Patent Publication Number: US-2023162944-A1

Title: Image enhancement based on charge accumulation reduction in charged-particle beam inspection

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of US application 63/005,074 which was filed on Apr. 3, 2020, and which is incorporated herein in its entirety by reference. 
    
    
     TECHNICAL FIELD 
     The embodiments provided herein relate to an image enhancement technology, and more particularly to inspection image enhancement based on charge accumulation reduction on a wafer in charged-particle beam inspection. 
     BACKGROUND 
     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 more important. 
     Pattern/structure displacements and dimension deviations from designs can be measured from a SEM image with sub-nanometer (nm) precision. These measurements can be helpful in identifying defects of manufactured ICs and in controlling manufacturing processes. Charge accumulation on a wafer during inspection may cause distortion, defocus, and abnormal grey level of a SEM image and thereby cause an error in measuring critical dimensions and overlays and detecting defects from the SEM image. 
     SUMMARY 
     The embodiments provided herein disclose a particle beam inspection apparatus, and more particularly, an inspection apparatus using a charged particle beam. 
     In some embodiments, a method for enhancing an inspection image in a charged-particle beam inspection system is provided. The method comprises acquiring a plurality of test images of a sample that are obtained at different landing energies, determining distortion levels for the plurality of test images, determining a landing energy level that enables the sample to be in a neutral charge condition during inspection based on the distortion levels, and acquiring an inspection image based on the determined landing energy level. 
     In some embodiments, an image enhancing 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 test images of a sample that are obtained at different landing energies, determining distortion levels for the plurality of test images, determining a landing energy level that enables the sample to be in a neutral charge condition during inspection based on the distortion levels, and acquiring an inspection image based on the determined landing energy level. 
     In some embodiments, a non-transitory computer readable medium that stores a set of instructions that is executable by at least one processor of a computing device to perform a method for enhancing an image is provided. The method comprises acquiring a plurality of test images of a sample that are obtained at different landing energies, determining distortion levels for the plurality of test images, determining a landing energy level that enables the sample to be in a neutral charge condition during inspection based on the distortion levels, and acquiring an inspection image based on the determined landing energy level. 
     In some embodiments, a method for identifying an optimum landing energy in a charged-particle beam inspection system is provided. The method comprises acquiring a plurality of test images of a sample that are obtained at different landing energies, determining distortion levels for the plurality of test images, wherein determining distortion levels comprises comparing a first test image with a first reference image corresponding to the first test image based on positions of features in the first test image and the first reference image, and determining a landing energy level that enables the sample to be in a neutral charge condition during inspection based on the distortion levels. 
     In some embodiments, a method for enhancing an inspection image in a charged-particle beam inspection system is provided. The method comprises acquiring a first test image and a second test image of a sample, wherein the first test image and the second test image are obtained at different landing energies, determining a first distortion level for the first test image and a second distortion level for the second test image, determining a landing energy level that enables a distortion level to be substantially zero when inspecting the sample, the determination of the landing energy level being based on the first distortion level, the second distortion level, and the different landing energies, and acquiring an inspection image based on the determined landing energy level. 
     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 
         FIG.  1    is a schematic diagram illustrating an example electron beam inspection (EBI) system, consistent with embodiments of the present disclosure. 
         FIG.  2    is a schematic diagram illustrating an example electron beam tool that can be a part of the electron beam inspection system of  FIG.  1   , consistent with embodiments of the present disclosure. 
         FIG.  3 A  is an example comparison of feature positions in an inspection image taken under a neutral charge condition with reference feature positions. 
         FIG.  3 B  is an example comparison of feature positions in an inspection image taken under a negative charge condition with reference feature positions. 
         FIG.  3 C  is an example comparison of feature positions in an inspection image taken under a positive charge condition with reference feature positions. 
         FIG.  4    is a block diagram of an example image enhancement apparatus, consistent with embodiments of the present disclosure. 
         FIG.  5    is an example test region on a sample, consistent with embodiments of the present disclosure. 
         FIG.  6    illustrates an example method of measuring a distortion amount, consistent with embodiments of the present disclosure. 
         FIG.  7    is an example graph for identifying a landing energy corresponding to a neutral charge condition, consistent with embodiments of the present disclosure. 
         FIG.  8    is a process flowchart representing an example method for enhancing an image in a multi-beam inspection system, consistent with embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     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 may be similarly applied. Furthermore, other imaging systems may be used, such as optical imaging, photo detection, x-ray detection, etc. 
     Electronic devices are constructed of circuits formed on a piece of silicon called a substrate. 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 fit on the substrate. For example, an IC chip in a smart phone can be as small as a thumbnail and yet may include over 2 billion transistors, the size of each transistor being less than 1/1000th the size of a human hair. 
     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 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. 
     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. 
     Critical dimensions of patterns/structures measured from a SEM image can be used when identifying defects of manufactured ICs. For example, shifts between patterns or edge placement variations, which are determined from measured critical dimensions, can be helpful in identifying defects and in controlling manufacturing processes. When there is imbalance between incoming primary electrons and outgoing secondary electrons, charge can accumulate on a wafer during inspection. Such charge accumulation may cause significant distortion, defocus, and abnormal grey level of a SEM image and thereby cause an error in measuring critical dimensions from the SEM image. 
     Some embodiments of the present disclosure provide a technique for identifying an energy level that enables the ability to balance a charge on the sample during inspection. Inspecting the sample based on the identified energy level can assist with providing a more accurate SEM image and thus enables detecting defects of a sample with higher accuracy and efficiency. In the present disclosure, identifying a neutral energy level and inspecting a sample based on the identified neutral energy level can be automated. 
     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. 
     Reference is now made to  FIG.  1   , which illustrates an example 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 main chamber  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. 
     EFEM  30  includes a first loading port  30   a  and a second loading port  30   b . EFEM  30  may include additional loading port(s). First loading port  30   a  and second loading port  30   b  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 . 
     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 load-lock 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. 
     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 , load-lock chamber  20 , and EFEM  30 , it is appreciated that controller  50  can be part of the structure. 
     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. 
     Reference is now made to  FIG.  2   , which illustrates a schematic diagram illustrating an example electron beam tool  40  that can be a part of the example charged particle beam inspection system  100  of  FIG.  1   , consistent with embodiments of the present disclosure. An electron beam tool  40  (also referred to herein as apparatus  40 ) comprises an electron source  101 , a gun aperture plate  171  with a gun aperture  103 , a pre-beamlet forming mechanism  172 , a condenser lens  110 , a source conversion unit  120 , a primary projection optical system  130 , a sample stage (not shown in  FIG.  2   ), a secondary imaging system  150 , and an electron detection device  140 . Primary projection optical system  130  can comprise an objective lens  131 . Electron detection device  140  can comprise a plurality of detection elements  140 _ 1 ,  140 _ 2 , and  140 _ 3 . Beam separator  160  and deflection scanning unit  132  can be placed inside primary projection optical system  130 . It may be appreciated that other commonly known components of apparatus  40  may be added/omitted as appropriate. 
     Electron source  101 , gun aperture plate  171 , condenser lens  110 , source conversion unit  120 , beam separator  160 , deflection scanning unit  132 , and primary projection optical system  130  can be aligned with a primary optical axis  100 _ 1  of apparatus  100 . Secondary imaging system  150  and electron detection device  140  can be aligned with a secondary optical axis  150 _ 1  of apparatus  40 . 
     Electron source  101  can comprise a cathode, an extractor or an anode, wherein primary electrons can be emitted from the cathode and extracted or accelerated to form a primary electron beam  102  that forms a crossover (virtual or real)  101   s . Primary electron beam  102  can be visualized as being emitted from crossover  101   s.    
