Dosage-controlled voltage contrast inspection in charged-particle beam systems and methods thereof

Abstract

Systems and methods (400; 700) of detecting a defect (638) in a sample (250) using a charged-particle beam apparatus are disclosed. A method (400; 700) for inspecting a sample (250) using a charged-particle beam apparatus includes causing a region of the sample (250) comprising a plurality of features (632) to be charged to a first polarity, causing the region to switch from the first polarity to a second polarity at a switching condition, the switching condition comprising a switching dosage of primary charged particles, forming an inspection image of the region from signal charged particles generated upon interaction of the switching dosage of primary charged particles with the plurality of features (632), and determining whether a feature (632) of the plurality of features (632) is defective based on a gray level value of the feature (632) in the inspection image.

Description

DOSAGE-CONTROLLED VOLTAGE CONTRAST INSPECTION IN CHARGED-PARTICLE BEAM SYSTEMS AND METHODS THEREOFCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63 / 626,484 which was filed on 29 January 2024, and which is incorporated herein in its entirety by reference.TECHNICAL FIELD

[0002] The embodiments provided herein disclose a charged-particle beam apparatus, and more particularly systems and methods for improving voltage contrast defect detection and defect classification.BACKGROUND

[0003] In manufacturing processes of integrated circuits (ICs), unfinished or finished circuit components are inspected to ensure that they are manufactured according to design and are free of defects. Inspection systems utilizing optical microscopes or charged particle (e.g., electron) beam microscopes, such as a scanning electron microscope (SEM) can be employed. As the complexity in device architecture increases, accurate inspection of 3D structures has become more important. Although voltage contrast inspection techniques may be employed to detect buried physical or electrical defects in such complex device structures, the existing techniques suffer from low defect sensitivity, incapability of detecting high-resistance defects, or inaccurately classifying defect types, thereby negatively impacting the defect capture rate, detection sensitivity, inspection throughput, among other things.SUMMARY

[0004] One aspect of the present disclosure is directed to a method of detecting a defect in a sample using a charged-particle beam apparatus. The method includes causing a region of a sample comprising a plurality of features to be charged to a first polarity, causing the region to switch from the first polarity to a second polarity at a switching condition, the switching condition comprising a switching dosage of primary charged particles, forming an inspection image of the region from signal charged particles generated after interaction of the switching dosage of primary charged particles with the plurality of features, and determining whether a feature of the plurality of features is defective based on a gray level value of the feature in the inspection image.

[0005] Another aspect of the present disclosure is directed to a method for inspecting a sample using a charged-particle beam apparatus. The method includes causing a region of a sample comprising a plurality of features to be charged to a first polarity by irradiating the region with a first dosage of primary charged particles, causing the region to be charged to a second polarity by scanning the regionwith a plurality of scans of a second dosage of primary charged particles, forming a plurality of inspection images from signal charged particles generated after interaction of primary charged particles of each scan of the plurality of scans with the region, determining whether a feature is defective based on a comparison of gray level value of the plurality of features in an inspection image of the plurality of inspection images, and after determining, identifying a characteristic of a defect based on the inspection image and a corresponding dosage of primary charged particles.

[0006] Yet another aspect of the present disclosure is directed to a charged-particle beam apparatus for sample inspection or defect detection. The apparatus comprises a charged-particle source configured to emit charged particles, the emitted charged particles forming a primary charged-particle beam, wherein the charged-particle beam apparatus is configured to cause a region of a sample comprising a plurality of features to be charged to a first polarity, and cause the region to switch from the first polarity to a second polarity at a switching condition, the switching condition comprising a switching dosage of primary charged particles. The apparatus further includes a controller including circuitry configured to acquire an inspection image of the region from signal charged particles generated upon interaction of the switching dosage of primary charged particles with the plurality of features, and determine whether a feature is defective based on a gray level value of the feature in the inspection image.

[0007] Yet another aspect of the present disclosure is directed to a charged-particle beam apparatus for sample inspection or defect detection. The apparatus comprises a charged-particle source configured to emit charged particles, the emitted charged particles forming a primary charged-particle beam, wherein the charged-particle beam apparatus is configured to cause a region of a sample comprising a plurality of features to be charged to a first polarity by irradiating the region with a first dosage of primary charged particles, and cause the region to be charged to a second polarity by scanning the region with a plurality of scans of a second dosage of primary charged particles. The apparatus further includes a controller including circuitry configured to acquire a plurality of inspection images from signal charged particles generated after interaction of primary charged particles of each scan of the plurality of scans with the region, determine whether a feature is defective based on a comparison of gray level value of the plurality of features in an inspection image of the plurality of inspection images, and identify, after determining, a characteristic of a defect based on the inspection image and a corresponding dosage of primary charged particles.

[0008] Yet another aspect of the present disclosure is directed to a non-transitory computer readable medium storing a set of instructions that is executable by one or more processors of a charged-particle beam apparatus to cause the charged-particle beam apparatus to perform a method for inspecting a sample. The method comprises activating a charged-particle source configured to emit charged particles, the emitted charged particles forming a primary charged-particle beam, irradiating a region of a sample comprising a plurality of features using the primary charged-particle beam to charge the region to a first polarity, irradiating the region using the primary charged-particle beam to switch from the first polarity to a second polarity at a switching condition, the switching condition comprising a switching dosage ofprimary charged particles, forming an inspection image of the region from signal charged particles generated after interaction of the switching dosage of primary charged particles with the plurality of features, and determining whether a feature of the plurality of features is defective based on a gray level value of the feature in the inspection image.

[0009] Yet another aspect of the present disclosure is directed to a non-transitory computer readable medium storing a set of instructions that is executable by one or more processors of a charged-particle beam apparatus to cause the charged-particle beam apparatus to perform a method for inspecting a sample. The method comprises activating a charged-particle source configured to emit charged particles, the emitted charged particles forming a primary charged-particle beam, irradiating a region of a sample comprising a plurality of features using a first dosage of charged particles of the primary charged- particle beam to charge the region to a first polarity, scanning the region with a plurality of scans of a second dosage of charged particles of the primary charged-particle beam to charge the region with a second polarity, forming a plurality of inspection images from signal charged particles generated after interaction of primary charged particles of each scan of the plurality of scans with the region, determining whether a feature is defective based on a comparison of gray level value of the plurality of features in an inspection image of the plurality of inspection images, and after determining, identifying a characteristic of a defect based on the inspection image and a corresponding dosage of primary charged particles.

[0010] Other advantages of the embodiments of the present disclosure will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of the present invention.BRIEF DESCRIPTION OF FIGURES

[0011] Fig. 1 is a schematic diagram illustrating an exemplary electron beam inspection (EBI) system, consistent with embodiments of the present disclosure.

[0012] Fig. 2 is a schematic diagram illustrating an exemplary electron beam tool that can be a part of the exemplary electron beam inspection system of Fig. 1, consistent with embodiments of the present disclosure.

[0013] Fig. 3A is a schematic diagram illustrating a staircase structure of an exemplary 3D NAND memory device.

[0014] Fig. 3B is a schematic illustration of exemplary defects in a multi-layered structure and their corresponding images as seen using an electron beam inspection system of Fig. 1.

[0015] Fig. 3C is a schematic illustration of exemplary defects in a piping structure.

[0016] Fig. 4A is a process flowchart for an exemplary method for sample inspection, consistent with embodiments of the present disclosure.

[0017] Fig. 4B is a schematic illustration of an exemplary methodology for defect detection used in sample inspection, consistent with embodiments of the present disclosure.

[0018] Figs. 5A and 5B illustrate schematic illustrations of exemplary inspection images including an array of features.

[0019] Fig. 6A is a schematic illustration of an exemplary methodology for defect detection used in sample inspection, consistent with embodiments of the present disclosure.

[0020] Fig. 6B illustrates a schematic representation of the number of defects detected with respect to the applied dosage or acquired inspection image, consistent with embodiments of the present disclosure.

[0021] Fig. 6C illustrates a schematic illustration of an exemplary inspection image including an array of features, consistent with embodiments of the present disclosure.

[0022] Fig. 7 is a process flowchart for an exemplary method of detecting a defect, consistent with embodiments of the present disclosure.DETAILED DESCRIPTION

[0023] Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the drawings. The following description refers to the accompanying drawings in which the same numbers in different drawings represent the same or similar elements unless otherwise represented. The implementations set forth in the following description of exemplary embodiments do not represent all implementations consistent with the invention. Instead, they are merely examples of apparatuses, systems, and methods consistent with aspects related to subject matter that may be recited 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 disclosure is not so limited. Other types of charged-particle beams (e.g., including protons, ions, muons, or any other particle carrying electric charges) may be similarly applied. Furthermore, other imaging systems may be used, such as optical imaging, photodetection, x-ray detection, extreme ultraviolet inspection, deep ultraviolet inspection, or the like.

[0024] Electronic devices are constructed of circuits formed on a piece of silicon called a substrate. The semiconductor material may include, for example, silicon (Si), gallium arsenide (GaAs), indium phosphide (InP), or silicon germanium (SiGe), or the like. Many circuits may be formed together on the same piece of silicon and are called integrated circuits or ICs. With advancements in technology, the size of these circuits has decreased dramatically so that many more of them can fit on the substrate. For example, an IC chip in a smart phone can be as small as a fingernail and yet may include over 2 billion transistors, the size of each transistor being less than one-thousandth the width of a human hair.

[0025] Making these extremely small ICs is a complex, time-consuming, and expensive process, often involving hundreds of individual steps. Errors in even one step have the potential to result in defects in the finished IC, thereby rendering it useless. Thus, one goal of the manufacturing process is to avoid such defects to maximize the number of functional ICs made in the process, that is, to improve the overall yield of the process.

[0026] One component of improving yield is monitoring the chip making process to ensure that it is producing a sufficient number of functional integrated circuits. One way to monitor the process is to inspect the chip circuit structures at various stages of their formation. Inspection can be carried out using a scanning electron microscope (SEM). A SEM can be used to image these extremely small structures, in effect, taking a “picture” of the structures. The image can be used to determine if the structure was formed properly and also if it was formed in the proper location. If the structure is defective, then the process can be adjusted so the defect is less likely to recur. Defects may be generated during various stages of semiconductor processing. For the reason stated above, it is important to find defects accurately, efficiently, and as early as possible. To enhance throughput (e.g., the number of samples processed per hour), it is desirable to conduct inspection as quickly as possible.

