Method and system for overlay measurement using charged particle inspection device

Abstract

Systems and methods for measuring overlay of a sample under scans performed by a charged particle beam inspection device include obtaining a first detector signal in response to a first scan of a first target of the sample, and obtaining a second detector signal in response to a second scan of a second target of the sample; determining a first transformed signal and a second transformed signal by performing Fourier transform on the first detector signal and the second detector signal; and determining an overlay value of the sample based on the first transformed signal and the second transformed signal.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP application 22172200.2 filed on May 6, 2022, and the entire contents of which are incorporated herein by reference. Technical Field

[0003] The description herein relates to the field of image inspection devices, and more particularly to overlay measurements using charged particle inspection devices. Background Art

[0004] An image inspection device (e.g., a charged particle beam device or a light beam device) is capable of generating a two-dimensional (2D) image of a wafer substrate by detecting particles (e.g., photons, secondary electrons, backscattered electrons, image electrons, or other types of electrons) generated when a beam (e.g., a charged particle beam or a light beam) generated by a source associated with the inspection device strikes a surface of the wafer substrate. In the semiconductor industry, various image inspection devices are used for semiconductor wafers for various purposes, such as wafer processing (e.g., electron beam direct write lithography systems), process monitoring (e.g., critical dimension scanning electron microscopes (CD-SEMs)), wafer inspection (e.g., electron beam inspection systems), or defect analysis (e.g., defect review SEMs, or DR-SEMs, and focused ion beam systems, or FIBs).

[0005] In semiconductor manufacturing, integrated circuits may be manufactured as one or more stacked material layers (e.g., silicon, silicon dioxide, metal, etc.) on a wafer. Each layer of material may include a design pattern (referred to herein as a "pattern layer") for forming components of the integrated circuit (e.g., transistors, contacts, etc.). The manufacture of each layer involves transferring the pattern from a mask to the wafer surface through a photolithography process. The position of each pattern layer relative to the previous pattern layer (referred to herein as "alignment") may affect the characteristics or quality of the manufactured integrated circuit.

[0006] Overlay refers to the planar, vector offset, displacement or misalignment of a pattern layer relative to its adjacent pattern layer. For example, two intra-pattern reference points (e.g., center points) may be selected for two patterns in two adjacent pattern layers, respectively, and the overlay between the two adjacent pattern layers may refer to the planar, vector displacement between the two intra-pattern reference points. Large overlay may cause problems or failures in the manufactured integrated circuit. Therefore, high-precision overlay measurement plays an important role in reducing overlay. Summary of the invention

[0007] Embodiments of the present disclosure provide systems and methods for measuring the overlay of a sample scanned by a charged particle beam inspection device. In some embodiments, the system may include a charged particle beam inspection device configured to scan the sample, and a controller including a circuit system. The controller may be configured to obtain a first detector signal in response to a first scan of a first target of the sample and a second detector signal in response to a second scan of a second target of the sample, determine a first transform signal and a second transform signal by performing Fourier transform on the first detector signal and the second detector signal, and determine the overlay value of the sample based on the first transform signal and the second transform signal.

[0008] In some embodiments, a non-transitory computer-readable medium may store an instruction set that is executable by at least one processor of a device to cause the device to perform a method. The method may include obtaining a first detector signal responsive to a first scan of a first target of a sample and a second detector signal responsive to a second scan of a second target of the sample; determining a first transform signal and a second transform signal by performing a Fourier transform on the first detector signal and the second detector signal; and determining an overlay value of the sample based on the first transform signal and the second transform signal.

[0009] In some embodiments, a method for measuring overlay of a sample under scanning performed by a charged particle beam inspection device may include: obtaining a first detector signal in response to a first scan of a first target of the sample and a second detector signal in response to a second scan of a second target of the sample; determining a first transform signal and a second transform signal by performing Fourier transform on the first detector signal and the second detector signal; and determining the overlay value of the sample based on the first transform signal and the second transform signal.

[0010] In some embodiments, a system may include a charged particle beam inspection apparatus configured to scan a sample, and a controller including a circuit system. The controller may be configured to obtain a detector signal responsive to the scanning of a target of the sample, determine a first transform signal by performing a Fourier transform on the detector signal, determine a second transform signal by converting the first transform signal, and determine an overlay value of the sample based on the first transform signal, the second transform signal, a first predetermined amplitude value, and a second predetermined amplitude value.

[0011] In some embodiments, a non-transitory computer readable medium may store an instruction set executable by at least one processor of a device to cause the device to perform a method. The method may include obtaining a detector signal in response to a scan of a target of a sample scanned by a charged particle beam inspection device, determining a first transform signal by performing a Fourier transform on the detector signal, determining a second transform signal by converting the first transform signal, and determining an overlay value of the sample based on the first transform signal, the second transform signal, a first predetermined amplitude value, and a second predetermined amplitude value.

[0012] In some embodiments, a method for measuring overlay of a sample under scanning performed by a charged particle beam inspection device may include obtaining a detector signal in response to scanning of a target of the sample scanned by the charged particle beam inspection device, determining a first transform signal by performing a Fourier transform on the detector signal and determining a second transform signal by converting the first transform signal, and determining an overlay value of the sample based on the first transform signal, the second transform signal, a first predetermined amplitude value, and a second predetermined amplitude value. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 is a schematic diagram illustrating an example charged particle beam inspection (CPBI) system consistent with certain embodiments of the present disclosure.

[0014] Figure 2 is a schematic diagram showing an example charged particle beam tool consistent with some embodiments of the present disclosure, which may be Figure 1 An example of a portion of a charged particle beam inspection system.

[0015] Figure 3 is a schematic diagram illustrating an example measurement process of surface and sub-surface structures using a charged particle beam tool consistent with some embodiments of the present disclosure.

[0016] Figure 4 is a schematic diagram showing an example of a first target and a second target fabricated on a sample consistent with some embodiments of the present disclosure.

[0017] Figure 5 is a graph illustrating example visualizations of first and second detector signals consistent with some embodiments of the present disclosure.

[0018] Figure 6 is a schematic diagram showing an example target fabricated on a sample consistent with some embodiments of the present disclosure.

[0019] Figure 7 is a schematic diagram showing an example arrangement of targets fabricated on a sample consistent with some embodiments of the present disclosure.

[0020] Figure 8is a flow chart illustrating an example method of overlay measurement consistent with some embodiments of the present disclosure.

[0021] Fig. 9 is a flow chart illustrating another example method of overlay measurement consistent with some embodiments of the present disclosure. DETAILED DESCRIPTION

[0022] Reference will now be made in detail to example embodiments, examples of which are shown in the accompanying drawings. The following description refers to the accompanying drawings, wherein, unless otherwise specified, the same numbers in different drawings represent the same or similar elements. The implementations set forth in the following description of example embodiments do not represent all implementations consistent with the present disclosure. Instead, they are merely examples of devices and methods consistent with aspects related to the subject matter described in the appended claims. Without limiting the scope of the present disclosure, some embodiments may be described in the context of providing detection systems and detection methods in systems utilizing electron beams ("e-beams"). However, the present disclosure is not so limited. Other types of charged particle beams (e.g., including protons, ions, muons, or any other particles carrying a charge) may be similarly applied. In addition, systems and methods for detection may be used in other imaging systems, such as optical imaging, photon detection, x-ray detection, ion detection, and the like.

[0023] Electronic devices consist of circuits formed on a semiconductor material called a substrate. The semiconductor material may include, for example, silicon, gallium arsenide, indium phosphide, or silicon germanium. Many circuits can be formed together on the same piece of silicon and are called an integrated circuit, or IC. The size of these circuits has been greatly reduced so that more circuits can fit on a substrate. For example, the IC chip in a smartphone may be as small as a thumbnail, but may contain more than 2 billion transistors, each less than one thousandth the size of a human hair.

[0024] Manufacturing these ICs, which have extremely small structures or components, is a complex, time-consuming, and expensive process that typically involves hundreds of individual steps. An error in even one step can cause a defect in the finished IC, rendering it unusable. Therefore, one goal of the manufacturing process is to avoid such defects in order to maximize the number of functional ICs manufactured in the process; that is, to increase the overall yield of the process.

[0025] One element of improving yield is monitoring the chip manufacturing process to ensure that a sufficient number of functional integrated circuits are produced. One way to monitor the process is to inspect the chip circuit structures at various stages of their formation. Inspection can be performed using a scanning charged particle microscope ("SCPM"). For example, the scanning charged particle microscope can be a scanning electron microscope (SEM). The scanning charged particle microscope can be used to image these extremely small structures, in effect taking a "photograph" of the wafer structure. The image can be used to determine if the structure is formed correctly in the correct location. If the structure is defective, the process can be adjusted to reduce the likelihood that the defect will occur again.

