Patent Publication Number: US-2021193437-A1

Title: Multiple charged-particle beam apparatus with low crosstalk

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of U.S. application 62/950,774 which was filed on Dec. 19, 2019, and which is incorporated herein in its entirety by reference. 
    
    
     TECHNICAL FIELD 
     The embodiments provided herein disclose a multi-beam apparatus, and more particularly a multi-beam charged particle microscope with enhanced inspection throughput by reducing crosstalk between detection elements of a charged-particle detector. 
     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. Although multiple electron beams may be used to increase the throughput, the crosstalk between detection elements of a secondary electron detector may limit the throughput desired, rendering the inspection tools inadequate for their desired purpose. 
     SUMMARY 
     One aspect of the present disclosure is directed to a method performed by a multi-beam apparatus to form images of a sample. The method may comprise generating a plurality of secondary electron beams from a plurality of probe spots on the sample along a primary-optical axis upon interaction with a plurality of primary electron beams. The method may further comprise focusing the plurality of secondary electron beams on a focus plane, and positioning a detection surface of a secondary electron detector with respect to the focus plane. The plurality of secondary electron beams may comprise an array of secondary electron beams. 
     The method may further comprise adjusting an orientation of the plurality of primary electron beams interacting with the sample, wherein adjusting the orientation of the plurality of primary electron beams comprises rotating the plurality of primary electron beams around the primary optical axis, and may adjust an orientation of the array of secondary electron beams. The method may further comprise directing, using a beam separator, the plurality of secondary electron beams towards the secondary electron detector along a secondary optical axis. The method may further comprise adjusting an electrical excitation of a stigmator to compensate astigmatism aberration of the plurality of secondary electron beams. 
     The secondary electron detector may be disposed downstream of a secondary electron projection system configured to focus the plurality of secondary electron beams on the focus plane, wherein the secondary electron detector comprises a plurality of detection elements, and wherein a detection element of the plurality of detection elements is associated with a corresponding secondary electron beam of the plurality of secondary electron beams. Positioning the detection surface of the secondary electron detector may comprise adjusting a tilting angle between the detection surface and the focus plane, wherein adjusting the tilting angle may comprise reducing the tilting angle between the detection surface of the secondary electron detector and the focus plane. Reducing the tilting angle may comprise adjusting the position of the secondary electron detector such that the detection surface of the secondary electron detector substantially coincides with the focus plane. 
     Adjusting the position of the secondary electron detector may comprise dynamically adjusting the tilting angle based on a collection efficiency of the secondary electron detector or adjusting the tilting angle to a predetermined value of the tilting angle. Adjusting the position of the secondary electron detector may comprise adjusting the tilting angle in one or more planes with reference to the secondary optical axis. 
     Another aspect of the present disclosure is directed to a method performed by a multi charged-particle beam apparatus to form images of a sample. The method may comprise generating a plurality of secondary electron beams from a plurality of probe spots on the sample along a primary optical axis upon interaction with a plurality of primary electron beams, adjusting an orientation of the plurality of primary electron beams interacting with the sample, forming images of the plurality of probe spots of the sample on a final image plane; and adjusting a position of a secondary electron detector with reference to a position of the final image plane. 
     Another aspect of the present disclosure is directed to a multi-beam apparatus for inspecting a sample using a plurality of primary electron beams configured to form a plurality of probe spots on the sample. The multi-beam apparatus may include a secondary electron projection system. The secondary electron projection system may be configured to receive a plurality of secondary electron beams resulting from the formation of the probe spots, form images of the plurality of probe spots on the sample on a final image plane, and a charged-particle detector configured to detect the plurality of secondary electron beams, wherein a position of the charged-particle detector is set based on a position of the final image plane. 
     The plurality of secondary electron beams may comprise an array of secondary electron beams. The multi-beam apparatus may include an objective lens configured to focus the plurality of primary electron beams on the sample and form images of the plurality of probe spots on an intermediate image plane along a primary optical axis. The multi-beam apparatus may further include a beam separator configured to direct the plurality of secondary electron beams towards the charged-particle detector along the secondary optical axis. The multi-beam apparatus may further comprise a stigmator configured to compensate astigmatism aberration of the plurality of secondary electron beams. The charged-particle detector may comprise a secondary electron detector disposed downstream of the secondary electron projection system and may comprise a plurality of detection elements, and wherein a detection element of the plurality of detection elements is associated with a corresponding secondary electron beam of the plurality of secondary electron beams. 
     A setting of a position of the secondary electron detector may comprise an adjusted tilting angle between a detection plane of the secondary electron detector and the final image plane. The setting of the position of the secondary electron detector may comprise a reduced tilting angle between the detection plane and the final image plane and wherein the reduced tilting angle comprises the setting of the position of the secondary electron detector such that the detection plane substantially coincides with the final image plane. The setting of the position of the secondary electron detector may comprise a dynamically adjusted tilted angle based on a collection efficiency of the secondary electron detector or the setting of the position of the secondary electron detector may comprise a predetermined value of the tilting angle. The final image plane may comprise a curved plane. 
     Another aspect of the present disclosure is directed to a multi-beam apparatus comprising a secondary electron projection system comprising a stigmator configured to influence paths of a plurality of secondary electron beams generated from a plurality of probe spots on a sample; and a secondary electron detector configured to detect the plurality of secondary electron beams, wherein a position of the secondary electron detector is adjusted in relation with reference to a secondary optical axis based on a position of a final image plane of the plurality of probe spots. The stigmator may comprise an electrical or a magnetic multi-pole lens. 
     Another aspect of the present disclosure is directed to a non-transitory computer readable medium storing a set of instructions that is executable by one or more processors of a multi-beam apparatus to cause the multi-beam apparatus to perform a method of forming images of a sample. The method may comprise generating a plurality of secondary electron beams from a plurality of probe spots of a plurality of primary electron beams on the sample along a primary optical axis, acquiring images of the plurality of probe spots of the sample on a final image plane using a secondary electron detector, and adjusting a position of the secondary electron detector based on a position of the final image plane. The apparatus may further perform forming an intermediate image of the plurality of probe spots on an intermediate image plane substantially perpendicular to a primary optical axis using an objective lens, and directing the plurality of secondary electron beams towards the secondary electron detector along the secondary optical axis using a beam separator. The multi-beam apparatus may further perform adjusting an orientation of the plurality of primary electron beams interacting with the sample, wherein adjusting the orientation of the plurality of primary electron beams may comprise rotating the plurality of primary electron beams around the primary optical axis, and adjusting a tilting angle between a detection plane of the secondary electron detector and the final image plane. The multi-beam apparatus may further perform adjusting an electrical excitation of a stigmator to compensate astigmatism aberration of the plurality of secondary electron beams. 
     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 exemplary electron beam inspection (EBI) system, consistent with embodiments of the present disclosure. 
         FIG. 2  is a schematic diagram illustrating an exemplary electron beam tool that can be a part of the exemplary electron beam inspection system of  FIG. 1 , consistent with embodiments of the present disclosure. 
         FIGS. 3A and 3B  are schematic diagrams illustrating exemplary forces experienced and their influence on the path of the secondary electron beams in a multi-beam apparatus, consistent with embodiments of the present disclosure. 
         FIGS. 3C and 3D  are schematic diagrams illustrating exemplary projections of the secondary electron beams before entering and after exiting the beam deflector, respectively, of the multi-beam apparatus, consistent with embodiments of the present disclosure. 
         FIG. 3E  is a schematic diagram illustrating an exemplary secondary projection imaging system comprising stigmators in a multi-beam apparatus, consistent with embodiments of the present disclosure. 
         FIG. 4A  is a schematic diagram illustrating an exemplary configuration of an electron optics system and the path of secondary electron beams in a multi-beam apparatus, consistent with embodiments of the present disclosure. 
         FIGS. 4B and 4C  are schematic diagrams illustrating exemplary projections of the secondary electron beams on a final image plane and an electron detector, respectively, of the multi-beam apparatus, consistent with embodiments of the present disclosure. 
         FIG. 5  is a schematic diagram illustrating an exemplary configuration of an electron optics system and the path of secondary electron beams in a multi-beam apparatus, consistent with embodiments of the present disclosure. 
         FIG. 6  is a schematic diagram illustrating an exemplary configuration of an electron detector in a multi-beam apparatus, consistent with embodiments of the present disclosure. 
         FIGS. 7A and 7B  are schematic diagrams illustrating exemplary projections of the secondary electron beams before entering and after exiting the beam deflector, respectively, consistent with embodiments of the present disclosure. 
         FIG. 8  is a process flowchart representing an exemplary method of forming an image of a sample using multiple beams in a multi-beam inspection system, consistent with embodiments of the present disclosure. 
         FIG. 9  is a process flowchart representing an exemplary method of forming images of a sample using multiple beams in a multi-beam inspection system, consistent with embodiments of the pre sent 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, thereby rendering it useless. Thus, one goal of the manufacturing process is to avoid such defects to maximize the number of functional ICs made in the process, that is, to improve the overall yield of the process. 
     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). An 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. 
     Although a multiple charged-beam particle imaging system, such as a multi-beam SEM, may seemingly offer the advantage of high wafer inspection throughput, it may encounter several challenges related to focusing secondary electron beams generating from the sample. Because defocused beams have a larger cross-section and a larger footprint on the detection elements compared to focused beams, each of the multiple detection elements may receive secondary electrons from a corresponding secondary electron beam and other adjacent beams. Consequently, the imaging signal of one detection element may comprise a main component originating from the corresponding secondary electron beam and a crosstalk component originating from adjacent electron beams. The crosstalk component, among other things, may deteriorate the fidelity of the imaging signal, and therefore, negatively impact the inspection throughput as well as the resolution. 
     To mitigate the occurrence of crosstalk, an aperture mechanism may be employed in a secondary imaging system to block off peripheral secondary electrons, or the size of the detection elements may be reduced, among other things. However, blocking off peripheral electrons or reducing the size of the detection elements may reduce the total number of electrons incident on the electron detector, and therefore may negatively impact the collection efficiency, inspection throughput, or inspection resolution. 
     In currently existing multi-beam SEMs, although a beam separator such as a Wien filter may isolate primary and secondary electrons, it may defocus secondary electron beams (astigmatism aberration) and may also deform the secondary electron beam array, both resulting in occurrence of crosstalk, among other things. One of the several ways to mitigate the occurrence of crosstalk may include using one or more stigmators to compensate the beam astigmatism and beam array deformation. However, such a configuration may render the secondary projection system very complicated to operate and maintain, thereby adversely affecting the inspection throughput. In addition, the focus plane of secondary electron beams may not align with the detection plane of the electron detectors, causing the secondary electron beams to further defocus on the detection plane and induce crosstalk. Therefore, it may be desirable to compensate the beam astigmatism and adjust the position of electron detectors by a mechanism that allows alignment of the electron detectors with the focus plane of secondary electron beams. 