     Source conversion unit  120  may comprise an image-forming element array (not shown in  FIG.  2   ), an aberration compensator array (not shown), a beam-limit aperture array (not shown), and a pre-bending micro-deflector array (not shown). The image-forming element array can comprise a plurality of micro-deflectors or micro-lenses to form a plurality of parallel images (virtual or real) of crossover  101   s  with a plurality of beamlets of primary electron beam  102 .  FIG.  2    shows three beamlets  102 _ 1 ,  102 _ 2 , and  102 _ 3  as an example, and it is appreciated that the source conversion unit  120  can handle any number of beamlets. 
     In some embodiments, source conversion unit  120  may be provided with beam-limit aperture array and image-forming element array (both are not shown). The beam-limit aperture array may comprise beam-limit apertures. It is appreciated that any number of apertures may be used, as appropriate. Beam-limit apertures may be configured to limit sizes of beamlets  102 _ 1 ,  102 _ 2 , and  102 _ 3  of primary-electron beam  102 . The image-forming element array may comprise image-forming deflectors (not shown) configured to deflect beamlets  102 _ 1 ,  102 _ 2 , and  102 _ 3  by varying angles towards primary optical axis  100 _ 1 . In some embodiments, deflectors further away from primary optical axis  100 _ 1  may deflect beamlets to a greater extent. Furthermore, image-forming element array may comprise multiple layers (not illustrated), and deflectors may be provided in separate layers. Deflectors may be configured to be individually controlled independent from one another. In some embodiments, a deflector may be controlled to adjust a pitch of probe spots (e.g.,  102 _ 1 S,  102 _ 2 S, and  102 _ 3 S) formed on a surface of sample  1 . As referred to herein, pitch of the probe spots may be defined as the distance between two immediately adjacent probe spots on the surface of sample  1 . 
     A centrally located deflector of image-forming element array may be aligned with primary optical axis  100 _ 1  of electron beam tool  40 . Thus, in some embodiments, a central deflector may be configured to maintain the trajectory of beamlet  102 _ 1  to be straight. In some embodiments, the central deflector may be omitted. However, in some embodiments, primary electron source  101  may not necessarily be aligned with the center of source conversion unit  120 . Furthermore, it is appreciated that while  FIG.  2    shows a side view of apparatus  40  where beamlet  102 _ 1  is on primary optical axis  100 _ 1 , beamlet  102 _ 1  may be off primary optical axis  100 _ 1  when viewed from a different side. That is, in some embodiments, all of beamlets  102 _ 1 ,  102 _ 2 , and  102 _ 3  may be off-axis. An off-axis component may be offset relative to primary optical axis  100 _ 1 . 
     The deflection angles of the deflected beamlets may be set based on one or more criteria. In some embodiments, deflectors may deflect off-axis beamlets radially outward or away (not illustrated) from primary optical axis  100 _ 1 . In some embodiments, deflectors may be configured to deflect off-axis beamlets radially inward or towards primary optical axis  100 _ 1 . Deflection angles of the beamlets may be set so that beamlets  102 _ 1 ,  102 _ 2 , and  102 _ 3  land perpendicularly on sample  1 . Off-axis aberrations of images due to lenses, such as objective lens  131 , may be reduced by adjusting paths of the beamlets passing through the lenses. Therefore, deflection angles of off-axis beamlets  102 _ 2  and  102 _ 3  may be set so that probe spots  102 _ 2 S and  102 _ 3 S have small aberrations. Beamlets may be deflected so as to pass through or close to the front focal point of objective lens  131  to decrease aberrations of off-axis probe spots  102 _ 2 S and  102 _ 3 S. In some embodiments, deflectors may be set to make beamlets  102 _ 1 ,  102 _ 2 , and  102 _ 3  land perpendicularly on sample  1  while probe spots  102 _ 1 S,  102 _ 2 S, and  102 _ 3 S have small aberrations. 
     Condenser lens  110  is configured to focus primary electron beam  102 . The electric currents of beamlets  102 _ 1 ,  102 _ 2 , and  102 _ 3  downstream of source conversion unit  120  can be varied by adjusting the focusing power of condenser lens  110  or by changing the radial sizes of the corresponding beam-limit apertures within the beam-limit aperture array. The electric currents may be changed by both, altering the radial sizes of beam-limit apertures and the focusing power of condenser lens  110 . Condenser lens  110  may be an adjustable condenser lens that may be configured so that the position of its first principle plane is movable. The adjustable condenser lens may be configured to be magnetic, which may result in off-axis beamlets  102 _ 2  and  102 _ 3  illuminating source conversion unit  120  with rotation angles. The rotation angles may change with the focusing power or the position of the first principal plane of the adjustable condenser lens. Accordingly, condenser lens  110  may be an anti-rotation condenser lens that may be configured to keep the rotation angles unchanged while the focusing power of condenser lens  110  is changed. In some embodiments, condenser lens  110  may be an adjustable anti-rotation condenser lens, in which the rotation angles do not change when the focusing power and the position of the first principal plane of condenser lens  110  are varied. 
     Electron beam tool  40  may comprise pre-beamlet forming mechanism  172 . In some embodiments, electron source  101  may be configured to emit primary electrons and form a primary electron beam  102 . In some embodiments, gun aperture plate  171  may be configured to block off peripheral electrons of primary electron beam  102  to reduce the Coulomb effect. In some embodiments, pre-beamlet-forming mechanism  172  further cuts the peripheral electrons of primary electron beam  102  to further reduce the Coulomb effect. Primary-electron beam  102  may be trimmed into three primary electron beamlets  102 _ 1 ,  102 _ 2 , and  102 _ 3  (or any other number of beamlets) after passing through pre-beamlet forming mechanism  172 . Electron source  101 , gun aperture plate  171 , pre-beamlet forming mechanism  172 , and condenser lens  110  may be aligned with a primary optical axis  100 _ 1  of electron beam tool  40 . 
     Pre-beamlet forming mechanism  172  may comprise a Coulomb aperture array. A center aperture, also referred to herein as the on-axis aperture, of pre-beamlet-forming mechanism  172  and a central deflector of source conversion unit  120  may be aligned with primary optical axis  100 _ 1  of electron beam tool  40 . Pre-beamlet-forming mechanism  172  may be provided with a plurality of pre-trimming apertures (e.g., a Coulomb aperture array). In  FIG.  2   , the three beamlets  102 _ 1 ,  102 _ 2  and  102 _ 3  are generated when primary electron beam  102  passes through the three pre-trimming apertures, and much of the remaining part of primary electron beam  102  is cut off. That is, pre-beamlet-forming mechanism  172  may trim much or most of the electrons from primary electron beam  102  that do not form the three beamlets  102 _ 1 ,  102 _ 2  and  102 _ 3 . Pre-beamlet-forming mechanism  172  may cut off electrons that will ultimately not be used to form probe spots  102 _ 1 S,  102 _ 2 S and  102 _ 3 S before primary electron beam  102  enters source conversion unit  120 . In some embodiments, a gun aperture plate  171  may be provided close to electron source  101  to cut off electrons at an early stage, while pre-beamlet forming mechanism  172  may be also provided to further cut off electrons around a plurality of beamlets. Although  FIG.  2    demonstrates three apertures of pre-beamlet forming mechanism  172 , it is appreciated that there may be any number of apertures, as appropriate. 
     In some embodiments, pre-beamlet forming mechanism  172  may be placed below condenser lens  110 . Placing pre-beamlet forming mechanism  172  closer to electron source  101  may more effectively reduce the Coulomb effect. In some embodiments, gun aperture plate  171  may be omitted when pre-beamlet forming mechanism  172  is able to be located sufficiently close to source  101  while still being manufacturable. 
     Objective lens  131  may be configured to focus beamlets  102 _ 1 ,  102 _ 2 , and  102 _ 3  onto a sample  1  for inspection and can form three probe spots  102 _ 1   s ,  102 _ 2   s , and  102 _ 3   s  on surface of sample  1 . Gun aperture plate  171  can block off peripheral electrons of primary electron beam  102  not in use to reduce Coulomb interaction effects. Coulomb interaction effects can enlarge the size of each of probe spots  102 _ 1   s ,  102 _ 2   s , and  102 _ 3   s , and therefore deteriorate inspection resolution. 