[0026] The working principle of a scanning charged particle microscope (e.g., SEM) is similar to that of a camera. A camera takes a picture by receiving and recording the intensity of light reflected or emitted from a person or object. A scanning charged particle microscope takes a picture by receiving and recording the energy or number of charged particles (e.g., electrons) reflected or emitted from a wafer structure. Typically, the structure is made on a substrate (e.g., a silicon substrate) that is placed on a platform called a stage for imaging. Before taking such a "picture", a charged particle beam can be projected onto the structure, and when the charged particles are reflected or emitted ("leave") from the structure (e.g., from the wafer surface, from the structure below the wafer surface, or both), the detector of the scanning charged particle microscope can receive and record the energy or number of these charged particles to generate an inspection image. To take such a "picture", the charged particle beam can scan the wafer (e.g., in a line-by-line or zigzag manner), and the detector can receive the leaving charged particles from the area (called the "beam spot") under the projection of the charged particle beam. The detector can receive and record the exiting charged particles from each beam spot at a time, and merge the recorded information of all beam spots to generate an inspection image. Some scanning charged particle microscopes use a single charged particle beam (called a "single-beam scanning charged particle microscope", such as a single-beam SEM) to take a single "photo" to generate an inspection image, while some scanning charged particle microscopes use multiple charged particle beams (called a "multi-beam scanning charged particle microscope", such as a multi-beam SEM) to take multiple "sub-photos" of the wafer in parallel and stitch them together to generate an inspection image. By using multiple charged particle beams, the SEM can provide more charged particle beams to the structure to obtain these multiple "sub-photos", so that more charged particles are ejected from the structure. Therefore, the detector can receive more exiting charged particles at the same time, generating inspection images of the wafer structure with higher efficiency and faster speed.

[0027] In order to control the quality of the manufactured semiconductor structure, various overlay measurement techniques can be used. Generally, optical tools can be used to measure overlay. For example, a broadband beam can be irradiated onto the surface of a sample. The surface can include a specially designed and manufactured structure (also referred to as a "target" herein). The target can include a first layer (e.g., a top layer) and a second layer (e.g., a bottom layer) below the first pattern layer. Optical scattering measurement tools can be used to measure the reflection or diffraction of the broadband light reflected by the target. The reflection or diffraction can have various characteristics, such as different wavelengths, polarizations, angles of incidence, phases, or other optical characteristics, from which unknown properties of the sample (e.g., overlay) can be determined.

[0028] For example, the overlay of a target can be determined based on a phase difference between diffraction of a first layer (e.g., a top layer) and a second layer (e.g., a layer below the first layer), each of which includes a specific structure (e.g., a grating). Overlay determined using such a target can be referred to as diffraction-based overlay ("DBO"). In order to measure diffraction-based overlay, a programmed offset can be used to manufacture the structure (e.g., a grating) in the first player and the second player. The programmed offset between the two layers herein can refer to a designed (known) plane vector displacement between the two layers. The programmed offset can be used to eliminate or reduce defects in optical scattering measurements.

[0029] There are several technical challenges with optical-based overlay measurement techniques. The first challenge is that as the target pitch (e.g., grating pitch) decreases and the spacing between adjacent pattern layers increases, the reflected or diffracted signal becomes weaker. "Pitch" in the present disclosure refers to the minimum center-to-center distance between interconnects in a manufactured integrated circuit, which can be used as an indicator of the integration level of the integrated circuit. The second challenge is that selecting the wavelength of a broadband light beam for optical-based overlay measurement techniques can be complex because each wavelength may produce different measurement results. The third challenge is that the measurement results of optical-based overlay measurement techniques may be sensitive to subtle tilts in the area between target lines (e.g., grating lines). These challenges may increase the uncertainty and inaccuracy of overlay measurements.

[0030] Embodiments of the present disclosure may provide methods, devices, and systems for non-optical overlay measurement. In some disclosed embodiments, a scanning charged particle microscope (e.g., SEM) may be used to perform overlay measurement using one or more targets. The scanning charged particle microscope may inject a charged particle beam (e.g., an electron beam) onto the surface of one or more targets, each target comprising a first layer (e.g., a top layer) and a second layer (e.g., below the first layer). Each of the first layer and the second layer may include a similar pattern (e.g., a grating with the same pitch and programming offset). The incident charged particle beam may interact with the pattern in the first layer and the pattern in the second layer to generate secondary electrons and backscattered electrons. The detector may detect the emitted secondary electrons and backscattered electrons to generate a signal. By analyzing the signal, the overlay between the first layer and the second layer may be determined. Compared with optical-based overlay measurement techniques, non-optical overlay measurement can reduce or eliminate the above challenges and can greatly improve the accuracy of overlay measurement.

[0031] For the sake of clarity, the relative sizes of the components in the figures may be exaggerated. In the following description of the figures, the same or similar reference numerals denote the same or similar components or entities, and only the differences with respect to the various embodiments are described.

[0032] As used herein, unless expressly stated otherwise, the term "or" encompasses all possible combinations unless not feasible. For example, if a component is specified to include A or B, then unless expressly stated otherwise or not feasible, the component may include A, or B, or A and B. As a second example, if a component is specified to include A, B, or C, then unless expressly stated otherwise or not feasible, the component may include A, or B, or C, or A and B, or A and C, or B and C, or A and B and C.

[0033] Figure 1 An exemplary charged particle beam inspection (CPBI) system 100 consistent with some embodiments of the present disclosure is shown. The CPBI system 100 can be used for imaging. For example, the CPBI system 100 can use an electron beam for imaging. Figure 1 As shown, the CPBI system 100 includes a main chamber 101, a load / lock chamber 102, a beam flow tool 104, and an equipment front end module (EFEM) 106. The beam flow tool 104 is located in the main chamber 101. The EFEM 106 includes a first loading port 106a and a second loading port 106b. The EFEM 106 may include additional loading ports. The first loading port 106a and the second loading port 106b receive a wafer front opening transport box (FOUP) containing a wafer (e.g., a semiconductor wafer or a wafer made of other materials) or a sample to be inspected (wafers and samples can be used interchangeably). A "batch" is a plurality of wafers that can be loaded and processed as a batch.

[0034] One or more robots (not shown) in the EFEM 106 can transport the wafer to the load / lock chamber 102. The load / lock chamber 102 is connected to a load / lock vacuum pump system (not shown), which removes gas molecules in the load / lock chamber 102 to reach a first pressure lower than atmospheric pressure. After reaching the first pressure, one or more robots (not shown) can transport the wafer from the load / lock chamber 102 to the main chamber 101. The main chamber 101 is connected to a main chamber vacuum pump system (not shown), which removes gas molecules in the main chamber 101 to reach a second pressure lower than the first pressure. After reaching the second pressure, the wafer will be inspected by the beam tool 104. The beam tool 104 can be a single beam system or a multi-beam system.

[0035] The controller 109 is electrically connected to the beam tool 104. The controller 109 may be a computer that can perform various controls of the CPBI system 100. Figure 1 The controller 109 is shown in FIG. 1 as being external to the structure including the main chamber 101 , the load / lock chamber 102 , and the EFEM 106 , but it will be appreciated that the controller 109 may be part of the structure.

[0036] In some embodiments, the controller 109 may include one or more processors (not shown). The processor may be a general or special electronic device capable of manipulating or processing information. For example, the processor may include any number of any combination of the following: a central processing unit (or "CPU"), a graphics processing unit (or "GPU"), an optical processor, a programmable logic controller, a microcontroller, a microprocessor, a digital signal processor, an intellectual property (IP) core, a programmable logic array (PLA), a programmable array logic (PAL), a general array logic (GAL), a complex programmable logic device (CPLD), a field programmable gate array (FPGA), a system on a chip (SoC), an application-specific integrated circuit (ASIC), and any type of circuit capable of data processing. The processor may also be a virtual processor, which includes one or more processors distributed on multiple machines or devices coupled via a network.

[0037] In some embodiments, the controller 109 may also include one or more memories (not shown). The memory may be a general or specific electronic device capable of storing processor-accessible code and data (e.g., via a bus). For example, the memory may include any number of any combination of the following: random access memory (RAM), read-only memory (ROM), optical disk, magnetic disk, hard disk, solid-state drive, flash drive, secure digital (SD) card, memory stick, compact flash (CF) card, or any type of storage device. The code may include an operating system (OS) and one or more applications (or "apps") for a specific task. The memory may also be a virtual memory, which includes one or more memories distributed on multiple machines or devices coupled via a network.

[0038] Figure 2 An example imaging system 200 is shown in accordance with an embodiment of the disclosure. Figure 2 The beam tool 104 can be configured for use with the CPBI system 100. The beam tool 104 can be a single beam device or a multi-beam device. Figure 2 As shown, the beam tool 104 includes a motorized sample stage 201, and a wafer holder 202 supported by the motorized sample stage 201 for holding a wafer 203 to be inspected. The beam tool 104 also includes an objective lens assembly 204, a charged particle detector 206 (which includes charged particle sensor surfaces 206a and 206b), an objective lens aperture 208, a converging lens 210, a beam limiting aperture 212, a gun aperture 214, an anode 216, and a cathode 218. In some embodiments, the objective lens assembly 204 may include a modified swinging objective delayed immersion lens (SORIL), which includes a pole piece 204a, a control electrode 204b, a deflector 204c, and an excitation coil 204d. The beam tool 104 may also include an energy dispersive X-ray spectrometer (EDS) detector (not shown) to characterize the material on the wafer 203.

[0039] A primary charged particle beam 220 (or simply "primary beam 220"), such as an electron beam, is emitted from the cathode 218 by applying an accelerating voltage between the anode 216 and the cathode 218. The primary beam 220 passes through the gun aperture 214 and the beam limiting aperture 212, both of which can determine the size of the charged particle beam entering the converging lens 210, which is located below the beam limiting aperture 212. The converging lens 210 focuses the primary beam 220 before the beam enters the objective lens aperture 208 to set the size of the charged particle beam before entering the objective lens assembly 204. The deflector 204c deflects the primary 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 beam 220 to different locations on the top surface of the wafer 203 at different time points in sequence to provide data for image reconstruction of different parts of the wafer 203. In addition, the deflector 204c can also be controlled to deflect the primary beam 220 to different sides of the wafer 203 at a specific position at different time points to provide data for stereoscopic image reconstruction of the wafer structure at that position. In addition, in some embodiments, the anode 216 and the cathode 218 can generate multiple primary beams 220, and the beam tool 104 can include multiple deflectors 204c to simultaneously project multiple primary beams 220 to different parts / sides of the wafer to provide data for image reconstruction of different parts of the wafer 203.