     Some embodiments of the present disclosure are directed to systems and methods of forming an image of a sample. The method may include generating secondary electron beams from probe spots formed by interaction of primary electron beams with regions of the sample. The generated secondary electron beams may pass through a beam separator configured to deflect the secondary electron beams towards an electron detection device. The deflection of the beam separator may cause astigmatism aberration of the secondary electron beams. The method may include compensating the astigmatism aberration by adjusting an electrical excitation of a stigmator configured to apply a correcting magnetic field or a correcting electrical field to the secondary electron beams. The method may further include forming an image of the probe spots on an image plane downstream of the stigmator and adjusting a position of the electron detector based on a position of the image plane. The ability to adjust the position of the electron detector may allow a user to focus the secondary electron beams on the detection surface of the detector, thereby minimizing the crosstalk, increasing detection efficiency, and therefore maintaining high inspection throughput. 
     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 exemplary electron beam inspection (EBI) system  100  consistent with embodiments of the present disclosure. As shown in  FIG. 1 , charged particle beam inspection system  100  includes a main chamber  10 , a load-lock chamber  20 , an electron beam tool  40 , and an equipment front end module (EFEM)  30 . Electron beam tool  40  is located within 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 of an exemplary electron beam tool  40  that can be a part of the exemplary charged particle beam inspection system  100  of  FIG. 1 , consistent with embodiments of the present disclosure. Electron beam tool  40  (also referred to herein as apparatus  40 ) may comprise an electron source  201 , a source conversion unit  220 , a primary projection optical system  230 , a secondary projection imaging system  250 , and an electron detection device  240 . It may be appreciated that other commonly known components of apparatus  40  may be added/omitted as appropriate. 
     Although not shown in  FIG. 2 , in some embodiments, electron beam tool  40  may comprise a gun aperture plate, a pre-beamlet forming mechanism, a condenser lens, a motorized sample stage, a sample holder to hold a sample (e.g., a wafer or a photomask). 
     Electron source  201 , source conversion unit  220 , deflection scanning unit  232 , beam separator  233 , and primary projection optical system  230  may be aligned with a primary optical axis  204  of apparatus  40 . Secondary projection imaging system  250  and electron detection device  240  may be aligned with a secondary optical axis  251  of apparatus  40 . 
     Electron source  201  may include 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  202  that forms a primary beam crossover (virtual or real)  203 . Primary electron beam  202  can be visualized as being emitted from primary beam crossover  203 . 
     In some embodiments, source conversion unit  220  may be configured to form a plurality of images of crossover  203  by influencing a plurality of beamlets of primary electron beam  202  (such as primary beamlets  211 ,  212 , and  213 ). Source conversion unit  220  may comprise a beam-limit aperture array and a deflector array. The beam-limit aperture array may form primary beamlets  211 ,  212 , and  213 , and the deflector array may be configured to deflect the primary beamlets to form a plurality of images of crossover  203 . In some embodiments, source conversion unit  220  may comprise an aperture lens array, a beam-limit aperture array, and an imaging lens. The aperture lens array may comprise an aperture-lens forming electrode plate and an aperture lens plate positioned below the aperture-lens forming electrode plate. In this context, “below” refers to the structural arrangement such that primary electron beam  202  traveling downstream from electron source  201  irradiates the aperture-lens forming electrode plate before the aperture lens plate. The aperture-lens forming electrode plate may be implemented via a plate having an aperture configured to allow at least a portion of primary electron beam  202  to pass through. The aperture lens plate may be implemented via a plate having a plurality of apertures traversed by primary electron beam  202  or multiple plates having plurality of apertures. The aperture-lens forming electrode plate and the aperture lens plate may be excited to generate electric fields above and below the aperture lens plate. The electric field above the aperture lens plate may be different from the electric field below the aperture lens plate so that a lens field is formed in each aperture of the aperture lens plate, and the aperture lens array may thus be formed. One aperture lens of the aperture lens array may focus one primary beamlet. 
     In some embodiments, the beam-limit aperture array may comprise beam-limit apertures. It is appreciated that any number of apertures may be used, as appropriate. The beam-limit aperture array may be configured to limit diameters of individual primary beamlets  211 ,  212 , and  213 . Although  FIG. 2  shows three primary beamlets  211 ,  212 , and  213  as an example, however, it is appreciated that source conversion unit  220  may be configured to form any number of primary beamlets. 
     In some embodiments, an imaging lens may comprise a collective imaging lens configured to focus primary beamlets  211 ,  212 , and  213  on an intermediate image plane. The imaging lens may have a principal plane orthogonal to primary optical axis  204 . The imaging lens may be positioned below beam-limit aperture array and may be configured to focus primary beamlets  211 ,  212 , and  213  such that the beamlets form a plurality of images of crossover  203  on the intermediate image plane. 
     Primary projection system  230  may be configured to project the images (virtual or real) onto sample  208  and form plural probe spots thereon. Primary projection optical system  230  may comprise an objective lens  231 , a deflection scanning unit  232 , and a beam separator  233 . Beam separator  233  and deflection scanning unit  232  may be positioned inside primary projection optical system  230 . Objective lens  231  may be configured to focus beamlets  211 ,  212 , and  213  onto sample  208  for inspection and can form three probe spots  211 S,  212 S, and  213 S, respectively, on surface of sample  208 . In some embodiments, beamlets  211 ,  212 , and  213  may land normally or substantially normally on sample  208 . In some embodiments, focusing by the objective lens may include reducing the aberrations of the probe spots  211 S,  212 S, and  213 S. 
     In response to incidence of primary beamlets  211 ,  212 , and  213  on probe spots  211 S,  212 S, and  213 S on sample  208 , secondary electrons may emerge from sample  208  and generate three secondary electron beams  261 ,  262 , and  263 . Each of secondary electron beams  261 ,  262 , and  263  typically comprise secondary electrons (having electron energy ≤50 eV) and backscattered electrons (having electron energy between 50 eV and the landing energy of primary beamlets  211 ,  212 , and  213 ). 
     Electron beam tool  40  may comprise beam separator  233 . Beam separator  233  may be of Wien filter type comprising an electrostatic deflector generating an electrostatic dipole field E 1  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 E 1  on an electron of beamlets  211 ,  212 , and  213  is equal in magnitude and opposite in direction to the force exerted on the electron by magnetic dipole field B 1 . Beamlets  211 ,  212 , and  213  can therefore pass straight through beam separator  233  with zero deflection angles. 
     Deflection scanning unit  232  may be configured to deflect beamlets  211 ,  212 , and  213  to scan probe spots  211 S,  212 S, and  213 S over three small scanned areas in a section of the surface of sample  208 . Beam separator  233  may direct secondary electron beams  261 ,  262 , and  263  towards secondary projection system imaging  250 . Secondary projection imaging system  250  can focus secondary electron beams  261 ,  262 , and  263  onto detection elements  241 ,  242 , and  243  of electron detection device  240 . Detection elements  241 ,  242 , and  243  may be configured to detect corresponding secondary electron beams  261 ,  262 , and  263  and generate corresponding signals used to construct images of the corresponding scanned areas of sample  208 . 
     In  FIG. 2 , three secondary electron beams  261 ,  262 , and  263  respectively generated by three probe spots  211 S,  212 S, and  213 S, travel upward towards electron source  201  along primary optical axis  204 , pass through objective lens  231  and deflection scanning unit  232  in succession. The three secondary electron beams  261 ,  262 , and  263  are diverted by beam separator  233  (such as a Wien Filter) to enter secondary projection imaging system  250  along secondary optical axis  251  thereof. Secondary projection imaging system  250  may focus the three secondary electron beams  261 ,  262 , and  263  onto electron detection device  240  which comprises three detection elements  241 ,  242 , and  243 . Therefore, electron detection device  240  can simultaneously generate the images of the three scanned regions scanned by the three probe spots  211 S,  212 S, and  213 S, respectively. In some embodiments, electron detection device  240  and secondary projection imaging system  250  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  231 , deflection scanning unit  232 , beam separator  233 , secondary projection imaging system  250  and electron detection device  240 , may form one detection system. 
     In some embodiments, secondary projection imaging system  250  will be shown and described together with the entire detection system, as illustrated in  FIG. 2 . With reference to  FIG. 2 , only three secondary electron beams  261 ,  262 , and  263 , with respect to three probe spots  211 S,  212 S, and  213 S are shown, although there may be any number of secondary electron beams. Although not illustrated, secondary projection imaging system  250  may include components such as a zoom lens, a projection lens, a secondary beam-limit aperture array, and anti-scanning deflection unit, among other things, all aligned with secondary optical axis  251 . Detection elements  241 ,  242 , and  243  of electron detection device  240  may be placed along a plane normal to secondary optical axis  251 . In some embodiments, the position and orientation of electron detection device  240  may be adjustable. Zoom lens, projection lens, and objective lens  231  may together project a surface of sample  208  onto a focus plane of the secondary electron beams, i.e. focus the secondary electron beams  261 ,  262 , and  263  to form secondary-electron spots on detection elements  241 ,  242 , and  243 , respectively, when deflection scanning unit  232  is off. 
     As is commonly known in the art, the emission of secondary electrons obeys Lambert&#39;s law and has a large energy spread. While the energy of a secondary electron may be up to 50 eV, most have an energy of approximately 5 eV, depending on the sample material, among other things. The landing energy of the primary electron beamlets, such as the energy of beamlet  211  as it lands on sample  208 , may be in the range of 0.1 keV to 5 keV. The landing energy may be adjusted by varying either or both of the potential of primary electron source  201  or the potential of sample  208 , among other things. The excitation of objective lens  231  may be adjusted to provide the corresponding focusing power for the three beamlets. Further, for reduced aberrations, objective lens  231  may be a magnetic or an electromagnetic compound lens configured to rotate the beamlets and affect the landing energy. Because the size, the position, or the magnification of the secondary electron spots formed by the secondary electron beams  261 , 262 , and  263  on detection elements  241 ,  242 , and  243  may vary, the secondary electron spots may partially enter a detection element adjacent to the corresponding detection element. The secondary electrons detected by the adjacent detection elements may generate image overlaps, for example, causing deterioration of image resolution and reduction in collection efficiency. The image signal from one detection element may include information from more than one scanned region of sample  208 , resulting in loss of resolution due to crosstalk. 
     In some embodiments, controller  50  may comprise an image processing system that includes an image acquirer (not shown) and a storage (not shown). The image acquirer may comprise one or more processors. For example, the image acquirer may comprise a computer, server, mainframe host, terminals, personal computer, any kind of mobile computing devices, and the like, or a combination thereof. The image acquirer may be communicatively coupled to electron detection device  240  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  240  and may construct an image. The image acquirer may thus acquire images of sample  208 . 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 an imaging signal received from electron detection device  240 . 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 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  208 . The acquired images may comprise multiple images of a single imaging area of sample  208  sampled multiple times over a time sequence. 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  208 . 
     In some embodiments, controller  50  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  211 ,  212 , and  213  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  208 , and thereby can be used to reveal any defects that may exist in the wafer. 