[0027] The working principle of a SEM is similar to a camera. A camera takes a picture by receiving and recording brightness and colors of light reflected or emitted from people or objects. A SEM takes a “picture” by receiving and recording energies or quantities of electrons reflected or emitted from the structures. Before taking such a “picture,” an electron beam may be provided onto the structures, and when the electrons are reflected or emitted (“exiting”) from the structures, a detector of the SEM may receive and record the energies or quantities of those electrons to generate an image. To take such a “picture,” some SEMs use a single electron beam (referred to as a “single -beam SEM”), while some SEMs use multiple electron beams (referred to as a “multi-beam SEM”) to take multiple “pictures” of the wafer. By using multiple electron beams, the SEM may provide more electron beams onto the structures for obtaining these multiple “pictures,” resulting in more electrons exiting from the structures. Accordingly, the detector may receive more exiting electrons simultaneously, and generate images of the structures of the wafer with a higher efficiency and a faster speed.

[0028] With continuous scaling and increased design and process complexity, there is an increasing need for semiconductor manufacturing process control. This need calls for not only advance methods and more capable tools, but also additional intra-wafer, inter-wafer, and across-lot sampling in order to capture process variations and / or changes in process signatures. Integrated circuits (ICs) are expected to perform more complex tasks with higher efficiency and faster processing speeds with each new process technology node, which necessitates complex device architectures to accommodate a higher density of active devices. A “technology node,” in the context of semiconductor device industry (e.g., “10 nm”), refers to the smallest size of a feature, such as a gate of a transistor or a half-pitch of a metal line, which can be reproducibly printed on a semiconductor wafer.

[0029] In semiconductor devices, buried defects such as voids or particles may cause full opens and leakages (shorts), or in some cases, a partial open, or a partial leakage. Existing voltage contrast (VC) inspection techniques, used to detect such defects, involve flood exposure of negatively charged particles e.g., electrons, on a surface and rely on differences in surface potential measurements of structures on the surface. The gray level of a pixel representing a surface region with high surfacepotential is higher (appears brighter in a SEM image) than the gray level of the pixel representing a lower surface potential region.

[0030] Existing VC inspection or VC imaging techniques using an electron-beam apparatus such as a SEM, operate in either a positive or negative voltage contrast mode. In either mode, the features on a wafer under inspection, as seen in a top-down view in a SEM apparatus (e.g., contact pads 340 of Fig. 3A), are raised to an electric potential by pre-charging the surface of the wafer with charged-particles (e.g., electrons). Because they appear in different contrasts, the floating and grounded conductors can therefore be distinguished by visual inspection based on the gray level values. Although existing single scan inspection techniques may be used to effectively determine whether there is a defect or not, they may be inadequate in classifying the defects or in determining the location of the defect in a layered or a stacked structure. In other words, while existing VC inspection techniques may differentiate a defective feature from a non-defective feature, they may not be used to further classify the defects such as differentiating between a hard electrical open and a high-resistance defect. High-resistance defects do not generate a lot of signal electrons, and are therefore, harder to detect by existing single-scan VC inspection techniques. While the sensitivity for detection of high-resistance defects is maximum at a neutral mode, the existing inspection techniques are extremely unstable under neutral conditions.

[0031] Some embodiments of the present disclosure are directed to apparatuses and methods for detecting a defect in a sample by voltage contrast inspection. The method may include pre-charging a sample using a charged-particle beam with a high average beam current to a first polarity. The polarity of the sample may be switched from the first polarity to a second polarity, opposite of the first polarity, at a switching condition by applying an incremental dosage of charged particles. The method may further include acquiring an inspection image at each applied dosage of the incremental dosage and performing defect detection in each acquired image to determine whether there is a defect based on the gray level value of a feature in the inspection image at the switching condition.

[0032] Relative dimensions of components in drawings may be exaggerated for clarity. Within the following description of drawings, the same or like reference numbers refer to the same or like components or entities, and only the differences with respect to the individual embodiments are described. As used herein, unless specifically stated otherwise, the term “or” encompasses all possible combinations, except where infeasible. For example, if it is stated that a component may include A or B, then, unless specifically stated otherwise or infeasible, the component may include A, or B, or A and B. As a second example, if it is stated that a component may include A, B, or C, then, unless specifically stated otherwise or infeasible, 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] Reference is now made to Fig. 1, which illustrates an exemplary electron beam inspection (EBI) system 100 consistent with embodiments of the present disclosure. As shown in Fig. 1, charged particle beam inspection system 100 includes a main chamber 10, a load-lock chamber 20, an electron beam tool 40, and an equipment front end module (EFEM) 30. Electron beam tool 40 is located within mainchamber 10. While the description and drawings are directed to an electron beam, it is appreciated that the embodiments are not used to limit the present disclosure to specific charged particles.

[0034] EFEM 30 includes a first loading port 30a and a second loading port 30b. EFEM 30 may include additional loading port(s). First loading port 30a and second loading port 30b receive wafer front opening unified pods (FOUPs) that contain wafers (e.g., semiconductor wafers or wafers made of other material(s)) or samples to be inspected (wafers and samples are collectively referred to as “wafers” hereafter). One or more robot arms (not shown) in EFEM 30 transport the wafers to load-lock chamber 20.

[0035] Load-lock chamber 20 is connected to a load / lock vacuum pump system (not shown), which removes gas molecules in load-lock chamber 20 to reach a first pressure below the atmospheric pressure. After reaching the first pressure, one or more robot arms (not shown) transport the wafer from loadlock chamber 20 to main chamber 10. Main chamber 10 is connected to a main chamber vacuum pump system (not shown), which removes gas molecules in main chamber 10 to reach a second pressure below the first pressure. After reaching the second pressure, the wafer is subject to inspection by electron beam tool 40. In some embodiments, electron beam tool 40 may comprise a single-beam inspection tool. In other embodiments, electron beam tool 40 may comprise a multi-beam inspection tool.

[0036] Controller 50 may be electronically connected to electron beam tool 40 and may be electronically connected to other components as well. Controller 50 may be a computer configured to execute various controls of charged particle beam inspection system 100. Controller 50 may also include processing circuitry configured to execute various signal and image processing functions. While controller 50 is shown in Fig. 1 as being outside of the structure that includes main chamber 10, loadlock chamber 20, and EFEM 30, it is appreciated that controller 50 can be part of the structure.

[0037] In some embodiments, controller 50 may include one or more processors (not shown). A processor may be a generic or specific electronic device capable of manipulating or processing information. For example, the processor may include any combination of any number of a central processing unit (or “CPU”), a graphics processing unit (or “GPU”), an optical processor, a programmable logic controllers, a microcontroller, a microprocessor, a digital signal processor, an intellectual property (IP) core, a Programmable Logic Array (PLA), a Programmable Array Logic (PAL), a Generic Array Logic (GAL), a Complex Programmable Logic Device (CPLD), a Field- Programmable Gate Array (FPGA), a System On Chip (SoC), an Application-Specific Integrated Circuit (ASIC), a neural processing unit (NPU), and any type of circuit capable of data processing. The processor may also be a virtual processor that includes one or more processors distributed across multiple machines or devices coupled via a network.

[0038] While the present disclosure provides examples of main chamber 10 housing an electron beam inspection system, it should be noted that aspects of the disclosure in their broadest sense, are not limited to a chamber housing an electron beam inspection system. Rather, it is appreciated that the foregoing principles may be applied to other chambers as well.

[0039] Reference is now made to Fig. 2, which illustrates a schematic diagram illustrating an exemplary configuration of electron beam tool 40 that can be a part of the exemplary charged particle beam inspection system 100 of Fig. 1, consistent with embodiments of the present disclosure. Electron beam tool 40 (also referred to herein as apparatus 40) may comprise an electron emitter, which may comprise a cathode 203, an extractor electrode 205, a gun aperture 220, and an anode 222. Electron beam tool 40 may further include a Coulomb aperture array 224, a condenser lens 226, a beam-limiting aperture array 235, an objective lens assembly 232, and an electron detector 244. Electron beam tool 40 may further include a sample holder 236 supported by motorized stage 234 to hold a sample 250 to be inspected. It is to be appreciated that other relevant components may be added or omitted, as needed.

[0040] In some embodiments, electron emitter may include cathode 203, an anode 222, wherein primary electrons can be emitted from the cathode and extracted or accelerated to form a primary electron beam 204 that forms a primary beam crossover 202. Primary electron beam 204 can be visualized as being emitted from primary beam crossover 202.

[0041] In some embodiments, the electron emitter, condenser lens 226, objective lens assembly 232, beam-limiting aperture array 235, and electron detector 244 may be aligned with a primary optical axis 201 of apparatus 40. In some embodiments, electron detector 244 may be placed off primary optical axis 201, along a secondary optical axis (not shown).

[0042] Objective lens assembly 232, in some embodiments, may comprise a modified swing objective retarding immersion lens (SORIL), which includes a pole piece 232a, a control electrode 232b, a beam manipulator assembly comprising deflectors 240a, 240b, 240d, and 240e, and an exciting coil 232d. In a general imaging process, primary electron beam 204 emanating from the tip of cathode 203 is accelerated by an accelerating voltage applied to anode 222. A portion of primary electron beam 204 passes through gun aperture 220, and an aperture of Coulomb aperture array 224, and is focused by condenser lens 226 so as to fully or partially pass through an aperture of beam-limiting aperture array 235. The electrons passing through the aperture of beam-limiting aperture array 235 may be focused to form a probe spot on the surface of sample 250 by the modified SORIL lens and deflected to scan the surface of sample 250 by one or more deflectors of the beam manipulator assembly. Secondary electrons emanated from the sample surface may be collected by electron detector 244 to form an image of the scanned area of interest.

[0043] In objective lens assembly 232, exciting coil 232d and pole piece 232a may generate a magnetic field. A part of sample 250 being scanned by primary electron beam 204 can be immersed in the magnetic field and can be electrically charged, which, in turn, creates an electric field. The electric field may reduce the energy of impinging primary electron beam 204 near and on the surface of sample 250. Control electrode 232b, being electrically isolated from pole piece 232a, may control, for example, an electric field above and on sample 250 to reduce aberrations of objective lens assembly 232 and control focusing situation of signal electron beams for high detection efficiency, or avoid arcing to protect sample. One or more deflectors of beam manipulator assembly may deflect primary electron beam 204to facilitate beam scanning on sample 250. For example, in a scanning process, deflectors 240a, 240b, 240d, and 240e can be controlled to deflect primary electron beam 204, onto different locations of top surface of sample 250 at different time points, to provide data for image reconstruction for different parts of sample 250. It is noted that the order of 240a-e may be different in different embodiments.

