additionally, in substrate plan view 1738, the white circles representing buried layer features are offset laterally (along x-axis 1772) from their corresponding buried layer features by a distance 1724. Distances 1739 and 1724 may vary with depth (e.g., of the buried features) and beam tilt angle 1722. Distance 1724 may be greater than distance 1739 (e.g., in magnitude) because distance 1724 corresponds to a buried feature layer having a greater depth than the top feature layer corresponding to distance 1739.

[0119] Fig.14A Also depicted are a cross-sectional view 1740 and a substrate plan view 1748 of a tilt angle calibration feature, wherein a non-selective recess has a non-zero recess depth and has a negligible lateral offset at a non-zero beam tilt angle (i.e., beam tilt angle 1722). Additionally, in substrate plan view 1748, the white circles representing buried layer features are offset laterally (along x-axis 1772) from their corresponding buried layer features by a distance 1724. Distances 1739 and 1724 in substrate plan view 1748 may be substantially similar to distances 1739 and 1724 in substrate plan view 1738, wherein the difference between cross-sectional view 1730 and cross-sectional view 1740 is the selectivity of the recess. As depicted in cross-sectional view 1730, a selective recess may involve recessing a top layer feature while the bulk or global substrate surface is not recessed, whereas, as depicted in cross-sectional view 1740, a non-selective recess may involve a locally recessed substrate surface. A non-selectively recessed region may include one or more tilt angle calibration features. For example, a set of tilt angle calibration features having programmed overlay values ​​may be included in one or more regions of the non-selective recess.

[0120] The cross-sectional views 1700, 1720, 1730, and 1740 are depicted relative to a z-axis 1776 and an x-axis 1772. The substrate plan views 1708, 1728, 1738, and 1748 are depicted relative to an x-axis 1772 and a y-axis 1774. As with other common axes described herein, common axes are provided for ease of description. In the cross-sectional views 1700, 1720, and 1730, the buried feature has a buried depth 1704. In the cross-sectional view 1740, the buried feature has a buried depth 1704 relative to an unrecessed wafer surface 1742, and has a buried depth 1732 relative to a local wafer surface. In the cross-sectional view 1730, the top feature has a recess depth 1734, a fill depth 1732 (which is the difference between the buried depth 1704 and the recess depth 1734), a thickness 1737, and the recess has a recess width 1736. Although fill depth 1732 is depicted as the difference between the top of the top feature and buried depth 1704, and thickness 1737 of the top feature is depicted as the difference between the top of the top feature and the bottom of the top feature, fill depth 1732 may otherwise be equal to thickness 1737, depending on the relative depths of the top feature layer and the buried feature layer. For example, the buried feature and the top feature may be vertically separated by one or more layers, the bottom of the top feature layer may be coplanar with the top of the buried feature, the top feature may extend vertically below the top of the buried feature, etc. Similarly, although recess width 1736 is depicted as being similar in width to the width of the top feature, recess width 1736 (e.g., the lateral extent of the recess) may be greater or less than the width of the top feature (larger or smaller, e.g., a few nanometers). In cross-sectional views 1720, 1730, and 1740 and substrate plan views 1728, 1738, and 1748, beam tilt angle 1722 of an incident electron beam (e.g., of an SEM system) is θ t .

[0121] Fig. 14B Paired tilt angle calibration features in the SEM objective are depicted. Fig. 14B Depicted are cross-sectional views 1720 and 1730 (which are substantially similar except for recess depth 1734) and their corresponding substrate plan views 1728 and 1738. This pair of tilt angle calibration features are provided as exemplary features that may be included in an SEM target 1770 and may be used to determine beam tilt angle 1722.