[0040] 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 beam 220 may be immersed in the magnetic field and may be charged, thereby generating an electric field. The electric field reduces the energy of the primary beam 220 that strikes near the surface of the wafer 203 before the primary beam 220 collides with the wafer 203. The control electrode 204b is electrically isolated from the pole piece 204a and controls the electric field on the wafer 203 to prevent the wafer 203 from slightly doming and to ensure proper beam focusing.

[0041] After receiving the primary beam 220, a secondary charged particle beam 222 (or "secondary beam 222"), such as a secondary electron beam, may be emitted from a portion of the wafer 203. The secondary beam 222 may form a beam spot on the sensor surfaces 206a and 206b of the charged particle detector 206. The charged particle 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 beam 222 and the resulting beam spot may vary depending on the external or internal structure of the wafer 203. In addition, as described above, the primary beam 220 may be projected onto different locations on the top surface of the wafer or different sides of the wafer at a specific location to generate secondary beams 222 (and resulting beam spots) of different intensities. Therefore, by mapping the intensity of the beam spot with the location of the wafer 203, the processing system can reconstruct an image reflecting the internal or surface structure of the wafer 203.

[0042] The imaging system 200 can be used to inspect a wafer 203 on a motorized sample stage 201 and includes a beam tool 104, as described above. The imaging system 200 can also include an image processing system 250, which includes an image acquisition device 260, a memory 270, and a controller 109. 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 type of mobile computing device, etc., or a combination thereof. The image acquisition device 260 can be connected to the detector 206 of the beam tool 104 through a medium such as an electrical conductor, a fiber optic cable, a portable storage medium, IR, Bluetooth, the Internet, a wireless network, radio, or a combination thereof. The image acquisition device 260 can receive a signal from the detector 206 and can construct an image. Therefore, the image acquisition device 260 can acquire an image of the wafer 203. The image acquisition device 260 can also perform various post-processing functions, such as generating a contour, superimposing an indicator on the acquired image, etc. The image acquisition device 260 can perform brightness and contrast adjustments on the acquired image, etc. The memory 270 may be a storage medium such as a hard disk, cloud storage, random access memory (RAM), other types of computer-readable memory, etc. The memory 270 may be coupled to the image acquirer 260 and may be used to save the scanned raw image data as a raw image, a post-processed image, or other auxiliary processed images. The image acquirer 260 and the memory 270 may be connected to the controller 109. In some embodiments, the image acquirer 260, the memory 270, and the controller 109 may be integrated together as a control unit.

[0043] In some embodiments, the image acquirer 260 may acquire one or more images of the sample based on the imaging signal received from the detector 206. The imaging signal may correspond to a scanning operation for performing charged particle imaging. The acquired image may be a single image including multiple imaging regions. The single image may be stored in the memory 270. The single image may be an original image that may be divided into multiple regions. Each region may include an imaging region that includes a feature of the wafer 203.

[0044] Figure 3 is a schematic diagram illustrating an example measurement process of surface and subsurface structure using a charged particle beam tool (e.g., a scanning charged particle microscope) consistent with some embodiments of the present disclosure. A scanning charged particle microscope (“SCPM”) generates a primary charged particle beam (e.g., Figure 2 The primary charged particle beam 220 in the embodiment of the present invention can be, for example, a primary charged particle beam. Figure 3 In the process, electrons of the primary electron beam 302 are projected onto the surface of a sample 304. The sample 304 may be any material, such as a non-conductive resist, a silicon dioxide layer, a metal layer, or any stacked combination of any dielectric or conductive material.

[0045] The electrons of the primary electron beam 302 may penetrate the surface of the sample 304 to a certain depth (e.g., from a few nanometers to a few microns) and interact with particles of the sample 304 in the interaction volume 306. Some of the electrons of the primary electron beam 302 may elastically interact with the particles in the interaction volume 306 (e.g., in the form of elastic scattering or collisions) and may be reflected or rebounded from the surface of the sample 304. The elastic interaction conserves the total kinetic energy of the interacting entities (e.g., the electrons of the primary electron beam 302 and the particles of the sample 304), wherein the kinetic energy of the interacting entities is not converted into other forms of energy (e.g., thermal energy, electromagnetic energy, etc.). Such reflected electrons generated by the elastic interaction may be referred to as backscattered electrons (BSEs), such as Figure 3 BSE 308 in the sample 304. Some electrons in the primary electron beam 302 may interact inelastically with particles in the interaction volume 306 (e.g., in the form of inelastic scattering or collisions). Inelastic interactions do not conserve the total kinetic energy of the interacting entities, wherein some or all of the kinetic energy of the interacting entities may be converted into other forms of energy. For example, through inelastic interactions, the kinetic energy of some electrons in the primary electron beam 302 may cause electron excitation and result in the generation of electrons that leave the surface of the sample 304, which may be referred to as secondary electrons (SEs), such as Figure 3 SE 310 in. Figure 3 As shown, some SE 310 (eg, SE with sufficient energy) may eventually leave the surface of sample 304 and reach the detector ( Figure 3304) and some SEs 310 (e.g., SEs with insufficient energy) may eventually leave and re-enter the surface of sample 304 (e.g., when the surface of sample 304 is positively charged). The BSE and SE yield or emission rate may depend on factors such as the electron energy of primary electron beam 302 and the material under examination. The electron energy of primary electron beam 302 may be determined in part by its accelerating voltage (e.g., Figure 2 The number of BSEs and SEs may be more or less (or even the same) than the injected electrons of the primary electron beam 302.

[0046] For example, the sample 304 may include a first layer (e.g., a resist layer above a wafer surface, Figure 3 ) and a second layer (e.g., a pattern layer below the wafer surface, Figure 3 308. The primary electron beam 302 may also penetrate the first layer to reach the particles in the second layer and interact with the particles in the second layer to generate BSE 308, and the BSE 308 generated at different positions of the targets in the second layer may reflect the geometric information of the targets in the second layer.

[0047] Consistent with some embodiments of the present disclosure, a computer-implemented method for measuring overlay of a sample under scanning performed by a charged particle beam inspection device may include obtaining a first detector signal in response to a first scan of a first target of the sample and a second detector signal in response to a second scan of a second target of the sample. As used herein, obtaining may refer to receiving, admitting, granting access, acquiring, retrieving, receiving, reading, accessing, collecting, or any operation for inputting data. In some embodiments, the charged particle beam inspection device may include a scanning electron microscope. The sample may include a wafer.

[0048] For example, the charged particle beam inspection device may be an imaging system (e.g., Figure 2 The sample may be a wafer (e.g., a wafer having a fabricated structure (e.g., a circuit) on its surface. Figure 2In some embodiments, the first target and the second target may be two specially designed and manufactured structures. For example, the first target and the second target may be independent of the manufacturing circuit on the wafer and have no functional relationship with the manufacturing circuit on the wafer. In some embodiments, the first target and the second target may be manufactured at one or more free spaces on the wafer that are not occupied by the manufacturing circuit. In some embodiments, the first target and the second target may be adjacent to each other. In some other embodiments, the first target and the second target may be separated from each other by other manufacturing structures on the sample. In some embodiments, the first target and the second target may be manufactured at a specific wafer.

[0049] The first detector signal and the second detector signal may be a detector of a charged particle inspection device (e.g., Figure 2 The detector 206 in the detector 206) outputs signals in response to the first scan and the second scan, respectively. In some embodiments, the first scan and the second scan can be the same scan. For example, the first target and the second target can be scanned by a single charged particle beam (e.g., a charged particle beam of a single beam inspection device) or a single charged particle beamlet (e.g., a charged particle beam of a multi-beam inspection device) in the same field of view. In some embodiments, the first scan and the second scan can be different scans. As an example, if the charged particle inspection device is a single beam inspection device (e.g., a single beam SEM), the first target can be scanned first and then the second target can be scanned. As another example, if the charged particle inspection device is a multi-beam inspection device (e.g., a multi-beam SEM), the first target and the second target can be scanned simultaneously by two different beamlets.

[0050] During scanning of the sample, when the primary beam (e.g. Figure 2 After charged particles (e.g., electrons) in the primary beam 220 hit the sample surface, secondary charged particles (e.g., Figure 3 SE 310 in ) or backscattered charged particles (e.g. Figure 3 and directed to a detector (e.g., Figure 2 In some embodiments, one of the secondary electrons or the backscattered electrons may be emitted from the first target and directed to the detector to generate a first detector signal, and one of the secondary electrons or the backscattered electrons may be emitted from the second target and directed to the detector to generate a second detector signal.

[0051] In some embodiments, the first detector signal and the second detector signal may be values ​​representing the sum or count of detection electrons emitted from the first target and the second target, respectively. In some embodiments, the first detector signal and the second detector signal may be values ​​representing the sum of charges of detection electrons emitted from the first target and the second target, respectively. In some embodiments, the first detector signal and the second detector signal may be visualized.

[0052] In some embodiments, the first target may include a first pattern layer and a second pattern layer located below the first pattern layer. The second target may include a third pattern layer and a fourth pattern layer located below the third pattern layer. The pitch value of the first pattern layer and the second pattern layer may be equal to the first pitch value of the first target. The pitch value of the third pattern layer and the fourth pattern layer may also be equal to the second pitch value of the second target. In some embodiments, the first pattern layer, the second pattern layer, the third pattern layer, and the fourth pattern layer may each include a grating.

