CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of US application 63/417,945 which was filed on October 20, 2022 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The embodiments provided herein relate to an inspection image enhancement technology, and more particularly to inspection image enhancement by concurrently performing a focusing sequence and local alignment.

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 physical sizes of IC components continue to shrink, accuracy and yield in defect detection become increasingly important. Inspection images such as SEM images can be used to identify or classify a defect(s) of the manufactured ICs. To improve defect detection performance, an inspection image enhancement technique that can improve throughput of the inspection systems is desired.

SUMMARY

[0004] The embodiments provided herein disclose a particle beam inspection apparatus, and more particularly, an inspection method of concurrent focus and image alignment of an inspection image.

[0005] Some embodiments provide a method for enhancing an inspection image. The method comprises acquiring a plurality of inspection images according to a plurality of focus conditions for an area of a sample containing a pattern, determining whether an inspection image of the plurality of inspection images has a focus index within a first threshold range, and in response to a determination that the inspection image has a focus index within the first threshold range, performing an image alignment using the inspection image.

[0006] In some embodiments, the method comprises setting a plurality of focus conditions for an area of a sample containing a pattern, acquiring a plurality of inspection images according to the plurality of focus conditions, determining a focus index for each of the plurality of inspection images, identifying, among the plurality of inspection images, an inspection image of which focus index is determine to be within a first threshold range, estimating an alignment parameter for the inspection image of which focus index is determined to be within the first threshold range, identifying among the plurality of inspection images one or more inspection images of which focus index is determined to be within a second threshold range, selecting a focus condition associated with an inspection image determined to be within the second threshold range having the largest focus index, and performing an image alignment and focus correction based on the estimated alignment parameter and selected focus condition.

[0007] In some embodiments, an apparatus for enhancing an inspection image is provided. The apparatus comprises a memory storing a set of instructions, and at least one processor configured to execute the set of instructions to cause the apparatus to perform: acquiring a plurality of inspection images according to a plurality of focus conditions for an area of a sample containing a pattern, , determining whether an inspection image of the plurality of inspection images has a focus index within a first threshold range, and in response to a determination that the inspection image has a focus index within the first threshold range, performing an image alignment using the inspection image.

[0008] In some embodiments, the apparatus comprises a memory storing a set of instructions, and at least one processor configured to execute the set of instructions to cause the apparatus to perform: setting a plurality of focus conditions for an area of a sample containing a pattern, acquiring a plurality of inspection images according to the plurality of focus conditions, determining a focus index for each of the plurality of inspection images, identifying, among the plurality of inspection images, an inspection image of which focus index is determine to be within a first threshold range, estimating an alignment parameter for the inspection image of which focus index is determined to be within the first threshold range, identifying among the plurality of inspection images one or more inspection images of which focus index is determined to be within a second threshold range, selecting a focus condition associated with an inspection image determined to be within the second threshold range having the largest focus index, and performing an image alignment and focus correction based on the estimated alignment parameter and selected focus condition.

[0009] In some embodiments, a non-transitory computer readable medium comprising a set of instructions that is executable by one or more processors of a computing device to cause the computing device to perform a method for enhancing an inspection image is provided. The method comprises acquiring a plurality of inspection images according to a plurality of focus conditions for an area of a sample containing a pattern, determining whether an inspection image of the plurality of inspection images has a focus index within a first threshold range, and in response to a determination that the inspection image has a focus index within the first threshold range, performing an image alignment using the inspection image.

[0010] In some embodiments, the non-transitory computer readable medium comprises a set of instructions that is executable by one or more processors of a computing device to cause the computing device to perform a method for enhancing an inspection image, the method comprising: setting a plurality of focus conditions for an area of a sample containing a pattern, acquiring a plurality of inspection images according to the plurality of focus conditions, determining a focus index for each of the plurality of inspection images, identifying, among the plurality of inspection images, an inspection image of which focus index is determine to be within a first threshold range, estimating an alignment parameter for the inspection image of which focus index is determined to be within the first threshold range, identifying among the plurality of inspection images one or more inspection images of which focus index is determined to be within a second threshold range, selecting a focus condition associated with an inspection image determined to be within the second threshold range having the largest focus index, and performing an image alignment and focus correction based on the estimated alignment parameter and selected focus condition.

[0011] 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

[0012] The above and other aspects of the present disclosure will become more apparent from the description of exemplary embodiments, taken in conjunction with the accompanying drawings.

[0013] FIG. 1 is a schematic diagram illustrating an example charged-particle beam inspection system, consistent with embodiments of the present disclosure.

[0014] FIG. 2 is a schematic diagram illustrating an example multi-beam tool that can be a part of the example charged-particle beam inspection system of FIG. 1, consistent with embodiments of the present disclosure.

[0015] FIG. 3 is a block diagram of an example concurrent autofocus and local alignment system, consistent with embodiments of the present disclosure.

[0016] FIG. 4A illustrates example inspection areas consistent with embodiments of the present disclosure.

[0017] FIG. 4B illustrates an example inspection image batch collection according to various focus conditions, consistent with embodiments of the present disclosure.

[0018] FIG. 4C illustrates an example first focus index threshold range, consistent with embodiments of the present disclosure.

[0019] FIG. 4D illustrates an example second focus index threshold range, consistent with embodiments of the present disclosure.

[0020] FIG. 5. illustrates an example procedure of local alignment, consistent with embodiments of the present disclosure.

[0021] FIG. 6 illustrates an example alignment parameter estimation, consistent with embodiments of the present disclosure.

[0022] FIG. 7 is a process flowchart representing an example concurrent autofocus and local alignment method, 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 accompanying 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. Instead, they are merely examples of apparatuses and methods consistent with aspects related to the disclosed embodiments as recited in the appended claims. For example, although some embodiments are described in the context of utilizing electron beams, 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, including but not limited to, optical imaging, photon detection, x- ray detection, ion detection, etc.

[0024] Electronic devices are constructed of circuits formed on a piece of semiconductor material called a substrate. The semiconductor material may include, for example, silicon, gallium arsenide, indium phosphide, or silicon germanium, or the like. Many circuits may be formed together on the same piece of silicon and are called integrated circuits or ICs. The size of these circuits has decreased dramatically so that many more of them can be fit on the substrate. For example, an IC chip in a smartphone can be as small as a thumbnail and yet may include over 2 billion transistors, the size of each transistor being less than l/1000th the size of a human hair.

[0025] Making these ICs with extremely small structures or components is a complex, timeconsuming, 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, 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 an inspection tool such as, for example, a scanning charged-particle microscope (SCPM). For example, an SCPM may be a scanning electron microscope (SEM). A SCPM inspection tool can be used to image these extremely small structures, in effect, taking a “picture” of the structures of the wafer to generate an inspection image. The inspection image can be used to determine if the structure was formed properly in the proper location. If the structure is defective, then the process can be adjusted, so the defect is less likely to recur. The inspection image can also be used for auto focusing and local alignment of the IC structure.