     Beam separator  160  may be a beam separator of Wien filter type comprising an electrostatic deflector generating an electrostatic dipole field El and a magnetic dipole field B 1  (both of which are not shown in  FIG.  2   ). If they are applied, the force exerted by electrostatic dipole field El on an electron of beamlets  102 _ 1 ,  102 _ 2 , and  102 _ 3  is equal in magnitude and opposite in direction to the force exerted on the electron by magnetic dipole field B 1 . Beamlets  102 _ 1 ,  102 _ 2 , and  102 _ 3  can therefore pass straight through beam separator  160  with zero deflection angles. 
     Deflection scanning unit  132  can deflect beamlets  102 _ 1 ,  102 _ 2 , and  102 _ 3  to scan probe spots  102 _ 1   s ,  102 _ 2   s , and  102 _ 3   s  over three small scanned areas in a section of the surface of sample  1 . In response to incidence of beamlets  102 _ 1 ,  102 _ 2 , and  102 _ 3  at probe spots  102 _ 1   s ,  102 _ 2   s , and  102 _ 3   s , three secondary electron beams  102 _ 1   se ,  102 _ 2   se , and  102 _ 3   se  may be emitted from sample  1 . Each of secondary electron beams  102 _ 1   se ,  102 _ 2   se , and  102 _ 3   se  can comprise electrons with a distribution of energies including secondary electrons (energies ≤50 eV) and backscattered electrons (energies between 50 eV and landing energies of beamlets  102 _ 1 ,  102 _ 2 , and  102 _ 3 ). Beam separator  160  can direct secondary electron beams  102 _ 1   se ,  102 _ 2   se , and  102 _ 3   se  towards secondary imaging system  150 . Secondary imaging system  150  can focus secondary electron beams  102 _ 1   se ,  102 _ 2   se , and  102 _ 3   se  onto detection elements  1401 ,  1402 , and  1403  of electron detection device  140 . Detection elements  140 _ 1 ,  140 _ 2 , and  140 _ 3  can detect corresponding secondary electron beams  102 _ 1   se ,  102 _ 2   se , and  102 _ 3   se  and generate corresponding signals used to construct images of the corresponding scanned areas of sample  1 . 
     In  FIG.  2   , three secondary electron beams  102 _ 1   se ,  102 _ 2   se , and  102 _ 3   se  respectively generated by three probe spots  102 _ 1 S,  102 _ 2 S, and  102 _ 3 S, travel upward towards electron source  101  along primary optical axis  100 _ 1 , pass through objective lens  131  and deflection scanning unit  132  in succession. The three secondary electron beams  102 _ 1   se ,  102 _ 2   se  and  102 _ 3   se  are diverted by beam separator  160  (such as a Wien Filter) to enter secondary imaging system  150  along secondary optical axis  150 _ 1  thereof. Secondary imaging system  150  focuses the three secondary electron beams  102 _ 1   se    102 _ 3   se  onto electron detection device  140  which comprises three detection elements  140 _ 1 ,  140 _ 2 , and  140 _ 3 . Therefore, electron detection device  140  can simultaneously generate the images of the three scanned regions scanned by the three probe spots  102 _ 1 S,  102 _ 2 S and  102 _ 3 S, respectively. In some embodiments, electron detection device  140  and secondary imaging system  150  form one detection unit (not shown). In some embodiments, the electron optics elements on the paths of secondary electron beams such as, but not limited to, objective lens  131 , deflection scanning unit  132 , beam separator  160 , secondary imaging system  150  and electron detection device  140 , may form one detection system. 
     In some embodiments, a controller (e.g., controller  50  of  FIG.  1   ) 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 detection device  140  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 detection device  140  and may construct an image. The image acquirer may thus acquire images of sample  1 . 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. 
     In some embodiments, the image acquirer may acquire one or more images of a sample based on one or more imaging signals received from electron detection device  140 . 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 or may involve multiple images. The single image may be stored in the storage. 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 sample  1 . The acquired images may comprise multiple images of a single imaging area of sample  1  sampled multiple times over a time sequence or may comprise multiple images of different imaging areas of sample  1 . The multiple images may be stored in the storage. In some embodiments, controller  50  may be configured to perform image processing steps with the multiple images of the same location of sample  1 . 
     In some embodiments, the controller may include measurement circuitries (e.g., analog-to-digital converters) to obtain a distribution of the detected secondary electrons. The electron distribution data collected during a detection time window, in combination with corresponding scan path data of each of primary beamlets  102 _ 1 ,  102 _ 2 , and  102 _ 3  incident on the wafer surface, can be used to reconstruct images of the wafer structures under inspection. The reconstructed images can be used to reveal various features of the internal or external structures of sample  1 , and thereby can be used to reveal any defects that may exist in the wafer. 
     In some embodiments, the controller may control a motorized stage (not shown) to move sample  1  during inspection. In some embodiments, the controller may enable the motorized stage to move sample  1  in a direction continuously at a constant speed. In other embodiments, the controller may enable the motorized stage to change the speed of the movement of sample  1  over time depending on the steps of scanning process. In some embodiments, the controller may adjust a configuration of primary projection optical system  130  or secondary imaging system  150  based on images of secondary electron beams  102 _ 1   se ,  102 _ 2   se , and  102 _ 3   se.    
     Although  FIG.  2    shows that electron beam tool  40  uses three primary electron beams, it is appreciated that electron beam tool  40  may use two or more number of primary electron beams. The present disclosure does not limit the number of primary electron beams used in apparatus  40 . 
     Reference is now made to  FIG.  3 A , which is an example comparison of feature positions in an inspection image taken under a neutral charge condition with reference feature positions. In this disclosure, a neutral charge condition can refer to a sample status during inspection where primary electrons incident on a sample is balanced with secondary electrons emitting from the sample and thus no charge accumulates on the sample. A feature can refer to a pattern or structure formed on a sample. In  FIG.  3 A , a first inspection image  300  can be obtained by a charged-particle beam inspection system (e.g., electron beam inspection system  100  of  FIG.  1   ). For example, first inspection image  300  can be an electron beam image generated based on an electron detection signal from electron detection element  140 . First inspection image  300  can be an inspection image of a sample comprising multiple features. In  FIG.  3 A , positions  301  of features on first inspection image  300  are indicated as circles. It will be appreciated that patterns of features are not indicted in first inspection image  300  of  FIG.  3 A  for purposes of simplicity. 
       FIG.  3 A  also shows reference positions  302  (e.g., indicated as squares) of features superimposed on inspection image  300 . According to embodiments of the present disclosure, reference positions  302  of features on a sample can be obtained from a reference image corresponding to the sample. In some embodiments, a reference image can be a ground truth image of a sample. A ground truth image can include a raw image of a wafer or die containing the corresponding pattern or can include a ground truth wafer map measured from a wafer or die containing the corresponding pattern, among others. In some embodiments, a reference image can comprise a wafer design layout of a corresponding sample, such as in a in Graphic Database System (GDS) format, Graphic Database System II (GDS II) format, an Open Artwork System Interchange Standard (OASIS) format, a Caltech Intermediate Format (CIF), etc. The wafer design layout may be based on a pattern layout for constructing the wafer. In some embodiments, a reference image, among others, may comprise feature information stored in a binary file format representing planar geometric shapes, text, and other information related to wafer design layout. As shown in  FIG.  3 A , when a sample is inspected under a neutral charge condition, positions  301  of features on first inspection image  300  match reference positions  302  of features in a reference image. 
     However, when inspecting a sample under a charge build-up condition such as a negative charge condition or a positive charge condition, an inspection image can be distorted. In this disclosure, a negative charge condition can refer to a sample status during inspection where an amount of primary electrons incident on a sample is larger than that of secondary electrons emitting from the sample and thus negative charge accumulates on the sample. Similarly, a positive charge condition can refer to a sample status during inspection where an amount of primary electrons incident on a sample is smaller than that of secondary electrons emitting from the sample and thus positive charge accumulates on the sample. 