     In some embodiments, controller  50  may control a motorized stage (not shown) to move sample  208  during inspection. In some embodiments, controller  50  may enable the motorized stage to move sample  208  in a direction continuously at a constant speed. In other embodiments, controller  50  may enable the motorized stage to change the speed of the movement of sample  208  over time depending on the steps of scanning process. In some embodiments, controller  50  may adjust a configuration of primary projection optical system  230  or secondary projection imaging system  250  based on images of secondary electron beams  261 ,  262 , and  263 . 
     In some embodiments, electron beam tool  40  may comprise a beamlet control unit  225  configured to receive primary beamlets  211 ,  212 , and  213  from source conversion unit  220  and direct them towards sample  208 . Beamlet control unit  225  may include a transfer lens configured to direct primary beamlets  211 ,  212 , and  213  from the image plane to the objective lens such that primary beamlets  211 ,  212 , and  213  normally or substantially normally land on surface of sample  208 , or form the plurality of probe spots  221 ,  222 , and  223  with small aberrations. Transfer lens may be a stationary or a movable lens. In a movable lens, the focusing power of the lens may be changed by adjusting the electrical excitation of the lens. 
     In some embodiments, beamlet control unit  225  may comprise a beamlet tilting deflector configured to may be configured to tilt primary beamlets  211 ,  212 , and  213  to obliquely land on the surface of sample  208  with same or substantially same landing angles ( 0 ) with respect to the surface normal of sample  208 . Tilting the beamlets may include shifting a crossover of primary beamlets  211 ,  212 , and  213  slightly off primary optical axis  204 . This may be useful in inspecting samples or regions of sample that include three-dimensional features or structures such as side walls of a well, or a trench, or a mesa structure. 
     In some embodiments, beamlet control unit  225  may comprise a beamlet adjustment unit configured to compensate for aberrations such as astigmatism and field curvature aberrations caused due to one or all of the lenses mentioned above. Beamlet adjustment unit may comprise an astigmatism compensator array, a field curvature compensator array, and a deflector array. The field curvature compensator array may comprise a plurality of micro-lenses to compensate field curvature aberrations of the primary beamlets  211 ,  212 , and  213 , and the astigmatism compensator array may comprise a plurality of micro-stigmators to compensate astigmatism aberrations of the primary beamlets  211 ,  212 , and  213 . 
     In some embodiments, the deflectors of the deflector array may be configured to deflect beamlets  211 ,  212 , and  213  by varying angles towards primary optical axis  204 . In some embodiments, deflectors farther away from primary optical axis  204  may be configured to deflect beamlets to a greater extent. Furthermore, deflector 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.,  221 ,  222 , and  223 ) formed on a surface of sample  208 . 
     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  208 . In some embodiments, the deflectors may be placed on the intermediate image plane. 
     In some embodiments, controller  50  may be configured to control source conversion unit  220 , beamlet control unit  225 , and primary projection optical system  230 , as illustrated in  FIG. 2 . 
     Although not illustrated, controller  50  may be configured to control one or more components of electron beam tool  40  including, but not limited to, electron source  201  and components of source conversion unit  220 , primary projection optical system  230 , electron detection device  240 , and secondary projection imaging system  250 . Although  FIG. 2  shows that electron beam tool  40  uses three primary electron beamlets  211 ,  212 , and  213 , it is appreciated that electron beam tool  40  may use two or more primary electron beamlets. The present disclosure does not limit the number of primary electron beamlets used in apparatus  40 . 
     Backscattered electrons and secondary electrons can be emitted from the part of sample  208  upon receiving primary electron beamlets  211 ,  212 , and  213 , for example. Beam separator  233  can direct the secondary or backscattered electron beam(s), to a sensor surface of electron detection device  240 . The detected electron beams can form corresponding beam spots on the sensor surface of electron detection device  240 . Electron detection device  240  can generate signals (e.g., voltages, currents) that represent the intensities of the received beams, and provide the signals to a processing system, such as controller  50 . The intensity of secondary or backscattered electron beams, and the resultant beam spots, can vary according to the external or internal structure of sample  208 . Moreover, as discussed above, primary electron beamlets  211 ,  212 , and  213  can be deflected onto different locations of the top surface of sample  208  to generate secondary or scattered electron beams (and the resultant beam spots) of different intensities. Therefore, by mapping the intensities of the beam spots with the locations of sample  208 , the processing system can reconstruct an image that reflects the internal or external structures of wafer sample  208 . 
     Reference is now made to  FIGS. 3A and 3B , which are schematic diagrams of exemplary forces experienced by the secondary electrons and the influence of such forces on the path of the secondary electron beams in a multi-beam apparatus, consistent with embodiments of the present disclosure.  FIG. 3A  shows secondary electrons  371 ,  372 , and  373  of an exemplary secondary electron beam  361  passing through a Wien filter (e.g., beam separator  233  of  FIG. 2 ). It is appreciated that although a secondary electron beam comprises a beam of electrons, only three discrete secondary electrons  371 ,  372 , and  373  are shown for illustrative purposes. Secondary electrons  371 ,  372 , and  373  may be visualized as traveling along the Z1-axis (not shown), extending in-and-out of the plane of the paper. The electric field along X1-axis, and the magnetic field along Y1-axis, are represented as E and B, respectively. 
     In the Wien Filter, electrical potential reduces along the direction of electric field E. As a result, energy of off-axis secondary electron  372  of secondary electron beam  361  is higher than on-axis secondary electron  371 , and energy of on-axis secondary electron  371  is higher than off-axis secondary electron  373 , if secondary electrons  371 ,  372  and  373  have same energies before entering the Wien Filter. Hence, magnetic force Fm 2  exerted on secondary electron  372  is stronger than magnetic force Fm 1  exerted on electron  371 , and magnetic force Fm 1  is stronger than magnetic force Fm 3  exerted on electron  373 . The difference in magnetic forces (Fm 3 &lt;Fm 1 &lt;Fm 2 ) experienced by secondary electrons (e.g., secondary electrons  371 ,  372 , and  373 ), while passing through the Wien filter, may deflect secondary electrons by dissimilar deflection angles. The difference in deflection angles of electrons of a secondary electron beam may cause beam astigmatism, among other things. 
     Reference is now made to  FIG. 3B , which shows an exemplary configuration  300  comprising a secondary electron beam  361  passing through a beam separator  333  disposed along X1-axis. It is appreciated that secondary electron beam  361  and beam separator  333  may be similar or substantially similar to secondary electron beam  261  and beam separator  233 , respectively, of  FIG. 2 . 
     In some embodiments, secondary electron beam  361  may have a substantially circular cross-section before entering beam separator  333 .  FIG. 3B  shows a projection of a substantially circular cross-section of secondary electron beam  361  on a plane  310 . In some embodiments, plane  310  may be located along Z1-axis, between objective lens  231  and beam separator  333 . The projection of secondary electron beam  361  on plane  310  may be substantially circular in cross-section before interaction with an electromagnetic field  335  in beam separator  333 . Upon interaction with electromagnetic field  335  of beam separator  333 , secondary electron beam  361  may be deflected and the cross-section of secondary electron beam  361  on plane  320  may be modified to a non-circular profile as a result of beam astigmatism. 
     Reference is now made to  FIGS. 3C and 3D , which illustrate schematic diagrams of arrays of secondary electron beams on planes  310  and  320 , respectively. As shown in  FIG. 3C , array  350  may comprise a 3×3 rectangular array of secondary electron beams originating from a sample (e.g., sample  208  of  FIG. 2 ) and directed towards beam separator  333 . For example, array  350  represents a square array comprising nine secondary electron beams  361 ,  362 ,  363 ,  364 ,  365 ,  366 ,  367 ,  368 , and  369 . In some embodiments, the square array may comprise fewer electron beams, such as in a 2×2 array, or more secondary electron beams such as in a 5×5 array, as appropriate. In some embodiments, the array may comprise a rectangular, a circular, a spiral, an elliptical array, a symmetric array, or an asymmetric array of secondary electron beams directed towards beam separator  333 . 
       FIG. 3C  illustrates an exemplary array  350  of secondary electron beams  361 - 369  and their projections on plane  310  before passing through the electric field E (along X1-axis direction) and the magnetic field B (along Y1-axis direction) of beam separator  333  (e.g., beam separator in  FIG. 3A ). Plane  310  may be a plane substantially parallel to the plane comprising X1- and Y1-axes, and substantially perpendicular to Z1-axis (not shown). The Z1-axis may be visualized as extending in-and-out of the paper. 
     In some embodiments, array  350  may comprise a square array of projections of secondary electron beams  361 - 369 . As shown in  FIG. 3C , an outline  360  of the square array of secondary electron beams  361 - 369 , each having a circular cross-section is represented by the dashed lines. In some embodiments, array  350  may be oriented at an angle θ with reference to X1- and Y1-axes such that it is aligned along X1′- and Y1′-axes. The orientation of array  350  with reference to X1- and Y1-axes may change with the landing energy of the corresponding primary electron beams (e.g., primary beamlets  211 ,  212 , and  213  of  FIG. 2 ) if the objective lens comprises a magnetic lens, among other things. As used herein, landing energy of the electron beams may be defined as the energy of the electrons of the primary electron beams as they impact the sample. The landing energy of the primary electron beams is equal to the potential difference between the electron emission source and the stage/sample, and therefore may be adjusted by changing either or both of these two potentials. 
     As shown in  FIG. 3C , different secondary electron beams may traverse beam separator  333  at different transverse locations along the direction (X1-axis) of electric field E thereof. Consequently, a beam with a larger X1 coordinate may have a higher energy than a beam with a small X1 coordinate, and therefore may experience a stronger magnetic force. For secondary electron beams  361 - 369 , the difference in magnetic force may cause difference in deflection angles and may result in beam array deformation as shown in  FIG. 3D . 
       FIG. 3D  illustrates an exemplary array  370  of exiting secondary electron beams  361   a - 369   a  and their projections on plane  320 . Exiting secondary electron beams  361   a - 369   a  respectively correspond to secondary electron beams  361 - 369  after passing through beam separator  333 . Because beam separator  333  generates beam astigmatism, as discussed above, exiting secondary electron beams  361   a - 369   a  may have a non-circular cross-section, different from circular cross-sections of incident secondary electron beams  361 - 369 . For example, the cross-section of exiting secondary electron beams  361   a - 369   a  may comprise an oval, an elliptical, or a non-circular shape. In some embodiments, exiting secondary electron beams  361   a - 369   a  may be deflected towards secondary projection imaging system  250  along secondary optical axis  251 . 
     In some embodiments, because beam separator  333  generates beam array deformation, as explained above, the beam array of exiting secondary electron beams  361   a - 369   a  may change into a diamond shape from the square shape of the beam array of incident secondary electron beams  361 - 369 , as shown in  FIG. 3D . Outline  360   a  of deformed array  370  of exiting secondary electron beams  361   a - 369   a  having a diamond shape is represented by the dash-dot lines. For comparison,  FIG. 3D  includes outline  360  of array  350  representing incident secondary electron beams  361 - 369 . 