[0027] As the physical sizes of IC components continue to shrink, accuracy and yield in defect detection become more important. Inspection images such as SCPM images can be used for metrology measurements (e.g., to identify or classify a defect(s)) of the manufactured ICs. Measurements including, but not limited to, critical dimensions of the inspection image may be used to identify defects on the wafer. In order to perform metrology measurements more accurately, it is desired to obtain inspection images that are accurately focused and properly aligned. To obtain accurate inspection images, image enhancement techniques such as auto focusing and local alignment are usually performed. Auto focusing is a technique to automatically determine when an image is out of focus and instantly initiate a focusing operation to obtain an in-focus inspection image. During auto focusing, multiple inspection images with different focus conditions can be acquired for a certain area of a sample and the optimal focus condition for the area can be determined based on sharpness indexes of the acquired multiple inspection images. Because a defect can be identified or detected by comparing an inspection image with a corresponding reference image, obtaining an accurate alignment between the inspection image and the reference image is desired to improve defect detection performance. Local alignment is a technique employed to align an inspection image to a reference image. Current applications of image enhancement such as auto focus and local alignment are done sequentially and independently. For clearer inspection images, auto focusing is often performed at greater magnifications and thus a smaller field of view, whereas local alignment is performed with a larger field of view for comprehensive structural comparison. This requires a different set of images for both methods and therefore increases the time and effort associated with image enhancement. Thus, defect detection throughput for ICs is decreased. A more efficient and expeditious image enhancement method is therefore desired for more efficient and rapid defect detection.

[0028] Embodiments of the disclosure may provide a concurrent autofocus and image alignment technique. According to some embodiments of the present disclosure, an accurate and expeditious concurrent autofocus and image alignment technique with improved throughput may be provided. According to some embodiments of the present disclosure, an acquired inspection image may be used for auto focus and image alignment techniques. According to some embodiments of the present disclosure, a plurality of acquired inspection images may be used for auto focus and image alignment techniques.

[0029] 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. [0030] FIG. 1 illustrates an example electron beam inspection (EBI) system 100 consistent with embodiments of the present disclosure. EBI system 100 may be used for imaging. As shown in FIG. 1, EBI system 100 includes a main chamber 101, a load/lock chamber 102, a beam tool 104, and an equipment front end module (EFEM) 106. Beam tool 104 is located within main chamber 101. EFEM 106 includes a first loading port 106a and a second loading port 106b. EFEM 106 may include additional loading port(s). First loading port 106a and second loading port 106b 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 may be used interchangeably). A “lot” is a plurality of wafers that may be loaded for processing as a batch.

[0031] One or more robotic arms (not shown) in EFEM 106 may transport the wafers to load/lock chamber 102. Load/lock chamber 102 is connected to a load/lock vacuum pump system (not shown) which removes gas molecules in load/lock chamber 102 to reach a first pressure below the atmospheric pressure. After reaching the first pressure, one or more robotic arms (not shown) may transport the wafer from load/lock chamber 102 to main chamber 101. Main chamber 101 is connected to a main chamber vacuum pump system (not shown) which removes gas molecules in main chamber 101 to reach a second pressure below the first pressure. After reaching the second pressure, the wafer is subject to inspection by beam tool 104. Beam tool 104 may be a single -beam system or a multi-beam system.

[0032] A controller 109 is electronically connected to beam tool 104. Controller 109 may be a computer configured to execute various controls of EBI system 100. Controller 109 may also include processing circuitry configured to execute various signal and image processing functions. While controller 109 is shown in FIG. 1 as being outside of the structure that includes main chamber 101, load/lock chamber 102, and EFEM 106, it is appreciated that controller 109 may be a part of the structure.

[0033] In some embodiments, controller 109 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, a hardware accelerator, 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), and any type 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.

[0034] In some embodiments, controller 109 may further include one or more memories (not shown). A memory may be a generic or specific electronic device capable of storing codes and data accessible by the processor (e.g., via a bus). For example, the memory may include any combination of any number of a random-access memory (RAM), a read-only memory (ROM), an optical disc, a magnetic disk, a hard drive, a solid-state drive, a flash drive, a security digital (SD) card, a memory stick, a compact flash (CF) card, or any type of storage device. The codes and data may include an operating system (OS) and one or more application programs (or “apps”) for specific tasks. The memory may also be a virtual memory that includes one or more memories distributed across multiple machines or devices coupled via a network.

[0035] FIG. 2 illustrates a schematic diagram of an example multi-beam tool 104 (also referred to herein as apparatus 104) and an image processing system 290 that may be configured for use in EBI system 100 (FIG. 1), consistent with embodiments of the present disclosure.

[0036] Beam tool 104 comprises a charged-particle source 202, a gun aperture 204, a condenser lens 206, a primary charged-particle beam 210 emitted from charged-particle source 202, a source conversion unit 212, a plurality of beamlets 214, 216, and 218 of primary charged-particle beam 210, a primary projection optical system 220, a motorized wafer stage 280, a wafer holder 282, multiple secondary charged-particle beams 236, 238, and 240, a secondary optical system 242, and a charged- particle detection device 244. Primary projection optical system 220 may comprise a beam separator 222, a deflection scanning unit 226, and an objective lens 228. Charged-particle detection device 244 may comprise detection sub-regions 246, 248, and 250.

[0037] Charged-particle source 202, gun aperture 204, condenser lens 206, source conversion unit 212, beam separator 222, deflection scanning unit 226, and objective lens 228 may be aligned with a primary optical axis 260 of apparatus 104. Secondary optical system 242 and charged-particle detection device 244 may be aligned with a secondary optical axis 252 of apparatus 104.

[0038] Charged-particle source 202 may emit one or more charged particles, such as electrons, protons, ions, muons, or any other particle carrying electric charges. In some embodiments, charged- particle source 202 may be an electron source. For example, charged-particle source 202 may include a cathode, an extractor, or an anode, wherein primary electrons may be emitted from the cathode and extracted or accelerated to form primary charged-particle beam 210 (in this case, a primary electron beam) with a crossover (virtual or real) 208. For ease of explanation without causing ambiguity, electrons are used as examples in some of the descriptions herein. However, it should be noted that any charged particle may be used in any embodiment of this disclosure, which is not limited to electrons. Primary charged-particle beam 210 may be visualized as being emitted from crossover 208. Gun aperture 204 may block off peripheral charged particles of primary charged-particle beam 210 to reduce Coulomb effect. The Coulomb effect may cause an increase in size of probe spots.

[0039] Source conversion unit 212 may comprise an array of image-forming elements and an array of beam-limit apertures. The array of image-forming elements may comprise an array of microdeflectors or micro-lenses. The array of image-forming elements may form a plurality of parallel images (virtual or real) of crossover 208 with a plurality of beamlets 214, 216, and 218 of primary charged- particle beam 210. The array of beam-limit apertures may limit the plurality of beamlets 214, 216, and 218. While three beamlets 214, 216, and 218 are shown in FIG. 2, embodiments of the present disclosure are not so limited. For example, in some embodiments, the apparatus 104 may be configured to generate a first number of beamlets. In some embodiments, the first number of beamlets may be in a range from 1 to 1000. In some embodiments, the first number of beamlets may be in a range from 200- 500. In an exemplary embodiment, an apparatus 104 may generate 400 beamlets.

[0040] Condenser lens 206 may focus primary charged-particle beam 210. The electric currents of beamlets 214, 216, and 218 downstream of source conversion unit 212 may be varied by adjusting the focusing power of condenser lens 206 or by changing the radial sizes of the corresponding beam-limit apertures within the array of beam-limit apertures. Objective lens 228 may focus beamlets 214, 216, and 218 onto a wafer 230 for imaging, and may form a plurality of probe spots 270, 272, and 274 on a surface of wafer 230.