[0044] Backscattered electrons (BSEs) and secondary electrons (SEs) can be emitted from the part of sample 250 upon receiving primary electron beam 204. A beam separator can direct the secondary or scattered electron beam(s), comprising backscattered and secondary electrons, to a sensor surface of electron detector 244. The detected secondary electron beams can form corresponding beam spots on the sensor surface of electron detector 244. Electron detector 244 can generate signals (e.g., voltages, currents) that represent the intensities of the received secondary electron beam spots, and provide the signals to a processing system, such as controller 50. The intensity of secondary or backscattered electron beams, and the resultant secondary electron beam spots, can vary according to the external or internal structure of sample 250. Moreover, as discussed above, primary electron beam 204 can be deflected onto different locations of the top surface of sample 250 to generate secondary or scattered electron beams (and the resultant beam spots) of different intensities. Therefore, by mapping the intensities of the secondary electron beam spots with the locations of sample 250, the processing system can reconstruct an image that reflects the internal or external structures of wafer sample 250.

[0045] In some embodiments, controller 50 may comprise an image processing system that includes an image acquirer (not shown) and a storage (not shown). The image acquirer may comprise one or more processors. For example, the image acquirer may comprise a computer, server, mainframe host, terminals, personal computer, any kind of mobile computing devices, and the like, or a combination thereof. The image acquirer may be communicatively coupled to electron detector 244 of apparatus 40 through a medium such as an electrical conductor, optical fiber cable, portable storage media, IR, Bluetooth, internet, wireless network, wireless radio, among others, or a combination thereof. In some embodiments, the image acquirer may receive a signal from electron detector 244 and may construct an image. The image acquirer may thus acquire images of regions of sample 250. The image acquirer may also perform various post-processing functions, such as generating contours, superimposing indicators on an acquired image, and the like. The image acquirer may be configured to perform adjustments of brightness and contrast, etc. of acquired images. In some embodiments, the storage may be a storage medium such as a hard disk, flash drive, cloud storage, random access memory (RAM), other types of computer readable memory, and the like. The storage may be coupled with the image acquirer and may be used for saving scanned raw image data as original images, and post-processed images.

[0046] In some embodiments, controller 50 may include measurement circuitries (e.g., analog-to- digital converters) to obtain a distribution of the detected secondary electrons and backscattered electrons. The electron distribution data collected during a detection time window, in combination with corresponding scan path data of a primary beam 204 incident on the sample (e.g., a wafer) surface, can be used to reconstruct images of the wafer structures under inspection. The reconstructed images canbe used to reveal various features of the internal or external structures of sample 250, and thereby can be used to reveal any defects that may exist in the wafer.

[0047] In some embodiments, controller 50 may control motorized stage 234 to move sample 250 during inspection. In some embodiments, controller 50 may enable motorized stage 234 to move sample 250 in a direction continuously at a constant speed. In other embodiments, controller 50 may enable motorized stage 234 to change the speed of the movement of sample 250 over time depending on the steps of scanning process.

[0048] As is commonly known in the art, interaction of charged particles, such as electrons of a primary electron beam with a sample may generate signal electrons containing compositional and topographical information about the probed regions of the sample. Secondary electrons (SEs) may be identified as signal electrons with low emission energies, and backscattered electrons (BSEs) may be identified as signal electrons with higher emission energies. Because of their low emission energy, an objective lens assembly may direct the SEs along electron paths and focus the SEs on a detection surface of in-lens electron detector placed inside the SEM column. BSEs traveling along electron paths may be detected by the in-lens electron detector as well. In some cases, BSEs with large emission angles, however, may be detected using additional electron detectors, such as a backscattered electron detector, or remain undetected, resulting in loss of sample information needed to inspect a sample or measure critical dimensions.

[0049] Detection and inspection of some defects in semiconductor fabrication processes, such as buried particles during photolithography, metal deposition, dry etching, or wet etching, among other things, may benefit from inspection of surface features as well as compositional analysis of the defect particle. In such scenarios, information obtained from secondary electron detectors and backscattered electron detectors to identify the defect(s), analyze the composition of the defect(s), and adjust process parameters based on the obtained information, among other things, may be desirable for a user.

[0050] Reference is now made to Fig. 3A, which illustrates a schematic diagram of an exemplary memory device 300 having a “staircase” structure. Device 300 may include multiple memory cells vertically stacked on a substrate 310. Device 300 may be fabricated, for example, by depositing multiple alternating layers of dielectric materials, such as oxides and nitride films 325. Memory device 300 may include horizontal word-lines 320, which may be formed by backfilling with a conductive material, such as tungsten, after the sacrificial layers, e.g., nitride films in the stack, have been removed. Memory cell formation in device 300 may further include a staircase etch of the dielectric film pairs and metal fill of contact channels 330 comprising a contact pad 340 for enabling a bit-line bus contact (not illustrated in Fig. 3A) to the word-lines. Multiple word-line lithography steps with repeated vertical step etching and 2D trimming at each staircase may be performed to provide the “up and down” shape of the WL staircase used in 3D NAND devices. This series of process steps requires precise etch step profiling, trim etch uniformity, and pull-back CD control for the WL contact. The length of the WL staircase may increase as more memory cells are vertically stacked to improve efficiency, storagedensity, among other things. Device 300 may include a film stack that may be >64 layers thick, or >96 layers thick, or in some cases, even >124 layers thick. A vertical channel hole (not illustrated) may further be created with a high aspect ratio (HAR) etch, through the entire film stack. In some cases, the HAR may be >100:1. Individual memory cells within layers may be electrically connected through word-line replacement metal fills, provided by materials such as tungsten, for example. The word-line metal filling may include void-free filling of complex, narrow, lateral structures with minimal stress on the device stack. In practice, fabricating a 3D NAND device is extremely challenging and it may be desirable to detect physical or electrical defects generated during or after fabrication of such complex device structures.

[0051] Detecting buried defects in vertical high-density structures, such as 3D NAND flash memory device 300, can be challenging. One of several ways to detect buried or on-surface electrical defects in such devices is by using a voltage contrast method in a SEM. In this method, electrical conductivity differences in materials, structures, or regions of a sample cause contrast differences in SEM images thereof. In the context of defect detection, an electrical defect under the sample surface may generate a charging variation on the sample surface, so the electrical defect can be detected by a contrast in the SEM image of the sample surface. To enhance the voltage contrast, a process called pre-charging or flooding may be employed in which the region of interest of the sample may be exposed to a large beam current before an inspection using a small beam current but with high imaging resolution. For the inspection, some of the advantages of flooding may include reduction of charging of the wafer to minimize distortion of images due to the charging, and in some cases, increase of charging of the wafer to enhance difference of defective and surrounding non-defective features in images, among other things. Some inspection systems, such as a SEM, are equipped to detect defects of a wafer using the voltage contrast method and may be operated in multiple modes such as a flooding mode to highlight the defect, followed by an inspection mode to detect the defect. In the flooding mode, it may be preferable to allow maximum electrons to pass through an aperture and maximize the beam current of the primary electron beam irradiating the sample, to enhance the voltage contrast. In the inspection mode, however, a small probe spot having a small beam current may be desirable for high resolution imaging.

[0052] In semiconductor devices, buried defects such as voids may cause full opens or hard opens, high-resistance defects and presence of unwanted particles may cause leakages (shorts), or in some cases, a partial open or a partial leakage. Existing voltage contrast (VC) inspection techniques, used to detect such defects, involve flood exposure of negatively charged particles e.g., electrons, on a surface and relying on differences in surface potential measurements of structures on the surface. The gray level of a pixel representing a surface region with high surface potential is higher (appears brighter in a SEM image) than the gray level of the pixel representing a lower surface potential region. The gray levels of structures are compared to a reference gray level to detect a defect.

[0053] Defect detection is an important aspect in manufacturing of semiconductor devices. Early detection, preferably at multiple stages of fabrication, enables a source of defects to be identified and eliminated before large numbers of wafers are affected. In some cases, VC inspection techniques such as, but not limited to, passive voltage contrast, biased voltage contrast, capacitive-coupled voltage contrast, or electron beam probing, may be used for failure analysis as well. Existing VC inspection or VC imaging techniques using an electron-beam apparatus, such as a SEM, operate in either a positive or negative voltage contrast mode. In either mode, the features on a wafer under inspection, as seen in a top-down view in a SEM apparatus (e.g., contact pads 340 of Fig. 3A), are raised to an electric potential by pre-charging the surface of the wafer with charged-particles (e.g., electrons). Because they appear in different contrasts, the floating and grounded conductors can therefore be distinguished by visual inspection. An example of a floating conductor or an electrical open 305, and an example of a high-resistance defect 308 is shown in Fig. 3B. For comparison, an example of a grounded conductor (e.g., contact channel 330) is also shown in Fig. 3B.

[0054] In a positive voltage contrast mode, the floating conductors are charged to a more positive voltage than the grounded conductors. In a negative voltage contrast mode, however, the floating conductors are charged to a more negative voltage than the grounded conductors. In positive voltage contrast mode, as shown in exemplary secondary electron images of Fig. 3B, the electrically insulated structures, e.g., an unconnected gate contact, may be charged up positively because the emitted secondary electrons cannot be compensated. The insulated structures may appear dark in the secondary electron image (e.g., feature 305A corresponding to electrical open 305) because a portion of the emitted electrons may not reach the secondary electron detector due to the retarding electric field near the surface. On the other hand, a grounded structure (e.g., contact channel 330 electrically connected to word-line 320) is not charged and may appear brighter in the secondary electron image because of the higher secondary electron yield and more detectable emitted electrons. In negative voltage contrast mode, an electrical hard open may appear bright in the secondary electron image and a grounded feature may appear darker.