[0122] The SEM target 1770 is comprised of regions 1754 separated by spacing distances 1762. The SEM target 1770 may have a size 1760 in one or more dimensions. Each of the regions 1754 of the SEM target 1770 may include features (e.g., buried features and top features) that have an overlay relationship (e.g., set overlay). For illustrative purposes, the regions 1754 are labeled with overlay values, which may correspond to one or more lateral overlay offsets. In the illustrative example, the regions 1754 are arranged in a linear array (although other arrangements previously described may be used), with the regions 1754 in a column having similar overlay values ​​that increase over each row. For example, overlay values ​​of -5 nm to 5 nm in steps of 1 nm may be used in both the x-axis 1772 direction and the y-axis 1774 direction. The regions 1754 in each row may have a consistent recess depth that may vary between rows. For example, a first row may have a negligible recess depth (e.g., which may correspond to Fig. 12A and 12B ), while the second row may have a non-negligible concavity depth (e.g., corresponding to Fig.13A and 13B 1754). Each row may correspond to a different depression depth. Alternatively, two or more rows may be dedicated to a feature set for measuring beam tilt angles, while other rows of the SEM target 1770 may be dedicated to features for measuring other parameters, such as resolution, contrast, overlay accuracy, etc. In the SEM target 1770, regions 1754 containing features with similar overlay values ​​but different depression depths may be positioned substantially adjacent. Substantially adjacent may include adjacent without intermediate features, adjacent with intermediate fields or features, adjacent with intermediate boundaries, scattered, etc. The SEM target 1770 may include regions 1754 containing substantially similar features with or without variations in depression depth and overlay values ​​between regions. The SEM target 1770 may include regions 1754 containing various features, such as regions 1754 (e.g., two or more rows) containing a first feature set (e.g., a first pattern) and other regions 1754 (e.g., two or more rows) containing a second feature set (e.g., a second pattern). The SEM target 1770 may include regions 1754 having various patterns, such as two-dimensional patterns, one-dimensional patterns, patterns with different feature sizes, patterns with various feature shapes (including different shapes of buried features and top features), etc. The SEM target 1770 may include regions 1754 that are substantially dedicated to measuring setup-acquisition overlay relationships.

[0123] Multiple regions 1754, when taken together, may comprise an overlay target. An overlay target may comprise features that may be used to measure a set-get overlay relationship. Features comprising an overlay target may be substantially similar features, but differ in overlay (e.g., set overlay), such as within a certain range. For example, a row of regions 1754 may comprise an overlay target having overlay values ​​varying between -5nm and 5nm. Features of an overlay target may comprise features having similar burial depths, recess depths, CDs, pitches, and the like. SEM target 1770 may comprise multiple overlay targets that are substantially similar to one another, but have different recess depths, such as a first overlay target having a negligible recess depth and a second overlay target having a non-negligible recess depth (such as recess depth 1744). Multiple set-get relationships may be determined based on multiple overlay targets.

[0124] Fig.15 An example graph depicting the set-acquire relationship between overlays acquired using a tilt angle calibration feature of a SEM target is shown. SEM images can be acquired from SE (secondary electron) detection (e.g., via the SE channel of the SEM) and BSE (backscattered electron) detection (e.g., via the BSE channel of the SEM), including simultaneous acquisitions. The SE channel image can be used to detect top layer pattern features, while the BSE channel image can be used to detect underlying pattern features (e.g., buried features). By combining information from the two images, the overlay between the top and bottom layer patterns can be derived. Fig.15 Points 1806 of an overlay target are depicted having set overlay values ​​(e.g., produced overlay) along x-axis 1802 and acquired overlay values ​​(e.g., measured overlay) along x-axis 1804. Each of points 1806 corresponds to an overlay measured for a beam tilt configuration, such as Fig. 12A , 12B , 13A, 13B and 14. Dashed line 1810 represents a linear fit to point 1806 and can represent a setup-acquire overlay relationship for a beam tilt structure. Although only one measurement value is depicted for each setup overlay value (e.g., the overlay produced), multiple measurements can be performed, including multiple measurements for each beam tilt structure or measurements for multiple beam tilt structures having the same setup overlay value. An intercept 1812 can be determined based on a linear fit. Fits other than linear fits can be used. The linear fit can be optimized using different fit optimizations, such as least mean squares, linear regression, and the like. The intercept 1812 may be related to the beam tilt angle and the recess depth, including via a geometric relationship. The value of the intercept 1812 can be related to the beam tilt angle and the depth using the following equation 1 or a similar equation:

[0125] Intercept = OV 真实 +d*tanθ t(1)

[0126] where intercept is the y-intercept of the beam tilt structure setup-acquisition overlay relationship, OV 真实 can be a true overlay (e.g., a fabrication overlay determined based on wafer fabrication rather than design layout), d is the depth of the beam tilt structure, and θ t is the beam tilt angle.