[0041] Beam separator 222 may be a beam separator of Wien filter type generating an electrostatic dipole field and a magnetic dipole field. In some embodiments, if they are applied, the force exerted by the electrostatic dipole field on a charged particle (e.g., an electron) of beamlets 214, 216, and 218 may be substantially equal in magnitude and opposite in a direction to the force exerted on the charged particle by magnetic dipole field. Beamlets 214, 216, and 218 may, therefore, pass straight through beam separator 222 with zero deflection angle. However, the total dispersion of beamlets 214, 216, and 218 generated by beam separator 222 may also be non-zero. Beam separator 222 may separate secondary charged-particle beams 236, 238, and 240 from beamlets 214, 216, and 218 and direct secondary charged-particle beams 236, 238, and 240 towards secondary optical system 242.

[0042] Deflection scanning unit 226 may deflect beamlets 214, 216, and 218 to scan probe spots 270, 272, and 274 over a surface area of wafer 230. In response to the incidence of beamlets 214, 216, and 218 at probe spots 270, 272, and 274, secondary charged-particle beams 236, 238, and 240 may be emitted from wafer 230. Secondary charged-particle beams 236, 238, and 240 may comprise charged particles (e.g., electrons) with a distribution of energies. For example, secondary charged-particle beams 236, 238, and 240 may be secondary electron beams including secondary electrons (energies < 50 eV) and backscattered electrons (energies between 50 eV and landing energies of beamlets 214, 216, and 218). Secondary optical system 242 may focus secondary charged-particle beams 236, 238, and 240 onto detection sub-regions 246, 248, and 250 of charged-particle detection device 244. Detection subregions 246, 248, and 250 may be configured to detect corresponding secondary charged-particle beams 236, 238, and 240 and generate corresponding signals (e.g., voltage or current) used to reconstruct an SCPM image of structures on or underneath the surface area of wafer 230. The generated corresponding signals may also include a spectrum of frequencies, wherein a low-frequency regime contains information about larger features of wafer 230 and a high-frequency regime contains information about finer (i.e., sharper) features of wafer 230.

[0043] The generated signals may represent intensities of secondary charged-particle beams 236, 238, and 240 and may be provided to image processing system 290 that is in communication with charged- particle detection device 244, primary projection optical system 220, and motorized wafer stage 280. The movement speed of motorized wafer stage 280 may be synchronized and coordinated with the beam deflections controlled by deflection scanning unit 226, such that the movement of the scan probe spots (e.g., scan probe spots 270, 272, and 274) may orderly cover regions of interests on the wafer 230. The parameters of such synchronization and coordination may be adjusted to adapt to different materials of wafer 230. For example, different materials of wafer 230 may have different resistance-capacitance characteristics that may cause different signal sensitivities to the movement of the scan probe spots.

[0044] The intensity of secondary charged-particle beams 236, 238, and 240 may vary according to the external or internal structure of wafer 230, and thus may indicate whether wafer 230 includes defects. Moreover, as discussed above, beamlets 214, 216, and 218 may be projected onto different locations of the top surface of wafer 230, or different sides of local structures of wafer 230, to generate secondary charged-particle beams 236, 238, and 240 that may have different intensities. Therefore, by mapping the intensity of secondary charged-particle beams 236, 238, and 240 with the areas of wafer 230, image processing system 290 may reconstruct an image that reflects the characteristics of internal or external structures of wafer 230.

[0045] In some embodiments, image processing system 290 may include an image acquirer 292, a storage 294, and a controller 296. Image acquirer 292 may comprise one or more processors. For example, image acquirer 292 may comprise a computer, server, mainframe host, terminals, personal computer, any other appropriate mobile computing devices. Image acquirer 292 may be communicatively coupled to charged-particle detection device 244 of beam tool 104 through a medium such as an electric conductor, optical fiber cable, portable storage media, IR, Bluetooth, internet, wireless network, or wireless radio. In some embodiments, image acquirer 292 may receive a signal from charged-particle detection device 244 and may construct an image. Image acquirer 292 may thus acquire SCPM images of wafer 230. Image acquirer 292 may also perform various post-processing functions, including, but not limited to, generating contours and superimposing indicators on an acquired image. Image acquirer 292 may be configured to perform adjustments of brightness and contrast of acquired images. In some embodiments, storage 294 may be a storage medium including, but not limited to, a hard disk, flash drive, cloud storage, random access memory (RAM), or other types of computer-readable memory. Storage 294 may be coupled with image acquirer 292 and may be used for saving scanned raw image data as original images and post-processed images. Image acquirer 292 and storage 294 may be connected to controller 296. In some embodiments, image acquirer 292, storage 294, and controller 296 may be integrated together as one control unit.

[0046] In some embodiments, image acquirer 292 may acquire one or more SCPM images of a wafer based on an imaging signal received from charged-particle detection device 244. An imaging signal may correspond to a scanning operation for conducting charged particle imaging. An acquired image may be a single image comprising a plurality of imaging areas. The single image may be stored in storage 294. The single image may be an original image that may be divided into a plurality of regions. Each of the regions may comprise one imaging area containing a feature of wafer 230. The acquired images may comprise multiple images of a single imaging area of wafer 230 sampled multiple times over a time sequence. The multiple images may be stored in storage 294. In some embodiments, image processing system 290 may include circuitry configured to perform image processing steps with the multiple images of the same location of wafer 230.

[0047] In some embodiments, image processing system 290 may include measurement circuitry (e.g., analog-to-digital converters) configured to obtain a distribution of the detected secondary charged particles (e.g., secondary electrons). The charged-particle distribution data collected during a detection time window, in combination with corresponding scan path data of beamlets 214, 216, and 218 incident on the wafer surface, may be used to reconstruct images of the wafer structures under inspection. The reconstructed images may be used to reveal various features of the internal or external structures of wafer 230, and thereby may be used to reveal any defects that may exist in the wafer.

[0048] In some embodiments, the charged particles may be electrons. When electrons of primary charged-particle beam 210 are projected onto a surface of wafer 230 (e.g., probe spots 270, 272, and 274), the electrons of primary charged-particle beam 210 may penetrate the surface of wafer 230 for a certain depth, interacting with particles of wafer 230. Some electrons of primary charged-particle beam 210 may elastically interact with (e.g., in the form of elastic scattering or collision) the materials of wafer 230 and may be reflected or recoiled out of the surface of wafer 230. An elastic interaction conserves the total kinetic energies of the bodies (e.g., electrons of primary charged-particle beam 210) of the interaction, in which the kinetic energy of the interacting bodies does not convert to other forms of energy (e.g., heat or electromagnetic energy). Such reflected electrons generated from elastic interaction may be referred to as backscattered electrons (BSEs). Some electrons of primary charged- particle beam 210 may inelastically interact with (e.g., in the form of inelastic scattering or collision) the materials of wafer 230. An inelastic interaction does not conserve the total kinetic energies of the bodies of the interaction, in which some or all of the kinetic energy of the interacting bodies converts to other forms of energy. For example, through the inelastic interaction, the kinetic energy of some electrons of primary charged-particle beam 210 may cause electron excitation and transition of atoms of the materials. Such inelastic interaction may also generate electrons exiting the surface of wafer 230, which may be referred to as secondary electrons (SEs). Yield or emission rates of BSEs and SEs may depend on the material under inspection and the landing energy of the electrons of primary charged- particle beam 210 landing on the surface of the material, among other factors. The energy of the electrons of primary charged-particle beam 210 may be imparted in part by its acceleration voltage (e.g., the acceleration voltage between the anode and cathode of charged-particle source 202 in FIG. 2). The quantity of BSEs and SEs may be more or fewer (or even the same) than the injected electrons of primary charged-particle beam 210.