[0055] Referring to Fig. 3B, which further illustrates a schematic representation of a gray level value signal of a contact to a non-defective word-line and to a defective word line, upon application of a dosage of charged particles, consistent with embodiments of the present disclosure. In Fig. 3B, image features 305A and 3O8A represent gray level value signals of a defective feature (hard open and high- resistance defects, respectively) and feature image 325A represents a gray level value signal of a nondefective feature. In this context, a “non-defective” feature is referred to as a feature, a structure, or a device that does not have or is not associated with a physical or an electrical defect. In some cases, a physical defect such as an under-etched metal line or an over-etched dielectric film, may result in an electrical defect. In this context, a “defective” feature is referred to as a feature, a structure, or a device that has or is associated with one or more physical or electrical defects. An example of a defective feature may include a structure that fails to make a desired electrical connection to an underlyingstructure. As used herein, a gray level value of a feature refers to a gray scale level of the feature as observed in an image (e.g., a SEM image). As an example, in an 8-bit grayscale image, there may be 256 discrete gray scale levels and each pixel may be assigned a gray scale value between “0” and “255,” where gray level 0 indicates a dark pixel and gray level 255 indicates a bright pixel. In the context of this disclosure, a “dosage” refers to the total number of charged particles a feature may be exposed to. In some embodiments, the charged particles may comprise electrons, for example in a SEM. In cases where the dosage comprises electrons, the dosage may be expressed as the total number of electrons, or the total charge, in Coulombs, carried by the total number of electrons. It is to be appreciated that the charge of an electron may be 1.6 x 1019Coulombs. A dosage of charged particles may be applied to by, for example, applying a voltage signal configured to supply a desired number of charges or charged particles to a contact.

[0056] As explained earlier, existing (VC) inspection techniques include a high-dosage (i.e., high beam current) pre-scan flooding or charging up the sample region of interest followed by a low-dosage inspection scan to detect the signal electrons generated from features on the sample. The low-dosage inspection scan may be either a positive mode scan or a negative mode scan. In either case, the inspection technique includes a single inspection scan. Although existing single scan inspection techniques may be used to effectively determine whether there is a defect, they may be inadequate in classifying the defects, or determining the location of the defect in a layered or a stacked structure. In other words, while existing VC inspection techniques may differentiate a defective feature from a nondefective feature, they may not be used to further classify the defects such as differentiating between a hard electrical open and a high-resistance defect.

[0057] Fig. 3C illustrates a schematic of an exemplary logic device structure 370 including examples of electrical opens or high-resistance defects. In the context of this disclosure, a high-resistance defect is referred to as a partially-open electrical defect having a resistance of 1 MQ (mega-ohms) or higher. Structure 370 may include, but is not limited to, multiple metal layers (M2, M3, or M4) interconnected through one or more vias (VI, V2, or V3). One or more physical or electrical defects 305_l, 305_2, or 305_3 may occur during fabrication of structure 370. Defect 305_l represents an “unlanding” defect or an electrical open, wherein conducting via V3 fails to form an electrical continuity between metal M4 and metal M3. Defect 305_2 represents a “single line open” (SLO) defect, causing electrical discontinuity within metal M3. Defect 305_3 represents another unlanding defect, wherein conducting via V2 fails to form an electrical continuity between metal M3 and metal M2. Typically, in existing VC inspection techniques, a user may set the pre-charging dosage based on the highest value to capture all the defects. While this approach may efficiently capture a vast majority of the defects, it may be incapable of further classification of the defects including the type of defect, or the location of the defect with respect to a reference feature. In this example, each defect 305_l, 305_2, and 305_3 may appear as a dark feature (electrical open or defective), without further classification of the type or location ofthe defects. Therefore, it may be desirable to provide systems and methods of voltage contrast inspection for detecting and classifying electrical defects based on the applied dosage of charged particles.

[0058] Reference is made to Fig. 4A, which illustrates a process flowchart representing an exemplary method 400 of sample inspection for defect detection using a charged-particle beam apparatus, consistent with embodiments of the present disclosure. One or more steps of method 400 may be performed by controller 50 of EBI system 100, as shown in Fig. 2, for example. Here, controller 50 may instruct a module of a charged-particle beam apparatus to activate a charged-particle source to generate primary charged particle beam (e.g., electron beam), or to apply an incremental dosage of charged particles to a region of interest of a sample (e.g., sample 250 of Fig. 2), or acquire an inspection image based on detected signal charged particles generated from the sample, or carry out other related functions.

[0059] In step 410, a charged-particle source of a charged-particle beam apparatus such as a SEM, is activated to emit charged particles (e.g., electrons). The charged particles may form a charged-particle beam (e.g., primary charged-particle beam 204 of Fig. 2). The electron source may be activated by a controller (e.g., controller 50 of Fig. 2). For example, the electron source may be controlled to emit primary electrons to form an electron beam along a primary optical axis (e.g., primary optical axis 201 of Fig. 2). The electron source may be activated remotely, for example, by using a software, an application, or a set of instructions for a processor of a controller to power the electron source through a control circuitry. The primary electron beam may pass through a Coulomb aperture array (e.g., Coulomb aperture array 224 of Fig. 2) and a beam-limit aperture array (not shown) to adjust the beam size or beam current of the primary electron beam and form a probing beam incident on the sample (e.g., sample 250 of Fig. 2).

[0060] Further, in step 410, a region of interest (ROI) of the sample is irradiated with an incremental dosage of charged particles from the primary charged-particle beam. The ROI may include a plurality of features, such as contacts or contact pads (e.g., contact pad 340 of Fig. 3A) to a plurality of wordlines (e.g., word-line 320 of Fig. 3A). Irradiating the ROI with charged particles includes irradiating each feature of interest in the region of interest with a first dosage of charged particles such as, for example, electrons of a primary electron beam in a SEM tool. Irradiating the features of interest may enable identification of defects based on the charges stored, which forms a gray level value contrast signal of the feature in an acquired image, such as a SEM image. The primary charged-particle beam may be a high-dosage electron beam configured to inspect the sample for defects. As used herein, a high-dosage electron beam includes a primary electron beam with a high average beam current. In some embodiments, the average beam current may be in the range of 100 nA-500 nA, or 200 nA -500 nA, or 300 nA -500 nA, or 400 nA-500 nA. In some embodiments, the average beam current of the high- dosage electron beam may be higher, as appropriate. The high-dosage electron beam may function as a flooding or a pre-scanning beam as well as a probing or an inspection beam.

[0061] “Incremental dosage,” as used herein, refers to a progressive increase in the dosage of charged particles, applied via irradiation of a beam of the charged particles, on the surface of a sample for multiscan inspection. In such cases, if a ROI is scanned by a charged-particle beam “n” number of times, where n is a positive integer and n > 2, the dosage for the n* scan (Dn) is higher than the dosage for the (n-l)* scan (Dni) by a known factor. In some embodiments, the dosage on the substrate (e.g., wafer) may be fixed and cumulative over a number of scans. For example, if the dosage on the substrate for the first scan is Di , the cumulative dosage on the substrate after the second scan may be twice the dosage for the first scan, mathematically represented as (D2) = 2*(Di), or the cumulative dosage on the substrate after the third scan may be the sum of the dosages for the first, the second, and the third scan (D3 = Di + D2 + D3), or the cumulative dosage on the substrate after the fourth scan may be the sum of the dosages for the first, the second, the third, and the fourth scans (D4 = Di + D2 -1- D3 + D4), and so on.

[0062] In some embodiments, there may be substantially no time delay between successive scans in a multi-scan inspection. In some embodiments, the time delay between successive scans may be negligible, in the order of several microseconds or less. For example, if the scanning frequency for a ROI is 1 hertz (Hz) and the time delay is 100 microseconds (us), the total time to scan the ROI ten times may be 10.0009 seconds, thereby adding 900 ps to the total scan time. In some embodiments, the time delay may be adjusted based on factors including, but not limited to, charging-discharging rate of the features, size of the ROI, number of defects, location of the defects, type of defects, etc. In some embodiments, the dosages Di, D2, and D3 may be the same (DI=D2=DS), in which case, dosage for the fourth scan D4 may be equal to three times the dosage for the first scan (D4 = 3*Di). In some embodiments, the dosage increments Di, D2-D1, and D3-D2 may be different, in which case, D4= Di + D2 + D3. In some embodiments, dosage for the fourth scan D4 may be greater than the sum of the preceding dosages Di , D2, and D3 (D4 > Di -1- D2 + D3). It is to be appreciated that although only dosages for four scans are discussed above, there may be fewer or more scans, as appropriate.

[0063] In step 420, an inspection image of the region of interest corresponding to each applied dosage of the incremental dosages may be acquired, as illustrated in Fig. 4B. Inspection images 452, 453, 454, 455, and 460, also referred to herein as frames, correspond to applied dosages Di, D2, D3, D4, and Dn, respectively. Because the dosage is maintained throughout each individual scan of the ROI, each inspection image or each frame corresponds to a scan. For example, the fifth frame may correspond to the fifth scan with a dosage D5 of charged particles, where D5 = D4 + D3 -1- D2 + Dn An inspection image (e.g., inspection image 452, 453, 454, 455, or 460) may include a SEM inspection image formed based on signal particles (e.g., secondary electrons, backscattered electrons, or the like) generated upon interaction of the charged particles with the features in the ROI. The acquired images may be saved in a database to allow retrieval for comparison and defect inspection at a later stage.

[0064] In step 430 of Fig. 4A, defect detection may be performed for each inspection image to generate a corresponding defect signature for each acquired inspection image. Performing defect detection may include, but is not limited to, performing a die-to-die comparison, or performing a cell-to-cellcomparison, or performing a die-to-database pixel. A “defect signature,” as used herein, refers to a spatial pattern or a map of defects identified on a sample. The defect signature may be used as an identifying characteristic of a sample.

[0065] In a die-to-die mode, inspection images of two or more dies may be compared to detect defects within an inspection image. For example, a multi-scan inspection of a sample comprising multiple dies may include acquiring 25 frames of each die on a wafer and defects may be identified within each frame for every die. In such a scenario, for example, the 20thframe for every die may be compared to identify a defect. The comparison of frames or inspection images may include visual inspection to identify the defect based on a gray level value of the feature, for example.

[0066] In a cell-to-cell mode, defect detection may be based on the locations of features within a cell or a repeating unit of a die. For example, a multi-scan inspection of a ROI comprising cells may include acquiring multiple images of the cells. In an inspection image of a cell including a plurality of arrayed features, presence of a feature at an unintended location or absence of a feature at an expected location of an array may indicate the existence of a defect. The intended or the expected location may be based on the characteristics of the arrayed pattern of features within a cell or in other similar cells.

[0067] In a die-to-database pixel counting mode, inspection image of a die may be compared to a reference image or a reference layout of the die. The reference image or reference layout may be a database image or a reference feature layout. The reference feature information may be obtained from an information file comprising a wafer design layout in Graphic Database System (GDS) format, Graphic Database System II (GDS II) format including a graphical representation of the features on the wafer surface, or an Open Artwork System Interchange Standard (OASIS) format. The feature layout may be based on a pattern layout for constructing devices on the wafer. The wafer design layout may correspond to one or more photolithography masks or reticles used to transfer features from the photolithography masks or reticles to the wafer. GDS information file or OASIS information file may comprise feature information stored in a binary file format representing planar geometric shapes, text, and other information related to wafer design layout.