     In a multi-beam inspection apparatus, such as a multi-beam SEM, using a Wien filter (e.g., beam separator  333  of  FIG. 3B ) to separate primary electron beams (e.g., primary beamlets  211 ,  212 , and  213  of  FIG. 2 ) from secondary electron beams (e.g.,  261 ,  262 , and  263  of  FIG. 2 ) may cause astigmatism aberration, and beam array deformation, among other things. The imaging resolution in a multi-beam SEM may depend on, among other things, crosstalk of secondary electron beams detected by an electron detection device (e.g., electron detection device  240  of  FIG. 2 ) such as a secondary electron detector. The crosstalk may depend on the focus of the secondary electron beams incident on the electron detection device. If each secondary electron beam is focused such that it may be detected by the corresponding detection element of the electron detection device, the crosstalk will be zero. Beam astigmatism of beam separator may be one of several factors impacting the focus of a secondary electron beam, which may cause beam profile distortion, among other things. A defocused electron beam may have a large incidence spot on a secondary electron detector. Beam array deformation of beam separator may be one of several factors impacting beam array shape of secondary electron beams, which may cause beam pitch distortion on the electron detection device, among other things. A deformed beam array may not match detection element array of the electron detection device. In conventional multi-beam SEMs, the defocused secondary electron beam or a secondary beam in a deformed beam array may be incident upon multiple detection elements of the electron detection device. In other words, each of the multiple detection elements may receive secondary electrons from a corresponding secondary electron beam and other adjacent beams. Consequently, the imaging signal of one detection element may comprise a main component originating from the corresponding secondary electron beam and a crosstalk component originating from adjacent electron beams. The crosstalk component, among other things, may deteriorate the fidelity of the imaging signal. Therefore, it is desirable to minimize crosstalk between multiple detection elements to enhance image resolution and enhance detection efficiency or collection efficiency of electron detection devices to enhance the imaging throughput. 
     Some of the several ways to mitigate the occurrence and impact of crosstalk, among other things, may include using an aperture mechanism in secondary projection imaging system  250  to block off peripheral secondary electrons, or reducing the size of the corresponding detection elements of electron detection device. However, reducing the number of secondary electrons incident on the electron detector by blocking the peripheral electrons or by reducing the size of the corresponding detection elements may negatively impact the collection and the detection efficiency, thereby reducing the inspection throughput, among other things. 
     In conventional SEM systems, a secondary projection imaging system in a multi-beam apparatus may include one or more stigmators to compensate the beam astigmatism aberration and the beam array deformation so as to minimize the crosstalk.  FIG. 3E  illustrates a schematic diagram of a secondary projection imaging system  350  in a multi-beam apparatus (e.g., apparatus  40  of  FIG. 2 ). Secondary projection imaging system  350  may comprise stigmators  351  and  352  configured to compensate the beam array deformation and astigmatism aberration of individual exiting secondary electron beams  361   a ,  362   a , and  363   a  passing through secondary projection imaging system  350 . In some embodiments, stigmator  352  may be placed at or close to the crossover of exiting secondary electron beams  361   a ,  362   a , and  363   a  and configured to compensate the beam astigmatism aberration. Stigmator  351  may be placed away from the crossover and may be configured to compensate the beam array shape deformation. 
     In addition, though stigmators  351  and  352  may be configured to minimize the crosstalk by compensating the astigmatism aberration and beam array deformation, using multiple stigmators may increase the structural and operational complexity of secondary projection imaging system  350 , thereby negatively impacting the inspection throughput, among other things. Although only three exiting secondary electron beams are illustrated in the cross-sectional view in  FIG. 3E , it is appreciated that there may be any number of exiting secondary electron beams, as needed. 
     In multi-beam inspection systems, astigmatism aberration or beam array deformation of secondary electron beams may cause crosstalk, and therefore, negatively impact inspection throughput and resolution, among other things. Astigmatism aberration or beam array deformation may be caused by several factors including, but are not limited to, using a Wien filter as beam separator (e.g., beam separator  333  of  FIG. 3B ), as discussed in  FIGS. 3A-3E , using a beam deflector (e.g., deflection scanning unit  232  of  FIG. 2 ), or using a secondary projection imaging system (e.g., secondary projection imaging system  250  of  FIG. 2 ). 
     Reference is now made to  FIG. 4A , which illustrates a schematic diagram of a multi-beam inspection apparatus  400  that can be a part of the exemplary charged particle beam inspection system  100  of  FIG. 1 , consistent with embodiments of the present disclosure. Multi-beam inspection apparatus  400  (also referred to herein as apparatus  400 ) may comprise a primary projection optical system (analogous to primary projection optical system  230  of  FIG. 2 ) including objective lens  431  and beam separator  433 . Apparatus  400  may further comprise a secondary projection imaging system  450  (analogous to secondary projection imaging system  250  of  FIG. 2 ) and an electron detection device  440 , among other relevant components. It may be appreciated that other commonly known components of apparatus  40  may be added/omitted as appropriate. 
     Objective lens  431  may be substantially similar to and may perform substantially similar functions as objective lens  231  of  FIG. 2  including, but is not limited to, focusing primary electron beams or beamlets onto sample  408  for inspection and forming corresponding probe spots. Beam separator  433  may be substantially similar to and may perform substantially similar functions as beam separator  233  of  FIG. 2  including, but is not limited to, directing secondary electron beams  461 ,  462  towards secondary projection system imaging  450 . Although,  FIG. 4A  illustrates only two secondary electron beams  461  and  462 , it is appreciated that the number of secondary electron beams generated from sample  408  and directed towards secondary projection system imaging  450  may be more or fewer, as appropriate. Objective lens  431  and beam separator  433  may be aligned with a primary optical axis  404  of apparatus  400 , and secondary projection imaging system  450  and electron detection device  440  may be aligned with a secondary optical axis  451  of apparatus  400 . Secondary optical axis  451  may form a non-zero angle with primary optical axis  404 . In some embodiments, the angle between primary optical axis  404  and secondary optical axis  451  may be determined based on the desired deflection of secondary electron beams  461  and  462  by beam separator  433 , among other things. 
     Upon interaction of the primary electron beams or beamlets with sample  408 , secondary electrons or backscattered electrons may be generated from sample  408 . The generated secondary electrons and backscattered electrons may travel in the opposite direction of primary electron beams along primary optical path  404 . Apparatus  400  may be operated in a secondary electron inspection mode or a backscattered electron inspection mode, or both. In a secondary electron inspection mode, electron detection device  440  may be configured to detect secondary electron beams  461  and  462 . Objective lens  431  may be configured to focus secondary electron beams  461  and  462  generated from corresponding probe spots on sample  408  onto plane SP 1  and therefore form intermediate images  461   i  and  462   i  of the probe spots thereon. Hence, plane SP 1  is an intermediate image plane of the probe spots. In some embodiments, intermediate image plane SP 1  may be formed between beam separator  433  and objective lens  431 , and may be substantially perpendicular to primary optical axis  404 . Intermediate image plane SP 1  may be the focus plane of secondary electron beams  461  and  462  before entering the electrical field region or magnetic field region of beam separator  433 . 
     Beam separator  433  may be configured to deflect secondary electron beams  461  and  462  towards secondary projection imaging system  450 . In  FIG. 4A , after exiting beam separator  433 , secondary electron beams  461  and  462  become exiting secondary electron beams  461   a  and  462   a  (analogous to  361   a  and  362   a  of  FIG. 3D ), and intermediate images  461   i  and  462   i  become virtual intermediate images  461 R and  462 R on a virtual intermediate image plane SP 1 -R. Virtual intermediate images  461 R and  462 R may be objects for secondary projection imaging system  450 , and virtual intermediate image plane SP 1 -R may be object plane for secondary projection imaging system  450 . In some embodiments, virtual intermediate image plane SP 1 -R may be rotated by an angle with reference to intermediate image plane SP 1  such that it is not perpendicular to primary optical axis  404 . In some embodiments, virtual intermediate image plane SP 1 -R may be rotated by an angle based on the deflection angle of secondary electron beams. As shown in  FIG. 4A , on-axis secondary electron beam  461  may be deflected substantially parallel to and along secondary optical axis  451 , and virtual intermediate image plane SP 1 -R may be not substantially perpendicular to secondary optical axis  451 . 
     In conventional multi-beam inspection systems, electron detection device  440  and secondary projection imaging system  450  may be placed substantially perpendicular to secondary optical axis  451 . Secondary projection imaging system  450  may be configured to focus exiting secondary electron beams  461   a  and  462   a  onto plane SP 2  and therefore, image virtual intermediate images  461 R and  462 R thereon, i.e. forming images  461 Ri and  462 Ri of the probe spots on plane SP 2 . Hence, plane SP 2  is a final image plane of the probe spots or a final focus plane of secondary electron beams  461  and  462 . In some embodiments, virtual intermediate image plane SP 1 -R may not be substantially perpendicular to secondary optical axis  451 , and accordingly, final image plane SP 2  may not be perpendicular to secondary optical axis  451 . As a result, final image plane SP 2  may not overlap with a detection surface  440 D of electron detection device  440 . Detection surface  440 D may be substantially perpendicular to secondary optical axis  451 . Detection surface  440 D may comprise an electron-collecting or an electron-receiving surface of a detection element of electron detection device  440 . In some embodiments, detection elements of electron detection device  440  may be arranged such that the detection surfaces of all detection elements form a single coplanar detection surface  440 D. 
     In some embodiments, final image plane SP 2  may form an angle α with reference to detection surface  440 D. The mismatch of final image plane SP 2  and detection surface  440 D of electron detection device  440  may result in exiting secondary electron beams to be defocused on detection surface  440 D of electron detection device  440  and the exiting secondary beam array to be deformed on detection surface  440 D of electron detection device  440 . The defocused secondary electron beams and deformed secondary beam array may cause crosstalk and reduce collection or detection efficiency and resultantly may negatively impact the inspection resolution and inspection throughput, among other things. 
     Reference is now made to  FIGS. 4B and 4C , which illustrate schematic diagrams of exemplary projections of exiting secondary electron beams  461   a - 469   a  on final image plane SP 2  and on detection surface  440 D of electron detection device  440 , respectively, consistent with embodiments of the present disclosure. exiting secondary electron beams  461   a - 469   a  may form an array  460  on final image plane SP 2 , as shown in  FIG. 4B . In some embodiments, array  460  may comprise a diamond, a rectangular, or a square array. Projections of focused exiting secondary electron beams  461   a - 469   a  may be substantially circular in cross-section on final image plane SP 2 . 
       FIG. 4C  illustrates a projection of exiting secondary electron beams  461   a - 469   a  on detection surface  440 D of electron detection device  440 . Projections of exiting secondary electron beams  461   a - 469   a  on detection surface  440 D of electron detection device  440  may be represented by an array  470 . In some embodiments, the tilt angle of final image plane SP 2  with reference to detection surface  440 D may cause projections of one or more exiting secondary electron beams  461   a - 469   a  on detection surface  440 D non-circular. The tilt angle of final image plane SP 2  with reference to detection surface  440 D may cause the projection of exiting secondary electron beams  461   a - 469   a  to form a deformed array  470  on detection surface  440 D. Deformed array  470  may comprise a tilted array of non-circular projections of exiting secondary electron beams  461   a - 469   a . In the context of this disclosure, a deformed array (e.g., deformed array  470  of  FIG. 4C ) may comprise an array of projections of defocused secondary electron beams or an array having an outline of projections of out-of-focus secondary electron beams different compared to the outline of in-focus secondary electron beams. 