[0049] The images generated by SCPM may be used for defect inspection. For example, a generated image capturing a test device region of a wafer may be compared with a reference image capturing the same test device region. The reference image may be predetermined (e.g., by simulation) and include no known defect. If a difference between the generated image and the reference image exceeds a tolerance level, a potential defect may be identified. For another example, the SCPM may scan multiple regions of the wafer, each region including a test device region designed as the same and generate multiple images capturing those test device regions as manufactured. The multiple images may be compared with each other. If a difference between the multiple images exceeds a tolerance level, a potential defect may be identified.

[0050] FIG. 3 is a block diagram of an example concurrent auto focus and image alignment system according to some embodiments of the present disclosure. In some embodiments, a concurrent auto focus and alignment system 300 comprises one or more processors and memories. It is appreciated that in various embodiments concurrent auto focus and alignment system 300 may be part of or may be separate from a charged-particle beam inspection system (e.g., EBI system 100 of FIG. 1). In some embodiments, concurrent auto focus and alignment system 300 may include one or more components (e.g., software modules or circuitry) that can be implemented in controller 109 or system 290 as discussed herein. In some embodiments, concurrent auto focus and alignment system 300 may include or may be associated with user interface(s) for receiving user input(s) or for presenting information to a user, such as a displayer, a keyboard, a mouse, or a controller. As shown in FIG. 3, concurrent auto focus and alignment system 300 may comprise an auto focus component 300A and an alignment component 300B. In some embodiments, auto focus component 300A can comprise an inspection area processor 310, an inspection image acquirer 320, an evaluator 330 and a focus parameter selector 340. In some embodiments, alignment component 300B can comprise a selected image acquirer 350, a reference image acquirer 360, and an alignment parameter estimator 370. Concurrent auto focus and alignment system 300 can further comprise a parameter applier 380.

[0051] According to some embodiments of the present disclosure, inspection area processor 310 may be configured to set a plurality of focus conditions for subsequent imaging of an inspection area. Inspection area processor 310 may determine an inspection area for subsequent imaging with the set plurality of focus conditions. In some embodiments, an inspection area of a sample can be selected considering one or more factors including, but not limited to, a unique pattern, a detected pattern edge, or a field of view. A unique pattern and detected pattern edge may provide a basis of comparison to a pattern of a reference image for local alignment. In some embodiments, a detected pattern edge may be discernible from an image background and indicates a boundary of the pattern. In some embodiments, a field of view of an inspection area is dependent on sample and pattern conditions. The field of view may provide an inspection area containing the unique pattern and pattern edges to compare to the pattern of the reference image for local alignment. FIG. 4A illustrates example inspection areas consistent with embodiments of the present disclosure. As shown in FIG. 4A, inspection area 401 contains a unique pattern 401_1 that is discernible from the image background, and inspection area 402 contains a unique pattern 402_l that is discernible from the image background. In this example, inspection area 402 has a larger field of view than inspection area 401.

[0052] In some embodiments, a focal plane or a focal point may be determined according to the set focus condition. In some embodiments, a focus condition may be set by adjusting a current of objective lens 228, a current of condenser lens 206, a bias voltage of stage 280, or a vertical position of stage 280, etc. In some embodiments, a current applied to objective lens 228 can be controlled to adjust an accelerating voltage of beamlets 214, 216, and 218. Adjusting the accelerating voltage of beamlets 214, 216, 218 may change a vertical position of cross-over points of beamlets 214, 216, and 218, and thus change a relative position of focal points of beamlets 214, 216, and 218 with respect to a surface of wafer 230. In some embodiments, a current applied to condenser lens 206 can be controlled to adjust the beam size (i.e., diameter) of primary charged-particle beam 210. Adjusting the beam size of primary charged-particle beam 210 adjusts the amount of electrons in primary charged-particle beam 210 that pass through to objective lens 228. In some embodiments, a bias voltage applied to stage 280 can be controlled to change the accelerating voltage of beamlets 214, 216, and 218 near the surface of wafer 230 and thereby change the relative position of focal points of beamlets 214, 216, and 218 with respect to the surface of wafer 230. In some embodiments, adjusting the vertical position of stage 280 will adjust the vertical position of the surface of wafer 280, attached to stage 280, relative to beamlet 214, 216, 218 focal points. In some embodiments, a focus condition may comprise different combinations of parameters related to a current of objective lens 226, a current of condenser lens 206, a bias voltage of stage 280, or vertical position of stage 280, etc. It is appreciated that adjustments for a focus condition are not so limited as recited.

[0053] An exemplary sample set of focus condition for an inspection area of a sample is illustrated in FIG. 4B. In FIG. 4B, a sample surface 410 is irradiated with a beamlet such as a first beamlet 420, a second beamlet 421, a third beamlet 422, or a fourth beamlet 423 having different focus conditions. It is appreciated that each beamlet 420, 421, 422, and 423 is a separate illustrative example and not occurring at the same time. Beamlets 420, 421, 422, and 423 each have a corresponding focus condition by which focal points 430, 431, 432, and 433 are determined. It is appreciated that the number of focus conditions is for demonstrative purposes and is not limited as illustrated in FIG. 4B. Moreover, while each beamlet 420, 421, 422, and 423 represents a different focus condition, multiple beamlets may be used for a focus condition. While the embodiments described herein are directed to a multi-beam environment, it is appreciated that a single beam environment may be used to set a focus condition. It is appreciated that beamlets 420, 421, 422, and 423 may represent a different focus condition applied to a single beam in sequence (e.g., such as a single beam in a single beam environment or a center beam in a multi-beam environment). Furthermore, while FIG. 4B illustrates the relative position between sample surface 410 and focal points 430, 431, 432, and 433, it is also appreciated that the vertical position of sample surface 410 may be adjusted while maintaining a constant position of focal points 430, 431, 432, and 433. In some embodiments, inspection area processor 310 may be a component that includes circuitry to change current in objective lens 228, a bias voltage of stage 280, or a vertical position of stage 280, etc.

[0054] Referring back to FIG. 3, inspection image acquirer 320 may acquire an inspection image. In some embodiments, an inspection image may be a SCPM image of a sample or a wafer. In some embodiments, an inspection image may be acquired from the apparatus of FIG. 1 or FIG. 2. In some embodiments, inspection image acquirer 320 may obtain an inspection image from a storage device, system, or database storing the inspection image. In some embodiments, inspection image acquirer 320 may acquire a plurality of inspection images of an inspection area according to a plurality of focus conditions set by inspection area processor 310. In some embodiments, inspection image acquirer 320 may be connected to a computer-readable memory or storage device to store focus conditions for each inspection image. Inspection image acquirer 320 may acquire a plurality of inspection images 430_l, 431_1 , 432_1, and 433_1 of FIG. 4B according to a plurality of focus conditions set by inspection area processor 310. It is appreciated that a plurality of inspection images may contain a greater or fewer number of inspection images than illustrated in FIG. 4B. In some embodiments, the inspection images may include a pattern 440.