[0068] Further, in step 430, data associated with all the frames may be merged and the repeating defects from different frames may be identified as a defect with a label. The label may further include data indicating the frame or frames comprising the defect. In some embodiments, the merged data may be a spatial map of defects, or may be presented in a tabulated format, or any other suitable format.

[0069] In step 440, the identified defect(s) may be classified based on a characteristic of the defect. The defect classification may include, but are not limited to, hard opens, high-resistance defects, partially open, shorts, partially leaky, or a blinking defect. As used herein, a blinking defect refers to an electrical defect which may appear or disappear between frames. As an example, a blinking defect may appear in the second frame but may disappear in the third frame or any successive frame. In some cases, although not necessary, the defect may reappear in a later frame, giving it the “blinking” characteristic. As another example, a blinking defect may appear in the first frame, disappear in the next frame, andreappear in a later frame with a higher dosage of applied charged particles. A blinking defect may be caused due to sudden electrical breakdown of a feature. A feature may be slowly charged up by exposing it to an irradiation of charged particles, but may fail to hold any more charges due to a dielectric breakdown, for example, the discharge may be rapid. If the breakdown is not fatal, the feature may be charged up again at a later stage with a higher dosage applied, but may exhibit a similar failure mode again, thereby appearing as a “blinking” defect. Fig. 4B illustrates an exemplary blinking defect 472 first appearing in inspection image 454, disappearing in inspection image 455, and reappearing in inspection image 460. Dotted circle 472M indicates the absence (should only be used as a visual aid to indicate the expected location of defect 472) of defect 472 from its original location identified in inspection image 454.

[0070] Defect identification and detection in existing VC inspection techniques may be inadequate to consistently capture blinking defects because defects are identified based on a visual inspection difference between two frames and the other frames are disregarded for inspection. In the proposed method for sample inspection, however, the output data comprises information associated with each inspection image and defect detection is performed in each inspection image. Defects are identified and classified based on a comparison of defect signature from each inspection image, thereby significantly improving the capture rate of defects.

[0071] In some embodiments, characteristics of a defect may include the time of appearance of a defect, or the frame in which the defect first appeared, or a threshold dosage (Dt) causing the defect to appear in a frame, or stability of a defect, or blinking period of a defect, among other characteristics. The threshold dosage (Dt), as used herein, refers to the minimum dosage of charged particles irradiated on a sample for a defect to be detected in an inspection image. In some embodiments, the time of appearance of the defect in the image or the frame in which the defect first appeared may be used to determine the threshold dosage. As illustrated schematically in Fig. 4B, the number of defects detected may increase with an increase in the dosage applied. Accordingly, the number of defects detected in inspection image 455 corresponding to dosage D4 may be higher than the number of defects detected in inspection image D3 or D2 or Di because D4>D3>D2>DI. In other words, the threshold dosage of detection of a defect may be based on the data associated with the inspection image the defect first appears in.

[0072] In some embodiments, the threshold dosage (Dt) may be based on a defect type, or a defect location, or detection sensitivity of a charged-particle detector, among other factors. In some embodiments, the threshold dosage may be determined based on the capacitance of the features associated with a defect. As an example, referring to structure 370 of Fig. 3C, the capacitance associated with V3 and M4 is smaller compared to the capacitance associated with V3, M4, and M3, therefore, the detection dosage applied to detect defect 305_l may be smaller compared to the detection dosage applied to detect defect 305_2. As a further example, the detection dosage to detect defect 305_3, which is the electrical open between V2 and M2, may be higher than the detection dosages for 305_l and305_2, thereby allowing identification of the location of a defect in a structure based on information associated with threshold dosage (Dt) or capacitance of the associated features.

[0073] In some embodiments, defect characteristics may include defect stability or blinking period. If a defect consistently appears in each frame of the multiple frames acquired, the defect may be a stable defect. If a defect is a blinking defect, such as blinking defect 472, the blinking period of blinking defect 472 may be determined based on the time difference between its first and second appearances. The time information may be obtained based on the scan speed, scan frequency, or time delay between scans.

[0074] High-resistance defects, such as defect 308 illustrated in Fig. 3B, typically do not generate a lot of signal electrons and are therefore harder to detect by existing single-scan VC inspection techniques. This is because the thin resistive layer (e.g., oxide), either remaining or partially broken during the inspection scanning, provides a tunneling path for the electrons, resulting in low sensitivity for detection. The voltage contrast difference between a non-defective feature (e.g., contact channel 330 of Fig. 3B) and a defective feature (e.g., contact channel 318 with a high-resistance defect 308) in negative mode or positive mode VC inspection is low, as illustrated in corresponding feature images 325A and 3O8A, respectively. Therefore, it may be desirable to provide systems and methods to enhance the detection sensitivity for high-resistance defects by enhancing the difference in signal intensity between nondefective and defective features.

[0075] Reference is now made to Figs. 5A and 5B, which illustrate schematics of exemplary inspection images 510 and 520 of a ROI in single positive mode scan and a single negative mode scan, respectively. Exemplary inspection image 510 shows a uniform array of features detected using a single positive mode scan VC inspection technique and inspection image 520 shows a uniform array of features detected using a single negative mode scan VC inspection technique. In either mode, the ROI may be flooded with a high-dosage of charged particles to pre-scan the ROI surface to enhance the voltage contrast signal. Features 512 and 522 represent a normal contact or a non-defective feature in inspection images 510 and 520, respectively. Features 518 and 528 represent a defective feature (e.g., high- resistance defect) in inspection images 510 and 520, respectively. As previously described, because the sensitivity for detecting high-resistance defects in existing VC inspection techniques is low, the difference in voltage contrast signal is low, rendering the existing techniques inadequate for detection or identification, much less, classification of such defects.

[0076] Reference is now made to Figs. A and 6B, which illustrate an exemplary methodology 600 for sample inspection and a schematic plot of an exemplary plot of the number of defects and dosage (frames) for defect detection and classification, respectively, consistent with embodiments of the present disclosure. Methodology 600 may be used to enhance the detection sensitivity for high- resistance defects by enhancing the voltage contrast between non-defective and defective features.

[0077] In some embodiments, high-resistance defects such as defect 308, may be detected at a neutral mode, also referred to herein as a neutral condition. In the context of this disclosure, “neutral mode” refers to a mode or a condition when an electric potential of the sample surface is neither negative norpositive. A “neutrally charged” surface or a “substantially neutrally charged” sample refers to a surface or a sample with no net charge and where the negative and positive charges cancel each other. In this context, a negative mode of a sample refers to a condition where the signal electron yield of a conducting feature (e.g., a metal) is smaller than the signal electron yield of an insulating feature (e.g., an oxide) and a positive mode of a sample surface refers to a condition where the signal electron yield of a conducting feature is larger than the signal electron yield of an insulating feature. In the neutral mode, the signal electron yield of an insulating feature and a conducting feature are substantially similar. Here, the signal electron yield refers to a ratio of the total number of signal electrons generated (e.g., secondary electrons and backscattered electrons) to the total number of primary electrons irradiating the sample.

[0078] As previously described, high-resistance defects are harder to detect in a single scan VC inspection mode (negative mode or positive mode) because the signal intensity and signal sensitivity is low. The sensitivity for detection of high-resistance defects is maximum at a neutral mode, however, the existing inspection techniques are extremely unstable under neutral conditions, rendering the existing techniques inadequate for defect detection, identification, or classification.

[0079] In some embodiments, methodology 600 for defect detection may comprise a pre-charge step 605, an inspection scan step 625, and a defect classification step 650. Pre-charge step 605 may include exposing a ROI of a sample with a charged-particle beam to pre-charge the features within the ROI. Flooding the sample with charged particles such as electrons, may negatively charge the surface, thus enhancing the signal and voltage contrast of features in the ROI.

[0080] Inspection step 625 may comprise irradiating the ROI of a sample with a first dosage of charged-particle beam, such as an electron beam, to cause the features in the ROI to have a first polarity. In some embodiments, the first polarity refers to a negative state in which the sample surface is negatively charged. However, in some embodiments, the first polarity may refer to a positive state in which the sample surface is positively charged. In some embodiments, the ROI may be irradiated with a first dosage of charged-particles in a negative mode or in a positive mode. In the negative mode, the landing energy of the primary charged-particle beam on a sample may be high and a potential of a control electrode (e.g., control electrode 232b of Fig. 2) may be negative. In the positive mode, the landing energy of the primary charged-particle beam on a sample may be low but the potential of the control electrode may be positive. The potential of the control electrode may be configured to adjust the surface electric field, which may impact the accumulation of charges on the surface. The adjustment of surface electric field and the landing energy of the primary charged particles irradiating the ROI may influence the signal charged-particle yield. As previously described, in the negative mode, the signal electron yield of a conducting feature (e.g., a metal) is smaller than the signal electron yield of an insulating feature (e.g., an oxide) and in the positive mode, the signal electron yield of a conducting feature is larger than the signal electron yield of an insulating feature. In some embodiments, the landing energy of the primary charged particles and the potential of the control electrode may be adjusted toadjust the polarity and the beam current may be adjusted to adjust the dosage of the charged particles. The negative state, also referred to herein as the negative condition or negative charge of the surface, may negatively charge the features (e.g., contacts), such that the features appear as dark voltage contrast features.

[0081] In some embodiments, the first dosage of charged particles may comprise electrons forming a primary electron beam with a high average beam current. In some embodiments, the average beam current may be in the range of 100 nA-500 nA, or 200 nA-500 nA, or 300 nA-500 nA, or 400 nA-500 nA. In some embodiments, the average beam current of the high-dosage electron beam may be higher, as appropriate.

[0082] Inspection step 625 may further comprise scanning the ROI with a second dosage of primary charged particles to cause the polarity of the sample surface to switch or transition from the first polarity to a second polarity, opposite of the first polarity. If the sample pre-scan in pre-charge step 605 charges the sample surface negatively (first polarity), the second dosage of primary charged particles may be configured to switch the polarity to charge the surface positively (second polarity), and vice versa. In some embodiments, scanning the ROI with a second dosage may include scanning in the positive mode. In such a case, the landing energy of the primary charged-particle beam on a sample may be adjusted to be low and the potential of the control electrode may be positive.