     As illustrated in  FIG. 4C , the size of the projection of an exiting secondary electron beam on detection surface  440 D of electron detection device  440  may be based on a distance between the focus position on the final image plane SP 2  of secondary electron beams  461   a - 469   a  and detection surface  440 D. For example, on-axis exiting secondary electron beam  461   a  may appear smaller than the off-axis exiting secondary electron beams  462   a  because on-axis exiting secondary electron beam  461   a  on final image plane SP 2  is closer to detection surface  440 D compared to off-axis exiting secondary electron beam  462   a . The final image plane SP 2  may be formed at an angle with respect to detection surface  440 D such that exiting secondary electron beams may be in focus, for example, upstream of electron detection device  440 , at or close to detection surface  440 D of electron detection device  440 , or downstream of electron detection device  440 . In the context of this disclosure, “upstream” or “downstream” may refer to the location of a system element with reference to another element along the path of the secondary electron beam. For example, if element A is downstream of element B, it is appreciated that element A is located after element B along the secondary electron beam path. If element A is upstream of element B, it is appreciated that element A is located before element B along the secondary electron beam path. It is appreciated that array  460  may comprise beam astigmatism and beam array deformation due to beam separator  433 , resulting in a non-circular projection of one or more exiting secondary electron beams. It is appreciated that array  470  comprises projection of modified secondary electron beams  461   a - 469   a  that are not corrected for astigmatism aberration, resulting in a non-circular projection of one or more modified secondary electron beams. It is to be also appreciated that while final image plane SP 2  is shown tilted at an angle α with reference to Z2-axis, it may be tilted in any of X2-, Y2-, or Z2-axis, or a combination thereof. 
     The deformation of secondary electron beams array and defocused secondary electron beams incident on electron detection device  440  may cause a reduction in collection efficiency, or crosstalk, among other things. For example, if the pitches and sizes of electron detectors (e.g., detection elements  241 ,  242 , and  243  of  FIG. 2 ) cannot cover the deviation of position of exiting secondary electron beams  461   a - 469   a  in deformed array  470 , the image signal from one electron detector may include information from more than one exiting secondary electron beams, thus crosstalk may occur. In some embodiments, only a portion of electrons of an exiting secondary electron beam may be collected by the corresponding electron detector, negatively impacting the inspection throughput and resolution. 
     In existing multi-beam inspection tools such as a multi-beam SEM, some of the challenges encountered include, among other things, limitations in collection efficiency of each secondary electron beam of secondary electron beam array and crosstalk among secondary electron beams of secondary electron beam array. One of several factors that may cause reduction in collection efficiency is a large spot size of the secondary electron beam incident on the detection surface of the electron detector. The size of the beam incident on the electron detector depends on, among other things, the location of the electron detector with reference to the final focus plane of the secondary electron beams. For example, the size of the secondary electron beam incident on the electron detector may increase as the distance between the position of the secondary electron beam on the final focus plane of the secondary electron beams and the electron detector increases. Therefore, it may be desirable to configure the electron detection device to make its detection surface overlap with the final image plane of the secondary electron beams as much as possible. 
     In some embodiments, the tilt angle of final image plane SP 2  of the secondary electron beams with reference to the detection surface  440 D may change with the rotation angle of virtual intermediate image plane SP 1 -R with reference to intermediate image plane SP 1 . The rotation angle may change with the position of intermediate image plane SP 1  and excitations of beam separator  433 . The position of intermediate image plane SP 1  and excitations of beam separator  433  may change with landing energies or probe currents of primary electron beams on sample  408 , among other things. Hence, the tilt angle of final image plane SP 2  of the secondary electron beams with reference to detection surface  440 D may vary based on the application conditions, among other things. Therefore, it may be desirable to minimize the tilt angle within the range of the application conditions by tilting the electron detection device by a fixed optimal angle or an adjustable angle with respect to secondary optical axis  451 , as shown in  FIG. 5 , while maintaining the inspection throughput. 
     Reference is now made to  FIG. 5 , which illustrates a schematic diagram of a multi-beam inspection apparatus  500  that can be a part of the exemplary charged particle beam inspection system  100  of  FIG. 1 , consistent with embodiments of the present disclosure. Multi-beam inspection apparatus  500  (also referred to herein as apparatus  500 ) may comprise a primary projection optical system (analogous to primary projection optical system  230  of  FIG. 2 ) including objective lens  531  and beam separator  533 , among other things. Apparatus  500  may further comprise a secondary projection imaging system  550  (analogous to secondary projection imaging system  250  of  FIG. 2 ) and an electron detection device  540  having a detection surface  540 D configured to detect secondary electrons, among other components. It may be appreciated that other components of apparatus  40  may be added/omitted, as appropriate. 
     In some embodiments, in apparatus  500 , primary electron beamlets (not shown, e.g., primary electron beamlets  211  and  212  of  FIG. 2 ) traveling along primary optical axis  504  may land on a surface of sample  508 . Upon interaction with sample  508 , secondary electrons or backscattered electrons may be generated from sample  508 . The generated secondary electrons and backscattered electrons may travel in the opposite direction of primary electron beams along primary optical axis  504 . Apparatus  500  may be operated in a secondary electron inspection mode or a backscattered electron inspection mode, or both, based on the energy of the electrons generated from sample  508 . In a secondary electron inspection mode, electron detection device  440  may be configured to detect secondary electron beams  461  and  462 . In some embodiments, objective lens  531  may be substantially similar to and may perform substantially similar functions as objective lens  431  of  FIG. 4A . Objective lens  531  may be configured to focus secondary electron beams  561  and  562  generated from corresponding probe spots on sample  508  and to form images of the probe spots on intermediate image plane SP 1 . 
     Secondary electron beams  561  and  562  are directed towards beam separator  533  (e.g., a Wien filter) configured to isolate primary electron beams and secondary electron beams, for example, based on the energy, or velocity, among other things. Beam separator  533  may be configured to deflect secondary electron beams  561  and  562  and form exiting secondary electron beams  561   a  and  562   a , respectively. Exiting secondary electron beams  561   a  and  562   a  may be directed towards secondary projection imaging system  550  along Z2-axis, also referred to herein as secondary optical axis  551 , as illustrated in  FIG. 5 . Exiting secondary electron beam  561   a  may comprise an on-axis exiting secondary electron beam and modified secondary electron beam  562   a  may comprise an off-axis exiting secondary electron beam. In the context of this disclosure, “on-axis” may refer to electron beams that are substantially parallel, aligned, or coincident with the reference axis, and “off-axis” may refer to electron beams that are non-parallel or not aligned with the reference axis. It is appreciated that in a multi-beam configuration, there may be more than one off-axis primary electron beams and corresponding exiting secondary electron beams. 
     In some embodiments, secondary projection imaging system  550  may comprise a stigmator  555  configured to compensate astigmatisms of secondary electron beams such as  561  and  562 . One of the several factors that may cause astigmatism is the beam astigmatism generated by beam separator  533 , as discussed in reference to  FIGS. 3A-3E . The beam astigmatism may refer to deformation of the electron beam profile caused when the electrons in the beam pass through deflection fields of beam separator  533 . 
     Stigmators such as stigmator  555  may be configured to apply a correcting quadrupole field to the secondary electron beams as the beams pass through. The quadrupole field experienced by the secondary electrons may be adjusted by adjusting the electrical excitation of stigmator  555 . Adjusting the electrical excitation of stigmator  555  may include, but is not limited to, adjusting a voltage or a coil current applied to one or more poles of stigmator  555 . In some embodiments, stigmator  555  may be placed on or close to a cross-over plane of secondary electron beams. In some embodiments, though not illustrated, secondary projection imaging system  550  may further include a zoom lens, a projection lens, anti-scanning deflection unit, and the like. 
     In some embodiments, adjusting the electrical excitation of stigmator  555  may comprise adjusting the profile of exiting secondary electron beams  561   a  and  562   a  from a non-circular cross-section to a substantially circular cross-section. In some embodiments, the electrical excitation of stigmator  555  may be adjusted to adjust the profile of one or more secondary electron beams, as desired. In some embodiments, the electrical excitation of stigmator  555  may be adjusted based on the application, the desired analysis, the sample, the desired throughput, among other things. 
     Secondary projection imaging system  550  may be configured to focus exiting secondary electron beams  561   a  and  562   a  on final image plane SP 2 , also referred to herein as final image plane. Final image plane SP 2  may comprise a focus plane of exiting secondary electron beams  561   a  and  562   a . The profile of focused exiting secondary electron beams  561   a  and  562   a  may be substantially circular, as shown in  FIG. 5 , after exiting stigmator  555  of secondary projection imaging system  550 . In some embodiments, final image plane SP 2  may comprise a flat focus plane such that the exiting secondary electron beams are focused on the same flat plane. 
     In practice, however, exiting secondary electron beams of a beam array may be focused on a curved focus plane comprising a plurality of flat focus planes. One of the several reasons for non-coplanar focus of exiting secondary electron beams may include, but is not limited to, field curvature aberration. In the context of this disclosure, field curvature aberration, also known as Petzval field curvature, may refer to as an imaging artifact or an aberration in which a flat object normal to the optical axis cannot be brought properly into focus on a flat image plane. When visible light is focused through a lens, the image plane produced by the lens is a curved Petzval surface. The image can be focused over a large number of focus planes to produce either a sharp focus on the edges or in the center of the image. When the specimen is viewed in a microscope, it either appears sharp and crisp in the center or on the edges of the view field, but not both. This artifact is commonly referred to as field curvature or curvature of field and the aberration caused thereby is known as field curvature aberration. 
     In some embodiments, final image plane SP 2  may be non-perpendicular to secondary optical axis  551 , in part because virtual intermediate image plane SP 1 -R formed by objective lens  531  and beam separator  533  is not perpendicular to secondary optical axis  551 , among other things. In conventional multi-beam inspection systems, an electron detector may be placed perpendicular to a secondary optical axis, while the final image plane is non-perpendicular to secondary optical axis. Such a configuration may result in one or more exiting secondary electron beams to be defocused on detection surface of electron detection device, and deformation of exiting secondary electron beam array in the direction of the tilt of the final image plane. The beam defocus and beam array deformation may cause a reduction in collection efficiency, and a reduction in throughput, among other things. 
     In multi-beam inspection systems such as apparatus  500 , occurrence of crosstalk and reduction in collection efficiency may be mitigated by tilting electron detection device  540  to make its detection surface overlap with the final image plane SP 2 . The tilting angle of electron detection device  540  may be fixed at a value optimized for all of application conditions or may be adjustable. In some embodiments, the position or orientation of electron detection device  540  may be adjustable along X2-, Y2-, or Z2-axes, or a combination thereof. Adjustable electron detection device  540  may be placed downstream of secondary projection imaging system  550 . 