[0055] Referring back to FIG. 3, according to some embodiments, evaluator 330 may be configured to determine a focus index of an acquired inspection image. In some embodiments, evaluator 330 can be implemented as software that may determine a focus index of an inspection image after the inspection image acquirer 320 acquires the inspection image. In some embodiments, evaluator 330 may determine a focus index of each inspection image of a plurality of inspection images one by one in the order of acquisition by the inspection image acquirer 320. In some embodiments, evaluator 330 may determine the focus index of an inspection image based on image resolution or sharpness. In some embodiments, evaluator 330 may determine the focus index by determining the contrast in an inspection image (e.g., inspection images 430_l, 431_1 , 432_1, and 433_1) between the background and pattern (e.g., pattern 440 of FIG. 4B). An inspection image containing a more clearly defined pattern from the background compared to a second inspection image with a less defined pattern will give insight as to the focal plane setting relative to the sample surface of each inspection image. In some embodiments, the focus index for each inspection image may be determined as a sharpness index. In some embodiments, the sharpness index may be determined by several mathematical operations including, but not limited to, a Fourier transform, Contrast-to-gradient, or Derivative method. In an example embodiment, the sharpness index determined by a Fourier transform method may provide a comparison of the low and high frequency signals generated by detector sub-regions 246, 248, and 250 for each inspection image. According to this example embodiment, an inspection image that is sharper, and thus more in focus, will indicate a wider range of high frequency signal magnitude in the Fourier transform, and thus indicate a larger sharpness index. An inspection image of which focal point is closer to sample surface 410 has higher frequency signal magnitude in the Fourier transform, and thus a greater sharpness index. Similarly, an inspection image with a more clearly defined pattern will have a focal point closer to the surface compared to another inspection image with a less defined pattern. In this example, inspection image 433_1 has focal point 433 closest to sample surface 410, and thus the largest focus index. Similarly, inspection image 430_l has focal point 430 farthest from sample surface 410, and thus the smallest focus index.

[0056] In some embodiments, evaluator 330 may identify an inspection image with a focus index within a first threshold range. In some embodiments, a focus index within a first threshold range may indicate that a corresponding inspection image has a level of sharpness or resolution that can be used for image alignment. FIG. 4C illustrates an example focus index graph 450 in which an x-axis represents a focus condition and a y-axis represents a focus index. In FIG. 4C, each focus index according to a corresponding focus condition is plotted. Focus indexes for inspection images 430_l, 431_1, 432_1, and 433_1 are also illustrated in FIG. 4C. FIG. 4C further illustrates an example first focus index threshold range 460. In some embodiments, the identified inspection image with a focus index within first threshold range 460 can be forwarded to selected image acquirer 350 for image alignment. In some embodiments, after an inspection image with a focus index within first threshold range 460 has been identified, evaluator 330 may not determine whether subsequent inspection images have a focus index within first threshold range 460. As illustrated FIG. 4C, inspection image 431_1 is the first identified inspection image with a focus index within first threshold range 460. Inspection image 431_1 may then be used by selected image acquirer 350 for image alignment. In some embodiments, evaluator 330 may not identify whether any subsequent inspection images have focus index values within first threshold range 460. For example, evaluator 330 may not determine whether inspection image 432_1 has a focus index within first threshold range 460.

[0057] In some embodiments, evaluator 330 may identify an inspection image with a focus index within a second threshold range. In some embodiments, a focus index within a second threshold range may indicate that a corresponding inspection image has a level of resolution or sharpness that may be used for focus optimization. FIG. 4D illustrates an example second focus index range 470 that is presented on the same graph 450 of FIG. 4C. As shown in FIG. 4D, second focus index range 470 may be narrower than first focus index range 460. In some embodiments, evaluator 330 may identify a plurality of inspection images with focus index values within second threshold range 470. In some embodiments, if evaluator 330 cannot identify an inspection image with a focus index within second threshold 470, evaluator 330 may provide feedback to inspection area process 310 and inspection image acquirer 320 to adjust focus conditions and acquire more inspection images. In some embodiments, identified inspection images with a focus index within second threshold range 470 can be forwarded to focus parameter selector 340. Evaluator 330 may identify inspection images 432_1 and 433_1 as having focus index values within second threshold range 470. In some embodiments, evaluator 330 does not identify if inspection images 432_1 and 433_ 1 have focus index values within first threshold range 460. In some embodiments, inspection images 432_1 and 433_1 can be forwarded to focus parameter selector 340. [0058] Referring back to FIG. 3, according to some embodiments of the present disclosure, focus parameter selector 340 may select a focus condition to be used for subsequent imaging. In some embodiments, focus parameter selector 340 can identify an inspection image that has a focus index within second threshold range 470, and can select a focus condition that was used in taking the identified inspection image. In some embodiments, focus parameter selector 340 may be configured to communicate with inspection image acquirer 320 to acquire a selected focus condition. In some embodiments, focus parameter selector 340 can be implemented as software or a component including circuitry that may acquire a selected focus condition from inspection image acquirer 320 for an inspection image with a focus index within second threshold range 470. In some embodiments, focus parameter selector 340 may identify the largest focus index value of an inspection image from a plurality of inspection images with focus index values within second threshold range 470. In some embodiments, focus parameter selector 340 can select a focus condition that was used in taking the identified inspection image. In some embodiments, focus parameter selector 340 may acquire a focus condition from inspection image acquirer 320 for an inspection image with the largest focus index value out of a plurality of inspection images with focus index values within second threshold range 470. As illustrated in FIG. 4D, inspection images 432_1 and 433_1 may be forwarded to focus parameter selector 340. Focus parameter selector 340 may identify inspection image 433_1 as having the largest focus index and can select a focus condition set for taking inspection image 433_1.

[0059] Referring back to FIG. 3, according to some embodiments of the present disclosure, selected image acquirer 350 may acquire an inspection image that is determined to have a focus index value within first threshold range 460. In some embodiments, selected image acquirer 350 may acquire an inspection image from evaluator 330 in parallel or concurrently with the auto focus sequence by autofocus component 300A. In some embodiments, selected image acquirer 350 may use an inspection image acquired during an auto focusing process by auto focus component 300A for a local image alignment. For example, selected image acquirer 350 may receive inspection image 431_1 that is acquired by inspection image acquirer 320 and determined to have a focus index with first threshold range 460.