[0127] For example, based on Fig. 14B The beam tilt angle is determined by the SEM target structure. For the beam tilt calibration feature represented by the cross-sectional view 1700, whose programmed overlay values ​​range from -5nm, -4nm, ... to +5nm, the intercept can be expressed as given by the following equation 2:

[0128] Intercept 1 = OV 真实 +d1*tanθ t (2)

[0129] where intercept 1 is the intercept of the set-get overlay relationship determined based on the beam tilt calibration feature having programmed overlay values, and where d1 is the buried depth 1704. For the beam tilt calibration feature represented by cross-sectional view 1730, whose programmed overlay values ​​range from -5 nm, -4 nm, ... to +5 nm, the intercept can be expressed as given by Equation 3 below:

[0130] Intercept2 = OV 真实 +d2*tanθ t (3)

[0131] Wherein intercept2 is the intercept of the set-to-get relationship determined based on the beam tilt calibration feature having a programmed overlay value and a recess depth, and where d2 is the recess depth 1734 .

[0132] Given the above, the beam tilt angle can then be solved, such as by using Equation 4 below:

[0133]

[0134] Wherein the beam tilt angle can be determined based on the depth difference between the top feature and the buried feature when the top feature is not recessed and when the top feature is recessed, and the intercept difference of the set-get relationship based on the programmed overlay value of the corresponding beam tilt angle calibration feature. Using more than two beam tilt angle calibration feature sets (e.g., three or more beam tilt angle calibration feature sets with different recess depths) for beam tilt angle calibration can improve tilt angle calibration robustness, accuracy, etc. Using two beam tilt angle calibration feature sets (e.g., a beam tilt angle calibration feature set with negligible recess depth and a beam tilt angle calibration feature set with non-zero recess depth) can provide sufficient beam tilt angle calibration while minimizing the complexity and size of the manufacturing process of the beam tilt angle feature.

[0135] The beam tilt angle can be characterized by the beam tilt angle along multiple axes. Based on the above method, the beam tilt angle can be calibrated for each lateral direction. For example, the tilt angle along the x direction is You can use Settings-Get x-overlay to calibrate, and the tilt angle along the y direction The y-overlay relationship can be used for calibration. Based on the tilt angles in the x and y directions, the beam tilt angle azimuth direction can be calculated, such as by determining

[0136] By determining the intercept and depth, the beam tilt angle can be empirically solved because the setup-get overlay relationship of Equation 1 can be used to generate two or more equations and solve for θ t In this way, the real overlay (e.g. OV 真实 ) and can be ignored. Since true overlay may be difficult to obtain, determination of a measurable characteristic based on the tilt angle calibration feature is advantageous. In some embodiments, the depth d may be measured as accurately as possible, including by non-destructive techniques, including ellipsometers, reflectometers, and AFM (atomic force microscopes), to determine the beam tilt angle.

[0137] Fig.16 A method of SEM target generation is illustrated. Each of these operations is described in detail below. The operations of method 1900 presented below are intended to be illustrative. In some embodiments, method 1900 may be accomplished with one or more additional operations not described and / or without one or more operations discussed. Additionally, the operations of method 1900 may be performed in Fig.16The order illustrated in and described below is not intended to be limiting. In some embodiments, one or more portions of method 1900 may be implemented in one or more processing devices (e.g., one or more processors) (e.g., by simulation, modeling, etc.). The one or more processing devices may include one or more devices that perform some or all of the operations of method 1900 in response to instructions electronically stored on an electronic storage medium. For example, the one or more processing devices may include one or more devices configured by hardware, firmware, and / or software to be specifically designed to perform one or more operations of method 1900.

[0138] In operation 1902, fabrication specifications are obtained. The fabrication specifications may correspond to a design layout. The fabrication specifications may include feature depths. The fabrication specifications may include identification of key features. The fabrication specifications may include feature materials. The fabrication specifications may include SEM imaging parameters such as LE, magnification, etc.

[0139] In operation 1904, the burial depth is determined. The burial depth can be determined by manufacturing specifications based on the characteristic depth. The burial depth can be determined by non-destructive techniques, including ellipsometers, reflectometers, and AFM (atomic force microscope).