     In some embodiments, electron detection device  540  may be disposed along a detection plane (not shown). In some embodiments, the detection plane of electron detection device  540  may form an angle α with final image plane SP 2 , as illustrated in  FIG. 4A . Adjusting the position of electron detection device  540  may comprise adjusting the angle α between the detection plane of electron detection device  540  and final image plane SP 2 . In some embodiments, adjusting the angle α may comprise tilting electron detection device  540  along one or more axes by a tilting angle to reduce the angle α between the detection plane of electron detection device  540  and final image plane SP 2 . In some embodiments, the angle α may be reduced such that the principal plane of electron detection device  540  may substantially coincide with final image plane SP 2 . In this context, “substantially coincident” planes may refer to overlapping or almost overlapping planes such that the angle between the planes is less than 5°. In a preferred embodiment, the angle between almost overlapping planes is between 0° to 1°. 
     In some embodiments, electron detection device  540  may comprise an array of detection elements (e.g., detection elements  241 - 243  of  FIG. 2 ). Electron detection device  540  may comprise a rectangular array, a square array, a triangular array, a circular array, or an irregular array of detection elements. In some embodiments, detection elements of electron detection device  540  may comprise a scintillator, a solid-state detector, a scintillator-photomultiplier assembly, among other things. In some embodiments, detection surface  540 D of detection elements of electron detection device  540  may represent the surface on which the secondary electron beam may be incident. 
     In some embodiments, controller  50  may be configured to communicate with and adjust the movement of electron detection device  540 . Controller  50  may be configured to cause dynamic adjustment of the position and orientation of electron detection device  540  based on the determined collection efficiency of electron detection device  540 . For example, if the collection efficiency increases by tilting electron detection device  540  along an axis, controller  50  may continue tilting electron detection device  540  along the axis. 
     In some embodiments, the position or orientation of electron detection device  540  may be adjusted by adjusting the tilting angle in one or more axes based on the application, the desired analysis, the sample, the desired throughput, among other things. In some embodiments, the tilting angle may be adjusted by a predetermined value based on the application, the desired analysis, the sample, the desired throughput, among other things. The predetermined value of tilting angle may be an optimal tilting angle based on a range of landing energies, probe currents, and on-sample electric field, among other things. 
     In some embodiments, electron detection device  540  may be aligned with secondary optical axis  551  such that the geometric center of detection surface  540 D of electron detection device  540  intersects with secondary optical axis  551 , as illustrated in  FIG. 5 . The spot size of a secondary electron beam incident on a detection element may depend on a scalar distance between its positions on final image plane SP 2  and detection surface  540 D. Electron detection device  540  may be placed to minimize the scalar distances of all the secondary electron beams. 
     Reference is made to  FIG. 6 , which illustrates an exemplary configuration of an electron detection device  640  and projection of exiting secondary electron beams incident thereon, consistent with embodiments of the present disclosure. Electron detection device  640  may be substantially similar or perform substantially similar functions as electron detection device  540  of  FIG. 5 . In some embodiments, electron detection device  640  may include an array of detection elements  641 - 649  configured to detect secondary electrons of exiting secondary electron beams  661   a - 669   a . In some embodiments, each exiting secondary electron beam may have a corresponding detection element, as shown in  FIG. 6 . Such a configuration may provide some advantages, including, but are not limited to, reduced crosstalk, increased throughput, or higher collection efficiency. 
       FIG. 6  illustrates projections of tilting secondary electron beams  661   a - 669   a  on a detection surface  640 D of electron detection device  640 . Detection surface  640 D may be configured to receive or collect secondary electrons of exiting secondary electron beams  661   a - 669   a  exiting secondary projection imaging system (e.g., secondary projection imaging system  550  of  FIG. 5 ), and containing information about probed regions of a sample (e.g., sample  508  of  FIG. 5 ). It is appreciated that substantially circular cross-sections of projections of tilting secondary electron beams  661   a - 669   a  on detection surface  640 D indicate that tilting secondary electron beams  661   a - 669   a  are substantially focused on detection surface  640 D. One of the several methods to substantially focus exiting secondary electron beams  661   a - 669   a  and enhance collection efficiency, among other things, may include compensating the astigmatism aberration using a stigmator (e.g., stigmator  555  of  FIG. 5 ) and adjusting the position or orientation of detection surface  640 D of electron detection device  640  to reduce the angle α formed between final image plane SP 2  and a detection surface (e.g., detection surface  540 D of  FIG. 5 ). In some embodiments, to maximize the collection efficiency, among other things, the angle α may be reduced such that detection surface  640 D substantially coincides with final image plane SP 2 . 
     In some embodiments, the surface area of two or more detection elements  641 - 649  may be similar. The surface area or the electron collection area may be based on the size or cross-section of the incident exiting secondary electron beams  661   a - 669   a . For example, the surface area of detection element  641  may be larger than the size of the incident exiting secondary electron beam  661   a  to collect and detect substantially all secondary electrons of exiting secondary electron beam  661   a , thereby maximizing collection efficiency, and inspection throughput, among other things. In some embodiments, the position or orientation of electron detection device  640  may be adjusted such that substantially all secondary electrons of an exiting secondary electron beams  661   a - 669   a  may be collected by their corresponding detection elements  641 - 649 , to maximize collection efficiency and inspection throughput, among other things. 
     In some embodiments, detection elements  641 - 649  may be arranged in an array, the array comprising a square array, a rectangular array, a circular array, a triangular array, an elliptical array, or the like. The detection elements arranged in an array may have a uniform or a non-uniform pitch along X2-axis or Y2-axis. It is appreciated that though  FIG. 6  illustrates electron detection device  640  comprising nine detection elements  641 - 649  configured to collect secondary electrons of nine exiting secondary electron beams  661   a - 669   a  generated by a 3×3 array of primary electron beamlets incident on a surface of the sample (e.g., sample  508  of  FIG. 5 ), more or fewer detection elements may be employed, based on a number of secondary electron beams generated, among other things. 
     In some embodiments, in addition to compensating the astigmatism aberration and tilting electron detector (e.g., electron detection device  540  of  FIG. 5 ), orientation of primary electron beamlets (e.g., primary beamlets  211 - 213  of  FIG. 2 ) with reference to X1- and Y1-axes may be adjusted such that the generated secondary electron beams  361 - 363  may be aligned in the deflection direction of beam separator  333 . It is appreciated that although  FIG. 2  illustrates only three electron beamlets, at least two or more electron beams may be used, as appropriate. For example, a 3×3 array of nine primary electron beams may be used to generate a 3×3 array of secondary electron beams (e.g., secondary electron beams  361 - 369  of  FIG. 3C ). It is to be appreciated that beam separators  233 ,  333 ,  433 , and  533  may be substantially similar and may perform substantially similar functions. 
     In some embodiments, adjusting the orientation of primary electron beamlets may include rotating the primary electron beamlets around primary optical axis (e.g., primary optical axis  204  of  FIG. 2 ) such that the resultant secondary electron beams may be rotated correspondingly to form a secondary electron beam array  740 , as shown in  FIG. 7A . Secondary electron beam array  740  may comprise a substantially square 3×3 array of secondary electron beams  761 ,  762 ,  763 ,  764 ,  765 ,  766 ,  767 ,  768 , and  769 , originating upon interaction of primary electron beamlets with a sample (e.g., sample  208  of  FIG. 2 ), and directed towards beam separator (e.g., beam separator  533  of  FIG. 5 ). Secondary electron beam array  740 , as illustrated in  FIG. 7A , may represent a projection of an array of secondary electron beams  761 - 769  on a plane  710  before entering beam separator  533 . Plane  710  may be a plane substantially parallel to the plane comprising X1- and Y1-axes (e.g., X1- and Y1-axes shown in  FIG. 4A ), and substantially perpendicular to Z1-axis (e.g., Z1-axis shown in  FIG. 4A ). As a visual aid, Z1-axis may be visualized as extending in-and-out of the paper. 
     In some embodiments, the angle of rotation of primary electron beamlets to cause formation of secondary electron beam array  740  of secondary electron beams  761 - 769  may be determined based on, but is not limited to, the orientation of source conversion unit  220 , among other things. The orientation of source conversion unit  220  to adjust the rotation of primary electron beamlets may be adjusted such that secondary electron beam array  740  is aligned in the direction of deflection of secondary electron beams  761 - 769  by beam separator  533 . In some embodiments, the orientation of source conversion unit  220  may be predetermined to an optimal value based on factors including, but are not limited to, the application, the desired analysis, the sample, objective lens excitation, landing energy of primary electron beamlets, among other things. In some embodiments, however, the orientation of source conversion unit  220  may be adjusted dynamically based on the collection efficiency of electron detectors, desired inspection throughput, the application, the desired analysis, the sample, among other things. 
       FIG. 7B  illustrates an exiting secondary electron beam array  760  representing a projection of an array of exiting secondary electron beams  761   a - 769   a  on a plane  720  downstream of beam separator (e.g., beam separator  533  of  FIG. 5 ), before entering secondary projection imaging system (e.g., secondary projection imaging system  550  of  FIG. 5 ). In this context, exiting secondary electron beams  761   a - 769   a  may refer to secondary electron beams deflected by the beam separator such that they are directed towards the secondary projection imaging system. In some embodiments, exiting secondary electron beams  761   a - 769   a  after exiting beam separator may be non-circular in cross-section, as illustrated in  FIG. 7B . For example, the cross-section of exiting secondary electron beams  361   a - 369   a  may comprise an oval, an elliptical, or a non-circular shape, based on the performance of the beam separator, among other things. 
     In some embodiments, exiting secondary electron beam array  760  may comprise a rectangular array (deformed array) of exiting secondary electron beams  761   a - 769   a  having non-circular cross-sections. One of several factors causing the variation of secondary electron beam cross-section and beam array deformation includes deflection performance of a beam separator (e.g., beam separator  533  of  FIG. 5 ) as shown and explained in  FIGS. 3A-3D , among other things. 
     Reference is now made to  FIG. 8 , which illustrates a process flowchart representing an exemplary method  800  of forming an image of a sample using multiple beams in a multi-beam inspection system, consistent with embodiments of the present disclosure. Method  800  may be performed by controller  50  of EBI system  100 , as shown in  FIG. 1 , for example. Controller  50  may be programmed to perform one or more blocks of method  800 . For example, controller  50  may apply an electrical signal to a stigmator (e.g., stigmator  555  of  FIG. 5 ) to adjust the quadrupole field thereof and compensate the astigmatism aberration of the secondary electron beams, and carry out other functions. 