[0053] For example, Figure 4 4 is a schematic diagram showing an example of a first target 402 and a second target 404 fabricated on a sample 400 consistent with some embodiments of the present disclosure. The sample 400 may be a silicon wafer substrate ( Figure 4 In some embodiments, the first target 402 and the second target 404 may be diffraction-based overlay targets. Figure 4 As shown, the first target 402 includes a first pattern layer 406 (indicated by the shaded area in the dotted box in the first target 402) and a second pattern layer 408 (indicated by the dashed area in the dotted box in the first target 402) located below the first pattern layer 406, and the second target 404 includes a third pattern layer 410 (indicated by the shaded area in the dotted box in the second target 404) and a fourth pattern layer 412 (indicated by the dashed area in the dotted box in the second target 404) located below the third pattern layer 410. Figure 4 In the embodiment, the first pattern layer 406, the second pattern layer 408, the third pattern layer 410 and the fourth pattern layer 412 may be of grating type (eg, line grating).

[0054] In some embodiments, the material of the first pattern layer 406 and the third pattern layer 410 may be polymethyl methacrylate (PMMA). Figure 4 As shown, the first pattern layer 406 and the third pattern layer 410 can be made on the PMMA layer 414 (indicated by the shaded area below the first pattern layer 406 and the third pattern layer 410) (for example, by coating, photolithography and etching processes). In some embodiments, the material of the second pattern layer 408 and the fourth pattern layer 412 can be copper. For example, Figure 4As shown, a second pattern layer 408 and a fourth pattern layer 412 may be fabricated (e.g., by coating, photolithography, and etching processes) on the sample 400. In some embodiments, a silicon dioxide layer 416 (represented by a white area) may separate the PMMA layer 414 from the second pattern layer 408, and may also separate the PMMA layer 414 from the fourth pattern layer 412. Figure 4 As shown, the first pattern layer 406 and the second pattern layer 408 are separated by a spacing distance d, and the third pattern layer 410 and the fourth pattern layer 412 are also separated by a spacing distance d.

[0055] exist Figure 4 In the figure, the first pattern layer 406, the second pattern layer 408, the third pattern layer 410, and the fourth pattern layer 412 may all have a pitch. For example, if the first pattern layer 406, the second pattern layer 408, the third pattern layer 410, and the fourth pattern layer 412 are of grating type, the pitch of any one of the first pattern layer 406, the second pattern layer 408, the third pattern layer 410, and the fourth pattern layer 412 may be represented by the distance between the centers of two adjacent lines of the grating (referred to as the “pitch value” herein).

[0056] In some embodiments, the pitch value of the first pattern layer 406 and the second pattern layer 408 may be equal to the first pitch value of the first target 402. The pitch value of the third pattern layer 410 and the fourth pattern layer 412 may also be equal to the second pitch value of the second target 404. In some embodiments, the first pitch value may be equal to the second pitch value. In some embodiments, the first pitch value may not be equal to the second pitch value. For example, Figure 4 As shown, each of the first pattern layer 406 and the second pattern layer 408 may have a first pitch value 418, and each of the third pattern layer 410 and the fourth pattern layer 412 may have a second pitch value 420. In some embodiments, the first pitch value 418 may be equal to the second pitch value 420.

[0057] In some embodiments, the first pattern layer may have a first offset relative to the second pattern layer, wherein the magnitude of the first offset may be equal to the overlay value (e.g., the size of the overlay of the sample) minus a predetermined offset value. The third pattern layer may have a second offset relative to the fourth pattern layer, wherein the magnitude of the second offset may be equal to the overlay value plus a predetermined offset value. The offset between two pattern layers used in this article refers to the horizontal distance between two corresponding structural portions on two adjacent pattern layers. For example, if the two corresponding structural portions are two corresponding grating lines, the offset between them may be the distance between the centers of the corresponding lines along the horizontal direction. In some embodiments, the offset may be expressed as a vector displacement having a magnitude and a direction.

[0058] For example, if Figure 4As shown, the first pattern layer 406 may have a first offset 422 (indicated by a left arrow between the centers of two corresponding grating lines) relative to the second pattern layer 408. The third pattern layer 410 may have a second offset 424 (indicated by a right arrow between the centers of two corresponding grating lines) relative to the fourth pattern layer 412. The first offset 422 and the second offset 424 may be represented as vector displacements having a magnitude and a direction. Figure 4 As shown, the first offset 422 and the second offset 424 may have opposite directions. Assuming that the horizontal direction to the right is Figure 4 In the figure, represents a positive direction, the first offset 422 may be a negative vector, and the second offset 320 may be a positive vector.

[0059] Figure 4 Each of the first offset 422 and the second offset 424 in can be determined based on two components. For example, assuming that the sample 400 has an overlay ( Figure 4 The overlay of the sample 400 may be represented as a vector having a magnitude (i.e., an overlay value) and a direction. Figure 4 As shown, assuming that the overlay is a positive vector (i.e., pointing to the right), the magnitude of the first offset 422 may be equal to the overlay value minus a predetermined offset value (e.g., a positive value), and the magnitude of the second offset 424 may be equal to the overlay value plus the predetermined offset value. The predetermined offset value may be a designed or programmed offset value. In an ideal case, if there are no manufacturing errors or inaccuracies, the overlay may be zero, wherein the first offset 422 and the second offset 424 may have the same magnitude (i.e., the predetermined offset value) and opposite directions.

[0060] It is worth noting that although Figure 4 The first offset 422 and the second offset 424 using the first pattern layer 406 and the third pattern layer 410 as static reference points are shown, but the present disclosure is not limited to preparing the first target 402 and the second target 404 in this way. For example, in order to prepare the first target 402 and the second target 404, the second pattern layer 408 and the fourth pattern layer 412 can be used as static reference points, wherein the first pattern layer 406 can be manufactured to be offset by a predetermined offset value relative to the second pattern layer 408 in a first direction, and the third pattern layer 410 can be manufactured to be offset by a predetermined offset value relative to the fourth pattern layer 412 in a second direction opposite to the first direction.

[0061] Figure 5 is a diagram 500 showing an example visualization of a first detector signal and a second detector signal consistent with some embodiments of the present disclosure. For example, Figure 5 The first detector signal in may be responsive to Figure 4 The first scan of the first target 402 is obtained, and Figure 5 The second detector signal in may be responsive to Figure 4 404 in the second target 404. In the graph 500, the horizontal axis may represent the distance (e.g., in pixels or nanometers), and the vertical axis may represent the magnitude of the first detector signal and the second detector signal. The first detector signal includes information corresponding to the electrons emitted from the first pattern layer 406 (e.g., amplitude and phase information) and information corresponding to the electrons emitted from the second pattern layer 408 (e.g., amplitude and phase information). The second detector signal includes information corresponding to the electrons emitted from the third pattern layer 410 (e.g., amplitude and phase information) and information corresponding to the electrons emitted from the fourth pattern layer 412 (e.g., amplitude and phase information). In some embodiments, an analysis may be performed based on the shapes of the first detector signal and the second detector signal to determine the overlay value described herein, which will be described below.

[0062] Consistent with some embodiments of the present disclosure, the computer-implemented method of measuring overlay may further include determining a first transformed signal and a second transformed signal by performing a Fourier transform on the first detector signal and the second detector signal. In some embodiments, a first period of the first transformed signal (e.g., a first sine sequence or a first cosine sequence) may correspond to a first pitch value of the first target. A second period of the second transformed signal (e.g., a second sine sequence or a second cosine sequence) may correspond to a second pitch value of the second target. For example, referring to Figure 4 , the first pitch value of the first target may be the first pitch value 418 , and the second pitch value of the second target may be the second pitch value 420 .

[0063] Consistent with some embodiments of the present disclosure, the computer-implemented method of measuring overlay may further include determining an overlay value of the sample based on the first transformed signal and the second transformed signal.

[0064] For example, the first detector signal and the second detector signal (eg, Figure 5 The first detector signal and the second detector signal in the detector 410 are Fourier transformed to determine the first transformation signal and the second transformation signal. If the first pitch value (e.g., the first pitch value 418) is equal to the second pitch value (e.g., the second pitch value 420), the first transformation signal S- and the second transformation signal S + It can be expressed by equation (1) and equation (2) respectively:

[0065] S - =a·sin(kx+θ a )+b·sin(kx+θ b- ) Equation (1)

[0066] S + =a·sin(kx+θ a )+b·sin(kx+θ b+ ) Equation (2)

[0067] In equation (1), a·sin(kx+θ a ) represents a sinusoidal sequence of a first signal corresponding to electrons emitted from the first pattern layer 406, where a represents the amplitude of the first signal, represents the period corresponding to the first pitch value (ie, equal to the second pitch value), θ a represents the phase term of the first signal. Similarly, in equation (1), b·sin(kx+θ b- ) represents a sinusoidal sequence of a second signal corresponding to electrons emitted from the second pattern layer 408, wherein b represents the amplitude of the second signal, represents the period corresponding to the second pitch value (ie, equal to the first pitch value), θ b- represents the phase term of the second signal. In equation (2), a·sin(kx+θ a ) represents a sinusoidal sequence of a third signal corresponding to electrons emitted from the third pattern layer 410, and b·sin(kx+θ b+ ) represents a sinusoidal sequence corresponding to a fourth signal of electrons emitted from the fourth pattern layer 412, wherein θ b+ represents the phase term of the fourth signal.