[0140] In operation 1906, a plurality of feature sizes, programming overlays, and / or shapes are determined. Feature sizes and programming overlays may be determined based on a manufacturing specification. Feature sizes may be determined based on a resolution limit of a manufacturing specification. Feature shapes may be determined based on feature shapes by a manufacturing specification.

[0141] In operation 1908, an alignment landing energy (LE) may optionally be determined. The beam tilt angle may be varied based on the LE even if all other SEM parameters remain constant. The alignment LE may be determined based on fabrication specifications, such as by selecting the LE most commonly used for overlay metrology.

[0142] In operation 1910, a SEM target design is generated based on the fabrication specifications. The SEM target may include regions containing features for measuring contrast and / or resolution, such as reference Figure 5A and 5B The SEM target may include an area containing features used to measure overlay accuracy, such as a reference Fig.9A and 9B The SEM target may include an area containing features for measuring beam tilt angles, such as reference Fig. 12A , 12B, 13A and 13B. The SEM target may include regions that include one or more overlay targets. The SEM target may include regions with different measurable structural features, such as recess depth, burial depth, pitch, etc. The SEM target may include various measurable structural features that may overlay a range of values, such as from the expected resolution limit (which may be less than CD) to the maximum fabricated feature size.

[0143] In operation 1912, a performance indicator may be determined based on one or more SEM images of the fabricated SEM target design. The performance indicator may be determined for one or more LEs, wherein a SEM image may be acquired for each of the one or more LEs. The performance indicator may be determined by comparing measured parameters of the SEM image (e.g., contrast, resolution, overlay accuracy, acquisition overlay, etc.) with modeled values ​​or other expected values ​​of the SEM image parameters. The performance indicator may be used to determine an indication of SEM performance. The performance indicator may be used to align a SEM system.

[0144] As described above, method 1900 (and / or other methods and systems described herein) is configured for SEM target generation.

[0145] Fig.17 is a diagram of an example computer system CS that can be used for one or more operations described herein. The computer system CS includes a bus BS or other communication mechanism for transmitting information and a processor PRO (or multiple processors) coupled to the bus BS for processing information. The computer system CS also includes a main memory MM, such as a random access memory (RAM) or other dynamic storage device, which is coupled to the bus BS for storing information and instructions to be executed by the processor PRO. The main memory MM can also be used to store temporary variables or other intermediate information during the execution of instructions by the processor PRO. The computer system CS also includes a read-only memory (ROM) ROM or other static storage device coupled to the bus BS for storing static information and instructions of the processor PRO. Storage devices SD such as magnetic disks or optical disks are provided and coupled to the bus BS for storing information and instructions.

[0146] The computer system CS can be coupled to a display DS via a bus BS, such as a cathode ray tube (CRT) or a flat panel or touch panel display, for displaying information to a computer user. An input device ID including alphanumeric keys and other keys is coupled to the bus BS for passing information and command selections to the processor PRO. Another type of user input device is a cursor control CC, such as a mouse, trackball or cursor direction keys, for passing direction information and command selections to the processor PRO and controlling cursor movement on the display DS. 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)), allowing the device to specify a position in a plane. A touch panel (screen) display can also be used as an input device.

[0147] In some embodiments, in response to one or more sequences of one or more instructions included in the processor PRO execution main memory MM, the part of one or more methods described herein can be performed by the computer system CS. Such instructions can be read from another computer readable medium (such as storage device SD) into the main memory MM. The instruction sequence included in the execution main memory MM causes the processor PRO to perform the process steps (operations) described herein. One or more processors in a multi-processing arrangement can also be adopted to execute the instruction sequence included in the main memory MM. In some embodiments, hard-wired circuitry can be used instead of software instructions or used in combination with software instructions. Therefore, the description herein is not limited to any specific combination of hardware circuitry and software.