     In step  810 , a plurality of secondary electron beams may be generated from a sample (e.g., sample  208  of  FIG. 2 ). A charged particle source (e.g., electron source  201  of  FIG. 2 ) may be activated to generate a charged particle beam (e.g., primary electron beam  202  of  FIG. 2 ). The electron source may be activated by a controller (e.g., controller  50  of  FIG. 2 ). For example, the electron source may be controlled to emit primary electrons to form an electron beam along a primary optical axis (e.g., primary optical axis  204  of  FIG. 2 ). The electron source may be activated remotely, for example, by using a software, an application, or a set of instructions for a processor of a controller to power the electron source through a control circuitry. 
     A plurality of primary electron beamlets (e.g., primary beamlets  211 ,  212 , and  213  of  FIG. 2 ) may be generated from the primary electron beam and focused on the sample using an objective lens (e.g., objective lens  231  of  FIG. 2 ). The focused primary electron beamlets, upon interaction with the sample, may form a plurality of probe spots on the sample and generate a plurality of secondary electron beams (e.g., secondary electron beams  361 - 369  of  FIG. 3C ). The generated secondary electron beams may be directed towards a beam separator (e.g., beam separator  233  of  FIG. 2 ) configured to deflect the secondary electron beams towards a secondary projection imaging system (e.g., secondary projection imaging system  250  of  FIG. 2 ). The secondary electron beams entering the beam separator may be deflected to travel along a secondary optical axis (e.g., secondary optical axis  251  of  FIG. 2 ) to form exiting secondary electron beams (e.g., modified secondary electron beams  361   a - 369   a  of  FIG. 3 ). 
     The deflection of a secondary electron beam may be related to the position of the secondary electron beam with reference to the primary optical axis along which beam separator is placed, among other things. For example, off-axis secondary electron beams away from the center of X1-Y1-axes, may be deflected by a larger distance than an on-axis secondary electron beam. 
     In a multi-beam inspection apparatus, such as a multi-beam SEM, using a Wien filter (e.g., beam separator  333  of  FIG. 3B ) to separate primary electron beams from secondary electron beams may cause beam astigmatism aberration, and beam array deformation, among other things. The imaging resolution in a SEM may depend on, among other things, focus of the secondary electron beams incident on a detection element (e.g., detection element  241  of  FIG. 2 ) of an electron detection device (e.g., electron detection device  240  of  FIG. 2 ) such as a secondary electron detector, quality of the imaging signals received, collection efficiency and detection efficiency of electron detection devices. One of several factors impacting the focus of a secondary electron beam may be astigmatism aberration, which may cause beam profile distortion, or beam array deformation, among other things. A defocused electron beam may have a large incidence spot on a secondary electron detector. In conventional multi-beam SEMs, the defocused electron beam may be incident upon multiple detection elements of the secondary electron detector. In other words, each of the multiple detection elements may receive secondary electrons from a corresponding secondary electron beam and other adjacent beams. Consequently, the imaging signal of one detection element may comprise a main component originating from the corresponding secondary electron beam and a crosstalk component originating from adjacent electron beams. The occurrence of crosstalk may reduce the collection efficiency and the inspection throughput, among other things. 
     In some embodiments, a stigmator (e.g., stigmator  555  of  FIG. 5 ) may be configured to compensate astigmatism aberration caused by the beam separator. One or more stigmators may be configured to apply a correcting magnetic or electric field to the secondary electron beams as the beams pass through the secondary projection imaging system. The magnetic or electric field experienced by the secondary electrons may be adjusted by adjusting the electrical excitation of stigmator. Adjusting the electrical excitation of stigmator may include, but is not limited to, adjusting a voltage or a coil current applied to one or more poles of the stigmator. 
     In some embodiments, adjusting the electrical excitation of the stigmator may comprise adjusting the profile of exiting secondary electron beams from a non-circular cross-section to a substantially circular cross-section. 
     In step  820 , the plurality of secondary electron beams may be focused on a focus plane (e.g., final image plane SP 2  of  FIG. 5 ). The final image plane SP 2  may comprise a focus plane of exiting secondary electron beams. The profile of focused exiting secondary electron beams may be substantially circular after exiting the stigmator. Final image plane SP 2  may comprise a flat focus plane such that the exiting secondary electron beams are focused on the same flat focus plane. In practice, however, exiting secondary electron beams of a beam array may be focused on a curved focus plane comprising a plurality of flat focus planes. One of the several reasons for non-coplanar focus of modified secondary electron beams may include, but is not limited to, field curvature aberration. 
     Flat focus image plane SP 2  may be non-perpendicular to the secondary optical axis, in part because virtual intermediate image plane SP 1 -R formed by the objective lens and beam separator is not perpendicular to the secondary optical axis, among other things. In conventional multi-beam inspection systems, an electron detector may be placed perpendicular to the secondary optical axis, while final image plane SP 2  is non-perpendicular to secondary optical axis. Such a configuration may result in one or more exiting secondary electron beams to be defocused on detection surface (e.g., detection surface  540 D of  FIG. 5 ) of electron detection device, for example, and deformation of exiting secondary electron beam array on the detection surface in the direction of the tilt of final image plane SP 2 . The beam defocus and beam array deformation may cause a reduction in collection efficiency, an increase in crosstalk, and a reduction in throughput, among other things. 
     In step  830 , the detection surface of the electron detection device may be positioned with respect to the focus plane. In some embodiments, the position of the electron detection device with respect to the focus plane may be adjusted. The electron detection device may be disposed along a detection plane. In some embodiments, the detection plane of electron detection device may form an angle α with final image plane SP 2 , as illustrated in  FIG. 4A . Adjusting the position of electron detection device may comprise adjusting the angle α between the detection plane of electron detection device and final image plane SP 2 . Adjusting the angle α may comprise tilting electron detection device along one or more axes by a tilting angle to reduce the angle α between the detection plane of electron detection device and final image plane SP 2 . The angle α may be reduced such that the detection plane of electron detection device  540  and final image plane SP 2  may coincide. 
     Reference is now made to  FIG. 9 , which illustrates a process flowchart representing an exemplary method  900  of forming an image of a sample using multiple beams in a multi-beam inspection system, consistent with embodiments of the present disclosure. Method  900  may be performed by controller  50  of EBI system  100 , as shown in  FIG. 1 , for example. Controller  50  may be programmed to perform one or more blocks of method  900 . For example, controller  50  may apply an electrical signal to a stigmator (e.g., stigmator  555  of  FIG. 5 ) to adjust the electric or magnetic field and compensate the astigmatism aberration of the secondary electron beams, and carry out other functions. 
     In step  910 , a plurality of primary electron beamlets (e.g., primary beamlets  211 ,  212 , and  213  of  FIG. 2 ) may be generated from a primary electron beam (e.g., primary electron beam  202  of  FIG. 2 ) and focused on the sample using an objective lens (e.g., objective lens  231  of  FIG. 2 ). The focused primary electron beamlets, upon interaction with the sample, may form a plurality of probe spots on the sample and generate a plurality of secondary electron beams (e.g., secondary electron beams  361 - 369  of  FIG. 3C ). The generated secondary electron beams may be directed towards a beam separator (e.g., beam separator  233  of  FIG. 2 ) configured to deflect the secondary electron beams towards a secondary projection imaging system (e.g., secondary projection imaging system  250  of  FIG. 2 ). The secondary electron beams entering the beam separator may be deflected along a secondary optical axis (e.g., secondary optical axis  251  of  FIG. 2 ) to form exiting secondary electron beams (e.g., modified secondary electron beams  361   a - 369   a  of  FIG. 3 ). 
     In step  920 , an orientation of the primary electron beamlets may be adjusted such that the resulting secondary electron beams are aligned with the direction of deflection of the secondary electron beams by the beam separator. Adjusting the orientation of the primary electron beamlets may comprise rotating the primary electron beamlets around the primary optical axis such that the resultant secondary electron beams may be rotated correspondingly to form a secondary electron beam array aligned along the X1- and Y1-axes. 
     The angle of rotation of primary electron beamlets to cause formation of secondary electron beam array may be determined based on including, but is not limited to, the orientation of source conversion unit (e.g., source conversion unit  220  of  FIG. 2 ), among other things. The orientation of source conversion unit to adjust the rotation of primary electron beamlets may be adjusted such that secondary electron beam array is aligned in the direction of deflection of secondary electron beams by the beam separator. 
     In some embodiments, a stigmator (e.g., stigmator  555  of  FIG. 5 ) may be configured to compensate astigmatism aberration caused by the beam separator. One or more stigmators may be configured to apply a correcting magnetic or electric field to the secondary electron beams as the beams pass through the secondary projection imaging system. The magnetic or electric field experienced by the secondary electrons may be adjusted by adjusting the electrical excitation of stigmator. Adjusting the electrical excitation of stigmator may include, but is not limited to, adjusting a voltage or a coil current applied to one or more poles of the stigmator. 
     In step  930 , an image of the plurality of probe spots of the sample may be formed on a final image plane (e.g., final image plane SP 2  of  FIG. 5 ). Final image plane SP 2  may comprise a focus plane of exiting secondary electron beams. The profile of focused exiting secondary electron beams may be substantially circular after exiting the stigmator. Final image plane SP 2  may comprise a flat focus plane such that the exiting secondary electron beams are focused on the same flat focus plane. In practice, however, exiting secondary electron beams of a beam array may be focused on a curved focus plane comprising a plurality of flat focus planes. 
     In step  940 , a position of the electron detection device with reference to the position of the final image plane may be adjusted. The electron detection device may be disposed along a detection plane. In some embodiments, the detection plane of electron detection device may form an angle α with final image plane SP 2 , as illustrated in  FIG. 4A . Adjusting the position of electron detection device may comprise adjusting the angle α between the detection plane of electron detection device and final image plane SP 2 . Adjusting the angle α may comprise tilting electron detection device along one or more axes by a tilting angle to reduce the angle α between the detection plane of electron detection device and final image plane SP 2 . The angle α may be reduced such that the detection plane of electron detection device  540  and final image plane SP 2  may coincide. 
     A non-transitory computer readable medium may be provided that stores instructions for a processor of a controller (e.g., controller  50  of  FIG. 1 ) to carry out image inspection, image acquisition, activating charged-particle source, adjusting electrical excitation of stigmators, adjusting landing energy of electrons, adjusting objective lens excitation, adjusting secondary electron detector position and orientation, stage motion control, beam separator excitation, etc. 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 embodiments may further be described using the following clauses: 
     1. A method performed by a multi-beam apparatus to form images of a sample, the method comprising: 
     generating a plurality of secondary electron beams from a plurality of probe spots on the sample along a primary-optical axis upon interaction with a plurality of primary electron beams; 
     focusing the plurality of secondary electron beams onto a focus plane; and 
     positioning a detection surface of a secondary electron detector with respect to the focus plane. 
     2. The method of clause 1, wherein the plurality of secondary electron beams comprises an array of secondary electron beams. 
     3. The method of any one of clauses 1 and 2, further comprising adjusting an orientation of the plurality of primary electron beams interacting with the sample. 
     4. The method of clause 3, wherein adjusting the orientation of the plurality of primary electron beams adjusts an orientation of the array of secondary electron beams. 
     5. The method of any one of clauses 3 and 4, wherein adjusting the orientation of the plurality of primary electron beams comprises rotating the plurality of primary electron beams around the primary optical axis. 