[0068] Phase term θ b- and θ b+ It can be expressed by equation (3) and equation (4) respectively:

[0069] θ b- =θ a +φ-ψ Equation (3)

[0070] θ b+ =θ a +φ+ψ Equation (4)

[0071] In equations (3)-(4), ψ represents the phase term contributed by the predetermined offset value described herein, and φ represents the phase term contributed by the overlay value. The phase value φ may represent the first transformed signal S + and the partial phase difference between the second transformed signal S-. Since the predetermined offset value is known, the value of ψ can be derived in equations (3)-(4).

[0072] Assume that S_ in equation (1) and S in equation (2) +can be equivalent to two signals in complex space, as shown in equations (5)-(6), and the difference signal S± can be determined as a differential signal (for example, by subtraction), as shown in equation (7):

[0073]

[0074] in

[0075]

[0076] CΔ=2·b·sinψEquation (10)

[0077] In equations (5)-(10), C - , C + and C Δ are called the first amplitude, the second amplitude and the third amplitude respectively. - and C + By Fourier transforming the first detector signal and the second detector signal (eg, Figure 5 The amplitudes of the first detector signal and the second detector signal in C are related, and the amplitudes of the first detector signal and the second detector signal are measurable, so C - It can be determined from equation (5) and the amplitude of the measured first detector signal, C + can be determined based on equation (6) and the measured amplitude of the second detector signal.

[0078] C Δ can be determined in different ways. For example, in determining C - and C + Afterwards, S ± can be determined analytically using equation (7), based on which C can also be determined analytically Δ As another example, the first detector signal and the second detector signal (eg, C Δ Figure 5 The difference detector signal can be determined by the difference between the first detector signal and the second detector signal in (e.g., by subtraction), and then a Fourier transform can be applied to the difference detector signal. In this case, C Δ The norm of the difference detector signal which is a Fourier transform may be determined.

[0079] Based on equations (5)-(10), equations (1)-(4) can be transformed into a quadratic function of tanφ expressed by equation (11), where φ is the only unknown variable:

[0080]

[0081] in

[0082]

[0083] In some embodiments, to determine the overlay value, the computer-implemented method may include determining a first amplitude value of the first transformed signal, a second amplitude value of the second transformed signal, and a third amplitude value associated with a difference between the first transformed signal and the second transformed signal. Then, a phase value representing a partial phase difference between the first transformed signal and the second transformed signal may be determined. Thereafter, an overlay value for the sample may be determined based on the phase value and the first pitch value (i.e., equal to the second pitch value).

[0084] For example, the first transformed signal and the second transformed signal may be S- and S- described in association with equations (1) and (2), respectively. + The first amplitude value, the second amplitude value, and the third amplitude value may be C described in association with equations (5)-(12) - , C + and C Δ The phase value may be the value of φ determined by solving equation (11). If the first pitch value (e.g., first pitch value 418) and the second pitch value (e.g., second pitch value 420) are equal and known (e.g., equal to the value of P), then the overlay value may be determined as

[0085] Combination Figure 4-Figure 5 The exemplary method described by equations (1)-(12) can manufacture the first target and the second target at one or more free spaces on the wafer that are not occupied by the fabricated circuits. When there is not enough space on the wafer to manufacture two targets, a single target can also be used to measure the overlay of the sample, which will be described below.

[0086] In accordance with some embodiments of the present disclosure, another computer-implemented method of measuring overlay of a sample scanned by a charged particle beam inspection device may include obtaining a detector signal responsive to a scan of a target of the sample. In some embodiments, the charged particle beam inspection device may include a scanning electron microscope. The sample may include a wafer.

[0087] For example, the charged particle beam inspection device may be an imaging system (e.g., Figure 2 The sample may be a wafer (e.g., a wafer having a fabricated structure (e.g., a circuit) on its surface. Figure 2 In some embodiments, the target may be a specially designed and manufactured structure. For example, the target may be independent of the circuits manufactured on the wafer and have no functional relationship with the circuits manufactured on the wafer. In some embodiments, the target may be manufactured at free space on the wafer that is not occupied by the manufactured circuits.

[0088] The detector signal may be a signal generated by a detector of a charged particle detection device (e.g. Figure 2 In some embodiments, the target may be scanned by a charged particle beam (e.g., a charged particle beam of a single beam inspection device) or a charged particle beamlet (e.g., a charged particle beamlet of a multi-beam inspection device). During scanning of the sample, the primary beam (e.g., Figure 2 After charged particles (e.g., electrons) in the primary beam 220 in the sample hit the surface of the sample, secondary charged particles (e.g., Figure 3 310) or backscattered charged particles (e.g., Figure 3 and directed to at least one of the BSEs 308 in FIG. Figure 2 In some embodiments, at least one of secondary electrons or backscattered electrons may be emitted from the target and directed to the detector to generate a detector signal.

[0089] In some embodiments, the detector signal may be a value representing a sum or count of detected electrons emitted from a target. In some embodiments, the detector signal may be a value representing a sum of charges of detected electrons emitted from a target. In some embodiments, the detector signal may be visualized.

[0090] In some embodiments, the target may include a first pattern layer and a second pattern layer located below the first pattern layer. The pitch value of the first pattern layer and the second pattern layer may be the same as the pitch value of the target. The first pattern layer may not have a predetermined offset relative to the second pattern layer. In some embodiments, each of the first pattern layer and the second pattern layer may include a grating.

[0091] For example, Figure 6 6 is a schematic diagram showing an example target 602 fabricated on a sample 600 consistent with some embodiments of the present disclosure. The sample 600 may be a silicon wafer substrate ( Figure 6 In some embodiments, target 602 may be a diffraction-based overlay target. Figure 6 As shown, the target 602 includes a first pattern layer 606 (indicated by the shaded area within the dotted box in the target 602) and a second pattern layer 608 (indicated by the dotted area within the dotted box in the target 602) located below the first pattern layer 606. Figure 6 In the embodiment, the first pattern layer 606 and the second pattern layer 608 may be of grating type (e.g., line grating). In some embodiments, the first pattern layer 606 may be made of polymethyl methacrylate (PMMA) material. Figure 6As shown, the first pattern layer 606 can be manufactured (e.g., by coating, photolithography, and etching processes) on the PMMA layer 614 (indicated by the shaded area below the first pattern layer 606 and the third pattern layer 610). In some embodiments, the second pattern layer 608 can be made of a copper material. For example, Figure 6 As shown, the second pattern layer 608 can be fabricated on the sample 600 (e.g., by coating, photolithography, and etching processes). In some embodiments, a silicon dioxide layer 616 (represented by the white area) can separate the PMMA layer 614 from the second pattern layer 608. Figure 6 As shown, the first pattern layer 606 and the second pattern layer 608 are separated by a spacing distance d.

[0092] exist Figure 6 In the embodiment, both the first pattern layer 606 and the second pattern layer 608 may have a pitch. For example, if the first pattern layer 606 and the second pattern layer 608 are of grating type, the pitch of the first pattern layer 606 and the second pattern layer 608 may be represented by the distance between the centers of two adjacent lines of the grating (referred to herein as the "pitch value"). In some embodiments, the pitch value of the first pattern layer 606 and the second pattern layer 608 may be equal to the pitch value of the target. For example, Figure 6 As shown, the first pattern layer 606 and the second pattern layer 608 may each have a pitch value 618 .

[0093] In some embodiments, the first pattern layer can have an offset relative to the second pattern layer, wherein the magnitude of the offset can be equal to the overlay value (e.g., the size of the overlay of the sample) plus or minus a predetermined offset value. In some embodiments, the offset can be represented as a vector displacement having a magnitude and a direction.

[0094] For example, if Figure 6 As shown, the first pattern layer 606 may have an offset 622 relative to the second pattern layer 608 (indicated by a left arrow between the centers of two corresponding grating lines, in FIG. Figure 6 The offset 622 may be represented as a vector displacement having a magnitude and a direction. Assume that the horizontal direction to the right is Figure 6 represents a positive direction, then the offset 622 can be a negative vector. Assuming that the sample 600 has an overlay ( Figure 6 The overlay of the sample 600 can be represented as a vector having a magnitude (i.e., an overlay value) and a direction. Figure 6As shown, assuming that the overlay is a positive vector (i.e., pointing to the right), the magnitude of the offset 622 may be equal to the overlay value plus or minus a predetermined offset value (e.g., a positive value). The predetermined offset value may be a designed or programmed offset value. In an ideal case, if there are no manufacturing errors or inaccuracies, the overlay may be zero, where the magnitude of the offset 622 may be equal to the predetermined offset value. Figure 6 , the predetermined offset value is zero, and the offset 622 represents the overlay of the sample.

[0095] Consistent with some embodiments of the present disclosure, the computer-implemented method of measuring overlay may further include determining a first transformed signal by performing a Fourier transform on the detector signal, and determining a second transformed signal by converting the first transformed signal. In some embodiments, a period of the first transformed signal (e.g., a first sine sequence or a first cosine sequence) and a period of the second transformed signal (e.g., a second sine sequence or a second cosine sequence) may correspond to a pitch value of the target. For example, referring to Figure 6 , the target pitch value may be pitch value 618 .

[0096] In accordance with some embodiments of the present disclosure, the computer-implemented method of measuring overlay may also include determining an overlay value of the sample based on the first transformed signal, the second transformed signal, the first predetermined amplitude value, and the second predetermined amplitude value. In some embodiments, the first predetermined amplitude value and the second predetermined amplitude value may be associated with two targets adjacent to the target.