[0083] Switching the polarity of the sample, for example from a negative state to a positive state, may comprise transitioning the polarity by applying incremental dosage of primary charged particles in the positive mode. Incremental dosage, as previously described, refers to the progressive increase in the dosage of charged particles. In switching the polarity, the incremental dosage may be applied in the opposite mode of the existing state. For example, if the sample is negatively charged, an incremental dosage of charged particles may be applied in the positive mode such that the sample is eventually positively charged. In the positive mode, the signal electron yield of the conducting feature is larger than the signal electron yield of the insulating feature, accordingly, the non-defective contacts (conducting) may appear as bright voltage contrast features and the defective contacts (high-resistance or non-conducting) may appear as dark.

[0084] Inspection step 625 may further comprise acquiring a plurality of inspection images, each image corresponding to a dosage of the incremental dosage. As illustrated in Fig. 6A, inspection image 610 of the ROI may correspond to the negative state of the sample, inspection image 612 of the ROI may correspond to dosage Di of incremental dosage, inspection image 614 of the ROI may correspond to a higher dosage D2 of incremental dosage, inspection image 616 of the ROI may correspond to an even higher dosage D3 of incremental dosage, and inspection image 618 of the ROI may correspond to dosage Dn. The polarity of the ROI may be increasingly positive from inspection image 610 to inspection image 618.

[0085] In transitioning from the negative state to the positive state, via application of charged particles in the positive mode, the polarity of the ROI may switch from negative to positive at a switchingcondition comprising a switching dosage of charged particles. Switching dosage, as used herein, refers to a dosage of the charged particles which causes the polarity of the ROI to switch from one polarity to the opposite polarity. In some embodiments, switching dosage may be a certain beam current, or a narrow distribution of beam currents, or a wider distribution of beam currents. An exemplary distribution of beam current for the switching dosage is described later with reference to Fig. 6B.

[0086] Inspection step 625 of methodology 600 may further comprise performing defect detection in each inspection image. Defect detection may be performed for each inspection image to generate a corresponding defect signature for each acquired inspection image. Performing defect detection may include, but is not limited to, performing a die-to-die comparison, or performing a cell-to-cell comparison, or performing a die-to-database pixel.

[0087] Referring to Fig. 6B, which illustrates an exemplary plot of a number of defects detected with respect to the dosage or the inspection image, consistent with embodiments of the present disclosure. The number of defects detected are represented on the vertical y-axis and the frame number or the inspection image corresponding to a dosage of the incremental dosage is represented on the horizontal x-axis.

[0088] In inspection image 610, the ROI of the sample is in a negative state due to the negative prescan condition. In such a state, upon detection, a defect such as an electrical open, also referred to as a hard open (e.g., defect 305 of Fig. 3B) may appear as a bright feature with a high gray level value. This is because in the negative mode, the signal electron yield of an insulating feature, such as an open defect, is larger than the signal electron yield of a conducting feature. In inspection image 612, corresponding to dosage Di of charged particles, the number of defects detected may be larger in comparison with defects detected in inspection image 610 due to the higher dosage of applied primary charged particles.

[0089] In some embodiments, dosage D2 may be the switching dosage Dsat which the polarity of the sample or the ROI of the sample switches from negative to positive. As previously alluded to, switching dosage Dsmay be a certain dosage or a range of dosages, as illustrated in Fig. 6B, and may represent a switching condition. The sample may be neutrally charged at the switching condition. In other words, the net charge or the polarity of the sample is substantially zero at the switching condition. In some embodiments, as illustrated in Fig. 6B, switching dosage Dsrange may comprise a leading edge and a trailing edge. The leading edge of switching dosage Dsrange may indicate the onset of appearance of a high-resistance defect. Although not illustrated, a plurality of inspection images may be acquired between inspection image 612 and inspection image 614 to enable defect detection at every dosage step of the incremental dosage applied to the sample, thereby enabling capturing the high-resistance defect at the neutral state.

[0090] In existing VC inspection techniques, a high-resistance defect may appear as a dark feature, similar to a normal contact, resulting in poor detection sensitivity and a low defect capture rate. In inspection images 610 and 612, a high-resistance defect and a normal contact may appear as a dark feature and an electrical open may appear as a bright feature. The voltage contrast difference betweena normal contact and an electrical open is significant, resulting in high sensitivity. However, because of the low voltage contrast difference between a normal contact and a high-resistance defect, the detection sensitivity for high-resistance defects is poor, resulting in low defect capture rate and a low inspection throughput.

[0091] In the negative state and in the positive state, corresponding to inspection images 610 and 618, respectively, a high-resistance defect may be indistinguishable from a normal contact because the difference in signal intensity is low. A high-resistance defect and a normal contact may both appear dark in inspection image 610 and may both appear bright in inspection image 618, rendering the detection of such defects challenging. For a high-resistance defect, however, the rate of transition from a negative polarity to a positive polarity is slower in comparison to a normal contact. Therefore, at the neutral condition or the switching condition, a high-resistance defect (e.g., defect 638) may appear as a darker feature in comparison to a normal contact (e.g., feature 632), as illustrated in Fig. 6C, thereby allowing the defect to be captured, identified, or classified.

[0092] Methodology 600 may further comprise a defect classification step 650. Upon determining, defects may be identified and classified based on a comparison of defect signature from each inspection image. Defects may be classified based on one or more defect characteristics. In some embodiments, characteristics of a defect may include the time of appearance of a defect, or the frame in which the defect first appeared, or a threshold dosage (Dt) causing the defect to appear in a frame, or stability of a defect, or blinking period of a defect, among other characteristics. The threshold dosage (Dt), as used herein, refers to the minimum dosage of charged particles irradiated on a sample for a defect to be detected in an inspection image. In some embodiments, the time of appearance of the defect in the image or the frame in which the defect first appeared may be used to determine the threshold dosage. As illustrated schematically in Fig. 4B, the number of defects detected may increase with an increase in the dosage applied. Accordingly, the number of defects detected in inspection image 455 corresponding to dosage D4 may be higher than the number of defects detected in inspection image D3 or D2 or Di because D4>D3>D2>Di. In other words, the threshold dosage of detection of a defect may be based on the data associated with the inspection image the defect first appears in. In some embodiments, the threshold dosage (Dt) may be based on a defect type, or a defect location, or detection sensitivity of a charged-particle detector, among other factors. In some embodiments, the threshold dosage may be determined based on the capacitance of the features associated with a defect. In some embodiments, defect characteristics may include defect stability or blinking period. If a defect consistently appears in each frame of the multiple frames acquired, the defect may be a stable defect. If a defect is a blinking defect (such as blinking defect 472), the blinking period of blinking defect 472 may be determined based on the time difference between its first and second appearances. The time information may be obtained based on the scan speed, scan frequency, or time delay between scans.

[0093] Reference is now made to Fig. 7, which illustrates an exemplary flowchart representing an exemplary method 700 of sample inspection for defect detection using a charged-particle beamapparatus, consistent with embodiments of the present disclosure. One or more steps of method 700 may be performed by controller 50 of EBI system 100, as shown in Fig. 2, for example. Here, controller 50 may instruct a module of a charged-particle beam apparatus to activate a charged-particle source to generate primary charged particle beam (e.g., electron beam), or acquire an inspection image based on detected signal charged particles generated from the sample, or carry out other related functions.

[0094] In step 710, a region of a sample comprising a plurality of features may be charged to a first polarity. A charged-particle source of a charged-particle beam apparatus such as a SEM, is activated to emit charged particles (e.g., electrons). The charged particles may form a charged-particle beam (e.g., primary charged-particle beam 204 of Fig. 2 or a primary electron beam). The electron source may be activated by a controller (e.g., controller 50 of Fig. 2). For example, the electron source may be controlled to emit primary electrons to form an electron beam along a primary optical axis (e.g., primary optical axis 201 of Fig. 2). The electron source may be activated remotely, for example, by using a software, an application, or a set of instructions for a processor of a controller to power the electron source through a control circuitry. The primary electron beam may pass through a Coulomb aperture array (e.g., Coulomb aperture array 224 of Fig. 2) and a beam-limit aperture array (not shown) to adjust the beam size or beam current of the primary electron beam and form a probing beam incident on the sample (e.g., sample 250 of Fig. 2). The primary electron beam may be configured to irradiate a ROI of a sample with a high dosage of primary electrons to pre-charge the sample.

[0095] In some embodiments, the first polarity refers to a negative state in which the sample surface is negatively charged. However, in some embodiments, the first polarity may refer to a positive state in which the sample surface is positively charged. In some embodiments, the ROI may be irradiated with a first dosage of charged-particles in a negative mode or in a positive mode. In the negative mode, the landing energy of the primary charged-particle beam on a sample may be high and a potential of a control electrode (e.g., control electrode 232b of Fig. 2) may be negative. In the positive mode, the landing energy of the primary charged-particle beam on a sample may be low but the potential of the control electrode may be positive. In some embodiments, the landing energy of the primary charged particles and the potential of the control electrode may be adjusted to adjust the polarity and the beam current may be adjusted to adjust the dosage of the charged particles. In some embodiments, the average beam current may be in the range of 100 nA-500 nA, or 200 nA-500 nA, or 300 nA-500 nA, or 400 nA- 500 nA.

[0096] In step 720, the polarity of the ROI may be switched from the first polarity to a second polarity at a switching condition. The switching condition may comprise a switching dosage of primary charged particles. The ROI may be scanned with a second dosage of primary charged particles to cause the polarity of the sample surface to transition from the first polarity to a second polarity, opposite of the first polarity. If the sample pre-scan charges the sample surface negatively (first polarity), the second dosage of primary charged particles may be configured to switch the polarity to charge the surface positively (second polarity), and vice versa. In some embodiments, scanning the ROI with a seconddosage may include scanning in the positive mode. In such a case, the landing energy of the primary charged-particle beam on a sample may be adjusted to be low and the potential of the control electrode may be positive. Switching the polarity of the sample, for example from negative state to a positive state, may comprise transitioning the polarity by applying incremental dosage of primary charged particles in the positive mode. Incremental dosage, as previously described, refers to the progressive increase in the dosage of charged particles. In switching the polarity, the incremental dosage may be applied in the opposite mode of the existing state. For example, if the sample is negatively charged, an incremental dosage of charged particles may be applied in the positive mode such that the sample is eventually positively charged. In the positive mode, the signal electron yield of the conducting feature is larger than the signal electron yield of the insulating feature, accordingly, the non-defective contacts (conducting) may appear as bright voltage contrast features and the defective contacts (high-resistance or non-conducting) may appear as dark.