     As shown in  FIG.  3 B  illustrating an example comparison of feature positions in an inspection image under a negative charge condition with reference feature positions, a second inspection image  310  taken under a negative charge condition can expand. In  FIG.  3 B , a distance (e.g., d 1 ) between two positions among positions  311  of features on second inspection image  310  is greater than a reference distance (e.g., d 2 ) between two reference positions  302  of corresponding features. As shown in  FIG.  3 C  illustrating an example comparison of feature positions in an inspection image under a positive charge condition with reference feature positions, a third inspection image  320  taken under a positive charge condition can shrink. As shown in  FIG.  3 C , a distance (e.g., di) between two positions among positions  321  of features on third inspection image  320  is smaller than a reference distance (e.g., d 2 ) between two reference positions  302  of corresponding features. While image expansion and image contraction are discussed as a type of distortion occurred due to charge accumulation on a sample (e.g., as shown  FIGS.  3 B,  3 C, and  6   ) in this disclosure, it is appreciated that a different type of distortion such as defocus, pattern shape distortion (e.g., pillow-shape or asymmetric-trapezoid distortions), etc. can also occur due to the charge accumulation. 
     As explained with respect to  FIG.  3 A  to  FIG.  3 C , charge accumulation on a sample may bend electron beams that are used to scan the sample and result in significant distortion of a feature position or displacement on an inspection image. Such distortion may lead to errors in detecting critical dimensions, edge displacements, etc. from an inspection image. Embodiments of the present disclosure can provide techniques to determine a landing energy that enables the ability to balance a charge on the sample and thereby to provide a more accurate SEM image. 
       FIG.  4    is a block diagram of an example image enhancement apparatus  400 , consistent with embodiments of the present disclosure. It is appreciated that in various embodiments image enhancement apparatus  400  may be part of or may be separate from a charged-particle beam inspection system (e.g., electron beam inspection system  100  of  FIG.  1   ). In some embodiments, image enhancement apparatus  400  may be part of controller  50  and may include an image acquirer, measurement circuitry, or storage, or the like. In some embodiments, image enhancement apparatus  400  may comprise an image processing system and may include an image acquirer, storage, or the like. 
     As illustrated in  FIG.  4   , image enhancement apparatus  400  may include a test image acquirer  410 , test image analyzer  420 , an inspection condition controller  430 , and an inspection image acquirer. 
     Test image acquirer  410  is configured to receive a plurality of test images, consistent with embodiments of the present disclosure. A test image can be an inspection image for a region of a sample. A plurality of test images can be taken at different landing energies. In some embodiments, a plurality of test images can be taken for different test regions of a sample. For example, multiple test regions can be chosen for testing and a corresponding test image can be taken for each test region. In some embodiments, a plurality of test images for different test regions of a sample can be taken at the same time, such as via a multi-beam SEM. In this case, testing regions for the plurality of test images can be spaced apart such that one test region is not affected by an electron beam for other test region during testing. In some other embodiments, a plurality of test images can be taken for a region of a sample sequentially. In some embodiments, test image acquirer  410  may generate a test image based on a detection signal from electron detection device  140  of electron beam tool  40 . In some embodiments, test image acquirer  410  may be part of or may be separate from an image acquirer included in controller  50 . In some embodiments, test image acquirer  410  may obtain a test image generated by an image acquirer included in controller  50 . In some embodiments, test image acquirer  410  may obtain a test image from a storage device or system storing the test image. In some embodiments, to reduce a processing time and resource, a test image can be obtained for a small portion of a sample. 
     According to embodiments of the present disclosure, a test region on a sample can be chosen such that an image distortion such as image expansion or image contraction is measured from its corresponding test image.  FIG.  5    illustrates an example of a test region  501  on a sample  500 , consistent with embodiments of the present disclosure. As shown in  FIG.  5   , test region  501  can comprise multiple features  502  therein and a center  503  of test region  501  is also indicated for an illustration purpose. While  FIG.  5    illustrates regularly arranged features  502 , it will be appreciated that features  502  included in test region  501  may not be regularly arranged Similarly, features  502  included in test region  501  can have different shapes even though  FIG.  5    illustrates features  502  having the same shape. In some embodiments, an area of test region  501  can correspond to a field of view of a primary electron beam (e.g., beamlets  102 _ 1 ,  102 _ 2 , or  102 _ 3 ). Choosing a region having multiple features  502  as test region  501  is one of several ways to enable measuring image distortion (e.g., image expansion or image extraction) of a test image based on feature displacements. In some embodiments, a sample can include an area that is designed or designated to be a test region and that  includes multiple features  502 , which is advantageous in determining a distortion level of a test image therefrom. 
     When a plurality of test images are taken for different test regions of sample  500 , a plurality of test regions on different portions of sample  500  can be chosen for the plurality of test images. Similarly, each of the plurality of test regions can have multiple features (e.g.,  502 ). In some embodiments, choosing a plurality of test regions having similar patterns or features can be advantageous in comparing displacement measurements (e.g., distortion levels) of a plurality of test images corresponding to the plurality of test regions. In some embodiments, a sample can include a plurality of areas that are designed or designated to be test regions and that include multiple features  502  having the same shape as each other, which is advantageous in comparing distortion levels of test images therefrom. In some embodiments, the plurality of areas can include a feature at the same relative position in each area. In some embodiments, a distance between adjacent two test regions can be large enough that one test region is not affected by a primary electron beam for the other test region during testing. 
     Referring back to  FIG.  4   , test image analyzer  420  is configured to determine whether test image  501  is distorted and to measure the distortion amount. According to embodiments of the present disclosure, test image analyzer  420  can analyze test images by referring to reference images corresponding to the test images. According to embodiments of the present disclosure, information file  440  can contain reference images corresponding to test images. Information file  440  may be any means of storing information, such as a file, a set of files, a database, a set of databases, etc. Information file  440  can, for example, include reference images of test regions for the test images. In some embodiments, a reference image contained in information file  440  can be a ground truth image of a corresponding test region. A ground truth image can include a raw image of a wafer or die containing the corresponding pattern or can include a ground truth wafer map measured from a wafer or die containing the corresponding pattern, among others. In some embodiments, a reference image contained in information file  440  can be in Graphic Database System (GDS) format, Graphic Database System II (GDS II) format, an Open Artwork System Interchange Standard (OASIS) format, a Caltech Intermediate Format (CIF), etc. In some embodiments, a reference image contained in information file  440  can comprise a wafer design layout of a corresponding test region. The wafer design layout may be based on a pattern layout for constructing 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 a wafer. In some embodiments, a reference image in GDS or OASIS, among others, may comprise feature information stored in a binary file format representing planar geometric shapes, text, and other information related to wafer design layout. 
     For illustration and simplicity purposes, operations of test image analyzer  420  will be explained under the assumption that inspection images  300 ,  310 , and  320  of  FIG.  3 A  to  FIG.  3 C  are test images. With respect to  FIG.  3 A , it is assumed that positions  301  are feature positions on a first test image  300  and reference positions  302  are corresponding feature positions on a first reference image corresponding to first test image  300 . As shown in  FIG.  3 A , positions  301  of features on first test image  300  match corresponding reference positions  302  of features. In this example, test image analyzer  420  can determine that first test image  300  is not distorted. 
     With respect to  FIG.  3 B , it is assumed that positions  311  are feature positions on a second test image  310  and reference positions  302  are corresponding feature positions on a second reference image corresponding to second test image  310 . As shown in  FIG.  3 B , positions  311  of features on second test image  310  do not match corresponding reference positions  302  of features and thus test image analyzer  420  can determine that second test image  310  is distorted. In some embodiments, based on a comparison of feature distances on second test image  310  and a second reference image, test image analyzer  420  can determine whether second test image  310  has expanded or shrunk For example, test image analyzer  420  can compare a first distance d 1  between two feature positions  311  on second test image  310  with a second distance d 2  between two reference feature positions  302  on a second reference image corresponding to the two feature positions  311 . In this example, since first distance d 1  is greater than second distance d 2 , test image analyzer  420  can determine that second test image  310  has expanded. 