[0148] The terms "computer-readable medium" and / or "machine-readable medium" used herein refer to any medium that participates in providing instructions to the processor PRO for execution. Such a medium can take 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 SD. Volatile media include dynamic memory, such as main memory MM. Transmission media include coaxial cables, copper wires and optical fibers, including wires that constitute bus BS. 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. Computer-readable media can be non-transient, such as floppy disks, foldable disks, hard disks, tapes, any other magnetic media, CD-ROMs, DVDs, any other optical media, punch cards, paper tapes, any other physical media with hole patterns, RAM, PROMs and EPROMs, flash EPROMs, any other memory chips or memory cartridges. Non-transient computer-readable media can have instructions recorded thereon. When executed by a computer, instructions can implement any operation described herein. For example, transient computer-readable media can include carrier waves or other propagating electromagnetic signals.

[0149] Various forms of computer-readable media may be involved in carrying one or more sequences of one or more instructions to the processor PRO for execution. For example, the instructions may initially be carried on a disk of a remote computer. The remote computer may load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. The modem local to the computer system CS may receive data on the telephone line and convert the data into an infrared signal using an infrared transmitter. An infrared detector coupled to the bus BS may receive the data carried in the infrared signal and place the data on the bus BS. The bus BS carries the data to the main memory MM, from which the processor PRO retrieves and executes the instructions. Before or after execution by the processor PRO, the instructions received by the main memory MM may optionally be stored on a storage device SD.

[0150] The computer system CS may also include a communication interface CI coupled to the bus BS. The communication interface CI provides a bidirectional data communication coupling with a network link NDL connected to the local network LAN. For example, the communication interface CI may 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 CI may be a local area network (LAN) card to provide a data communication connection with a compatible LAN. A wireless link may also be implemented. In any such implementation, the communication interface CI sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information.

[0151] The network link NDL typically provides data communication to other data devices through one or more networks. For example, the network link NDL can provide a connection to the host computer HC through a local network LAN. This can include data communication services provided through the global packet data communication network (now generally called the "Internet" INT). The local network LAN (Internet) can use electrical signals, electromagnetic signals or optical signals that carry digital data streams. The signals through the various networks and the signals on the network data link NDL and through the communication interface CI (carrying digital data to and from the computer system CS) are exemplary forms of carrier waves that transport information.

[0152] The computer system CS can send messages and receive data, including program code, through (multiple) networks, network data links NDL and communication interfaces CI. In the Internet example, the host computer HC can send the request code of the application program through the Internet INT, the network data link NDL, the local network LAN and the communication interface CI. For example, one such download application can provide all or part of the method described herein. The received code can be executed by the processor PRO when it is received, and / or stored in the storage device SD or other non-volatile storage device for later execution. In this way, the computer system CS can obtain the application code in the form of a carrier wave.

[0153] The embodiments of the present disclosure can be further described by the following clauses.

[0154] 1. A method for evaluating a scanning electron microscope (SEM) system, comprising:

[0155] Access SEM images of two or more sets of overlaid targets,

[0156] Each of the overlay target sets includes a buried feature and a top feature, the buried feature is at a buried depth,

[0157] wherein, in at least one of the two or more sets of overlay targets, the top feature is recessed, each of the recesses having a corresponding recess depth,

[0158] wherein top features of two or more sets of overlaid targets have different recess depths; and

[0159] The beam tilt angle of the SEM system is determined based on SEM images of two or more sets of overlaid targets.

[0160] 2. The method of clause 1, wherein determining the beam tilt angle comprises determining the beam tilt angle based on an overlay setup-acquire relationship determined for two or more sets of overlay targets.

[0161] 3. The method of clause 2, wherein determining the beam tilt angle based on the set-get relationship of the overlay comprises:

[0162] obtaining a measurement of at least one of a depth of a depression, a difference between depths of the depressions, or a combination thereof;

[0163] determining an intercept value based on fitting of set-acquire relationships for overlays of two or more sets of overlay targets; and

[0164] The beam tilt angle is determined based on the intercept value and a measurement of at least one of the notch depth, the difference between the notch depths, or a combination thereof.

[0165] 4. A method according to item 1, wherein accessing the SEM image also includes accessing additional SEM images captured at different landing energies, and wherein determining the beam tilt angle also includes determining the beam tilt angle corresponding to the different landing energies based on the SEM images captured at the different landing energies.

[0166] 5. The method of clause 4, wherein determining the beam tilt angles corresponding to different landing energies comprises determining the beam tilt angles based on a setup-acquire relationship for overlay determined for two or more sets of overlay targets at different landing energies.