[0097] Switching dosage Dsrefers to the dosage of charged particles (e.g., electrons) that causes the polarity of the sample or the ROI of the sample to switch from negative to positive. Switching dosage Dsmay be a certain dosage or a range of dosages, as illustrated in Fig. 6B, and may represent a switching condition. The sample may be neutrally charged at the switching condition. In other words, the net charge or the polarity of the sample is substantially zero at the switching condition. In some embodiments, switching dosage may be a certain beam current, or a narrow distribution of beam currents, or a wide distribution of beam currents.

[0098] In step 730, an inspection image of the ROI may be formed from signal charged particles such as, but not limited to, secondary electrons, backscattered electrons, etc., generated upon interaction of the switching dosage of primary charged particles with the plurality of features. In some embodiments, one or more inspection images may be acquired, each image corresponding to an applied dosage of the incremental dosage. In some embodiments, a charged particle detector may be used to detect signal charged particles generated from the ROI of the sample upon interaction with the charged particles of the primary charged-particle beam. An image may be formed based on the detected signal charged particles. The image formed may comprise a SEM image, or other suitable image in gray scale.

[0099] In step 740, one or more defects may be detected in the inspection image corresponding to the switching dosage. Defect detection may be performed for the inspection image to generate a corresponding defect signature. Performing defect detection may include, but is not limited to, performing a die-to-die comparison, or performing a cell-to-cell comparison, or performing a die-to- database pixel. In a die-to-die mode, inspection images of two or more dies may be compared to detect defects within an inspection image. For example, a multi-scan inspection of a sample comprising multiple dies may include acquiring 25 frames of each die on a wafer and defects may be identified within each frame for every die. In such a scenario, for example, the 20thframe for every die may be compared to identify a defect. The comparison of frames or inspection images may include visual inspection to identify the defect based on a gray level value of the feature, for example. In a cell-to-cellmode, defect detection may be based on the locations of features within a cell or a repeating unit of a die. For example, a multi-scan inspection of a ROI comprising cells may include acquiring multiple images of the cells. In an inspection image of a cell including a plurality of arrayed features, presence of a feature at an unintended location or absence of a feature at an expected location of an array may indicate the existence of a defect. The intended or the expected location may be based on the characteristics of the arrayed pattern of features within a cell or in other similar cells. In a die-to- database pixel counting mode, inspection image of a die may be compared to a reference image or a reference layout of the die. The reference image or reference layout may be a database image or a reference feature layout. The reference feature information may be obtained from an information file comprising a wafer design layout in Graphic Database System (GDS) format, Graphic Database System II (GDS II) format including a graphical representation of the features on the wafer surface, or an Open Artwork System Interchange Standard (OASIS) format.

[0100] A defect may be detected based on the gray level values of the features in the acquired inspection image(s). In the negative state and in the positive state, a high-resistance defect may be indistinguishable from a normal contact because the difference in signal intensity is negligible. A high- resistance defect and a normal contact may both appear dark in an inspection image (e.g., inspection image 610 of Fig. 6A), or they may both appear bright in an inspection image (e.g., inspection image 618 of Fig. 6A), rendering the detection of such defects challenging. For a high-resistance defect, however, the rate of transition from a negative polarity to a positive polarity is slower in comparison to a normal contact. Therefore, at the neutral condition or the switching condition, a high-resistance defect (e.g., defect 638) may appear as a darker feature in comparison to a normal contact (e.g., feature 632), as illustrated in Fig. 6C, thereby allowing determining whether there is a defect based on a gray level value of the feature.

[0101] Some of the advantages of dosage-controlled voltage contrast defect detection include improved inspection throughput, higher defect detection sensitivity, classification of defects, improved detection signal, identification of the location of defects, compatibility with single beam and multibeam inspection apparatuses, among other things.

[0102] A non-transitory computer readable medium may be provided that stores instructions for a processor of a controller (e.g., controller 50 of Fig. 1) to carry out image inspection, image acquisition, activating charged-particle source, irradiating a region of a sample with a first dosage of charged particles, inspecting a region of a sample with a second dosage of charged particles different from the first dosage, determining whether there is a defect based on a gray level value of a feature from an acquired image of the region of the sample, and the functionalities described in methods 400 and 700. Common forms of non-transitory media include, for example, a floppy disk, a flexible disk, hard disk, solid state drive, magnetic tape, or any other magnetic data storage medium, a Compact Disc Read Only Memory (CD-ROM), any other optical data storage medium, any physical medium with patterns of holes, a Random Access Memory (RAM), a Programmable Read Only Memory (PROM), and ErasableProgrammable Read Only Memory (EPROM), a FLASH-EPROM or any other flash memory, Non- Volatile Random Access Memory (NVRAM), a cache, a register, any other memory chip or cartridge, and networked versions of the same.