     In some embodiments, based on a distance between a feature position  311  on second test image  310  and a reference feature position  302  corresponding to the feature, test image analyzer  420  can determine a distortion amount. As shown in  FIG.  3 B , a distortion amount can be determined based on a third distance d 3  between centers of a feature position  311  on second test image  310  and a corresponding reference feature position  302 . In some embodiments, because an absolute distortion amount can vary depending on feature position  311  on second test image  310  as shown in  FIG.  3 B , a distortion amount (e.g., d 3 ) of a feature position  321  at a criteria location (e.g., a certain distance from a center) of second test image  310  can be used as a distortion amount. In some embodiments, if a feature does not exist at a criteria location, a distortion amount can be estimated based on measured distortion amounts of feature positions  311  in the second test image  310 . Thereby, an adequate comparison between distortion amounts of multiple test images can be obtained. In some embodiments, a distortion amount for second test image  310  can be determined based on an average of displacement amounts (e.g., third distance d 3 ) for multiple features in the second test image  310 . In this example, test image analyzer  420  can determine that second test image  310  is distorted (e.g., expanded) and that a distortion amount corresponds to a third distance d 3  or average of displacements amounts for features in the second test image  310 . 
     With respect to  FIG.  3 C , it is assumed that positions  321  are feature positions on a third test image  320  and reference positions  302  are corresponding feature positions on a third reference image corresponding to third test image  320 . As shown in  FIG.  3 C , positions  321  of features on third test image  320  do not match corresponding reference positions  302  of features and thus test image analyzer  420  can determine that third test image  320  is distorted. In some embodiments, based on a comparison of feature distances on third test image  320  and a third reference image, test image analyzer  420  can determine whether third test image  320  has expanded or shrunk For example, test image analyzer  420  can compare a first distance d 1  between two feature positions  321  on third test image  320  with a second distance d 2  between two reference feature positions  302  on a third reference image corresponding to the two feature positions  321 . In this example, since first distance dl is smaller than second distance d 2 , test image analyzer  420  can determine that third test image  320  has contracted. 
     In some embodiments, based on a distance between a feature position  321  on third test image  320  and a reference feature position  302  corresponding to the feature, test image analyzer  420  can determine a distortion amount. As shown in  FIG.  3 C , a distortion amount can be determined based on a third distance d 3  between centers of a feature position  321  on third test image  320  and a corresponding reference feature position  302 . In some embodiments, because an absolute distortion amount can vary depending on feature position  321  on third test image  320  as shown in  FIG.  3 C , a distortion amount (e.g., d 3 ) of feature position  321  at a criteria location (e.g., a certain distance from a center) of third test image  320  can be used as a distortion amount. In some embodiments, if a feature does not exist at a criteria location, a distortion amount can be estimated based on measured distortion amounts of feature positions  321  in third test image  320 . Thereby, an adequate comparison between distortions amounts of multiple test images can be obtained. In some embodiments, a distortion amount for third test image  320  can be determined based on an average of displacement amounts (e.g., third distance d 3 ) for multiple features in the third test image  320 . In this example, test image analyzer  420  can determine that third test image  320  is distorted (e.g., contracted) and that a distortion amount corresponds to a third distance d 3  or average of displacements amounts for features in the third test image  320 . 
     While determining a distortion level of a test image (e.g.,  300 ,  310 , or  320  of  FIGS.  3 A to  3 C ) has been explained by aligning a test image with a reference image such that centers of the test image and the reference image match, it will be appreciated that any method for determining a distortion level can be applied to embodiments of the present disclosure.  FIG.  6    illustrates an example method of measuring a distortion level, consistent with embodiments of the present disclosure. As shown in  FIG.  6   , a distortion amount can be analyzed and determined by aligning a test image  610  and a corresponding reference image such that a feature position  611  on test image  610  at one corner (e.g., top-left corner) matches a corresponding feature position  302  on the reference image. In this example, a distortion amount can be determined based on a third distance d 3  between centers of a feature position  611  on test image  610  located at another corner (e.g., a diagonally opposite corner to the one corner; bottom-right corner in  FIG.  6   ) and a corresponding reference feature position  302 . Although  FIG.  6    illustrates the example method of measuring a distortion level of test image  610  taken under a negative charge condition, it is noted that the same method can be applied to measure a distortion level of a test image taken under a positive charge condition or a neutral charge condition. 
     As discussed above, test image analyzer  420  is configured to analyze a plurality of test images that are acquired by test image acquirer  410 . According to embodiments of the present disclosure, based on the determined distortion tendency (e.g., expansion or contraction) and distortion amount (e.g., displacement amount), test image analyzer  420  is configured to determine a landing energy that enables a sample to be in a neutral charge condition during inspection.  FIG.  7    is an example graph  700  for identifying a landing energy corresponding to a neutral charge condition, consistent with embodiments of the present disclosure. In  FIG.  7   , a landing energy is indicated with a voltage V that is applied to primary electron beams, e.g., in order to accelerate or decelerate the electron beams during a test. Because an electron has a constant charge value, the voltage applied to the electron can be an indication of the electron&#39;s energy when the electron lands on the sample. 
     As shown in  FIG.  7   , test results T 1  to T 7  are indicated in the graph  700 . In this example, test results T 1  to T 7  can be distortion amounts determined by test image analyzer  420  from seven test images (e.g., test image  310  or  320  of  FIG.  3 B or  3 C ). For example, a first, second, third, and seventh test results T 1 , T 2 , T 3 , and T 7  can be obtained from four test images similar to second test image  310  of  FIG.  3 B  in that the test results T 1 , T 2 , T 3 , and T 7  indicate image expansion Similarly, a fourth, fifth, and sixth test results T 4 , T 5 , and T 6  can be obtained from three test images similar to third test image  320  of  FIG.  3 C  in that the test results T 4 , T 5 , and T 6  indicate image contraction. As shown in  FIG.  7   , each test result T 1  to T 7  is positioned in the graph  700  according to its corresponding landing energy and distortion amount. For example, first test result T 1  represents a distortion amount of a test image that is taken with a landing energy of 300V. Similarly, second to seventh test results T 2  to T 7  are indicated in the graph  700 . In some embodiments, when test results T 1  to T 7  are obtained from test images for different test regions, the distortion amounts for each test results T 1  to T 7  on graph  700  can be normalized values or distortion amounts at a criteria location in order for fair comparison among test results T 1  to T 7 . While  FIG.  7    illustrates 7 test results, it will be appreciated that any number of test results can be applied to embodiments of the present disclosure. 
     It may not be possible to get a test image (e.g., first test image  300  of  FIG.  3 A ) that is not distorted. According to embodiments of the present disclosure, test image analyzer  420  is configured to determine a landing energy (aka, a neutral landing energy in this disclosure) enabling a sample to be in a neutral charge condition during inspection based on test results (e.g., T 1  to T 7 ). In some embodiments, test image analyzer  420  can estimate a neutral landing energy by interpolation a curve of test results (e.g., T 1  to T 7 ) on graph  700 . For example, an interpolation line L 1  connecting test results T 1  to T 7  can be defined as shown in  FIG.  7    and a neutral landing energy E 1  or E 2  can be obtained at an intersection between interpolation line L 1  and a neutral charge condition line L 2 , which is a horizontal line indicating a zero displacement. In this example, two landing energies E 1  and E 2  are estimated as neutral landing energies for the sample. 