[0060] According to some embodiments, reference image acquirer 360 may acquire a reference image corresponding to an inspection image that is acquired by selected image acquirer 350. For example, reference image acquirer 360 may acquire a reference image corresponding to inspection image 431_1 that is forwarded to selected image acquirer 350. In some embodiments, reference image acquirer 360 may acquire a reference image in parallel or concurrently with the auto focus sequence by auto focus component 300A. In some embodiments, a reference image may be a layout file for a wafer design corresponding to the inspection image. The layout file may be a golden image or in a Graphic Database System (GDS) format, Graphic Database System II (GDS II) format, an Open Artwork System Interchange Standard (OASIS) format, or a Caltech Intermediate Format (CIF). The wafer design may include patterns or structures for inclusion on the wafer. The patterns or structures may be mask patterns used to transfer features from the photolithography masks or reticles to a wafer. In some embodiments, a layout in GDS or OASIS format, among other formats, may comprise feature information stored in a binary file format representing planar geometric shapes, text, and other information related to the wafer design. In some embodiments, a reference image can be an image rendered from the layout file. FIG. 5 illustrates a reference image 401 corresponding to selected inspection image 431_1. As shown in FIG. 5, reference image 401 includes a pattern 401_l corresponding to pattern 440 of selected inspection image 432_1.

[0061] Referring back to FIG. 3, according to some embodiments of the present disclosure, alignment parameter estimator 370 may estimate an alignment parameter between an inspection image and a corresponding reference image. In some embodiments, the inspection image can be an inspection image acquired by selected image acquirer 350, and the reference image can be a reference image acquired by reference image acquirer 360. Alignment parameter estimator 370 may estimate an alignment parameter in parallel or concurrently with the auto focus sequence by auto focus component 300A. In some embodiments, an alignment parameter may be pattern matching information between an inspection image and a reference image. FIG. 5 illustrates inspection image 431_1 aligned with reference image 401 such that inspection image 431_1 is overlaid reference image 401 as illustrated in composite image 510. According to some embodiments of the present disclosure, alignment parameter estimator 370 may estimate alignment parameter(s) to achieve the local image alignment. In some embodiments, alignment parameter estimator 370 may estimate a displacement parameter in an X-Y plane in which a position of the pattern of the inspection image may be offset from a corresponding point of the corresponding pattern of the reference image. According to some embodiments of the present disclosure, alignment parameter(s) may be determined such that an alignment between an inspection image and a reference image may be as close as possible. FIG. 6 illustrates an example alignment parameter 610 provided by alignment parameter estimator 370 such that inspection image pattern 440 matches the corresponding pattern 401_l of the reference image.

[0062] Referring back to FIG. 3, according to some embodiments of the present disclosure, parameter applier 380 may apply parameters acquired by focus parameter selector 340 or alignment parameter estimator 370. In some embodiments, parameter applier 380 may apply selected focus condition acquired from focus parameter selector 340 when acquiring subsequent inspection images. In some embodiments, parameter applier 380 may apply selected focus condition acquired from focus parameter selector 340 to inspection area processor 310. In some embodiments, parameter applier 380 may apply focus condition to adjust a current of objective lens 228, a bias voltage of stage 280, or a vertical position of stage 280, etc. In some embodiments, parameter applier 380 can apply estimated alignment parameter(s) 610 to subsequent inspection images. In some embodiments, estimated alignment parameter(s) 610 can be applied to inspection images that are taken according to the selected focus condition. In some embodiments, parameter applier 380 may apply estimated alignment parameter(s) 610 to inspection image acquirer 320. In some embodiments, parameter applier 380 may apply a displacement parameter in an X-Y plane to shift an inspection image so the inspection image pattern matches the reference image pattern.

[0063] FIG. 7 is a process flowchart representing an example concurrent autofocus and image alignment method, consistent with embodiments of the present disclosure. The steps of method 700 may be performed by a system (e.g., system 300 of FIG. 3) executing on or otherwise using the features of a computing device, e.g., controller 109 of FIG. 1. It is appreciated that the illustrated method 700 may be altered to modify the order of steps and to include additional steps.

[0064] In step S710, a plurality of focus conditions for an inspection area may be set. Step S710 can be performed by, for example, inspection area processor 310. In some embodiments, an inspection area may be a SCPM image of a sample or a wafer. In some embodiments, criteria for an inspection area of a sample may include, but are not limited to, a unique sample pattern, a field of view, or a detected pattern edge. In some embodiments, a focus condition may be set by adjusting a current of objective lens 228, a bias voltage of stage 280, or a vertical position of stage 280, etc. In some embodiments, a focus condition may comprise different combinations of parameters related to a current of objective lens 226, a bias voltage of stage 280, or a vertical position stage 280, etc. It is appreciated that adjustments for a focus condition are not so limited as recited.

[0065] In step S720, an inspection image may be acquired. Step S720 can, for example, be performed by inspection image acquirer 320. In some embodiments, an inspection image may be a SCPM image of a sample or a wafer. In some embodiments, an inspection image may be acquired from the apparatus of FIG. 1 or FIG. 2. In some embodiments, an inspection image may be acquired from a storage device, system, or database storing the inspection image. In some embodiments, a plurality of inspection images of an inspection area according to a plurality of focus conditions set by inspection area processor 310 may be acquired.

[0066] In Step S730, a focus index of an inspection image may be evaluated. Step S730 may, for example, be performed by evaluator 330 or other image focus processing software. In some embodiments, a focus index of each inspection image of a plurality of inspection images may be evaluated one by one in the order of acquisition by the inspection image acquirer 320. In some embodiments, the focus index of an inspection image may be determined as image resolution or sharpness. In some embodiments, the focus index of an inspection image may be determined by comparing the contrast in an inspection image between the background and pattern (e.g., pattern 440 of FIG. 4B) of an acquired inspection image (e.g., inspection images 430_l, 431_1, 432_1, and 433_1). In some embodiments, the focus index for each inspection image may be determined as a sharpness index. In some embodiments, the sharpness index may be determined by several mathematical operations including, but not limited to, a Fourier transform, Contrast-to-gradient, or Derivative method. In some embodiments, an inspection image with a focus index within a first threshold range may be identified. In some embodiments, the identified inspection image with a focus index within first threshold range 460 can be forwarded to selected image acquirer 350 for local image alignment. In some embodiments, after an inspection image with a focus index within first threshold range 460 is identified, no subsequent inspection images are identified within first threshold range 460. In some embodiments, an inspection image with a focus index value within a second threshold range may be identified. In some embodiments, a plurality of inspection images with focus index values within second threshold range 470 may be identified. In some embodiments, if an inspection image with a focus index within second threshold 470 cannot be identified, feedback may be provided to inspection area process 310 and inspection image acquirer 320 to adjust focal plane settings and acquire more inspection images. In some embodiments, identified inspection images with a focus index value within second threshold range 470 can be forwarded to focus parameter selector 340.

[0067] In Step S740, a focus condition of an inspection image may be selected to be used for subsequent imaging. Step S740 may, for example, be performed by a focus parameter selector 340. In some embodiments, an inspection image that has a focus index within second threshold range 470 may be identified. In some embodiments, focus conditions of the identified inspection image may be acquired from a computer-readable memory or storage device connected to inspection image acquirer 320. In some embodiments, an inspection image from a plurality of inspection images that has a largest focus index value within second threshold range 470 may be identified. In some embodiments, focus conditions for an inspection image out of a plurality of inspection images that has the largest focus index value within second threshold range 470 may be acquired from a computer-readable memory or storage device connected to inspection image acquirer 320.