[0097] For example, a Fourier transform may be performed on the detector signal to determine a first transformed signal S1 represented by equation (13):

[0098] S1=a·sin(x+θ a )+b·sin(x+θ b ) Equation (13)

[0099] In equation (13), a·sin(x+θ a ) represents a sinusoidal sequence of a first signal corresponding to electrons emitted from the first pattern layer 606, whose period corresponds to the pitch value (e.g., the pitch value 618), where a represents the amplitude of the first signal, θ a represents the phase term of the first signal. Similarly, in equation (13), b·sin(x+θ b ) represents a sinusoidal sequence of a second signal corresponding to electrons emitted from the second pattern layer 608, whose period corresponds to the pitch value (e.g., the pitch value 618), where b represents the amplitude of the second signal, θ b represents the phase term of the second signal.

[0100] For example, the first transformed signal S1 represented by equation (13) can be converted (for example, by a·sin(x+θa ) and b·sin(x+θ b ) are summed to generate a second transformed signal S2 represented by equation (14):

[0101]

[0102] In equation (14), c represents the third amplitude value (for example, representing the size of the sum signal), It represents the two waves a·sin(x+θ a ) and b·sin(x+θ b ) is the phase term contributed by the sum of the overlay value and the phase difference (θ a -θ b ). By making S1 and S2 equal, we can deduce the following conclusions:

[0103]

[0104] c 2 =a 2 +b 2 +2·a·b·cos(θ a -θ b ) Equation (16)

[0105]

[0106] In some embodiments, The value of can be determined based on the measurement of the detector signal, the Fourier transform of the detector signal is the first transformed signal S1. It should be noted that the first amplitude value a and the second amplitude value b cannot be solved by equations (13)-(17) themselves. In some embodiments, a and b in equations (16)-(17) can be replaced by a first predetermined amplitude value a' and a second predetermined amplitude value b', and both a' and b' can be solved according to equations (1)-(12).

[0107] For example, Figure 7 is a schematic diagram showing an example arrangement 700 of targets fabricated on a sample consistent with some embodiments of the present disclosure. Figure 7 As shown, the large white box indicates that in the sample (e.g., Figure 5 Samples 500 or Figure 6 A device (e.g., an integrated circuit) is fabricated on a sample 600 of FIG. 1 . In the present disclosure, the space between two adjacent large white boxes may be referred to as a scribe line. As an example, arrangement 700 depicts one horizontal scribe line and two vertical scribe lines. In the scribe lines, a first target and a second target pair may be fabricated. Figure 7 As an example, the first goal may be similar to Figure 4The first target 402 (having a first offset 422) is manufactured by the method of FIG. Figure 4 The first target in the arrangement 700 may be represented by a small white box, while the second target in the arrangement 700 may be represented by a small black box, such as Figure 7 as shown in the legend.

[0108] To determine the overlay value of the fabricated device shown in arrangement 700, a third target (or "in-device target") may be fabricated within the device, such as Figure 7 The third target can be manufactured without programming offset, similar to Figure 6 The overlay value of the device (e.g., represented by offset 622) can be determined using the example methods described in association with equations (13)-(17), where a, b, and (θ a -θ b ) is unknown. In this case, the example methods described in association with equations (1)-(12) may be used to generate first and second predetermined amplitude values ​​a′ and b′ for a pair of first and second targets adjacent to a third target.

[0109] For example, refer to Figure 7 , the example methods described in association with equations (13)-(17) may be applied to the third target 702 to generate the tanφ value represented in equation (17), where a, b, and (θ associated with the third target 702 a -θ b ) is unknown. However, the exemplary method described in association with equations (1)-(12) may be applied to a pair of first target 704 and second target 706, where a first predetermined amplitude value a′ (corresponding to the value of a in equations (1)-(12)) and a second predetermined amplitude value b′ (corresponding to the value of b in equations (1)-(12)) may be determined (e.g., derived from equations (8)-(9), where C - , C + , C Δ , ψ and φ are known).

[0110] The first predetermined amplitude value a′ and the second predetermined amplitude value b′ may be used to determine the unknown values ​​a and b associated with the third target 702, respectively. For example, the unknown values ​​a and b associated with the third target 702 may be determined by interpolating the first predetermined amplitude value a′ and the second predetermined amplitude value b′ of a plurality of pairs of first targets and second targets, including the pair of the first target 704 and the second target 706. As another example, the unknown values ​​a and b associated with the third target 702 may be determined as the first predetermined amplitude value a′ and the second predetermined amplitude value b′ determined from the pair of the first target 704 and the second target 706.

[0111] In addition, based on the amplitude of the detector signal corresponding to the third target 702, the amplitude (e.g., corresponding to c in equation (14)) of the Fourier transform signal of the detector signal (e.g., corresponding to S2 in equation (14)) can be determined. Referring to equation (16), for the third target 702, c has been determined, and a and b have been determined based on a′ and b′ as described herein, therefore, (θ) can also be determined. a -θ b The overlay value of target 702 (indicating the overlay value of the manufacturing device where target 702 is located) can be determined as Where P represents a known pitch value of the third target 702 (e.g., similar to Figure 6 Pitch value 618).

[0112] Alternatively, to determine the overlay value of target 702, after determining the values ​​of a, b, and c (e.g., in the same manner as Figure 7 In the manner described in relation to the above), equation (16) can be used to determine (θ a -θ b ). Next, θ may be determined based on the secondary electronic signal corresponding to target 702. a , where the secondary electronic signal may represent a value of (e.g., using edge detection techniques) from a first pattern layer of target 702 (e.g., similar to Figure 6 The secondary electrons emitted by the first pattern layer 606) have a significant peak value. a and After the value of , we can also determine θ from equation (17) b The value of (because θ b is the only remaining unknown parameter). Then, the overlay value of target 702 (representing the overlay value of the manufactured device where target 702 is located) can be determined as Where P represents a known pitch value of the third target 702 (e.g., similar to Figure 6 Pitch value 618).

[0113] For example, Figure 88 is a flow chart illustrating an example method 800 for overlay measurement consistent with some embodiments of the present disclosure. The method 800 may be performed by a controller that may be coupled to a charged particle beam inspection apparatus (eg, the charged particle beam inspection system 100). For example, the controller may be Figure 2 The controller 109 in FIG. 8 can be programmed to implement the method 800 .

[0114] In step 802, the controller may respond to a sample (e.g., Figure 4 The first target of the sample 400) (e.g., Figure 4 A first detector signal (eg, Figure 5 ), and in response to a second target of the sample (e.g., Figure 4 A second detector signal (eg, Figure 5 In some embodiments, the charged particle beam inspection apparatus may include a scanning electron microscope. For example, the sample may include a wafer.

[0115] In some embodiments, the first target may include a first pattern layer (eg, Figure 4 a first pattern layer 406) and a second pattern layer (eg, Figure 4 The second target may include a third pattern layer (e.g., Figure 4 A third pattern layer 410) and a fourth pattern layer located below the third pattern layer (eg, Figure 4 The pitch value of the first pattern layer and the second pattern layer may be equal to the first pitch value of the first target (eg, Figure 4 The pitch values ​​of the third pattern layer and the fourth pattern layer may also be equal to the second pitch value of the second target (e.g., Figure 4 In some embodiments, the first pattern layer, the second pattern layer, the third pattern layer, and the fourth pattern layer may each include a grating (eg, a line grating).

[0116] In some embodiments, the first pattern layer may have a first offset relative to the second pattern layer (eg, Figure 4 The first offset 422 of the first pattern layer may be equal to the overlay value (eg, the size of the overlay of the sample) minus the predetermined offset value. The third pattern layer may have a second offset relative to the fourth pattern layer (eg, Figure 4 A second offset 424) may be provided, wherein the magnitude of the second offset may be equal to the overlay value plus a predetermined offset value.

[0117] At step 804, the controller may determine a first transformed signal (eg, S described in association with equation (1)) by performing a Fourier transform on the first detector signal and the second detector signal. - ) and a second transformed signal (e.g., S described in association with equation (2) + In some embodiments, the first period of the first transformed signal (eg, described in association with equation (1)) ) may correspond to a first pitch value of a first target (e.g., Figure 4 The first pitch value 418 of the first transformation signal, and the second period of the second transformation signal (eg, the period described in relation to equation (2) ) may correspond to a second pitch value of a second target (e.g., Figure 4 In some embodiments, the first pitch value may be equal to the second pitch value.

[0118] At step 806, the controller may determine an overlay value of the sample based on the first transform signal and the second transform signal (e.g., based on tanφ described in association with equations (3)-(12)). In some embodiments, when the first pitch value may be equal to the second pitch value (e.g., both are pitch value P) to determine the overlay value in step 806, the controller may determine a first amplitude value of the first transform signal (e.g., C described in association with equations (5)-(12)). - ), a second amplitude value of the second transformed signal (eg, C described in association with equations (5)-(12)) + ) and the difference between the first transformed signal and the second transformed signal (e.g., S described in association with equations (5)-(12) ± ) associated with a third amplitude value (e.g., C described in association with equations (5)-(12) Δ ). The controller may then determine a phase value representing a partial phase difference between the first transformed signal and the second transformed signal (e.g., φ described in conjunction with equations (1)-(12)). Thereafter, the controller may determine an overlay value of the sample based on the first amplitude value and the first pitch value (e.g., ).

[0119] Fig. 9 is a flow chart illustrating another example method 900 for overlay measurement consistent with some embodiments of the present disclosure. The method 900 may be performed by a controller that may be coupled to a charged particle beam inspection apparatus (eg, the charged particle beam inspection system 100). For example, the controller may be Figure 2 The controller 109 in FIG. 100 may be programmed to implement the method 900 .