     Referring back to  FIG.  4   , inspection condition controller  430  is configured to set an inspection condition for inspecting the sample according to the determination of test image analyzer  420 , consistent with embodiments of the present disclosure. According to embodiments of the present disclosure, an inspection condition can include a landing energy of a primary electron beam for inspecting the sample. A neutral landing energy (e.g., E 1  or E 2 ) can be a material or property specific parameter and therefore a neutral landing energy determined from some portion (e.g., test region) of a sample can be used to inspect a whole sample. In some embodiments, a neutral landing energy determined for a sample having a certain material can also be used to inspect another sample having the same material. In some embodiments, inspection condition controller  430  can set a landing energy for inspecting the sample to a neutral landing energy E 1  or E 2  determined by test image analyzer  420 , which enables avoiding charge accumulation on a sample. 
     In some embodiments, setting a landing energy to a neutral landing energy E 1  or E 2  may not be allowed, for example, due to inspection requirements, restraints, etc. For example, a landing energy may not be set greater than a certain level because a sample may start getting damaged from an electron beam current with a higher level of energy. A landing energy may not be set smaller than a certain level because secondary electron beams may not be adequately emitted with a lower level of energy. Or a landing energy may not be set smaller than a certain level in order to get an inspection image having a desired resolution. Therefore, in some embodiments, a landing energy for inspecting a sample can be set as close as to a neutral landing energy E 1  or E 2 . And inspection condition controller  430  can further perform an inspection tool calibration to suppress or compensate charging on a sample during inspection in addition to controlling a landing energy of a primary electron beam. For example, other inspection conditions such as a primary beam current dose on a sample can be also adjusted. 
     According to embodiments of the present disclosure, inspection image acquirer  450  can acquire an inspection image of the sample. An inspection image can be acquired by using the landing energy set by inspection condition controller  430 . In some embodiments, inspection image acquirer  450  may generate an inspection image of the sample based on a detection signal from electron detection device  140  of electron beam tool  40 . In some embodiments, inspection image acquirer  450  may be part of or may be separate from an image acquirer included in controller  50 . In some embodiments, inspection image acquirer  450  may obtain the inspection image generated by an image acquirer included in controller  50 . In some embodiments, inspection image acquirer  450  may obtain the inspection image from a storage device or system storing the inspection image. 
     As discussed above, setting a landing energy to a neutral landing energy E 1  or E 2  may not be allowed, for example, due to inspection requirements, restraints, etc, or the estimated neutral landing energy E 1  or E 2  may not be accurate. Therefore, charge can still accumulate on a sample during inspection with a landing energy set by inspection condition controller  430  and the inspection image taken therefrom can still have distortion. 
     According to embodiments of the present disclosure, image enhancement apparatus  400  can further comprise an image corrector  460  as shown in  FIG.  4   . Image corrector  460  can be configured to perform image correction to compensate charge accumulation effects. In some embodiments, image corrector  460  can correct an inspection image by referring to a reference image corresponding to an inspection image of a sample. For example, image corrector  460  can compare the reference image contained in information file  440  with an inspection image acquired by inspection image acquirer  450  and correct errors on the inspection image. In some embodiments, a reference image can be an image for a whole sample. 
     In some other embodiments, image corrector  460  can correct an inspection image by applying a predetermined offset to the inspection image. A predetermined offset can be obtained from multiple experiments. In some embodiments, multiple experimental inspection images can be taken with the landing energy set by inspection condition controller  430  and an error amount (e.g., distortion amount or displacement amount) for each experimental inspection image can be determined, e.g., by comparison with a reference image. An offset can be determined based on an average of error amounts for multiple experimental inspection. In some embodiments, to reduce a processing time and resource, each experimental inspection image can be obtained for a small portion of a sample. In some embodiments, multiple experimental inspection images can be taken at the same time similar to test images. In some embodiments, a plurality of test regions for testing can be also used for multiple experimental inspection images. 
     According to embodiments of the present disclosure, operations of image enhancement apparatus  400  can be automated. According to embodiments of the present disclosure, e.g., when image processing time and resource for test image analysis or experimental inspection image analysis is sufficiently small, identifying a neutral landing energy for a sample, inspecting a sample with a landing energy based on the neutral landing energy, and correcting an inspection image taken therefrom can be performed in real time. 
       FIG.  8    is a process flowchart representing an example method for enhancing an image in a multi-beam inspection system, consistent with embodiments of the present disclosure. For illustrative purposes, a method for enhancing an image will be described referring to image enhancing apparatus  400  of  FIG.  4   . 
     In step  5810 , a plurality of test images (e.g.,  300 ,  310 , or  320  of  FIGS.  3 A- 3 C ) can be obtained. Step  5810  can be performed by, for example, test image acquirer  410 , among others. A test image can be an inspection image for a region of a sample. A plurality of test images can be taken at different landing energies. In some embodiments, a plurality of test images can be taken for different test regions of a sample at the same time, such as via a multi-beam SEM. In this case, testing regions for the plurality of test images can be spaced apart such that one test region is not affected by an electron beam for other test region during testing. In some other embodiments, a plurality of test images can be taken for a region of a sample at different times sequentially. In some embodiments, to reduce a processing time and resource, a test image can be obtained for a small portion of a sample. 
     According to embodiments of the present disclosure, a test region on a sample can be chosen such that an image distortion (e.g., image expansion or image contraction) is measured from its corresponding test image.  FIG.  5    illustrates an example of a test region  501  on a sample  500 , consistent with embodiments of the present disclosure. As shown in  FIG.  5   , test region  501  can comprise multiple features  502  therein, and a center  503  of test region  501  is also indicated for an illustration purpose. While  FIG.  5    illustrates regularly arranged features  502 , it will be appreciated that features  502  included in test region  501  may not be regularly arranged Similarly, features  502  included in test region  501  can have different shapes even though  FIG.  5    illustrates features  502  having the same shape. In some embodiments, an area of test region  501  can correspond to a field of view of a primary electron beam (e.g., beamlets  102 _ 1 ,  102 _ 2 , or  102 _ 3 ). Choosing a region having multiple features  502  as test region  501  is one of several ways to enable measuring image distortion (e.g., image expansion or image extraction) of a test image based on feature displacements. In some embodiments, a sample can include an area that is designed or designated to be a test region and that includes multiple features  502 , which is advantageous in determining a distortion level of a test image therefrom. 
     When a plurality of test images are taken for different test regions of sample  500 , a plurality of test regions on different portions of sample  500  can be chosen for the plurality of test images. Similarly, each of the plurality of test regions can have multiple features (e.g.,  502 ). In some embodiments, a sample can include a plurality of areas that are designed to be test regions and that include multiple features  502  having the same shape as each other, which is advantageous in comparing distortion levels of test images therefrom. In some embodiments, the plurality of areas can include a feature at the same relative position in each area. In some embodiments, choosing a plurality of test regions having similar patterns or features can be advantageous in comparing displacement measurements from a plurality of test images corresponding to the plurality of test regions. 
     In step S 820 , acquired test images are analyzed. Step S 820  can be performed by, for example, test image analyzer  420 , among others. In step S 820 , a distortion level (e.g., distortion tendency, distortion amount, etc.) can be determined. According to embodiments of the present disclosure, test images can be analyzed by referring to reference images corresponding to the test images. In some embodiments, a reference image can be a ground truth image of a corresponding test region. A ground truth image can include a raw image of a wafer or die containing the corresponding pattern or can include a ground truth wafer map measured from a wafer or die containing the corresponding pattern, among others. In some embodiments, a reference image can be in Graphic Database System (GDS) format, Graphic Database System II (GDS II) format, an Open Artwork System Interchange Standard (OASIS) format, a Caltech Intermediate Format (CIF), etc. In some embodiments, a reference image can comprise a wafer design layout of a corresponding test region. The wafer design layout may be based on a pattern layout for constructing 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 a wafer. In some embodiments, a reference image in GDS or OASIS, among others, may comprise feature information stored in a binary file format representing planar geometric shapes, text, and other information related to wafer design layout. 