[0068] In Step S750, which can occur concurrently or in parallel to Step S740, an inspection image that has a focus index value within first threshold range 460 may be acquired by selected image acquirer 350 and a reference image may be acquired by reference image acquirer 360. In some embodiments, an inspection image that has a focus index value within first threshold range 460 may be used from evaluator 330. In some embodiments, an inspection image that has a focus index value within first threshold range 460 may be used from inspection image acquirer 320.

[0069] In Step S760, which can occur concurrently or in parallel to Step S740, an alignment parameter between an inspection image and a corresponding reference image may be estimated. Step S760 may, for example, be performed by alignment parameter estimator 370. In some embodiments, the inspection image may be acquired from selected image acquirer 350 and the reference image may be acquired from reference image acquirer 360. In some embodiments, the inspection image may be overlaid the reference image. In some embodiments, an image alignment estimate may be pattern matching information between the inspection image and the reference image. In some embodiments, an image alignment estimate may provide a displacement parameter in an X-Y plane in which a position of the pattern of the inspection image may be offset from a corresponding point of the corresponding pattern of the reference image. In some embodiments, image alignment parameter(s) may be determined such that an alignment between the inspection image and the reference image may be as close as possible. [0070] In Step S770, a selected focus condition and estimated alignment parameter may be applied to an inspection area processor 310 and inspection image acquirer 320 for subsequent inspection image acquisition. Step S770 may, for example, be performed by parameter applier 380. In some embodiments, a selected focus condition applied to inspection area processor 310 may include an adjustment to current in objective lens 228, an adjustment in bias voltage to stage 280, or an adjustment to vertical position of stage 280, etc. for subsequent inspection image acquisition. In some embodiments, an estimated image alignment parameter(s) may be applied to inspection image acquirer 320 for subsequent inspection image acquisition. In some embodiments, a displacement parameter in an X-Y plane may be applied to inspection image acquirer 320 to shift an inspection image so the inspection image pattern matches the reference image pattern.

[0071] A non-transitory computer readable medium may be provided that may store instructions for a processor of a controller (e.g., controller 109 of FIG. 1) to perform inspection image area evaluation, inspection image acquisition, stage positioning, beam focusing, electric field adjustments, objective lens adjusting, activating charged-particle source, method 700, and other executable functions in the charged particle system relating to the concurrent autofocus and image alignment method. 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 Erasable Programmable 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.

[0072] The embodiments may further be described using the following clauses:

1. A method for enhancing an inspection image, comprising: acquiring a plurality of inspection images according to a plurality of focus conditions for an area of a sample containing a pattern; determining whether an inspection image of the plurality of inspection images has a focus index within a first threshold range; and in response to a determination that the inspection image has a focus index within the first threshold range, performing an image alignment using the inspection image.

2. The method of clause 1, wherein a first focus condition and a second focus condition of the plurality of focus conditions set a first focal point and a second focal point respectively, and the first focal point and the second focal point have a different relative position with respect to a sample.

3. The method of clause 1 or 2, wherein the plurality of focus conditions includes a current value of an objective lens, a voltage bias applied to a stage, or a change in a vertical position of a stage.

4. The method of any one of clauses 1 to 3, wherein performing an image alignment using the inspection image comprises: acquiring a reference image corresponding to the inspection image; and estimating an alignment parameter by comparing the inspection image with the reference image.

5. The method of any one of clauses 1 to 4, further comprising: determining a focus index of each of the plurality of inspection images; identifying, among the plurality of inspection images, one or more inspection images that have a focus index within a second threshold range; selecting an inspection image having a largest focus index among the identified one or more inspection images; and selecting, among the plurality of focus conditions, a focus condition associated with the selected inspection image.

6. The method of clause 5, wherein the second threshold range is narrower than and is included within the first threshold range.

7. The method of any one of clauses 1-6, wherein the area of the sample containing the pattern is selected to include a unique sample pattern, a discernible pattern edge, or field of view.

8. A method for enhancing an inspection image, comprising: setting a plurality of focus conditions for an area of a sample containing a pattern; acquiring a plurality of inspection images according to the plurality of focus conditions; determining a focus index for each of the plurality of inspection images; identifying, among the plurality of inspection images, an inspection image of which focus index is determined to be within a first threshold range; estimating an alignment parameter for the inspection image of which focus index is determined to be within the first threshold range; identifying, among the plurality of inspection images, one or more inspection images of which focus index is determined to be within a second threshold range; selecting a focus condition associated with an inspection image determined to be within the second threshold range having the largest focus index; and performing an image alignment and focus correction based on the estimated alignment parameter and selected focus condition.

9. The method of clause 8, wherein estimating an alignment parameter for the inspection image of which focus index is determined to be within the first threshold range, identifying, among the plurality of inspection images, one or more inspection images of which focus index is determined to be within a second threshold range, and selecting a focus condition associated with an inspection image determined to be within the second threshold range having the largest focus index occur concurrently or in parallel.

10. The method of clause 8 or 9, wherein a first focus condition and a second focus condition of the plurality of focus conditions set a first focal point and a second focal point respectively, and the first focal point and the second focal point have a different relative position with respect to a sample. 11. The method of any one of clauses 8 to 10, wherein the plurality of focus conditions includes a current value of an objective lens, a voltage bias applied to a stage, or a change in a vertical position of a stage.

12. The method of any one of clauses 8 to 11, wherein performing an image alignment using the inspection image comprises: acquiring a reference image corresponding to the inspection image; and estimating an alignment parameter by comparing the inspection image with the reference image.

13. The method of clause 8 or 9, wherein the second threshold range is narrower than and is included within the first threshold range.

14. The method of any one of clauses 8-12, wherein the area of the sample containing the pattern is selected to include a unique sample pattern, a discernible pattern edge, or field of view.

15. An apparatus for enhancing an inspection image, comprising: a memory storing a set of instructions; and at least one processor configured to execute the set of instructions to cause the apparatus to perform: acquiring a plurality of inspection images according to a plurality of focus conditions for an area of a sample containing a pattern; determining whether an inspection image of the plurality of inspection images has a focus index within a first threshold range; and in response to a determination that the inspection image has a focus index within the first threshold range, performing an image alignment using the inspection image.

16. The apparatus of clause 15, wherein a first focus condition and a second focus condition of the plurality of focus conditions set a first focal point and a second focal point respectively, and the first focal point and the second focal point have a different relative position with respect to a sample.

17. The apparatus of clause 15 or 16, wherein the plurality of focus conditions includes a current value of an objective lens, a voltage bias applied to a stage, or a change in a vertical position of a stage.

18. The apparatus of any one of clauses 15 to 17, wherein, in performing the image alignment using the inspection image, the at least one processor is configured to execute the set of instructions to cause the apparatus to further perform: acquiring a reference image corresponding to the inspection image; and estimating an alignment parameter by comparing the inspection image with the reference image.

19. The apparatus of any one of clauses 15 to 18, wherein the at least one processor is configured to execute the set of instructions to cause the apparatus to further perform: determining a focus index of each of the plurality of inspection images; identifying, among the plurality of inspection images, one or more inspection images that have a focus index within a second threshold range; selecting an inspection image having a largest focus index among the identified one or more inspection images; and selecting, among the plurality of focus conditions, a focus condition associated with the selected inspection image.

20. The apparatus of clause 19, wherein the second threshold range is narrower than and is included within the first threshold range.