[0167] 6. The method of clause 5, wherein determining the beam tilt angle based on a set-to-acquire relationship for overlay determined for two or more sets of overlay targets at different landing energies comprises:

[0168] obtaining a measurement of at least one of a depth of a depression, a difference between depths of the depressions, or a combination thereof;

[0169] Determining intercept values ​​for different landing energies based on fitting of set-acquire relationships for overlays of two or more sets of overlay targets at different landing energies; and

[0170] Beam tilt angles corresponding to the different landing energies are determined based on the intercept values ​​for the different landing energies and the measured values ​​of at least one of the recess depths, the differences between the recess depths, or a combination thereof.

[0171] 7. The method of clause 4, further comprising adjusting the SEM alignment based on at least one of the beam tilt angles corresponding to different landing energies.

[0172] 8. The method of clause 1, further comprising adjusting the SEM alignment based on the beam tilt angle.

[0173] 9. The method of clause 1, wherein the SEM system comprises a secondary electron detector.

[0174] 10. A method according to clause 1, wherein the SEM image is generated by detecting secondary electrons.

[0175] 11. A method according to clause 1, wherein the SEM system comprises a backscattered electron detector.

[0176] 12. A method according to clause 1, wherein the SEM image is generated by detecting backscattered electrons.

[0177] 13. A method according to clause 1, wherein the SEM image is generated by detecting secondary electrons and backscattered electrons.

[0178] 14. A method according to clause 1, wherein the two or more sets of overlay targets are adjacent to each other.

[0179] 15. A method for evaluating a scanning electron microscope (SEM) system, comprising:

[0180] Access SEM images of multiple cells containing various patterns,

[0181] wherein each unit within the plurality of units comprises a pattern having a corresponding pitch,

[0182] wherein the pattern includes buried features at a buried depth, and

[0183] wherein the SEM image includes images of multiple cells within a single field of view (FOV); and

[0184] For at least one cell within a single FOV, a relationship between its pitch and at least one of contrast, resolution, or a combination thereof is determined.

[0185] 16. The method according to item 15 also includes comparing the relationship between the pitch and contrast, resolution or a combination thereof of at least one of the determined cells within the single FOV with a simulation of the relationship between the pitch and contrast, resolution or a combination thereof to obtain information about the alignment of the SEM.

[0186] 17. A method according to item 15, wherein determining the relationship between pitch and at least one of contrast, resolution, or a combination thereof includes determining at least one of contrast, resolution, or a combination thereof for multiple of a plurality of cells, wherein the corresponding pitch varies between multiple of the multiple cells.

[0187] 18. The method of clause 15, further comprising determining a performance indicator of SEM beam quality based on a relationship between pitch and at least one of contrast, resolution, or a combination thereof.

[0188] 19. A method according to item 15, wherein accessing SEM images includes accessing SEM images captured at different landing energies, and wherein determining the relationship between pitch and at least one of contrast, resolution, or a combination thereof includes determining the relationship between pitch and at least one of contrast, resolution, or a combination thereof for different landing energies.

[0189] 20. The method of clause 19, further comprising determining a performance indicator of SEM beam quality based on a relationship between pitch and at least one of contrast, resolution, or a combination thereof for different landing energies.

[0190] 21. The method of clause 18 or 20, further comprising adjusting an alignment of the SEM based on at least one SEM performance indicator.

[0191] 22. The method of clause 19, wherein the corresponding pitch of each cell is selected based on simulations such that a relationship between pitch and at least one of contrast, resolution, or a combination thereof varies between different landing energies.

[0192] 23. A method according to clause 15, wherein determining the contrast comprises determining the ratio between the intensity change and the number of pixels.

[0193] 24. A method for evaluating a scanning electron microscope (SEM) system, comprising:

[0194] Access SEM images of multiple cells containing various patterns,

[0195] wherein each unit within the plurality of units comprises a pattern having a corresponding pitch,

[0196] wherein the pattern includes buried features and top features separated by buried depths,

[0197] The buried feature and the top feature are separated by an overlay offset in a direction perpendicular to the buried depth.

[0198] wherein the corresponding pitch varies between at least some of the plurality of cells, and

[0199] wherein the SEM image includes images of multiple cells within a single field of view (FOV); and

[0200] The relationship between pitch and overlay accuracy was determined based on SEM images of multiple cells with corresponding pitches.