     For illustration and simplicity purposes, step S 820  will be explained under the assumption that inspection images  300 ,  310 , and  320  of  FIG.  3 A  to  FIG.  3 C  are test images. As shown in  FIG.  3 A , positions  301  of features on first test image  300  correspond to reference positions  302  of features. In this example, it will be determined that first test image  300  is not distorted. 
     As shown in  FIG.  3 B , positions  311  of features on second test image  310  do not match corresponding reference positions  302  of features and thus it will be determined that second test image  310  is distorted. In some embodiments, based on a comparison of feature distances on second test image  310  and a second reference image, whether second test image  310  has expanded or shrunk is determined. For example, a first distance d 1  between two feature positions  311  on second test image  310  can be compared with a second distance d 2  between two reference feature positions  302  on a second reference image corresponding to the two feature positions  311 . In this example, since first distance d 1  is greater than second distance d 2 , it will be determined that second test image  310  has expanded. 
     In some embodiments, based on a distance between a feature position  311  on second test image  310  and a reference feature position  302  corresponding to the feature, a distortion amount can be determined. As shown in  FIG.  3 B , a distortion amount can be determined based on a third distance d 3  between centers of a feature position  311  on second test image  310  and a corresponding reference feature position  302 . In some embodiments, because an absolute distortion amount can vary depending on feature position  311  on second test image  310  as shown in  FIG.  3 B , a distortion amount (e.g., d 3 ) of a feature position  321  at a criteria location (e.g., a certain distance from a center) of second test image  310  can be used as a distortion amount. In some embodiments, if a feature does not exist at a criteria location, a distortion amount can be estimated based on measured distortion amounts of feature positions  311  in the second test image  310 . Thereby, an adequate comparison between distortion amounts of multiple test images can be obtained. In some embodiments, a distortion amount for second test image  310  can be determined based on an average of displacement amounts (e.g., third distance d 3 ) for multiple features in the second test image  310 . In this example, it will be determined that second test image  310  is distorted (e.g., expanded) and that a distortion amount corresponds to a third distance d 3  or average of displacements amounts for features in the second test image  310 . 
     As shown in  FIG.  3 C , positions  321  of features on third test image  320  do not match corresponding reference positions  302  of features and thus it will be determined that third test image  320  is distorted. In some embodiments, based on a comparison of feature distances on third test image  320  and a third reference image, whether third test image  320  has expanded or shrunk is determined. For example, a first distance d 1  between two feature positions  321  on third test image  320  can be compared with a second distance d 2  between two reference feature positions  302  on a third reference image corresponding to the two feature positions  321 . In this example, since first distance dl is smaller than second distance d 2 , it will be determined that third test image  320  has contracted. 
     In some embodiments, based on a distance between a feature position  321  on third test image  320  and a reference feature position  302  corresponding to the feature, a distortion amount can be determined. As shown in  FIG.  3 C , a distortion amount can be determined based on a third distance d 3  between centers of a feature position  321  on third test image  320  and a corresponding reference feature position  302 . In some embodiments, because an absolute distortion amount can vary depending on feature position  321  on third test image  320  as shown in  FIG.  3 C , a distortion amount (e.g., d 3 ) of feature position  321  at a criteria location (e.g., a certain distance from a center) of third test image  320  can be used as a distortion amount. In some embodiments, if a feature does not exist at a criteria location, a distortion amount can be estimated based on measured distortion amounts of feature positions  321  in third test image  320 . Thereby, an adequate comparison between distortion amounts of multiple test images can be obtained. In some embodiments, a distortion amount for third test image  320  can be determined based on an average of displacement amounts (e.g., third distance d 3 ) for multiple features in the second test image  320 . In this example, it will be determined that third test image  320  is distorted (e.g., contracted) and that a distortion amount corresponds to a third distance d 3  or average of displacements amounts for features in the third test image  320 . 
     While determining a distortion level of a test image (e.g.,  300 ,  310 , and  320  of  FIGS.  3 A to  3 C ) has been explained by aligning a test image with a reference image such that centers of the test image and the reference image match, it will be appreciated that any method for determining a distortion level can be applied to embodiments of the present disclosure.  FIG.  6    illustrates an example method of measuring a distortion level, consistent with embodiments of the present disclosure. As shown in  FIG.  6   , a distortion amount can be analyzed and determined by aligning a test image  610  and a corresponding reference image such that a feature position  611  on test image  610  at one corner (e.g., top-left corner) matches a corresponding feature position  302  on the reference image. In this example, a distortion amount can be determined based on a third distance d 3  between centers of a feature position  611  on test image  610  located at another corner (e.g., a diagonally opposite corner to the one corner; bottom-right corner in  FIG.  6   ) and a corresponding reference feature position  302 . Although  FIG.  6    illustrates the example method of measuring a distortion level of test image  610  taken under a negative charge condition, it is noted that the same method can be applied to measure a distortion level of a test image taken under a positive charge condition or a neutral charge condition. 
       FIG.  7    is an example graph  700  for identifying a landing energy corresponding to a neutral charge condition, consistent with embodiments of the present disclosure. In  FIG.  7   , a landing energy is indicated with a voltage V that is applied to primary electron beams, e.g., in order to accelerate or decelerate the electron beams during a test. Because an electron has a constant charge value, the voltage applied to the electron can be an indication of the electron&#39;s energy when the electron lands on the sample. 
     As shown in  FIG.  7   , test results T 1  to T 7  are indicated in the graph  700 . In this example, test results T 1  to T 7  can be distortion amounts. As shown in  FIG.  7   , each test result T 1  to T 7  is positioned in the graph  700  according to its corresponding landing energy and distortion amount. In some embodiments, when test results T 1  to T 7  are obtained from test images for different test regions, the distortion amounts for each test results T 1  to T 7  on graph  700  can be normalized values or distortion amounts at a criteria location in order for fair comparison among test results T 1  to T 7 . According to embodiments of the present disclosure, in step  5820 , a landing energy (aka, a neutral landing energy in this disclosure) enabling a sample to be in a neutral charge condition during inspection can be determined based on test results (e.g., T 1  to T 7 ). In some embodiments, a neutral landing energy can be estimated by interpolation a curve of test results (e.g., T 1  to T 7 ) on graph  700 . For example, an interpolation line L 1  connecting test results T 1  to T 7  can be defined as shown in  FIG.  7    and a neutral landing energy E 1  or E 2  can be obtained at an intersection between interpolation line L 1  and a neutral charge condition line L 2 , which is a horizontal line indicating a zero displacement. In this example, two landing energies E 1  and E 2  are estimated as neutral landing energies for the sample. 
     Referring back to  FIG.  8   , in step S 830 , an inspection condition for inspecting the sample can be controlled according to the analysis in step S 820 . Step S 830  can be performed by, for example, inspection condition controller  430 , among others. According to embodiments of the present disclosure, an inspection condition can include a landing energy of a primary electron beam for inspecting the sample. A neutral landing energy (e.g., E 1  and E 2 ) can be material or property specific parameter, and therefore a neutral landing energy determined from some portions (e.g., test region) of a sample can be used to inspect a whole sample. In some embodiments, a neutral landing energy determined for a sample having a certain material can also be used to inspect another sample having the same material. In some embodiments, a landing energy for inspecting the sample can be set to a neutral landing energy E 1  or E 2  determined in step S 820 , which enables avoiding charge accumulation on a sample. 
     In some embodiments, setting a landing energy to a neutral landing energy E 1  or E 2  may not be allowed, for example, due to inspection requirements, restraints, etc. Therefore, in some embodiments, a landing energy for inspecting a sample can be set as close as to a neutral landing energy E 1  or E 2 . And an inspection tool calibration can be further performed to suppress or compensate charging on a sample during inspection in addition to controlling a landing energy of a primary electron beam. For example, other inspection conditions such as a primary beam current dose on a sample can also be adjusted. 
     In step  5840 , an inspection image of the sample can be acquired. Step S 8740  can be performed by, for example, inspection image acquirer  420 , among others. Inspection image can be acquired by using the landing energy set in step S 830 . 