     6. The method of any one of clauses 1-5, further comprising directing, using a beam separator, the plurality of secondary electron beams towards the secondary electron detector along a secondary optical axis. 
     7. The method of any one of clauses 1-6, further comprising adjusting an electrical excitation of a stigmator to compensate astigmatism aberration of the plurality of secondary electron beams. 
     8. The method of any one of clauses 1-7, wherein the secondary electron detector is disposed downstream of a secondary electron projection system configured to focus the plurality of secondary electron beams on the focus plane. 
     9. The method of clause 8, wherein the secondary electron detector comprises a plurality of detection elements, and wherein a detection element of the plurality of detection elements is associated with a corresponding secondary electron beam of the plurality of secondary electron beams. 
     10. The method of any one of clauses 8 and 9, wherein positioning the detection surface of the secondary electron detector comprises adjusting a tilting angle between the detection surface and the focus plane. 
     11. The method of clause 10, wherein adjusting the tilting angle comprises reducing the tilting angle between the detection surface of the secondary electron detector and the focus plane. 
     12. The method of clause 11, wherein reducing the tilting angle comprises adjusting the position of the secondary electron detector such that the detection surface of the secondary electron detector substantially coincides with the focus plane. 
     13. The method of clause 12, wherein adjusting the position of the secondary electron detector comprises dynamically adjusting the tilting angle based on a collection efficiency of the secondary electron detector. 
     14. The method of any one of clauses 12-13, wherein adjusting the position of the secondary electron detector comprises adjusting the tilting angle to a predetermined value of the tilting angle. 
     15. The method of any one of clauses 12-14, wherein adjusting the position of the secondary electron detector comprises adjusting the tilting angle in one or more planes with reference to the secondary optical axis. 
     16. A method performed by a multi charged-particle beam apparatus to form images of a sample, the method comprising: 
     generating a plurality of secondary electron beams from a plurality of probe spots on the sample along a primary optical axis upon interaction with a plurality of primary electron beams; 
     adjusting an orientation of the plurality of primary electron beams interacting with the sample; 
     forming images of the plurality of probe spots of the sample on a final image plane; and 
     positioning a detection surface of a secondary electron detector with respect to a position of the final image plane. 
     17. The method of clause 16, wherein the plurality of secondary electron beams comprises an array of secondary electron beams. 
     18. The method of any one of clauses 16 and 17, wherein adjusting the orientation of the plurality of primary electron beams adjusts an orientation of the array of secondary electron beams. 
     19. The method of any one of clauses 16-18, wherein adjusting the orientation of the plurality of primary electron beams comprises rotating the plurality of primary electron beams around the primary optical axis. 
     20. The method of any one of clauses 16-19, further comprising directing, using a beam separator, the plurality of secondary electron beams towards the secondary electron detector along a secondary optical axis. 
     21. The method of any one of clauses 16-20, further comprising adjusting an electrical excitation of a stigmator to compensate astigmatism aberration of the plurality of secondary electron beams. 
     22. The method of any one of clauses 16-21, wherein the secondary electron detector is disposed downstream of a secondary electron projection system configured to form the images of the plurality of probe spots on the final image plane. 
     23. The method of clause 22, wherein the secondary electron detector comprises a plurality of detection elements, and wherein a detection element of the plurality of detection elements is associated with a corresponding secondary electron beam of the plurality of secondary electron beams. 
     24. The method of any one of clauses 22 and 23, wherein positioning the detection surface of the secondary electron detector comprises adjusting a tilting angle between a detection plane of the secondary electron detector and the final image plane. 
     25. The method of clause 24, wherein adjusting the tilting angle comprises reducing the tilting angle between the detection plane of the secondary electron detector and the final image plane. 
     26. The method of clause 25, wherein reducing the tilting angle comprises adjusting the position of the secondary electron detector such that the detection plane of the secondary electron detector substantially coincides with the final image plane. 
     27. The method of clause 26, wherein adjusting the position of the secondary electron detector comprises dynamically adjusting the tilting angle based on a collection efficiency of the secondary electron detector. 
     28. The method of any one of clauses 26-27, wherein adjusting the position of the secondary electron detector comprises adjusting the tilting angle to a predetermined value of the tilting angle. 
     29. The method of any one of clauses 24-28, wherein adjusting the position of the secondary electron detector comprises adjusting the tilting angle in one or more planes with reference to the secondary optical axis. 
     30. A multi-beam apparatus for inspecting a sample using a plurality of primary electron beams configured to form a plurality of probe spots on the sample, the multi-beam apparatus comprising: 
     a secondary electron projection system configured to:
         receive a plurality of secondary electron beams resulting from the formation of the probe spots, and form images of the plurality of probe spots on the sample on a final image plane; and       

     a secondary electron detector configured to detect the plurality of secondary electron beams, wherein a position of the charged-particle detector is set based on a position of the final image plane. 
     31. The multi-beam apparatus of clause 30, wherein the plurality of secondary electron beams comprises an array of secondary electron beams. 
     32. The multi-beam apparatus of any one of clauses 30-31, further comprising an objective lens configured to focus the plurality of primary electron beams on the sample and form images of the plurality of probe spots on an intermediate image plane along a primary optical axis. 
     33. The multi-beam apparatus of any one of clauses 30-32, further comprising a beam separator configured to direct the plurality of secondary electron beams towards the secondary electron detector along a secondary optical axis. 
     34. The multi-beam apparatus of any one of clauses 30-33, further comprising a stigmator configured to compensate astigmatism aberration of the plurality of secondary electron beams. 
     35. The multi-beam apparatus of any one of clauses 30-34, wherein the secondary electron detector is disposed downstream of the secondary electron projection system. 
     36. The multi-beam apparatus of any one of clauses 30-35, wherein the secondary electron detector comprises a plurality of detection elements, and wherein a detection element of the plurality of detection elements is associated with a corresponding secondary electron beam of the plurality of secondary electron beams. 
     37. The multi-beam apparatus of any one of clauses 30-36, wherein a setting of a position of the secondary electron detector comprises an adjusted tilting angle between a detection plane of the secondary electron detector and the final image plane. 
     38. The multi-beam apparatus of clause 37, wherein the setting of the position of the secondary electron detector comprises a reduced tilting angle between the detection plane and the final image plane. 
     39. The multi-beam apparatus of clause 38, wherein the reduced tilting angle comprises the setting of the position of the secondary electron detector such that the detection plane substantially coincides with the final image plane. 
     40. The multi-beam apparatus of any one of clauses 37-39, wherein the setting of the position of the secondary electron detector further comprises a dynamically adjusted tilting angle based on a collection efficiency of the secondary electron detector. 
     41. The multi-beam apparatus of any one of clauses 37-40, wherein the setting of the position of the secondary electron detector further comprises a predetermined value of the tilting angle. 
     42. The multi-beam apparatus of any one of clauses 30-41, wherein the final image plane comprises a curved plane. 
     43. A multi-beam apparatus, comprising: 
     a secondary electron projection system comprising a stigmator configured to influence paths of a plurality of secondary electron beams generated from a plurality of probe spots on a sample; and 
     a secondary electron detector configured to detect the plurality of secondary electron beams, wherein a position of the secondary electron detector is determined based on a position of a final image plane of the plurality of probe spots. 
     44. The apparatus of clause 43, wherein the secondary electron projection system is configured to focus the plurality of secondary electron beams and form the final image plane. 
     45. The apparatus of any one of clauses 43 and 44, wherein the stigmator comprises an electric or a magnetic multi-pole lens. 
     46. The apparatus of any one of clauses 43-45, wherein an adjustment of an electrical excitation of the stigmator compensates astigmatism aberration of the plurality of secondary electron beams. 
     47. The apparatus of any one of clauses 43-46, further comprising an objective lens configured to: 
     focus a plurality of primary electron beams to form the plurality of probe spots on the sample; and 
     form images of the plurality of probe spots on an intermediate image plane substantially perpendicular to a primary optical axis. 
     48. The apparatus of any one of clauses 43-47, further comprising a beam separator configured to direct the plurality of secondary electron beams towards the secondary electron detector along a secondary optical axis. 
     49. The apparatus of any one of clauses 43-48, wherein the secondary electron detector is disposed downstream of the secondary electron projection system. 
     50. The apparatus of clause 49, wherein the secondary electron detector comprises a plurality of detection elements, and wherein a detection element of the plurality of detection elements is associated with a corresponding secondary electron beam of the plurality of secondary electron beams. 
     51. The apparatus of any one of clauses 49 and 50, wherein an adjustment of the position of the secondary electron detector comprises adjustment of a tilting angle between a detection plane of the secondary electron detector and the final image plane. 
     52. The apparatus of clause 51, wherein the adjustment of the position of the secondary electron detector comprises a reduction of the tilting angle between the detection plane and the final image plane. 
     53. The apparatus of clause 52, wherein the reduction of the tilting angle comprises adjustment of the position of the secondary electron detector such that the detection plane substantially coincides with the final image plane. 
     54. The apparatus of any one of clauses 51-53, wherein the adjustment of the position of the secondary electron detector further comprises a dynamic adjustment of the tilting angle based on a detection efficiency of the plurality of secondary electron beams of the secondary electron detector. 
     55. The apparatus of any one of clauses 51-54, wherein the adjustment of the position of the secondary electron detector further comprises setting the tilting angle to a predetermined value. 
     56. A non-transitory computer readable medium storing a set of instructions that is executable by one or more processors of a multi-beam apparatus to cause the multi-beam apparatus to perform a method of forming images of a sample, the method comprising: 
     generating a plurality of secondary electron beams from a plurality of probe spots of a plurality of primary electron beams on the sample along a primary-optical axis; 
     acquiring images of the plurality of probe spots of the sample on a final image plane using a secondary electron detector; and 
     positioning a detection surface of the secondary electron detector based on a position of the final image plane. 
     57. The non-transitory computer readable medium of clause 56, wherein the set of instructions that is executable by one or more processors of the multi-beam apparatus causes the multi-beam apparatus to further perform: 
     forming, using an objective lens, an intermediate image of the plurality of probe spots on an intermediate image plane substantially perpendicular to a primary optical axis; and 
     directing, using a beam separator, the plurality of secondary electron beams towards the secondary electron detector along a secondary optical axis. 
     58. The non-transitory computer readable medium of clause 57, wherein the set of instructions that is executable by one or more processors of the multi-beam apparatus causes the multi-beam apparatus to further perform: 
     adjusting an orientation of the plurality of primary electron beams interacting with the sample, wherein adjusting the orientation of the plurality of primary electron beams comprises rotating the plurality of primary electron beams around the primary optical axis; and 
     adjusting a tilting angle between a detection plane of the charged-particle detector and the final image plane. 
     59. The non-transitory computer readable medium of clause 58, wherein the set of instructions that is executable by one or more processors of the multi-beam apparatus causes the multi-beam apparatus to further perform adjusting an electrical excitation of a stigmator to compensate astigmatism aberration of the plurality of secondary electron beams. 
     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.