[0103] The embodiments of the present disclosure may further be described using the following clauses:1. A method for inspecting a sample using a charged-particle beam apparatus, the method comprising: causing a region of a sample comprising a plurality of features to be charged to a first polarity; causing the region to switch from the first polarity to a second polarity at a switching condition, the switching condition comprising a switching dosage of primary charged particles; forming an inspection image of the region from signal charged particles generated after interaction of the switching dosage of primary charged particles with the plurality of features; and determining whether a feature of the plurality of features is defective based on a gray level value of the feature in the inspection image.2. The method of clause 1 , wherein charging the region to the first polarity comprises flooding the region with a first dosage of primary charged particles emitted from a charged-particle source of the charged-particle beam apparatus.3. The method of any one of clauses 1 and 2, wherein the first dosage of primary charged particles is configured to pre-charge the region.4. The method of any one of clauses 1-3, wherein the switching dosage is smaller than the first dosage of primary charged particles.5. The method of any one of clauses 1-4, further comprising irradiating the region with a second dosage of primary charged particles configured to form a probing beam.6. The method of any one of clauses 1-5, further comprising adjusting an electric field associated with the region to adjust a net charge on the irradiated region.7. The method of clause 6, wherein adjusting the electric field comprises: adjusting a landing energy of the primary charged particles on the sample; and adjusting accumulation of the signal charged particles on the sample.8. The method of any one of clauses 1-7, wherein the first polarity of the region comprises a negative charge and the second polarity of the region comprises a positive charge.9. The method of any one of clauses 1-8, wherein the region is neutrally charged at the switching condition.10. The method of any one of clauses 1 -9, wherein a gray level value of a defective feature is higher than a gray level value of a non-defective feature.11. The method of clause 10, further comprising identifying a defect associated with the defective feature based on the switching dosage of primary charged particles.Z112. The method of clause 11, wherein the defect comprises an electrical open or a high-resistance defect.13. The method of any one of clauses 1-12, wherein the feature comprises a contact pad configured to form an electrical connection to a capacitor.14. The method of clause 13, wherein the capacitor comprises a word-line of a memory device.15. A method for inspecting a sample using a charged-particle beam apparatus, the method comprising: causing a region of a sample comprising a plurality of features to be charged to a first polarity by irradiating the region with a first dosage of primary charged particles; causing the region to be charged to a second polarity by scanning the region with a plurality of scans of a second dosage of primary charged particles; forming a plurality of inspection images from signal charged particles generated after interaction of primary charged particles of each scan of the plurality of scans with the region; determining whether a feature is defective based on a comparison of gray level value of the plurality of features in an inspection image of the plurality of inspection images; and after determining, identifying a characteristic of a defect based on the inspection image and a corresponding dosage of primary charged particles.16. The method of clause 15, further comprising switching from the first polarity to the second polarity at a switching dosage of primary charged particles, wherein the switching dosage comprises at least one applied dosage of the second dosage of primary charged particles.17. The method of clause 16, wherein the switching dosage is smaller than the first dosage of primary charged particles.18. The method of any one of clauses 15-17, wherein the first dosage of primary charged particles is configured to pre-charge the region.19. The method of any one of clauses 15-18, wherein the second dosage of primary charged particles is configured to form a probing beam.20. The method of any one of clauses 15-19, further comprising adjusting an electric field associated with the region to adjust a net charge on the irradiated region.21. The method of clause 20, wherein adjusting the electric field comprises: adjusting a landing energy of the primary charged particles on the sample; and adjusting accumulation of the signal charged particles on the sample.22. The method of any one of clauses 15-21, wherein the first polarity of the region comprises a negative charge and the second polarity of the region comprises a positive charge.23. The method of any one of clauses 16-22, wherein the region is neutrally charged at the switching dosage.24. The method of any one of clauses 16-23, wherein a gray level value of a defective feature is higher than a gray level value of a non-defective feature in the inspection image corresponding to the switching dosage.25. The method of clause 24, further comprising identifying a defect associated with the defective feature based on the switching dosage of primary charged particles.26. The method of clause 25, wherein the defect comprises an electrical open or a high-resistance defect.27. The method of any one of clauses 15-26, wherein the feature comprises a contact pad, the contact pad configured to form an electrical connection to a capacitor.28. The method of clause 27, wherein the capacitor comprises a word-line of a memory device.29. The method of any one of clauses 15-28, wherein the plurality of scans comprises a first scan and a second scan, and wherein a dosage of the second scan is higher than a dosage of the first scan.30. The method of clause 29, wherein the dosage for the second scan is a sum of the first scan and the second scan.31. The method of any one of clauses 29 and 30, wherein the plurality of scans further comprises a third scan, and wherein a dosage of the third scan is a sum of the dosages for the first scan, the second scan, and the third scan.32. A charged-particle beam apparatus comprising: a charged-particle source configured to emit charged particles, the emitted charged particles forming a primary charged-particle beam, wherein the charged-particle beam apparatus is configured to: cause a region of a sample comprising a plurality of features to be charged to a first polarity, and cause the region to switch from the first polarity to a second polarity at a switching condition, the switching condition comprising a switching dosage of primary charged particles; and a controller including circuitry configured to: acquire an inspection image of the region from signal charged particles generated upon interaction of the switching dosage of primary charged particles with the plurality of features, and determine whether a feature is defective based on a gray level value of the feature in the inspection image.33. The apparatus of clause 32, wherein the charged-particle beam apparatus is configured to charge the region to the first polarity by irradiation of the region with a first dosage of charged particles of the primary charged-particle beam configured to pre-charge the region.34. The apparatus of any one of clauses 32 and 33, wherein the first dosage of primary charged particles is configured to pre-charge the region.35. The apparatus of any one of clauses 32-34, wherein the switching dosage is smaller than the first dosage of primary charged particles.36. The apparatus of any one of clauses 32-35, wherein the charged-particle beam apparatus is configured to charge the region to the second polarity by irradiation of the region with a second dosage of charged particles of the primary charged-particle beam configured to probe the region.37. The apparatus of any one of clauses 32-36, the charged-particle beam apparatus is further configured to adjust an electric field associated with the region to adjust a net charge on the irradiated region.38. The apparatus of clause 37, wherein the adjustment of the electric field associated with the region comprises: an adjustment of a landing energy of the primary charged particles on the sample; and an adjustment of accumulation of the signal charged particles on the sample.39. The apparatus of any one of clauses 32-38, wherein the first polarity of the region comprises a negative charge and the second polarity of the region comprises a positive charge.40. The apparatus of any one of clauses 32-39, wherein the charged-particle beam apparatus is configured to neutrally charge the region at the switching condition.41. The apparatus of any one of clauses 32-40, wherein a gray level value of a defective feature is higher than a gray level value of a non-defective feature.42. The apparatus of clause 41, wherein the controller including circuitry is further configured to identify a defect associated with the defective feature based on the switching dosage of primary charged particles.43. The apparatus of clause 42, wherein the defect comprises an electrical open or a high-resistance defect.44. The apparatus of any one of clauses 32-43, wherein the feature comprises a contact pad configured to form an electrical connection to a capacitor.45. The apparatus of clause 44, wherein the capacitor comprises a word-line of a memory device.46. A charged-particle beam apparatus comprising: a charged-particle source configured to emit charged particles, the emitted charged particles forming a primary charged-particle beam, wherein the charged-particle beam apparatus is configured to: cause a region of a sample comprising a plurality of features to be charged to a first polarity by irradiating the region with a first dosage of primary charged particles, and cause the region to be charged to a second polarity by scanning the region with a plurality of scans of a second dosage of primary charged particles; and a controller including circuitry configured to: acquire a plurality of inspection images from signal charged particles generated after interaction of primary charged particles of each scan of the plurality of scans with the region, determine whether a feature is defective based on a comparison of gray level value of the plurality of features in an inspection image of the plurality of inspection images, andidentify, after determining, a characteristic of a defect based on the inspection image and a corresponding dosage of primary charged particles.47. The apparatus of clause 46, wherein the charged-particle beam apparatus is further configured to cause a switch from the first polarity to the second polarity at a switching dosage of primary charged particles, wherein the switching dosage comprises at least one applied dosage of the second dosage of primary charged particles.48. The apparatus of clause 47, wherein the switching dosage is smaller than the first dosage of primary charged particles.49. The apparatus of any one of clauses 46-48, wherein the first dosage of primary charged particles is configured to pre-charge the region.50. The apparatus of any one of clauses 46-49, wherein the second dosage of primary charged particles is configured to probe the region.51. The apparatus of any one of clauses 46-50, wherein the charged-particle beam apparatus is further configured to adjust an electric field associated with the region to adjust a net charge on the irradiated region.52. The apparatus of clause 51, wherein an adjustment of the electric field associated with the region comprises: an adjustment of a landing energy of the primary charged particles on the sample; and an adjustment of accumulation of the signal charged particles on the sample.53. The apparatus of any one of clauses 46-52, wherein the first polarity of the region comprises a negative charge and the second polarity of the region comprises a positive charge.54. The apparatus of any one of clauses 46-53, wherein the charged-particle beam apparatus is further configured to neutrally charge the region at the switching dosage.55. The apparatus of any one of clauses 46-54, wherein a gray level value of a defective feature is higher than a gray level value of a non-defective feature in the inspection image corresponding to the switching dosage.56. The apparatus of clause 55, wherein the controller including circuitry is further configured to identify a defect associated with the defective feature based on the switching dosage of primary charged particles.57. The apparatus of clause 56, wherein the defect comprises an electrical open or a high-resistance defect.58. The apparatus of any one of clauses 46-57, wherein the feature comprises a contact pad, the contact pad configured to form an electrical connection to a capacitor.59. The apparatus of clause 58, wherein the capacitor comprises a word-line of a memory device.60. The apparatus of any one of clauses 46-59, wherein the plurality of scans comprises a first scan and a second scan, and wherein a dosage of the second scan is higher than a dosage of the first scan.61. The apparatus of clause 60, wherein the dosage for the second scan is twice the dosage for the first scan.62. The apparatus of any one of clauses 60 and 61 , wherein the plurality of scans further comprises a third scan, and wherein a dosage of the third scan is a sum of the dosages for the first scan and the second scan.63. A non-transitory computer readable medium storing a set of instructions that is executable by one or more processors of a charged-particle beam apparatus to cause the charged-particle beam apparatus to perform a method for inspecting a sample, the method comprising: activating a charged-particle source configured to emit charged particles, the emitted charged particles forming a primary charged-particle beam; irradiating a region of a sample comprising a plurality of features using the primary charged- particle beam to charge the region to a first polarity; irradiating the region using the primary charged-particle beam to switch from the first polarity to a second polarity at a switching condition, the switching condition comprising a switching dosage of primary charged particles; forming an inspection image of the region from signal charged particles generated after interaction of the switching dosage of primary charged particles with the plurality of features; and determining whether a feature of the plurality of features is defective based on a gray level value of the feature in the inspection image.64. A non-transitory computer readable medium storing a set of instructions that is executable by one or more processors of a charged-particle beam apparatus to cause the charged-particle beam apparatus to perform a method for inspecting a sample, the method comprising: activating a charged-particle source configured to emit charged particles, the emitted charged particles forming a primary charged-particle beam; irradiating a region of a sample comprising a plurality of features using a first dosage of charged particles of the primary charged-particle beam to charge the region to a first polarity; scanning the region with a plurality of scans of a second dosage of charged particles of the primary charged-particle beam to charge the region with a second polarity; forming a plurality of inspection images from signal charged particles generated after interaction of primary charged particles of each scan of the plurality of scans with the region; determining whether a feature is defective based on a comparison of gray level value of the plurality of features in an inspection image of the plurality of inspection images; and after determining, identifying a characteristic of a defect based on the inspection image and a corresponding dosage of primary charged particles.65. The non-transitory computer readable medium of clause 64, wherein the method further comprises: inspecting at least two inspection images of the plurality of inspection images;determining whether the defective feature is present in the at least two inspection images; and after determining, storing in a database, information associated with the at least two inspection images and the corresponding dosages.66. The non-transitory computer readable medium of clause 65, wherein the method further comprises inspecting each image of the plurality of inspection images.

[0104] It will be appreciated that the embodiments of the present disclosure are not limited to the exact construction that has been described above and illustrated in the accompanying drawings, and that various modifications and changes may be made without departing from the scope thereof. The present disclosure has been described in connection with various embodiments, other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

[0105] The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A method for inspecting a sample using a charged-particle beam apparatus, the method comprising: causing a region of a sample comprising a plurality of features to be charged to a first polarity; causing the region to switch from the first polarity to a second polarity at a switching condition, the switching condition comprising a switching dosage of primary charged particles; forming an inspection image of the region from signal charged particles generated after interaction of the switching dosage of primary charged particles with the plurality of features; and determining whether a feature of the plurality of features is defective based on a gray level value of the feature in the inspection image.

2. A charged-particle beam apparatus comprising: a charged-particle source configured to emit charged particles, the emitted charged particles forming a primary charged-particle beam, wherein the charged-particle beam apparatus is configured to: cause a region of a sample comprising a plurality of features to be charged to a first polarity, and cause the region to switch from the first polarity to a second polarity at a switching condition, the switching condition comprising a switching dosage of primary charged particles; and a controller including circuitry configured to: acquire an inspection image of the region from signal charged particles generated upon interaction of the switching dosage of primary charged particles with the plurality of features, and determine whether a feature is defective based on a gray level value of the feature in the inspection image.

3. The apparatus of claim 2, wherein the charged-particle beam apparatus is configured to charge the region to the first polarity by irradiation of the region with a first dosage of charged particles of the primary charged-particle beam configured to pre-charge the region.

4. The apparatus of claim 2, wherein the first dosage of primary charged particles is configured to pre-charge the region.

5. The apparatus of claim 2, wherein the switching dosage is smaller than the first dosage of primary charged particles.

6. The apparatus of claim 2, wherein the charged-particle beam apparatus is configured to charge the region to the second polarity by irradiation of the region with a second dosage of charged particles of the primary charged-particle beam configured to probe the region.

7. The apparatus of claim 2, the charged-particle beam apparatus is further configured to adjust an electric field associated with the region to adjust a net charge on the irradiated region.

8. The apparatus of claim 7, wherein the adjustment of the electric field associated with the region comprises: an adjustment of a landing energy of the primary charged particles on the sample; and an adjustment of accumulation of the signal charged particles on the sample.

9. The apparatus of claim 2, wherein the first polarity of the region comprises a negative charge and the second polarity of the region comprises a positive charge.

10. The apparatus of claim 2, wherein the charged-particle beam apparatus is configured to neutrally charge the region at the switching condition.

11. The apparatus of claim 2, wherein a gray level value of a defective feature is higher than a gray level value of a non-defective feature.

12. The apparatus of claim 11, wherein the controller including circuitry is further configured to identify a defect associated with the defective feature based on the switching dosage of primary charged particles.

13. The apparatus of claim 12, wherein the defect comprises an electrical open or a high-resistance defect.

14. The apparatus of claim 2, wherein the feature comprises a contact pad configured to form an electrical connection to a capacitor.

15. A non-transitory computer readable medium storing a set of instructions that is executable by one or more processors of a charged-particle beam apparatus to cause the charged-particle beam apparatus to perform a method for inspecting a sample, the method comprising: activating a charged-particle source configured to emit charged particles, the emitted charged particles forming a primary charged-particle beam;irradiating a region of a sample comprising a plurality of features using a first dosage of charged particles of the primary charged-particle beam to charge the region to a first polarity; scanning the region with a plurality of scans of a second dosage of charged particles of the primary charged-particle beam to charge the region with a second polarity; forming a plurality of inspection images from signal charged particles generated after interaction of primary charged particles of each scan of the plurality of scans with the region; determining whether a feature is defective based on a comparison of gray level value of the plurality of features in an inspection image of the plurality of inspection images; and after determining, identifying a characteristic of a defect based on the inspection image and a corresponding dosage of primary charged particles.

Images