[0201] 25. The method of clause 24, further comprising comparing the relationship between pitch and overlay accuracy with a simulation of the relationship between pitch and overlay accuracy.

[0202] 26. The method of clause 24, further comprising determining a performance indicator of SEM beam quality based on a relationship between pitch and overlay accuracy.

[0203] 27. The method of clause 24, wherein accessing the SEM images comprises accessing SEM images captured at different landing energies, and wherein determining the relationship between pitch and overlay accuracy comprises determining the relationship between pitch and overlay accuracy for different landing energies.

[0204] 28. The method of clause 27, further comprising determining a performance indicator of SEM beam quality based on a relationship between pitch and overlay accuracy for different landing energies.

[0205] 29. The method of clause 26 or 28, further comprising adjusting an alignment of the SEM based on the performance indicator.

[0206] 30. The method of clause 27, wherein the corresponding pitch of each cell is selected based on simulations such that the relationship between pitch and overlay accuracy varies between landing energies.

[0207] 31. A method according to clause 15 or 24, wherein adjacent cells comprise patterns having different corresponding pitches.

[0208] 32. One or more non-transitory machine-readable media having instructions thereon that, when executed by a processor, are configured to perform the method of any of clauses 1 to 31.

[0209] 33. A scanning electron microscope (SEM) target for evaluating a SEM system, comprising:

[0210] A plurality of regions on a wafer, each region containing a buried feature having a measurable structural characteristic,

[0211] The buried feature is buried under the top feature, and the buried feature has a corresponding buried depth.

[0212] wherein at least some of the top features are recessed, the top features having corresponding recess depths,

[0213] wherein the first area set includes a first set of engraved objects, and the second area set includes a second set of engraved objects,

[0214] The first set of engraved targets has a different depth of depression than the second set of engraved targets, and

[0215] The first set of engraved targets and the second set of engraved targets are adjacent to each other.

[0216] 34. The SEM target of clause 33, wherein the top features and the buried features have substantially similar sizes and substantially similar shapes.

[0217] 35. The SEM target of clause 33, wherein recessed sections of the top features have substantially similar side wall angles (SWA).

[0218] 36. The SEM target of clause 33, wherein the recess depths of top features in a given area are substantially similar.

[0219] 37. The SEM target of clause 33, wherein the first set of regions comprising the first set of engraved targets and the second set of regions comprising the second set of engraved targets respectively comprise sets of regions where top features have different lateral offsets relative to buried features in a direction perpendicular to the recess depth.

[0220] 38. The SEM target of clause 37, wherein the different lateral offsets of the first set of regions comprising the first set of engraved targets and the different lateral offsets of the second set of regions comprising the second set of engraved targets are substantially similar.

[0221] 39. A SEM target according to clause 33, wherein a set-acquire relationship for overlay can be determined for a first set of overlay targets and a second set of overlay targets.

[0222] 40. The SEM target of clause 33, wherein an intercept of the set-acquire relationship of the overlay can be determined for the first set of overlay targets and the second set of overlay targets.

[0223] 41. The SEM target of clause 33, wherein top features of the first set of engraved targets are not recessed.

[0224] 42. The SEM target of clause 41, wherein the first set of engraved targets has a recess depth of substantially zero.

[0225] 43. The SEM target of clause 41, wherein the depth of the recesses of the second set of engraved targets is measurable by non-destructive measurement techniques.

[0226] 44. The SEM target of clause 41, wherein the recess depth of the second set of engraved targets is greater than one tenth of the buried depth of the buried features.

[0227] 45. The SEM target of clause 41, wherein the second set of engraved targets has a recess depth greater than 10 nanometers.

[0228] 46. ​​The SEM target of clause 41, wherein the recess depth of the second set of engraved targets is substantially equal to the buried depth of the buried features.

[0229] 47. The SEM target of clause 41, wherein the second set of engraved targets has a recess depth greater than 200 nanometers.

[0230] 48. A SEM target according to clause 33, wherein the burial depth corresponds to a depth to which the SEM is aligned.

[0231] 49. The SEM target of clause 48, wherein the burial depth corresponds to a depth for SEM alignment based on backscattered electrons.