21. The apparatus of any one of clauses 15-20, wherein the area of the sample containing the pattern is selected to include a unique sample pattern, a discernible pattern edge, or field of view.

22. An apparatus for enhancing an inspection image, comprising: a memory storing a set of instructions; and at least one processor configured to execute the set of instructions to cause the apparatus to perform: setting a plurality of focus conditions for an area of a sample containing a pattern; acquiring a plurality of inspection images according to the plurality of focus conditions; determining a focus index for each of the plurality of inspection images; identifying, among the plurality of inspection images, an inspection image of which focus index is determined to be within a first threshold range; estimating an alignment parameter for the inspection image of which focus index is determined to be within the first threshold range; identifying, among the plurality of inspection images, one or more inspection images of which focus index is determined to be within a second threshold range; selecting a focus condition associated with an inspection image determined to be within the second threshold range having the largest focus index; and performing an image alignment and focus correction based on the estimated alignment parameter and selected focus condition.

23. The apparatus of clause 22 wherein the at least one processor is configured to execute the set of instructions to cause the apparatus to further perform concurrently or in parallel: estimating an alignment parameter for the inspection image of which focus index is determined to be within the first threshold range; identifying, among the plurality of inspection images, one or more inspection images of which focus index is determined to be within a second threshold range; and selecting a focus condition associated with an inspection image determined to be within the second threshold range having the largest focus index.

24. The apparatus of clause 22 or 23, wherein a first focus condition and a second focus condition of the plurality of focus conditions set a first focal point and a second focal point respectively, and the first focal point and the second focal point have a different relative position with respect to a sample.

25. The apparatus of any one of clauses 22 to 24, wherein the plurality of focus conditions includes a current value of an objective lens, a voltage bias applied to a stage, or a change in a vertical position of a stage. 26. The apparatus of any one of clauses 22 to 25, wherein, in performing an image alignment using the inspection image, the at least one processor is configured to execute the set of instructions to cause the apparatus to further perform: acquiring a reference image corresponding to the inspection image; and estimating an alignment parameter by comparing the inspection image with the reference image.

27. The apparatus of clause 22 or 23, wherein the second threshold range is narrower than and is included within the first threshold range.

28. The apparatus of any one of clauses 22-26, wherein the area of the sample containing the pattern is selected to include a unique sample pattern, a discernible pattern edge, or field of view.

29. A non-transitory computer readable medium comprising a set of instructions that is executable by one or more processors of a computing device to cause the computing device to perform a method for enhancing an inspection image, the method comprising: acquiring a plurality of inspection images according to a plurality of focus conditions for an area of a sample containing a pattern; determining whether an inspection image of the plurality of inspection images has a focus index within a first threshold range; and in response to a determination that the inspection image has a focus index within the first threshold range, performing an image alignment using the inspection image.

30. The non-transitory computer readable medium of clause 29, wherein a first focus condition and a second focus condition of the plurality of focus conditions set a first focal point and a second focal point respectively, and the first focal point and the second focal point have a different relative position with respect to a sample.

31. The non-transitory computer readable medium of clause 29 or 30, wherein the plurality of focus conditions includes a current value of an objective lens, a voltage bias applied to a stage, or a change in a vertical position of a stage.

32. The non-transitory computer readable medium of any one of clauses 29 to 31 wherein, in performing an image alignment using the inspection image, the set of instructions that is executable by one or more processors of the computing device to cause the computing device to further perform: acquiring a reference image corresponding to the inspection image; and estimating an alignment parameter by comparing the inspection image with the reference image.

33. The non-transitory computer readable medium of any one of clauses 29 to 32, wherein the set of instructions that is executable by one or more processors of the computing device to cause the computing device to further perform: determining a focus index of each of the plurality of inspection images; identifying, among the plurality of inspection images, one or more inspection images that have a focus index within a second threshold range; selecting an inspection image having a largest focus index among the identified one or more inspection images; and selecting, among the plurality of focus conditions, a focus condition associated with the selected inspection image.

34. The non-transitory computer readable medium of clause 33, wherein the second threshold range is narrower than and is included within the first threshold range.

35. The non-transitory computer readable medium of clauses 29-34, wherein the area of the sample containing the pattern is selected to include a unique sample pattern, a discernible pattern edge, or field of view.

36. A non-transitory computer readable medium comprising a set of instructions that is executable by one or more processors of a computing device to cause the computing device to perform a method for enhancing an inspection image, the method comprising: setting a plurality of focus conditions for an area of a sample containing a pattern; acquiring a plurality of inspection images according to the plurality of focus conditions; determining a focus index for each of the plurality of inspection images; identifying, among the plurality of inspection images, an inspection image of which focus index is determined to be within a first threshold range; estimating an alignment parameter for the inspection image of which focus index is determined to be within the first threshold range; identifying, among the plurality of inspection images, one or more inspection images of which focus index is determined to be within a second threshold range; selecting a focus condition associated with an inspection image determined to be within the second threshold range having the largest focus index; and performing an image alignment and focus correction based on the estimated alignment parameter and selected focus condition.

37. The non-transitory computer readable medium of clause 36 wherein the set of instructions that is executable by one or more processors of the computing device to cause the computing device to perform concurrently or in parallel: estimating an alignment parameter for the inspection image of which focus index is determined to be within the first threshold range; identifying, among the plurality of inspection images, one or more inspection images of which focus index is determined to be within a second threshold range; and selecting a focus condition associated with an inspection image determined to be within the second threshold range having the largest focus index.

38. The non-transitory computer readable medium of clause 36 or 37, wherein a first focus condition and a second focus condition of the plurality of focus conditions set a first focal point and a second focal point respectively, and the first focal point and the second focal point have a different relative position with respect to a sample.

39. The non-transitory computer readable medium of clauses 36 to 38, wherein the plurality of focus conditions includes a current value of an objective lens, a voltage bias applied to a stage, or a change in a vertical position of a stage.

40. The non-transitory computer readable medium of clauses 36 to 39, wherein, in performing an image alignment using the inspection image, the set of instructions that is executable by one or more processors of the computing device to cause the computing device to perform: acquiring a reference image corresponding to the inspection image; and estimating an alignment parameter by comparing the inspection image with the reference image.

41. The non-transitory computer readable medium of clause 36 or 37, wherein the second threshold range is narrower than and is included within the first threshold range.

42. The non-transitory computer readable medium of any one of clauses 36-40, wherein the area of the sample containing the pattern is selected to include a unique sample pattern, a discernible pattern edge, or field of view.

[0073] Block diagrams in the figures may illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer hardware or software products according to various exemplary embodiments of the present disclosure. In this regard, each block in a schematic diagram may represent certain arithmetical or logical operation processing that may be implemented using hardware such as an electronic circuit. Blocks may also represent a module, segment, or portion of code that comprises one or more executable instructions for implementing the specified logical functions. It should be understood that in some alternative implementations, functions indicated in a block may occur out of the order noted in the figures. For example, two blocks shown in succession may be executed or implemented substantially concurrently, or two blocks may sometimes be executed in reverse order, depending upon the functionality involved. Some blocks may also be omitted. It should also be understood that each block of the block diagrams, and combination of the blocks, may be implemented by special purpose hardware-based systems that perform the specified functions or acts, or by combinations of special purpose hardware and computer instructions.

[0074] 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.