[0120] In step 902, the controller may respond to a sample (e.g., Figure 6 of samples 600) (e.g., Figure 6 The detector signal is obtained by scanning a target 602) (e.g., by a single beam inspection device or a multi-beam inspection device). In some embodiments, the charged particle beam inspection device may include a scanning electron microscope. For example, the sample may include a wafer.

[0121] In some embodiments, the first target may include a first pattern layer (eg, Figure 6 A first pattern layer 606) and a second pattern layer (eg, Figure 6 The pitch value of the first pattern layer and the second pattern layer may be equal to the target pitch value (eg, Figure 6 The first pattern layer may not have a predetermined offset relative to the second pattern layer. In some embodiments, the first pattern layer and the second pattern layer may both include a grating (eg, a line grating).

[0122] In step 904, the controller may determine a first transform signal (e.g., S1 described in association with equation (13)) by performing a Fourier transform on the first detector signal, and determine a second transform signal (e.g., S2 described in association with equation (14)) by converting the first transform signal. In some embodiments, the period of the first transform signal and the period of the second transform signal may correspond to the pitch value of the target (e.g., Figure 6 Pitch value 618).

[0123] In step 906, the controller may determine an overlay value of the sample (e.g., an overlay value corresponding to tanφ described in association with equations (17)-(20)) based on the first transformed signal, the second transformed signal, the first predetermined amplitude value, and the second predetermined amplitude value. In some embodiments, the first predetermined amplitude value (e.g., a′ determined using equations (1)-(9)) and the second predetermined amplitude value (e.g., b′ determined using equations (1)-(9)) may be associated with two targets adjacent to the target. For example, a target (e.g., Figure 6 The target 602 described in connection with the present invention may be located in a first space (eg, a die) sufficient to manufacture a single target, and two targets (eg, Figure 4 The first target 402 and the second target 404 described in association may be located in a second space (eg, a scribe line street) adjacent to the first space and are sufficient to manufacture both targets. The first space and the second space are both on the same sample.

[0124] A non-transitory computer-readable medium may be provided that stores a processor (e.g., Figure 1 109) to perform overlay measurement (e.g., Figure 8 Method 800 or Fig. 9 Method 900), data processing, database management, graphic display, operation of image inspection apparatus or other imaging equipment, detection of defects on a sample, etc. Common forms of non-transitory media include, for example, floppy disks, flexible disks, hard disks, solid-state drives, magnetic tapes or any other magnetic data storage media, CD-ROMs, any other optical data storage media, any physical media with a pattern of holes, RAM, PROM and EPROM, FLASH-EPROM or any other flash memory, NVRAM, cache, registers, any other memory chip or cartridge memory, and network versions thereof.

[0125] This embodiment can be further described in the following terms:

[0126] 1. A system comprising:

[0127] a charged particle beam inspection device configured to scan the sample; and

[0128] A controller comprising a circuit system configured to:

[0129] obtaining a first detector signal responsive to a first scan of a first target of the sample and a second detector signal responsive to a second scan of a second target of the sample;

[0130] determining a first transformed signal and a second transformed signal by performing a Fourier transform on the first detector signal and the second detector signal; and

[0131] An overlay value of the sample is determined based on the first transformed signal and the second transformed signal.

[0132] 2. The system of clause 1, wherein a first period of the first transform signal corresponds to a first pitch value of the first target, and a second period of the second transform signal corresponds to a second pitch value of the second target.

[0133] 3. The system of any one of clauses 1 to 2, wherein the charged particle beam inspection device comprises a scanning electron microscope and the sample comprises a wafer.

[0134] 4. The system according to any one of clauses 1 to 3, wherein the first target comprises a first pattern layer and a second pattern layer located below the first pattern layer,

[0135] The second target includes a third pattern layer and a fourth pattern layer located below the third pattern layer,

[0136] The pitch value between the first pattern layer and the second pattern layer is equal to the first pitch value of the first target, and

[0137] A pitch value between the third pattern layer and the fourth pattern layer is equal to a second pitch value of the second target.

[0138] 5. The system of clause 4, wherein each of the first pattern layer, the second pattern layer, the third pattern layer, and the fourth pattern layer comprises a grating.

[0139] 6. A system according to any one of clauses 4-5, wherein the first pattern layer has a first offset relative to the second pattern layer, wherein the magnitude of the first offset is equal to the overlay value minus a predetermined offset value,

[0140] The third pattern layer has a second offset relative to the fourth pattern layer, wherein a magnitude of the second offset is equal to the overlay value plus the predetermined offset value.

[0141] 7. The system of any of clauses 4 to 5, wherein the first pitch value is equal to the second pitch value.

[0142] 8. The system of clause 7, wherein the controller is further configured to:

[0143] determining a first amplitude value of the first transformed signal, a second amplitude value of the second transformed signal, and a third amplitude value associated with a difference between the first transformed signal and the second transformed signal;

[0144] determining a phase value representing a partial phase difference between the first transformed signal and the second transformed signal; and

[0145] The overlay value of the sample is determined based on the phase value and the first pitch value.

[0146] 9. A non-transitory computer readable medium storing a set of instructions executable by at least one processor of a device to cause the device to perform a method comprising:

[0147] obtaining a first detector signal responsive to a first scan of a first target of a sample scanned by a charged particle beam inspection apparatus and a second detector signal responsive to a second scan of a second target of the sample;

[0148] determining a first transformed signal and a second transformed signal by performing a Fourier transform on the first detector signal and the second detector signal; and

[0149] An overlay value of the sample is determined based on the first transformed signal and the second transformed signal.

[0150] 10. The non-transitory computer-readable medium of clause 9, wherein a first period of the first transform signal corresponds to a first pitch value of the first target and a second period of the second transform signal corresponds to a second pitch value of the second target.

[0151] 11. The non-transitory computer-readable medium of any of clauses 9-10, wherein the charged particle beam inspection device comprises a scanning electron microscope and the sample comprises a wafer.

[0152] 12. The non-transitory computer readable medium of any one of clauses 9-11, wherein the first target comprises a first pattern layer and a second pattern layer located below the first pattern layer,

[0153] The second target includes a third pattern layer and a fourth pattern layer located below the third pattern layer,

[0154] The pitch value between the first pattern layer and the second pattern layer is equal to the first pitch value of the first target, and

[0155] A pitch value between the third pattern layer and the fourth pattern layer is equal to a second pitch value of the second target.

[0156] 13. The non-transitory computer readable medium of clause 12, wherein each of the first pattern layer, the second pattern layer, the third pattern layer, and the fourth pattern layer comprises a grating.

[0157] 14. The non-transitory computer-readable medium of any one of clauses 12-13, wherein the first pattern layer has a first offset relative to the second pattern layer, wherein a magnitude of the first offset is equal to the overlay value minus a predetermined offset value,

[0158] The third pattern layer has a second offset relative to the fourth pattern layer, wherein a magnitude of the second offset is equal to the overlay value plus the predetermined offset value.

[0159] 15. The non-transitory computer-readable medium of any of clauses 12-14, wherein the first pitch value is equal to the second pitch value.

[0160] 16. The non-transitory computer-readable medium of clause 15, wherein determining the overlay value of the sample based on the first transformed signal and the second transformed signal comprises:

[0161] determining a first amplitude value of the first transformed signal, a second amplitude value of the second transformed signal, and a third amplitude value associated with a difference between the first transformed signal and the second transformed signal;

[0162] determining a phase value representing a partial phase difference between the first transformed signal and the second transformed signal; and

[0163] The overlay value of the sample is determined based on the phase value and the first pitch value.

[0164] 17. A computer implemented method for measuring the overlay of a sample under scanning performed by a charged particle beam inspection apparatus, comprising:

[0165] obtaining a first detector signal responsive to a first scan of a first target of the sample and a second detector signal responsive to a second scan of a second target of the sample;

[0166] determining a first transformed signal and a second transformed signal by performing a Fourier transform on the first detector signal and the second detector signal; and

[0167] An overlay value of the sample is determined based on the first transformed signal and the second transformed signal.

[0168] 18. The computer-implemented method of clause 17, wherein a first period of the first transformed signal corresponds to a first pitch value of the first target and a second period of the second transformed signal corresponds to a second pitch value of the second target.

[0169] 19. The computer-implemented method of any of clauses 17-18, wherein the charged particle beam inspection device comprises a scanning electron microscope and the sample comprises a wafer.

[0170] 20. The computer-implemented method of any of clauses 17-19, wherein the first target comprises a first pattern layer and a second pattern layer located below the first pattern layer,

[0171] The second target includes a third pattern layer and a fourth pattern layer located below the third pattern layer,

[0172] The pitch value between the first pattern layer and the second pattern layer is equal to the first pitch value of the first target, and

[0173] A pitch value between the third pattern layer and the fourth pattern layer is equal to a second pitch value of the second target.

[0174] 21. The computer-implemented method of clause 20, wherein each of the first pattern layer, the second pattern layer, the third pattern layer, and the fourth pattern layer comprises a grating.

[0175] 22. A computer-implemented method according to any of clauses 20-21, wherein the first pattern layer has a first offset relative to the second pattern layer, wherein a magnitude of the first offset is equal to the overlay value minus a predetermined offset value,

[0176] The third pattern layer has a second offset relative to the fourth pattern layer, wherein a magnitude of the second offset is equal to the overlay value plus the predetermined offset value.

[0177] 23. The computer-implemented method of any of clauses 20-22, wherein the first pitch value is equal to the second pitch value.