[0232] 50. The SEM target of clause 33, wherein at least some of the recess depths are made by at least partially etching areas of the wafer corresponding to the top features.

[0233] 51. The SEM target of clause 33, wherein at least some of the recess depths are made by partially filling recessed areas of the wafer corresponding to top features.

[0234] 52. A method of making a SEM target of any one of clauses 33 to 51.

[0235] 53. One or more non-transitory machine-readable media having instructions thereon that, when executed by a processor, are configured to fabricate the SEM target of any of items 33 to 51.

[0236] Although the concepts disclosed herein may be used for manufacturing with substrates such as silicon wafers, it should be understood that the disclosed concepts may be used with any type of manufacturing system, such as those used for manufacturing on substrates other than silicon wafers.

[0237] In addition, combinations and sub-combinations of disclosed elements may include separate embodiments.For example, one or more of the operations described above may be included in separate embodiments, or they may be included together in the same embodiment.

[0238] The above description is intended to be illustrative, rather than limiting. It will therefore be apparent to those skilled in the art that modifications may be made as described without departing from the scope of the claims set forth below.

Claims

1. One or more non-transitory machine-readable media having instructions thereon, which when executed by at least one processor cause the at least one processor to perform a method of evaluating a scanning electron microscope (SEM) system, the method comprising: Access SEM images of two or more sets of overlaid targets, Each of the overlay target sets includes buried features and top features, wherein the buried features are at a buried depth, wherein in at least one of the two or more sets of overlay targets, the top feature is recessed, each of the recesses having a corresponding recess depth, wherein the recess depths of the top features of the two or more sets of overlay targets are different; as well as A beam tilt angle of a SEM system is determined based on the SEM images of the two or more sets of overlaid targets. 2 . The medium of claim 1 , wherein determining the beam tilt angle comprises determining the beam tilt angle based on a setup-to-acquire relationship for overlay determined for the two or more sets of overlay targets.

3. The medium of claim 2, wherein determining the beam tilt angle based on the setup-get relationship for overlay comprises: obtaining a measurement of at least one of the recess depths, a difference between the recess depths, or a combination thereof; determining an intercept value based on a fit of the set-acquire relationship for overlay for the two or more sets of overlay targets; as well as The beam tilt angle is determined based on the intercept value and the measured value of at least one of a notch depth, a difference between the notch depths, or a combination thereof.

4. The medium of claim 1 , wherein accessing the SEM image further comprises accessing additional SEM images captured at different landing energies, and wherein determining the beam tilt angle further comprises: Beam tilt angles corresponding to the different landing energies are determined based on the SEM images captured at the different landing energies.

5. The medium of claim 4, wherein determining the beam tilt angles corresponding to the different landing energies comprises: The beam tilt angle is determined based on a set-to-get relationship for overlay determined for the two or more sets of overlay targets at the different landing energies.

6. The medium of claim 5, wherein determining the beam tilt angle based on the set-up-acquire relationship for overlay determined for the two or more sets of overlay targets at the different landing energies comprises: obtaining a measurement of at least one of the recess depths, a difference between the recess depths, or a combination thereof; determining intercept values ​​for the different landing energies based on fitting the set-get relationships for overlay at the different landing energies for the two or more sets of overlay targets; as well as The beam tilt angles corresponding to the different landing energies are determined based on the intercept values ​​for the different landing energies and the measured values ​​of at least one of a depression depth, a difference between the depression depths, or a combination thereof.

7. The medium of claim 4, wherein the method further comprises adjusting SEM alignment based on at least one of the beam tilt angles corresponding to the different landing energies.

8. The medium of claim 1, wherein the method further comprises adjusting SEM alignment based on the beam tilt angle.

9. The medium of claim 1, wherein the SEM system comprises a secondary electron detector.

10. The medium of claim 1, wherein the SEM image is generated by detecting secondary electrons.

11. The medium of claim 1 , wherein the SEM system comprises a backscattered electron detector.

12. The medium of claim 1, wherein the SEM image is generated by detecting backscattered electrons.

13. The medium of claim 1, wherein the SEM image is generated by detecting secondary electrons and backscattered electrons.

14. The medium of claim 1, wherein the two or more sets of overlay targets are adjacent to each other.

Images