     As discussed above, setting a landing energy to a neutral landing energy E 1  or E 2  may not be allowed, for example, due to inspection requirements, restraints, etc, or the estimated neutral landing energy E 1  or E 2  may not be accurate. Therefore, charge can still accumulate on a sample during inspection with a landing energy set in step S 830  and the inspection image taken therefrom can still have distortion. 
     According to embodiments of the present disclosure, the method can further comprise step S 850 . In step S 850 , image correction can be performed to compensate charge accumulation effects. In some embodiments, an inspection image can be corrected by referring to a reference image corresponding to an inspection image of a sample. For example, the reference image can be compared with an inspection image acquired in step S 840 , and errors on the inspection image can be corrected based on the comparison. Here, a reference image can be an image for a whole sample. 
     In some embodiments, an inspection image can be corrected by applying a predetermined offset to the inspection image. A predetermined offset can be obtained from multiple experiments. In some embodiments, multiple experimental inspection images can be taken with the landing energy set in step  5830  and an error amount (e.g., distortion amount or displacement amount) for each experimental inspection image can be determined, e.g., by comparison with a reference image. An offset can be determined based on an average of error amounts for multiple experimental inspection. In some embodiments, to reduce a processing time and resource, each experimental inspection image can be obtained for a small portion of a sample. 
     Aspects of the present disclosure are set out in the following numbered clauses: 
     1. A method for enhancing an inspection image in a charged-particle beam inspection system, the method comprising: 
     acquiring a plurality of test images of a sample that are obtained at different landing energies; 
     determining distortion levels for the plurality of test images; 
     determining a landing energy level that enables the sample to be in a neutral charge condition during inspection based on the distortion levels; and 
     acquiring an inspection image based on the determined landing energy level. 
     2. The method of clause 1, further comprising:
         correcting the inspection image based on a reference image corresponding to the inspection image.       

     3. The method of clause 1 or 2, wherein each of the acquired plurality of test images of the sample corresponds to a test region of a plurality of test regions of the sample. 
     4. The method of any one of clauses 1-3, wherein determining distortion levels for the plurality of test images comprises determining a first distortion level for a first test image among the plurality of test images based on a first reference image corresponding to the first test image. 
     5. The method of clause 4, wherein the first distortion level comprises information that indicates whether the first test image expands or contracts. 
     6. The method of clause 4, wherein the first distortion level comprises a first distortion amount based on a displacement between a feature on the first test image and a corresponding feature on the first reference image. 
     7. The method of any one of clauses 1-6, wherein determining a landing energy level enabling the sample to be in a neutral charge condition comprises estimating the landing energy level enabling a distortion amount to be zero based on the distortion levels. 
     8. The method of clause 4, wherein the determination of the first distortion level is based on a comparison between a first distance of two features on the first test image and a second distance of corresponding two features on the first reference image. 
     9. The method of clause 3, wherein each of the plurality of test regions comprises multiple features. 
     10. The method of clause 1, further comprising correcting the inspection image by applying a predetermined offset to the inspection image. 
     11. The method of clause 10, wherein the predetermined offset is determined based on an error amount of an experimental inspection image corresponding to a portion of the sample acquired based on the determined landing energy level. 
     12. An image enhancing apparatus 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 test images of a sample that are obtained at different landing energies;
           determining distortion levels for the plurality of test images;   determining a landing energy level that enables the sample to be in a neutral charge condition during inspection based on the distortion levels; and   acquiring an inspection image based on the determined landing energy level.   
               

     13. The apparatus of clause 12, wherein the at least one processor is configured to execute the set of instructions to cause the apparatus to further perform:
         correcting the inspection image based on a reference image corresponding to the inspection image.       

     14. The apparatus of clause 12 or 13, wherein each of the acquired plurality of test images of the sample corresponds to a test region of a plurality of test regions of the sample. 
     15. The apparatus of any one of clauses 12-14, wherein determining distortion levels for the plurality of test images comprises determining a first distortion level for a first test image among the plurality of test images based on a first reference image corresponding to the first test image. 
     16. The apparatus of clause 15, wherein the first distortion level comprises information that indicates whether the first test image expands or contracts. 
     17. The apparatus of clause 15, wherein the first distortion level comprises a first distortion amount based on a displacement between a feature on the first test image and a corresponding feature on the first reference image. 
     18. The apparatus of any one of clauses 13-17, wherein determining a landing energy level enabling the sample to be in a neutral charge condition comprises estimating the landing energy level enabling a distortion amount to be zero based on the distortion levels. 
     19. A non-transitory computer readable medium that stores a set of instructions that is executable by at least one processor of a computing device to perform a method for enhancing an image, the method comprising: 
     acquiring a plurality of test images of a sample that are obtained at different landing energies; 
     determining distortion levels for the plurality of test images; 
     determining a landing energy level that enables the sample to be in a neutral charge condition during inspection based on the distortion levels; and 
     acquiring an inspection image based on the determined landing energy level. 
     20. The computer readable medium of clause 19, wherein the set of instructions that is executable by at least one processor of the computing device to further perform:
         correcting the inspection image based on a reference image corresponding to the inspection image.       

     21. The computer readable medium of clause 19 or 20, wherein each of the acquired plurality of test images of the sample corresponds to a test region of a plurality of test regions of the sample. 
     22. The computer readable medium of any one of clauses 19-21, wherein determining distortion levels for the plurality of test images comprises determining a first distortion level for a first test image among the plurality of test images based on a first reference image corresponding to the first test image. 
     23. The computer readable medium of clause 22, wherein the first distortion level comprises information that indicates whether the first test image expands or contracts. 
     24. The computer readable medium of clause 22, wherein the first distortion level comprises a first distortion amount based on a displacement between a feature on the first test image and a corresponding feature on the first reference image. 
     25. The computer readable medium of any one of clauses 19-24, wherein determining a landing energy level enabling the sample to be in a neutral charge condition comprises estimating the landing energy level enabling a distortion amount to be zero based on the distortion levels. 
     26. A method for identifying an optimum landing energy in a charged-particle beam inspection system, the method comprising: 
     acquiring a plurality of test images of a sample that are obtained at different landing energies; 
     determining distortion levels for the plurality of test images, wherein determining distortion levels comprises comparing a first test image with a first reference image corresponding to the first test image based on positions of features in the first test image and the first reference image; and 
     determining a landing energy level that enables the sample to be in a neutral charge condition during inspection based on the distortion levels. 
     27. The method of clause 26, further comprising: 
     correcting an inspection image that is obtained based on the determined landing energy level based on a reference image corresponding to the inspection image. 
     28. The method of clause 26, further comprising correcting the inspection image by applying a predetermined offset to the inspection image. 
     29. The method of any one of clauses 26-28, wherein each of the acquired plurality of test images of the sample corresponds to a test region of a plurality of test regions of the sample. 
     30. A method for enhancing an inspection image in a charged-particle beam inspection system, the method comprising: 
     acquiring a first test image and a second test image of a sample, wherein the first test image and the second test image are obtained at different landing energies; 
     determining a first distortion level for the first test image and a second distortion level for the second test image; 
     determining a landing energy level that enables a distortion level to be substantially zero when inspecting the sample, the determination of the landing energy level being based on the first distortion level, the second distortion level, and the different landing energies; and 
     acquiring an inspection image based on the determined landing energy level. 
     31. The method of clause 30, wherein the determination of the landing energy level includes performing an interpolation based on the first distortion level, the second distortion level, and the different landing energies. 
     32. The method of clause 30 or 31, further comprising:
         correcting the inspection image based on a reference image corresponding to the inspection image.       

     33. The method of clause 30 or 31, further comprising correcting the inspection image by applying a predetermined offset to the inspection image. 
     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, among other things, image inspection, image acquisition, stage positioning, beam focusing, electric field adjustment, beam bending, condenser lens adjusting, activating charged-particle source, beam deflecting, and method  800 . 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. 
     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. 
     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.