[0178] 24. The computer-implemented method of claim 23, wherein determining the overlay value of the sample based on the first transformed signal and the second transformed signal comprises:

[0179] determining a first amplitude value of the first transformed signal, a second amplitude value of the second transformed signal, and a third amplitude value associated with a difference between the first transformed signal and the second transformed signal;

[0180] determining a phase value representing a partial phase difference between the first transformed signal and the second transformed signal; and

[0181] The overlay value of the sample is determined based on the phase value and the first pitch value.

[0182] 25. A system comprising:

[0183] a charged particle beam inspection device configured to scan the sample; and

[0184] A controller comprising a circuit system configured to:

[0185] obtaining a detector signal responsive to a scan of the target of the sample;

[0186] determining a first transformed signal by performing a Fourier transform on the detector signal, and determining a second transformed signal by converting the first transformed signal; and

[0187] An overlay value of the sample is determined based on the first transformed signal, the second transformed signal, a first predetermined amplitude value, and a second predetermined amplitude value.

[0188] 26. The system of clause 25, wherein the first predetermined amplitude value and the second predetermined amplitude value are associated with two objects adjacent to the object.

[0189] 27. The system of any one of claims 25 to 26, wherein a period of the first transform signal and a period of the second transform signal correspond to a pitch value of the target.

[0190] 28. The system of any of clauses 25-27, wherein the charged particle beam inspection device comprises a scanning electron microscope and the sample comprises a wafer.

[0191] 29. The system according to any one of clauses 25-28, wherein the target comprises a first pattern layer and a second pattern layer located below the first pattern layer,

[0192] The pitch value of the first pattern layer and the second pattern layer is equal to the pitch value of the target, and

[0193] The first pattern layer has no predetermined offset relative to the second pattern layer.

[0194] 30. The system of clause 29, wherein each of the first pattern layer and the second pattern layer comprises a grating.

[0195] 31. A non-transitory computer readable medium storing a set of instructions executable by at least one processor of a device to cause the device to perform a method comprising:

[0196] obtaining a detector signal in response to scanning of a target of a sample scanned by a charged particle beam inspection device;

[0197] determining a first transformed signal by performing a Fourier transform on the detector signal, and determining a second transformed signal by converting the first transformed signal; and

[0198] An overlay value of the sample is determined based on the first transformed signal, the second transformed signal, a first predetermined amplitude value, and a second predetermined amplitude value.

[0199] 32. The non-transitory computer-readable medium of clause 31, wherein the first predetermined amplitude value and the second predetermined amplitude value are capable of being associated with two targets adjacent to the target.

[0200] 33. The non-transitory computer-readable medium of any of clauses 31-32, wherein a period of the first transform signal and a period of the second transform signal correspond to a pitch value of the target.

[0201] 34. The non-transitory computer-readable medium of any of clauses 31-33, wherein the charged particle beam inspection device comprises a scanning electron microscope and the sample comprises a wafer.

[0202] 35. The non-transitory computer readable medium of any one of clauses 31-34, wherein the target comprises a first pattern layer and a second pattern layer located below the first pattern layer,

[0203] The pitch value of the first pattern layer and the second pattern layer is equal to the pitch value of the target, and

[0204] The first pattern layer has no predetermined offset relative to the second pattern layer.

[0205] 36. The non-transitory computer-readable medium of any of clauses 31-35, wherein each of the first pattern layer and the second pattern layer comprises a grating.

[0206] 37. A computer implemented method for measuring the overlay of a sample under scanning performed by a charged particle beam inspection apparatus, comprising:

[0207] obtaining a detector signal responsive to a scan of the target of the sample;

[0208] determining a first transformed signal by performing a Fourier transform on the detector signal, and determining a second transformed signal by converting the first transformed signal; and

[0209] An overlay value of the sample is determined based on the first transformed signal, the second transformed signal, a first predetermined amplitude value, and a second predetermined amplitude value.

[0210] 38. The computer-implemented method of clause 37, wherein the first predetermined amplitude value and the second predetermined amplitude value are associated with two targets that are adjacent to the target.

[0211] 39. A computer-implemented method according to any of clauses 37-38, wherein a period of the first transformed signal and a period of the second transformed signal correspond to a pitch value of the target.

[0212] 40. The computer-implemented method of any of clauses 37-39, wherein the charged particle beam inspection device comprises a scanning electron microscope and the sample comprises a wafer.

[0213] 41. A computer-implemented method according to any of clauses 37-40, wherein the target comprises a first pattern layer and a second pattern layer located below the first pattern layer,

[0214] The pitch value of the first pattern layer and the second pattern layer is equal to the pitch value of the target, and

[0215] The first pattern layer has no predetermined offset relative to the second pattern layer.

[0216] 42. The computer-implemented method of clause 41, wherein the first pattern layer and the second pattern layer each comprise a grating.

[0217] The block diagram in the figure shows the possible architecture, function and operation of the system, method and computer hardware or software product according to various example embodiments of the present disclosure. In this regard, each box in the flow chart or block diagram can represent a part of a module, segment or code, which includes one or more executable instructions for implementing a specified logical function. It should be understood that in some alternative implementations, the functions indicated in the box may not occur in the order shown in the figure. For example, two boxes displayed in succession can be executed or implemented substantially at the same time, or sometimes two boxes can be executed in reverse order, depending on the functions involved. Some boxes can also be omitted. It should also be understood that each box in the block diagram and the combination of boxes can be implemented by a special hardware system that performs a specified function or action, or by a combination of special hardware and computer instructions.

[0218] It should be understood that the embodiments of the present disclosure are not limited to the exact constructions described above and shown in the drawings, and that various modifications and changes may be made without departing from the scope thereof.

Claims

1. A system comprising: a charged particle beam inspection device configured to scan the sample; as well as A controller comprising a circuit system configured to: obtaining a first detector signal responsive to a first scan of a first target of the sample and a second detector signal responsive to a second scan of a second target of the sample; determining a first transformed signal and a second transformed signal by performing a Fourier transform on the first detector signal and the second detector signal; as well as An overlay value of the sample is determined based on the first transformed signal and the second transformed signal. 2 . The system of claim 1 , wherein a first period of the first transform signal corresponds to a first pitch value of the first target, and a second period of the second transform signal corresponds to a second pitch value of the second target.

3. The system of claim 1, wherein the charged particle beam inspection device comprises a scanning electron microscope and the sample comprises a wafer.

4. The system of claim 1, wherein the first target comprises a first pattern layer and a second pattern layer located below the first pattern layer, The second target includes a third pattern layer and a fourth pattern layer located below the third pattern layer, The pitch value of the first pattern layer and the second pattern layer is equal to the first pitch value of the first target, and A pitch value of the third pattern layer and the fourth pattern layer is equal to a second pitch value of the second target. 5 . The system of claim 4 , wherein each of the first pattern layer, the second pattern layer, the third pattern layer, and the fourth pattern layer comprises a grating.

6. The system of claim 4, wherein the first pattern layer has a first offset relative to the second pattern layer, wherein a magnitude of the first offset has a magnitude equal to the overlay value minus a predetermined offset value, The third pattern layer has a second offset relative to the fourth pattern layer, wherein a magnitude of the second offset has a magnitude equal to the overlay value plus the predetermined offset value.

7. The system of claim 4, wherein the first pitch value is equal to the second pitch value.

8. The system of claim 7, wherein the controller is further configured to: determining a first amplitude value of the first transformed signal, a second amplitude value of the second transformed signal, and a third amplitude value associated with a difference between the first transformed signal and the second transformed signal; determining a phase value representing a partial phase difference between the first transformed signal and the second transformed signal; as well as The overlay value of the sample is determined based on the phase value and the first pitch value.

9. A non-transitory computer-readable medium storing a set of instructions executable by at least one processor of a device to cause the device to perform a method comprising: obtaining a first detector signal responsive to a first scan of a first target of a sample scanned by a charged particle beam inspection apparatus and a second detector signal responsive to a second scan of a second target of the sample; determining a first transformed signal and a second transformed signal by performing a Fourier transform on the first detector signal and the second detector signal; as well as An overlay value of the sample is determined based on the first transformed signal and the second transformed signal.

10. The non-transitory computer readable medium of claim 9, wherein a first period of the first transform signal corresponds to a first pitch value of the first target, and a second period of the second transform signal corresponds to a second pitch value of the second target.

11. The non-transitory computer readable medium of claim 9, wherein the charged particle beam inspection device comprises a scanning electron microscope and the sample comprises a wafer.

12. The non-transitory computer readable medium of claim 9, wherein the first target comprises a first pattern layer and a second pattern layer located below the first pattern layer, The second target includes a third pattern layer and a fourth pattern layer located below the third pattern layer, The pitch value of the first pattern layer and the second pattern layer is equal to the first pitch value of the first target, and A pitch value of the third pattern layer and the fourth pattern layer is equal to a second pitch value of the second target. 13 . The non-transitory computer readable medium of claim 12 , wherein each of the first pattern layer, the second pattern layer, the third pattern layer, and the fourth pattern layer comprises a grating.

14. The non-transitory computer readable medium of claim 12, wherein the first pattern layer has a first offset relative to the second pattern layer, wherein a magnitude of the first offset has a magnitude equal to the overlay value minus a predetermined offset value, The third pattern layer has a second offset relative to the fourth pattern layer, wherein a magnitude of the second offset has a magnitude equal to the overlay value plus the predetermined offset value.

15. The non-transitory computer readable medium of claim 12, wherein the first pitch value is equal to the second pitch value.

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